Danielle Collins - Motion Control Tips

Control Techniques announces new servo drive

October 15, 2018 By Danielle Collins Leave a Comment

Control Techniques, part of the Nidec group of companies, has revealed its new generation of advanced servo drive technologies.

The new Digitax HD servo series delivers ultimate motor control performance and flexibility from a uniquely compact package. Targeting high axis count automation systems, Digitax HD provides all of the benefits of a modular system with a common DC bus, with standalone drive flexibility. The drives’ wide power range provides 6.2 – 451.4 lb-in (0.7 – 51 Nm) with 3x peak torque (1.5 A – 16 A and 48 A peak).

The new series is focused on high overload, pulse duty applications but also provides continuous servo control, plus induction motor control, and is initially available with two functionality levels. The new Digitax HD servo solutions launched in the Americas on October 15 at Pack Expo.

Digitax M753 is designed as an optimized amplifier for high performance centralized control with EtherCAT integrated on-board and simple rotary switches for fast network address assignment. Alternatively, the flexible Digitax M751 base option allows design engineers to add up to two option modules from the existing Unidrive M range, such as PROFINET, Ethernet/IP, or an IEC61131 high performance motion controller for decentralized machine control.

Minimum size

For both single and multi-axis configurations, the Digitax HD series offers industry-leading compactness – the M753 EtherCAT variant is just 1.6 inch (40 mm) wide – that is 5 axes across the width of a piece of paper or 7 axes across if you turn it to landscape. The drive is also designed to fit within 8 inch deep (200 mm) enclosures.

Digitax HD is the most compact 400V servo drive available in the world

Its patented Ultraflow™ system allows machine builders to further reduce cabinet size by up to 50 percent through expelling heat from the drive directly outside the enclosure. This approach offers the further benefit of enabling drives to be stacked without the need for a large air channel between them.

Maximum performance

High dynamic applications will benefit immensely from Digitax HD’s 300 percent peak performance pulse-duty overload capabilities, along with its 62 μs current loop and 16 kHz switching frequency. Its flexible speed and position feedback interface supports a wide range of feedback technologies, from robust resolvers to the latest single cable digital encoder technologies.

Quick and easy installation

Digitax HD boasts a wealth of features and accessories designed to make installation and commissioning as easy as possible. Features include easy-access pluggable connectors and a dedicated multi-axis paralleling kit for rapid installation; integrated braking resistor and electronic motor name plate for faster setup; and quick commissioning using the Unidrive M Connect PC tool or an optional SD card. Machine Control Studio provides a flexible and intuitive IEC61131 environment for programming automation and motion control features.

Flexibility

The Digitax HD servo series flexibly adapts to your architecture of choice, be it centralized motion control, distributed intelligence, or any combination of the two. Support for all main industrial field-buses guarantees easy integration into any production line.

Complementary motor line-up

Working in tandem with the Digitax HD series, delivering class-leading performance with a wide torque range, up to 750 lb-in (85 Nm) with 3x peak, rated speeds from 1,000 to 6,000 rpm, several inertia levels, and a broad selection of feedback options, Unimotor hd offers the perfect fit for high dynamic applications.

Control Techniques

What is meant by the term General Motion Control?

October 11, 2018 By Danielle Collins Leave a Comment

There are numerous interpretations of what the motion control market entails in terms of products included and industries served, but there are some commonalities among the various definitions.

First, a motion control system includes at least three basic components — a motor, a drive, and a controller. Second, motion control systems are primarily used in discrete industries such as packaging and semiconductor manufacturing, as opposed to process industries such as chemical manufacturing and power generation.

No matter how it’s defined, the motion control market is huge, ranging anywhere from $9 billion to $12 billion, depending on which products and industries are included. With very large industries, the number of market players and drivers add significant uncertainty and complexity to everything from product planning to market analysis. One way to address this complexity is to divide the market into smaller segments, based on similar product groups or industry categories. Take, for example, the passenger vehicle market. Most passenger vehicle manufacturers and industry analysts recognize at least three subsegments of the market — cars, trucks, and sport-utility vehicles (and most break the market into even smaller segments).

Similarly, to address the size and complexity of the motion control market, manufacturers and analysts have divided the market into two subsegments: general motion control (GMC) and computer numerical control (CNC).

