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ATV630 negative PID feedback or reference. Is it possible? e.g. pressure sensor range -1bar...+10bars

Hello. There are two parts concerning PID regulator:
1. Scalling of PID reference and PID feedback for being processed by PID regulator
2. Displaying values on HMI in desired range that respects negative number

Part 1 in ATV630 is the same as in ATV61.
But the second part is different. ATV61 display allows to define (create) new user variable and to use offset, miltiplier and divisor. ATV6xx HMI does not have such a possibility.

Please bear in mind that:
a) PID reference and PID feedback are always processed by microcontroller in positive values and it is done by digital computing (also in ATV61 it is done like that).
b) If PID reference is not entered via HMI, but comes from analog input or via the network ( PLC), then PID reference is always in positive value. The only exception is ATV61-71 HMI that allows to create user variable with negative offset.
c) ATV6xx HMI allows to change system units, but the selection is limited to predefined units, like bars, milibars, 0.1bars, mmH2O, etc. HMI does not offer versatile unit definition. Nevertheless, if your PID feedback represents outlet pressure or inlet pressure or installation pressure, then for monitoring purpose you can see even negative value, see appendix 1. But do not forget that PID reference can only be possitive as ATV6xx HMI does not allow to enter negative value.

Conclusion: In ATV6xx, PID reference that comes from AIx, network or ATV6xx HMI can only be possitive. Likewise PID feedback processed by PID regulator can be possitive only.
The only negative value you can observe is in Display menu 4.2 Application parameters if you configure sensor e.g to outlet pressure according to appendix 1. Do not mix min PID feedback with lowest process, as they are two different variables.
Having pressure sensor that gives measuremnt related to atmosferic pressure (or any other reference point), fro example -1bar to +10bars, it is important to recalculate the measured range to positive values, e.g.
-1 bar related to atmosferic pressure is in fact absolute vaccum and therefore consider as 0bars as absolute pressure. And +10bars relative to atmosferic will corresponds to 10+1bars (=11bars). In ATV630 therefore configure min feedback as 0, max feedback as 11bars.
 

Apendix 1:
In 5.4 menu :
Complete setting - sensor assignement - Outlet pressure assignement= Ai2,
then : AI2 lowest process can be set to negative value.


Note: This FAQ validated for ATV6xx version lower or equal to 1.8. In future ATV versions (>1.8) the structure of PID may change.  
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