Motors, Drives & Embedded State Machines

An actuator system includes load, mechanism, transmission, motor, drive, power source, feedback, brake, and software. The motor alone is not the axis.

Peak and RMS torque

For cycle segments with torque \(T_i\) lasting \(t_i\), a simple RMS estimate is

If an axis needs 4 N·m for 1 s, 1 N·m for 3 s, and 0 for 2 s, \(T_{RMS}=\sqrt{(16+3)/6}=1.78\) N·m. Peak capability must cover 4 N·m; continuous thermal capacity must cover the duty-cycle demand under real cooling. Check motor and drive speed-torque curves, bus voltage, regenerative energy, and ambient derating.

Reflected inertia and ratio

A reducer can improve torque matching but reflects load inertia by approximately \(J_L/N^2\) to the motor for reduction ratio \(N\). Higher ratio reduces reflected load but raises motor speed, transmission losses, compliance, and sometimes backlash. Optimize the complete move, not just torque.

Make software behavior explicit

Define states such as PowerOff, Initializing, Homing, Ready, Running, Paused, Faulted, Recovering, Maintenance. Each state owns outputs; each transition has a trigger, guard, action, timeout, and failure destination.

Example: Ready → Running requires start request, guards closed, axis referenced, part present, downstream ready, no active fault, and safety function ready. A start request is not permission by itself.

Separate control from safety

Standard control software sequences production. Safety-related control performs specified risk-reduction functions with suitable architecture and validation. Do not recreate a safety function as a casual Boolean in application code.

Exercise

Size a motor-reducer for a rotary indexer through acceleration, cruise, dwell, and deceleration. Calculate peak and RMS torque, maximum speed, reflected inertia, and stopping energy. Then draw its state machine including failed homing, sensor disagreement, drive fault, power loss, and manual recovery.