Proceedings of, 4th International Conference on Production Automation and Mechanical Engineering held on 3-4 August 2018 Montreal,Canada ISBN:9780998900063 33 Conceptual Design of a Six Degree of Freedom Parallel Robotic Ankle Rehabilitation System Fatemeh Alavi Department of Mechanical Engineering, Tarbiat Modares University, Nasr Bridge, Tehran, Iran Davoud Karimi Department of Mechanical Engineering, Iranian Research Organization for Science and Technology (IROST), Ahmadabad Mostofi, Tehran, Iran Abstract— Ankle joint is a critical member of human body that is prone to several injuries. Joint sprain, spasticity and contracture, foot drop, as well as neurological disorders are among many problems that may occur for this stable joint. Rehabilitation of ankle joint is therefore highly demanded. Due to their accuracy, repeatability, its ability to measure and record data, its capability to link with virtual reality media and the possibility of telerehabilitation, robotic rehabilitation systems are more advantageous compared to traditional rehabilitation methods. In this paper, conceptual design of an ankle joint robotic rehabilitation instrument is presented. Necessary parameters for the design of an ankle rehabilitation system are determined. Required number of degrees of freedom (DOF) is determined from the analysis of the biomechanics of ankle joint complex (AJC) such that the designed robot is anatomically compatible with the AJC. The required workspace volume is also determined from the study of range of motion (ROM) of AJC. It is shown that for a robotic ankle rehabilitation system to be anatomically compatible with AJC, six DOF is required. Based on the determined workspace and the common force, torque applied by the patient in a rehabilitation exercise, the maximum value of the force in the actuator’s space is determined by employing a numerical method. The stroke of the actuators to provide the required workspace is also investigated. Based on the nature of rehabilitation exercises for AJC, the required control strategy is presented. Stewart platform as a mechanism, which provides anatomically compatible rehabilitation routines is recommended in this investigation. It is shown that the actuator stroke of at least 237 mm, capable of withstanding 1851 N is required for such a mechanism. Keywords— ankle joint, robotic rehabilitation system, Stewart platform, conceptual design. INTRODUCTION Ankle joint injuries is one the most common musculoskeletal problems for athletes and elderly people. It has been published that 92 percent of basketball players have ever experienced ankle joint injury, among which, 83 percent injured more than twice [1]. 37 percent of athletes have ever suffered from ligament injuries more than twice [1]. Ankle joint sprain, in which the ankle rotates beyond its range of motion (ROM) and consequently the joint’s ligaments are stretched more that their tolerable displacement, is one of the most injuries of this joint. Besides mechanical injuries such as ankle sprain, neurological problems such as foot drop frequently occur after stroke. In foot drop, the patient is unable to control his or her foot during the gait. Muscular problems are also among common problems of ankle joint. Muscle spasticity and contracture may happen after stroke. Muscle atrophy is very common after joint healing process. All the mentioned problems require rehabilitation procedure, especially for athletes to come back to their normal activities as soon as possible. Rehabilitation helps the joint perform much like before injury. In contrast to the conventional physical techniques, robotic rehabilitation systems have been proven to be advantageous in that they can provide more accurate and repeatable exercises, they provide tele-rehabilitation, physical property measurement and recording is possible during treatment. Ankle joint robotic systems can fall into two types. The first type is wearable. This type can be worn and acts like an exoskeleton. The second type is platform- based, which is mounted on the ground and the foot is usually fixed on its platform. The first type has a limitation on its weight as the patient is required to carry the robot. The second type however, does not have this limitation as its weight is supported on the ground. Finding and fixing the anatomical axis of tibia with respect to the ankle is a challenge in platform-based robots. Liu, Gao, Yue, and Lu [1] proposed a three degree of freedom (DOF) parallel- kinematic robot for ankle joint complex (AJC). The robot is shown in Fig. 1. As shown in this figure, the actuators are equipped with electrical motors with position feedback. The position feedback is transmitted to the host computer. The force as measured by a sensor mounted on the upper platform is also sent to the host computer. The schematic architecture of the robot is shown in Fig 1.