In all machines incorporating parts with relative motion, friction is present. Friction is a natural resistance to relative motion between two contacting bodies. At particular locations in machines, friction is an undesirable property. Friction can lead to bad performance of for example a servo system. At other locations, friction plays an important role and is desirable, as is the case for example in actions as walking and grasping and in a brake system.
In most systems, friction has a negative influence on the performance of the system. Under certain conditions, friction in a system can result in effects like a steady state error, tracking errors, especially nearby velocity reversals (reversal bump effect: getting stuck when moving through zero velocity), limit cycles (hunting) and/or stick-slip.
To overcome these negative effects, insight in friction phenomena, and proper models of these phenomena are useful. This chapter will give attention to these aspects of friction and describe the friction models that are available in the 20-sim library.