Kollmorgen - Motion Control Tips

New dual-axis AKD2G servo drive from Kollmorgen

November 29, 2018 By Miles Budimir Leave a Comment

Kollmorgen’s new dual-axis AKD2G Servo Drive is one of the most power-dense industrial servo drives on the market. It substantially reduces mains wiring, fuses, and filtering to save significant panel space and cost. The AKD2G features expandable I/O (without increasing the size), removable memory, dual-channel STO, and the first drive-resident graphic display in the industry.

Other highlights include: Workbench interface, optimized hybrid connector, and fast speed with its multicore Compute Engine Servo on a Chip. The AKD2G’s current loop update rate of 1.28 µsec accommodates changing load conditions immediately. Velocity and position loop updates are at 62.5 µsec and 125 µsec respectively.

The AKD2G also offers plug-and-play compatibility to Kollmorgen motors, including the company’s recently released AKM2G. The AKM2G is a configurable, high-performance synchronous servo motor with increased power density and improved torque/speed ratio, allowing OEMs to get up to 30% more performance out of an equivalent size motor or achieve the same performance with a smaller motor. Together, the AKD2G and AKM2G create a powerful servo motor system solution.

For more information, visit www.kollmorgen.com.

Difference between continuous ratings and holding continuous loads (an ac servo-motor terminology primer)

October 31, 2018 By Lisa Eitel Leave a Comment

For applications primarily needing the holding of a load with little movement, there are special servo-motor sizing considerations. But confusion often arises from the terminology used to describe such designs — especially from the common use of the term stall in the servo-motor industry. Here we clarify the definition in context and then relate it to the axis’ effective RMS force-torque calculation for proper motor sizing.

By Hurley Gill • Senior applications and systems engineer | Kollmorgen

Expansion of closed-loop feedback systems into nontraditional servo-motor designs has prompted more demand for specialized motor functions. Some of these servo-motor applications require forces or torques to maintain a load for a time that’s quite long relative to the axis’ motion profile. This necessitates clarification of terminology — especially of terms that aren’t synonyms or only equivalent under specific operating conditions or events … but used interchangeably by those incorporating other motor types.

Potentially inconsistent communication can cause incorrect interpretations between parties and degrade motor sizing, machine programming, machine startup, and the troubleshooting of events or product-process issues.

Image credit: sykono

For example, the terms stall or stall torque never appear in specifications for induction motors. But the terms are often used to describe conditions arising when load on the motor exceeds the full-load or breakdown torque capacity. Non-servo-motors (such as induction motors) in anything other than proper working condition are said to be in a locked-rotor or stalled (zero rpm) state. If an applied load exceeding the motor’s continuous full-load torque persists, the power demand will as well … and the motor will overheat and eventually burn up.

If a motor is stalling (0_rpm < actual rpm < full-load rpm) it draws a current greater than its continuous capacity … and if this condition persists, the motor’s windings will overheat. Such abnormal stall, stalled, or stalling conditions of an open-loop style induction motor aren’t a typical part of any normal application — so any induction motor under any one of these conditions is:

affected by the motor’s ability to dissipate its heat losses relative to its load demand and
will cause the motor’s windings to overheat and burn up if an overload continues.

A stalling motor may continue to run for a while at a slower-than-rated speed, but when it stops running (due to the tripping of an overload protection device or because it burned up) it is not stalled. After all, no unpowered or burned-up motor can recognize a mechanical load.

Shown here are Kollmorgen AKM2G ac synchronous servo motors. Unlike asynchronous induction motors, such servo motors’ torque, velocity, and position are limited by drive loop gains and limits and the peak current limit of the drive’s foldback circuitry.

Servo motor designs — where a stopped condition might be by design

In contrast, the servo-controlled condition of purposefully holding position under a load (or maintaining a torque/force against a load) can be a normal function of a motion application … and one that’s completely different than our induction-motor example above. Unfortunately, it’s also a motor state that’s associated with and communicated by the terms stall, stalled, stalling, or locked rotor. So when a servo motor (being a closed-loop system) is properly sized for a specific operating condition, it will just do what it’s commanded to do and nothing else. When properly sized and operating normally (as sized) such a servo motor can handle its commanded load (to hold position under a load or maintain a torque/force against a load) within the defined event or motion profile — without threat of overheating.

In fact, most confusion in the servo-motor industry about these terms and their meanings stems from the fact that there’s no designated terminology to describe when a servo-motor is improperly sized or improperly used outside of its specifications. Describing such application issues with terms that should be reserved for open-loop systems generates confusion — mostly because of mixed interpretations and understandings of different styles of motor systems — such as open versus closed-loop control.

Because the terms in question are sometimes used interchangeably, let’s first describe how they’re used in the non-servo (induction) motor industry.

Significance of the locked-rotor expression for induction motors

For a powered open-loop induction motor, the term locked rotor is actually the condition or procedure for determining the maximum possible starting current (locked-rotor current or LRC) drawn by the motor while developing maximum starting torque (locked-rotor torque or LRT). This maximum starting current and its resulting maximum starting torque are typically measured in a lab environment with the motor’s rotor locked in place — hence the term locked rotor.

Locked-rotor current (LRC) is typically found on an induction motor’s nameplate identified as the locked-rotor amps (LRA), which is the maximum possible starting current drawn by the motor at zero speed when the power is first applied (and slip is at its maximum).

In actual induction-motor applications, this is the maximum possible current the motor can endure for a short (intermittent) period of time when power is first applied to the motor. That’s before the motor’s rotor accelerates to reduce slip (the Δrpm between armature field and rotor) to bring the motor up to a balanced operating point of equilibrium with the applied load — preferably within its continuous rating. Induction-motor open-loop intermittent currents exceeding the motor’s continuous capability are typically seen during acceleration (when power is first applied) and occasionally during process load disturbances … but overall root mean square (RMS) currents seen by the motor over time must remain within the motor’s continuous capability.

The performance of a servo motor (as that of other motors) is also affected by its ability to dissipate its heat losses — though it’s much less likely to overheat due to an overload than other motor types. That’s because servo controls and feedback along with closed-loop settings and limits from the drive amplifier (and potentially other controller programming) keep the motor within a safe operating range. Unlike the overload condition of an open-loop induction motor, the typical servo motor can be and is controlled to operate intermittently above its continuous capacity.

But just like the open-loop induction motor, the RMS currents seen by the servo motor over time must remain within the motor’s continuous capability. Otherwise, the motor’s windings will overheat. Intermittent overload states on a servo motor perform specific functions and are purposeful. So when these states are included in a design, engineers must account for them during motor-drive selection to ensure proper axis operation during normal machine operation, maintenance, safety events, and potential failures.

Servo-motor rating techniques used today were developed years ago — when most applications didn’t experience loading that was substantial (in relation to the application’s overall motion profile) during zero movement. So when one of these nontypical applications is under consideration, its requirements during these special conditions or events are evaluated separately. Results from this separate set of calculations may or may not override the application’s effective RMS torque calculation.

One specialized servo-motor use in robotic, industrial, and factory automation processes is to hold a specific torque/force against a load with little to no motor movement. The application may be as simple as a holding clamp or maintaining a vertical load against gravity (where a holding brake would increase process times or degrade precision) … or the application could be maintaining a torque/force against a load for testing … or dynamically holding a part in place for some process … or expelling a high-viscosity liquid in a controlled manner.

Among other things, a key element for sizing a servo-motor-drive combination for such use is the duration of this (effectively continuous) load with little to no actuator movement within its motion profile — or for some specific event — relative to:

the motor and windings’ thermal time constants and
the drive capabilities.

If the servo motor is properly sized and operating with proper drive-system settings for a given application, then it won’t overheat, trip a protection device, or burn up. Under such planned servo-motor utilization, the worst-case condition for the servo motor may actually be the motor’s continuous operation against its applied load (due to gravity or otherwise) during normal operation. Or the worst-case condition may be during a situation with a downed machine or line (or maintenance operation) that’s excessive in relation to calculated effective RMS torque/force requirement for the axis based on its motion profile.

Unlike an open-loop asynchronous induction motor (capable of sacrificing itself while trying to satisfy the needs of its load) a closed-loop servo motor’s torque, velocity, and position are controlled and limited by:

the drive’s current/velocity/position loop gains and limits and
the peak current limit of the drive’s foldback circuitry and programming over time — which is I²t foldback, typically set = Ic(motor) or Ic(drive).

Thus, even when the servo-motor may appear to be in a physically stalled or locked-rotor state, when properly sized and programmed, it is being specifically controlled within its continuous capacity and thus within the motor’s ability to dissipate its own heat losses. However, to describe this operation, especially when there is an axis issue, our subject words/terms are often communicated, with different meanings or interpretations in mind.

The servo-motor industry’s term stall often appears as a subscript on torque/force expressions and in other nomenclature to indicate a servo motor’s maximum obtainable continuous torque (Tc) and its resulting continuous current (Ic) requirement.

The continuous torque and its resulting Ic is at a specified ambient temperature (with an even steady-state heat-loss distribution throughout the motor windings) to deliver the torque capacity based on a specific temperature rise and heat sink (mounting plate) size — without overheating motor windings.

