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     Stepper motors are employed in a variety of applications across the engineering spectrum because they are inexpensive, simple to operate, and offer high torque at low speeds. The primary drawbacks of stepper motors are lack of position feedback and stalls; the user assumes that the stepper 'got there.' The addition of an encoder to the system can mitigate these drawbacks but at the cost of increased move and settle time. Galil's new Closed Loop Stepper Firmware overcomes these drawbacks while keeping move and settle times low. The result is precise motion and positioning with the low speed/high torque benefits of a stepper motor.

     Galil's Closed Loop Stepper (CLS) Firmware employs a Control Filter somewhat similar to standard PID Control. The Control Diagram in Figure 1 lays out the basics. An encoder, either directly coupled to the motor or to the load, relays actual position data to the filter. From this actual position and the profiled reference position, a position error is calculated and input into the CLS Control Filter. Here, a step pulse frequency is generated based on position error and a handful of user settable gains. The step pulses are fed into a stepper motor driver and motor, causing motion of the load and end encoder. At a servo update rate of 1kHz, this control loop constantly monitors position error and adjusts the stepper pulse frequency to minimize it.

Figure 1: Closed Loop Stepper Firmware Control Diagram

     Stepper motors are still bound by Physics and that means that their other primary drawback, stalls, must be accounted for. Galils CLS firmware allows the user to set a maximum pulse frequency for each axis and avoid stalls during operation. In addition, to eliminate the dither that can result from a constantly 'hunting' servo system, the CLS firmware includes a deadband function which deactivates the CLS filter when the position error is within a user defined range. An added benefit to this control scheme is that if the encoder is coupled to the load, backlash due to gearing or other mechanics is already accounted for. If the Load isn't in position, the CLS filter will simply keep putting out step pulses until it is.

     Galil continues to provide cost effective and innovative solutions for motion control in both industry and research. The new Closed Loop Stepper Firmware is available on the DMC-40x0 series and provides yet another tool to those looking for the most flexible and robust motion solutions. Contact a Galil Applications Engineer or email for more information.

For the full application note, see Application Note 3503.