To realize ideal force control of robots that interact with a human, a very precise actuating system with zero impedance is desired. For such applications, a rotary series elastic actuator (RSEA) has been introduced recently. This paper presents the design of RSEA and the associated control algorithms. To generate joint torque as desired, a torsional spring is installed between a motor and a human joint, and the motor is controlled to produce a proper spring deflection for torque generation. When the desired torque is zero, the motor must follow the human joint motion, which requires that the friction and the inertia of the motor be compensated. The human joint and the body part impose the load on the RSEA. They interact with uncertain environments and their physical properties vary with time. In this paper, the disturbance observer (DOB) method is applied to make the RSEA precisely generate the desired torque under such time-varying conditions. Based on the nominal model preserved by the DOB, feedback and feedforward controllers are optimally designed for the desired performance, i.e., the RSEA: (1) exhibits very low impedance and (2) generates the desired torque precisely while interacting with a human. The effectiveness of the proposed design is verified by experiments.

Precise and large torque generation, back drivability, low output impedance, and compactness of hardware are important requirements for human assistive robots. In this paper, a compact rotary series elastic actuator (cRSEA) is designed considering these requirements. To magnify the torque generated by an electric motor in the limited space of the compact device, a worm gear is utilized. However, the actual torque amplification ratio provided by the worm gear is different from the nominal speed reduction ratio due to friction, which makes the controller design challenging. In this paper, the friction effect is considered in the model of cRSEA, and a robust control algorithm is designed to precisely control the torque output in the presence of nonlinearities such as the friction. The mechanical design and dynamic model of the proposed device and the design of a robust control algorithm are discussed, and actuation performance is verified by experiments. Experimental results with a human subject are also presented to show the performance of the cRSEA while interacting with humans.

For a physical human-robot interaction (pHRI) system such as an exoskeleton, it has been an important issue to control the interaction force between the user and the actuators because it directly determines the accuracy of the realized impedance. However, the accurate force control of the exoskeleton system has not been fully achieved due to the difficulties caused by uncertainties from pHRI. To overcome this problem, in this study, the series elastic actuator (SEA) was operated by a time delay control (TDC), which directly compensated the uncertainties without an accurate model of them, realizing the designed model dynamics in the actuator. However, in spite of the successful application of TDC, the delays between the desired and model dynamics severely degraded the force control performance of the SEA. Thus, a new strategy named model-inverse time delay control was proposed by introducing a virtual reference predicted by the inverse of model dynamics in TDC. The proposed method significantly improved performance of the force control, and it was verified by demonstrating zero impedance control with pHRI, free oscillation of a pendulum, and force control in the exoskeleton.