Basics | Design World Motion Handbook

How to address leakage current from motor to ground?

September 14, 2018 By Miles Budimir Leave a Comment

All electric motors operate with inherent losses from a number of different areas. Some are inherently mechanical (think friction losses as heat) while others are electrical, from current in conductors, also known as I²R losses, to losses due to hysteresis and eddy currents in the iron components of a motor.

One type of loss that is especially pernicious is leakage current from the motor to ground. The fact is that some small amount of current can flow from the motor to ground in all electrical equipment, usually as a result of some type of fault in the equipment or cabling. The amount for the most part is small, perhaps on the order of milliamps, but it is present nonetheless and poses a safety risk to human operators.

Several sources of leakage current are shown here including from the motor itself as well as from the motor cable. (Image via TDK)

Leakage currents can flow through the insulation surrounding conductors. Electrically speaking, insulators have both resistance and capacitance, and leakage current can flow through both. Old or damaged insulation means a lower resistance and hence more potential current flow. Also, the longer the conductor the more capacitance, and therefore more leakage current flow. Typical allowable leakage currents in IT and medical equipment, for instance, are in the range of a milliamp or two to sub-milliamps.

So what is likely to cause leakage currents? For one, unshielded cables. Also, any other sources of electrical noise that can couple to the motor or motor cabling. Remember, leakage current can be an indicator of trouble in conductors and insulators. A common way to check for leakage current is to use a clamp-type current meter that fits around a conductor and gives a read-out of the current. The best ways to prevent leakage current are to always use shielded cables on motor installations and to make sure that these cables are grounded at both ends. Also, check cables and insulation for damage and replace ones that are broken or damaged.

Power and free conveyors: Where do they excel?

September 11, 2018 By Danielle Collins Leave a Comment

Power and free conveyors are designed for manufacturing environments where products need to be transported in a non-linear fashion — that is, where materials aren’t necessarily delivered in the order they were loaded or at the same pace. Traditional “linear” conveyors, on the other hand, lack the flexibility to handle manufacturing environments where different production processes run at different cadences or where various materials have different flow paths.

Because of their flexibility in handling and delivering products, power and free conveyors are also referred to as “asynchronous” or “non-linear” conveyors.

The defining feature of a power and free conveyor is that it consists of two tracks — an upper track and a lower track. The upper track is powered by a chain, and the lower track is unpowered. Trolleys, which carry the load, run on the lower track, supported by rolling wheels. Mechanical devices — often referred to as “pusher dogs” — on the powered track engage with the trolleys to move them and disengage with the trolleys to stop them. The pusher dogs are engaged and disengaged by cam action caused by a trolley in front or by a stop blade positioned along the powered chain. Air-activated stops, triggered by switches, can also be used to control the movement of trolleys.

In a power and free conveyor, the upper track is powered by a chain and the lower track is unpowered. Trolleys (two of which are shown here) ride on the lower track and are driven by devices on the powered track called “pusher dogs” that engage and disengage with the trolleys.
Image credit: Cisco-Eagle

Much like cars on a road, conveyor “traffic” can also be merged or diverted among multiple conveyor lines, and both sharp turns and elevation changes can be executed. And unlike traditional linear conveyors, power and free versions can allow products to accumulate, or “stack up” by holding trolleys stationary while other trolleys “catch up” and join them. The trolleys can then be released in the required sequence. All of this is accomplished through mechanical linkages via the pusher dogs and trolleys, without requiring complex automation.

To maximize space, especially during accumulation, trolleys can be connected via rigid load bars that set the spacing between trolleys at as little as 6 inches (depending on the size and shape of the load). And storage density can be maximized with trolleys that hold the load diagonally relative to the conveyor path.

Trolleys positioned diagonally can maximize space and increase storage density.
Image credit: Ultimation Industries, LLC

Circuits of conveyors up to 300 feet can be controlled with just one motor, and complex “traffic” patterns and timing sequences can be controlled via sensors triggered by a PLC or PC-based controller. In addition, bar codes or RFID tags on the trolleys can be used to identify individual loads and carry instructions for sequencing.

