Avoiding stick-slip in position and force control through feedback

The avoidance of stick-slip motion at low velocities through feedback control is discussed. Simplified single-joint robot models are derived for position and force control. It is shown that both models can be represented by the same differential equation. Most prior work in control used friction models which depend only on the current value of velocity. This type of analysis indicates that stick-slip can be avoided only through velocity feedback. The tribology literature, however, indicates that friction also depends on the past history of motion. To include this dependence, a state variable friction model is used in conjunction with the position and force control models. Analysis reveals the existence of a second regime of stable, low-velocity motion associated with position feedback gains above a critical value. This is an important result because the accuracy of position data is typically much better than that of velocity data at these speeds