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.

We report on the model-based approach to dynamic trotting and pronking gait generation for a leg-wheel transformable robot. The rolling spring-loaded inverted pendulum (R-SLIP) model served as the template for robot locomotion by programming the robot's motion according to the stable fixed-point trajectories of the model. Two strategies are developed to match the robot leg motions to the virtual model leg: First, using the two active degrees of freedom on each leg to simulate the passive spring effect of the R-SLIP model leg. Second, installing a torsion spring on the leg-wheel module to render the leg-wheel morphology identical to that of the model leg. This model-based approach to dynamic behavior generation for the robot is experimentally evaluated. The robot can successfully generate an R-SLIP-like stable pronking gait with a flight phase.

The ability of the robot to negotiate the terrain strongly depends on its morphology and control strategy. Wheeled morphology is extremely good on the flat terrain, and the legged morphology has great mobility on the rough terrain, as proved by the legged animals. Thus, as the terrain nowadays is highly mixed with natural rough terrain, artificial rough terrain (ex: stair, bumps, etc), and artificial flat terrain (ex: road), designing a good robotic platform which can operated on all three categories may be a good solution. In the last few decades researchers have come up with two different approaches to elevate robots' adaption to the environment: one is to design special mechanisms to overcome uneven terrain, and the other is to focus on behavioral development of a given robot to adapt different situations. Both approaches exist, but only a few works tried to solve the problem from both aspects simultaneously.