Small-Scale Hydraulics for Human Assist Devices

Abstract

Motivation: Human assist devices such as hand tools and orthotics require high force, low speed, compact size, and light weight, which match hydraulics. Traditionally, hydraulic systems are used in applications that require large amounts of power so components are large and heavy. To apply hydraulic technologies to human assist devices, traditional hydraulic components must be scaled down to appropriate power levels, that is, from thousands of watts to about 100 W. To apply small-scale (10–100 W) hydraulics to human assist devices, three steps were taken. First, a hydraulic ankle foot orthosis (AFO) was built and tested to understand the feasibility of using small-scale hydraulics in human assist devices. Second, a small-scale electrohydraulic actuator (EHA) system was built to identify the gaps between the desired small-scale hydraulic components and the smallest off-the-shelf hydraulic components. Third, basic fluid mechanics and structural equations were used to model the efficiency of small-scale hydraulic components, which is the key to miniaturize traditional hydraulic systems. Results: The AFO platform showed that sufficient torque and range of motion can be realized with a hydraulic system but confirmed the need for small hydraulics to reduce the weight and bulk. The EHA system showed that the smallest off-the-shelf components are oversized for a small-scale hydraulic system and identified the need for custom small-scale hydraulic components. The efficiency models showed that reasonable efficiencies are achievable for small-scale hydraulic components, but different design rules are required.