Gravity Balancing of a Human Leg using an External Orthosis Abbas Fattah, Ph.D., and Sunil K. Agrawal, Ph.D., Professor Abstract���Gravity balancing is often used in industrial ma- chines to decrease the required actuator efforts during motion. In this paper, we present a new design for gravity balancing of the human leg using an external orthosis. This external orthosis is connected to the human leg on the shank and its other end is fixed to a walking frame. The major issues addressed in this paper are: (i) Design for gravity balancing of the human leg and the orthosis, (ii) Kinematic compatibility of the human leg and the external orthosis during walking, (iii) Comparison of the joint torque trajectories of the human leg with and without external orthosis, and (iv) Effects of variation of the link lengths and masses of the human leg on the inertia of the external orthosis. We illustrate feasible 2D and 3D designs of the external orthosis through computer simulations. Fabrication of this design will be the subject of future work. I. INTRODUCTION In recent years, passive gravity balancing orthoses have been proposed for the upper arm ([1], [2], [3]). However, orthoses for the lower extremity are typically powered. The authors have proposed a design of an exoskeleton for full or partial gravity-balancing of a human leg during motion ([4],[5]). The device is worn by the user and segments of the exoskeleton are strapped to the corresponding segments of the human leg. However, there are some issues with the existing exoskeletons ( [6], [7], [8], [9]) which motivate us to look at alternative designs. One such issue is the alignment of the human leg and exoskeleton segments. Also, it is hard to get full extension of the knee due to singular configuration of the exoskeleton. Robotic devices are developed to assist pa- tients with lower extremity using alternate designs [10], [11], [12]). Aoyagi et al. developed a robotic device, PAM(Pelvic Assist Manipulator), that assists the pelvic motion during gait training on a treadmill [10]. PAM consists of a pair of 3 DOF pneumatic robots. Galvez et al. proposed a sensorized orthosis that measured shank kinematics and therapist forces during locomotor training [11]. The orthosis is attached to one of the legs. Surdilovic et al. developed the String-Man, a tension controlled wire-drive system which stabilizes the torso of a subject during stepping on a treadmill [12]. In this paper, we present a new design for gravity bal- ancing of the human leg using an external orthosis. The key contribution of this paper is the design of an external orthosis to avoid issues with the existing exoskeletons, such This work was supported by NIH grant # 1 RO1 HD38582-01A2 A. Fattah was with Mechanical Systems Laboratory, Department of Me- chanical Engineering, University of Delaware, Newark, DE 19716, U.S.A. A. Fattah is now with Department of Mechanical Engineering, Isfahan University of Technology, Isfahan, Iran. fattah@cc.iut.ac.ir S. K. Agrawal is with Mechanical Systems Laboratory, Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, U.S.A. agrawal@udel.edu as joint and segment misalignment. This orthosis is designed for a two degree-of-freedom (DOF) motion of the human leg in the sagittal plane, i.e., flexion and extension at the hip and knee during walking and hip abduction/adduction motion. The foot is considered as a point mass at the end of the shank segment. The external orthosis connected at the shank, together with the human leg, creates a kinematic closed loop. The kinematic loop constraint can be satisfied during walking by choosing appropriate link lengths for the external orthosis. Gravity balancing of the human leg and the external orthosis is achieved by making the potential energy of the combined system, human and the machine, to be configuration invariant. First, the potential energy of the system is written in terms of the joint angles of the human leg and the external orthosis. Loop constraint equations are then substituted in the potential energy to express dependent joint angles in terms of independent ones. Finally, the coefficients of joint angle dependent terms in this expression are made to vanish to make the potential energy invariant, thereby achieve a gravity balanced system. These conditions are satisfied by choosing appropriate inertia parameters of the segments of the orthosis and addition of proper springs. The main advantages of the design of human leg with external orthosis as compared to the existing exoskeletons are as follows: (i) This design has a better alignment between the human leg and the orthosis. (ii) We can also get the full extension of the knee with this design. (iii) it is required less hardware such as force-torque sensors between human leg and orthosis to compute the joint torques. However, there are also some drawbacks for this design such as: (a) It increases the inertia of the system, which may be a drawback during fast walking. (b) Modeling of the human leg with the external orthois, which makes a kinematic closed loop, is more complicated than the modeling of the existing exoskeletons, namely, open loop system. The organization of this paper is as follows: Section II describes the kinematic compatibility of the human leg and the external orthosis during walking. Gravity balancing of the human leg and the external orthosis is described in Section III. Some feasible designs are then presented in Section IV. Joint torque computation is studied in Section V followed by sensitivity analysis of the results described in Section VI. II. KINEMATIC COMPATIBILITY The external orthosis is designed for three DOF motion of the human leg, namely, two DOF motion in the sagittal plane, i.e., flexion and extension at the hip and knee and one DOF for hip abduction/adduction (See Fig. 1). The human leg has 2007 IEEE International Conference on Robotics and Automation Roma, Italy, 10-14 April 2007 FrB8.3 1-4244-0602-1/07/$20.00 ��2007 IEEE. 3755