Stepper motor avoids misstepsBy Susi | Published on Nov 24,2015
Boone, NC —Given the small and delicate nature of the components it manufactures, International Resistive Company (IRC), a subsidiary of TT Group PLC, had to think carefully about the design of its testing and handling equipment. In particular, engineers required that the motor shaft maintain position and come to a dead stop in order to meet positioning tolerances.
As an example, one test process calls for the positioning of a rotary wheel that handles round resistors. "These resistors go through short-time electrical overload two times and are exposed to six times the amount of current for which they're rated. Each part gets tested both before and after. We count on the motors to help the equipment hold tolerances within a few thousands of a inch," says Roy Keller, electronic design technician.
IRC engineers didn't even consider servo motors for this application, because of the "jitter" that servo motors sometimes exhibit when trying to hold position. Although this movement is very small, Keller says that when bending small resistors, or testing their contacts, any movement at all can interfere with and invalidate the test results. Gain tuning or the addition of mechanical brakes to manage the problem were not options for IRC engineers.
Ordinary stepper motors, on the other hand, do not have jitter. But they can miss a step when the rotor lags behind or moves ahead of the excited winding, potentially causing an unscheduled shutdown of the production line or worse.
Seemingly having their cake and eating it too, engineers replaced the existing open-loop step motors with AlphaStep motors from Oriental Motors (Torrance, CA). The motor contains a built-in resolver that continuously monitors motor shaft position. Under normal conditions, the driver operates in open-loop mode similar to an ordinary stepper motor. But when the rotor position gets out of sync with the excited winding by more than 1.8 degrees, the motor switches to closed-loop control to maintain synchronicity.
Another alternative would be to put an encoder on a conventional step motor—but cost was an issue for IRC. "An encoder for a regular step motor costs between $400 and $500. In contrast, the price tag on the small ASC66 AlphaStep is less than $650 including the resolver, and the ASC98 is under $900," Keller says. "Servo motors for these applications can cost two or three times as much."
In addition to testing equipment, IRC uses the stepper motors in materials handling applications. For example, one AlphaStep drives a conveying system with parallel rails, in which the inner rail moves up, lifts, and drops parts over the outer rail.
The motor drives the cam in one direction, rotates 60o, and then returns to the start position. It raises and lowers the teeth of the conveyor, the motions of which have to be very precise given that the resistors are on the order of just a few mils thick. In another application—a machine that bends and tests the resistor—a motor drives a gearbox that converts the motor's circular motion into vertical and horizontal motion.
"It does 1,000 steps per revolution with all the forming and testing keyed via the step count," says Keller.
In each operation, the motor stops precisely. Keller says, "The resolver counts the steps and will try to make up any missed ones. If the motor can't complete its steps, it will stop and send an alarm, instead of continuing to miss steps and possibly damage the equipment."
Another manufacturing application at IRC consists of filling resistors—some as small as 0.250-inch long, with concrete that functions as an insulator. In this case, Keller uses a geared AlphaStep motor. "The geared version allows us to increase the number of steps, and at its coarsest setting gives 2,500 steps [as compared to 1,000 for the non-geared version] for extreme precision. That's important, because the resistor shell must be filled to a specific level. The geared motor also gives us 78 inch-lbs of torque for this application to achieve rapid fill times," he says.
"The primary advantage of the AlphaStep is that we get feedback only when we need it, at an economical price," says Keller."
How the motor technologies
Type of control
Dead stop capability?
Achieved through tuning, filters,or mechanical brake
AlphaStep motor with built-in resolver
Open loop during normal operation; closed loop to maintain synchronicity
Table source: Oriental Motor