Such use of the term stall makes for a meaning that’s entirely different from the common definition of the word — in which stall means to stop and:

when an induction motor can no longer move at its designed rpm against its applied load — with an appearance of a locked rotor (for a fully stalled or stopped condition) or
stalling — on its way to be stopped — or
stalling — running at an rpm that’s slower than rated, but not at zero rpm.

So it’s clear the word stall to describe induction-motor systems has a different meaning than stall used in the servo industry.

Widespread misconceptions about the term stall means that even some manufacturers’ publications incorrectly state that stall means zero rpm or no rotor movement.

Proper use of stall expressions for sizing ac brushless servo motors

Now the question is: How does one size an ac (brushless PM) servo motor so it won’t overheat at the windings — especially for designs needing continuous holding torque/force with little or no movement for a duration that makes the axis’ motion-profile effective-RMS calculation invalid?

As noted before, this invalidation is due to what would otherwise result in an uneven heat-loss distribution within the motor.

We’ll answer the question using an application example. First though, recall how manufacturer ratings for continuous current Ic(motor) are determined. We’ll assume RMS current with sine-wave commutation, but some manufacturers use other means to express Ic. Typically, servo motors are rated to establish maximum continuous capacity with heat distribution of internal winding losses that’s even throughout the motor. This means during the rating process, electrical cycles within the motor move at a speed fast enough to make for even distribution of heat due to internal losses … but slow enough to ensure jX_L and core losses are essentially zero within the motor. This physical test speed is usually around 1 to 4_rps (revolutions per second) for motors but may be slower or higher — as a function of the motor’s pole pairs. Recall that jX_Lis inductive reactance.

Most manufacturers define continuous Torque (Tc) and resulting current (Ic) at this or similar speeds. The published continuous torque and current specifications are also often identified as stall torque (Tc_stall) and Stall current (Ic) regardless of the type of commutation and resulting current units.

Note the difference between stall as it’s used in the context of controlled closed-loop dyno tests (for determining a servo motor’s maximum continuous capability) versus our definitions for stall, stalled, and stalling as they describe overload conditions exceeding an induction motor’s maximum capability —whether the rotor has stopped moving or not.

Also consider what the servo drive’s output is doing, when a servo motor is held under some load with little or no movement. Because we are using sinusoidal commutation for our example, the controlled three-phase ac output is effectively at a standstill — presenting a continuously PWMed non-moving three-phase output with a value (appropriate to the servo motor’s commutation position) equal to what would essentially be a frozen signal at that position location. This may be envisioned in your mind’s eye as an instantaneous snapshot of the moving sine waves.

The main difference between the two major commutation methods is that:

Six-step or Block (unmodified trapezoidal) commutations only allows current flow through two of the three motor phases at any given time — for 2-ON 1-OFF at all times

Sine-wave or sinusoidal commutation allows for current flow through three motor phases at the same time (when appropriate) and each electrical cycle of the motor is presented as a sine-wave to the servo motor.

The two different commutation methods require a different representation of the motor’s torque constant (Kt). For more information, download a PDF white paper titled, AC Permanent Magnet and PMDC Brushless Servo Motor Parameters and Conversions and available at www.kollmorgen.com.

Most rotary servo-motor designs today have good thermal conductivity between motor windings, laminations, and frame — especially with epoxy encapsulation. That said, each design has a different thermal conductivity between windings and frame … and modeling those values accurately necessitates significant thermal modeling or actual measurements and testing. So for the purpose of this article, we’ll assume each motor winding is a standalone mounted coil — and that no coil sees a thermal advantage of transferring heat to the area of another winding or coil in the motor.

For our example design, each winding can dissipate 75_watts.

Under the condition of a fully loaded servo motor, the two worst-case commutation positions are:

All (100%) current (I_actual = Ic x √2) going through one winding and 50% through the remaining two windings — as shown in Figure B
All applicable (86.6%) current going through only two windings (I_actual = Ic x cos(30°) x √2) — as shown in Figure C.

These are the worst-case conditions the motor’s windings would see if the drive’s Ipk elapsed time (controlled by an I²t circuit/program) folds back to the motor’s published continuous current Ic_stall — established with the even heat-loss distribution of a slow rpm.

Here we select a motor with a capability Tc => √2 x T_hold. For the sake of simplicity, we ignore statistical failure probabilities.

So for the condition shown in Figure B, assume Ic(motor)_stall = 10_Arms and a 10_Nm load. Also assume the commutation position and load necessitate 10_Arms through phase U to hold that stationary position. Then phase U actually has 14.14 DC-Amps [10_Arms x 2] continuously PWMed through it. That means the U winding must try to dissipate (14.14² x RmØ) watts-loss versus (10² x RmØ) watts-loss — twice its capacity — which of course it cannot continuously do.

This is the second of our two worst-case commutation positions … when all applicable (86.6%) current is going through two windings.

Similarly, for condition shown in Figure C, each of the two coils tries to dissipate (12.247² x RmØ) watts-loss versus the (10² x RmØ) watts-loss — 50% over each coil’s capacity.

Calculations show it clearly — a critical factor when sizing a servo motor is the effectively stand-still current (to hold a load still) relative to the motor’s ability to dissipate its windings’ losses under such conditions.

So, we need a motor that has a continuous torque rating (Tc) equal to the √2 x T_hold required — not because we need any additional torque from the motor, but because we need each of the motor’s windings to reliably handle (for an effectively continuous period of time) what would otherwise be an instantaneous peak-crest of a moving sine-wave current.

Under these conditions (assuming nominal values and no margin) using a servo motor rated Tc (stall) = 10_Nm in an application requiring 10_Nm to indefinitely hold a vertical load is insufficient. However, choosing a slightly larger motor capable of a Tc (stall) => 14.14_Nm would be sufficient.

Additionally, if the drive is also rated in terms of Arms, it would only need to produce the continuous RMS current required to produce 10_Nm by the motor (approximately 10_Arms, if the servo motor’s Kt = 1_Nm/Arms).

We can prove this by first determining the watts-loss capability at continuous rating — as shown in Figure A. Then we compare the wattage-dissipation capability against the two worst-case commutation points of the three-phase motor coils where current is 100%, 50%, 50% (Figure B) and 86.6%, 86.6%, 0% (Figure C).

Given the motor information:

Tc(stall) = 10_Nm

Ic(stall) = 10_Arms

Kt=1.0_Nm/Arms

Rm(L-L)_25°C = 1.006_ohms, where Rm(L-L)_150°C = (1.006_ohms x 1.491); and Rm Ø_150°C = 1.5/2 = 0.75_ohms

Temp (ultimate winding temperature for continuous operation) = 150°C

Temp-Rise (max. from a 25°C ambient) = 150°C – 25°C = 125°C

So based on this information, application conditions, and our assumption that each motor winding is a standalone phase (Ø) branch or coil (with no advantage of heat transfer to another winding’s thermal area) each winding can dissipate up to 75_watts.

If we consider the first of the two worst-case commutation positions with all (100%) current (I_actual = Ic x √2), we can conclude the watts-loss (total) will still equal 225_watts (Figure B); but the specific wattage needing to be dissipated through one winding (Ø_UN) is 100% over the previously calculated thermal capability of 75_watts (Figure A) and the other two windings are each at 50% capacity.

In this case (Figure B), to keep any of this motor’s windings from overheating due to this specific commutation position we would need to limit the Ic(drive) to 70.7% of the Ic(motor). For our example, if the given 10_Nm holding load requirement is maintained, this motor selection will NOT accomplish the job without overheating. Hence, a possible solution would be to select a motor with a capability: Tc => √2 x T_hold, preferably with about the same motor torque constant (Kt) so the maximum required application rpm may be maintained without changing the drive.

If we consider the second worst-case commutation position with all available (86.6%) current going through only two windings (I_actual = cos(30°) x Ic(motor) x √2) we see watts-loss (total) will again equal 225_watts (Figure C); but the specific wattage needing to be dissipated through windings (Ø_UN) and (Ø_WN) is 50% over the previously calculated thermal capability of 75_watts (Figure A) for each winding.

In this case (Figure C) we would only need to limit the RMS drive current (Ic(drive)) to 81.6% [100 x (75w/112.5w)^1/2] of the Ic(motor) to keep the motor’s windings from overheating due to this specific commutation position. That yields a W_loss(Ø_UN) = W_loss(Ø_WN) = ((10_Arms / √1.5 x cos(30°)) x √2)² x 0.75 = 75_watts.

Still, if the 10_Nm holding load requirement is not re-specified to a lower value, this motor selection won’t accomplish the job — as with the case depicted in Figure B. For this specific condition, we could select a motor with the capability Tc => √1.5 x T_hold. However, this still allows a 33.3% wattage overload of the first commutation condition as in Figure B.

A better solution is to select a motor with a capability Tc => √2 x T_hold as detailed in Figure B.

Understanding stall — for proper servo-motor calculations

Understanding the meaning of the term stall in a servo-motor context lets engineers correctly consider the specifics of an axis’ motion profile and load demands over relative load demand times (versus total cycle times). That way, dominant factors can be determined and analyzed for sizing calculations, machine-axis programming, and troubleshooting — for normal operation or otherwise. These dominant factors allow reasonable consideration between the results: RMS calculations and any effectively constant or constant, loads held for a long time relative to the axis’ total motion-profile time, the motor’s thermal time constants: TCT_motor and TCT_winding, and the servo drive’s I²t foldback algorithm.

