Static Friction (Stiction) and Stribeck Effect

静摩擦与 Stribeck 效应Advanced

Getting something moving takes extra force to beat static friction first, and once it starts, friction actually drops before rising again.

Static friction, or stiction (a blend of 'static' and 'friction'), is the force that must be overcome to start relative motion between two surfaces at rest; the maximum static friction is usually larger than the kinetic friction once the surfaces are already sliding. The Stribeck effect is named after the German engineer Richard Stribeck, whose lubricated-bearing experiments published in 1901–1902 showed that friction in a lubricated contact doesn't stay constant with speed — it first drops from its static value as speed increases, reaches a minimum, and then rises again (the rising part comes mainly from viscous friction). Plotted as friction versus speed, this is the Stribeck curve. In robots, friction changes most sharply at low speed and at direction reversals in gearboxes and joints, which can produce a stick-then-suddenly-slip-then-stick-again behavior called stick-slip, causing low-speed creeping and small positioning errors. When compensating for joint friction or performing parameter identification, a combined Coulomb-plus-viscous-plus-Stribeck model is commonly fit to the data — for example, friction-identification work on the ergoCub humanoid's harmonic-drive joints uses this as its baseline model.

ExampleGive a harmonic-drive joint a very small velocity command: at first the motor current rises while the joint doesn't move at all (static friction hasn't been overcome yet); once it breaks free, friction drops sharply and the joint suddenly overshoots, the controller pulls it back, and the cycle repeats — the encoder reading traces a sawtooth pattern, which is low-speed stick-slip.

Also called: Stiction, Stribeck Curve, Stribeck Friction
Related: Coulomb Friction · Viscous Friction · Friction Compensation · Strain Wave Gear (Harmonic Drive) · System Identification · Friction Coefficient (Coefficient of Friction)
Sources: Wikipedia: Stribeck curve
Wikipedia: Stiction
Sorrentino et al., Physics-Informed Learning for the Friction Modeling of High-Ratio Harmonic Drives (arXiv:2410.12685)