Variable structure control of constrained dynamic systems

A modified computed torque controller has been developed by N.H. McClamroch and D. Wang (1987). If the mathematical model of the robot is exact, the modified computed torque can simultaneously control the robot motion and contact force in an accurate way. However, there may exist uncertainties in the model, such as flexibility of joints and links, joint friction, and an inexact surface model. It is shown that the modified computed torque controller may result in an unstable closed-loop system for the system with uncertainties. This difficulty can be overcome by using a variable structure controller. The controller is robust in that it is insensitive to variations in the plant parameters and to external disturbances of contact force