Danielle Collins - Motion Control Tips

Kevin Barker selected as new president of Beckhoff Automation LLC

February 14, 2019 By Danielle Collins Leave a Comment

Barker to manage operations of US Beckhoff subsidiary

Kevin Barker (pictured) becomes the new president of Beckhoff USA, succeeding Aurelio Banda.

Beckhoff Automation is pleased to announce that Kevin Barker has been appointed the new president of Beckhoff Automation LLC to manage Beckhoff business operations across the US. Barker takes over the leadership role that Aurelio Banda has held for four years. Banda led Beckhoff Automation LLC revenue growth and business developments to new heights, finishing fiscal year 2018 with nearly $80 million in US sales, new field offices and more employees than ever.

As the new president, Barker will oversee all sales, engineering, marketing and administrative operations from the headquarters of Beckhoff Automation LLC in Savage, Minn. (Minneapolis area). This location also houses the main technical support hub and warehouse for Beckhoff customers in the US. Prior to joining Beckhoff, Barker worked at Yaskawa America, Inc. as the director of sales in the company’s motion division.

“As a proven manager of large, cross-functional teams, and an esteemed expert on motion control, EtherCAT and general automation technologies, Kevin Barker is well prepared to guide Beckhoff Automation LLC to the next phase of business development,” said Hans Beckhoff, managing director and founder of Beckhoff Automation. “Kevin’s expertise and abilities align very well with our philosophy to implement leading automation technology based on Beckhoff’s PC Control principles, in close collaboration with customers. As the leadership hand-off from Aurelio Banda is completed, Beckhoff Automation LLC operates on its strongest foundation ever with significant revenue growth, major investments in new regional offices and the hiring of numerous skilled employees across the US.”

Over an eight-year tenure at Yaskawa, Barker led multi-national sales teams with the responsibility to develop complex industrial markets, large sales channels and advanced motion control offerings, including the company’s lineup of EtherCAT servo drives. Barker, who earned a Bachelor of Science in Business Administration from Illinois Wesleyan University, also held director-level positions at Mitsubishi Electric Automation and Omron Electronics earlier in his career.

According to Barker, the prospects for developing the market in general automation technology and a rapidly evolving portfolio of motion control solutions were major factors in his decision to join Beckhoff Automation and lead the subsidiary into a new era.

“I have dedicated my entire career to helping machine builders, manufacturers and other high-tech companies succeed on a global scale with leading-edge automation technologies. Beckhoff Automation is the logical progression of my journey and an exciting new chapter,” Barker said. “I am honored to lead the US subsidiary of such a profoundly innovative automation company. Further, I am ready to take charge with a strategic vision that empowers all US-based employees, lifting the high bar set at Beckhoff Automation, the inventor of EtherCAT and pioneer in PC-based control technology.”

Barker succeeds Aurelio Banda who has served as CEO and president of Beckhoff in North America for four years and guided the company to consecutive double-digit profit growth. Banda was a driving force behind the Beckhoff growth across the US, Canada and Mexico. During his tenure, Beckhoff became the preferred automation partner for many highly recognizable Fortune 500 companies across numerous industries, such as packaging, entertainment, semiconductors, metal cutting and material handling/logistics. This has proven to be the catalyst for business success and a key point in Banda’s legacy at Beckhoff as he leaves on the best of terms.

“I have had the privilege to lead Beckhoff in North America, starting with a vision to expand our market, which has been shaped into the reality today of deeply entrenched market penetration with even more well-established US manufacturers and machine builders. My time with Beckhoff Automation has been an immensely rewarding experience that I will never forget,” Banda said. “I am excited now to pursue new endeavors, and envision Kevin Barker continuing to drive the momentum Beckhoff has built to date in numerous high-tech industries throughout the United States.”

Beckhoff Automation

What is an IK rating, and how does it apply to motion control products?

February 13, 2019 By Danielle Collins Leave a Comment

If you deal with electrical components, including electrical enclosures such as control cabinets, you’re probably familiar with IP (International Protection) ratings, which specify the enclosure’s ability to prevent the ingress of solid objects and liquids. But you may be less familiar with the IK rating system, which defines the enclosure’s ability to protect its components from external mechanical impacts, as measured by the impact energy, in joules.

Like IP ratings, IK ratings specify how much protection an enclosure provides for its contents. But rather than protection from ingress of solid objects and liquids, an IK rating indicates the amount of protection the enclosure provides against external mechanical impacts.
Image credit: Hoffmann

A joule is a very small amount of energy. A common example of one joule is the energy required to lift a small apple (100 g, or 0.1 kg) against gravity (9.81 m/s², or approximately 10 m/s²), to a height of 1 meter.