Most industry statistics show that general motion control (GMC) accounts for just over 50 percent of the overall motion control market, while computer numerical control (CNC) accounts for just under half.

The basic function of a general motion control system is to control the acceleration, speed, and/or position of a single motor or of multiple coordinated motors. From an OEM or end-user perspective, GMC systems provide general-purpose, coordinated motion. They are typically used in the industrial automation market, but exclude military, aerospace, and consumer products applications.

Semiconductor manufacturing is one of the key industries that general motion control systems serve, along with packaging, material handling, and a host of others.

In contrast, computer numerical control (CNC) systems provide multi-axis coordinated motion and are typically used for contoured moves that require interpolation. CNC systems are often designed to meet the specific requirements of the machine tool industry, although they can be used in other industries.

CNC systems are often tailored to the specific requirements of the machine tool industry.
Image credit: Gebr. Heller Maschinenfabrik GmbH

Note that GMC systems can also be used in machine tool applications, especially for processes such as loading/unloading parts and for coarse positioning moves not involved with actual material processing.

One thing that is not widely agreed upon among industry analysts and manufacturers is which components are included in a general motion control system. Case in point, ARC Advisory Group considers the components that make up a GMC system to be the motor, drive, controller, and any integrated feedback devices. Siemens, a leading manufacturer of motion control components and systems, includes two additional components in their definition: software that is developed specifically for the application (which, in some cases, would be included in the controller in ARC’s definition) and the mechanical system — belt, chain, ball screw, rack and pinion, etc. — that transmits work to the load.

Most experts agree that a GMC system consists of a motor, drive, controller, and feedback. Siemens’ definition of a GMC system also includes the application-specific software and the mechanical system.
Image credit: Siemens Industry, Inc.

It is recognized, however, that general motion control systems are most often based on servo motors and drives, although some manufacturers and analysts consider stepper-based systems to be part of the GMC market. (It could be argued that open-loop stepper systems — which are by far more common than closed-loop steppers — do not qualify as GMC systems since they don’t rely on a feedback device.) The most common definitions of general motion control also include both stand-alone drive amplifiers and motor-integrated drives, as well as stand-alone, PLC-based, and PC-based motion controllers.

Don’t confuse the term “general motion control,” as described here, with the term “generic motion control,” which is a software platform from B&R Automation.

What are coreless DC motors?

October 9, 2018 By Danielle Collins Leave a Comment

A typical brushed DC motor consists of an outer stator, typically made of either a permanent magnet or electromagnetic windings, and an inner rotor made of iron laminations with coil windings. A segmented commutator and brushes control the sequence in which the rotor windings are energized, to produce continuous rotation.

A typical DC motor consists of an outer, permanent magnet stator and an inner rotor made of iron laminations with windings.
Image credit: maxon motor ag

Coreless DC motors do away with the laminated iron core in the rotor. Instead, the rotor windings are wound in a skewed, or honeycomb, fashion to form a self-supporting hollow cylinder or “basket.” Because there is no iron core to support the windings, they are often held together with epoxy. The stator is made of a rare earth magnets, such as Neodymium, AlNiCo (aluminum-nickel-cobalt), or SmCo (samarium-cobalt), and sits inside the coreless rotor.

A coreless DC motor does away with the iron core in the rotor. Instead, the rotor windings are wound in a skewed, or honeycomb fashion to form a self-supporting hollow cylinder. The stator magnet sits inside the coreless rotor.
Image credit: maxon motor ag

Other terms for coreless DC motors include “air core,” “slotless,” and “ironless.”

The brushes used in coreless DC motors can be made of precious metal or graphite. Precious metal brushes (silver, gold, platinum, or palladium) are paired with precious metal commutators. This design has low contact resistance and is often used in low-current applications. When sintered metal graphite brushes are used, the commutator is made of copper. The copper-graphite combination is more suitable for applications requiring higher power and higher current.

Coreless DC motors have a rotor that is hollow and self-supporting, which reduces mass and inertia.
Image credit: Portescap

The construction of coreless DC motors provides several advantages over traditional, iron core DC motors. First, the elimination of iron significantly reduces the mass and inertia of the rotor, so very rapid acceleration and deceleration rates are possible. And no iron also means no iron losses, giving coreless designs significantly higher efficiencies (up to 90 percent) than traditional DC motors. The coreless design also reduces winding inductance, so sparking between the brushes and commutator is reduced, increasing motor life and reducing electromagnetic interference (EMI).

