Monday, March 16, 2009

Model and Friction Compensation

Friction Models and Friction Compensation
H. Olsson† K.J. Åström† C. Canudas de Wit‡
M. Gäfvert† P. Lischinsky††
Friction occurs in all mechanical systems,e.g. bearings,
transmissions, hydraulic and pneumatic cylinders, valves, brakes
and wheels. Friction appears at the physical interface between
two surfaces in contact. Lubricants such as grease or oil are often
used but the there may also be a dry contact between the
surfaces. Friction is strongly influenced by contaminations. There
is a wide range of physical phenomena that cause friction, this
includes elastic and plastic deformations, fluid mechanics and
wave phenomena, and material sciences

Friction phenomena
Static models
Dynamic models
Comparison of the Bliman-Sorine and the LuGre
Control Systems Applications

Friction Compensation
There are many ways to compensate for friction. A very simple
way to eliminate some effects of friction is to use a dither signal,
that is a high frequency signal that is added to the control signal.
An interesting form of this was used in gyroscopes for auto pilots
in the 1940s. There the dither signal was obtained simply by
a mechanical vibrator, see J41K. The effect of the dither is that it
introduces extra forces that makes the system move before the
stiction level is reached. The effect is thus similar to removing
the stiction. A modern version is the Knocker, introduced in J32K,
for use in industrial valves. The effects of dither in systems with
dynamic friction HLuGreI was recently studied in J43K.

Friction Models and Friction Compensation
Karl J. Åström
Slide Content
1. Introduction
2. Friction Models
3. The LuGre Model
4. Effects of Friction on Control Systems
5. Friction Compensation
6. Summary

Friction Models and Friction Compensation
1. Introduction
2. Friction Models
3. The LuGre Model
4. Effects of Friction on Control Systems
5. Friction Compensation
6. Summary

Static Models


Model Based Friction Compensation in a DC Motor
Tegoeh Tjahjowidodo, Farid Al-Bender, Hendrik Van Brussel

1 Introduction
Friction modeling and identification is a prerequisite for
the accurate control of electromechanical systems. In the
literature, identification of friction in a motor system
usually considers only classical friction models, such as
Coulomb and Viscous friction. Presliding motion, which is
apparent in many friction investigations, is usually
neglected. The presliding regime is taken into account in
some advanced models, such as LuGre model and the most
recent Generalized Maxwell-Slip (GMS) model.
Unfortunatelly, LuGre does not accommodate the unique
behavior of presliding faithfully. The GMS model manages
to overcome those difficulties by modeling friction as a
Maxwell-Slip model where the slip elements satisfy a
certain, new state equations [1,2].
Once the friction models have been optimized, position
control incorporating friction compensation is performed
[1,3]. For this purpose, the inertial force and friction
behavior are compensated for using a feedforward control,
while a simple (PID) feedback part is included to track setpoint
changes and to suppress unmeasured disturbances.

2 Modeling and Results


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