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.
Highlights
Generating dynamic motion for biped robots is challenging due to the hybrid and non-linear dynamics of legged locomotion
In the first part of our work, we investigate whether the virtual point (VP) control mechanism can counteract external perturbations introduced by a single drop on the ground level
The ground reaction forces (GRF) vectors intersect at a virtual point below the center of mass (VPB), whose magnitude is reported as 30 cm for running over a ground level drop of 10 cm at 5 m s−1 (Drama et al, 2020)
Summary
Generating dynamic motion for biped robots is challenging due to the hybrid and non-linear dynamics of legged locomotion. The literature presents two main approaches to motion planning: the first applies trajectory optimization with whole-body dynamics (Koenemann et al, 2015). Postural stability is crucial in motion planning; the trunk is underactuated, and its motions can be controlled only indirectly. With the control approaches above, the biped robots nowadays able to maintain an upright trunk and walk steadily on flat terrain (Kim et al, 2007; Sheng et al, 2013; Ding et al, 2018). Sustaining trunk stability becomes difficult under external perturbations such as changes in ground level, since the control mechanism needs to regulate the additional change in the system’s energy (Tokur, 2019). Perturbations can be either local, like a single step up/down, or global, as in up/downhill terrain
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