Motors - Motion Control Tips

Washdown servo motors for industrial applications from KEB America

September 20, 2018 By Miles Budimir Leave a Comment

KEB America’s new STEEL IT epoxy coating for servo motors is now available for the North American market. When motors are used in applications that are exposed to moisture and chemicals – such as food and beverage processing – they require special design considerations to ensure they will withstand the environment. The new STEEL IT epoxy coating option from KEB America provides a washdown servo motor with many of the benefits of a stainless steel motor, but at a fraction of the cost.

KEB DL3 Servo Motors provide dynamic control for positioning and cycling tasks. With excellent torque density and a range of options for voltage and power, they can easily fit into many industrial applications. The motors are CE and cUL certified, built to IP65 environmental protection standards (IP64 at shaft seal) and have lifetime lubrication. With the addition of the new STEEL IT coating, the motors are able to withstand exposure to caustic solutions and washdown procedures, much like a full stainless steel motor. This makes KEB Servo Motors an economical and cost-effective choice for machine builders and integrators.

Other options for KEB Servo Motors include a range of available encoder types, holding brakes, or fail-safe braking. Planetary gearheads from KEB are a great compliment to the DL3 Servo Motor, particularly for applications that require precise positioning and maximum efficiency. The S6 Servo Drive completes the system, making it easy to integrate precise motion control into a new or existing automated process.

Additional motion control components are available from KEB including robotic-grade encoder cables, modular gearboxes in a variety of configurations and gearing types, and software for maintenance and commissioning.

For more information, visit www.kebblog.com.

NORD launches next generation two-stage bevel gear motors

September 18, 2018 By Miles Budimir Leave a Comment

NORD’s new SK 920072.1 two-stage helical bevel gear motor (mounted with a NORD motor) is a drive solution for a wide range of light duty conveying, processing and manufacturing applications. Identified by its high-strength and light-weight design, NORD’s newest aluminum gear motor is a cost-effective replacement for cast iron units. Two housing styles offer options for multiple applications and environmental conditions.

The next size of premium efficient two-stage bevel gearboxes features a new compact, more efficient modular design. Covering applications for torques up to 443 in.-lb (50 Nm) and offering up to 97 percent gear efficiency, the SK 920072.1 and SK 930072.1 are superior alternatives to worm gear units in light conveying and processing applications, as well as for food and beverage industries.

Like other NORD gearmotors, the new size comes in two housing designs – the SK 920072.1 has optimized thermal properties due to its large surface area. The SK 930072.1 is ideal for washdown applications because of its closed housing design.

The new NORD size has a power range of 0.16 to 1.50 hp (0.12 to 1.1 kW) and speed ratios from 3.03 to 47.67. They provide exceptional adaptability for all mounting positions as far as input, output and motor options. Available with either NEMA or IEC input, a NORD motor or brake motor, both the SK 920072.1 and SK 930072.1 gear units are well suited for use in the material handling industry, car wash, as well as for sanitary and hygienic environments.

Available for the SK 930072.1 model is NORD’s NSD TUPH surface conversion, a revolutionary process only offered by NORD. The aluminum alloy reducer housing and modular accessories are electrically catalyzed producing a strong corrosion resistant base layer (or molecular surface conversion). The additional top layer provides enhanced sealing, additional corrosion protection and a brighter finish.

Unlike painted cast iron gear units, the extremely strong base layer (six to seven times harder than the original aluminum alloy) provides superior corrosion resistance and scratch resistance, and it will not chip, flake or rust.

With NORD’s leak-free design, longer gear and bearing life, quiet operation, and precise output torque capabilities, SK 920072.1 and SK 930072.1 helical bevels deliver high efficiency operations, low maintenance, and longer service life.

For more information, visit www.nord.com.

How to specify motion components for cleanroom environments: Part 2

September 5, 2018 By Danielle Collins Leave a Comment

In this two-part article, we cover the products, materials, and features that designers and engineers should specify when choosing motion components for cleanroom environments.