Image credit: Thossaphol

Solid understanding of the motor’s worst-case commutation positions when holding a continuous load with effectively no movement (and standstill PWM drive commutation that comes with this condition) is core to proper motor and drive sizing. Holding a torque against a load (external or otherwise) for intervals that are long in relation to motion-profile times or thermal time constants can (if corrections aren’t made) make for erroneous RMS conclusions.

Side note: Similar considerations are required for intermittent torque requirements that are high relative to a motion profile’s times and demands and any proposed motor’s thermal time constants. This is a topic unto itself. Google Kollmorgen — Managing PM AC Servo-motor Overloads: Thermal Time Constant to download a white paper on this topic.

In real-world applications, the torque multiplier may be conservative considering the good thermal conductivity between today’s motor windings, laminations, and frame. Nevertheless, the author’s experience and collected feedback on motor designs and applications over many years suggests that the √2 multiplier for iron-core rotary servo motors imparts a safety margin of approximately 9 to 11%. Though this information has not been specifically verified, the worst-case scenario with the √2 multiplier seemingly offers enough margin to overcome typical manufacturing tolerances of ±10%. So selecting a motor with a continuous capability equal to the calculated continuous requirement (or slightly above) using the √2 torque multiplier is reasonable.

The importance of good communication is paramount to avoid confusion of the word stall (and its derivatives) and the term stall defined within the servo-motor industry as part of normal parameterization, operation, or otherwise. Image credit: Thossaphol

However, for ironless-core motors, no margin is assumed … and for these motors, design engineers are advised to specifically consider each manufacturer’s stall rating definition. The ac servo-motor industry’s stall is a limited term with a specific definition, but not so fixed that it couldn’t be redefined in part or in whole — especially for special-purpose servo-motors or those of a certain design — ac PM ironless-core servo motors, for example.

Kollmorgen | kollmorgen.com

Hurley Gill is Senior applications and systems engineer at Kollmorgen located in Radford, Va. He’s a 1978 Engineering Graduate of Virginia Tech who has been engaged in the motion control industry since 1980. He can be reached at hurley.gill@kollmorgen.com.

Since its founding in 1916, Kollmorgen’s innovative solutions have brought big ideas to life, kept the world safer, and improved peoples’ lives. Today, its world-class knowledge of motion systems and components, industry-leading quality, and deep expertise in linking and integrating standard and custom products continually delivers breakthrough motion solutions that are unmatched in performance, reliability, and ease-of-use. This gives machine builders around the world an irrefutable marketplace advantage and provides their customers with ultimate peace-of-mind.

For more information visit www.kollmorgen.com, email support@kollmorgen.com, or call (540) 633-3545.

Kollmorgen to demo new AKD2G Servo Drive & AKM2G Servo Motor System at Pack Expo 2018

September 28, 2018 By Miles Budimir Leave a Comment

Kollmorgen will be exhibiting at Pack Expo, October 14-17, 2018, in Chicago, IL. Attendees are invited to visit booth N-5251 to see new-to-the-show demos of the company’s motion technology, including the new AKD2G servo drive and AKM2G servo motor system solution, hygienic motor solutions for packaging applications, and state-of-the-art robotics.

AKD2G Servo Drive: The dual-axis AKD2G is one of the most power-dense industrial servo drives on the market. It substantially reduces mains wiring, fuses, and filtering to save significant panel space and cost. Highlights include: plug-and-play compatibility to Kollmorgen motors and Workbench interface, optimized hybrid connector, and speed – its multicore Compute Engine, Servo on a Chip, is far more powerful than other servo drives. The AKD2G’s current loop update rate of 1.28 µsec accommodates changing load conditions immediately. Velocity and position loop updates are also market leading at 62.5 µsec and 125 µsec.

AKM2G Servo Motor: A configurable, high-performance synchronous servo motor with increased power density and improved torque/speed ratio so OEMs can get up to 30% more performance out of an equivalent size motor or achieve the same performance with a smaller motor. With six frame sizes with up to five stack lengths and performance rates 0.3 and 10 kW, and selectable options such as feedback, mounting, and performance capabilities, the AMK2G can easily be dropped into existing designs.

Mobile and Robotic Arm Solutions (MARS): An interactive demo of a robotic arm enabled by Kollmorgen Automated Guided Vehicle (AGV) software, hardware, and heavily integrated frameless motion technology. Experience the way Kollmorgen is merging the world of modern robotics and AGVs to develop smarter, more dynamic, and versatile robots.

Frameless Gearmotor: Learn how Kollmorgen’s award-winning TBMTM frameless motors are optimized for fit and performance with industry standard strain wave gearing and industry-leading torque density to enable smaller, lighter, faster robots.

AKMHTM Hygienic Motor: The superior hygienic design stands up to the most challenging washdown environments with an IP69K certification and ultra-durable, food grade stainless steel. The AKMH reduces cleaning time and the risk of food recalls by eliminating the cracks and crevices where contamination can occur. Learn how this exceptional hygienic motor can increase OEE at packaging and processing facilities while increasing food safety and productivity.

For more information, visit www.kollmorgen.com.

Kollmorgen website: New tools, technical support, and online community

May 19, 2018 By Lisa Eitel Leave a Comment

Kollmorgen’s new www.kollmorgen.com now has site enhancements that include streamlined navigation and improved onsite search capability. The company incorporated feedback from customers to create an improved user experience where visitors can more easily find the motion products, industry-specific solutions, and technical support they’re looking for with a minimal number of clicks.

True responsive design and layout allows for optimal viewing when accessed on tablets and mobile devices. The new site also integrates the Kollmorgen Developer Network (KDN), a one-stop-shop for product support and learning.

“We are excited about our newly-refreshed website and the improved experience it provides customers, partners, developers, and others,” said Josh Inman of Kollmorgen. “In addition to even more robust content, visitors can easily access essential information with just a few clicks.”

• 3D Models: Online modeling tool to configure 2D/3D product views with download capability in most major formats

• Video Gallery: An extensive library of product demos, application examples, programming tutorials, and more

• Kollmorgen Developer Network (KDN): Quick access to technical support for each product. Ask questions and get feedback from the online community

• Improved Search: Quickly and easily find/download information about products, including specifications, FAQs, technical support, videos, and more

For more information visit www.kollmorgen.com or email support@kollmorgen.com.

Thermal time constants and managing PMAC servo motor overloads

February 10, 2018 By Lisa Eitel Leave a Comment

When high intermittent power density is required, using classic RMS calculations and speed-torque performance curves as the sole selection methods may be problematic. That’s because such approaches might cause motor or drive undersizing.

By Hurley Gill • Senior applications and systems engineer | Kollmorgen

Using classic performance curves with RMS calculations is perfectly acceptable for most servo applications. However, if an application’s intermittent torque is large relative to its motor’s continuous capability for some relative time period, the thermal time constants of the proposed setup need consideration. Tight or enclosed design spaces can exacerbate thermal issues.

This article presents visuals for understanding how significant dynamic effects are on servo motor thermal time constants — especially when the application requires an I_actual > I_continuous for an extended time.

Servo motors generate heat due to internal losses, and each motor’s distinct ability to dissipate its internally generated heat determines its rated continuous capacity … more after the jump.

Many factors need attention during machine design planning. Servo motor and drive selection for a given application affects the mechanism’s chance of success for delivering top performance under all conditions — including normal operation, E-stops, and other anticipated events. Where a motor’s torque and current must exceed its continuous capability for an extended time, using a simplified graphical approach (as in Figure B later in this article) can prevent design issues.

Conventional servo motor applications see various velocity demands, with torque requirements that enter and exit the intermittent capability over the defined motion profile. Traditionally, peak currents exceeding a servo motor’s continuous capability help the motor meet acceleration and deceleration requirements. Motion profiles most often need these peak currents for short periods of time (often in the millisecond range) not exceeding the typical maximum of 4 to 5 seconds available from a drive amplifier.

In these routine cases of intermittent-duty operation, it’s usually unnecessary to select a motor with a continuous-duty capability that satisfies the application’s peak-torque requirement. Instead we simply:

Use a root mean square (RMS) equation to find the application’s effective continuous torque (Trms) and velocity (Nrms) requirement and then …
Ensure that this equivalent operating requirement falls within the continuous (and thus thermal) capability of the chosen motor chosen …
While verifying that the required peak Torque (Tpk_required) is less than available peak torque (Tpk_available) from the selected motor and drive, at its needed rpm.

Special application conditions

The expansion of closed-loop motion control technology into unconventional applications often results in conditions that exceed ordinary intermittent-duty operation. However, even conventional applications sometimes present special conditions.

For example, a design may specify that in the event of an emergency stop (E-stop) all controlled motion must halt before the removal of mains power and within a specific amount of time. For most applications, this isn’t usually an issue … but on large machines with significant kinetic energy, time needed to bring an axis’ motion to a stop can easily exceed the typical maximum 4-to-5 seconds of available peak current from a paired motor-drive combination [Ic(drive) equal to approximately Ic(motor)].

