Background: The ankle joint complex (AJC) is of fundamental importance for balance, support, and propulsion. However, it is particularly susceptible to musculoskeletal and neurological injuries, especially neurological injuries such as drop foot following stroke. An important factor in ankle dysfunction is damage to the central nervous system (CNS). Correspondingly, the fundamental goal of rehabilitation training is to stimulate the reorganization and compensation of the CNS, and to promote the recovery of the motor system’s motor perception function. Therefore, an increasing number of ankle rehabilitation robots have been developed to provide long-term accurate and uniform rehabilitation training of the AJC, among which the parallel ankle rehabilitation robot (PARR) is the most studied. The aim of this study is to provide a systematic review of the state of the art in PARR technology, with consideration of the mechanism configurations, actuator types with different trajectory tracking control techniques, and rehabilitation training methods, thus facilitating the development of new and improved PARRs as a next step towards obtaining clinical proof of their rehabilitation benefits.Methods: A literature search was conducted on PubMed, Scopus, IEEE Xplore, and Web of Science for articles related to the design and improvement of PARRs for ankle rehabilitation from each site’s respective inception from January 1999 to September 2020 using the keywords “ parallel”, “ ankle”, and “ robot”. Appropriate syntax using Boolean operators and wildcard symbols was utilized for each database to include a wider range of articles that may have used alternate spellings or synonyms, and the references listed in relevant publications were further screened according to the inclusion criteria and exclusion criteria.Results and discussion: Ultimately, 65 articles representing 16 unique PARRs were selected for review, all of which have developed the prototypes with experiments designed to verify their usability and feasibility. From the comparison among these PARRs, we found that there are three main considerations for the mechanical design and mechanism optimization of PARRs, the choice of two actuator types including pneumatic and electrically driven control, the covering of the AJC’s motion space, and the optimization of the kinematic design, actuation design and structural design. The trajectory tracking accuracy and interactive control performance also need to be guaranteed to improve the effect of rehabilitation training and stimulate a patient’s active participation. In addition, the parameters of the reviewed 16 PARRs are summarized in detail with their differences compared by using figures and tables in the order they appeared, showing their differences in the two main actuator types, four exercise modes, fifteen control strategies, etc., which revealed the future research trends related to the improvement of the PARRs.Conclusion: The selected studies showed the rapid development of PARRs in terms of their mechanical designs, control strategies, and rehabilitation training methods over the last two decades. However, the existing PARRs all have their own pros and cons, and few of the developed devices have been subjected to clinical trials. Designing a PARR with three degrees of freedom (DOFs) and whereby the mechanism’s rotation center coincides with the AJC rotation center is of vital importance in the mechanism design and optimization of PARRs. In addition, the design of actuators combining the advantages of the pneumatic-driven and electrically driven ones, as well as some new other actuators, will be a research hotspot for the development of PARRs. For the control strategy, compliance control with variable parameters should be further studied, with sEMG signal included to improve the real-time performance. Multimode rehabilitation training methods with multimodal motion intention recognition, real-time online detection and evaluation system should also be further developed to meet the needs of different ankle disability and rehabilitation stages. In addition, the clinical trials are in urgent need to help the PARRs be implementable as an intervention in clinical practice.

Robot-assisted physical therapy can encourage a subject's voluntary participation to achieve rapid motor function recovery. To enhance the subject's cooperation and training safety during training sessions, we develop a new ankle robotic system with three rehabilitation training strategies based on an admittance controller, namely, patient-passive compliance exercise, isotonic exercise and patient-active exercise, which fully consider the patient's muscle strength level and different stages of recovery. The developed parallel 2-UPS/RRR ankle rehabilitation robot is actuated by two servo linear actuators and one servo motor and has three rotational degrees of freedom. The mechanism is simple and ensures that the rotation center of the ankle joint complex from different patients coincides with the robot's rotation center. All three strategies are based on the admittance controller and its derivatives, and the controller's output is obtained based on proportional and time-shifting methods according to the continuous measured torque. In particular, the developed 2-UPS/RRR ankle robotic system covers the whole-stage compliance rehabilitation training, ensuring the safety of full-cycle training sessions. Experiments of the three rehabilitation training strategies were carried out on five healthy subjects, with the results showing good compliance and trajectory tracking performance.