In part 1, we looked at how to reduce particle generation due to friction between moving components. In this second part, we’ll look at another source of contamination — outgassing — and how to minimize it.

Objective #2: Minimize outgassing

Silicon wafer processing is extremely sensitive to contamination, including both particulates and outgassing.
Image credit: WaferPro

In cleanroom applications that involve the manufacture of LEDs, optics, or glass, or the processing of silicon wafers, a second type of contamination can damage the process or the product: outgassing.

Outgassing is the desorption of vapors or gasses, either from within a material or from the surface of a material. ISO standard 14644-8 addresses outgassing in cleanroom environments and specifically refers to airborne molecular contamination (AMC) as:

The presence in the atmosphere of a cleanroom or controlled environment of molecular (chemical, non-particulate) substances in the gaseous or vapor state that may have a deleterious effect on the product, process, or equipment in the cleanroom or controlled environment.

These types of airborne molecular contaminants take the form of molecular vapor. They’re smaller than particles and easily pass through HEPA and ULPA filters, making them particularly difficult to control once released.

Minimize outgassing through material selection

The first criteria when choosing motion components for cleanroom environments where outgassing is a concern is to ensure the material has a smooth surface, which makes it more difficult for airborne molecular contaminants to adhere to the surface. Stainless steel and anodized aluminum are the preferred materials, although some epoxy paints are suitable for environments where low-outgassing is required.

Fortunately, numerous motion control components, such as linear guides, motors, and gearboxes, can be made of stainless steel. When possible, plastic end caps on linear guides and screws should also be replaced with stainless steel versions to further minimize the use of plastics. And standard steel fastening hardware, such as screws or pins, should be replaced with stainless steel versions.

Although epoxy paint is sometimes suitable for motor and gearbox housings, stainless steel versions are better options.
Image credit: Wittenstein

When the required material has a high outgassing tendency, other solutions exist. For example, epoxy paint is suitable in some cleanroom environments and can be applied to the housings of linear actuators, motors, and gearboxes. And when steel is required — to maintain hardness, durability or load-carrying capacity — nickel plating can reduce the tendency of steel to outgas. (Note: Some nickel-plating formulas include a PTFE, or Teflon^®, top coat. The use of Teflon is generally advised against, so be sure to choose a nickel plating formula that does not include Teflon.)

Good: Nickel-plated steel, epoxy coated aluminum
Better: Stainless steel or anodized aluminum; minimal use of plastics
Look for: Smooth surfaces (no textured paints)

Reduce outgassing from lubricants

A wide variety of cleanroom-rated lubricants are suitable for bearings, motors, gearboxes, and other motion components.
Image credit: NSK

When it comes to outgassing, lubricants are one of the major offenders, readily spewing molecules of water and oil vapor into the atmosphere. Unfortunately, in the majority of motion systems, the use of components that require lubrication simply can’t be avoided. When lubrication is required, ensure the lubricant is suitable for cleanroom environments. Cleanroom compatible formulas that minimize outgassing while protecting bearing surfaces from rust and friction are widely available.

Good: Cleanroom-rated lubricants
Look for: Opportunities to minimize the need for lubrication through component selection

Feature image credit: Lin Engineering, Inc.

QTR Series torque motors from Tecnotion

September 4, 2018 By Miles Budimir Leave a Comment

The QTR motor family from Tecnotion, long known as “The Linear Motor Company,” entered the rotary torque motor market last year with a new design for torque motors. The new design means the magnetic design is optimized for maximum flux density and copper fill to provide high torque to volume ratios.

Manufactured in volume, these new Tecnotion torque motors feature encapsulated windings and include two types of thermistors as standard features at competitive pricing. Tecnotion offers motor sizes from 68 to 160 mm diameters. These motors work with any servo drives on the market.

Torque motors from Tecnotion are extremely compact, provide an extremely stiff drive train, eliminate the need for time-consuming mounting procedures, are virtually maintenance free and can be integrated directly into the machine structure, while the spacious open inner diameter enables wire and cable feed-through. The entire series also comes with completely sealed stators and integrated temperature protection and measurement sensors.

