Identification and modeling of friction forces at a hydraulic parallel link manipulator joints

Compensation for friction is essential for accurate assembly. Friction identification and modeling is the first step towards compensation for its effect. We propose an identification method and modeling of friction forces at the prismatic joints of a six d.o.f. hydraulically powered parallel link manipulator. First, we introduce a mathematical basis for friction forces identification at every prismatic joint, and then we identify friction forces at the prismatic joints using pressure sensors experimentally. The proposed method does not use any additional instruments or sensors, neither does it need any special design of the manipulator joints and hence, it is cost effective and reliable. We also present an experimental evaluation of friction forces at every prismatic joint based on the proposed method. A suitable model for friction forces at the joints is determined. The results show that friction forces may differ from one actuator to another as every actuator has its own characteristics. This assures the need for proper compensation for every actuator friction forces.