Novel and Electro-Hydrostatic Actuation for Robotic Applications

Hydraulic actuation is particularly attractive for robotics because it is mature, robust, high speed and offers inherently high power and force densities. However, its implementation in this field has been relatively restrained because of size (central pumps, actuators and fluid distribution networks), leakage, complex control dynamics and cost. The team of Active Signal Technologies (AST), Cornell University and Moog intends to address these shortcomings with a novel approach to electrohydrostatic actuation (EHA) featuring a compact central pump and smaller distributed pumps in order to optimize system efficiency, weight and size. Current EHA technology as employed on the F-35 Joint Strike Fighter provides all-electric power across a heavy copper bus to relatively large actuators integrated with local hydraulic pumps. The hybrid system proposed here for robotics will employ lower fluid pressure and alternative fluids such as water to maximize the use of plastic and lightweight composite parts (such as PMMA/PDMS sandwich structures) in actuator construction, and will parse actuation tasks between its central and local power units. System simulations run by Cornell in Phase I will quantify the pressure and power requirements. The challenge for hydraulics in the field of mobile robots is to achieve reciprocating motion for pumps and valves at the meso scale because there is a performance gap in available technology in the middle-range between solid state actuation (small, high frequency, high force and low displacement) and linear motors (large, lower frequency and force, and high displacement). The heart of the proposed new system is a revolutionary electromagnetic driver that powers the piston of the pump. Instead of a conventional solenoid configuration where the induced magnetic field produces a highly nonlinear mono-directional force on the core, or the linear stepper motor with its alternately attracting and repelling shear forces, the novel AST driver takes advantage of additive push-pull forces in-line with the motion. The resultant compact device which will be prototyped in Phase I has very high force per unit of moving mass, is fully reversible and generates high work output by virtue of relatively high forces at all points in the travel. It also scales readily in the dimension range between central (~0.15m) and remote (~0.05m) pumps. A number of ways to minimize pump noise will be investigated including passive damping via plastic construction materials, gas-charged accumulators and absorber elements at the end walls of the cylinder. Design of the hydraulic actuators themselves will build upon an extensive body of work conducted at Cornell"s Laboratory for Intelligent Machine Systems (LIMS). This will include new hydraulically driven McKibben-type artificial muscle and use of biomimetic hydraulic valving schemes to enable the phenomenon of"dangle"which will allow a robotic limb to transition smoothly to swaying to preserve momentum or respond passively to external loads.