A Study of a Compliant Hydraulic Actuator for Running Robots

In the Iegged locomotion research area, it is known that efficient running or hopping in either animals or Iegged robots requires leg actuator compliance. Springy legs interacting with body mass gives a natural hopping/running frequency. Servo-hydraulics is highly suitable for robot leg actuation due to its high power density and fast response. In this paper, we investigate using a hydraulic accumulator connected to a hydraulic cylinder to provide both actuation and the required leg compliance. This approach is experimentally applied to a bipedal hopping robot, and closed loop leg position control is implemented. A non-linear simulation model is used to explain the main findings from the experimental results. The effect of friction in this type of compliant hydraulic actuator is found to be very significant. An error-time factor is introduced to enable an understanding of the friction effect and aids component selection for this application.