This requirement, though generally not demanding a larger motor, often requires a higher continuous current drive to ensure the needed peak current (Ipk_required) is available during an E-stop deceleration. For large machines, E-stops requirements in the 20 to 40-second time range are somewhat common.

Many pulse width modulation (PWM) drives are designed with foldback overload circuits or an algorithm using the thermal time constant of the copper coil (TCT_coil) — whether the current is folded back to the continuous current of the drive or motor. But to satisfy some of these atypical servo applications, the typically paired motor-drive combination that would otherwise be selected is unsatisfactory.

For some large machines (such as this one for printing) E-stops of 20_seconds to 40_seconds are often acceptable.

Servo motor application operating conditions and potential events must be addressed during sizing and selection. Consider some examples:

There could be a vertical-axis requirement necessitating a servo motor capable of holding a load greater than its continuous capability for a specific amount of time before engagement of a static brake. (That’s especially relevant as it’s typically undesirable to cycle a static holding brake through engagement and disengagement during normal production cycles.)
There could be an axis requirement that necessitates a motion system to handle undesired events in which load becomes stuck or otherwise hindered from movement. Here, the motor must be capable of surviving some peak current for the full duration of the commanded move (even when not properly functioning).

Whether it’s easy or impossible to replace a motor once it’s installed (in a normal environment, in space or underwater, or in an area exposed to radiation) it’s always best to select a servo motor-drive combination that minimizes the risk of failure due to atypical events or normal operating requirements — as these in turn maximize reliability and safety.

Overload and the effect of power losses

Depending on the complexity of requirements, many of these applications need torques (and thus currents) above the motor’s continuous capability (Ic or I_rated) at the application’s required rpm (such as Npk or Nrms). Hence, the potential limiting or control of the motor’s power losses must be considered so that the motor can execute the necessary work or specific event — while the motor’s insulation system is protected from thermal overload.

At this juncture, the motor’s demanded current (I_actual) is greater than the motor’s continuous rated current (Ic) capability for a significant enough period of time relative to the motor’s overall thermal time constant (TCT_motor), that the TCT_motor becomes dominated by the TCT_coil due to the relative heat transfer rates between the used non-homogeneous materials.

For these specific cases or events under evaluation, the previously referenced RMS calculations over a given motion profile are usually invalid, though still calculated to ensure overall product selection requirements; because events needing overload situations can differ greatly from one application to the next.

To satisfy applications with potential events or conditions requiring a peak current (I_actual) to deliver a peak torque (Tpk) for a qualified period of time, we also need to determine if the motor’s winding/coil can sustain the overload current needed without sustaining motor-insulation damage.

Available life of a motor’s insulation (based on its continuous rating) is halved for every 10° C the motor experiences beyond its continuous rating.

We can estimate the time to rated ultimate temperature (t_ ultimate) of the motor’s coil/winding from a cold start (ambient) using:

t_ultimate = -TCT_coil(mounted) x ln[1-(W_loss(rated)/W_loss(actual))]

t_ult. = -TCT_winding x ln[1-(Ic²/I_actual²)]

where W_loss(rated) is substituted with: Ic² or I_rated²

and W_loss(actual) with: I_actual²

Note that for these conditions, I_actual is greater than the motor’s Ic (continuous rated current of the servo motor at low or stall rpm). What’s more, under this condition, actual W_loss will continue to exceed rated values — potentially causing thermal runaway if there’s a delay in power removal.

Formula assumptions for motor heat

Of course, the above substitution (with the appropriate I² for watts in both the numerator and denominator) assumes constant power dissipation with a constant applied [step input] current. That value — due to the actual winding temperature rise from ambient temperature (e.g. Rm(25° C)) to the target temperature based on W_loss(actual) — is incorrect. However, that value offers a conservative approach with Rm(hot) assumed constant, over solving a dynamic nonlinear differential equation. What’s more, even further manipulation of the formula is required for real-world application of the motor starting from a non-ambient temperature based on an Irms value.

However, whether the equation is manipulated or not, the preformed calculations tend to be done at only one or two points when needed. Unfortunately, that means that the substantial effect of actual current (I_actual) exceeding continuous capability (Ic) is often missed.

So in this article, we present a plot in Figure A to demonstrate the effects of demanded Watts_loss exceeding the design’s continuous capability. The plot also lets us graphically determine a relative (if not effective) TCT for a specific condition under evaluation. That in turn provides a way to overcome design challenges where the ratio: I_actual (under evaluation) / Ic (continuous current) is greater than one and known for the production of the application’s needed torque (T_required).

Figure A — Shown here is the thermal time constant (TCT) effects as I_actual is increased to beyond Ic. Click to enlarge.

This article also lets us compare effective TCT_motor and TCT_coil(air) under their specific condition using Figure B.

Most of today’s servo motors have excellent thermal conductivity between the motor windings, laminations, and frame — especially in motors with epoxy encapsulation. That said, these non-homogeneous materials still have vastly different heat-transfer capabilities.

However, under the subject condition, the thermal time constant of the motor’s mounted coil (TCT_coil(mounted)) is the dominant factor. In addition, the TCT_coil(air) is likely too conservative to be of reasonable use, as it’s calculated from the magnet wire’s specific mass without any consideration of it being mounted within the motor’s frame.

The TCT_coil(mounted) value (henceforth identified as the TCT_winding) represents the first level of materials contact of the thermally nonhomogeneous motor materials — more specifically, at the interfaces between the coil and epoxy and air and laminations.

Because I_actual (under evaluation) > Ic, the published TCTs [coil(air), winding (coil mounted), and motor] are no longer constant as when I_actual <= Ic, the thermal time constant under consideration is dynamically changing with the motor’s actual watts loss (W_loss(actual)). For example, when the actual current (under evaluation) is <= Ic, the published TCTs for a given servo motor may have a relative range of TCT_coil(air) = 25_seconds, TCT_winding = 60_seconds, and TCT_motor = 600_seconds; however, when I_actual > Ic the effective TCTs will be significantly reduced from those published as a function: W_loss(actual) verses W_loss(rated).

Effects of overloads on thermal time constants (TCT)

Figure A shows the significance of I_actual greater than Ic(motor) under a specific condition by the resulting percentage of W_loss(actual) / W_loss(rated) — both plotted against the calculated thermal time constant (TCT) multiplier. For example, an I_actual = 5xIc requires that the winding dissipate 2,500% (25x) more watts than its rated continuous capability.

Figure B — Shown here is the thermal time constant (TCT) multiplier for I_actual (from slightly greater than Ic up to 4.5Ic). Click to enlarge.

Figure B lets us graphically determine a specific TCT and the time to ultimate temperature for specific situations. This is through application of the graph’s X-axis’ corresponding (TCT) multiplier as a function of needed I_actual against a known TCT — and multiplying that result by five to reach the time to ultimate temperature. To illustrate:

Question One: Irrespective of the drive’s ability to supply current, can our motor handle 20_seconds of a peak current = 3xIc assuming we begin with an ambient temperature of 25° C? Assuming we’re using the initially proposed motor, the application’s normal operation spurs a TCT_winding = TCT_coil(mounted) = 60_seconds.

Answer One: Using Figure B, we simply go to the vertical scale on the left at 3 (3xIc) and move horizontally until we intersect with the curve. Then we read the corresponding X-axis multiplier on the semi-log scale and apply its value to the published TCT_winding. This intersection for 3Ic occurs at about 0.023 on the X-axis log scale … so at 3Ic, the effective TCT_winding(3Ic) = 0.023 x 60 = ~1.38_sec.

Note that 95% of the thermal rise will occur in 3x (thermal time constant) or ~4.14_sec [3×1.38_sec.], where 5xTCT = 99.3% of rise or 6.9_sec (the total time to ultimate rated winding temperature).

So, for this application, we’ll need to select either a larger motor or a motor with a longer TCT_winding … or change the application requirements.

As an aside, if we had used the formula for calculating t_ ultimate = -TCT_winding x ln[1-(Ic/I_actual)2]; t_ ulimate = -60_sec. x ln [1-(1 /3)2] = 7.06_seconds … yielding a TCT_winding(3Ic) = 7.06/5 or ~1.41_ seconds.

Question Two: Because 3Ic is impossible for 30_seconds with the target motor of Question One, can we use an Ipk of 2Ic for 20_seconds?

Answer Two: Again, using Figure B, the intersection for 2Ic occurs at about 0.057 on the X-axis log scale. Thus, at 2Ic, effective TCT_winding(2Ic) = 0.057 x 60 = ~3.42_sec. Knowing that 5 x TCT = 99.3% of the temperature rise is ~17.1_sec, it is still less than the proposed specification: 20_seconds.

So even with this changed specification: 2Ic for 20_seconds, we’ll need to select a larger motor or one with a longer TCT_winding or again change the specification for the condition under consideration.

Note: Smaller multiples of Ic will present a less physically dominant TCT_winding over the motor’s overall thermal time constant (TCT_motor). Where I_actual/Ic approaches unity (1), the motor’s other thermally non-homogeneous materials (as of the aluminum housing, for example) come into play.