With continuous torque ratings ranging from 0.27 Nm to 63.5 Nm and speed ranges from 190 rpm to 6,918 rpm, these motors can play a pivotal role in increasing the availability and cost-effectiveness of many different applications. Typical industry applications include semiconductor, printing, robotics, and machine tooling, among others.

To find out more about QTR Series torque motors, visit www.electromate.com/direct-drive-motors.html.

Frameless brushless dc motors in hybrid solid-state LIDAR Applications

August 31, 2018 By Lisa Eitel Leave a Comment

Edited by Terry Persun || Use of light detection and ranging (LiDAR) in the automotive industry is growing exponentially. The two most common variations are solid-state LiDAR and hybrid spinning LiDAR.

Solid-state LiDAR are non-spinning systems. Some employ a phased array to steer the laser beam sans moving parts; others include a tiny mirror to steer the laser beam. Still another solid-state design called flash LiDAR illuminates entire scenes in a single flash to allow detection of returned light via a two-dimensional array of sensors — similar to the operation of a digital camera. With few moving parts, these methods provide cost-effective LiDAR functionality.

Vehicles with long-range LiDAR can anticipate the need to brake earlier — to reduce the necessary deceleration to a rate less likely to injure occupants. Frameless dc motors can make these LiDAR designs more compact.

Sometimes manufacturers put all the LiDAR electronics on a single chip, including the laser, any beam-steering circuitry, detectors for the return signal, and supporting computer power. But single-chip solutions for solid-state LiDAR are vulnerable to environmental challenges such as heat, cold, humidity, rain, and corrosive substances. End users needing to repair systems often find replacement ICs costly. What’s more, manufacturers are increasingly looking to offer lower-cost modular options that allow individual component swap-outs.

The latter is significant because a typical solid-state LiDAR setup provides a single fixed-in-place field of view of 120° or less. That means it takes at least four units to cover a full 360° safety zone. That contrasts with spinning LiDAR systems, which cover 360° operation with no blind spots due to gaps or overlapping … problems inherent to solid-state systems.

In addition, safety rules dictate that LiDAR must not generate laser output capable of damaging a person’s eyesight — even if that person were to look directly at the LiDAR unit for a number of seconds.

Reasons for solid-state hybrid LiDAR

Leveraging the advantages of both solid-state and spinning LiDAR is hybrid LiDAR. These designs have a field of view that is seamless and repeatable. Plus safety regulations permit moving laser sources to emit higher-power beams than stationary ones. So more powerful lasers let hybrid spinning LiDAR units map wide ranges and (when incorporated into systems for autonomous operation) help vehicles avoid the need to make short stops — and avoid the increased risk of body trauma to passengers. So even while solid-state LiDAR becomes increasingly common, demanding applications (including self-driving cars) will employ robust hybrids with superior long-range operation, higher resolution, and broader fields of view than other systems.

As we’ll see, trimming the cost and footprint of these LiDAR designs sometimes means using alternatives to the usual brushless dc (BLDC) motors employed for their spinning action.

Motors parts that LiDAR systems don’t need

Recall that brushless dc motors are more powerful than brushed motors — and avoid the friction and voltage drop associated with brushes for commutation. That’s why brushless dc motors are used in LiDAR systems. Spinning LiDAR designs usually turn at 200 to 500 rpm. The mirror assembly the motor turns is very light, so torque isn’t typically an issue. But many of today’s LiDAR systems use off-the-shelf (OtS) NEMA 17 and NEMA 23 motors — which can make for unnecessarily bulky builds.

Brushless dc motors are essential to hybrid LiDAR systems. But as the automotive industry works to miniaturize LiDAR systems, the built-in components of standard motor offerings (which are useful in other designs) are sometimes a detriment.

That’s because most OtS brushless dc motors are structurally and mechanically self-supporting. Endcaps suspend the rotor inside the stator; then the LiDAR’s moving assembly couples to an output shaft. In fact, motor ends can easily account for up to 50% of a motor’s overall length.

