A Power Transmission Design for an Untethered Hydraulic Ankle Orthosis

Introduction- Ankle-Foot Orthoses (AFO) can oer assistance to people who have impaired gait due to lower limb and ankle impairments. A powered AFO would fully replace ankle function. A powered AFO should be lightweight, compact and untethered. An hydraulic power transmission design for an AFO has been proposed. The hydraulic power transmission design consists of a battery, a DC motor, a piston pump, a double acting cylinder, and a moment arm acting at the ankle. Methods- The Hydraulic Ankle Foot Orthosis (HAFO) was designed with a method that works from ankle kinetics through each component's "across variables" and "through variables". The HAFO analysis was performed using a complimentary approach allowing for comparison component sizing. A longer moment arm at the ankle required a higher fluid flow rate and cylinder shaft speed but allows for lower system pressure and motor torque. The HAFO was analyzed with a model using an object-oriented approach that allows for manipulation of component parameters to better understand system behavior and efficiency. This simulated analysis was augmented and authenticated by a hardware prototype which provided "real life" parameters for testing the model. The model was used to explore the elements of the transmission design. The hydraulic transmission system's efficiency was affected by component geometry. The system's efficiency increased with a decrease in the cracking pressure of the cap-side check valve and with an increase in cylinder bore diameter. Results- Motors are characteristically high speed and low torque there-fore combining a DC motor with a small displacement pump would result in a system with a low flow rate. A lower flow rate results in a small velocity at the ankle. This small velocity allows a small moment arm to be used. However a small moment arm and low pump displacement require a higher motor torque to create the desired force at the ankle. This is the tradeoff of the transmission as power must be conserved within each component. Design Recommendations- The cylinder bore diameter is set at half an inch to keep the system small and compact. Using an achievable motor speed of 6,000 rpm in combination with a pump displacement of 0.4 cc/rev results in a piston velocity of 0.2 m/s. A moment arm of 5 cm with this piston velocity achieves the desired ankle angular velocity. With a 5 cm moment arm and the half-inch cylinder, a motor torque of 1.2 Nm is needed to achieve maximum ankle torque. Due to this torque, the cylinder and pump will have to withstand 2,000 psi. Most hydraulic equipment is built to withstand at least 2,000 psi. A search did not find any currently available hydraulic cylinders of half-inch diameter or hydraulic piston pumps that operate at 6,000 rpm. These components would have to be customized for the system.