Rohnert Park, CA--Step motors find themselves in a variety of motion-control applications, from the mundane to the mission-critical. On board the X-Ray Astronomy Satellite, for instance, a step motor drives an essential telescope-calibration system. Out in the shop, step motors control the sheet-stock feeders used in metal stamping.
But they have their limits. Highly undamped--most exhibit damping ratios of 0.02 to 0.04--step motors suffer stability problems. "The dynamic model is like a mass on a spring," says Stuart Goodnick, manager of the power products group at Compumotor. And as with springs and masses, step motors can resonate in ways that lengthen their settling time, fight high-speed moves, and cause instability in full-step motors at about 1.0 rps.
A new drive from Compumotor, called Zeta, is the first to address these issues throughout the operating range, the company says. And it does so without changes to the motor.
It incorporates several patentable features that greatly improve damping at both low and high speed, increase throughput, and provide more available torque. Engineers believe the drive may open doors to new applications or even replace servo motors in some cases.
Zeta's secret lies in three separate technologies: electronic viscosity, active damping, and stall detection. Uniquely, all three operate off signals generated by the motor, thus foregoing the need for separate sensors and encoders.
Electronic viscosity dampens system ringing that occurs when trying to stop, decreases the duration of individual moves, and increases throughput. "We found a way to detune the current loop at low speed," says electronics engineer Christopher Botka. "The net effect is that we introduce a very strong viscous term." At speeds less than 3 rps, electronic viscosity increases the damping ratio from 0.20 to 0.25, cutting settling time by an order of magnitude compared to undamped systems.
By contrast, active damping only functions at more than 3 rps (built-in hysteresis prevents it from conflicting with electronic viscosity at the crossover point). It reduces vibration, counteracts mid-frequency resonance at 15 to 20 rps, and increases the usable torque of the motor. Unlike competing passive damping techniques, active damping can be applied while driving the motor and not just at the end of a move. It can also be used with microstepping drives, whereas passive damping cannot.
Active damping requires a feedback signal to form a closed-loop system similar to that of a servo motor. The signal is usually supplied by a tachometer or other sensor. While this technique has been used since the 1980's, the expense and fragility of tachometers has limited its use.
With Zeta, however, engineers developed a sensorless active damping method that uses the signals produced by the motor's back-EMF. "You know this back-EMF is there," says Botka, "and we asked, is there anything we can do with these signals to extract position?'"
And indeed there is. Zeta uses the motor-terminal voltages and currents as a sort of electronic observer, Goodnick explains. By looking at the signals being sent and the signals coming back, engineers found a way to extract rotor dynamics. "We extract phase information that tells how much torque the rotor is seeing," he says. "We scale the signal, find the velocity of the rotor, and then difference that with the command signal to create an active loop."
Zeta contrasts with traditional solutions such as friction rings and ferrofluidic dampers. The former reduce usable torque and eventually wear out, and the latter add inertia and cost--the price of a ferrofluidic damper might be four times the cost of the step motor itself. With a damping ratio of 0.45 at speeds greater than 3 rps, "Zeta exceeds what ferrofluidic dampers give you with no changes to the motor," says John Walewander, product planning manager.
Active damping also eliminates the need for customers to choose an over-sized motor to account for mid-frequency (15 to 20 rps) resonance, ringing, and transient behavior. "We usually recommend that customers dial in a 50% torque-safety margin," says Goodnick, "but you don't need that with this drive."
During the 18 months spent studying sensorless active damping, engineers also discovered a way to detect stalls. "We use the motor terminals to look for an impulse of energy that arises when the rotor loses synchronization with the stator," explains Botka. This feature can save customers the expense of adding an encoder.
While the company makes no specific claims regarding improved high-speed performance, during a demo performed for Design News, engineers ran a standard size-23 motor to more than 300 rps. That compares with a maximum of 50 to 150 rps for a typical motor and drive, they claim.
Skeptical of Zeta's abilities? Don't take Compumotor's word for it. Mary Slaw-ski, an advanced design engineer at 3M, St. Paul, MN, has used Zeta in an R&D project. The application demands a linear positioning accuracy of 0.002-inch with the motor turning at 0.9 rps. "We were using Compumotor's S-Drive, and you could hear it whining," says Slawski.
After trying Zeta, she believes the application can now run open-loop. "The anti-resonance circuitry is pretty incredible," she says. "I've never seen a step motor run that quiet that slow."
Additional details...Contact John Walewander, Parker Hannifin, 5500 Business Park Drive, Rohnert Park, CA 94928, (800) 358-9068.
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