Strategies for stabilizing a 3D dynamically walking robot

Oort, Gijs van (2005) Strategies for stabilizing a 3D dynamically walking robot.

Abstract:A dynamic walker is a system that makes use of its natural dynamics in order to walk in an energy-efficient way. In this report two models of dynamic bipedal walkers are described, and strategies are discussed that stabilize the models. Main focus is on sideways stabilization by means of lateral foot placement. The first model discussed is an 8-degrees-of-freedom bipedal walker, generated with 20-sim's 3D~Mechanics Editor. Each leg has a hip joint (forward/backward rotation), a splay joint (sideways rotation), a knee joint and an ankle joint (that lifts/lowers the foot). The feet are ellipsoids. The dimensions of the walker are inspired by humans, however, the walker has no upper body. With the help of an `aligner' block that prevents the walker from falling sideways, a simple controller was developed that stabilizes the walker in forward/backward direction. The hip joints are actuated, the splay and ankle joints are fixed and the knee joints are passive (they do have end stops and a locking mechanism). Also a controller was developed that controls the forward velocity of the walker by means of changing the ankle joint angle. Then the influence of the aligner block was gradually reduced and a controller (based on trajectory prediction) was added that should stabilize the walker in sideways direction. This controller did improve the behaviour with respect to sideways falling, but by manual tuning no set of parameters could be found that stabilizes the walker completely. The second model discussed is a `very simple 3D walker', consisting of a point mass as the hip, and two massless, stiff legs. Energy injection is done by a `toe-off' mechanism. Two different controllers were developed to stabilize the walker in three dimensions. One uses a normal discrete state feedback controller based on a linearized approximation of the model around a certain stable cycle (a limit cycle). The other uses a property common to all limit cycles, that results from symmetry: the fact that the sideways velocity exactly halfway the step should be zero. As this controller is not optimized for just one limit cycle, it can be used over a whole range of different limit cycles. Both controllers stabilize the walker well and have a reasonably large (but different) region of stability. The differences in performance are discussed and explained. It is expected that (a combination of) the controllers (with some adaptations) can also be used for more complex walker models.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:53 electrotechnology
Programme:Electrical Engineering MSc (60353)
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