In contrast, frameless motors omit additional mounting supports, plates, brackets, and couplings to let design engineers directly integrate loads into the rotor design; in addition, the stator can be seamlessly incorporated into the system for a smaller design that doesn’t sacrifice performance.

Sleeker automobiles with compact LiDAR motors

Aerodynamics and visual styling are paramount in automotive design. It’s easier for engineers to incorporate wider flatter shapes to car exteriors than tall narrow shapes. Frameless motors can deliver this kind of geometry — with large diameters and less height than comparable OtS brushless dc motors. In addition, a portion of the LiDAR mirror load can sink down into the frameless design’s stator. Both of these options help reduce the overall LiDAR height for a streamlined shape.

Frameless motors — as these from Lin Engineering — integrate into designs to reduce waste and redundant mounting supports, plates, and brackets. They also omit endcaps that can easily account for up to 50% of overall motor overall length.

Frameless motors aren’t constrained to standard sized motors, so give design engineers more freedom to tailor motor shape and size to the application. This makes for LiDAR systems with the smallest footprint possible. When paired with the lightweight sensors of solid-state technologies, the motors have less mass to move and lower torque requirements to satisfy — allowing for a much smaller motor to do the job.

Off-the-shelf motors may be oversized for LiDAR systems. In contrast, custom motors are often more compact and (if supplied in high volumes) save thousands of dollars in manufacturing and production over the long run. For more information, visit Lin Engineering at linengineering.com.

How to specify motion components for cleanroom environments: Part 1

August 29, 2018 By Danielle Collins Leave a Comment

In this two-part article, we cover the products, materials, and features that designers and engineers should specify when choosing motion components for cleanroom environments. In part 1, we look at how to reduce particle generation due to friction between moving components.

Designers of automation systems are tasked with many competing demands, such as balancing cost and performance, fitting motion components and systems into existing machine or process layouts, and designing for ease of manufacturing and assembly — to name a few. But when the system is to be used in a cleanroom, an additional layer of complexity is added, requiring designers to choose products that won’t degrade or compromise the cleanroom environment.

The level of “cleanliness” of a cleanroom is determined by the number of particles, broken down into six size ranges, that are present in a specified volume of air. In the United States, Federal Standard 209E is often used for cleanroom ratings, although it was officially canceled in 2001. This standard specifies cleanroom levels from 1 (best) to 100,000 (worst) in multiples of 10.

The current, and more universally-accepted, standard is ISO 14644-1, which defines cleanrooms on a scale from 1 (best) to 9 (worst). Each of the ISO classifications has a corresponding FS 209E level, with the exceptions of ISO classes 1 and 2, which are higher (better) than the highest FS 209E classification, and ISO class 9, which is lower (worse) than the lowest FS 209E classification.

ISO standard 14664-1 rates cleanrooms on a scale from 1 (best) to 9 (worst). Federal Standard 209E uses a scale from 1 (best) to 100,000 (worst) based on the number of allowable particles greater than or equal to 0.5 micron.
Image credit: Mecart Inc.

Objective #1: Reduce friction

One of the two primary sources of contamination, or particles, that can degrade a cleanroom environment is friction from moving components. (The other source is people.)

Virtually every motion system involves some amount of friction between sliding or rolling surfaces — whether from linear bearings, rotary bearings, or meshing gears. And when there is friction, particles are generated. Therefore, when specifying motion components for cleanroom environments, reducing friction should be the foremost objective.

Linear guides and drives

To minimize friction from linear guides, choose rolling contact rather than sliding contact and, when possible, avoid systems with high preload. For example, a non-preloaded miniature rail guide that uses two rows of recirculating balls emits significantly fewer particles than a preloaded standard guide with four rows of recirculating balls. And because there’s little chance of the bearing experiencing contamination in a cleanroom environment, use linear guides with low-friction or non-contact type seals.

Air bearings are another, although less common, method for guiding and supporting loads. In cleanroom applications, however, air bearings are often the best choice for low particle generation, since they are completely non-contact devices.

