Absolutely stable force control for telerobotic applications [electronic resource]
Keywords: Mechanical Engineering

The objective of bilateral telerobotic control architectures has long been to provide as tight a connection as possible between a human operator and the remote environment. Ideally the telerobot would be completely transparent allowing the user to feel as if they were directly touching the remote environment. The physical reality of remote manipulation requires the telerobot to consist of a master and a slave robot as well as a control system to connect the two devices. Unfortunately, all three of these systems have dynamics that limit the transparency of the overall telerobot. In particular, transparency is severely limited when using large industrial-like slave robots. The large internal friction forces, arising from high gear ratio actuators, as well as the large inertial forces, arising from the heavy linkages and reflected motor inertia, make it very difficult for the user to distinguish between forces arising from contact with the remote environment and forces arising from the natural dynamics of the slave robot. To improve transparency of telerobots using industrial-like slaves, force control can be applied around the slave device in an attempt to ensure the force applied to the environment by the slave tracks the desired force generated by the telerobotic control algorithm. Most force control algorithms however are plagued by instability when trying to make and maintain contact with stiff environments resulting in a persistent and potentially dangerous hammering of the environment by the slave known as contact instability. The focus of this thesis is to develop a force control algorithm that can be applied to an industrial-like slave device to improve the transparency of the overall telerobot without decreasing stability. Included in this thesis is an analysis of the dynamic interaction between the slave robot, the force control algorithm, the remote environment, and the human operator which shows that contact instability is caused by unmodeled yet fundamentally unavoidable lags in the control system such as amplifier roll-off or sensor bandwidth limitations. The analysis further shows that in order to have guaranteed stability with all possible combinations of human and environment impedances, a condition known as absolute stability, it is very difficult if not impossible to hide any of the slave's inertia from either the environment or the user. Based on this analysis, a model-based force control algorithm is developed that focuses control effort on rejecting the friction in the system without attempting to hide any of the slave's mass. The controller simulates a frictionless model of the robot used to provide the ideal trajectory the robot would take in response to the forces applied by both the user and the environment. A velocity controller is then used to make the robot track this idealized trajectory. This model-based force controller achieves perfect steady state force tracking when in contact, and is provably absolutely stable both for linear one degree of freedom (DOF) robots and nonlinear multi-DOF robots. This model-based force control algorithm represents a significant contribution to the field of telerobotics because it allows control engineers to utilize preexisting and well understood telerobotic control algorithms originally designed for slaves with minimal friction on telerobots using industrial-like slaves where friction is clearly a dominate force. Analytical and experimental results show that the addition of model-based force control around these industrial-like slaves improve the transparency of the telerobot, regardless of the specific telerobotic control architecture being used, without decreasing the overall robustness of the system.