In conclusion, this article illustrates the severity of the I_actual/Ic overload conditions. Where I_actual/Ic is high, one can fairly accurately estimate TCT_winding(new) or the time to ultimate temperature due to the inability of nonhomogeneous materials to transfer heat quickly enough to maintain TCT_motor dominance.

However, as the ratio: I_actual/Ic approaches one, the thermodynamic response engages two exponential functions — each with its own time resulting constant (of TCT_winding and TCT_motor). This blending of vastly different thermal time constants due to the non-homogeneous materials is beyond the scope of this article (and often necessitates further evaluation and understanding).

Kollmorgen | www.kollmorgen.com

Hurley Gill is Senior Applications and Systems Engineer at Kollmorgen in Radford, Va. He’s a 1978 Engineering Graduate of Virginia Tech who has been engaged in the motion control industry since 1980. He can be reached at hurley.gill@kollmorgen.com.

Designing motors with safety in mind

September 9, 2016 By Motion Control Tips Editor Leave a Comment

by Hilary Whiting, Manufacturing Engineer, Advanced Manufacturing, Kollmorgen

With scientifically verified hygienic design principles, motor manufacturers are designing motors for food and beverage applications that dramatically improve safety.

The food and beverage industry is one where safety is paramount. That’s why food and beverage companies and their machine suppliers are battling constantly to keep their products safe. These microscopic battles are fought with large quantities of water and chemical sanitation processes. Hygienic machine design is critical even down to the sub-component level in order to allow these processes to be effective in removing product residues as well as pathogens, and destroying bacterial intruders.

Motor H does not have any metal to metal joints. Instead, all joints are mated with gaskets. The gaskets are bright blue in color to contrast against food products.

Both the North American Meat Institute (NAMI) and the European Hygienic Engineering Design Group (EHEDG) have published lists of Hygienic Machine guidelines (See Table 1). One of the challenges in meeting these guidelines is the design of components such as motors, gearboxes, and sensors.

Table 1: NAMI guidelines table of continence *referenced from Fact Sheet, Sanitary Equipment Design, American Meat Institute, 2012

Kollmorgen set out to verify the benefits of hygienic machine design by partnering with the Virginia Tech Department of Food Science and Technology (VT FST). These hygienic design principles could then be applied to the design of servomotors used in the food and beverage industry.

Test motors were a representation of commonly used motors on food and beverage equipment.

Scientific testing
A series of tests were conducted to determine the sanitizability of three servomotor designs. The three motors tested were an industry standard IP67 wash down servomotor designated as Motor W, a stainless steel motor used in the food and beverage industry designated Motor C, and a stainless steel servomotor designed to comply with the NAMI hygienic design checklist and EHEDG design parameters designated Motor H. These motors represent the most common types used in food and beverage equipment.

Each motor was inoculated with ATCC 25922 strain of E. coli. The study focused on determining the sanitizability at areas of concern on the motor’s surfaces including labels and surface finish, seals and fasteners, and cabling. The team focused on these areas to see how design features affected the ability of the servomotor to be sanitized of E. coli. These areas were marked and isolated, then put in contact with 10 μl of E. coli for one hour. During this time, the motors were run at amperage that kept their surface temperatures between 33 °C and 38 °C, non-lethal temperatures for E. coli.

Sites were inoculated with E. coli and sampled prior to the cleaning and sanitation procedure (pre-wash) to determine a baseline. A typical industry cleaning and sanitation procedure supplied by a leading food production company was used to clean and sanitize each of the motors. The marked sites were sampled again post-wash to determine if the area was successfully sanitized. The test included three repetitions of this process to provide an appropriate data sample size. The reported study findings can be seen compiled in Table 3. This data could then be used to draw conclusions about the impact of design features on the sanitizability of the three motors.

Table 3: Pre-wash and post-wash E.coli sampling.

Label and surface finish testing results
The first sets of locations to be evaluated are those pertaining directly to labeling and surface finish on the motors. Motor W has labels etched onto the surface of the aluminum housing (Figure 1) and the surface is covered with food grade paint. The rough finish of the etched label was identified as high risk to harbor bacteria. Motor C has stamped labeling with a brushed surface finish on its housing (Figure 2). The brushed surface finish of Motor C was identified as an area of concern due to the textured surface finish, so was targeted as a sampling location by the test team. Motor H has a smooth stainless steel housing that holds a 32 μ-in. surface finish or better, even across the annealed label face (Figure 3). In the NAMI guidelines Principal 6.1 states; “It is recommended product contact surfaces textures shall not exceed 32 μ-inches…” The housings and labeling methods of Motor W and Motor C do not comply with this guideline, but Motor H is compliant. Both Motor W and Motor C were found to have E. coli present on their surfaces post-wash, whereas Motor H was not found to have any. So the lack of bacterial recovery post-wash from Motor H supports the NAMI design guidelines regarding surface finish and external materials as verified through the scientific testing.

Fig. 1: Motor W has an etched label covered with paint. The etching process creates a rough uneven surface that can create niches where bacteria and food product may be harbored.

Fig. 2: Motor C has a high-risk texture surface for product trapping and bacteria harboring. The uneven surface finish creates niches where microorganisms can hide.

Fig. 3: Motor H features an annealed label with a 32 μ- inch surface finish. Unlike etching, the annealed label preserves the metal’s surface finish. This smooth label was designed to avoid trapping bacteria or food.

Seal and fastener testing results
Seals and fasteners were the second group of test locations investigated. The NAMI guidelines caution against the use of fasteners above product areas in Principle 7 as well as stating that seals and O-rings must be designed to minimize product contact, Principle 2.9. Most standard fasteners create niches or spaces where microorganisms and product can be harbored. Principle 6 of the NAMI guidelines explicitly outlines the need for elimination of niches. Both Motor W and Motor C have external screws, a shaft seal, and several metal-to-metal joints (Figures 4 and 5) while Motor H has hygienically designed gasket joints instead of metal-to-metal joints, as well as a shaft seal, and does not use any external screws or fasteners. Motor W was found to have bacteria present post-wash on all of its junctures except the seal surface where the shaft was spinning. Motor C was found to have E. coli present on the outer juncture of the shaft seal. However, on the surface of Motor H, E. coli bacteria were not found to be present on any external sealing surface after sanitation. So here again, the lack of E. coli recovery supports the NAMI design guidelines regarding fasteners and junctures.

Fig. 4: Motor W has metal to metal joints that are covered by a layer of paint. The motor has eight screws in total.

Fig. 5: Motor C has metal to metal joints at both ends of the motor body. The end cover joint is secured with six screws as seen in Figure 2.

Cable and hardware testing results
The third area examined on the test motors was the cables and hardware. The NAMI guidelines do not explicitly address cables, but the general material guidelines can be applied in addition to Principle 6, the elimination of niches. Both Motor W and Motor C were found to have bacterial colonies on the cable and both cable fitting interfaces. The VT FST test team was not able to recover any E. coli on the cable or the cable junctures for Motor H, post-wash. The cable of Motor H was specifically designed to not harbor bacteria and withstand harsh wash down procedures. The cable connection to the motor is also sealed using a bushing interface instead of a metal to metal interface like Motor W and Motor C. Here, too, the lack of post-wash bacterial recovery on Motor H supports the NAMI guidelines regarding material specifications and niche elimination.

In summary, the hygienically designed Motor H outperformed Motor W and Motor C with regards to hygienic design. The VT FST team could not detect E. coli colonies on the exterior surfaces of Motor H post-wash while Motor W and C both had multiple positive sites after the sanitation procedure. The problem sites can be directly correlated to areas where hygienic design criteria were not strictly followed. Each bacterium that survives a wash down procedure may be a risk to food safety or quality, not only to food and beverage companies, but most importantly to the end consumers.

The data collected through this study points to the importance of system-wide hygienic design including purchased components. The importance of careful design on the servomotor component level can only be magnified when applied to entire machines and production lines.

The data also show that deviating from hygienic design standards corresponds to decreased motor sanitizability leading to increased contamination risk and the need for additional costly and time-consuming interventions. These risks translate to real world costs, whether it’s added sanitation time, increased numbers of cleaning interventions, reduced production time or increased risk of consumer harm when safeguards fail.

Reprint info >>

Kollmorgen
www.kollmorgen.com

Stepper motor or servomotor: Which should it be?

April 8, 2016 By Patrick Curran Leave a Comment

by Hurley Gill, Senior Systems and Application Engineer, Kollmorgen

Servomotor and stepper motor technologies are indispensable in today’s mechatronic machine designs. However, clear understanding of the advantages and disadvantages of both make it easier to identify which is best for a given application.

If a given motion task can be done with either a stepper or servomotor-based design, the application’s exact needs (present and future) for repeatability, accuracy and flexibility dictate which is most suitable. Other factors that can help engineers pick between the two include environment, life expectancy, operating noise and energy use. However, when a machine axis allows use of either technology, engineers must use specific knowledge of the machine function and foresight to pick.

The ultimate motion-design goal is to get target performance from a given machine while balancing capabilities and cost. Picking a servomotor or stepper motor for a given application affects the odds of a successful motion design, so machine builders must consider the technical advantages and disadvantages of both.

