Vacuum sensor lookup tables, units etc

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JW

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Apr 21, 2024, 6:29:29 PMApr 21
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Looking for some guidance on setting up vacuum sensors, as I'm just finishing off my head design which has close-coupled valves for each nozzle on the head, as well as the pressure sensor within a few centimetres of the nozzle vacuum line connector.

I can't seem to find the right page in the documentation regarding what OpenPnP expects to see pressure wise; do we need the controller (Duet 3 Mini5+ with RFF in my case) to report raw voltage, and then we use a transfer function or lookup table in OpenPnp to get to pressure?

Or, do we need to do the conversion in the controller and report back a pressure; in which case, which units? Pa, kPa, mBar? Guage/Absolute?

What format do we need to report back to OpenPnP in, if for example it's units of PSI, how should we report 7.43psi, as a string of "7.43", or "743" and we know that's psi * 100.

I guess there is an interface specification type page in the docs that exists, and I'm just not finding it! Any help greatly appreciated.

mark maker

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Apr 22, 2024, 2:58:03 AMApr 22
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For OpenPnP, these are just "numbers". 

In the vacuum sensing setup you set some ranges that are good/bad and that's it.

https://github.com/openpnp/openpnp/wiki/Setup-and-Calibration_Vacuum-Sensing

Traditionally for most controllers these  are "temperature" or rather thermistor readings, due to 3D printing ancestry. Therefore there is a ThermistorToLinearSensorActuator that you can use. Don't know if that would allow you to really map your readings to actual pressure (or underpressure).

_Mark

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JW

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Apr 26, 2024, 4:47:20 PMApr 26
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Thanks Mark, so essentially however the value is read and converted in the motion controller, whatever numerical value it transfers to OpenPnP is meaningless per se, bar the relative change in pressure when picking.

Need to get my head around how the readings are actually parsed next, presumably as ASCII text and the value extracted by regex...

mark maker

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Apr 27, 2024, 3:44:55 AMApr 27
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Exactly, meaningless and  doesn't even matter whether numbers go up or down with pressure (or underpressure).

And yes, there's the ACTUATOR_READ_REGEX to be configured.

Let Issues & Solutions propose one that you can then refine (usually there is some prefix in the controller response that lets you identify the right value).

".*myprefix:(?<Value>-?\d+\.\d+).*"

_Mark

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