The Zero-G robot is designed for running in zero-gravity. Normally it is not possible to run without gravity, because the upward motion that initiates the flight phase can not be reversed once contact with the ground is lost. One way to overcome this limitation is to run between two parallel rebound surfaces. In this case, the supporting forces generated in the collision with one surface reverse the vertical velocity generated during collision with the other surface. Such a configuration for running might occur in a space station, where the walls could act as the rebound surfaces.
The Zero-G robot consists of a double ended leg with a common hip connecting it to the body frame. The leg has two pneumatic cylinders which act as both air springs and thrust actuators during the stance phase. Two rubber feet are attached to the ends of the cylinder rods. A frame made out of half inch birch plywood holds electronics and sensors on the top level, air tank, batteries, and hip actuator on the middle level, and air bearings, leg and thrust actuators on the bottom level. Three air bearings attached to the bottom of the frame float the machine on the floor and allow it to slide with very little friction.
The machine uses a control strategy developed for control of hopping machines. Algorithms that control running in zero-gravity can be similar to those used in one-g, to the extent that behavior during ground collision in normal running is dominated by the need to reverse the vertical velocity of the body, rather than oppose the acceleration of gravity. Thus, the control algorithms developed for a bipedal running machine are adequate to control running between two walls in zero-gravity. This is possible because the running cycles of both machines are the same, and the absence of gravity has a small effect on the dynamics of the machine.