This video from Caldan Conveyor A/S demonstrates how power and free conveyors work, including their ability to accumulate, stage, merge, and divert products.

While most power and free conveyors are mounted overhead, above the working area, and carry products below the tracks, floor-mounted designs are available that carry the product above the tracks. This is especially useful when overhead space is a concern, or when potential contamination from the conveyor or product is an issue, as is often the case in cleanroom environments. And when overhead space is limited but floor-mounting is not an option, some manufacturers offer overhead versions with powered and non-powered tracks positioned side-by-side, rather than in a top-and-bottom configuration.

A floor-mounted power and free conveyor is useful when overhead space is limited, or when contamination from the conveyor or the product is an issue.
Image credit: Richards-Wilcox

Power and free conveyors are custom-designed for each application and can carry loads from just a few pounds to several hundred pounds. They’re used extensively in the automotive industry, where robot cells are sometimes used to load/unload products from the trolleys or to perform work on the parts being conveyed as they’re held stationary. And power and free designs are available in heat- and corrosion-resistant versions, so they can transport loads through processes such as washing and painting or through ovens.

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.

What are some different options for integrated servocontrols?

September 4, 2018 By Miles Budimir Leave a Comment

Integration in motion control is commonplace. Integrated servocontrols can refer to both the way in which the integration of the controller into a servo system (or motor) takes place as well as the control options and functions integrated into the controls themselves.

By integrated here we mean that the controller is integrated into the motor. This is a fairly common practice these days and in fact most manufacturers offer servomotors with some level of built-in controls. Such systems are often dubbed integrated servomotors. As for what exactly is integrated, there can be a range of components so that an integrated servomotor can include the motor itself along with any combination of a drive, controller, or encoder.

The benefits of integration are well known. They include the fact that they make for a more compact drive unit and eliminate the need for cables between the motor and drive, which simplifies installation and also lowers the overall system cost. The tight integration of controls with the motor and other components makes for a compact design, with the ability to fit into smaller spaces but also pack more into that same space than before. Another benefit of these systems is the elimination of control cabinets, with the control itself being distributed and local at the servomotor itself.

These ACSI EtherNet/IP integrated servos from Tolomatic feature a servomotor integrated with a drive and controller.

The kinds of options can mean which functions are included in the controls. For example, this includes basics like the number of inputs and outputs as well as signal types (analog or digital.) Other common options may include the type of control, such as current of speed control.

Other options can include specific control functions such as PID loop control, trajectory generation, program execution, I/O control and communication options such as whether the servocontroller operates as a stand-alone controller or connected into a network with other controllers.

Oriental Motor 60-mm DG II rotary actuator with AlphaStep AZ Series absolute encoder stepper motor

August 29, 2018 By Lisa Eitel Leave a Comment

Oriental Motor is pleased to expand the DGII Series product lineup and AlphaStep hybrid control system with the introduction of the new 60 mm DGII rotary actuator.

The DGII Series is a line of products that combine a high rigidity hollow rotary table with an AlphaStep absolute encoder AZ series closed loop stepper motor and driver. It retains the ease of use of a stepper motor, while also allowing for highly accurate positioning of large inertia loads.

The new DG II retains the ease of use of a stepper motor while allowing for highly accurate positioning of large inertia loads.

The smaller 60 mm frame size is equipped with an AlphaStep AZ Series absolute encoder stepper motor. This new rotary actuator is available to be used with 4 types of AZ Series DC input drivers … stored data type, pulse input driver with built-in RS-485 communications, pulse input type and multi-axis driver with EtherCAT communication.

It is a complete cost-effective accurate rotary actuator system that help eliminate the need for additional sensors. This new combination can work in traditional applications where step and direction are required, or, in applications where PLC or Touch Screens (HMI) are being used, without the need for additional modules connected to the PLC. The end result is a more efficient and energy efficient product that is compatible with a wider range of master controller types and control methods.