Machine designers shouldn’t limit use of steppers or servos based on personal preference or comfort level, but learn where each technology works best for actuating specific mechanisms. Today’s digital stepper-motor drives enhance flexibility, drive features and communication protocols with integrated circuits and simplified programming. The same is true of servomotor systems. Here, higher torque density as well as improved electronics, algorithms, and higher feedback resolution have also boosted system bandwidth capabilities, with lower initial and overall operating costs for many applications.

Where stepper motors do and don’t outperform servomotors
Stepper motors have several major advantages over servo systems. They are typically lower cost, have common NEMA mountings, offer lower torque options and require less costly cabling. In addition, their open-loop motion control component simplifies integration and use for machine builders and end users.

• Consider torque and speed. Steppers are typically sized for twice their continuous requirements to handle acceleration/deceleration torque—or for peak-torque requirements. In contrast, designers typically size servomotors to specific application velocities and torques for maximum intermittent acceleration or deceleration, holding torque (if applicable), and continuous RMS requirements over the complete motion profile.

The PMX series step motors offer performance and design flexibility in a high torque-to-dollar package. AKM series servomotors give designers flexibility to quickly co-engineer modifications to perfectly fit applications. It is available in eight frame sizes and more than 500,000 standard configurations.

In general, if an application requires high throughput, speed and bandwidth for disturbance correction, or high rpm with or without tight coordination between axes, servomotors are the best option. If point-to-point position performance or rpm requirements are modest (as a function of the process loads or expected disturbances), steppers may be a better choice. Additionally, when loads are within reason, a stepper’s ability to hold position (with holding torque under power and detent torque under no power) can be an advantage.

Speed-torque curves highlight the difference between stepper and ac permanent-magnet (PM) servomotors of equal volume. Steppers typically generate higher continuous torque at lower speeds than servomotors. However, servomotors produce intermittent peak torques in this same low-speed range and produce peak and continuous torques over a much wider (and higher) speed range.

performance-curve-chart-motors-same-volume

This performance-curve chart compares motors with approximately same volume.

• Consider some specific examples. Video inspection axes and automated machine axes for adjustment and setup often use steppers. That’s because these axes are easier to design into control systems and less expensive to setup. Plus, when such axes lock into a position, steppers can turn off (with an on/off function) to boost efficiency. Properly installed steppers are less prone to failure than servos because of their simplistic open-loop control, which only needs winding-to-drive matchup (This is in contrast to the motor-drive-to-mechanism tuning with feedback circuits required by closed-loop systems).

performance-curve-chart-motors-same-volume-speeds

This speed-torque chart compares motors with approximately same volume and speeds to highlight the difference between stepper and ac permanent-magnet (PM) servomotors. Steppers typically generate higher continuous torque at lower speeds than servos. However, servomotors produce intermittent peak torques into this same low-speed range and produce peak and continuous torques over a much wider (and higher) speed range.

• Today’s stepper drives offer distinct advantages. Newer design techniques have improved stepper-motor performance with built-in feedback; end-of-move damping (for reduced settling times while maximizing accuracy); soft starting (so there’s no quick motion or jerk on power-up); and anti-resonance modes—for optimizing torque, stability and noise reduction (audible or otherwise).

Other stepper drive capabilities include idle current reduction (or IRC for reduced motor heating during standstill) and easily controlled operation modes between full stepping, half stepping and microstepping.

Although most properly sized steppers are extremely accurate running open loop in the selected step mode, built-in feedback provides additional precision without the cost of an external feedback device. Microstepping techniques also smooth torque and motion at low speeds and boost resolution at high speeds (with smaller step sizes than full-step or half-step setups).

• Modern stepper motors also offer more capabilities than in the past. They now come in higher power ratings than earlier-generation steppers. Advances in design have led to smaller airgaps, larger and stronger magnets, and rotor oversizing. Increasing the rotor’s diameter generates more torque per unit volume. Here, physical frame size and stepper motor winding stays the same while the rotor’s diameter and inertia are bigger. Of course, the larger rotor inertia can affect acceleration and deceleration times for a given application. Even so, such motors let more applications use a given stepper frame size thanks to a lower inertia ratio of load (J_load) to rotor (Jm). Generally, stepper motor systems are sized with a J_load:Jm of less than 30:1, but with slower accelerations and decelerations and advanced microstepping techniques, inertia ratios of 200:1 are possible.

• One last item to mention here. Today’s modern stepper drives handle stall detection electronically, so optional feedback devices, when used, help with position correction due to component misalignment, noise and lost pulse (position) information. A stepper motor with feedback, depending on the stepper drive, has less velocity ripple and uses less power than open-loop equivalents … and higher residual torque at low speeds than an equivalent three-phase servomotor.

So designers must be discerning in regard to the work an axis does, because stepper motor applications requiring feedback can approach the cost of a servo system, and what may be an operational advantage in one application may be a disadvantage in another. Closed-loop stepper systems don’t typically outperform lower-priced servomotor systems, so designers should carefully consider their benefits and drawbacks for the application at hand.

Stepper motors have several major advantages over servo systems. PMX series step motors offer performance and design flexibility in a high torque-to-dollar package.

Where servomotors do and don’t outperform stepper motors
Servomotors have several advantages over steppers. They can develop a high torque output over a wide speed range on demand, and are available in wider torque ranges and higher voltages (up to 480 Vac). They respond to disturbances with torque far exceeding their continuous capability and use only the power needed to move an axis to the commanded motion. In addition, servomotors are compact.

• Servodrives offer advantages as well. Motor-drive compensation, or tuning, once considered the bane of servo use is now mostly history. Today, the newest digital servodrives have software and hardware capabilities for better user experience and remarkable compensation flexibility. In fact, some servo systems autotune themselves after automatically configuring the motor, drive and feedback. Servodrives that autotune adapt to the given motor-drive-mechanism setup while maintaining performance without the need of any manual control-loop fine tuning.

• Reconsider torque and speed. Although servomotors run at high speeds, they can accurately run at extremely low speeds under precise control, even down to 1 rpm and below with proper preparation. Steppers are accurate and usually more economical for low-speed applications that run at less than 1,000 rpm. However, above 1,000 rpm, stepper-motor torque begins to fall off due to core losses and magnetic circuit time constants.

In contrast, servomotors with comparable torque don’t start falling off until around 2,000 to 4,000 rpm or more. Direct-drive servo applications powering high-inertia loads typically run at less than 1,000 rpm, while typical servo applications with a mechanically advantaged drivetrain run at any number of speeds within the working capability envelope.

Servomotors have several advantages over steppers. They can generate high torque over a wide speed range on demand and come in versions for a wider torque range and higher voltages (up to 480 Vac).

If a given application needs a speed range between 1,000 to 3,000 rpm, the most suitable motor technology depends on requirements such as peak torque at speed, continuous (RMS) torque and speed, and repeatability.

At stall (low speeds less than 50 steps/sec or 15 rpm) or when holding a load with no movement, stepper motors—especially with oversized rotors—can produce more torque than servos for a given frame size (though newer servomotor designs are encroaching on this capability). All that torque lets steppers produce extremely accurate and stiff low-speed motion without a gearbox or other mechanical advantage.

In contrast, direct-drive servomotors with higher pole count and high-resolution feedback often drive industrial process applications requiring typical velocities of less than 1,000 rpm without a mechanical advantage such as a gearhead.

When a stepper motor is at rest, it uses continuous energy and there’s absolutely no movement (unless overloaded). When unpowered, for some applications a stepper motor’s detent torque can hold position.

In contrast, a servomotor is never at rest when on, due to constant closed-loop error correction. At which time it only uses the energy required to maintain its commanded position. Constantly varying position-loop error causes the servo’s output shaft to move back-and-forth (though on a scale that may be unnoticeable) while it continuously looks for minimum error. This continuous back-and-forth actuator motion is called hunting.

AKM series servomotors give designers flexibility to quickly coengineer modifications to perfectly fit applications. They’re available in eight frame sizes and more than 500,000 standard configurations.

A similar movement is dither —a purposely induced actuator movement, as in a valve to continuously overcome the issues of stiction. Hunting is unnoticeable in most applications involving only a few feedback counts (relative to the percent of total resolution), but can be unacceptable in others (though higher-resolution feedback devices reduce the typical hunting delta).

Where repeatability and resolution are issues, servomotors are the traditional choice. However, steppers are making inroads in these applications wherever the load is predictable or subject to only small external forces and disturbances, and where tight coordination between axes is unnecessary. Here, steppers running open loop can save 20 to 30% on initial machine expense over comparable servo solutions.

• Regarding torque, designers should select the motor that provides higher values at the application’s speeds. Engineers can determine which motor excels by comparing speed-torque curves. For a given price, most designers prefer servomotors. For constant or variable loads, servo systems recover from overload conditions where stepper systems cannot. Steppers output a lot of torque from a small package for speeds less than 1,000 rpm. In contrast, servos can handle torque requirements below and well above 1,000 rpm.

• Determining a system’s load inertia and inertia matching can help engineers pick between motor types. As a rule of thumb, steppers usually don’t exceed a 30:1 ratio of load inertia to motor inertia (J_load:Jm). In contrast, direct-drive servo systems with no compliance (except the driven steel) and high-resolution feedback can run 200 to 300:1 (J_load:Jm ratio) and higher—with quick response times relative to legacy feedback technologies.

