Design of quaternion controller based on Virtual Spring-Damper hypothesis for hydraulic manipulator

Systems that implement robot joints by using servo valve-controlled hydraulic actuators are known to be difficult to control because hydraulic systems are highly nonlinear. Although this problem is tough to solve, these systems continue to be of interest because their hydraulic manipulators have a high power-to-weight ratio, which can be used for a variety of tasks. In order to enable a hydraulic manipulator to perform these tasks, the position and orientation of its end-effector must be controlled. We have designed Virtual Spring-Damper (VSD) hypothesis based quaternion controller to control the orientation of a hydraulic manipulator end-effector. VSD hypothesis is a relatively simple calculation method that uses virtual force and joint damping to move the end-position of a robot arm to the desired position (control point). We extended VSD hypothesis for orientation control and connected 2 additional VSD models to the wrist and thumb in order to control the orientation of an end-effector with 3 degrees of freedom (DoF). We have proposed a quaternion controller to determine control points used as input for each VSD model. We have also designed VSD controller to calculate the control input of joints; this input is used for the movement to each control point. In order to verify the performance of the proposed control method, we present the results of experiments performed on a hydraulic manipulator with 7 DoF.