Planar Biped (1985-1990)


We built the Planar Biped, a planar two-legged running machine, to test the control of bipedal running using the one-leg algorithms.

The machine has two telescoping legs connected to the body by pivot joints at the hips. A hydraulic actuator within each leg works in series with a pneumatic spring. Together, they change the length of the leg and make the leg compliant along its long axis. The body is an aluminum frame, on which are mounted hip actuators and computer interface electronics. The arrangement of body, legs, hips, and actuators provides a means to control the position of the feet with respect to the body, to generate an axial thrust with each leg, and to provide hip torques during running.

The algorithms used to control machines that hop on one leg can be extended to control a planar biped, which runs on two legs. The basic approach is for the control system to designate an active leg and an idle leg. Because there is just one active leg at a time, the one-leg algorithms can be used to control the biped's behavior. These algorithms focus on controlling hopping height, forward running speed, and body posture. The idle leg is kept short while it is made ready for the next step. Using this approach, the planar biped runs with an alternating gait or a hopping gait, and can change gaits while running. We have used the planar biped to study locomotion on rough terrain, running at high speed, and gymnastic maneuvers. The control program that produces a flip uses open-loop control patterns in conjunction with the algorithms for normal running.

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