In the past, traditional servomotor setups (with mechanical advantage through power-transmission components) needed a J_load:Jm ratio in the range of 1:1 to 5:1 (which later became 1:1 to 8-10:1) to get quick response times and high acceleration or deceleration.

Today, a system that has the highest available feedback resolution and minimal compliance and/or backlash is capable of 1 to 15:1 and higher inertia ratios for many applications. These deliver the best operational efficiency with little additional risk. Higher inertia ratios are application dependent, not only relative to the target machine specification, but also in relation to the mechanism’s compliance and backlash. For direct-drive systems, required stiffness often demands larger shafts and bearings, and even the compliance of the machine fixture and motor stand comes into play.

Other parameters to help engineers decide
The next application factor to help engineers pick between servomotors and stepper motors is the need for coordination of axes. Applications that need axis coordination should use servo-controlled systems to leverage their tight synchronization capability (high bandwidth and high resolution allow for fast correction of signal disturbances and command changes). Properly sized open-loop stepper motor systems stay in sync without confirmation feedback, but are limited to point-to-point moves with only sequential or pseudo-coordination between axes possible.

• Next consider cabling and motor-drive adjustments. One recent change improving servo reliability (and minimizing maintenance) is the reduction in the number of power and feedback wires the average servomotor needs.

Manufacturers have also taken guesswork out of closed-loop system servo tuning (motor-drive-mechanism compensation) and determining when a system needs maintenance. Automated or calculated tuning techniques and built-in diagnostic programs help simplify these tasks for the user. In addition, most servodrives can use traditional Step and Direction inputs that have been used to interface to steppers for years. Servos using this capability are typically in a position mode that eliminates the potential loss or addition of commanded motor steps.

In fact, this is a good segue to discuss one of the most common issues with stepper motor systems when run on the edge of their capability—the loss or addition of motion steps relative to the number of commanded steps. The problem is most apparent during acceleration or deceleration.

Step losses typically result from overly large inertia (affecting acceleration) or higher-than-desirable friction. In contrast, the addition of steps typically results from noise or too large inertia (affecting deceleration). Due to the accumulation of extraneous (+) and lost (-) motion steps, it can take hours before some manufacturing tolerance is exceeded. Nonetheless, stepper motors are still simpler because they have fewer wires to connect with minimal amounts of motor-drive adjustment to get a system up and running.

• Accuracy and resolution are other parameters to help pick an axis design’s motor type. Note that theoretical and actual resolution differ for stepper systems. For example, a two-phase, full stepping, 1.8° step-angle motor may have 200 possible positions in one revolution (from 360°/1.8°), but whether the motor achieves that depends on how the motor was sized for the application. The same is true of half-stepping and microstepping motor-drive modes. A 1.8° microstepper specified as having 10 microsteps per each full step won’t necessarily find any position within 0.18°.

Additionally, several commanded microsteps can transpire before there’s enough torque buildup to overcome friction and load inertia to get moving. In a real-world situation, the motor could easily jump one or more microsteps beyond the number commanded and stabilize there. When positioning-resolution requirements must exceed 200 steps/rev, steppers can use a feedback encoder to get 1,000 steps/rev or better. Five-phase motors and (in some cases) microstepping motors can also improve the steps per revolution.

Servomotor resolution is theoretically infinite, but in closed-loop operation, system positioning depends primarily on the resolution of the feedback device, be it a sine encoder, resolver or a digital (TTL) type encoder. Today’s high resolution feedback devices can approach between 2²¹ (2,097,152) to 2²⁸ (268,435,456) counts per motor revolution, plus optional multi-turn capability to 4,096 turns. Multi-turn feedback capable devices are available for an axis’ absolute position on machine power-up, eliminating the initial axis power-up homing cycle.

• Repeatability is yet another parameter that differentiates servomotors and stepper motors. Servomotors are extremely repeatable because they run closed loop. But steppers can be just as repeatable in many applications, especially when running in one direction. That said, when an ICR mode is used or the load increases (like during a direction reversal) and exceeds the stepper’s capabilities, the situation changes. Similar to how a gearbox must take up backlash, the stepper must catch up to system command. During the first move in a new direction, motor accuracy is affected because the stepper is overcoming inertia and friction (effects of the load). Once that happens, the system regains its specified repeatability, but it may have lost or gained actual position steps over those commanded.

• Consider input power as well. A stepper motor is equivalent to an inductor in series with a resistance. So, the current that produces torque needs time to rise. This time limits the speed at any given voltage, so increasing motor speed in a given application may require higher voltages.

Servo system requirements are similar, but (within its capability envelope) the drive’s control loops supply required voltage and current to the servomotor to meet the load’s demand relative to the move command and feedback error. When a servomotor system is forced to work (for whatever reason) outside its operational envelope, even for a millisecond, it’s no longer under control and doesn’t operate as a servo.

Reprint info >>

Kollmorgen
www.kollmorgen.com

Standalone motion controller in small package from Kollmorgen

April 8, 2016 By Miles Budimir Leave a Comment

Kollmorgen’s new Programmable Controller Multi-axis Master (PCMM) is a powerful new motion controller that meets a broad range of OEM requirements for a wide range of machines. The PCMM delivers complete PLC and motion control functionality in a compact, easy-to-use controller. Integrated programming, configuration screens, and diagnostics in a single software package simplify and minimize setup time, resulting in faster time to market for OEMs.

The PCMM also offers superior scalability. It is versatile enough for a simple 1-axis machine as well as more complex machines with 100 or more axes. The PCMM often eliminates the need for large, expensive PCs. By using a single modular/scalable controller, OEMs are also able to reduce inventory, installation and design efforts. The compact size and multi-network connectivity of the PCMM help with modularity, meaning OEMs do not have to use multiple control platforms.

Key benefits and features of Kollmorgen’s PCMM include:

Higher performance motion and increased machine throughput
IEC 61131-3 Programmable Automation and Motion Controller
EtherCAT® Master for high-performance motion and device synchronization
Coordinated control of up to 128 axes
Synchronized path control, 250 usec for up to 32 axes
Communication over TCP/IP, MODBUS®, EthernetIP®, Profinet® to host PLC, Computer, or Network
Onboard Digital I/O with support for expansion I/O and other devices over EtherCAT®
Integrated Webserver for remote diagnostics and status checking
Removable SD memory card for simple back up and program loading
All-in-one IDE includes PLC and motion programming, system configuration and diagnostic tools, plus one-click simulation for rapid prototyping

The PCMM is ideal for use in a number of industries including packaging, printing, converting, material forming, general motion control, food and beverage, medical and robotics.

For more information, visit www.kollmorgen.com.

New vented connectors for Kollmorgen AKMH motors

March 25, 2016 By Miles Budimir Leave a Comment

Kollmorgen has developed new vented connectors in response to customer requests for robust connectors for their AKMH motors. (The AKMH hygienic servomotor is designed for long life and trouble-free operation in the most challenging environments – such as daily exposure to chemicals and high-pressure wash-downs.) One of the keys to AKMH’s durability is the integrated pressure equalization feature. A hybrid power/feedback cable incorporates a tube that allows the motor’s internal pressure to equalize with the electrical cabinet of the connected drive.

Kollmorgen’s test data showed that internal pressure of a sealed AKMH during a thermal cycle ranges from -5.5 to +6.0 PSI. Allowing high pressure to occur inside the motor can cause premature failure of the motor’s seals and result in the failure of the motor due to water ingress in a wash-down application, so the venting of a motor in a wash-down environment is critical. Kollmorgen considered several solutions to reduce internal motor pressure, eventually holding a design event that resulted in the development of the innovative new vented connectors. The winning design meets several key criteria:

• Maintains the venting feature of the AKMH
• Maintains IP69K rating of the motor and cable
• Maintains hygienic features of the motor
• Maintains caustic wash-down ratings of the motor and cable

The vented connector assembly is applied to the end of the cable that exits the AKMH motor, allowing the motor to vent at the connection point while maintaining the IP69K rating. (Venting is not required beyond this connection point.) This allows the use of either a wash-down mating cable, if the cable is continuing through a wash-down zone, or a non-wash-down cable if the connection point is placed outside of the wash-down area.

For more information visit www.kollmorgen.com.

Updated: Trends in electric motors Part II — Scott Evans of Kollmorgen sets record straight on motion design

March 17, 2016 By Lisa Eitel Leave a Comment

In a recent interview with Scott Evans, Director of Product Strategy at Kollmorgen (for our Motion Trends coverage of electric motors) we got particularly good insight on the changes of today’s motion design. Following is some of Scott’s perceptive commentary. Enjoy!

Several industries are spurring the biggest changes in motion design.

Motion design changes in electronic hardware

One obvious change in motion is the miniaturization of products by virtue of efficiency gains and miniaturization in electronic components. The main driver behind this is consumer electronics, because that is where the real volume is.

However, industries such as medical, aerospace and defense (A&D), and automotive (OEMs … not the manufacturing) — which have long favored embedded motion — are now driving the adoption of the consumer electronics in industrial and military products … and demanding their motion suppliers do the same.

