An asymmetric cable-driven mechanism for force control of exoskeleton systems

In this paper, an asymmetric cable-driven mechanism is proposed for accurate force control of exoskeleton systems with a compact structure. Inspired by the fact that the required forces in human motions are not symmetric in many cases, a spring-actuator type cable-drive mechanism is adopted, which enables a compact cable routing structure. The drive pulley is connected with the exoskeleton frame through a rotary series elastic mechanism to transmit the desired force to the human user. High performance in force control is achieved by advanced control algorithms, which combine a proportional and differential (PD) controller optimized by a linear quadratic (LQ) method with a disturbance observer (DOB) and a zero phase error tracking (ZPET) feedforward filter. The proposed system was tested for the elbow joint. Experimental results confirmed that the proposed system could generate and deliver accurate force to the human user even with external disturbances and modeling uncertainties introduced by human motions.