Advanced motion control: From classical PID to nonlinear adaptive robust control

The ever increasingly stringent performance requirements of modern mechanical systems have forced control engineers to look beyond traditional linear control theory for more advanced nonlinear controllers. During the past decade, a mathematically rigorous nonlinear adaptive robust control (ARC) theory has been developed and has been experimentally demonstrated achieving significant performance improvement in a number of motion control applications. This plenary paper first uses a simple motion control problem as an example to bring out the conceptual connection and nonlinear extension of the widely used PID controller structure to the developed ARC approach. Through this example, some of the key underlying working mechanisms of the ARC theory can be grasped easily. The paper then highlights how major issues in the precision motion control can be handled systematically and effectively with the ARC framework. The issues considered include (i) large variations of physical parameters of a system; (ii) unknown nonlinearities such as cogging and ripple forces of linear motors; (iii) dynamic uncertain nonlinearities with non-uniformly detectable unmeasured internal states (e.g., friction described by dynamic models in high precision motion controls); and (iv) control input saturation due to limited capacity of physical actuators. The precision motion control of a linear motor driven high-speed/high-acceleration industrial gantry is used as a case study and comparative experimental results are presented to illustrate the achievable performance and limitations of various ARC controllers in implementation.