Design of a Rotational Hydroelastic Actuator for a Powered Exoskeleton for Upper Limb Rehabilitation

The goal of this study was to validate the suitability of a novel rotational hydroelastic actuator (rHEA) for use in our new rehabilitation exoskeleton for the upper limbs, the Limpact. The rHEA consists of a rotational hydraulic actuator and a custom-designed symmetric torsion spring in a series-elastic configuration. For rehabilitation therapy and impairment quantification, both compliant impedance control and stiff admittance control modes are possible. In the validation experiments, the torque bandwidth of the rHEA was limited to 18 Hz for a desired 20 N·m reference signal (multisine, constant spectrum) due the transport delays in the long flexible tubes between the valve and cylinder. These transport delays also required changes to existing theoretical models to better fit the models on the measured frequency response functions. The (theoretical) measurable torque resolution was better than 0.01 N ·m and the (validated) delivered torque resolution below 1 N· m. After the validation experiments, further iterative improvements resulted in a spring design capable of a maximum output torque of 50 N·m with an intrinsic stiffness of 150 N· m/rad and a slightly higher bandwidth. With the design locked, the maximum measurable isometric torque is 100 N ·m. In conclusion, the rHEA is suitable for upper limb rehabilitation therapy as it matches the desired performance.