Serving Up Better Servos

With absolute position monitoring, digital communications, and ever higher torques, today’s servo motors offer motion control designers more options than ever.

By: Kristin Lewotsky, Contributing Editor

You know servos -- those dinky little motors that whiz things around. They offer exacting motion but they can’t handle real loads, right?

Wrong. Today’s servos are increasingly appearing in a wide range of applications, in some cases even displacing more traditional motors.

A servo motor consists of a motor -- typically AC brushless or DC brushless with a permanent magnet -- linked to a driver and a feedback loop. It is the feedback loop that gives the servo motor its characteristic precision motion. Servo motors typically offer very high power density, or high torque at low volume -- on the order of thousands of Newton-meters, in some cases.

Despite these torque numbers, servos have gotten tagged in the past as only being able to handle low-power applications. Not true, says John Mazurkiewicz, product manager at Baldor Electric Co. (Fort Smith, Arkansas). “People look at a servo motor and say ‘oh, that’s a cute product you have.’ They don’t realize that there’s a lot of power there,” he says. “We have one application on a printing press in which they’re moving a printing wheel that weighs 2000 lbs.” He cites another application in which a 4-in-diameter motor is being used to position logs in a sawmill. Servos are capable of a lot more than you’d expect and they’re getting better all the time.

Tiny But Mighty
In one example of the kinds of shifts underway, servo motors are starting to replace hydraulics in selected applications, says Pat Berkner, product sales manager at Parker Hannifin Corp. (Rohnert Park, California). “Companies are very concerned about the environmental aspects of hydraulic oil, so what we’re seeing is the mass conversion of hydraulics over to electromechanics, using electric servo motors with ball screw [actuators], and so on.” Such a system could open and close even a very heavy door, or move a ram in and out of a machine, Berkner says. “Now there’s no more hydraulic oil, it’s much cleaner.” Servo motors don’t constitute a universal panacea, of course--some applications simply require brute power--but for a variety of applications, servo motors present an unexpectedly effective solution.

One major trend from Europe, becoming more and more recognized in the US, is the use of servo motors for direct drive applications, says Ralph Baran, product manager for motors and mechatronics at Siemens Energy & Automation Inc. (Atlanta, Georgia). A direct drive motor, typically low speed and high torque, mounts directly to the machine mechanics, dispensing with the need for gearboxes and clutches. This makes systems five to ten percent more efficient, in applications ranging from textiles and paper manufacturing to plastics.

“We can control what the machine is doing in a more accurate way compared to an asynchronous motor/gear box that is running the same torque at the end of the day,” says Baran. That means improved quantity and/or quality of production, a direct benefit. In addition, the approach provides space savings, reduced maintenance demands, and lower noise. The latter is particularly important in Europe, where occupational health regulations mandate noise limitations and even increased employee wages for every hour over four that they are forced to wear hearing protection in a given shift.

Of course, there is always a tradeoff. In this case, it's a cost increase. That said, the improved operating efficiency and production quality can provide a return on investment surprisingly quickly. In one case study of a plastics extrusion line, Baran showed a potential cost savings of $26,000 (20,000 euros), which brought return on investment within less than two years.

Paradoxically, a trend in servo motors is toward smaller motors, enabled in part by the rare-earth materials now used in the permanent magnets of the motors. A smaller motor capable of providing the same performance as the larger servo motors of the past means a smaller footprint. This is a plus, Berkner says, in an industrial environment in which machine designers don’t necessarily plan for motor size, merely leaving it to be figured out later. Smaller motors also mean smaller cables.

“Where you had to have maybe a 3-in-diameter cable, now you can probably use a cable that’s more like 0.25-in diameter because it’s such a low power [consumption] device. What limited you before was the connectors, but now the connectors are getting smaller.” There is no official standards effort underway, but he notes that the industry appears to be gravitating toward one on its own.

Precision Positioning
Commanding a motor or actuator to move a set linear distance or through a set angular rotation is only useful if the system knows where it is to begin with. In the case of servos with incremental encoders, that means homing the machine at start-up each time. With a single axis of operation, this is a fairly quick process but in the case of a system with 16 or 25 axes, that can take as long as five or ten minutes, and we all know time is money on the production line. Linear motion systems moreover require limit switches or hard stops to prevent the units from going beyond the extent of travel and potentially hanging up the machine, or even causing damage. More switches mean higher cost, more electronic noise, and more things to break down.

As a result, another trend in servo motors is a shift to absolute encoders. An absolute encoder remembers its position, whether by battery-powered circuitry or a complex set of precision gears that Berkner characterizes as surprisingly free of backlash and other mechanical problems.

We live in the real world, though, which means that there are always tradeoffs. In this case, cost is the issue, Berkner notes, citing the need for intelligence in both encoder and driver. The silver lining? “The benefit of an absolute feedback device is that you can actually store motor data inside that device. If you buy a motor, you power up your drive and the drive talks to the motor. It says, ‘this is the kind of motor I am, here are my electrical characteristics.’ The drive knows exactly what to do with that and you’re running.”

Which brings us to another trend -- Ethernet. That protocol, along with other digital communications protocols busses, is increasingly being used for system communication and for communication between components. Siemens, for example, uses Ethernet-based communication between motor and drive, which allows them to work with so-called electronic nameplates that function as described above. “In the encoders, we save the complete technical information about each motor,” says Baran. “When you plug it into the drive system, the drive automatically reads out this information. It has all the information about the different parameters that have to be set up for driving this motor, which means it’s really a plug and play possibility. [Ethernet] gives us the possibility to realize new functions that haven’t been possible before.”

Of course, like anything, Ethernet carries challenges with it. Because it is a packet-based protocol, standard Ethernet is not appropriate for system communications that have emergency stop requirements -- a critical command could get backed up behind other, less important traffic. “It has to be deterministic,” says Mazurkiewicz. “It’s got to be done real time. If somebody comes in and disturbs a light curtain, he’s walking into the machine so you want to stop that machine now. With standard Ethernet, that won’t happen.” With Ethernet protocols modified to be deterministic, though, the stop command could be given priority, ensuring safety.

Different horses for different courses, so the saying goes. Servo motors will never be a universal solution -- there will always be applications for which an induction motor or a DC brushed motor with gearbox provide the optimal answer. With the ongoing advances in servo motor technology, though, servo systems are crossing into more application areas than ever.