Glenn R. Widmann

A force commanded impedance control scheme for robots with hard nonlinearities

This paper investigates a hybrid impedance control scheme that utilizes a desired force, rather than a desired position, as the commanded variable. This scheme is examined for a manipulator system in the presence of inherent hardware nonlinearities, and it demonstrates improved performance with respect to an explicit force control structure of a similar degree of robustness. Theoretical conclusions are supported experimentally on a customized Puma 560 Robotic Testbed facility, developed by the authors.

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. >

Practical aspects on robust control strategies of a single link flexible manipulator

An investigation is conducted of an adaptive approach to the control of a flexible robotic manipulator in the presence of changing payload. The approach is a combination of indirect adaptive control and gain scheduling. A lookup table of precalculated controller parameter vectors for a set of possible payloads is utilized. The payload in the lookup table closest to the actual payload provides the initial controller parameter vector that sets the starting adaptation point, and online recursive-least-squares identification of the plant parameters, together with online pole placement controller design, allows fast tuning of the adaptive controller. Simulation experiments, using realistic data obtained from the CSU experimental flexible manipulator testbed, show the feasibility of the approach.<<ETX>>

A new model for nonlinear friction compensation in the force control of robot manipulators

This paper introduces a new model which is suitable for nonlinear friction feedforward compensation in the force control of robot manipulators. This application is experimentally demonstrated.

Force control of robotic manipulators with structured uncertainties using variable structure control

Abstract A discrete variable structure tracking controller is developed for a wide class of linear and nonlinear systems with structured uncertainties. The proposed controller uses a pole placement controller as a kernel and compensates for the errors associated with uncertainties. The algorithm is computationally efficient and can be easily used as a robust backup controller for the higher performance, but sensitive, adaptive algorithms.

Force Control of Robots with Nonlinearities

This paper explores two practical issues related to the force control of manipulators. The first issue examined is how system stability is effected by commonly occurring manipulator nonlinearities, such as sampled-data, control signal saturation and slip-stick friction. It is shown that discretely implemented force control algorithms can drive the feedback force controlled manipulator into a limit cycle, even for a very small sampling period that by far satisfies Shannons sampling theorem. The bounds of stability are enhanced by the presence of control signal saturation and slip stick friction. The second issue investigated is the inclusion of a high gain inner position loop as a means to minimize the unpredictability in the steady state error due to slip-stick friction. In order to support the theoretical conclusions, experiments were performed with the PUMP 560 industrial robot testbed facility developed at Colorado State University.

Robustness Investigation of Adaptive Control Under Reference Model Switching for a Cylindrical Robot

Space robots are expected to play a significant role in future space missions. This requires a thorough investigation into the capabilities of such robots. A typical task that will need to be performed by the space robot is the capture and manipulation of large objects. This could increase the mass of the robot by as much as 100%. Additionally, the object capture task will require the robot to handle high dynamic disturbance. The control system of the robot must maintain stability under these harsh conditions. In this paper, the authors have conducted an investigation into the robustness properties of a new robot control strategy based on the Direct Model Reference Adaptive Control (MRAC) Algorithm under switching of the reference model. Experimental results of the investigation are presented for a three degree-of-freedom cylindrical robot. The new control scheme is demonstrated to be a powerful approach to the difficult problems that will be encountered in space robotics control.