The design of a stepper motor torque characterization system

Stepper motors are used in many devices in which precise motion control or positioning control is required. The dynamic torque characteristics of stepper motors become important to the design engineer when selecting a stepper motor to fit the dynamic design requirements of any prototype system. However, accurate dynamic torque data for stepper motors is usually not readily available. Many factors affect the dynamic characteristics of stepper motors, including external inertia, coupling type, driver type, driver voltage, phase current and the design of the particular stepper motor in question. These factors may all serve to alter the dynamic characteristics of a stepper motor as it is moved from one system to another. The stepper motor torque characterization system described in this thesis has been designed to provide accurate dynamic torque data and holding torque data for stepper motors operating in known environments.

One major design requirement for the stepper motor torque characterization system was automatic control over all test parameters and data logging. Other design requirements included the ability to test stepper motors up to 300 oz-in holding torque in half step or full step mode, and the ability to simulate the torque characteristics of either an accelerating or braking load applied to the stepper motor shaft.

The stepper motor torque characterization system described in this thesis meets the specified functional requirements. The torque characterization system can test stepper motors up to 400 oz-in holding torque at test speeds up to 1500 RPM. The theoretical accuracy of the torque characterization system has been held to +1.47 oz-in and -3.94 oz-in over the full scale of a test.

This thesis discusses the design methodology of the torque characterization system, and presents the reader with an overview of the features and operation. The system performance data and an evaluation of the system characteristics are included along with failure modes, recommendations, instructions and a troubleshooting guide.