Rockwell Automation Publication 750-AT006D-EN-P - January 2022
100
Chapter
7
Applications
Friction Compensation
Friction compensation applies a compensating directional torque or force to the motor to overcome the effects of friction in a mechanical
system, thus reducing the control effort required. Individual parameters have been defined to support compensation for different types of
friction, including
(Coulomb Friction), and
. A compensation window parameter is
also provided to mitigate motor dithering associated with conventional friction compensation methods.
of friction compensation is covered on
Types of Friction
The method used to compensate for friction varies depends on the type of friction.
Static Friction
It is not unusual for a mechanical load to have enough static friction, commonly called stiction, where the mechanical load refuses to move,
even with a significant position error. Position integral gain can be used to generate enough drive output to correct the error, but this
approach may not be responsive enough for some applications. An alternative is to use static friction compensation to break the stiction in
the presence of a nonzero velocity. This method is done by adding or subtracting a fixed torque level to the torque reference signal value
based on its current sign.
The static friction compensation value must be just under the value to overcome the stiction. A larger value results in dither, a phenomenon
that describes a rapid back and forth motion of the load centered on the commanded position as it overcompensates for the stiction. To
address the issue of dither when applying static friction compensation, a window around zero speed is established where the friction
compensation output signal is zero torque. When the velocity reference reaches the edges of this window, as determined by a trigger
velocity parameter, then a nonzero friction-compensation output-signal is momentarily applied.
A stiction parameter sets this torque level. Within the window, the position and velocity loop integrators are also disabled to avoid the
hunting effect that occurs when the integrators wind up. After the velocity reference reaches or exceeds the friction compensation window
value, the full static friction-compensation value is applied.
A friction compensation window value of zero effectively disables this feature. A nonzero friction compensation window effectively softens
the static friction compensation as it is applied to the torque reference and reduces the dithering and hunting effects that it can create. This
feature generally lets higher values of static friction compensation to be applied, which results in better control.
Sliding Friction
Sliding friction or Coulomb friction is the component of friction that is independent of speed as long as the mechanical load is moving. The
method of compensating for sliding friction is similar to the method for static friction. However, the torque level added to the torque
reference signal is always less than the level applied to overcome static friction and is determined by the sliding friction compensation
parameters.
Sliding friction compensation is applied only when the motor is being commanded to move. The static-friction-torque initial value is only
applied briefly after the window around zero velocity is exceeded by the velocity reference signal. After a time period set by a parameter, the
static friction decays to a level of sliding friction. Set this level of torque using the slip parameter. This mechanism can be thought of as
giving the drive a brief torque impulse or boost to start moving from the zero velocity.
Viscous Friction
Viscous friction is defined as the component of friction that increases linearly with the speed of the mechanical system. The method of
compensating for viscous friction is to multiply the configured viscous friction compensation value by the speed of the motor and apply the
result to the torque reference signal.
