New Trends in Motors
| By: Kristin Lewotsky, Contributing Editor
Today, you can configure a car online, specifying the exact color and features you want. You can purchase a smart phone and dictate nearly every aspect of it, from the memory to the material of the case to the color of the back, face, side, and inlay. Manufacturing is increasingly about customization. It makes sense that OEMs, integrators, and end-users look for the same degree of flexibility in the products they buy professionally as they do in their personal lives. And automation suppliers are delivering. Whether it involves value-added commodity products or customization options that deliver a competitive advantage, the latest trends in motors revolve around choice.
Motion applications can roughly be divided into two classes. The first is a high-volume low-margin operation such as consumer packaged goods or food and beverage manufacturing and packaging. Machine builders and end-users in this sector typically want commodity components that can be rapidly and easily sourced around the world. For every organization that has motion capabilities on its design staff, there are more who lack the in-house skills. They treat motion control as an enabling technology. They want it to be turnkey so that they can focus on the core value they bring to the market.
The second type of application is based around smaller niches that are typically more technology intensive. Examples might be semiconductor equipment or medical devices. Among the small collection of suppliers, the core functionality tends to be the same. As a result, OEMs in these types of sectors have to use every opportunity to differentiate their product in the market. They are more likely to look for custom solutions that will afford them a competitive advantage. Let’s take a look at the activities in both areas.
For the past decade, the focus has been around improving the performance of commodity motors for mainstream applications like packaging, food and beverage, and manufacturing. The theme has been one of incremental change rather than major leaps in technology. That makes sense for risk-averse markets with a high cost of downtime. Now, however, new trends are emerging.
The first is less about actual devices than about processes. There was a time organizations had deep engineering expertise in motion. With today's leaned-out staffing, they are more likely to look to their vendors to fill in the gaps. "For every standard request for quote we get, we have one where an OEM wants a specific connector, a specific shaft modification,” says Josh Bellefeuille, Product Line Manager and Stepper Motor Specialist at Kollmorgen (Radford, Virginia). “They want it done at the manufacturing level. They don’t want to do that value-add in-house.” That puts pressure on motion vendors to develop flexible manufacturing capabilities that enable them to customize quickly and efficiently while controlling cost.
As part of that process, manufacturers convert repeated custom orders into product lines. “If you go to any number of stepper motor manufacturers and you look at their “what’s new” pages, a lot of products are related to very specific niche applications type,” Bellefeuille says. “For medical, if someone needs a very, very smooth, very flat, quiet motor, we might optimize a winding, optimize the construction. It’s a standard motor but actually based on an OEM spec that we have received so often we have been able to put that into a ‘standard’ design.”
Examples include motors with pancake form factors and thicknesses on the order of 10 mm. They would be used in feeder modules in machines for semiconductor fabrication or food and beverage packaging machine. To address the medical device market, a class of motors has emerged specifically designed to run cool. This takes a combination of the windings and the drive that energizes them. Degrees of freedom in the windings include the number of phases, coil design, wire diameter, and winding density.
Although motor manufacturers frequently produce their own drives matched to their motors, here, too, new trends are emerging. With the availability of increasingly sophisticated and functional microcontrollers, customers needing specific motion properties can now build their own drives. These drives can only deliver benefit, though, if they are paired with motors matched to their characteristics. Increasingly, motion vendors are developing frameworks that enable them to realize custom motor designs rapidly and effectively. "The number of turns, the wire diameter is all going to affect the amount of current you can put in the motor,” Bellefeuille says. “It’s going to affect the current and inductance and input voltage, and those are all going to change the torque-speed curve. So for OEMs, having a manufacturer that can develop custom windings for them is a huge advantage.”
Twenty years ago, an engineer wanting to add motion to a machine would have bought a motor, coupler, gearbox, and actuator. Today, they’re just as likely to install positioning using a frameless motor formed of a separate rotor and stator, without a housing so that it can be built directly into the equipment.
It’s a matter of competitive advantage. “OEMs are buying rotors and stators¬ - or in the case of a linear motor, the magnet track and the forcer - and integrating them into their system, whether it’s a robot arm or a joint assembly or whatever,” says Rob Mastromattei, Business Development Director at Celera Motion (Bedford, Massachusetts). “These are ways they can differentiate themselves and win in their market segment.”
Consider ion implantation in semiconductors. It’s a step-and-scan process that involves positioning a wafer under a stationary ion beam for a specific dwell time. OEMs can make their customers money by building equipment that will process more chips per hour. Now, an equipment builder can’t change the basic physics of implantation. They can, however, give their customers and themselves a competitive advantage by speeding positioning.
A conventional servo motor operating at low speed requires a gearbox in order to deliver the high-acceleration motion required for ion implantation. Gearboxes introduce compliance, however, which increase settling time and slows down the process. Driving the axis directly using a frameless motor eliminates the problem while also reducing size and inertia. “You have got to be able to implant very, very accurately,” says Will Vinson, Business Unit Manager at Wittenstein (Bartlett, Illinois). “The only way to do it on some of these axes is to go direct drive with a frameless motor and design the machine around it.”
It’s an approach that can provide a differentiator for OEMs in a variety of application areas such as aerospace, communications, medical, scientific, and semiconductor. “OEMs that are really challenged in their market segments to be faster, lighter, more accurate, and offer better dynamics, are [using frameless motors] to do away with redundant parts,” says Mastromattei. “If you want to reduce the size, improve the stiffness, give it dynamic stability, change the excitation modes that can cause instability, OEMs have figured out that means you need a uniquely designed motion axis.”
