A Portable Powered Knee-Ankle- Foot Orthosis1

Abstract

With the population aging, stroke is becoming one of the leading causes of adult disability, such as gait impairment. Robots have been developed to overcome the limitations of manual therapy for rehabilitation, but most of them are bulky, expensive, and available only to big hospitals [1]. A significant portion of patients still have residual gait impairments, such as knee hyperextension and drop foot, after discharge from hospitals. Therefore, there is a great need for a home-based wearable robotic system for gait rehabilitation. Numerous robots have been developed specifically for the ankle joint to tackle the drop foot problem, such as the MIT ankle robot [2] or to aid the knee joint such as Tibion [3]. However, research aiming at providing active assistive torque to both the knee and ankle was very limited. The knee-ankle-foot orthosis (KAFO) presented in Ref. [4] is lightweight but not portable because of the tethered operation with pneumatic actuation system. We present an intelligent compact and modular powered knee-ankle-foot orthosis for chronic stroke patients to conduct gait rehabilitation at outpatient rehabilitation centers or at private homes. We developed a novel compact compliant actuator and linkage mechanism to achieve light-weight and modular design.Figure 1 shows the prototype of the robot. The modular system consists of an ankle foot module and a knee module. Each module is driven with the same compact compliant force controllable linear actuator, which is described in Sec. 3. It is known from human biomechanics that the range of motion of the lower limb joints is within 90 deg during normal walking. Therefore, a simple rocker-slider mechanism is used to achieve a compact design, which is optimized based on the human gait kinematics. Figure 2 shows the schematic diagram and design procedure of the robot. The design is verified with biomechanics analysis. A standard gait cycle data of a 70 Kg person walking in 1 m/s speed is adopted [5]. Simulation shows that with the optimized parameters, knee joint has a motion range of 89.8 deg. Maximum force is within the capacity of the actuator, thus, providing full assistance to human limbs is possible. The structure of the system is made of lightweight carbon fiber composite material. The total weight for the mechanical module is less than 3.5 Kg. The orthosis has a sensory array for gait pattern detection, muscle activity monitoring and assistive control. Potentiometers are employed to determine gait kinematics. Foot pressure sensor is used to detect gait phases during stance phase, together with inertia measurement unit (IMU), which is implemented to detect gait phases in swing phase. EMG electrodes are placed at the main muscle group to monitor the muscle activity pattern. With all the available information and control strategies, the system will deliver the optimal assistance. Quantitative measurement of the recovery progress is also provided as an evidence and record to evaluate the effectiveness of the rehabilitation therapy.One of the common major concerns with devices that provide torque assistance to patients with neuromuscular problems that include increased tone and spasticity is safety in the event that the subject resists the applied torque. Compliant actuator is required to provide safe and force controllable interaction to human limbs. The most widely used compliant actuator, the series elastic actuator (SEA), however, has to compromise between force transmission and compliance: large output and low compliance cannot be achieved simultaneously.We developed a novel SEA to overcome this limitation, extending a large range of output force while keeping a low intrinsic compliance [6]. As shown in Fig. 3, we use a very soft linear spring at the output to handle the low force range and ensure truly high intrinsic compliance and high force control fidelity. We introduce a torsion spring between the motor and the gear before the ball screw. Due to its high re