Rehabilitation robots, by necessity, have direct physical interaction with humans. Physical interaction affects the controlled variables and may even cause system instability. Thus, human-robot interaction control design is critical in rehabilitation robotics research. This paper presents an interaction control strategy for a gait rehabilitation robot. The robot is driven by a novel compact series elastic actuator, which provides intrinsic compliance and backdrivablility for safe human-robot interaction. The control design is based on the actuator model with consideration of interaction dynamics. It consists mainly of human interaction compensation, friction compensation, and is enhanced with a disturbance observer. Such a control scheme enables the robot to achieve low output impedance when operating in human-in-charge mode and achieve accurate force tracking when operating in force control mode. Due to the direct physical interaction with humans, the controller design must also meet the stability requirement. A theoretical proof is provided to show the guaranteed stability of the closed-loop system under the proposed controller. The proposed design is verified with an ankle robot in walking experiments. The results can be readily extended to other rehabilitation and assistive robots driven with compliant actuators without much difficulty.

Sliding mode control (SMC) is an attractive for nonlinear systems due to its invariance for both parametric and nonparametric uncertainties. However, the invariance of SMC is not guaranteed in a reaching phase. Integral SMC (ISMC) eliminates the reaching phase such that the invariance is achieved in an entire system response. To reduce chattering in ISMC, it was suggested that the switching element is smoothed by using a low-pass filter and an integral sliding variable is modified. This study discusses several crucial problems regarding the performance, modification, and improvement of ISMC. Firstly, the modification of the integral sliding variable is revealed to be unnecessary as it degrades the performance of a sliding phase; secondly, ISMC is shown to be a kind of global SMC; thirdly, it is manifested that \tcolor{a high-order ISMC design with super twisting} involves in a stability condition that may be infeasible in theory; finally, an efficient solution is suggested to attenuate chattering in ISMC without the degradation of tracking accuracy and the solution is extended to the case with uncertain control gain functions. Comprehensive simulation results have verified the arguments of this study.

Permanent magnet synchronous motors are extensively used in high-performance industrial applications. However, plenty of practical factors (cogging torques, load torques, friction torques, measurement error effects, dead-time effects, mechanical/electrical parameter perturbations, etc.) in the closed-loop servo system inevitably bring barriers to the high-performance speed regulation. All these factors can be regarded as generalized disturbances. Most approaches only focus on dealing with one single kind of disturbances. In practical cases, the servo system is affected by multiple sources of disturbances simultaneously, and these disturbances enter the system via different channels.To this end, this paper systematically analyzes several representative disturbances (particularly including their features and distribution in the practical servo system), and then, specifically puts forward a novel disturbance rejection framework based on the non-cascade structure. In this framework, a comprehensive disturbance observer is proposed to simultaneously and accurately estimate multiple disturbances so that the composite controller can have a corresponding part to compensate for multiple disturbances. Rigorous analysis of stability is established. Comparative experimental results demonstrate that the proposed method achieves a better speed dynamic response and a higher precision tracking performance in the presence of multiple disturbances.

The rapid advancement of robotics technology in recent years has pushed the development of a distinctive field of robotic applications, namely robotic exoskeletons. Because of the aging population, more people are suffering from neurological disorders such as stroke, central nervous system disorder, and spinal cord injury. As manual therapy seems to be physically demanding for both the patient and therapist, robotic exoskeletons have been developed to increase the efficiency of rehabilitation therapy. Robotic exoskeletons are capable of providing more intensive patient training, better quantitative feedback, and improved functional outcomes for patients compared to manual therapy. This review emphasizes treadmill-based and over-ground exoskeletons for rehabilitation. Analyses of their mechanical designs, actuation systems, and integrated control strategies are given priority because the interactions between these components are crucial for the optimal performance of the rehabilitation robot. The review also discusses the limitations of current exoskeletons and technical challenges faced in exoskeleton development. A general perspective of the future development of more effective robot exoskeletons, specifically real-time biological synergy-based exoskeletons, could help promote brain plasticity among neurologically impaired patients and allow them to regain normal walking ability.

Compliant actuators are significant for safe physical human-robot interaction. Series elastic actuator (SEA) is the most popular type of compliant actuators, which possesses several attractive features, such as low output impedance, back drivability, shock tolerance, smooth force transmission, and energy efficiency. This brief focuses on the adaptive command-filtered backstepping control (CFBC) design for robot arms driven by SEAs. The CFBC alleviates the complexity problem in integrator backstepping control and has attracted great attention. However, experimental results of adaptive CFBC have not been reported. In this brief, a simplified adaptive CFBC framework is developed for robot arms driven by SEAs, where discontinuous friction is compensated for by an adaptive mechanism. Closed-loop stability is rigorously established by the Lyapunov synthesis and time-scales separation. Experimental results based on a single-link SEA-driven robot arm have been provided to verify the effectiveness of the proposed control strategy.