0.1 kg * 10 m/s² * 1 m ≅ 1 kgm/s², or 1 J

Although the units for the joule (kgm/s²) are the same as the units for torque (1 kgm/s² = 1 Nm), a joule is a scalar quantity (it has magnitude only) of force applied through a distance, while torque is a vector quantity (it has both magnitude and direction) of force applied at a distance.

The IK rating was originally introduced as the European standard BS EN 50102 in 1995, but was replaced by the international standard IEC 62262 (and the renumbered European standard EN 62262) in 2002. Prior to these standards, an impact protection rating was often added to the end of an enclosure’s IP rating. For example, IP66(9) would mean the enclosure had an IP rating of 66 and an impact rating of 9. The IEC 62262 standard provides ten levels of protection, designated by two-digit codes, from 00 (no protection) to 10 (highest level of protection).

Image credit: APEM

An enclosure’s IK rating is determined by a testing procedure set out in IEC 62262. The testing procedure involves impacts administered by a pendulum-type device, which uses hammers of various weights to simulate different masses being dropped from increasing distances from the impacted surface. The impact tests, known as “Charpy tests,” are carried out for every surface that is exposed during normal usage of the enclosure. For an enclosure with an area to be tested that is less than 1 meter in length, three impacts are administered. For a tested area over 1 meter in length, five impacts are administered.

The Charpy test simulates the impact of dropping specified weights from various distances.
Image credit: TWI Ltd.

The IEC 62262 standard specifies that the locations of the impacts should follow a certain symmetry and should be distributed evenly over the surface, but the manufacturer has significant discretion in choosing where the impacts are located. And parts such as hinges and locks are excluded from testing. Therefore, it’s important to understand the manufacturer’s testing procedure, as the enclosure’s IK rating can be affected by where the impacts were applied.

In addition to how and where the impacts are administered, IEC 62262 also specifies how the enclosure should be mounted and the atmospheric conditions that should be present during testing. It also provides instructions for the size, dimensions, style, and materials of the hammers to be used for testing.

Note that an enclosure must meet both the stated ratings, IP and IK, together. For example, if an enclosure maintains its IP66 rating after passing the test for IK06 protection, it can be labeled as both IP66 and IK06. However, if, after the enclosure passes testing for IK08 protection, it only maintains IP54 protection, then it must be labeled as IK08 and IP54, or IK06 and IP66, but it cannot be labeled as IK08 and IP66.

3 things to consider when choosing a linear servo motor

February 8, 2019 By Danielle Collins Leave a Comment

Direct drive linear servo motors have seen a measurable increase in adoption over the past several years, thanks in part to end users’ demands for higher throughput and better precision. And although linear motors are most often recognized for their ability to provide a combination of high speeds, long strokes, and excellent positioning accuracy that isn’t possible with other drive mechanisms, they can also achieve extremely slow, smooth, and precise motion. In fact, linear motor technology provides such a wide range of capabilities — thrust force, velocity, acceleration, positioning accuracy, and repeatability — that there are few applications for which linear motors are not a suitable solution.

Linear motor variations include linear servo motors, linear stepper motors, linear induction motors, and thrust tube linear motors. When a linear servo motor is the best option for an application, here are three things to consider during the initial motor selection.

The “primary” consideration: Iron core or ironless?

Linear direct drive servo motors come in two main types, iron core or ironless, referring to whether the windings in the primary part (analogous to the stator in a rotary motor) are mounted in an iron lamination stack or in epoxy. Deciding whether the application requires an iron core or an ironless linear motor is typically the first step in design and selection.

Iron core linear motors are best suited for applications that require extremely high thrust forces. This is because the lamination of the primary part contains teeth (protrusions) that focus the electromagnetic flux toward the magnets of the secondary part (analogous to the rotor in a rotary motor). This magnetic attraction between the iron in the primary part and the permanent magnets in the secondary part allows the motor to deliver high forces.

In an iron core linear motor, the coils (windings) are wrapped around iron protrusions, referred to as “teeth.”
Image credit: Nippon Pulse America, Inc.

Ironless linear motors house the windings of the primary part (forcer) in epoxy.
Image credit: Aerotech Inc.

Ironless linear motors generally have lower thrust force capabilities, so they’re not suitable for the extremely high thrust requirements found in applications such as pressing, machining, or molding. But they excel at high-speed assembly and transport.

Ironless linear motors are sometimes referred to as “U-channel” or “air core” linear motors because their secondary part is often shaped like a “U,” with two magnet plates facing each other, separated by a spacer. The primary part (also referred to as a “forcer”) rides in the slot, or air gap, between the two magnet plates.

The downside of the iron core design is cogging, which degrades the smoothness of motion. Cogging occurs because the slotted design of the primary part causes it to have “preferred” positions as it travels along the magnets of the secondary part. To overcome the primary’s tendency to align with the magnets of the secondary, the motor has to produce more force, which causes a velocity ripple — referred to as cogging. This variation of force and velocity ripple degrades the smoothness of motion, which can be a significant issue in applications where the quality of motion during travel (not just final positioning accuracy) is important.

