Hydraulic actuator analysis for industrial robot multivariable control

This paper aims to obtain a general mathematical function for hydraulic actuators which can be used for the design of multivariable controllers. The paper shows that the current/torque relationships of linear and rotary actuators may be represented as first order, resulting overall in a third order input/output function for robots with large supply pressure. The model caters for the greatest source of instability in hydraulic systems, namely the compressibility of the hydraulic fluid. It also indicates that pressure feedback can be used to solve the problem of supply pressure variations resulting from multi-joint movements of robot arms. A large hydraulic robot, the Workmaster, formerly manufactured by Thorn EMI Robotics, is modeled without loss of generality.