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What to do if my pneumatic controller does not reach the pressure target?

Owned by Diogo Teles (Deactivated)
Last updated: Jan 08, 2021 by David Silivestru (Deactivated)
4 min read

Pneumatic controllers use a completely different scalar for reading and control. You can actually send targets to it without having any reading output (i.e. cable unplugged).

The control output can’t be adjusted with GDSLAB's calibration interface. It has a fixed relation between target requested and mV output to the proportional air valve inside the controller, the is a factory setting.



This default slope can be increased or decreased by applying a multiplier. If you right-click on the picture of the pneumatic controller in Object Display you will see the Valve A Slope for pneumatic channel 1 and Valve B slope for pneumatic channel 2 (if it exists). You can also apply an offset, which increases or decreases the target requested by a default % of the controller's full scale.

 

How to adjust the control offset

Type in the offset as the percentage of the controller’s full scale. (%FS)

For example, if you apply a slope of 1% in a 1000kPa controller and then target 100kPa, the controller will, in reality, send to the valve the voltage it would if it was requesting 110kPa.

 

How to adjust the control slope

Manual Adjustment

The slope value is by default 1 for non-unity devices and it should be equal to the sensitivity of the reference transducer (in kPa/mv) for Unity devices

You modify this value to change the relationship between the requested target and control signal.


Example: When targeting a value of 100 kPa the real pressure reading is 110kPa

  1. Find the ratio between reading value and target value. 110/100 = 1.1

  2. Multiply that relation by the current slope value. Assuming a slope of 1 then the new slope would be 1*1.1=1.1

  3. Target a value lower than the intended target pressure (pneumatic controllers are not as accurate when decreasing pressure, then target 100kPa you will get a pressure of 100kPa.

This process is iterative, normally the first slope alteration will not get you to the exact correct target. This happens for different reasons (accuracy, electrical noise in the controller, volumetric compression, etc). You will in most cases need to repeat these 3 steps. After a few times your slope, for this example, would probably look something like 1.038732 when you reach a point when the controller is at the limit of its control accuracy. Through this process expect to be within +-1kPa of the target for 1MPa controllers and +-2kPa for 2 MPa controllers.

 

Automatic Adjustment

For versions of GDSLAB from 2018 onwards, there is a new feature called Nudging. When this feature is enabled GDSLAB will automatically execute the 3 steps above in the background to adjust the control slope if it finds that the targeted value is different then the resultant pressure reading. This is done every 5 to 10 control cycles during a test (a control cycle is approximately 1 second in static systems).

 

To enable Nudging right click on the image of the pneumatic controller in PObject Display and then click on the Advanced Tab.

 

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