# Structure Design, Kinematics Analysis, and Effect Evaluation of a Novel Ankle Rehabilitation Robot
## 1 Introduction  
The ankle joint is a key driving force in the human body. Therefore, the ankle joint is the most vulnerable to damage, which can seriously affect people's walking ability and daily life. Several experimental studies on market products of ankle rehabilitation robots have been reported in the literature. [PERSON] et al. [1] proposed an ankle rehabilitation robot based on the 3-RRS spherical parallel mechanism. However, the axis of the kinematic pairs must have a high machining and assembly accuracy. This makes the project costs particularly high, which makes it difficult to promote in the market. [PERSON] et al. [2] proposed an RRR-PaRPS-RHJ parallel mechanism for ankle rehabilitation with three rotation degrees of freedom to overcome the rehabilitation motion decoupling. Obviously, the stiffness and carrying capacity of the decoupled configuration are far less than those of the parallel/hybrid mechanism. [PERSON] et al. [3] replaced the middle passive link with the lower limb of the patient in a parallel ankle robot. This design can match the anatomical ankle joint by placing four actuators above the end-effector (AaEE), but unexpected loads may be exerted, causing discomfort and safety issues. [PERSON] et al. [4] proposed one single DOF 3R open-chain, non-circular gear-linkage mechanism. It is only suitable for ankle plantar flexion and back extension, and it cannot realize the movement requirements of adduction/abduction and valgus/valgus of the ankle joint. [PERSON] et al. [5] proposed a hybrid ankle rehabilitation robot. It greatly improves the movement space and dexterity of the mechanism. [PERSON] et al. [6] designed two special serial spherical mechanisms as the constraint limb to construct several 3-DOF parallel mechanisms for addressing the issue of non-coincidence of the centersthrough redundantly actuated guarantees of no singularity, better dexterity, and enhanced stiffness within the prescribed workspace. [PERSON] et al. [7] proposed a 3-RRS lower-mobility, parallel ankle rehabilitation mechanism. This mechanism can rotate around the rotation center to match the ankle motion mechanism. [PERSON] et al. [8] proposed a redundantly actuated parallel mechanism, wherein the actuated method eliminated the configuration singularities and increased the dexterity. [PERSON] et al. [9] developed several parallel mechanisms with a central strut for ankle rehabilitation. The central strut configuration enhanced the mechanism's stiffness. [PERSON] et al. [10] proposed a soft parallel robot for ankle joint rehabilitation through a genetic algorithm, with an optimal robot global conditioning number. [PERSON] et al. [11] adopted a neural network algorithm for control of hydraulic manipulators. [PERSON] et al. [12] realized a 6-DOF Stewart platform-based system for ankle telerehabilitation control, able to be monitored remotely. However, the system is very complex for the mechanism with 6-DOF. [PERSON] et al. [13] analyzed a reconfigurable workspace and the torque capacity in order to improve the ankle rehabilitation robot's comfort and safety issues. [PERSON] et al. [14] proposed a fuzzy logic control scheme for a 2-DOF redundantly actuated parallel ankle rehabilitation robot. [PERSON] et al. [15] presented a framework for the analysis of a 3-RRR spherical parallel manipulator with coaxial input axes (coaxial SPM), with a focus on its infinite rotational movement capabilities and its effects on the manipulator's characteristics. It can be noted that the existing ankle rehabilitation robots have some limitations. First, most robots only have three degrees of freedom (DOFs) according to the ankle motion model, and they cannot be used for the size requirements of different adults' lower limbs. Second, for most spherical parallel robots, their rotation centers can easily shift away from the rotation center of the human ankle [1, 6, 16]. Such a shift can cause an uncomfortable feeling in the patients and can further cause a secondary injury of the ankle joint.