Hello. There are two parts concerning PID regulator:
1. Scaling 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 Altivar 630 (ATV6xx) is the same as in older ATV61.
But the second part is different. ATV61 display allows to define (create) new user variable and to use offset, multiplier 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 positive as ATV6xx HMI does not allow to enter negative value.
Conclusion: In ATV6xx, PID reference that comes from analog input AIx, network or ATV6xx HMI can only be positive. Likewise PID feedback processed by PID regulator can be positive 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 measurement related to atmospheric pressure (or any other reference point), for example -1bar to +10bars (in KPa it is -100kPa...+1000kPa), it is important to recalculate the measured range to positive values, e.g.
-1 bar (equal to -100kPa) related to atmospheric pressure is in fact absolute vacuum and therefore consider it as 0bars (0kPa) in absolute pressure representation. And +10bars (+1000kPa) relative to 10 x atmospheric will corresponds to 10+1bars (=11bars) or in KPa units it will be 1000+100=1100kPa. In ATV630 therefore configure min feedback as 0, max feedback as 11bars (1100kPa).
Appendix 1:
In 5.4 menu :
Complete setting - sensor assignment - Outlet pressure assignment= Ai2,
then : AI2 lowest process can be set to negative value.
Note: This FAQ validated for ATV6xx versions up to 3.8. In future ATV6xx versions (>3.8) the structure of PID may change.
