Motion Control Resources
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Recent Trends in Motors
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 07/27/2018
OEMs work to deliver equipment to end-users that delivers higher throughput and better operational equipment effectiveness (OEE) in a smaller footprint. In turn, equipment builders pressure motor manufacturers to deliver components that can support those goals.
More is the mantra of the modern marketplace. Consumers want more for less and manufacturers seek to give it to them. OEMs work to deliver equipment to end-users that delivers higher throughput and better operational equipment effectiveness (OEE) in a smaller footprint. In turn, equipment builders pressure motor manufacturers to deliver components that can support those goals. Add the performance demands of high-growth applications such as automated guided vehicles and collaborative robots, and you have the factors driving recent trends in motors.
“Customers are looking for ways to make their machines have more value to their customers,” says Chris Radley, senior manager global platform commercialization at Kollmorgen (Radford, Virginia). “They want improvements around uptime, operational efficiency and energy efficiency. How do they make sure that their machine is operating the maximum amount of time? How do they shrink it down in size?”
The answer is by taking advantage of the best that motor technology and the industry has to offer.
In the aftermath of the recession, many companies are operating with significantly smaller engineering departments. To compensate, they rely on their suppliers to assist. “If you try to do everything on your own, you need to invest in the building up of the know-how,” says Carsten Horn, Maxon Precision Motors (Fall River, Massachusetts). “If you partner with a company that has access to that technology and know-how, it can give you a big advantage in speed and quality of the product that you design and produce.” There are some challenges to this process though, he is careful to note. In particular, partnerships need to be managed. This can be complex and time-consuming, especially since the number of partners on a project can reach as high as four or five. “We take over a certain amount of design work and need to be integrated heavily through the product. If there are other partners also heavily involved, they also impact each other and that needs to be managed.”
Other organizations see engineering in-house as a way to differentiate themselves from the competition. The new technology makes it feasible, even with reduced resources. “Everything has changed in the last 10 years,” says Rob Mastromattei, senior director of business development and technical sales at Celera Motion (Bedford, Massachusetts). “It doesn't take a team of engineers meeting in the lobby with a team of sales people from fifteen different companies to make a decision anymore. The products have gotten smarter. Application information on how to put things together and how to design systems is available with the click of a mouse. The industry has just evolved so that you don't need the engineering horsepower that you needed ten or twenty years ago.”
Whether or not they needed assistance with their engineering, organizations demand performance to get an edge over the competition. They may need higher torque in the same footprint, or the same torque in a smaller footprint. They want greater efficiency, either to reduce energy demands or to support battery-operated systems, whether those are on automated guided vehicles (AGVs) or portable devices. The ever-increasing interest in modern manufacturing initiatives like industry 4.0 is pushing higher levels of integration.
To address performance demands, major manufacturers are modifying magnet materials and adjusting windings. These changes are not just to increase the magnitude of the magnetic flux but in some cases to change the flux paths entirely. The challenge, course, lies in not just designing the system but being able to manufacture it reliably and in volume.
To achieve a competitive advantage, system designers want motors with specific attributes. Here, motor manufacturers stand ready to give it to them. For all of the catalogs supported out in industry, a surprising number of manufacturers consider customization not just possible but common and readily available.
Customization can be roughly divided into two classes. The first involves small modifications around a stock design. This sort of customization might involve different flanges, modified hole patterns, changes to cabling connectors, gear ratios, etc. The function of the motor itself is generally unchanged but the mechanical features are modified to more readily integrate into the rest of the system. A motor manufacturer might design a modular housing, for example, that lends itself to easy modification.
“A lot of our industrial customers don’t necessarily want something designed from the ground up, they just want to meet certain mechanical requirements,” says Radley. “On our latest-generation motor, we made the front-end bell of the motor a separate piece in the housing. That lets us interchange that part whenever the customer needs us to. It’s not a big deal.”
