Our new Limpact exoskeleton is mechanically based on the design of the passive Dampace and will be powered by rotational hydro-elastic actuators (rHEAs), using impedance control. In this paper we describe the design of the rHEA, which is a novel, custom-designed combination of a rotational hydraulic actuator and a symmetric torsion spring. The rHEA can also be used as a springless hydraulic actuator for stiffer admittance control, or for isometric large-torque measurements of up to 100 Nm, by locking specific components in the design. Our implementation of HEA required alterations to the existing theoretical models to account for (1) our long flexible tubes between the valve and cylinder, and (2) the influence of the pressure feedback on the valve flow. These newly adapted models gave the best fits on the frequency response functions from our open- and closed-loop identification experiments, and might even provide a better fit for the data in the original publication of the theoretical models. Multi-sine identification showed the torque-tracking bandwidth restricted to 18 Hz for a constant spectral-density reference signal of 20 nm, mostly due the transport delays in the long flexible tubes. The measured torque resolution was better then 0.01 Nm. The delivered torque resolution was below 1 Nm, although at those small amplitudes, the output signal was accompanied by significant phase lead indicating some unaccounted for non-linearities in the actuator. When manipulated manually by forefinger and thumb, almost no distortion torques were felt during minimal-impedance and virtual-spring control. The symmetric torsion spring proved difficult to model correctly, and finding the best design became an iterative process. The spring in the prototype, used for the measurements as reported in this study, had a stiffness and maximum torque below those theoretically calculated, limiting the desired output to 22 Nm. With our latest spring design for the actuators in the Limpact, t- — he maximum output torque is increased to 50 Nm.

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