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5.8 Drift compensation / high accurate positioning
The high accurate positioning or the drift compensation can be used if external influences limit the position-
ing accuracy. These function can be critical because limit cycling
tion.
Which positioning errors can be compensated
1. Zero point adjustment of the valve. By this kind of error a constant offset between command and
feedback signal remains. This error is more or less constant.
2. Zero point deviation depending on the temperature. The same behavior as point 1, but the effect is
increasing slowly (over the temperature).
3. Position error caused by an external force. All control and servo valves have a typical pressure gain
characteristic. In case of external forces an output signal of 2...3 % has to be generated for the com-
pensation of this force. And this signal is proportional to the positioning error. In opposite to point one
and two the positioning error generated by forces can vary from cycle to cycle.
How does the drift compensation / high accurate positioning work?
The position errors should be compensated when the axis is near by the target position. The drift compensa-
tor generates a slowly changing output signal (integrating behavior) by which the a.m. errors can be elimi-
nated.
To prevent instabilities, the integrator value will be frozen when the output value is lower than the deactiva-
tion limit (DC:DV).
Drift compensation (zero point adjustment)
By this function position errors described below point one and two are eliminated.
High accurate positioning (external force compensation)
To compensate positions errors as described below point three.
Control bits via fieldbus:
Through the fieldbus it is possible to activate drift compensation as well as high accurate positioning.
This can be accomplished by using the following control bits:
DC_ACTIVE:
General activation of the drift compensation and high accurate positioning
DC_FEEZE:
Freezing of the static drift compensation value.
DC_F_POS:
Activation of the high accurate positioning (dynamic drift compensation).
The „limit cycling" is a small and permanent oscillation around the target position. The main reason are static frictions
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and the hysteresis of the valve.
desired accuracy is not achieved. In this case, the hydraulic system is the limiting factor in the accuracy.
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This is relevant for zero lapped control valves and servo valves.
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The static drift compensation to adjust the zero point and the freezing of this value should always be carried out at first.
Only by this it is possible to avoid or minimise overshooting of the target position.
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By proper parameter setting, this can be avoided under the boundary condition that the
POS-124-U
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could be caused by wrong parameteriza-
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24.01.2022
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