Consumer electronics also continues to drive down component cost, which lets all industries afford more sophisticated products that were previously unaffordable.

This is the Kollmorgen P5000 stepper drive, a micro-stepping drive to get high system performance from Kollmorgen’s POWERMAX II stepper motors.

Many motion companies will say there is movement towards de-centralization or towards centralization. Or they might say there is movement towards integration or de integration. We see ebb and flow between these, even at the same OEM …

… but it is our opinion that this is driven more by the specific application than a broad preference.

Motion design changes in software and communication

Although it feels like there are more Ethernet-based protocols than ever before, we are starting to see obvious winners and losers in this space. We see indications of de-facto consolidation and expect that to continue.

We also expect the open and consumer-driven protocols such as TSN will start to take root. This will help the smaller players, if they jump on board … or hurry their extinction.

Motion design changes in power transmission and mechanical components

Precise-and-automated machining continues to drive quieter, more precise and more efficient mechanicals. These gains let OEMs specify smaller transmissions, and make the motors and amplifiers that drive them smaller, too. Gearing technologies that were previously too expensive for most applications — for example, planetary, cycloidal, and even harmonic — are becoming more affordable, so are gaining wider adoption.

Kollmorgen UltraTRUE Series combines precise helical-crowned gearing with TRUE Planetary orientation for ultra-quiet gearheads that only exhibit 4 arc-min. of backlash.

Also, there are more Savvy OEMs …

Often, conversations are focused exclusively on technologies … but what is often overlooked is the increased sophistication even of small OEMs. For instance, these OEMs now appreciate the cost of ownership and the value of precision much more … versus just the acquisition cost, as in the past.

As service contracts and leasing arrangements (not unlike the jet-engine industry) start to become more popular, so will selection of components that increase the precision and life of a machine.

On cloud-enabling technologies and IIoT capabilities for motors

Inside our motors, we can supply embedded sensing technology, in addition to the velocity/position feedback, that provides data around torque/torsion/stress, humidity, pressure, etc. We find this of demand in high-value machines one might find in large HVAC and mills and mining; But the technology is young enough that widespread adoption in the typical factory is a few years off yet. More after the jump.

Kollmorgen’s Stainless Steel AKMH series motors with IP69 construction can withstand rigorous washdown in the toughest of hygienic environments.

We do see the ability of sensors to scavenge power off of inefficiencies in machinery (vibration, heat, light, etc.) to be self-powered to be a huge driver. But until they can scavenge enough power to drive a radio— and can store energy long enough to transmit what is in the buffer after the machine stops — this will not be adopted widely. Also, until we have industry standards that stratify, normalize, and harmonize data standards (scaling and units, for instance) and a safe-and-universally-standard way to communicate the data, even a self-powered sensor will not find wide adoption except in specific-duty (and closed) systems.

Drives and control systems have long had the ability to handle much more data, stratify it and even normalize it, then send to upstream systems. The process industries have been leveraging this capability for years and continue to drive.

Again, more-open systems such as OPC-UA will aid and abet wider use of these capabilities …

… and will someday be widely used even in discrete manufacturing.

Recent applications using motion components to improve design

Moore’s Law has afforded many applications that were unaffordable or inconceivable 10 years ago.

Motion has so improved that the error (actual motion vs. commanded motion) is controlled in the 1µs range, by the electronics mounted on—or a few meters from—the motion; so concerns like jitter and lag that were huge issues a few years ago, are no longer such large issues.

A popular discussion around drive-by-wire can be used as an example:

We can fly drones over targets of interest from a control center thousands of miles away, using cable-TV satellites, and COTS consumer electronics at both ends of the communiqué. The lag is less than a second. None of that was possible 10 years ago. The same technologies are being used in multi-plant processing such as refining; also being used in driverless vehicles such as for mining and exploration.

Even ultra-simple applications, such as slow labeling machines, are able to simplify their mechanicals dramatically because the higher-performance motion is more affordable. If an OEM can buy a stepper system, for example, for the same price he once paid for a mechanical clutch system, why wouldn’t he do that? Especially when customers expect accurate label placement and zero scrap, and longer machine life by virtue of fewer wear-components, but are not willing to pay a higher price for the machine?

Changes in motion and power-transmission design over the last decade

For specialty designs (in this context, designs made through partnering with suppliers for custom solutions) there have been some exciting trends as well. As reshoring picks up steam, we are seeing a spiral effect happening around mass-customization and artisanship.

First, I’ll describe what I mean by spiral. You’ve heard the terms referring to when something comes back around or comes full-circle. But if things come back around in a slightly different form, the situation is more like a spiral than a circle … as the circumstances exist on a slightly different plane.

Well, in the olden days before the Industrial Revolution, businesses thrived from the work of artisans (such as bakers, stained-glass-window manufacturers, cobblers, and blacksmiths). This mode of production gave way to mass-produced have-any-color-you-want-as-long-as-it’s-black production. But even mass-produced products were labor-intensive to make … manufacturers just got better organized and adept at outputting product with mass-parallel production with industrial-automation speeds. Once developing countries started catching up with the ability to mass-produce, but with cheaper labor, manufacturing jobs began to disappear from mature countries.

Today, affordable automation and CAD/CAM now lets companies reduce labor and bring manufacturing back home to places such as the U.S., but it’s not just mass-produced cookie-cutter production this time around. Instead, mass-customized products are now possible with minimal changeover time. This allows smaller batches and lower overhead costs.

Included in affordable automation are motion and PT. Not only are the automation components more affordable, they are more-precise, longer-living, and easier-to-use.

In summary, even motion suppliers are figuring out how to customize quickly and efficiently, and at minimal cost. So OEMs who have an idea, but find that commercial off-the-shelf (COTS) motion products are insufficient can now leverage co-engineering and customization in concert with the right supplier.

Other innovations on the horizon for motor-driven design

We believe a megatrend is coming …

End users will begin to steer clear of “specifying specific vendors” on the components and systems inside capital equipment they purchase. Why?

I. Baby boomers are retiring. With them, antiquated notions are retiring. Here they are in no particular order …

a. Buying equipment and maintaining it with expensive in-house staff

1. Specifying sub-optimized COTS components/systems in the name of reduced spares-and-repairs inventory

2. Relying (almost exclusively)upon particular vendors with closed systems

b. Programming with relay logic

c. The notion that “everything we do is secret sauce”

II. The above will be replaced with other approaches. Here they are in no particular order …

a. The notion of “robotics” will mean so much more, and be much-more-widely accepted than any time in our history

1. Mechanical designers will have real-world robotics- and machine-design experience right out of school

b. There will be more 21st-century coders and electronics experts

c. There will be a shift towards leasing equipment … and pushing responsibility for OEE and “perfect production” onto the machinery suppliers

1. This will spur best-in-class components and subsystems

2. The design objectives of quieter and smoother operation, more precise setups, and longer-life will drive designs

3. There will be more service contracts … so suppliers will begin to be paid based on uptime, OEE, production

• OEMs will own the maintenance, and spares-and-repairs

• Remote monitoring and predictive maintenance will be de rigueur

• Winners will produce machines that self-diagnose, self-prognose, even self-heal

4. OEMs will be much more willing to try new ideas and hacks than in the past

d. Anonymizing, normalizing, and harmonizing data — all in the spirit of sharing these data in a BIG way — will become critical to continuous improvement.

1. Mutual learning without collusion will become necessary. (This is arguably already necessary.)

Kollmorgen’s PMX Series stepper motors offer performance, quality and design flexibility in an affordable package.

New dual-axis AKD2G servo drive from Kollmorgen

Difference between continuous ratings and holding continuous loads (an ac servo-motor terminology primer)

Servo motor designs — where a stopped condition might be by design

Significance of the locked-rotor expression for induction motors

Widespread misconceptions about the term stall means that even some manufacturers’ publications incorrectly state that stall means zero rpm or no rotor movement.

Proper use of stall expressions for sizing ac brushless servo motors

The main difference between the two major commutation methods is that:

Six-step or Block (unmodified trapezoidal) commutations only allows current flow through two of the three motor phases at any given time — for 2-ON 1-OFF at all times

Sine-wave or sinusoidal commutation allows for current flow through three motor phases at the same time (when appropriate) and each electrical cycle of the motor is presented as a sine-wave to the servo motor.

The two different commutation methods require a different representation of the motor’s torque constant (Kt). For more information, download a PDF white paper titled, AC Permanent Magnet and PMDC Brushless Servo Motor Parameters and Conversions and available at www.kollmorgen.com.

Understanding stall — for proper servo-motor calculations

Kollmorgen to demo new AKD2G Servo Drive & AKM2G Servo Motor System at Pack Expo 2018

Kollmorgen website: New tools, technical support, and online community

Thermal time constants and managing PMAC servo motor overloads

Special application conditions

Designing motors with safety in mind

Stepper motor or servomotor: Which should it be?

Standalone motion controller in small package from Kollmorgen

New vented connectors for Kollmorgen AKMH motors

Updated: Trends in electric motors Part II — Scott Evans of Kollmorgen sets record straight on motion design

Related articles on trends in motor technology

DESIGN WORLD NETWORK

MOTION CONTROL TIPS