## Friction model definition

@FRICTION_MODEL_DEFINITION {
@FRICTION_MODEL_NAME {FctModlName}{
@FRICTION_MODEL_TYPE {ModelType}
@CHARACTERISTIC_RELATIVE_VELOCITY {vs}
@DYNAMIC_FRICTION_COEFFICIENT {μk}
@STATIC_FRICTION_COEFFICIENT {μs}
@NORMAL_FORCE_VARIATION {NormalForceVar}
@TIME_FUNCTION_NAME {TimFunName}
@LUGRE_MODEL_COEFFICIENTS {σ0, σ1, σ2, γ}
@STRATEGY_PARAMETERS {c1, c2, c3, c4}
}
}
}

### Introduction

The friction model definition describes the parameters associated with the friction between two bodies of the system. This model is used in conjunction with the planar contact joint, wheel element, relative rotation elements, and relative displacement elements. The forces generated at the point of contact can be divided into two parts: the normal contact forces and the tangential contact forces. The normal contact forces act in the direction normal to the plane tangent to the contacting bodies at the point of contact, whereas the friction contact forces act in this plane. The parameters associated with the evaluation of the friction forces are defined in this section, whereas the parameters required for the computation of the normal contact forces are defined by a contact model.

The friction behavior can be modeled through a number of different models.

• If ModelType = COULOMB, the classical Coulomb model for dry friction will be used.
• If ModelType = LUGRE, the LuGre model for friction will be used.

### NOTES

1. If a friction model is used with a relative rotation elements, the friction force is still given by the continuous friction law, where the relative tangential velocity is Vr = ρ dφ/dt, ρ is the radius of the joint, and φ the relative rotation. Fig.~\ref{FctModl: Rvj} depicts the friction force in a revolute joint.
2. For relative displacement or rotation elements, the normal contact force, Fn, at the frictional interface is set to the pre-load normal force, Pn.
• if NormalForceVar = CONSTANT, the pre-load normal force is constant in time.
• if NormalForceVar = TIME_VARYING, the pre-load normal force equals the value of time function, TimFunName.
3. It is possible to attach comments to the definition of the object; these comments have no effect on its definition.

### Sensors

Sensors, see section~\ref{Sensor}, can be defined to extract information about friction models.
1. A single SensorType is allowed for friction models.
• SensorType = FRICTION_DATA Evaluates four quantities relative to the friction model: the relative velocity, the friction force, the internal variable z of the LuGre model, and the friction coefficient.
2. No FrameName is allowed for this sensor.
3. No u or v values are accepted for the friction model.