Hierarchical Compliance Control of a Soft Ankle Rehabilitation Robot Actuated by Pneumatic Muscles

Abstract

Traditional compliance control of a rehabilitation robot is implemented in task space by using impedance or admittance control algorithms. The soft robot actuated by pneumatic muscle actuators (PMAs) is becoming prominent for patients as it enables the compliance being adjusted in each active link, which, however, has not been reported in the literature. This paper proposes a new compliance control method of a soft ankle rehabilitation robot that is driven by four PMAs configured in parallel to enable three degrees of freedom movement of the ankle joint. A new hierarchical compliance control structure, including a low-level compliance adjustment controller in joint space and a high-level admittance controller in task space, is designed. An adaptive compliance control paradigm is further developed by taking into account patient’s active contribution and movement ability during a previous period of time, in order to provide robot assistance only when it is necessarily required. Experiments on healthy and impaired human subjects were conducted to verify the adaptive hierarchical compliance control scheme. The results show that the robot hierarchical compliance can be online adjusted according to the participant’s assessment. The robot reduces its assistance output when participants contribute more and vice versa, thus providing a potentially feasible solution to the patient-in-loop cooperative training strategy.

Design and Modelling of a Compliant Ankle Rehabilitation Robot Redundantly Driven by Pneumatic Muscles

“…Our future work will focus on control strategies of using the robot for ankle rehabilitation treatment. This will include passive training based on robot pre-defined trajectory tracking control in which robust learning control [24] will be used and the active personalized training based on adaptive impedance control as well as adaptation of robot compliance [25].…”

A Review on Human–exoskeleton Coordination Towards Lower Limb Robotic Exoskeleton Systems

“…To provide adaptive impedance control, the stiffness parameters of the controller are calculated Online from the patient's estimated stiffness using two methods: complementary adaptive control and optimal adaptive control. Liu et al [119] have designed a two-level control strategy to encourage pilots to perform their best using adaptive compliance in a PMAs-driven ankle rehabilitation exoskeleton robot. The higher level is a task space adaptive admittance controller that considers the pilot's ability to adjust the admittance parameters.…”

A Trajectory Tracking Control of a Robot Actuated With Pneumatic Artificial Muscles Based on Hysteresis Compensation

“…As the affected limbs of patients are usually fragile, it is easy to cause secondary injury with the use of rigid-driven actuators. In addition, it is difficult to achieve force control for their unchangeable stiffness [6]. Therefore, the flexibility of the mechanism should be considered in the design of rehabilitation robots.…”