Object Inserting Operation with Admittance Control of A 4-DOF Hydraulic Manipulator with Rotary Vane Actuators

We present an admittance control scheme for an object insertion task with a four degree-of-freedom (DOF) hydraulic manipulator. We formulate a loose insertion problem into an admittance control of the manipulator and a strategy for the insertion task with our 4-DOF manipulator whose joints are double vane type hydraulic rotary actuators. We verify the performance of the insertion scheme experimentally. In case of electrically driven manipulators, impedance control based on the joint torque and insertion operation strategies have been applied. However, hydraulic actuators controlled by servo valves have difficulties in joint torque control due to the non-linearity between the input current to the servo valve and the output torque of the hydraulic manipulator. In this work, we applied the admittance control to the hydraulic manipulator based on the joint position control to ensure the stability of the external contact with a force/torque sensor. The hydraulic manipulator with the admittance control was used to insert a cylindrical object with a radius of 37.5 mm into a hole with a radius of 52.5 mm without information about the hole location. A spiral exploring motion while maintaining the contact is used for searching the insertion hole position. Our experiments show that the admittance control enabled the robot to maintain the contact stability with the external environment and that the inserting operation strategy ensures 100% success rate.