There are numerous methods that manufacturers use to reduce cogging. One common approach is to skew the position of the magnets (or the teeth), creating smoother transitions as the primary teeth travel across the secondary magnets. A similar effect can be achieved by changing the shape of the magnets to an elongated octagon.

To reduce cogging in iron core linear motors, some manufacturers skew the magnets (left) or shape them (right) to reduce the sharp change in attractive forces as the primary moves across the magnets.
Image credit: Parker Hannifin

Another method to reduce cogging is referred to as fractional winding. In this design, the primary contains more lamination teeth than there are magnets in the secondary, and the lamination stack has a special shape. Together, these two modifications work to cancel cogging forces. And of course, software always offers a solution. Anti-cogging algorithms allow servo drives and controllers to adjust the current supplied to the primary so that variations in force and velocity are minimized.

Ironless linear motors don’t experience cogging, since their primary coils are encapsulated in epoxy, rather than being wound around a steel lamination. And ironless linear servo motors have a lower mass (epoxy is lighter, albeit less stiff, than steel), allowing them to achieve some of the highest acceleration, deceleration, and maximum velocity values found in electromechanical systems. Settling times are typically better (lower) for ironless motors than for iron core versions as well. The lack of steel in the primary, and associated lack of cogging or velocity ripple, also means ironless linear motors can provide very slow, steady motion, typically with less than 0.01 percent speed variation.

What level of integration?

Like rotary motors, linear servo motors are just one component in a motion system. A complete linear motor system also requires bearings to support and guide the load, cable management, feedback (typically a linear encoder), and a servo drive and controller. Highly experienced OEMs and machine builders, or those who have very unique design or performance requirements, can build a complete system with in-house capabilities and off-the-shelf components from various manufacturers.

Linear motor system design is arguably simpler than the design of systems based on belts, rack and pinions, or screws. There are fewer components and fewer labor-intensive assembly steps (no aligning of ball screw supports or tensioning of belts). And linear motors are non-contact, so designers don’t have to worry about making provisions for lubrication, adjustments, or other maintenance of the drive unit. But for those OEMs and machine builders who are looking for a turnkey solution, there are myriad options for complete linear motor driven actuators, high-precision stages, and even Cartesian and gantry systems.

Complete linear motor stages – including base plate, linear motor, linear guides, encoder, and controls – are offered by numerous manufacturers.
Image credit: Primatics Inc.

For the DIY-ers, choosing the appropriate linear guides for an iron core linear motor needs to be done with consideration for the attractive forces between the primary and secondary parts, which can present a significant load on the linear guides. But this isn’t a concern for ironless linear motors, since the lack of iron in the primary part means there are no attractive forces between the primary and secondary.

Is the environment suitable for a linear motor?

Linear motors are often the preferred solution in difficult environments, such as cleanrooms and vacuum environments, since they have fewer moving parts and can be paired with almost any type of linear guide or cable management to meet the particle generation, outgassing, and temperature requirements of the application. And in extreme cases, the secondary (magnet track) can be used as the moving part, with the primary part (windings, including cables and cable management) remaining stationary.

Iron core linear motors leave the magnet track exposed, so metallic chips and other contamination can be a concern.
Image credit: Chieftek Precision USA

But if the environment will consist of metal chips, metallic dust, or metal particles, a linear servo motor may not be the best option. This is especially true for iron core linear motors because their design is inherently open, leaving the magnet track exposed to contamination. The semi-enclosed design of ironless linear motors provides better protection, but care should be taken to ensure the slot in the secondary part is not directly exposed to sources of contamination. There are design options for enclosing both iron core and ironless linear motors, but these can reduce a motor’s ability to dissipate heat, potentially trading one problem for another.

Feature image credit: ETEL S.A.

What are magnetoelastic sensors for torque measurement?

January 31, 2019 By Danielle Collins Leave a Comment

When a ferromagnetic material is subjected to a mechanical stress — caused by an applied force or torque — it experiences a change in magnetic flux. This phenomenon is known as the magnetoelastic effect, also referred to as the Villari effect, for the Italian physicist who discovered it in the late 1800’s.

In motion control applications, the torque produced by rotating equipment is determined either mathematically, based on the amount of current being drawn (a common method for servo-driven systems) or with the use of strain gauges. But calculated values are not exact representations of real-world results, and strain gauges require precise mounting and calibration to ensure accuracy. A relatively new type of sensor, based on the magnetoelastic effect, is a simple, low-cost technology that uses non-contact sensing to provide accurate torque measurements for rotating or stationary shafts.

