Principles of Small-Scale Hydraulic Systems for Human Assistive Machines

Abstract

The high power and force density of hydraulic actuators, along with the ability to distribute system weight through the separation of the power supply and actuators makes hydraulic technology ideal for use in human assistive machines. However, hydraulic systems often operate inefficiently due to throttling losses in the control valves and have increased viscous losses in small-scale applications as bore size is decreased. The objective of this research is to address the limitations of small-scale hydraulics using validated modeling techniques to optimize performance and minimize system weight. This research compares and contrasts the use of different hydraulic technology as well as develops detailed models of small-scale hydraulic components. These models are used to construct a software tool that optimizes the design of a hydraulic system using specified input requirements of actuation, conduit lengths, operating pressure, and runtime. A system-level energetics analysis provides estimates of efficiencies and weights, while a heat transfer analysis estimates the working fluid and component surface temperatures. In addition, the dynamic performance of different small-scale pump and valve controlled hydraulic systems are simulated to compare the cycle efficiencies, rise times, and flow rate capabilities as a function of duty cycles. The use of an accumulator, unloading valves, variable displacement pumps, and proportional pressure control are explored to improve the efficiency of the system during intermittent operation. In addition a small-scale, digital, high frequency switching valve is designed and simulated to reduce the throttling losses of a traditional proportional control valve. This body of knowledge is used to design, prototype, and performance test two hydraulic powered ankle-foot orthoses. The first orthosis is an untethered system that provides active gait assistance. Hydraulics allows the system to be separated into two parts as the actuator is secured to the ankle, and the portable electrohydraulic power supply is positioned on the lower back. The second orthosis emulates the dynamics of a passive ankle-foot orthosis providing torque assistance to bring the ankle to a neutral position. This device is specifically designed to reduce the time and resources in the clinical prescription of passive ankle-foot orthoses while providing more quantitative metrics.