PD-Based Fuzzy Sliding Mode Control of a Wheelchair Exoskeleton Robot

Wheelchair upper-limb exoskeletons can offer a new paradigm to assist people with neuromuscular dysfunction in their activities of daily living such as eating and drinking. A key challenge in their control is to ensure safe and comfortable interaction between the human upper limb and exoskeleton. Compared with industrial manipulators, exoskeletons suffer severe kinematic and dynamic uncertainties and external disturbances. Therefore, the selection of optimal control methods that can address the aforementioned challenge is required. In this article, a method combining proportional-derivative (PD) control, sliding mode control, and fuzzy logic control, i.e., PD-based fuzzy sliding mode control, is developed to deal with unmodeled dynamics and external disturbances in the human-exoskeleton system. The sliding mode control can be generally divided by the equivalent control law and the switching control law. For the basic equivalent control part, it adopts the PD controller due to its simplicity in controller design and parameter tuning. For the switching control part, it is replaced by fuzzy logic control to eliminate the chattering of control input such that the smooth motion of the system is achieved. Simulation and experiment results are provided to show the effectiveness of the proposed control method.