Investigation of nonlinearities in the force control of real robots

Some practical issues associated with the force control of manipulators are investigated. How the stability of a force controlled system is affected by a variety of inherent manipulator nonlinearities, such as control signal saturation, slip-stick friction, and sampled-data controller implementation is examined. In order to improve stable system performance, the inclusion of a high gain inner position loop with environmental force compensation is explored. It is demonstrated that the inclusion of the inner position loop will minimize the effects due to slip-stick friction and thereby improve the predictability in the steady-state error. A discrete-time, nonlinear robotic plant model suitable for force control investigations is developed and is demonstrated to provide an accurate prediction of actual system responses. Theoretical conclusions are supported by experiments performed with the PUMA 560 industrial robot testbed facility developed at Colorado State University