As the population ages, increasingly more individuals experience ankle disabilities caused by stroke and cerebral palsy. Studies on parallel robots for ankle rehabilitation have been conducted under this circumstance. This paper presents a novel parallel ankle rehabilitation robot with the key features of a simple configuration and actuator nonredundancy. The mechanical design is determined, and a prototype is built. Additionally, inverse position solution is addressed to calculate the workspace of the parallel robot. Jacobian matrices mapping the velocity and force from the active joint space to the task space are derived, and kinetostatic performance indices, namely, motion isotropy, force transfer ratio, and force isotropic radius are defined. Moreover, the inverse dynamic model is presented using the Newton–Euler formulation. Dynamic evaluation index, i.e., dynamic uniformity, is proposed according to the derived Jacobian matrix and inertia matrix. Based on the workspace analysis, the parallel robot demonstrates a sufficient workspace for ankle rehabilitation compared with measured range of motion of human ankle joint complex. The results of the kinetostatic and dynamic performance analysis indicate that the parallel robot possesses good motion isotropy, high force transfer ratio, large force isotropic radius, and relatively uniform dynamic dexterity within most of the workspace, especially in the central part. A numerical example is presented to simulate the rehabilitation process and verify the correctness of the inverse dynamic model. The simplicity and the performance of the proposed robot indicate that it has the potential to be widely used for ankle rehabilitation.

The ankle rehabilitation robot is essential equipment for patients with foot drop and talipes valgus to make up deficiencies of the manual rehabilitation training and reduce the workload of rehabilitation physicians. A parallel ankle rehabilitation robot (PARR) was developed which had three rotational degrees of freedom around a virtual stationary center for the ankle joint. The center of the ankle should be coincided with the virtual stationary center during the rehabilitation process. Meanwhile, a complete information acquisition system was constructed to improve the human-machine interactivity among the robot, patients, and physicians. The physiological motion space (PMS) of ankle joint in the autonomous and boundary elliptical movements was obtained with the help of the RRR branch and absolute encoders. The natural extreme postures of the ankle complex are the superposition of the three typical movements at the boundary motions. Based on the kinematic model of PARR, the theoretical workspace (TWS) of the parallel mechanism was acquired using the limit boundary searching method and could encircle PMS completely. However, the effective workspace (EWS) was smaller than TWS due to the physical structure, volume, and interference of mechanical elements. In addition, EWS has more clinical significance for the ankle rehabilitation. The PARR prototype satisfies all single-axis rehabilitations of the ankle and can cover most compound motions of the ankle. The goodness of fit of PMS can reach 93.5%. Hence, the developed PARR can be applied to the ankle rehabilitation widely.

The goal of this article is to present a number synthesis method for the self-adapting upper-limb rehabilitation exoskeletons. The human joint kinematics and the variability of joint axes around their supposed locations were analyzed; the axes misalignments caused by the wearing error and the movement coupling between joint rotations and axes motions in human joints were taken into account, and the kinematic incompatibility of upper-limb exoskeletons was investigated from an over-actuation perspective. Then, a number synthesis method of the self-adapting upper-limb exoskeletons was proposed by using the traditional degrees of freedom analysis approach of spatial multiloop chains. Applying this method, the basic chains of the self-adapting four-degrees of freedom shoulder–elbow and five-degrees of freedom shoulder–elbow–forearm exoskeletons were synthesized and classified into three and two groups, respectively. Finally, the feasible simplified chains of the self-adapting upper-limb exoskeletons were investigated with the consideration of the quasi-anthropopathic feature and structure simplicity, and several examples were enumerated. The proposed simplified chains could be used as the candidates for further studies on the performance analysis, the chain compare and selection, and structure design of the self-adapting upper-limb exoskeletons.