We report on the development of a motion generation strategy that allows the robot to transit from one stable running motion to another in one stride by actively changing leg stiffness in real time. Stable motion of the robot is generated based on the stable fixed-point trajectories of the spring-loaded inverted pendulum (SLIP) model. While the transition of the ordinary SLIP model with fixed parameters gradually converges if stable, a robot that uses force control to simulate the passive spring of the SLIP can actively modulate leg-spring stiffness. This enables the robot to switch instantly to another fixed-point trajectory of the SLIP model without going through multi-stride transition. The proposed method is implemented on a leg-wheel transformable robot, TurboQuad, and is evaluated experimentally. The results confirm that the robot can successfully transit between different fixed-point trajectories.

This report is on the design, control strategy, implementation, and performance evaluation of a novel leg-wheel transformable robot called TurboQuad, which can perform fast gait/mode coordination and transitions in wheeled mode, in legged trotting, and in legged walking while in motion. This functionality was achieved by including two novel setups in the robot that were not included in its predecessor, Quattroped. First, a new leg-wheel mechanism was used, in which the leg/wheel operation and its in situ transition can be driven by the same set of motors, so the actuation system and power can be utilized efficiently. Second, a bio-inspired control strategy was applied based on the central pattern generator and coupled oscillator networks, in which the gait/mode generation, coordination, and transitions can be integrally controlled. The robot was empirically built and its performances in the described three gaits/modes as well as the transitions among them were experimentally evaluated and will be discussed in this paper.