The key component in a magnetoelastic sensor is a ferromagnetic ring attached to the shaft being measured. Alternatively, if the shaft is ferromagnetic, a section of the shaft can be permanently magnetized to produce a circumferential magnetic field, eliminating the need for an external ring.

Magnetoelasticity is a property of ferromagnetic materials. In the case of non-ferromagnetic shafts, rings made of ferromagnetic material can be installed.
Image credit: MagCanica, Inc.

When torque (torsional stress) is applied, the magnetic moments inside the shaft (or ring) are reoriented, causing a magnetic flux to develop around outside circumference of the shaft. The strength of the magnetic field flux is linearly proportional to the stress — and therefore, the torque — on the shaft, and the polarity of the magnetic field indicates the direction of torque. Magnetic field sensors positioned around the shaft determine the amount and direction of torque based on this flux.

When torque is applied to a ferromagnetic shaft, a magnetic flux develops around the shaft.
Image credit: Methode Electronics

Magnetoelastic sensors are non-contact, so they don’t suffer from mechanical wear, giving them an extremely long operating life. And because the magnetoelastic properties of a material are highly reliable and don’t degrade, calibration isn’t necessary for sensors that use magnetoelastic technology.

If magnetoelastic sensors seem familiar, it may be because they are similar in function to magnetostrictive sensors. In fact, magnetoelasticity and magnetostriction are essentially inverse effects. Where magnetoelasticity deals with the interaction of magnetic fields and elastic properties of a material, magnetostriction deals with the change in dimensions of a material when a magnetic field is applied.

The most common uses for magnetoelastic torque sensors are in automotive and e-mobility applications, for sensing torque in transmissions and drivetrains. The next areas of growth for this technology are expected to be in the medical robotics and diagnostic equipment markets.

In this application, a magnetoelastic sensor is placed inside an all-wheel-drive coupling to measure torque directly at the shaft.
Image credit: BorgWarner

Magnetoelastic sensor technology can also be used to measure linear position, force, speed, or angle.

Despite their widespread use in automotive applications, magnetoelastic sensors haven’t achieved significant adoption in typical motion control applications. This is likely because servo systems, which require precise torque control, are able to quantify torque with sufficient range and accuracy by monitoring the motor’s current draw.

However, large, non-servo controlled motors, such as those used in processing applications and heavy machinery, can benefit from the use of magnetoelastic sensors to monitor torque for predictive and preventive maintenance purposes. Magnetoelastic technology can also be applied to tightening equipment used in assembly and food and beverage applications, where accurate, consistent, torque measurements are important.

Why is viscosity important for bearing lubrication?

January 10, 2019 By Danielle Collins Leave a Comment

Lubrication — or, more correctly, improper or insufficient lubrication — is one of the leading causes of equipment failures in industrial applications. Without it, sliding, rolling, or meshing surfaces experience significant friction, heat, and wear, leading to increased noise, loss of accuracy, and reduced equipment life.

One of the most important characteristics of a lubricant is its viscosity (or, in the case of grease lubricant, the viscosity of the base oil). But viscosity is analogous to friction in fluids. So why do you need friction (in lubrication) in order to reduce friction (in bearings)?

Tribology is the study and application of the principles of friction, lubrication, and wear between two surfaces in relative motion.

Viscosity = friction in fluids

Viscosity is a property of fluids and is caused by their internal resistance to shear. When a fluid is moving under laminar flow conditions, with no turbulence — as is the case with most bearing lubrication scenarios — microscopic layers of the fluid flow over one another, much like a stack of paper, with each sheet moving slightly faster than the one below it. Cohesive forces between these microscopically thin fluid layers must be overcome as the layers move past one another. The resistance caused by these cohesive forces is the primary determinant of the fluid’s viscosity.

In laminar flow, microscopic layers of fluid flow over one another. The layers’ internal resistance to this shear force gives rise to viscosity.
Image credit: Michael Fowler, virginia.edu

The role of viscosity in reducing bearing friction

Bearing surfaces, no matter how well they’re machined, finished, and cleaned, will always have asperities (peaks) and valleys. When two bearing surfaces are in contact (for example, ball and raceway or roller and raceway), the asperities of the surfaces interfere with each other. One of the primary roles of lubrication is to separate the surfaces and reduce or eliminate the interference of their asperities, which significantly decreases friction and wear.

But when the surfaces are stationary (or moving at very low speeds), the pressure between them essentially “squeezes” the lubrication from between the surfaces. There is a very thin lubricating film, but it is not sufficient to separate the asperities of the two surfaces, and significant contact still exists. This is referred to as boundary lubrication.

In boundary lubrication, friction, heat, and wear depend primarily on the interactions between the surfaces, although chemical reactions between the lubricant and the surfaces can also contribute to wear. The time spent in boundary lubrication conditions significantly affects bearing performance and life.