For more information about the Oriental Motor 60-mm DGII Series rotary actuator, call (800) 468-3982 or visit this deep link on orientalmotor.com.

Since its founding in Japan in 1885, Oriental Motor has been a world leader in motion control systems. For over a century, we have concentrated on technological advancement and product design improvement — an emphasis evident in the sophisticated devices we market today.

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.

For a given motion profile, when are other options for stopping insufficient — and a clutch or brake necessary?

August 19, 2018 By Lisa Eitel Leave a Comment

At the core of most motion axes are electric motors. Stopping loads on their axes can be done with the electric motor itself — called internal braking in certain contexts — or with an external clutch or brake. For the former, one simple approach is to simply cut motor voltage input and allow the axis to coast to stop. That’s acceptable where stops are infrequent — from a few times a minute (for designs running off small motors) to a few times an hour (for larger motor installations). Another option is to use controls to generate stopping torque in the motor via regenerative braking t convert kinetic into electrical energy; dynamic braking — the injection of dc current into the stator; or electric reversal as plugging.

But where such approaches are too slow — including all modern motion designs for high throughput — external brakes and clutches are required to get sufficiently quick stops or disengagement. This applies to conveyors, airport-baggage handlers, escalators, and elevators … as well as other axes that make frequent stops and starts — even as few as 10 cycles a minute in some cases. Where stops and starts happen at much higher cycle rates, motor inertia may degrade the quickness with which starts and stops are possible. So here, clutch-brakes are often more suitable — as they disengage the driven load from the motor to allow the former to run even while the brake engages and stops the load. Of course, though we focus on responsiveness here, failsafe design features are another main driver of brake and clutch inclusion.

Mechanical, electric, fluidic, and self-actuated clutches and brakes are suitable for different applications. For example, spring-set brakes benefit motion designs that slow loads with the motor before the brake engages … and they’re suitable as holding mechanisms. Control of electric brakes is easy, and they can keep pace to a thousand cycles per minute. Most air-actuated brakes and clutches are cool-running and hold with minimal input. Friction brakes with drum, disc, and cone geometries deliver e-brake functionality with failsafe holding.

Brake or clutch size and type depend on whether the axis at hand will make emergency stops or softer stops that sacrifice the clutch or brake to protect systems and loads from shock. Or sometimes it’s more essential that the brake deliver soft stops to prevent shifting loads and misalignment. After that, other criteria — cycle rates, thermal capacity, machine envelope, and MRO schedules — dictate final selection.

Miki Pulley’s BXR-LE electric spring-applied brakes are suitable for small and precise servomotor designs. A lightweight design optimizes servodrive performance and efficiency. Its voltage controller means the brake’s power consumption is stepped down to 7 Vdc after a split second of 24 Vdc for actuation. When compared to most other electric brakes, the BXR-LE brake consumes just a third the power (and generates just a third the heat) — though it’s half as thick. Speed is to 6,000 rpm; static friction torque is 0.044 ft.lb. to 2.36 ft.lb. and ambient operating temperature is 14° to 104°F. BXR-LE brake applications include those on end effectors, ballscrew actuators, XYZ positioning tables, and 3D printers.

Some tips: Size clutches and brakes to the machine axis’ motor torque. Where a brake must stop vertical loads, account for how motors can briefly draw current to output in excess of their rated torque. Consult performance curves in manufacturer PDFs for dynamic torque ratings at set speeds to match the brake or clutch to peak motor-output torque. Case in point: Consider an inclined conveyor with regularly spaced on-off cycles. Here, a power-off spring-set brake may suffice to prevent load crashes during power failures. But more complex conveyor installations to position discrete product of varied size — without jerking — may need multiple deceleration rates may need more sophisticated spring-set brake plus drive on the motor for stopping … or even a permanent-magnet brake for quick yet soft starts and stops.

