Some nonlinear control systems admit an exponential dichotomy (Coppel, 1966), that is to say, their dynamics can be decomposed into (exponentially) stable and unstable components. Walking robots fall into this category, and we call their unstable components divergent components of motion (DCM). The concept of DCM has been fruitfully applied to the linear inverted pendulum (LIP) for both walking pattern generation and balance feedback control. But DCMs can be found for other models as well! In this talk, we will discuss DCMs for the variable-height inverted pendulum (VHIP), an extension of the LIP where the controller can add height variations. Ideally, we would like our robot to behave as a LIP (nominal height) unless some perturbation occurs and the robot resorts to the height-variation strategy, if it has to. Deciding when to use or not this strategy may seem "smart" or predictive, but we will see that it can be implemented straightforwardly as linear feedback over a 4D DCM.
|Feedback control of a 4D DCM for the variable-height inverted pendulum|
|Walking trajectory generation with height variations|
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