Mitigating friction induced limit cycles by an intermediate flexure stage

Friction-induced limit cycling, termed hunting, bounds the positioning performance of precision systems. A friction isolator mitigates this issue through an intentionally passive compliance between the friction-inducing bearing and the actuator. This compliance omits the sudden change in friction force felt by the controller close to standstill. Many design parameters influence the performance of such a friction isolator, including its compliance and damping, system mass, travelled path, and control parameters. Currently, a general design guideline for these friction isolators is missing. This research presents a simulation environment with a metric for identifying hunting cycles, from which a design guideline is distilled. The simulations demonstrated, and experiments confirmed, that a limited controller bandwidth and a significant gap between static and dynamic friction forces can lead to hunting limit cycles. With a friction isolator in place, these hunting cycles are avoided under specific conditions. A parameter study revealed that the friction isolator’ drive stiffness should be kept as low as possible for optimal hunting cycle mitigation. On the other hand, the design of the friction isolator is constrained by parasitic frequencies of this mechanism and the stiffness in supporting directions. The experimental setup demonstrated that hunting cycles could be prevented with a friction isolator even with a control bandwidth of only 5 Hz, whereas the non-isolated system necessitated 3 times higher controller bandwidth. A 0.3 mm stroke of the friction isolator proved sufficient to prevent hunting. These experiments validate the suitability of the friction isolator as a solution for systems exhibiting hunting behavior.