Virtual Point Control for Step-Down Perturbations and Downhill Slopes in Bipedal Running

Abstract

Bipedal running is a difficult task to realize in robots, since the trunk is underactuated and control is limited by intermittent ground contacts. Stabilizing the trunk becomes even more challenging if the terrain is uneven and causes perturbations. One bio-inspired method to achieve postural stability is the virtual point (VP) control, which is able to generate natural motion. However, so far it has only been studied for level running. In this work, we investigate whether the VP control method can accommodate single step-down perturbations and downhill terrains. We provide guidelines on the model and controller parameterizations for handling varying terrain conditions. Next, we show that the VP method is able to stabilize single step-down perturbations up to 40 cm, and downhill grades up to 20–40° corresponding to running speeds of 2–5 ms−1. Our results show that the VP approach leads to asymmetrically bounded ground reaction forces for downhill running, unlike the commonly-used symmetric friction cone constraints. Overall, VP control is a promising candidate for terrain-adaptive running control of bipedal robots.

Positions of pivot points in quadrupedal locomotion: limbs and trunk global control in four different dog breeds

“…Finally, if the VPP is placed caudally to the proximal pivot, as depicted in dogs’ thoracic limb, the torque generated will absorb energy. In this case the leg must add energy axially to the system for the sake of periodicity (Blickhan et al, 2015; Drama and Badri-Spröwitz, 2020). Accordingly, the dogs under study here, except Whippets during walk, add energy axially to the system.…”

“…In bipeds, VPP as the target of control simplifies the balance of the trunk by changing an inverted pendulum control problem into a suspended one. Moreover, simulation studies based on this bio-inspired strategy showed that VPP control can balance not only the trunk but also generate stable bipedal gaits (Andrada et al, 2014; Drama and Badri-Spröwitz, 2020; Maus et al, 2010). So far, the existence of a VPP in quadrupedal locomotion has not been demonstrated experimentally.…”