Comparing clutch and torque-limiter functions

Torque limiters are not clutches, as they’re not designed to continuously slip. That’s an important distinction when design engineers are specifying slowing and disengaging technologies for mechanical designs. Some tips for optimal design selection: First consider whether a torque limiter will only protect against catastrophic failure or semi-regular overloads. That will indicate whether an economical friction-style torque limiter is sufficient or if the design necessitates a ball-detent design. The latter is usually more costly but capable of slipping and triggering a limit switch for shutoff and resetting to resume operation. Ball-detent torque limiters here can slip multiple times in their application … unlike friction torque limiters requiring reset.

When applying a torque limiter, confirm whether a zero-backlash design is needed. Note that torque limiters can generate heat at their frictional connections, so upon their activation, system shutdown is key. After it’s activated, the end user should inspect the torque limiter for wear and heat damage. He or she should also check its setting torque … as in some designs, that value will be diminished if the torque limiter has been run too long. As long as the torque limiter is still within design range, it’s safe from exhibiting more and more slipping. With some motion designs, a limit or proximity switch paired with controls can detect when the torque limiter slips — and shuts the system down to let the end user address the issue the torque limiter was protect (or protect against). || Insight into torque-limiter functions provided by KTR engineering services manager Chris Scholz.

Brake modules such as the mayr ROBA brake-checker and the ROBA torqcontrol monitor the condition of safety brakes — and can facilitate smooth deceleration of machines and devices. Shown here is a ROBA brake-checker module that without sensors. Electronics track current and voltage and recognize the movement of the armature disc to assess brake condition as well as temperature, wear, and tension path or tensile force reserve — in other words, whether the electromagnetic brake’s magnet has sufficient force to attract the armature disc. On reaching the tensile force reserve, a warning signal sounds to prompt service. There is a design for ac voltage; a coming version of the module will integrate brake input power supply (and replace a separate rectifier) for switching condition monitoring and brake control in one device.

Precision motion control FAQ: What is repeatability? (Part 2)

August 16, 2018 By Miles Budimir Leave a Comment

In this two-part series, we go over the basic definition of types of repeatability and their application in linear motion and actuation applications. Part 2 covers bidirectional repeatability. (Click here for Part 1.)

Information courtesy of IntelLiDrives Corp.

As noted in Part 1, repeatability can be defined as the variation in results for a series of moves or, more analytically, the width of the dispersion about the mean for a significant number of positioning trials. Repeatability, a statistical quality, is commonly defined for a normal distribution by the dispersion width corresponding to a number of standard deviations.

Unidirectional repeatability deals with motion in one direction only. Here, we cover motion in two directions and thus must consider bidirectional repeatability.

Bidirectional repeatability comes into play, for instance, in this RLTX-40 Roto-Linear actuator from IntelLiDrives.

A high level of unidirectional repeatability is relatively easy to achieve since backlash, the motion lost on reversal that contributes to bi-directional repeatability, does not affect unidirectional movement. Of course, approaching targets from a single direction sacrifices throughput times. Bidirectional repeatability is more demanding.

A high degree of bidirectional repeatability presupposes high-level unidirectional repeatability. Tolerances between drive train elements (lead screws/nuts, meshed gears, multi-piece couplings, etc.) must be closely controlled and preloads must be adjusted in order to limit backlash, which may be considered as a mechanical dead band in the motion system.

In programmable motion systems, it’s possible to remove backlash by means of a small incremental move before making normal moves in a given direction. Minimizing either the number of interacting drive train components (gear trains, belts/pulleys, etc.) or the play or looseness between components (which develops as components wear) will also reduce backlash.

In rolled ball screw systems, a backlash is typically less than 0.001 in., while high-precision ground ball screws exhibit backlash of less than 0.0001 in.

When high performance and maximum production throughput are required, bidirectional repeatability will usually be required as well.

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