We demonstrate in simulation that active force control applied to a passive spring-mass model for walking and running attenuates disturbances, while maintaining the energy economy of a completely passive system during steady-state operation. It is well known that spring-mass models approximate steady-state animal running, but these passive dynamic models are sensitive to disturbances that animals are able to accommodate. Active control can be used to add robustness to spring-mass walking and running, and most existing controllers add a fixed amount of energy to the system based on information from previous strides. Because spring-mass models are schematically similar to force control actuators, it is convenient to combine the two concepts in a single system. We show, in simulation, that the resulting system can attenuate sudden disturbances during a single stance phase by matching its toe force profile to that of the undisturbed spring-mass model.
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Koepl et al. (2010) studied this question.