This paper presents the implementation of impedance control for a hydraulically driven hexapod robot named COMET-IV, which can walk on uneven and extremely soft terrain. To achieve the dynamic behavior of the hexapod robot, changes in center of mass and body attitude must be taken into consideration during the walking periods. Indirect force control via impedance control is used to address these issues. Two different impedance control schemes are developed and implemented: single-leg impedance control and center of massbased impedance control. In the case of single-leg impedance control, we derive the necessary impedance and adjust parameters (mass, damping, and stiffness) according to the robot legs' configuration. For center of massbased impedance control, we use the sum of the forces of the support legs as a control input (represented by the body's current center of mass) for the derived impedance control and adjust parameters based on the robot body's configuration. The virtual forces from the robot body's moment of inertia are adapted to achieve optimal control via a linear quadratic regulator method for the proposed indirect attitude control. In addition, a compliant switching mechanism is designed to ensure that the implementation of the controller is applicable to the tripod sequences of force-based walking modules. Evaluation and verification tests were conducted in the laboratory and the actual field with uneven terrain and extremely soft surfaces. C 2011 Wiley Periodicals, Inc.

This article describes the proposed force-based walking method for hydraulically driven hexapod robot named COMET-IV, to walk on the large scale rough terrain. The trajectory is designed where foot step motion for each leg is decided by vertical force on the foot that is calculated from cylinder torque of thigh and shank. This proposed walking trajectory is established with compliant control strategy, which consists of force control based on position range from the trajectory motion signal. This force controller is dynamically control ON/OFF by proposed decision algorithms that derived from the changes of kinematic motion of the trajectory itself. In addition logical attitude (body) control is designed as a part of the decision control module that makes a pre-calculation of decision making based on leg sequence changes. For more stability dynamic swings raising control is derived from trajectory equations to perform a different degree of swing rising for each leg when the robot stepping on the different level of terrain. All proposed controllers are verified in the COMET-IV actual system with walking on the designed rough terrain platform consists of random levels of hard bricks and rubber pads.

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