Impedance Control of Hydraulic Actuation Systems With Inherent Backdrivability

The demand for high-performance impedance control in electro-hydraulic actuation systems is a mainstream issue in the robot actuation research community. Although various nonlinear model-based impedance controllers have been developed and showed good performance in recent years, they remain hindered by the high cost of force sensors, and accurate internal dynamics models need to be derived to overcome the non-backdrivability and complex dynamics of hydraulic systems. As a solution to these problems, this paper presents an impedance control approach for hydraulic systems that require no external force sensors or chamber pressure sensors with a pressure control servovalve or backdrivable servovalve. First, we show that a backdrivable servovalve brings inherent backdrivability to hydraulic systems, such that high-performance impedance control can be implemented without requirement of force sensors and exact knowledge on the dynamics of hydraulic chambers, owing to its torque-source property and low effective valve inertia on the link side. Second, utilizing the obtained backdrivability, we develop a passivity-guaranteed impedance controller. A momentum-based external torque observation algorithm is incorporated to render desired impedance over a wide range, without using force/torque sensor. The performance of the proposed backdrivable servovalve and impedance controller was verified through extensive experimental results.