Tail control decreases jamming of a mechanics-dominated legged robot on rough terrain

To develop robot locomotion principles which leverage mechanics to simplify control, we study a minimally aware quadrupedal RHex-type robot (~3kg, 20 cm long) walking on a rough terrain composed of blocks with Gaussian height distribution with mean and variance approximately leg length and one-quarter leg length respectively. This terrain resulted in failure modes due to limb jamming in protruding regions of the terrain. Performance was insensitive to gait symmetries (phasing and duty factor of limbs). To reduce jamming probability while maintaining the focus on minimal control and mechanics, we added a motor driven 10 cm long tail whose pitch and yaw actuation was uncoupled to limb dynamics. We experimented with a variety of open loop behaviors such as tapping, pulsing, and maintaining a constant angle with the body. We also developed a closed loop tapping scheme that triggered on IMU measured pitch and roll of the robot. Jamming probability could be reduced to near zero for open and closed loop schemes. The appropriate tail behaviors helped maintain either high or low “robot temperature” (variance of instantaneous velocity) relative to tail-less robot temperature.