Exoskeletons are devices used to enhance the load carrying capacity of human limbs as well as to help in regaining the capability of a weak limb. Hydraulic actuation of exoskeletons has gained interest among researchers due the potentials of high power density and energy recuperation allowing reduction of the mass and space used by the device (when compared to the traditional electrically actuated exoskeletons). However, developing a light and cost-effective design of such exoskeleton remains a key challenge.In this work, a novel design of knee exoskeleton is presented that satisfies the peak torque and angular speed requirements during a typical walking cycle while ensuring small size of components and a moderate power demand from the energy source. The design consists of two identical hydraulic cylinders with each of the four ports connected to either pressure source or tank via four 2/2-way valves resulting in 16 possible combinations of the force provided by the cylinders. A novel kinematic mechanism linking the linear actuation of the hydraulic cylinders and the angular rotation of the knee exoskeleton is developed that allows a threefold increment of transmission ratio for the phases in walking characterized by low speed and high torque. The mechanism further constrains the torque contribution of one of the cylinders to be one-half of the other. This results in 16 discrete uniform steps of the torque provided by the exoskeleton. This mechanism eliminates the need of multi-chamber cylinder in the hydraulic system as proposed in previous designs, thereby significantly reducing the cost of the device.The dynamic behaviour of this knee exoskeleton design is studied via a numerical model of the leg motion. For the simulations, apart from the knee motion (which is described by the exoskeleton dynamics), the motion data for the rest of the limb in a walking cycle is obtained from the publicly available human motion database “HUMOD”. A model predictive control strategy is employed to obtain the valve actuation sequences with the goal of tracking the desired knee motion while keeping valve position changes to a minimum. The simulation results indicate the capability of the knee exoskeleton to match the desired motion. However, the challenges related to pure force control of the hydraulic system is also revealed by the simulation model.

A Novel Design Concept of Digital Hydraulic Drive for Knee Exoskeleton

Paper Proceedings: https://asmedigitalcollection.asme.org/FPST/FPMC2021/volume/85239

2021 ASME/BATH Symposium on Fluid Power and Motion Control (FPMC2021)

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