The second type of customization involves changes to the electromagnetic elements of the motor. The shape and size of the rotor and stator can be completely different to change the magnetic properties. “We are seeing a lot of requests for increased power density relative to frame size,” says Jeff Nazzaro, product manager for servomotors and gear heads for Parker Hannifin (New Ulm, Minnesota). “They tell us the standard catalog product envelope sizes are too large and ask if we can do something different to increase the power in a smaller envelope.”
A motor might be made more copper centric, for example, with a greater number of turns and a thicker gauge of wire on the stator. In the case of permanent-magnet motors, the size, shape, and composition of the magnets can be modified. The result of these changes is a stronger magnetic flux that boosts the power density. The result can be either higher torque in the same package or the same torque in a smaller package.
The Parker Hannifin team worked with Brammo to build a custom motor for the company’s Empulse R electric motorcycle. The application required high power density and efficiency, as well as water cooling. “The flexibility of variable stack lengths and variable winding configurations allowed us to design to fit the magnetics that can be used in the best product base,” says Daniel Riegel, mechanical engineer at Brammo. In a recent speed test at Daytona, the motorcycle reached speeds of 176 MPH.
Custom designs need to go beyond pure performance. If the prototype works, there needs to be a straightforward path to volume production. Design for manufacturing requires examining each component from a cost standpoint. Time equals money in the manufacturing environment, so the process itself needs to be examined. Alignment features might be added, or elements to speed testing. Materials may also need to be revised. Nazzaro describes a motor project that used billet aluminum in the prototyping phase but switched to cast aluminum for manufacturing.
As system designers seek to differentiate their products from those of their competitors, they increasingly turn to frameless motors. In a frameless motor, the rotor and stator are shipped as separate parts that can be integrated directly into the machine (see Figure 1). Robot arms, for example, frequently use frameless motors for the elbow and wrist joints. A frameless motor develops torque and moves the joint without the need for a shaft. Frequently, gearing is integrated into the system.
“If the customer is shifting their development process to highly integrate their assembly to differentiate from their competitors, then frameless motor technology is an enabler for them,” says Mastromattei. “Being able to buy the magnetic components and integrate them into their own mechanics is an advantage for a lot of customers.”
The frameless approach provides a significant degree of design freedom but also requires engineering skill, both in design and also integration. Rotor and stator are delivered separately but must be aligned upon installation to very tight tolerances. Here, too, system designers turn to their suppliers for assistance. “The strength that customers in this space typically bring to market is they understand how to make that collaborative robot do what it does, deal with the haptics, the feedback that comes from the system,” says Radley. “They are experts in controls but they are not experts in the mechanics on how to translate what their software does into the actual physical motion. They like dealing with somebody like us because we don't look like a competitor. We look like a true supplier for them.”
Frameless motors aren’t always built into the equipment directly. For an application that demands a specialized form factor, a frameless motor can be placed in a customized housing for a more efficient development of a custom package. “When a customer comes to us with a motor request that doesn't match anything we have in the catalog, a frameless motor is often more adaptable than taking the inner workings of a standard industrial motor to convert it to something unique,” says Nazzaro. “We find the frameless motor components that achieve the power and the envelope the customer is asking for. Then, it’s just a matter of creating the features they need on the aluminum housing.”
Multiple factors drive demand for increasing levels of efficiency. In our wireless world, a growing number of applications require battery-operated systems. The more efficient the motor, the longer the battery life. This holds for medical devices and autonomous robots just as much as for the AGVs that roam around manufacturing facilities and warehouses. Increasing power density minimizes weight, which saves battery life.
One of the primary loss mechanisms in electric motors is resistive heating caused by eddy currents that form in the iron backing of the rotor and stator. To address this issue, these structures are typically formed of very thin laminations stacked together. Thin layers of insulation separate them to prevent eddy currents from hopping between laminations; another, less common approach is to use a powdered molded core. In this case, the eddy currents are restricted to the size of the individual metal grains. This approach reduces losses significantly. On the downside, its power densities are not as high as four motors designed around laminations. The approach tends to be more effective for smaller motors rather than larger ones.
