AB Analogue Card 1769-IF4 signal fluctuation

[Ehsan-ul haq]

Hi All,

I am working on a small PLC system having Compact Logix LE33 PLC having only two IF4 card. Some of these channels are used for 4 - 20 mAmps pressure transmitters and some vibration Tx. When system is operational these channels read the process variables correctly but as soon as system stops or pressure goes to zero, they start showing 3980 to 4001 value and under range bit becomes high in card diagnostics. All transmitters are 4 - 20 mAmps. There is only one source of 24 VDC. I have checked wiring which is as per manual and all commons are looped together.
I believe one common factor is effecting all these readings, cant figure out what. Any help would be appreciated. Thanks

 

[drbitboy]

Is there a question in there? What do you want help with?

Assuming a reading of 4000 (Engineering unit range, right?) means zero pressure, those readings of 3980 to 4001 indicate a signal of 3.98-4.001mA, which is within a whisker of zero pressure, and can be pretty convincingly explained as normal, i.e. typical, noise in the measurement. Whenever a zero pressure in the physical system being measured results in a digital reading below 4000, that Under-range bit will become 1, and that bit will be 0 when the noise, or the signal, return a signal of 4000 or higher.

The fact that the range readings at zero pressure is not centered on a reading of 4000 suggests that there could be a slight mis-calibration in the instrument zero, but unless that zero was re- and mis-calibrated to be at least ~4020, that Under-range bit would still come on occasionally.

That bit would also be 1 if the 4-20mA wiring were broken, but then the digitial reading would be around 3200, which is when it would be worthwhile raising an alarm. N.B that 3200 is well outside the range of noise generated when the pressure is zero.

Chasing noise is a time honored, and time-wasting, activity. Thank you for the opportunity.

 

[Ehsan-ul haq]

Thanks drbitboy for your response. Yes engineering units.
Actually I am using under and over range bits as healthy interlocks for the loops, that is why loops cant run as per logic if any of those bits is high.
As you have advised it is normal and it is worthwhile creating an alarm if digital reading goes to 3200, in case wire is broken, could you please advise which bit I can use as an interlock if digital value goes to 3200? if there is any. Thanks

 

[Steve Bailey]

Instead of using the under and over range bits in the module's data file, roll your own in logic.

Something along the lines of this:

My_Underrange_CH0 = CH0.data < 3975

 

[drbitboy]

please advise which bit I can use as an interlock if digital value goes to 3200? if there is any. Thanks

[Update: what @Steve Bailey said]


don't use a bit, use a compare i.e.

LES Pressure_reading_raw 3800 OTE Pressure_reading_invalid
​

Where

• Pressure_reading_raw is the reading from the IF4,
• Pressure_reading_invalid is a boolean flag,
• 3800 is my guess at an abnormally-low pressure reading that is not caused by noise.