Motor cogging, which is an issue in traditional DC motors due to the magnetic interaction of the permanent magnets and the iron laminations, is also eliminated, since there are no laminations in the ironless design. And in turn, torque ripple is extremely low, which provides very smooth motor rotation with minimal vibration and noise.

Because these motors are often used for highly dynamic movements (high acceleration and deceleration), the coils in the rotor must be able to withstand high torque and dissipate significant heat generated by peak currents. Because there’s no iron core to act as a heat sink, the motor housing often contains ports to facilitate forced air cooling.

Coreless DC motors are available in both cylindrical and flat (disc) configurations.
Image credit: MICROMO

The compact design of coreless DC motors lends itself to applications that require a high power-to-size ratio, with motor sizes typically in the range of 6 mm to 75 mm (although sizes down to 1 mm are available) and power ratings of generally 250 W or less. Coreless designs are an especially good solution for battery-powered devices because they draw extremely low current at no-load conditions.

Coreless DC motors are used extensively in medical applications, including prosthetics, small pumps (such as insulin pumps), laboratory equipment, and X-ray machines. Their ability to handle fast, dynamic moves also makes them ideal for use in robotic applications.

What are leading methods for VFD control of AC motors?

October 3, 2018 By Danielle Collins Leave a Comment

AC motors are often used in equipment that runs at constant speed regardless of load, such as fans, pumps, and conveyors. But when speed control is required, an AC motor is paired with a variable frequency drive (VFD), which regulates the motor’s speed by using one of two control methods — scalar control or vector control — to vary the frequency of the supplied voltage.

A scalar is a quantity that has only magnitude, such as mass or temperature. A vector is a quantity that has both magnitude and direction, such as acceleration or force.

In a VFD, AC power is converted to DC through the rectifier. The rectified power is then filtered and stored in the DC bus (link). The inverter converts it back to AC power with the proper frequency and voltage via pulse-width modulation.
Image credit: what-when-how.com

Scalar control methods

Scalar methods for VFD control work by optimizing the motor flux and keeping the strength of the magnetic field constant, which ensures constant torque production. Often referred to as V/Hz or V/f control, scalar methods vary both the voltage (V) and frequency (f) of power to the motor in order to maintain a fixed, constant ratio between the two, so the strength of the magnetic field is constant, regardless of motor speed.

The appropriate V/Hz ratio is equal to the motor’s rated voltage divided by its rated frequency. V/Hz control is typically implemented without feedback (i.e. open-loop), although closed-loop V/Hz control — incorporating motor feedback — is possible.

V/Hz control maintains a constant ratio between voltage (V) and frequency (Hz).
Image credit: Square D

V/Hz control is simple and low-cost, although it should be noted that the closed-loop implementation increases cost and complexity. Control tuning is not required but can improve system performance.

Speed regulation with scalar control is only in the range of 2 to 3 percent of rated motor frequency, so these methods aren’t suitable for applications where precise speed control is required. Open-loop V/Hz control is unique in its ability to allow one VFD to control multiple motors and is arguably the most-commonly implemented VFD control method.

Vector control methods

Vector control — also referred to as field oriented control (FOC) — controls the speed or torque of an AC motor by controlling the stator current space vectors, in manner similar to (but more complicated than) DC control methods. Field oriented control uses complex mathematics to transform a three-phase system that depends on time and speed to a two-coordinate (d and q) time-invariant system.

The stator current in an AC motor is made up of two components: the magnetizing component (d) of the current and the torque-producing component (q). With FOC, these two current components are controlled independently (each with its own PI controller). This allows the torque-producing component, q, to be kept orthogonal to the rotor flux for maximum torque production, and therefore, optimum speed control.

Vector control, or field-oriented control, converts 3-phase currents in a stationary reference frame to a 2-phase system (consisting of a flux component, d, and torque-producing component, q) with a rotating reference frame. Here, the torque-producing current (q) can be independently controlled to ensure maximum torque production. The system is then transformed back into a 3-phase system in a stationary reference frame for output to the motor.
Image credit: Freescale Semiconductor

Like scalar methods, vector VFD control methods can be open-loop or closed-loop. Open-loop vector control (also referred to as sensorless vector control) uses a mathematic model of the motor operating parameters, rather than using a physical feedback device. The controller monitors voltage and current from the motor and compares them to the mathematical model. It then corrects any errors by adjusting the current supplied to the motor, which adjusts the motor’s torque production accordingly. With senseless vector control, it’s important to have a very accurate mathematical model of the motor, and the controller must be tuned for proper operation.

