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UBC Theses and Dissertations

An investigation into the reduction of stick-slip friction in hydraulic actuators Owen, William Scott


The stick-slip friction phenomenon occurs during the switch from static to dynamic friction. Static friction is the force that opposes the sliding motion of an object at rest. Dynamic friction is the force that opposes the sliding motion of a moving object. Thus, near zero velocity, there is a switch from static to dynamic friction. Generally, static friction is greater than dynamic friction. In order to move an object the applied force must exceed the static friction. Once movement starts the friction force typically decreases as it switches to dynamic friction. However, if the applied force is still at the original magnitude, then the sudden increase in the resultant forces results in an increase in the object's acceleration; namely a jerky motion. In a similar manner, when an object is brought to rest the sudden increase in friction, as the switch from dynamic to static friction occurs, results in an abrupt and premature stopping of the object. Because of the rapidly changing and inconsistent nature of the friction force at low velocities, accurate and repeatable position control is difficult to achieve. In some cases the actuator position controller can reach a limit cycle (hunting effect). Friction compensation at low speeds has traditionally been approached through various control techniques. This work presents 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. As a result, simpler, linear control techniques at low speeds may then be utilized. Simulation and experimental results are presented to validate this approach and identify the operating limits for the rotational velocity. The experimental results validate the model. The results show that by rotating the piston, the friction is reduced and the Stribeck curve is eliminated. As the rotational velocity is increased the static friction from the axial motion approaches the static friction of the rotational motion. In order to eliminate the Stribeck curve, the rotating velocity must be located outside the range of the Stribeck area of the rotating friction - rotating velocity curve and into the full fluid lubrication regime.

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