Adaptive impedance control with variable target stiffness for wheel-legged robot on complex unknown terrain

Wheel-legged robots operating on the ground experience real-time interactions with the complex unknown terrain, which may lead to tilting of the whole body and instability if no regulated effort is made. Maintaining a horizontal posture of the whole body with changes in the terrain geometry via impedance control (IC) that is widely used in many fields is desirable to be realized. However, because the stiffness and location of the terrain relative to the robot are not known in advance, the force-tracking error occur when using IC, which is the main cause of robot tilting. In this paper, an adaptive variable impedance control (AVIC) method is proposed to minimize the force-tracking error for the forces of each leg that are exerted on the body, thereby maintaining a horizontal posture of the whole body and improving the stability. This control method is applied by adjusting the target stiffness to compensate for terrain uncertainties. In terms of the existence of the dynamic force tracking error, the proposed control method also allows the robot to adapt to changes to track the desired force. The theoretical analysis of the stability of the AVIC was demonstrated through a stable force-tracking application. The numerical and experimental results were compared to those obtained using IC, and the proposed control method was validated on complex, unknown terrain.