As bearing speed increases, lubrication is “pulled into” the contact zone.
Image credit: Wessex Petroleum Limited

As velocity increases, more lubrication is “pulled” into the space between the bearing surfaces, allowing a thicker lubrication film to develop. This causes the pressure on the lubrication film to increase, which, in turn, increases the lubricant’s viscosity (according to the lubricant’s pressure-viscosity coefficient). But there is a transition period where, as velocity increases, the surfaces are separated in some places, but in other places, interference between asperities still occurs. This is referred to as mixed lubrication.

Finally, when a sufficient velocity is reached, the lubricating layer becomes large enough to separate the asperities of the two surfaces, and the increased viscosity gives the lubricant sufficient film strength to support the load, by elastically deforming the bearing surface. This lubrication regime is referred to as elastohydrodynamic lubrication.

A Stribeck curve shows the development of the lubrication film and the resulting change in friction. As velocity increases, the bearing surfaces “pull in” more lubricant, creating a thicker lubricating layer under higher pressure, which results in higher viscosity. Finally, the layer is thick enough to separate the asperities of the surfaces, and the viscosity provides sufficient film strength for the lubricant to support the load.
Image credit: Anton Paar GmbH

Feature image credit: Exxon Mobil Corporation

Why is static friction greater than kinetic friction?

January 4, 2019 By Danielle Collins Leave a Comment

Solid surfaces are subjected to two types of friction: static friction and kinetic friction. Static friction acts when the surfaces are stationary — think of a box on the floor. Static friction is what keeps the box from moving without being pushed, and it must be overcome with a sufficient opposing force before the box will move. Kinetic friction (also referred to as dynamic friction) is the force that resists the relative movement of the surfaces once they’re in motion.

The static friction between two surfaces is always higher than the kinetic friction (at least, in practical, real-world applications). But why is this? To find out, let’s look at the causes behind each type of friction.

Static friction

There are several theories regarding the causes of static friction, and like most friction-related concepts, each one proves valid under some conditions, but fails under other circumstances. For real-world applications (especially those related to industrial machinery and motion control) the two most widely-accepted theories behind static friction have to do with the microscopic roughness of surfaces.

Static friction keeps the box from moving until the applied force (F) exceeds the static friction force (f_s).

Regardless of how “perfectly” a surface is machined, finished, and cleaned, it will inevitably have asperities — essentially “roughness,” consisting of peaks and valleys, much like a mountain range. (Technically the “peaks” are the asperities.) When two surfaces are in contact, it may appear that they have a large, well-defined area of contact, but in reality, contact occurs only at certain places — that is, where the asperities of both surfaces interfere. The sum of these small areas of contact between the asperities is referred to as the “real” or “effective” area of contact.

Because these individual areas of contact are very small, the pressure (pressure = force/area) between the surfaces at these points is very high. This extreme pressure allows adhesion to occur between the surfaces, via a process known as cold welding, which occurs at the molecular level. Before the surfaces can move relative to each other, the bonds that cause this adhesion must be broken.

Because the surfaces are microscopically rough, with asperities and valleys, the real contact area between them is much smaller than the apparent contact area.

In addition, the roughness of the surfaces means that at some locations, the asperities of one surface will settle into the valleys of the other surface — in other words, the surfaces will interlock. These interlocked areas must be broken or plastically deformed before the surfaces can move. In other words, abrasion must occur.

So, in most applications, static friction is caused by both adhesion and abrasion of the contacting surfaces.

Kinetic friction

Overcoming the static friction between two surfaces essentially removes both the molecular obstacles (cold welding between asperities) and, to some degree, the mechanical obstacles (interference between the asperities and valleys of the surfaces) to movement. Once movement is initiated, some abrasion continues to occur, but at a much reduced level than during static friction. And the relative velocity between the surfaces provides insufficient time for additional cold welding to occur (except in the case of extremely low velocity).

Once the static friction is overcome, the box will move. Now, the friction opposing movement is much lower and is referred to as kinetic friction (F_k).

With most of the adhesion and abrasion being overcome to induce movement, the resistance to motion between the surfaces is reduced, and the surfaces are now moving under the influence of kinetic friction, which is much lower than static friction.

Assumptions regarding friction

Friction is an incredibly complex force that manifests differently under various conditions, making it difficult to express in terms of physical laws and mathematic equations. However, there are three assumptions regarding friction that apply in most real-world situations:

The frictional force is proportional to the normal force.
The frictional force is independent of the apparent area of contact.
The frictional force is independent of the relative velocity.

You might notice that assumption #2 (friction is independent of area of contact) seems to contradict the idea presented earlier that because the points of contact between the asperities are very small, pressure between the surfaces is high and adhesion (via cold welding) occurs, which increases friction. But notice that assumption #2 specifies the “apparent” area of contact.