Closed-loop vector control uses an encoder to provide shaft position feedback, and this information is sent to the controller, which adjusts the supplied voltage to increase or decrease torque. This is the only method that allows direct torque control in all four quadrants of motor operation for dynamic braking or regeneration.

Vector control methods are more complex than scalar VFD control methods, but they offer significant benefits over scalar methods in some applications. For example, open-loop vector control enables the motor to produce high torque at low speeds, and closed-loop vector control allows a motor to produce up to 200 percent of its rated torque at zero speed, useful for holding loads at standstill. Closed-loop vector control also provides very accurate torque and speed control for industrial applications.

What are rotary unions and how are they used in motion applications?

September 25, 2018 By Danielle Collins Leave a Comment

Rotating components are common in motion applications — robot arms, winders, spools, spindles, and rotary tables are just a few examples. And while creating rotary motion is easy, when air or liquids need to be transferred from a stationary supply to a rotating component for cooling, heating, lubricating, or transmitting fluid power, the task becomes more complicated. This is where rotary unions come in.

Rotary unions provide an interface that allows fluid media — air, steam, or liquid — to be transferred between a rotating component and a static component. They’re designed to accommodate a wide variety of media conditions, including very high and low temperatures, flows, and pressures, including vacuum applications. And unlike other fluid transfer mechanisms, rotary unions work for applications involving any combination of rotation angles, including continuous rotation in one direction.

This rotary union can be used for supplying cooling from a stationary source directly to a machine tool spindle.
Image credit: Dublin Company

Rotary unions are referred to by a variety of names, including rotating unions, rotary joints, rotary swivels, and rotary couplings.

There are numerous designs and configurations for rotary unions — in fact, they’re often custom-designed for the application — but all rotary unions are made up of four basic parts: the housing, one or more bearings, the shaft, and one or more seals.

The housing is typically the stationary part and is connected to the media supply. The shaft is independent of the housing and rotates with the connected rotary equipment. Media can flow through the union radially or axially via ports in the housing. The rotating part is supported by radial bearings — typically one or more fully sealed deep groove ball bearings.

The most important component of the rotary union is arguably the sealing mechanism, which prevents or reduces leakage between the rotating and stationary components, and must do so with minimal friction and wear. Seal types range from simple lip seals to more complex spring-loaded mechanical seals that automatically adjust to minimize the pressure on the seal faces, reducing friction and wear.

Of course, the fluid transfer should ideally be leak-free, but in some applications, completely leak-free transfer is not possible. For these cases, the media is prevented from being released into the environment by a collection system that recovers the leaked media, protecting surrounding equipment and personnel.

This video from Dynamic Sealing Technologies Inc. demonstrates how rotary unions transfer fluid between rotating and stationary component, with examples of their use in various industries.

Whether choosing a standard, off-the-shelf rotary union or designing a custom product, the following factors should be considered:

type of media being transferred
speed of rotation
flow rate of media
pressure of media
temperature of media
allowable leakage

The type of media is especially important, since the housing, shaft, and seal materials must be compatible with the fluid flowing through the union. Fluid media can range from water to highly corrosive liquids and steam, and manufacturers address the challenges of each by offering a wide range of materials, not only for the housing and shaft, but also for the seals.

Although single- and dual-flow designs are most common, rotary unions can include multiple independent flow paths that allow different media to be transferred concurrently without mixing. It’s also common for slip rings or fiber optic rotary joints to be integrated with rotary unions, allowing the passage of both fluids and electrical or fiber optic cables.

This cross-section of a two-passage rotary union shows the two separate channels through which different media can pass without mixing.
Image credit: Kadant Inc.

In motion control systems, rotary unions can be found in robot joints, especially for transferring air or fluid to end-of-arm tooling. They’re also used for passing vacuum or air pressure through rotary air bearing stages, and for supplying coolant and lubrication to machine tool spindles.

Feature image credit: Moog

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.

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.

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