When it comes to linear drives, belts and chains should be avoided in cleanroom applications, due to the significant contact and wear they experience. Similarly, the meshing of gears in rack and pinion systems cause high friction and wear, so these should also be avoided. This means that ball screws are typically the default choice for linear drives in cleanroom applications.

However, ball screws require lubrication, and the rotation of the screw can cause lubrication to “sling” or “splatter,” contaminating the cleanroom environment. Using low-friction or non-contact seals (see above) will help keep the lubrication inside the ball nut and protect the cleanroom.

Like air bearings for linear guidance, linear motors provide a completely non-contact option for driving the load. But in their traditional setup, linear motors operate with the forcer (primary part) moving. This means the cables must move as well, and as we’ll discuss below, cables are another source of particle generation. A better configuration for cleanroom applications is to keep the forcer (primary part) and its cables stationary and allow the magnet track (secondary part) to move.

The combination of air bearing guides and a linear motor drive in this ABL 2000 stage from Aerotech provides completely non-contact movement, ideal for cleanroom applications.

Good: Rolling contact (rather than sliding contact) guides and drives
Better: Air bearing guides and linear motors
Look for: Low-friction or non-contact seals; cleanroom lubrication

Cables and cable management

Another source of friction and, therefore, particle generation, is the cable management system, including the cables themselves. Traditional round cables can generate particles when they rub against each other or against parts of the cable track. The best way to reduce particulates from cables and cable management systems is to use components and system design practices that reduce the amount of cabling required — for example, using an integrated motor-drive system instead of separate motor and drive components.

Cable carriers used in cleanrooms should have low vibration, low-abrasion fastening, and smooth interior surfaces to prevent abrasion of cables.
Image credit: igus Inc.

For the power, feedback, and data cables that are necessary in a motion control system, manufacturers offer cable designs with special low-friction coatings to minimize particulates and reduce outgassing. Similarly, several cable track manufacturers offer systems that reduce wear between chain sections through the use of abrasion-resistant joints. And for shorter lengths, so-called “trackless cables” are self-supporting flat cables that don’t require a cable track or carrier.

Trackless cables are self-supporting and don’t require cable carriers.
Image credit: W.L. Gore & Associates, Inc.

Good: Round cables with anti-friction coatings; cable carriers with abrasion-resistant joints
Better: Self-supporting flat cables
Look for: Opportunities to reduce cabling

Rotating equipment: motors and gearboxes

When it comes to rotating equipment in motion applications, the bad news is, motors and gearboxes use rotary bearings, and gearboxes require meshing teeth — all of which are sources of friction and particle generation. The good news is that these components are enclosed, so particles are less likely to “escape” and contaminate the cleanroom environment. And there is a wide range of cleanroom-compatible lubricants that can be used in the high-speed, high-load conditions found in motors and gearboxes.

To improve cleanroom compatibility, it is also possible to add a slight vacuum to the motor or gearbox housing. The vacuum serves to extract and remove particulates so they don’t have an opportunity to contaminate the cleanroom. Note that vacuum purge is also a good option for enclosed actuators that have a static (non-moving) seal. Although the enclosed design tends to keep particles inside the actuator, adding vacuum purge helps to achieve higher levels (class 100, 10, or 1) of cleanroom compatibility.

Vacuum purge is an effective way to ensure particulates from closed systems, such as this linear actuator, don’t have a chance to escape and degrade the cleanroom environment.
Image credit: THK Co., Ltd.

Good: Fully enclosed housings
Better: Vacuum purge
Look for: Cleanroom grease options

In part 2 of this article, we’ll look at another source of contamination — outgassing — and how to minimize it.

What are slip rings and why do some motors use them?

August 21, 2018 By Danielle Collins Leave a Comment

Slip rings — also referred to as rotary electrical joints, electric swivels, and collector rings — are devices that can transmit power, electrical signals, or data between a stationary component and a rotating component. The design of a slip ring will depend on its application — transmitting data, for example, requires a slip ring with higher bandwidth and better EMI (electromagnetic interference) mitigation than one that transmits power — but the basic components are a rotating ring and stationary brushes.

