A rotary motion control system inherently contains plant uncertainties such as unmodeled dynamics, nonlinear friction, plant parameter changes, and external disturbances. Precise motion control requires not only fast response time, high accuracy, and high precision, but also robust performance against internal uncertainties and external disturbances. This implies that the motion control system should follow quickly and precisely the reference command, and compensates adequately for various uncertainties and disturbances.
It is well known that the PID controller has limited capabilities in suppressing steady-state errors, compensating external disturbances and unmodeled nonlinear dynamics, following the prescribed trajectory, and so on. In order to compensate unmodeled nonlinear dynamics such as Coulomb and static friction, we added a friction compensator based on friction modeling and verified its effectiveness against nonlinear friction components through simulations and experiments. In order to provide robustness to the motion control system, we have included the disturbance observer (DOB) that is effective against rotary inertia changes, step and sinusoidal external disturbances.
For precise track-following performance, we designed and tested two kinds of feedforward tracking controllers: the zero phase error tracking controller (ZPETC) and the zero phase error tracking controller-alternative (ZPETC-alt). Since ZPETC and ZPETC-alt are not robust against internal and external disturbances, we have added the disturbance observer in the feedback loop and tested their performance under various conditions through simulations and experiments.
In order to achieve the ultimate tracking performance of the rotary motion control system, we designed the high-performance position controller (HPPC) that includes the PID controller, the friction compensator, the disturbance observer and the zero phase error tracking controller. We have verified its excellent tracking performance by simulations and experiments. This confirms that the combination of purpose-designed controllers can perform better than a single general purpose controller such as the PID controller.