The reduction of stick-slip friction in hydraulic actuators

The stick-slip friction phenomenon is observed near zero relative velocity, during the transition from static to dynamic friction, when static friction is greater than dynamic friction. This nonlinear change in friction force over a small change in velocity results in difficulties in achieving accurate and repeatable position control. In some cases, the actuator position controller reaches a limit cycle (hunting effect). Friction compensation at low speeds has traditionally been approached through various control techniques. This paper proposes an alternative solution, namely, friction avoidance. By rotating the piston and rod, the Stribeck region of the friction-velocity curve is avoided and the axial friction opposing the piston movement is approximately linearized. Simulation and experimental results are presented to validate this approach.