Manual Tuning of the position control loop: Cascaded controller

The cascaded controller can be tuned manually, which can become necessary if:

  • The auto-tuner cannot be run due to mechanical limitations, attached robot arms or other restrictions
  • The auto-tuner fails or does not give sufficient results for a very specific application scenario
  • For experimental purposes

As shown in 3.3, the cascaded controller implements two PI controllers, one on the position loop and one on the velocity loop. The one on the velocity loop uses different parameters than the pure velocity controller (the velocity controller which is not part of the cascaded position controller). You can tune both independently.

To avoid instabilities caused by the cascaded controller, it is recommended to use only one of the integrators at a time: Either the integrator on the position loop or the integrator on the velocity loop. When the integrator on the position loop is used, the gain for the velocity integrator should be set to 0. This configuration is refered to as PI-P. When the integrator on the velocity loop is used, the gain for the position integrator should be set to 0. This configuration is refered to as P-PI.

These initial parameters can be used as a starting point for cascaded position control-loop:

  • Position P: 0.05.
  • Position I: 0 for P-PI configuration. 0.5 otherwise.
  • Velocity P: 0.5;
  • Velocity I: 0 for PI-P configuration. 0.5 otherwise.

Using these initial values, controller gains could be manually tuned using different methods. See: Cascaded PID controller tuning

The simple PID controller has generally poorer dynamic performance than the cascaded PID controller. However, it can be used for experimental investigations as the easiest configuration. Manual tuning is mandatory.

The following initial parameters could be used:

  • Position P: 0.01
  • Position I: 0.02
  • Position D: 0.001

Using these initial values, controller gains could be manually tuned using your preferred methods. Classical tuning methods for PID controllers like Ziegler-Nichols can be applied here.

The PI-P cascade controller may look similar to the simple PID controller, however the structure is different and therefore a different tuning method should be used.