Closed-Loop Electric Drives & Cascaded Control
High-performance drives use nested current, speed, and position loops because electrical torque responds faster than mechanics; bandwidth separation, saturation management, sensing, and safe state make the cascade work.
A servo drive normally closes an inner current/torque loop, a slower speed loop, and sometimes a still slower position loop. The cascade matches physical time scales: the converter changes winding voltage quickly, current produces torque, torque accelerates inertia, and position integrates speed.
Loop responsibilities #
| Loop | Command | Measurement | Dominant limit |
|---|---|---|---|
| current | torque-producing current | phase/DC current | bus voltage, switching, thermal current |
| speed | velocity | encoder/resolver/estimate | torque/current and mechanical resonance |
| position | angle/displacement | position feedback | speed/acceleration/travel |
The inner loop must be substantially faster than its outer consumer so the outer loop can treat it as a controlled actuator. Exact bandwidth ratios depend on delay and resonances; blindly using a fixed decade rule is not a stability proof.
Saturation and anti-windup #
During acceleration the speed controller may demand more current than allowed. Without anti-windup its integrator continues accumulating, producing overshoot and delayed recovery after the limit clears. Likewise the current controller saturates when requested voltage exceeds the DC-bus/modulation limit, especially at high speed. Limit status must propagate outward so each loop knows what authority remains.
Feedforward and decoupling #
Velocity and acceleration feedforward can supply predictable friction and inertia torque while feedback corrects model error. FOC adds cross-coupling and back-EMF compensation. These improve tracking only when sign, units, timing, and parameters are correct; they do not replace a stable feedback loop.
Sensor and delay boundaries #
Quantization causes low-speed limit cycles; filtering adds phase lag; sample/compute/PWM delay consumes stability margin; encoder eccentricity creates periodic velocity ripple after differentiation. Sensorless observers have operating regions where state is weakly observable.
Commission from inside out #
- Verify current-sensor offset, polarity, phase mapping, and hardware trip with power limited.
- Tune/validate current response and voltage saturation.
- Verify feedback direction before enabling the speed integrator.
- Identify resonance and set speed bandwidth below unsafe modes.
- Add position control, limits, feedforward, and trajectory shaping.
- Test current limit, blocked rotor, feedback loss, overvoltage during deceleration, end stops, and restart state.
Capture command, feedback, error, controller output, every active limit, bus voltage, and fault state on the same timebase. Otherwise a cascade can make an outer-loop symptom look like an inner-loop tuning problem.