Using the box example from above: Assume you move the contents of the box to a different box with a larger footprint. The mass (and hence, the normal force) hasn’t changed, but the apparent area of contact is larger. Despite the larger apparent area of contact, the friction force remains the same, as predicted by assumption #2.

Why does rolling motion experience friction?

In theory, a ball makes point contact with the surface. But in reality, the ball (and/or the surface) deforms due to the load, and the contact area becomes elliptical.
Image credit: SKF

Note that the discussion above relates to non-lubricated, sliding surfaces. In theory, rolling surfaces, such as those found in most rotary and linear bearings (except plain bearings), should not encounter friction forces. But in real-world applications, three factors cause friction in rolling surfaces:

Micro-slip between the surfaces (the surfaces slide relative to one another)
Inelastic properties (i.e. deformation) of the materials
Roughness of the surfaces

What is current decay (aka recirculating current) in a stepper drive?

December 27, 2018 By Danielle Collins Leave a Comment

A common type of stepper drive is the chopper drive, so named because it rapidly switches the output voltage from the drive on and off — “chopping” it — to provide constant current to the motor. Chopper drives use one H-bridge per motor phase to drive voltage to the motor windings. A sense resistor at the bottom of the H-bridge monitors the current. When a set current level is reached, the H-bridge changes state to stop the current supply to the motor. After a set period of time (referred to as the “off time”), a new cycle begins, and the H-bridge once again drives current to the winding.

Although the H-bridge doesn’t supply current during the off time, current is maintained in the motor coils. While this stored current dissipates, or decays, it must have a path to flow through, or the field-effect transistors (FETs) in the H-bridge will be damaged. One option for a safe current path is to use freewheeling diodes in parallel with the FETs, but these add bulk and cost. Instead, the FETs themselves can be manipulated in one of two ways to provide a safe path for the decaying current.

An H-bridge is a circuit that contains four independently controlled switching elements (FETs) that direct the current flow to the load (in this case, the stepper motor).

In an H-bridge, diagonal switches are turned on to allow current to flow through. Alternating the sides of the “on” switches changes the motor’s direction of rotation.
Image credit: Texas Instruments

Turning on two FETs on the same side of the H-bridge causes very high current (referred to as “shoot through”) that could damage the FETs. Therefore, when the FETs on one side of the H-bridge need to be turned on sequentially, a method referred to as “break before make” is used.

This method disables the first FET for a set amount of time (typically a few hundred nanoseconds) before the second one is enabled, preventing shoot-through. During this brief “dead time,” when neither FET is enabled, internal body diodes on the FETs carry the current.

Current decay: Fast or slow

The two methods of current decay using the FETs are termed “fast decay” and “slow decay.” It’s important to note that, in this context, the terms “fast” and “slow” refer to the speed at which current decays, and are inversely related to how quickly the motor stops.

With fast decay, negative voltage is applied to the winding by switching on the opposite FETs and reversing the direction of current flow through the H-bridge. Fast decay enables the motor to respond quickly to changing step inputs, but it results in a slow stop of the motor. The downside of fast decay is that it causes high current ripple.

Although fast decay doesn’t provide truly recirculating current (current flows back to the power supply), it is sometimes referred to as a type of recirculating current.

With slow decay, two of the FETs (either both high-side or both low-side) in the H-bridge are turned on. This shorts the motor winding and allows the current to recirculate and slowly decay according to the motor’s L/R time constant. In a DC motor, the slow decay method causes the motor to stop rapidly because it shorts the motor’s back EMF.

Recirculating the current through the H-bridge provides slow decay (middle), while providing reverse current through the H-bridge to the power supply allows fast decay (right).
Image credit: Monolithic Power Systems

Microstepping drives find a compromise

Microstepping drives, which drive the motor windings with sinusoidal currents, typically use a method referred to as “mixed decay.” During the rising part of the current waveform, slow decay is sufficient to simulate the current sine wave as microsteps are issued, while providing high efficiency and low current ripple. But when the waveform is decreasing, slow decay doesn’t allow current to decrease fast enough to maintain the sinusoidal waveform, and fast decay would introduce unacceptable current ripple.

Fast decay produces significant current ripple, so microstepping applications use mixed decay.
Image credit: STMicroelectronics

Mixed decay solves this problem by beginning the downward part of the current waveform with fast decay enabled, and then switching to slow decay. This maintains the sinusoidal shape of the current while minimizing current ripple.

Feature image credit: STMicroelectronics

Can mechatronics engineering help alleviate the shortage of skilled manufacturing workers?

