Cascade force control of lower limb hydraulic exoskeleton for human performance augmentation

Recently the research on hydraulically actuated exoskeleton becomes an attractive topic for those application requirements of human performance augmentation. The control goal of this type of exoskeleton system is to minimize the human machine interaction force. And it becomes more challenging for hydraulically actuated lower limb exoskeleton where the multi-variable nonlinear dynamics is quite complicated and multiple walking phases are existing. Furthermore, since the exoskeleton is driven by the hydraulic actuators, the accurate output force tracking can not be easily realized due to the large compressibility of hydraulic oil. This paper focuses on the human machine interaction force control and the walking phase partition of the hydraulically actuated lower limb exoskeleton. Firstly, a cascade interaction force control strategy is proposed for a 3-DOF support leg which is the basic partitioned module of the lower limb exoskeleton. The spring model is built for the dynamics of human-machine interface, and a high level controller minimizing the integral of human-machine interaction force is designed to generate the desired joint trajectories of the exoskeleton which can be considered as the human motion intent. Subsequently, an independent joint based PID controller is developed in the low level to achieve the good tracking of the above generated human motion intent. Secondly, the exoskeleton system in different walking phases is partitioned into three serial chain manipulator modules. For each serial chain manipulator module, the proposed cascade interaction force controller is applied to minimize the human machine interaction force at the end effector. Finally, the walking experiments with 20Kg load on a practical hydraulically actuated lower limb exoskeleton are carried out to validate the effectiveness of the proposed approach.