Cascaded position control: How it works
What is cascaded position control?
Cascaded position control is the standard controller architecture for servo drives with position control. Instead of converting the position error directly into a torque command, several nested control loops are used—an outer position control loop, a middle speed control loop, and an inner current/torque control loop.
Why is this important?
A single PID controller that directly converts the position error into a torque command (known as a "simple PID") is mathematically simple but has significant limitations in practice. The cascaded architecture offers decisive advantages:
- Higher control quality — each control loop can be optimized independently
- Better stability — the fast internal control loop stabilizes the system before the slower external control loop intervenes.
- Natural limitation — Speed and current limits can be inserted directly between the cascade stages.
- Simpler tuning — tuning is done from the inside out, with each stage assumed to be stable
How does it work?
The three control loops operate at different frequencies — the inner control loop is the fastest:
1. External control loop — position controller (4 kHz): Compares the target position with the actual position and calculates a target speed using a PID controller. In normal operation, the PI-P configuration is used: proportional and integral components in the position controller, proportional component only in the speed controller.
2. Middle control loop — speed controller (4 kHz): Compares the target speed with the actual speed and calculates a target torque command. Anti-windup limitation protects the integrator from overflow.
3. Internal control loop — current/torque controller (16 kHz): Converts the torque command via field-oriented control (FOC) into PWM signals for the motor phases. On SOMANET drives, this control loop works with model predictive deadbeat control.
Tuning strategy: Tuning is always done from the inside out—first the speed controller (increase Kp to the stability limit, then reduce to 90%), then the position controller. Integral components are added last and carefully.
The parameters are stored in the Position Controller Object (0x2012): Sub-indices 1–4 for the position controller (Kp, Ki, Kd, integral limitation), sub-indices 5–8 for the speed controller.
How does SOMANET implement this?
Cascaded position control is the standard control mode on SOMANET drives. OBLAC Drives offers auto-tuning, which performs system identification and automatically calculates optimized gains for both cascade stages. The gains can then be fine-tuned manually.
In addition to cascade mode, a "simple PID" mode is available, which converts the position error directly into a torque command. However, Synapticon recommends cascade mode for production applications.
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