This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE/ASME TRANSACTIONS ON MECHATRONICS 1 An Adaptive Wearable Parallel Robot for the Treatment of Ankle Injuries Prashant K. Jamwal, Sheng Q. Xie, Senior Member, IEEE, Shahid Hussain, and John G. Parsons Abstract—This paper presents the development of a novel adap- tive wearable ankle robot for the treatments of ankle sprain through physical rehabilitation. The ankle robot has a bioinspired design, devised after a careful study of the improvement oppor- tunities in the existing ankle robots. Robot design is adaptable to subjects of varying physiological abilities and age groups. Ankle robot employs lightweight but powerful pneumatic muscle actua- tors (PMA) which mimics skeletal muscles in actuation. To address nonlinear characteristics of PMA, a fuzzy-based disturbance ob- server (FBDO) has been developed. Another instance of an adaptive fuzzy logic controller based on Mamdani inference has been devel- oped and appended with the FBDO to compensate for the transient nature of the PMA. With the proposed control scheme, it is possible to simultaneously control four parallel actuators of the ankle robot and achieve three rotational degrees of freedom. To evaluate the robot design, the disturbance observer, and the adaptive fuzzy logic controller, experiments were performed. The ankle robot was used by a neurologically intact subject. The robot–human interaction was kept as active–passive while the robot was operated on prede- fined trajectories commonly adopted by the therapists. Trajectory tracking results are reported in the presence of an unpredicted hu- man user intervention, use of compliant and nonlinear actuators, and parallel kinematic structure of the ankle robot. Index Terms—Adaptive fuzzy logic controller, fuzzy-based dis- turbance observer (FBDO), pneumatic muscle actuators (PMAs), wearable ankle rehabilitation robot. I. INTRODUCTION T HE ankle joint is a complex structure in the human muscu- loskeletal system and plays an important role in maintain- ing body balance during ambulation [1], [2]. Due to its location, the human ankle is frequently subjected to large loads which can reach up to several times of the body weight. The exposure to such large loads also means a higher likelihood of injuries. In fact, the ankle is the most common site of sprain injuries in the human body, with over 23 000 cases per day in the U.S. [3]. An- kle sprains are injuries that involve the overstretching or tearing of ligaments around the ankle and are often sustained during Manuscript received June 18, 2011; revised November 29, 2011, March 20, 2012, and July 8, 2012; accepted September 5, 2012. Recommended by Techni- cal Editor S. Martel. This work was supported by the National Natural Science Foundation of China under Grant 50975109. P. K. Jamwal is with Rajasthan Technical University, Kota 324010, India (e-mail: pjam025@aucklanduni.ac.nz). S. Q. Xie and S. Hussain are with the School of Engineering, The Univer- sity of Auckland, Auckland 1142, New Zealand (e-mail:s.xie@auckland.ac.nz; shus045@aucklanduni.ac.nz). J. G. Parsons is with the School of Nursing, The University of Auckland, Auckland 1142, New Zealand (e-mail: j.parsons@auckland.ac.nz). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMECH.2012.2219065 sporting or acts of daily living. Ankle sprains can be classified into several grades, ranging from mild overstretching to com- plete disruption of ankle ligaments. Depending on the severity of the sprain, the time required for recovery can range from 12 days to more than six weeks [4]. Researchers have reported that a significant number (>40%) of severe ankle sprains can develop into chronic ankle instability [4], [5], which makes the ankle more susceptible to further injuries in the future. Fur- thermore, there is considerable evidence following ankle injury of prolonged symptoms [6], [7], self-reported disability [7], diminished physical activity [8], and recurrent injury [9] are commonly reported for months and years after initial injury. There is also emerging evidence linking severe and repetitive ankle sprains to the development of ankle osteoarthritis [10], [11]. Functional instability of the ankle joint often results and is typically ascribed to sensorimotor, or neuromuscular, deficits that accompany ligamentous injury. The general rehabilitation program for ankle sprains is carried out in stages. The initial stage of treatment right after injury is considered the acute phase of rehabilitation and is focused on re- ducing effusion and swelling at the affected to promote healing of the injured tissues. A reduction in effusion can be achieved with elevation, application of ice, and compression. The af- fected ankle is also often immobilized. However, as prolonged immobilization of the ankle can lead to reduced range of motion (ROM), compounding of sensorimotor deficits and muscular at- rophy, the next phase of ankle rehabilitation typically involves ROM and muscle strengthening exercises. ROM exercises are normally carried out within the pain-free range of the patient to improve the ROM and reduce muscular atrophy. Research has also suggested that this has the ability to stimulate healing of torn ligaments [4]. The muscle strengthening phase is achieved once pain free weight bearing gait is possible. During this phase, ROM exercises are continued together with the commencement of muscle stretching and resistive exercises [4]. The resistance level of these strengthening exercises should be increased as the patient progresses with recovery. Muscle stretching is im- portant to assist the recovery of joint ROM while resistance training is used to improve the strength of muscles surrounding the ankle to prevent future injuries [12]. Finally, proprioceptive and balancing exercises should be carried out toward the end of the rehabilitation program (functional phase) to enhance the patients’ sense of joint position, thus giving them better foot and ankle coordination and improving their ability to respond to sudden perturbations at the ankle [4] and minimize the risk of further injury [13]. It has been documented that using conventional approaches, the recuperation is slow and tedious [14]. It is apparent from 1083-4435/$31.00 © 2012 IEEE