A complete slip ring assembly includes end caps, bearings, and other structural features. But the basic components of a slip ring are the ring and the brushes.
Image credit: Moog Inc.

If the rotation of one component involves a fixed number of revolutions, it may be possible to use spools with sufficient cable length and rotating speed to allow for the required revolutions, although cable management in this setup can be quite complex. But if one component rotates continuously, using cables to transmit signals between the rotating and stationary components isn’t practical or reliable in many cases.

Slip rings in AC motors

Image credit: brighthubengineering.com

In a version of the AC induction motor referred to as a wound rotor motor, slip rings are used not for transferring power, but for inserting resistance into the rotor windings. A wound rotor motor uses three slip rings — typically made of copper or a copper alloy — mounted to (but insulated from) the motor shaft. Each slip ring is connected to one of the three phases of rotor windings. The slip ring brushes, made of graphite, are connected to a resistive device, such as a rheostat. As the slip rings turn with the rotor, the brushes maintain constant contact with the rings and transfer the resistance to the rotor windings.

Slip rings on a wound rotor AC motor. Once the motor reaches operating speed, the brushes are lifted via the springs and the slip rings are short-circuited via the sliding contact bar.
Image credit: Wikipedia

Adding resistance to the rotor windings brings the rotor current more in-phase with the stator current. (Recall that wound rotor motors are a type of asynchronous motor, in which the rotor and stator electrical fields rotate at different speeds) The result is higher torque production with relatively low current. The slip rings are only used at start-up, however, due to their lower efficiency and drop-off of torque at full running speed. As the motor reaches its operating speed, the slip rings are shorted out and the brushes lose contact, so the motor then acts like a standard AC induction (aka “squirrel cage”) motor.

The slip rings in a wound rotor motor form a secondary, external circuit. Inserting resistance to this circuit allows the motor to produce very high torque at start-up, which is necessary for moving loads with high inertia.

Slip ring or commutator?

You may have noticed that the design and function of a slip ring sounds very similar to that of a commutator. While there are similarities between the two, there are critical distinctions between slip rings and commutators. Physically, a slip ring is a continuous ring, whereas a commutator is segmented. Functionally, slip rings provide a continuous transfer of power, signals, or data. Specifically, in AC motors, they transfer resistance to the rotor windings.

Commutators, on the other hand, are used in DC motors to reverse the polarity of current in the armature windings. The ends of each armature coil are connected to commutator bars located 180 degrees apart. As the armature spins, brushes supply current to opposing segments of the commutator and, therefore, to opposing armature coils.

Slip rings are used in virtually any application that includes a rotating base or platform, from industrial equipment such as index tables, winders, and automated welders, to wind turbines, medical imaging machines (CT, MRI), and even amusement park rides that have a turntable-style operation. Although the traditional application for slip rings was to transmit power, they can also transmit analog and digital signals from devices such as temperature sensors or strain gauges, and even data via Ethernet or other bus networks.

Feature image credit: Rotary Systems Inc.

Low inertia, high precision, hollow core voice coil motor

August 17, 2018 By Paul Heney Leave a Comment

A low inertia hollow core voice coil or moving coil linear motor from Moticont, the HVCM-051-025-019-01, features high acceleration, precise positioning to less than one micron (0.00004 in.), high reliability, and low cost. This cleanroom friendly, non-commutated motor is available off-the-shelf. The HVCM-051-025-019-01 hollow core voice coil linear motor has an open apertures of 0.750 in. (19.05 mm). The hollow core allows for optics, illumination, laser beams, or cables to pass through.

With a 0.50 in. (12.7 mm) stroke length, a continuous force rating of 4.8 lbs (21.3 N) and a peak force of 15.15 lbs (67.4 N) at a 10% duty cycle, this compact voice coil motor measuring just 2.000 in. (50.8 mm) dia. X 1.00 in. (25.4 mm) housing length, is suitable for applications such as: Positioning, medical diagnostics and testing, laser beam steering, dynamic vibration absorption, optical focusing, laser cutting & welding, scanning, assembly, and wafer handling.