December 18, 2018 By Danielle Collins Leave a Comment

If you went to college in the 1990’s or earlier and were interested in engineering, you generally had eight or ten fields of study to choose from:

— Mechanical Engineering

— Aerospace Engineering

— Electrical Engineering

— Computer Science (often included in the Engineering department)

— Civil Engineering

— Industrial Engineering

— Chemical Engineering

— Materials Science Engineering

— Nuclear Engineering

Those of us who landed in the automation industry in the past two decades typically came from either a mechanical engineering or an electrical engineering background. And while both mechanical and electrical engineering programs cover a broad range of topics within their respective fields, there has historically been little overlap between the two.

Case in point: Those of us from the mechanical side were great at calculating forces and inertias but had limited expertise in motor and control theory — much less the practical application of motors and controls to linear or rotational systems. And those of us from the electrical side were great at the hands-on integration of motors and controls, but typically lacked experience in the fundamentals of mechanical design, such as calculating beam deflection or tensile strength. If you found someone who had mastered both fields — mechanical and electrical engineering — they either had many years of experience under their belt, or they had, at some point in their career, been thrown into a “sink or swim” situation and learned out of necessity and self-preservation.

But the silos that were mechanical and electrical engineering programs are changing. There are now engineers coming out of both 2-year technical programs and 4-year degree programs who are educated and ready to hit the ground running with practical experience in mechanics, electrical hardware, electronics, controls, and software. This new, diverse type of graduate is the product of mechatronics engineering: a field of study that first became available in the late 1990s and has seen significant growth over the last decade.

The term “mechatronics” was first coined by an engineer at Yaskawa Electric Corporation, stemming from the combination of mechanics and electronics.

This diagram, developed by Kevin Craig, a leader in the field of mechatronics, shows the diversity of related disciplines that contribute to mechatronics.
Image credit: K. Craig

Mechatronics engineering programs can now be found at community colleges, technical schools, and universities. These programs range from two-year associate degrees or technical certificates with a focus on practical, hands-on training, to four-year degrees that cover everything from engineering theory to hands-on applications, to humanities and social-science courses that will give modern engineers the “soft” skills they’ll need alongside their technical expertise.

Regardless of the length of the program and the type of certificate or degree awarded, one thing the mechatronics engineering curricula all have in common is that they combine mechanical, electrical, and computer science disciplines with hands-on training in software, controls, sensors, and networking to produce students who can effectively work in advanced manufacturing facilities.

Mechatronics engineering is inherently tied to the manufacturing industry, although it also has application in other fields such as surgical robots, aerospace, and even agriculture. In fact, industry leader Siemens sees the need for mechatronics education as such as critical part of manufacturing’s future that it has developed the Siemens Mechatronic Systems Certification Program (SMSCP). This program, which can be integrated into existing programs at schools and universities, provides training and teaching resources to schools and instructors, along with certification for students to ensure they have the necessary competencies for specific job profiles in mechatronics disciplines.

The rise in popularity and availability of mechatronics engineering programs could be a godsend for the manufacturing industry. A recent study by Deloitte and The Manufacturing Institute found that the US manufacturing labor shortage could equal 2.4 million by 2028. Much of this shortage is attributed to three factors: the retirement of baby boomers, with fewer millennials to take their place; shifting skills requirements, which are increasingly dependent on technical training and technology degrees (although not necessarily four-year degrees); and a misrepresentation of manufacturing jobs, which will require a concerted effort from manufacturers to overcome.

The report from Deloitte debunks the theory that advanced technologies such as automation, AI, and IoT are eliminating jobs. Instead the report finds that more jobs have been created thanks to these technologies. From the report:

For more than two centuries, the manufacturing industry has adopted new technologies and provided new jobs for workers. Today, the industry is experiencing exciting and exponential change, as technologies such as artificial intelligence (AI), robotics, and Internet of Things (IoT) are rapidly changing the workplace. While some predicted that these new technologies would eliminate jobs, we have found the reverse—more jobs are actually being created.

The availability and attractiveness of mechatronics training and engineering degrees can provide a two-fold benefit to the manufacturing industry. First, it can entice students and younger workers to consider working in fields and industries they would not have otherwise considered. And more importantly, it can prepare them for the types of jobs that manufacturers are having the most difficulty filling — those that require a multidisciplinary approach to engineering, marrying traditional mechanical and electrical concepts with practical knowledge of software, motion control, sensors, and networking.

Gearbox service factor and service class explained

December 14, 2018 By Danielle Collins Leave a Comment

Sizing a gearbox (or gearmotor) for an industrial application typically begins with determining the appropriate service factor. In simple terms, the service factor is the ratio of the gearbox rated horsepower (or torque) to the application’s required horsepower (or torque). Service factors are defined by the American Gear Manufacturers Association (AGMA), based on the type of gearbox, the expected service duty, and the type of application.

