Reconfigurable Hydrostatics: Toward Multifunctional and Powerful Wearable Robotics

--Wearable and locomotive robot designers face multiple challenges when choosing actuation. Traditional fully actuated designs using electric motors are multifunctional but oversized and inefficient for bearing conservative loads and for being back-drivable. Alternatively, quasi-passive and underactuated designs reduce the size of motorization and energy storage, but are often designed for specific tasks. Designers of versatile and stronger wearable robots will face these challenges unless future actuators become very torque-dense, backdrivable and efficient. This paper explores a design paradigm for addressing this issue: reconfigurable hydrostatics. We show that a hydrostatic actuator can integrate a passive force mechanism and a sharing mechanism in the fluid domain and still be multifunctional. First, an analytical study compares how these two mechanisms can relax the motorization requirements in the context of a load-bearing exoskeleton. Then, the hydrostatic concept integrating these two mechanisms using hydraulic components is presented. A case study analysis shows the mass/efficiency/inertia benefits of the concept over a fully actuated one. Then, the feasibility of the concept is partially validated with a proof-of-concept that actuates the knees of an exoskeleton. The experiments show that it can track the vertical ground reaction force (GRF) profiles of walking, running, squatting, and jumping, and that the energy consumption is 6x lower . The transient force behaviors due to switching from one leg to the other are also analyzed along with some mitigation to improve them. Mobile robots that must bear their own weight have conflicting design requirements. For reasonable autonomy, their actuators should be lightweight and efficient, but at the same time they need good backdrivability for good physical interaction with their environment, e.g., with the ground for a legged robot or with the user for an exoskeleton; and still to be useful in various situations (multifunctional), they should have high strength and power levels. The recent research in legged robots and exoskeletons push mainly for lightly geared electric motors for their high velocity and relatively good torque density and backdrivability [1]-[3]. These actuators 1) have low inertia for better interactions and simpler force control, 2) have high transmission efficiency, and 3) enable good energy regeneration at batteries [1], [4]. For instance, the running legged robot Cheetah could regenerate most of its negative power to the battery, but 74% of its energy consumption was due to motor heating [1]. All authors are with the Department of Mechanical Engineering, Universit e de Sherbrooke, Qc, Canada.