Persons with transtibial (below-knee) amputations experience certain limitations associated with everyday walking. These include an increased metabolic cost, kinematic and kinetic asymmetries, and a higher rate of osteoarthritis in more proximal joints. Commercially available passive prosthetic feet do not provide a biological range of motion, and have stiffness profiles that are in contrast to the intact human ankle. The stiffness of these prosthetic feet decreases as the user moves their weight towards the toe, which is the opposite of the biological ankle, which has been shown to increase in stiffness during the progression of stance phase. As a result, these prosthetic feet are not able to properly store and return energy to the user during walking. To address these issues, we have developed a quasi-passive pneumatic prosthetic foot and ankle inspired by the intact ankle joint, capable of the appropriate range of motion and the ability to produce the torques experienced during level ground walking.
The prosthesis is comprised of a pneumatic cylinder in series with a fiberglass leaf spring. As the ankle dorsiflexes, energy is stored in the compression of air within the cylinder as well as the bending of the fiberglass spring. The ankle stores energy during the stance phase of walking, and returns it to the user during toe-off. Similar to the biological ankle, the pneumatic prosthetic foot becomes stiffer as the center of pressure progresses towards the toe during stance phase.The pneumatic prosthesis also contains a solenoid valve, which allows air to flow between the two sides of the cylinder at the appropriate time. The valve is closed during stance phase, to allow energy to be stored and returned, and is open during swing phase and heel-strike to foot-flat. A finite state control system is implemented on Raspberry Pi microcontroller in order to allow the prosthesis to behave appropriately. The control system operates solely based on the pressure being read on the compression side of the cylinder, and operates at around 800 Hz.
Contributors: Jeffrey Lee, Elliott Rouse