Development of rotary hydro-elastic actuator with robust internal-loop-compensator-based torque control and cross-parallel connection spring

Hydraulic actuation systems have high power-to-weight ratio and durability characteristics, but its the non-backdrivability and high output stiffness of hydraulic prevented it from being actively applied to interactive control systems, especially in robotics. This paper describes a force/torque control strategy to address these interactive hydraulic system control problems, with hydro-elastic actuation, which is a combination of a hydraulic actuator with a spring installed on the output side. This configuration simplifies the hydraulic actuator force/torque control problem into a motion control problem. To achieve high performance, first we introduced a robust internal- loop compensator (RIC) for hydraulic motion control, which has equivalent properties to a disturbance observer (DOB) and applicability in non-linear hydraulic actuator dynamics by decoupling the inner-loop compensator and outer-loop controller. This also minimizes sensor requirements and computation costs versus conventional hydraulic motion controllers. Based on this first strategy, second, we propose link-side motion feedback to compensate for undesirable velocity disturbances from the link side to provide high-accuracy torque control for the HEA system. Finally, we propose cross-parallel connection strategy to HEA spring to achieve high durability and deflection linearity Experimental results with custom-designed hardware showed torque controllability, backdrivability, low output impedance, and robustness in linking motion disturbance in time domain zero-torque and stiffness control experiments, without losing much of the high-power potential.