Hip Strategy Experiments

I took some video yesterday demonstrating new robot balance control that can perform a "hip strategy". Basically there are multiple objectives, one to keep the center of mass (COM) over the feet and another to keep the torso upright. Keeping the COM over the feet is more important, so after a large push the robot will decide to swing its torso forward to achieve that objective. Once it is stabilized, it can return to an upright posture. The video demonstrates pushing from both directions in the sagittal plane which is the more unstable direction.

The basic controller is similar to the one used in [1], which uses optimization to trade off between several objectives related to balance. In this case, there are objectives related to regulating the COM, regulating posture, regulating angular momentum, and minimizing joint torques and contact forces. Inequality constraints are imposed to keep the feet flat on the ground and avoid joint torque limits. This is a very generalizable and easy to implement approach to full body balance control.

[1] Y. Abe, M. da Silva, and J. Popovic, “Multiobjective control with frictional contacts,” Proceedings of the 2007 ACM SIGGRAPH/Eurographics symposium on Computer animation, 2007, pp. 249-258.