Biological Actuators are not just Springs Investigating Muscle Dynamics and Control Signals

While there is a trend in current robotics towards more biologically inspired actuators, most work emphasizes the elastic property of muscles and tendons. Although elasticity plays a major role in many forms of movements, particularly walking and running, other features of animal muscles might also affect or even dominate movement dynamics. In this paper we use the Hill-type muscle model, common in biomechanics, to investigate the relationship between muscle dynamics and control signals in simple goal-directed movements. We find that the various non-linearities of the model lead to desirable properties with regard to controllability, such as increased stability and robustness to noise, independence of position and stiffness, or near linearity in search space. We conclude that in our attempt to create robots exhibiting the same flexibility and robustness as animals we have to seek a balance between the complexity of actuators and the extent to which their natural dynamics can be exploited in a given task.