Maintaining floor-foot contact of a biped robot by force constraint position control

This paper presents a method of limiting referential joint torques so as to prevent tipping, sliding, twisting and excessively large ground reaction forces (GRF) at the foot This method only requires the referential torques, the manipulator Jacobian and the force sensor data as input, a ZMP trajectory is not necessary. The method works by using joint space position control and workspace force control in series. The torques output by the position controller are converted into the GRF´s that they would produce, this GRF is then limited and realized with force control. In effect the position controller becomes a reference force generator. Some of the advantages of this approach is that it doesn´t require a ZMP (Zero Moment Point) trajectory, each joint can be moved independently (assuming DOF ≤ 6), has a equivalence to pure position control and therefore equivalent positioning performance as long as no force limiting is required) and can constrain all 6 axes of the GRF. The method is verified experimentally with the MARI-3 biped robot as well as simulations of walking (flat and sloped ground) and jumping.