| Traditionally, control of haptic interfaces is achieved using open-loop impedance or admittance control. During open-loop control, the quality of haptic feedback is limited by the dynamics of the haptic device. Further improvements in quality of the haptic feedback require the use of closed-loop control techniques. Closed loop control of haptic interfaces is limited due to stability and cost considerations associated with closed loop force control. In this work, a non-linear disturbance observer for estimating human-machine contact forces during haptic interactions is presented. These estimated forces are then used for closed loop impedance control of the haptic interface. Globally exponential stability of the disturbance observer under the assumption of slowly varying disturbances is demonstrated. Finally, simulation and experimental results corresponding to disturbance observer based closed-loop control of a haptic interface are provided. Qualitative as well as quantitative comparison of the performance of disturbance observer-based control with traditional open and closed loop controllers is presented. In addition to control of haptic interfaces, the disturbance observer can be employed for contact force estimation between the slave and environment during teleoperation. |
