Steppers versus Servos: A Round-Table Discussion

Selecting the small motor

—by Richard Mandel

Significant technological advances over the last decade in motor and controller design, and the formulation of magnetic materials, have altered the divisions that affect motor selection. The marketplace now includes nanostepping motors, and systems that have qualities of both servos and steppers. Juxtapose that against the rising number of electric motors used in products and industry — for example, the average number of motors on a new car today is around 30, according to Dan Jones of Incremotion Associates. Moreover, that number is expected to rise, as the automobile industry refines steer-by-wire and brake-by-wire systems. What to choose?

To clarify some of these issues, Designfax conferred with George Beauchemin of MicroMo, Glenn Bythrow of Bayside Motion Group, Jan Bosteels of Advanced Motion Controls and Nick Johantgen of Oriental Motor.

If we go back five to ten years, what had been the strengths and weaknesses of stepper and servo motors?

George: If we speak of servos as brush-type DC motors, one of the selling points was that the stepper motor didn’t have sliding electrical contacts, so there was one less mode of failure compared to servos. Secondly, you didn’t need a positioning transducer, which increased reliability while lowering costs.

Nick: We used to think of servomotors in terms of acceleration rates and top-end speeds. The disadvantage occurred when the motor reached a final rest point — there would be dither, something that didn’t occur with stepper motors.

Jan: Ten years ago, steppers were easier for people because they offered positioning capability without having to close a servo loop, deal with feedback or shoot PID loops. The limits came in speed and acceleration, and probably in the power range. Servomotors were more expensive — at that time the common form was brush-commutated, which meant lower duty cycles and more maintenance. “Brushless” motors were still exotic and expensive.

Around five years ago more brushless motors were being built, sending the prices down. Automation engineers became more familiar with tuning PID loops, and less “afraid.” In addition, semiconductor developments were resulting in better DSPs and processors, bringing down the costs. Those are trends that made brushless more popular, but not necessarily at the cost of steppers. Even ten years ago, there were claims that brushless would take over the market from steppers, nobody is going to buy brushed motors — to the contrary, I don’t think those technologies are ever going to disappear. The application base keeps growing — hydraulic and pneumatic systems are moving into electromechanical solutions.

George: There are applications for all of these — brushed, brushless and stepper — that will never change. Take, for example, the fan motor in a laptop computer. You’d never use a stepper motor there. On the other hand, the paper drive motor in an inkjet printer seems to be a natural stepper application. Or in a portable medical infusion pump — most designers would prefer to use a stepper motor. With a servo, if the encoder feedback drops out, the servo might just take off and deliver a lethal dose. The tradeoff is that the stepper typically consumes a lot more power.

Nick: I’d say it was when the microprocessors became powerful enough and inexpensive enough — about five years ago — for servo problems to be corrected in the software. That’s when we started seeing things cross over.

Glenn: Right. The changes in the industry have been more in the controls than in the motors themselves, when the components in the controls could be reduced in size and efficiency went up. Then, steppers could be integrated with their controls.

As a result of this crossover, then, where do you see new applications for steppers and servos?

George: The automotive industry is one. The trend is towards a combined alternator and starter — that will probably become a brushless DC motor as time goes forward. Many other systems, like electric motor-driven brakes and steering, will also use brushless DC motors.

Jan: There are many new markets in the automotive industry. In office automation, particularly printers and copy machines, I see that market dominated by steppers because of the quantities involved. Even the control electronics will be easier to produce in the volumes required. With industrial automation, there it depends on the power involved. Electric motors in the 0-2 hp range have the largest overlap of steppers, brushed and brushless. Above two hp, the brushless servomotor will be more typical.

Glenn: We’ve observed that, with prices coming down over the last few years for both steppers and servos, most people would prefer to go with a servo if priced competitively, because the servo typically has a torque range that stretches to higher rpm. Our markets are now into newspaper printing and semiconductor manufacturing, among many others.

Nick: The lines are definitely grayer. Step motors are becoming more powerful. Servos were gaining in some markets over steppers, but built-in feedback mechanisms are helping steppers return to those same markets.

Jan: And there is a fourth motor technology that is becoming a player — that is induction motors. In the next 5-10 years we should be seeing 1 hp AC induction motors competing with brushed and brushless.

George: We may be seeing big changes in the appliance market, too, driven by the cost of energy. DC motors of a smaller nameplate horsepower can drive a washing machine or clothes dryer as well as a larger AC induction motor.

Jan: I once saw a statistic that 60-70% of the world’s electric power is utilized by electric motors. Nevertheless, concerns over selecting a motor based on power consumption, I suspect, is going to be greater for appliances over industrial automation. Different industries have different criteria, but as changes, such as the example of DC motors in washing machines, become mainstream and volumes increase, these criteria will change.

So, is the differentiation still application-specific?

Nick: Absolutely. We also look at it from the user’s knowledge level — what they know, what they’re comfortable with, and if they have potential and time to learn.

Jan: I agree with that.

George: A lot of applications are almost no-brainers for choice. And as the other gentlemen suggest, there are applications where the design team’s comfort zone decides what’s best.

Glenn: With companies in the marketplace having gone leaner and meaner, they’re demanding the expertise come from the vendor. The vendor is becoming partnered with the OEM company as part of the design team.

Jan: The unwritten criteria is, it’s a lot about education at the end. There are a lot of unquantifiable factors to each technology that may or may not be advantageous to an application. Engineers are like consumers — they are always educating themselves when they are purchasing technology for a product.