The restoration of the walking function of paraplegic patients requires long and extensive training. Physicians themselves are not able to provide this training due to the physical labour required. Robots and soft exoskeletons are in development to assist the physicians in this task, however, they are either bulky and restrictive or lack a method to actuate the knee joint. This research explores if a hydraulic system with elastic element is able to actuate a human knee joint during gait training.
The hydraulic system is required to provide a torque of 75Nm with a bandwidth of 4Hz, the large force bandwidth, and a torque of 20Nm over a bandwidth of 12Hz, the small force bandwidth. The system must fit on a knee joint and impose the least amount of restrictions on the patient. Concepts that were identified as possible solutions in a previous literature review included a hydrostatic transmission, a system with a master rotary pump and a rotary slave actuator, a dual-acting hydraulic cylinder, and a master slave hydraulic cylinder system. Preliminary open loop simulation results suggest that the Hydrostatic transmission could give better performance in Torque transmission, size, and complexity. The bandwidth performance for both concepts was equal.
The simulation model of the hydrostatic concept is expanded to include DC motor/driver dynamics, knee dynamics in swing phase, and ground reaction forces in stance phase. A physical test bed is designed to evaluate the simulation results with a real-world counterpart.
Open loop and closed loop with PI controller results showed a significant decrease in the output torque magnitude for the real-world system, up to a factor 13 smaller. The output torque is decreased by the higher friction, slow pressure build-up and leakages in the real-world system. These factors are in part caused by the design choices made for the test bed design, such as the over-the-counter parts and usage of a 3D printed spring. Failure of the spring was observed at high torque and low frequency input signal. This was made worse by the large initial overshoot of the system in the closed loop tests. A more precise control scheme than the PI controller should be designed to prevent breakage. The real-world system is stable and the bandwidth of the system was larger than the 4Hz that is required for the application. For future research a smaller custom hydraulic loop will need to be created to alleviate the friction and leakage issues. Only then will a hydraulic system with series elastic element actuate a human knee to help a paraplegic patient walk again.