Hybrid Control Scheme of a Hydraulically Actuated Lower Extremity Exoskeleton for Load-Carrying

In this paper, a lower extremity exoskeleton is developed to help human beings walk and carry heavy loads. The exoskeleton is actuated by a pump-based hydraulic actuation system. The hydraulic actuation system has a high speed on/off valve and a unidirectional cylinder with embedded springs on the cylinder rod. The hybrid control scheme, including two modes, i.e., position control and following control, is proposed to drive the exoskeleton system. The position control mode is employed to support the carrying load in the stance phase. The following control mode is used to shut down the DC motor to disable the pump and open the relief valve in the swing phase. In the position control, an online Gaussian process regression algorithm is proposed to estimate the human gait trajectory using the human robot interaction signals. A general position control strategy, i.e., proportion integration differentiation (PID), is utilized to control the exoskeleton to shadow the estimated human gait trajectory. In the following control, the operator can lead the mechanical legs with the help of embedded springs on the cylinder rod. Experiments are performed on the healthy human subject, who walks on the level ground at natural speed. The experimental results demonstrate that the proposed hybrid control strategy is suitable for the pump-based hydraulically actuated lower extremity exoskeleton.