Two 10-32 UNF-2B x 0.25 in. (6.4 mm) deep threaded mounting holes on 1.000 in. (25.4 mm) centers in the housing and coil makes for flexible and easy integration into new and existing applications. Low inertia, high acceleration, high speed, and zero cogging are benefits of this moving coil or voice coil linear motor. When used in closed loop applications with an optional position sensor, resolutions of less then 1 micron are achievable for millions of cycles. If a longer stroke, higher force, a smaller foot print, or customized mounting is required consult Moticont’s team of engineers. All of Moticont’s voice coil motors are available as complete plug-and-play systems, including, position sensor, controller, amplifier, power supply, and if required with a linear slide.

Moticont
www.moticont.com

How does stiffness affect piezo output force and displacement?

August 15, 2018 By Danielle Collins Leave a Comment

Piezo actuators operate on the inverse piezoelectric effect: when voltage is applied, the actuator contracts or expands. But when the actuator is blocked from moving (by a load applied in the direction of travel), it generates a force. This relationship between displacement and force in piezo materials is an inverse relationship. In other words, the more force an actuator must generate against a load, the less travel it can produce. A “free” actuator — one that experiences no resistance to movement — will produce its maximum displacement, often referred to as “free stroke,” and generate zero force.

Conversely, when an actuator is blocked from moving, it will produce its maximum force, which is referred to as the blocked, or blocking, force. Theoretically, when the actuator is blocked, it is working against a load with infinitely high stiffness. But materials with infinite stiffness don’t occur in the real world, so the blocking force is determined by applying a voltage to the actuator, with no load applied. This “free” operation, as described above, generates the actuator’s maximum displacement, or free stroke. A force is then applied to return the actuator to its original length. This force is measured and recorded as the blocking force.

The stiffness of the actuator is the ratio of blocking force to free stroke.

k_p = stiffness of piezo actuator

F_block = blocking force

ΔL_fs = free stroke (nominal displacement) with no external load

In piezo actuators, force and displacement are inversely related. Maximum, or blocked, force (F_block) occurs when there is no displacement. Likewise, at maximum displacement, or free stroke, (ΔL_fs) no force is generated. When an external load is applied (A), the stiffness of the load (k_e) determines the displacement (ΔL_A) and force (ΔF_A) that can be produced.
Image credit: APC International, Ltd.

There are two types of load that can be applied to a piezo actuator: a spring load, which increases as the actuator moves, or a constant load, which does not vary. The actuator’s ability to produce displacement and force depends on the type of load.

Piezo output force and displacement with a non-constant (spring) load

Many piezo applications, such as valve operation, introduce loads that vary with the piezo’s stroke — in other words, spring loads. When the stiffness of the applied spring load is low compared to the piezo stiffness, the piezo output force (F_eff) is small. In other words, a spring (load) with low stiffness provides very little “blocking” for the piezo to work against in order to generate a force. Therefore, the piezo output force is greatly reduced from its maximum, or blocking, force.

F_eff = force produced with spring load

k_e = stiffness of applied load

On the other hand, the spring reduces the actuator’s displacement only slightly:

ΔL = displacement 0f actuator with spring load

Performance of a piezo actuator with no load (A) versus an actuator with a low-stiffness spring load (B). Note that the displacement (ΔL) with the spring load is slightly less than the maximum displacement, or free stroke, (ΔL_fs), but the force produced (F_eff) is much less than the maximum, or blocked, force (F_block). The stiffness of the spring load (k_e) is shown by the blue line.
Image credit: PI Ceramic GmbH

A low-stiffness spring load — typically no more than 10 percent of the piezo stiffness — is generally recommended for preloading a piezo actuator to ensure the displacement is not severely reduced.

Piezo output force and displacement with constant load

When a constant load is applied, there is an initial deflection, or compression, of the actuatordue to the load. This means the zero point of the working curve is offset, or shifted, but the actuator’s stroke is unaffected.

