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.
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
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For industrial control applications, or even a simple assembly line, that machine can go almost 24/7 without a break. But what happens when the task is a little more complex? That’s where the “smart” machine would come in. The smart machine is one that has some simple (or complex in some cases) processing capability to be able to adapt to changing conditions. Such machines are suited for a host of applications, including automotive, aerospace, defense, medical, computers and electronics, telecommunications, consumer goods, and so on. This discussion will examine what’s possible with smart machines, and what tradeoffs need to be made to implement such a solution.