Impedance control of a hydraulically actuated robotic excavator

In robotic excavation hybrid position/force control has been proposed for b ucket digging trajectory following. In hybrid position/force control, the c ontrol mode is required to switch between position- and force-control depen ding on whether the bucket is in free space or in contact with the soil dur ing the process. Alternatively, impedance control can be applied such that one control mode is employed in both free and constrained motion. This pape r presents a robust sliding controller that implements impedance control fo r a backhoe excavator. The control law consists of three components: an equ ivalent control, a switching control and a tuning control. Given an excavat ion task in world space, inverse kinematic and dynamic models are used to c onvert the task into a desired digging trajectory in joint space. The propo sed controller is applied to provide good tracking performance with attenua ted vibration at bucket-soil contact points. From the control signals and t he joint angles of the excavator, the piston position and ram force of each hydraulic cylinder for the axis control of the boom, arm, and bucket can b e determined. The problem is then how to find the control voltage applied t o each servovalve to achieve force and position tracking of each electrohyd raulic system for the axis motion of the boom, arm, and bucket. With an obs erver-based compensation for disturbance force including hydraulic friction , tracking of the piston ram force and position is guaranteed using robust sliding control. High performance and strong robustness can be obtained as demonstrated by simulation and experiments performed on a hydraulically act uated robotic excavator. The results obtained suggest that the proposed con trol technique can provide robust performance when employed in autonomous e xcavation with soil contact considerations. (C) 2000 Elsevier Science B.V. All rights reserved.