By Jim Montague, Executive Editor
Stepper motors always have been the feisty "little engines that could" of the motion control world. So, it should be no surprise that some steppers are being festooned with or linked to software and components, such as PLCs, that allow them to do jobs traditionally done by servo motors.
For example, Roger Klisch, who owns Machine System Integrators near St. Paul., Minnesota, was asked by the University of Minnesota's Bell Museum of Natural History to help animate a wildlife exhibit. "One of these landscapes was a scene of an eagle in its natural environment, along with a gopher that was slightly popping out of its hole," says Klisch. "The request was to make the gopher move when a person walked by. One of the larger challenges was the budget—the customer had only $800 for this project. Since I am a small integrator and my costs are low, I decided to do it, and I stayed well within the budget."
To accomplish this feat, Klisch used one of AutomationDirect's (www.automationdirect.com) PLCs for the primary control and its stepper-output function to drive a small step motor attached to a simple, plastic rack and pinion. The pinion was on the stepper and the end of the rack was attached to the base of the gopher. "I used a small step drive and motor from Superior Electric and a photoelectric sensor to detect a person walking by the exhibit," says Klisch. "The photoelectric sensor was inconspicuous because it was buried within the landscape. It was a hit. The animated gopher taunted visitors and then ducked out of sight. It really added extra spice to an exhibit that can inspire kids to further explore the museum."
While the traditional view of stepper motors is of an open-loop device with no encoder or other positioner, some developers are using PCs to control steppers in some servo-like ways. This usually means using an encoder for feedback to help create a closed loop with the PC.
Christian Fritz, National Instruments' (www.ni.com) motion and mechatronics marketing manager, explains that stepper motors and servo motors are coming together because steppers are gaining sophisticated controls that allow them to act like servos, while at the same time servos are simply getting easier to use. "This is why some applications of steppers and servos are overlapping. There used to be clear differences when to apply one or the other, but that line isn't as clear anymore," says Fritz. "For instance, one of the main weaknesses of steppers has been eliminated by adding microstepping functions, which are handled by a built-in controller, and can help steppers operate at low speeds. Also, more steppers are using non-encoder feedback devices, such as sensing resistors, which can gather data about the rotor, help create closed-loop algorithms and then apply a filter to help the stepper eliminate noise and rough performance at low speeds."
For instance, CNC-enabled plasma cutting machine builder Dynatorch (www.dynatorch.com) in Paducah, Kentucky, recently implemented AutomationDirect's C0-05DD Micro PLC, so it could use a stepper motor and drive in a closed loop in its Technogon torch height controller (THC) for controlling a CNC plasma cutting head's height over a plate. The company reports that its cutting machine's control accuracy is less than .021 in., which produces consistent cuts and lessens the need for operator observation and intervention. Dynatorch adds that its low-cost PLC and stepper motor were key to making its CNC plasma cutting head possible and that its system is half the price of its competition.
Though commonly used in the medical device and factory automation realms, stepping motors branched out in recent years into the wafer-handling aspects of the semiconductor industry and recently into plasma TVs, LEDs and solar components manufacturing, according to Nick Johantgen, engineering manager at Oriental Motor (www.orientalmotor.com), who adds that future applications could be in small machine tools and food processes requiring washdown-enabling motors.
"We always work on making our stepping motors more efficient, but as they achieve higher resolution and smoother rotations, we find ways for them to compete with servos," says Johantgen. "A stepping motor is a positioning device that moves in discrete, incremental steps and runs in an open loop, and so stepping motors generate all their torque with each step. On the other hand, servos only apply enough current and generate enough torque to meet what is being required, and so they're more sophisticated and cost more, but they run cooler and can be used in continuous duty applications."