Coordinating Feet in Bipedal Balance

Abstract

Abstract-Biomechanical models of human standing balance in the sagittal plane typically treat the two ankle joints as a single degree of freedom. They describe the sum of the torques produced by the ankles, but do not predict what the contribution of each ankle will be. Similarly, balance algorithms for bipedal robots control the location of the overall center of pressure, but do not consider the individual centers of pressure under each foot. We present theoretical and experimental results showing an optimal solution to the problem of producing a single desired torque with redundant actuators, resulting in alignment of the individual centers of pressure under each foot. This produces a feedback gain structure not addressed in the biomechanics literature and a balance controller that is potentially more robust to unexpected changes in the region of support. We show that the feedback gain matrix of this controller has an unexpected structure -large off-axis integral gain elements indicate that the ankle torque that equalize the position of the center of pressure is determined primarily by information from the other foot. We also demonstrate controllers based on this design using the Sarcos Primus hydraulic biped.

Multiple balance strategies from one optimization criterion

“…This will require state estimation based on imperfect sensing and dealing with floor compliance. It also requires coordinating the action of both legs and feet [17].…”

Online gain switching algorithm for joint position control of a hydraulic humanoid robot

“…Therefore, electric motors and reduction gears may not be suitable for an adult-sized biped humanoid robot that must be robust to disturbances and uneven terrain. An alternative approach is to use hydraulics for biped humanoid robots [4][5][6][7][8]. Hydraulic actuators for biped humanoid robots can produce a large torque for their size, have efficient power distribution, easy force control and backdrivability, and have a low rotational inertia due to the absence of a reduction gear.…”

Comparison of Trunk and Leg Sway During Single Leg Stance

“…The BOS is formed by both feet if one stands bipedally or by the single standing foot if one stands unipedally. Postural sway has been intensively studied regardless of whether subjects stood on both feet [10,11] or on a single foot [9,12,13]. However, when postural sway was assessed, force plates were often used in taking the measurements, like ground reaction force, the strategy and velocity of the whole body sway [14,15].…”