Stepper, Torque & Micropositioning Motors

A stepper drive commands a rotating stator-field sequence. The rotor settles at an energy minimum relative to that field. Full-step, half-step, and microstep modes change current references; none guarantees that the mechanical load actually follows.

Static load angle #

For a simplified two-phase motor under sinusoidal excitation, torque is approximately

$$T=T_{max}\sin\delta,$$

where $\delta$ is displacement between commanded field and rotor. Load torque consumes angular margin. If acceleration plus disturbance exceeds available torque, the rotor slips one or more electrical equilibria and open-loop position becomes wrong without an electrical fault.

Current regulation #

Winding inductance limits how quickly current rises: $di/dt=(V-Ri)/L$. A higher DC bus with chopped current regulation reaches commanded current faster than a low-voltage resistive drive, improving high-speed torque. Driver current limit must match phase current definition and motor thermal rating.

Microstepping #

Ideal sine/cosine phase currents subdivide the electrical step and reduce low-speed ripple. Microstep count is command resolution, not guaranteed accuracy. Detent torque, friction, current mismatch, magnetic harmonics, and load torque cause nonuniform microstep positions.

Motion profile and resonance #

Instantly requesting high step frequency demands impossible acceleration. Use bounded acceleration/jerk profiles that fit torque-speed capability and reflected inertia. Mechanical resonance can amplify oscillation or cause stall in a mid-band; damping, current shaping, load changes, or feedback may be required.

Torque motors are designed for controlled torque at low or zero speed; micropositioning motors optimize fine motion and often rely on gearing or feedback. Classify them by required output and thermal duty, not by historical name alone.

Verify phase wiring, current waveforms, static direction, low-speed smoothness, acceleration margin, worst load, hot resistance, supply regeneration, and lost-step detection. If absolute position matters after disturbance or power loss, use a reference or encoder.