Zaber’s T-Series of computer-controlled positioning devices use stepper motors to achieve open loop position control. These devices turn by a constant angle, called a step, for every electrical impulse sent to them. This allows a system to be built without feedback, reducing total system cost. It is also possible to rotate the motor by an angle less than a step by using a technique called microstepping. This consists of using an analog current in the motor coil in place of full current switching. Stepper motor technology is well described in many books and websites. However, being incremental (as opposed to absolute) in nature, the stepper motor must initially be zeroed by going to a home sensor. Also, if the torque required becomes higher than the maximum torque (or stall load) of the motor, a stall condition will occur and the motor will stop turning. To avoid this situation, try lowering the speed, the acceleration, or the load. Stalling does not damage the motor. However, because there is no position feedback, the actual position of the device will become different from the position indicated by the controller.
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