Reconfigurable hydrostatics: Toward versatile and efficient load-bearing robotics

TL;DR

This work tackles the conflicting demands of load-bearing, backdrivability, and efficiency in mobile legged robotics by introducing reconfigurable hydrostatics. It combines an adjustable static passive force unit with an actuator-sharing unit implemented through hydraulic components (pump-accumulator and motorized valves), forming a fluidic differential that can service multiple joints and modes. Analytical mass/energy analysis shows RMS dynamic force reductions of $2.7\times$ to $7.8\times$, and a case study indicates Design D achieves similar actuation mass with $3.9\times$ lower power and $2.7\times$ lower reflected inertia compared to a fully actuated counterpart. A proof-of-concept on a knee exoskeleton demonstrates vertical GRF tracking for walking, running, squatting, and jumping, with walking energy savings up to $4.8\times$ and effective switching mitigations via valve strategies. Overall, the approach promises more transparent, adaptable, and energy-efficient load-bearing robotics, with future human trials and online valve synchronization needed to realize full gait-agnostic benefits.

Abstract

Wearable and legged 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 backdrivable. Alternatively, quasi-passive and underactuated designs reduce the amount 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 the effect of these two mechanisms on 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, experiments are conducted on robotic legs to demonstrate that the actuator concept can meet the expected performance in terms of force tracking, versatility, and efficiency under controlled conditions. The proof-of-concept can track the vertical ground reaction force (GRF) profiles of walking, running, squatting, and jumping, and the energy consumption is 4.8x lower for walking. The transient force behaviors due to switching from one leg to the other are also analyzed along with some mitigation to improve them.

Figures (18)

• Figure 1: Overview of the proposed multifunctional actuator. The same color coding is used through the paper for their corresponding variables and signals.
• Figure 2: Spring-based mechanisms in evolution of complexity and functionalities: a) clutchable spring, b) static balancing mechanism with adjustable attachment, c) a + large spring + adjustable $y_0$, d) c + parallel actuator. $\Delta y_\text{task}$ is the vertical travel needed for a given task and $l_0$ is the spring length at rest.
• Figure 3: The four generic designs compared in this analysis: A) fully actuated, B) with passive unit, C) with sharing unit, D) with both units.
• Figure 4: Vertical GRF to be generated by the actuation for walking, running, sit-to-stands and jumping for the generic designs A--D (blue, red and black lines are the dynamic forces for the right, left or for both legs, respectively, and yellow line for the static force offset when applicable.).
• Figure 5: Actuator proposition combining actuation sharing and an adjustable passive force unit into single topology for a lower-limb exoskeleton.