Design of a Two Degree-of-freedom Ankle-Foot Orthosis for Robotic Rehabilitation Abhishek Agrawal, Sai K. Banala, Sunil K. Agrawal, Stuart A. Binder-Macleod Abstract — An ankle-foot orthosis (AFO) is commonly used to help subjects with weakness of ankle dorsiflexor muscles due to peripheral or central nervous system disorders. Both these disorders are due to the weakness of the tibialis anterior muscle which results in lack of dorsiflexion assist moment. The deformity and muscle weakness of one joint in the lower extremity influences the stability of the adjacent joints, thereby requiring compensatory adaptations. We present an innovative ankle-foot orthosis (AFO) that was designed to allow two degree-of-freedom motion while serving to maintain proper foot position for subjects. The prototype AFO would introduce greater functionality over currently marketed devices by means of its inversion-eversion degree-of-freedom in addition to flexion/extension. The flexion/extension is controlled with the help of an actuator and inversion/eversion with a spring and a damper. I. I NTRODUCTION During level plane ambulation the ankle should be close to a neutral position (a right angle) each time the foot strikes the floor. Insufficient dorsiflexion may be the result of hyperactive plantarflexion muscles that produce a very high plantarflexion moment at the ankle, or weakness of the dorsiflexion muscles. This, affects the ability of the ankle to dorsiflex. Both of these cause the patient to make a forefoot contact instead of the normal “heel-strike”. If there is a weak push-off, the stride length reduces, and the gait velocity falls. Similarly, during the swing phase of the gait, the ankle is dorsiflexed to allow the foot to clear the ground while the extremity is advanced. Hyperactive or weak dorsiflexors may result in insufficient dorsiflexion, which must be compensated for by alterations in the gait patterns so that the toes do not drag. This insufficient dorsiflexion during the swing phase of the gait is termed as foot-drop. In addition to the toes dragging, the foot may become abnormally supinated, which may result in an ankle sprain or fracture, when the weight is applied to the limb. Foot-drop is commonly seen in subjects who have had a stroke or who have sustained a peroneal nerve injury. There are several possible treatments for foot-drop such as medicinal, orthotic, or surgical. Of these, orthotic treatment is the most common one. Orthotic devices are intended to support the ankle, correct deformities, and prevent further Abhishek Agrawal and Sai K. Banala are graduate students in the Department of Mechanical Engineering, University of Delaware, Newark, DE 19716. agrawala@me.udel.edu Sunil K. Agrawal is Professor, Department of Mechanical Engineering, University of Delaware, Newark, DE 19716. agrawal@me.udel.edu Stuart A. Binder-Macleod is Professor and Chair, Department of Physical Therapy, University of Delaware, Newark, DE 19716. sbinder@udel.edu occurrences. A key goal of orthotic treatment is to assist the patient in achieving a measure of normal function. Ferris et. al. [1] proposed an ankle-foot orthosis powered by artificial muscles. The orthosis has two pneumatic muscles to control the dorsiflexion and plantarflexion motion of the ankle. Yamamoto et. al. [2] developed a dorsiflexion assist, controlled by a spring. Dorsiflexion correction is achieved via the compression force of a spring within the assist device. Blaya [3] proposed an active ankle-foot orthosis with one degree-of-freedom. The active ankle foot orthosis comprises a force-controllable series elastic actuator (SEA) capable of controlling orthotic joint stiffness and damping for plantar and dorsiflexion ankle motions. There are a number of commercial ankle-foot orthoses manufactured. All these orthoses are single axis or are elastically deformable. The inversion-eversion motion in all of these orthoses are accommodated through the flexibility of the material, such as polypropelyene. The limitation in normal inversion-eversion adds to the discomfort and does not provide a natural motion to the ankle. In this paper, an ankle-foot orthosis with two degrees-of- freedom is proposed. The two motions incorporated are dorsiflexion-plantarflexion and inversion-eversion motion. Among these, the dorsiflexion-plantarflexion motion is ac- tively controlled by a dc servomotor. The inversion-eversion joint is passive with a torsion spring and a damper. The spring and the damper creates a virtual wall and restricts the motion beyond a certain range by the application of the spring force. The device is aimed to be used in two different scenarios - (i) it can be used as a standalone measurement device to measure forces and torques at the joints like KinCom and BioDex, and (ii) it can be an integral part of another rehabiltation device. So, if the position of the leg in the gait cycle is available at all times, then the motion of the ankle can be controlled more accurately. The organisation of the paper is as follows: Section II presents the kinematic and dynamic model of the foot. Section III describes the design of the proposed orthosis. Section IV presents the control law used to control the dorsiflexion-plantarflexion motion and results from simulation. II. MODEL OF THE FOOT The overall motion of the foot is complex and occurs around three axes and three planes. The abduction-adduction motion is small and hence is neglected in our study. The orientations of the other two joint axes are shown in Fig. 1. Fig. 1 shows the projection of joint axes in the inertial frame. The inertial frame is attached to the shank and the axis Proceedings of the 2005 IEEE 9th International Conference on Rehabilitation Robotics June 28 - July 1, 2005, Chicago, IL, USA 0-7803-9003-2/05/$20.00 ©2005 IEEE WeB01-04 41