Organizations also seek high-efficiency motors to minimize energy consumption in their facilities. The Software Motor Company has developed an updated version of the switched-reluctance motor to achieve efficiencies of 90% or better. In a switched-reluctance motor, the number of stator poles is different than the number of rotor poles. The switching electronics activate the stator windings in sequence. The rotor turns to try to align with the magnetic field of the stator poles. Because of the mismatch between poles, alignment never takes place and the rotor keeps turning.
The concept of the switched-reluctance motor has been around for more than a century, but it only became practical with the availability of microprocessors that permitted accurate control. The technology became the subject of renewed interest during the rare-earth magnet bubble, when it provided a magnet-free alternative to increasingly expensive induction motors for applications like pumps, fans, blowers, and compressors.
Conventional analog versions had issues with low power density, electronic noise, and torque ripple. In addition, the electronics themselves were expensive, partially offsetting the savings introduced by eliminating the magnets. SMC aims to solve those problems using several design approaches. Unlike most switched-reluctance motors, the SMC version features more rotor poles than stator poles (a 6-10 design) versus the opposite, which is more commonly used. More important, the SMC design uses digital versus analog processing for the controls.
“We didn’t just take this 100 year old motor design and add a bunch of stuff to it,” says Ryan Morris, executive chairman at the Software Motor Company (SMC; Sunnyvale, California). “We really redesigned it from the ground up. As it turned out, the software-defined control has enabled us to solve all these other problems like making it more power dense and making it dramatically more efficient.” When AC induction motors are oversized, as is frequently the case, they only achieve efficiencies of around 70%. The SMC motors demonstrate a flat efficiency curve of about 90% or better.
The company is currently focusing on smaller versions of the motor. The roadmap calls for releasing a 15 hp motor in the next few months, which will position them for environmental control of large buildings.
The combination of limited engineering skills and highly specific application requirements has led to a trend toward application-specific integrated motor assemblies. Examples include traction motors and steering motors for automated-guided vehicles and other mobile robots (see Figure 2).
“It’s challenging enough when you’re designing some sort of robotic system, to deal with multiple components that need to come together,” says Jeff Shearer, systems engineer at Allied Motion Technologies (Amherst, New York). “There’s an additional challenge if you are trying to source these components from multiple vendors – there are compatibility issues, fitting issues, etc. Traditionally, you would need an engineer who would understand each piece of the puzzle and bring everything together. With an integrated subassembly, OEMs who don’t have that expertise can buy a complete motion solution at once.”
These types of integrated subassemblies provide a number of advantages. In general, they tend to be more compact. They eliminate the need for external cabling between the motor, drive, and feedback components. Instead of making all of those connections, the OEM technician typically just needs to bolt on the physical assembly and make one electrical connection. This increases reliability and reduces points of failure.
Integrated assemblies bring other benefits. The individual elements are targeted to the application. If necessary, they will be selected and/or manufactured to meet standards. Because the manufacturer is able to source the components in volume, integrated subassemblies typically cost less than a collection of discrete components.
Exoskeletons are under development for applications ranging from assisting the disabled to powering soldiers to helping factory workers manage heavy loads. Particularly for nonmilitary applications, size and cost control are essential (see Figure 3).
“There is a list of requirements you need to fulfill for exoskeletons,” says Horn. “The biggest requirement is to minimize the size of the system. If you over specify the assembly, you add weight and cost. If you use a highly integrated version, you can minimize cost and get the size of the components so that they can fit under a person’s trousers. That’s the goal.”
Today’s applications are more demanding than ever. OEMs and other system designers have numerous options for the choice of motors to serve their needs. The process of specifying the solution begins with gathering as much detail as possible about the application. Calling up with the torque number will not give the best results. Provide as much detail as possible about the application and reach out to the motion vendors as early in the process as possible.