ΔL_N = offset of the zero point with constant load

F = applied load

Some force is generated when the load is applied, causing the initial deflection, but once the actuator responds to the application of the load, its remaining work goes into generating the displacement. The zero point of the blocking force (maximum force) is also shifted, but is magnitude is not affected.

When a constant load is applied, the zero point of the actuator’s displacement is shifted, but its total displacement, or free stroke, is not affected: ΔL’_fs = ΔL_fs
Image credit: PI Ceramic GmbH

Feature image credit: Noliac

What are the benefits of an active front end (AFE) drive?

August 8, 2018 By Danielle Collins Leave a Comment

A variable frequency drive, or VFD, delivers power from the source — typically the utility — to a motor in three basic steps:

A rectifier converts the AC power to DC
A DC bus receives, smooths, and stores the power
An inverter converts the DC power back into AC with the necessary frequency and voltage via pulse width modulation (PWM)

In a VFD, AC power is converted to DC through the rectifier (shown here as a standard 6-pulse version, consisting of six diodes). The rectified power is then filtered and stored in the DC bus. The inverter converts it back to AC power with the proper frequency and voltage for the motor, via pulse-width modulation.
Image credit: what-when-how.com

The rectifier is typically a 6-pulse type, so named because it consists of six diodes: one for when the voltage is positive and one for when the voltage is negative, on each of the three power phases. But while diodes are simple and cost-effective, they produce harmonics that are transferred back into the power system, resulting in additional heat and losses, and even causing erratic behavior of connected equipment.

Multiple rectifiers (i.e. 12-pulse, 18-pulse, or 24-pulse) can reduce harmonics, but as rectifier sections are added, footprint and cost go up. Another solution to reduce harmonics is to add passive filters, which introduce a low impedance to absorb harmonic frequencies. But passive filters are useful only in limited conditions, and they have a large footprint and generally use more energy than other alternatives. An active front end (AFE) not only reduces harmonics, but also provides other benefits that can reduce costs for the end user.

Rather than using diodes in the rectifier to convert the incoming AC power to DC, an active front end uses insulated gate bipolar resistors (IGBTs). IGBTs are devices whose switching is controlled electronically — hence the term “active” front end. The active front end monitors the input current waveform and shapes it to be sinusoidal, reducing total harmonic distortion (THD) to 5 percent or less. (Note that THD is only measured for lower-order harmonics. An LCL filter is necessary to reduce higher-order harmonics caused by the switching frequency of the IGBTs.)

An active front end (AFE) drive replaces the diodes in the rectifier with IGBTs, which significantly reduce harmonics and allow regenerated power to be fed back to the supply.
Image credit: Mesta Electronics, Inc.

Harmonic distortion is also the primary contributor to the system’s power factor in VFD applications. Power factor is determined by two variables. The first is the displacement, or phase angle, between the applied voltage and resulting current. For VFD operation, the voltage and current remain in phase, and the displacement between them (cos θ) is kept near unity (1).

The other factor is total harmonic distortion, which has an inverse relationship to the power factor — the higher the THD, the lower the power factor. So an active front end, which reduces harmonic distortion (for example from 45 percent to 5 percent) significantly improves the system’s power factor.

Power factor is essentially a measure of how efficiently the electrical power is being used to perform useful work. Utility companies often penalize industrial customers if their power factor falls below a set threshold, often in the range of 0.85 to 0.95.

Another benefit of active front end drives is their ability to handle regenerative power. The IGBT rectifier design allows four-quadrant operation, meaning the motor can produce either positive or negative torque (as occurs during braking or back driving), during either the forward or reverse motor rotation.

When torque and motor rotation are in different directions (i.e. clockwise torque and counterclockwise motor rotation, or vice-versa), the motor acts like a generator, and the power produced by the motor can be fed back into the electrical supply. This eliminates the need for large resistor banks to dissipate the regenerated energy and reduces costs by feeding the energy back to the line, where it can be reused.