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Drift compensation / high accurate positioning
The high accurate positioning or the drift compensation can be used in case of external influence which is
limiting the positioning accuracy. This function could be critical if limit cycling
the system behavior was not taken into account.
Which positioning errors can be compensated
1. Zero point adjustment of the valve. By this kind of failure a constant offset between command and
feedback signal remains. This failure is more or less constant.
2. Zero point failure depending on the temperature. The same behavior as point 1, but the failure is in-
creasing slowly (over the temperature).
3. Position failure 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 failure generated by the force signal can vary cycle to cycle.
How does it work?
The position errors should be compensated when the axis is near by the target position. The output signal is
going lower and lower but a system specific position error remains. At the activation point this function – a
slowly working integrator – is active. This integrator signal is added to the output signal and will compensate
offsets and other failure. To prevent instabilities, the integrator value will be frozen when the output value is
lower than the deactivation point.
Drift compensation (to compensate failure of the zero point adjustment)
To compensate position errors of point one and two.
High accurate positioning (used at external forces or general drift compensation)
To compensate positions errors of point three. Alternatively of point one, two and three.
Positioning modules without fieldbus:
Only one function is implemented to compensate the positioning error of point one, two and three. The acti-
vation is controlled by the parameterization of DC:AV parameter.
Positioning modules with fieldbus:
Two functions are implemented to compensate offset/temperature dependent and/or force dependent posi-
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tioning errors
. The activation is controlled by the parameterization of DC:AV parameter and the following
fieldbus control bits:
DC_ACTIVE:
General activation of the drift compensation and high accurate positioning.
DC_FEEZE:
Freezing of the static drift compensation value.
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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To prevent / minimize position overshoots the static drift compensation have to be done first.
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By proper parameter setting, this can be avoided under the boundary condition that the
POS-124-U-PFN-2030
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by wrong parameterization or
23.08.2016
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