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Pseudo Servos Coming on Strong
A whole slew of new motors is mimicking servo performance, thanks to new developments in software and hardware
Randy Frank -- Design News, January 9, 2005
The traditional operating area for stepper motors versus servo motors is extended when an encoder and algorithms are added. These psuedo servos still operate open loop, but they have the capability to function in closed loop and avoid getting out of step. While not new, they are growing in popularity, and they are expected to have a significant impact in some applications. Dan Jones, president of Incremotion Associates Inc., coined the term pseudo servo. "You had open loop and closed loop and now you have a number of interesting in betweens," says Jones.
Oriental Motor's AlphaSTEP integrates a feedback device that constantly monitors the motor shaft to detect missed steps and correct for loss of synchronism. Its newer AlphaSTEP PLUS combines the stepping motor with a built-in resolver concept with an integrated programmable motion controller and driver.
Nick Johantgen, engineering manager, Oriental Motor USA Corp., explains, "The AlphaStep has a built in resolver feedback mechanism, but it runs open loop like a conventional stepper motor until it notices that it is about to miss steps (within a ±1.8 window)." If operation is outside of this window, the feedback mechanism provides information to get to the correct position.
Using encoder-based feedback to determine position error, QuickSilver Control Inc.'s (QCI) SilverDust controller dynamically varies the motor torque to achieve the desired position. With this approach, QCI claims to offer servo systems at stepper prices and enhancements to either type of motor. They commutate a high pole motor (step motor) to form a high torque servo motor that does not generate torque when the load is removed. Software in an advanced digital signal processor (DSP) provides a tuning procedure that permits 100:1 reflected inertial mismatch. In addition, an Anti-Hunt™ command eliminates typical servo dithering.
Compumotor uses a standard stepper motor driver with more electronics to provide increased functionality without adding a sensor to the motor. "When we've asked our customers the reason that they wanted to put a feedback device on the back of a step motor, it wasn't for dynamic performance," says Marketing manager John Walewander. "It was mostly for detecting stall." The digital circuitry in Parker Compumotor's Gemini GT6 provides both encoderless stall detection and active damping. Active damping keeps the motor from vibrating so more usable torque is available and more of the motor's curve can be used without requiring as much safety margin.
According to Oriental's Johantgen, the packaging industry, could be one of the areas that benefit from pseudo servos in 2005.
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| No additional physical sensors are used in Parker Compumotor's Gemini GT6 products. They use signals from the motor and software to provide active damping and encoderless stall detection. |
| Characteristic | Stepper Motor | Pseudo Servo | Servo Motor |
| Operation | Open loop | Open and closed loop | Closed loop |
| Speed | Low | Increased* | High (2 to 4X*) |
| Power | Low | Increased* | High (2 to 4X*) |
| Torque at high speed | Low | Increased* | High |
| Resolution | 200 to 25,000 steps/revolution | Same as stepper with encoder feedback as required | 2,000 to 10,000 pulses/rev from encoder |
| Position accuracy | Can lose steps | Encoder feedback prevents losing steps | Constantly correcting to minimize error |
| Web Resources | ||
| //Check out the links below for more info// |
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Oriental Motor AlphaSTEP and AlphaSTEP PLUS:
http://rbi.ims.ca/4385-516 |
QuickSilver Controls, Inc. SilverDust Controller and Driver:
http://rbi.ims.ca/4385-517 |
Compumotor Gemini GT6 Stepper Drive/Controller:
http://rbi.ims.ca/4385-518 |