That’s the good news. The bad news is that it’s not as easy as it sounds. Fortunately, a number of vendors offer frameless motors or kit motors, with matched rotor/stator pairs. They are available various form factors, including arcs and R-Theta configurations for rotationally symmetric applications (see Figure 1). Even with matched hardware, frameless motors require a clear understanding of specifying and applying motor technology. At a time in-house engineering skills may not encompass motion control, the idea of co-engineering services once again comes into play.
“There are a lot of things that need to be considered,” says Vinson. “There is all the workmanship when you actually put the motor together. You need to be careful when you design the bearing system. Cooling can be very challenging. The support of experts is very important.”
As with the stepper discussion above, once an OEM decides to use a frameless motor solution, they should work with their vendors from the design phase on up. It’s the best way to avoid problems and misconceptions. The process can begin and end there, or the vendor can actually build and supply subassemblies. The OEM reaps a competitive advantage without any of the risk.
In addition to the shift toward frameless motors, Mastromattei sees increased use of voice-coil motors (VCM) and voice-coil actuators. The technology is not new – it’s been available for around 40 years. It’s more a case of the right technology showing up at the right time. “One of the things that’s driving this is the fact that people can get very small, high precision, high resolution encoders to go with the VCM,” he says. “Before, you had to buy a $200 dollar VCM and a $2,000 dollar encoder. Now, you can get a VCM for a few hundred bucks and an encoder for a few hundred dollars and be positioning with nanometer resolution.”
Beyond the trend toward customization, there are some new motor technologies waiting in the wings. George Bennett, Motor Consultant and Founder of Optimal Motion (Murrieta, California) points to a Minnesota startup working on a variable-reluctance motor designed for easy maintenance. Asset owners can easily replace coils, for example, without removing the motor from the machine. “I’m not really comfortable with the idea of variable reluctance because the acoustics problem, but the idea of replicable parts so that you don’t have to take the whole machine apart to fix the motor, that’s a great idea,” says Bennett.
He is also part of a team developing a new type of motor called the field-modulation motor. In general, servo motors have a hard time developing torque at low speeds. They typically require the addition of a gearbox, which adds size, cost, weight, maintenance, and additional points of failure. Gearboxes also introduce backlash, making them non-starters for certain applications.
Stepper motors suffer from the reverse problem of servo motors. They generate plenty of torque at low speed but inductance limits their high-speed performance. They also suffer from a phenomenon known as mid-range resonance in which resonances cause the torque of the motor to fall to zero abruptly at certain speeds. Mutual inductance also limits the phase count, which in turn puts a cap on both torque and resolution.
The field modulation motor, designed by Carl Copeland, president of Floor 36, Inc. (Fort Worth, Texas) provides an alternative to both classes of motors. It features 18 phases, 36 poles, and a 3-D magnetic flux path entirely different from conventional motors (see Figure 2). The high pole count allows individual staggering of the phase alignment. The result of this is zero detent torque. As a result, the motor can provide higher torque at low speeds than a servo motor but it can also run faster than a classic stepper motor. It has very low inertia and a resolution of 0.909°. It is not intended to replace either motor type but to offer a viable mid-range solution for applications like pick-and-place assembly and robotics.
As an example, consider a three-phase motor operating at 5000 RPM to generate 22 Nm of torque. This translates to about 15 HP. Now, let’s compare it to an equivalent field-modulation motor. You can think of the 18-phase field modulation motor as essentially emulating six separate three-phase motors and attached to a single output shaft. The trade-off is speed – it would deliver 130 Nm of torque but at around 830 RPM for roughly the same output power. The effect of the field modulation approach is analogous to that of rewinding the motor with thinner wire, but without the increased inductance that method would introduce. “The motor allows you to build a driver that has one sixth of the current requirements of a standard 3 phase motor for a given torque at low speed,” says Bennett. “Looking at it from a driver circuit perspective, you get 6x more torque per amp of current at the low end than the three-phase servo motor.” For the three-phase motor to deliver that much torque at 800 RPM, it would need a gearbox, with all the trade-offs that entails.
The field modulation motor does have a drawback in that instead of driving three phases, the amplifier has to drive 18. “The cost is comparable for the driver because instead of three big H bridges you have 18, but they’re 1/6 the size,” Bennett says. “Between that and the pricing break for volume, you wind up with about the same cost overall.” The smaller parts also deliver an advantage in terms of form factor. Instead of the standard large box, drive manufacturers and end-users can develop smaller drives, flat packages, and more to better suit the requirements of the application.
The team has already built eight prototypes, including a 60 HP, 42 frame size. Floor 36 has been granted multiple patents on the IP. Look for the technology to be available for licensing by the end of 2016.
The variety and demands of today’s applications puts OEMs under increasing pressure to meet performance requirements. With a focus on customization, motor and motion manufacturer deliver the flexibility design teams need. Vendors can do everything from value-added manufacturing to supplying engineering services. With alternatives ranging from specialized windings to frameless motors to entirely new designs, today’s motor solutions gives OEMs and end-users easy access to the performance they need.
Thanks go to Dan Jones, President of Incremotion Associates (Westlake Village, California), for useful conversations.