Image credit: Cone Drive

While service factors may seem to be very specific, with thousands of combinations of gearbox types and applications each assigned its own numerical value, the criteria used to determine these values are based not on testing and empirical data, but rather on extensive review and analysis of gearbox manufacturers’ experience.

In general, the horsepower (or torque) rating of a gear tooth is based on the durability of the gear surface — its resistance to pitting — or on its bending fatigue. As the service factor of a gearbox is increased, the relationship between the gear teeth life (based on durability of the gear surface) and load is proportional to the increase in service factor, raised to the 8.78 power. In other words, if the service factor is increased by 30 percent (from 1.0 to 1.30, for example), the gear tooth life will increase 10 times (1.30^8.78 = 10.01).

To determine the gearbox service factor, start by consulting a set of tables or charts provided by the manufacturer, based on the type of gearing (worm, spiral bevel, helical, etc.). These tables list a wide range of applications (conveyors, cranes, winders, saws, blowers, etc.), each with (typically) three levels of service duty the gearbox is expected to see: zero to 3 hours per day; 3 to 10 hours per day; or greater than 10 hours per day. Each of these application-service duty combinations is assigned a recommended service factor.

Service factors by application type and gearbox service duty, per AGMA guidelines.
Image credit: Regal Beloit Corporation

Remember, the gearbox service factor is much like a safety factor to ensure the gearbox meets the application requirements, taking into account typical operating conditions known to exist for various types of applications. Once the AGMA-recommended service factor is determined, consider other, non-typical, working conditions that can cause additional stress and wear on the gear teeth, bearings, or lubrication. If any of these conditions exist, increase the service factor accordingly to ensure a sufficient safety margin and life of the gearbox.

Some conditions that may require an increase in the service factor are:

Elevated temperatures
Extreme shock loads or vibrations
Non-uniform loads (cutting versus conveying, for example)
Cyclic loads (frequent starts and stops)
High peak versus continuous loads

Once the appropriate gearbox service factor is determined, multiply the service factor by the horsepower (or torque) required for the application, and the result is the output horsepower (or torque) required by the gearbox.

How does service class differ from service factor?

In some cases, manufacturers cite gearbox “service classes” rather than service factors. Service classes are designated as I, II, or III, and are generally translated to numerical service factors of 1.0, 1.4, and 2.0, respectively, to be used in gearbox sizing calculations. It’s common that even if a manufacturer publishes service classes for general application types, they also publish the more specific service factors for specific applications as well.

Suggested service factors based on service class.
Image credit: STOBER Drives Inc.

Why don’t some catalogs list gearbox service factors?

Servo-rated gearboxes require detailed sizing to choose inertia, output torque, and speed for proper servo system operation.
Image credit: Wittenstein

Using service factor to guide the selection of a gearbox is appropriate for applications driven by traditional AC induction motors. But because gearbox output torque, speed, and inertia are much more critical for the proper operation of a servo system, sizing a so-called “servo-rated” gearbox requires a more detailed and exact method. For gearboxes that are used in servo systems, the primary emphasis in the sizing process is on required torque and inertia match.

Improved tails allow better cleanings on sanitary conveyor platform

December 12, 2018 By Danielle Collins Leave a Comment

Perhaps the most important feature of a sanitary belt conveyor is the ability for it to be thoroughly cleaned. Dorner’s new AquaGard 7350 V2 Series with tip-up tails checks off that box…and then some.

The AquaGard 7350 V2 with tip-up tails provides ample access to the inside of the conveyor for effective, fast cleanings and easy maintenance. Tails can be tipped up in just a few seconds with no tools, and then retracted and locked back into place with the same minimal effort.

Tip-up tails are standard on the AquaGard 7350 V2 straight-belt models and are also available on straight-running modular belt units. They come with improved strength in the tail for more rigidity and no flexing, as well as supporting the belt/chain closer to the tail for improved performance.

Debuting at PACK EXPO 2018, the AquaGard 7350 V2 is built for numerous sanitary applications in baking, snack food, pharmaceutical, pet food, packaging, and other industries that require wipe-down and occasional washdown cleanings of the conveyor. The new conveyor comes in straight belt, as well as modular belt straight and curve models. Belted LPZ, modular belt LPZ, and positive drive models will be available in the coming months.

The AquaGard 7350 V2 is the safest, most advanced modular curve chain conveyor in the industry today. The modular belt curve conveyor has no openings greater than the international standard of 4 mm, even within the curves, which increases safety by eliminating pinch points for operators. Added safety measures are also achieved by covering the upper and lower chain edges and fully containing the drive system, which reduces catenary belt sag and conveyor noise.

Space reduction is another improvement on the AquaGard 7350 V2. The modular belt conveyor system is designed to maximize available plant space by keeping the footprint as compact as possible. Infeed and outfeed sections are a compact 18 in., further saving valuable floor space.

Dorner