How to configure AI card to match field reading with DCS reading?
We have recently installed ABB DCS 800 XA system in our plant. Since installation we are facing the problem of field instrument reading connected as AI to DCS AI815 card doesn't match with the reading displayed in DCS. We have calibrate our filed instrument, it is ok. We have checked the scaling in DCS. It is alos OK.
For example : ENdress & Hauser FLowmeter connected as 4-20mA inout to DCS.
At zero flow it the flowmeter it shows 41 m3/Hr flow in DCS. Please help to resolve the problem.
Earlier we were using same flow meter with siemens DCS PCS7 it was showing correct value.
You have to connect test unit with 4mA to the input of AI815. Then check the value in DCS. If still shows other then 0, you have to check various things:
- in control builder, hardware configuration of this input channel. In online mode you can see directly the value of the input. At this hardware configuration you have to set range which should be 0...XX in your case. Also check 4-20mA setting, also SQR characteristic, offset and so on - these are hardware configuration of the input module
- then the value of this input is going in application software. It could be configured by means of calculations until it is displayed on the screen.
There are too many possibilities, have to check the way of the signal, both hardware and software
It doesn't matter Siemens or ABB, we are talking for current signal and common controller's issues.
1. Try 4 mA to input = min range (e.g. 0 m3/h), 12 mA = half range (e.g. 500 m3/h), 20 mA = full range (e.g. 1000 m3/h)
2. Check settings for AI815 (attahed document). Pay attention on:
- Shunt mode = internal
- Linearization code channel = No linearization
- fraction = 1
3. Check cabling - some TU doesn't support 4-wire transmitters directly (you should connect signal- to L-). In the attached document you can see typical connections.
AI815+TU810 - 4-wire transmitter for channel 1 you have to connect:
signal+ = B1, signal - = A1
signal+ = C2, signal - = A1,A2
I agree with below answer from kstoilov. As long as the Range (Min & Max values) of AI card channel is correct, the indication in DCS only representation of the current (4-20mA) comining from the field transmitter.
I would like to suggest the following simple and routine troubleshooting points:
1. Check the current output (4-20mA) signal from flow meter. Is it 4mA when it is showing 0m³/hr at local display?
2. The flowmeter you are using to measure the flow is a magnetic flowmeter? If yes, when the tube is empty, the current output from the transmitter will not be 4mA. You need to set the empty pipe detection to 4mA.
3. The other possible cause would be noise, causing this error to the measurement indication. Hence please check the current at AI card as well.
There is nothing else to configure in the AI card. You only set the minumum (4mA = 0 L/min and 20mA = maximum L/Min).
- How much of a percentage error is 41 m3/Hr. ?(ie what is the max range)
- You say that you read 4.1 mA instead of 4.000 mA, but is that 4.0501 or 4.1499 mA ?
- Is the 41m3/hr measurement consistent with reading 4.1mA ?
- What readings do you get if you use a signal generator and inject 4.0mA into the AI terminals ? Into the cable ?
- Are you 100% positive that the Siemens PLC was displaying the true reading and no-one had done some other "tweaks" in software to make the signal read "correctly" ?
This likely is not a new problem. From your comments, on your old Siemens PCS7 you simply adjusted the A/D scaling so that 4.1mA read as zero m3/Hour. Both the Instrument and the AI card are working as expected.
If you are looking for a 0.3% error on an analogue signal, then thats pretty close to the expected precision and personally I wouldn't usually bother trying to get any better accuracy. However, if you need a more accurate result then the issue is somewhere in your field wiring. If you inject 4.00 mA into the end of the field cabling and you dont get 4mA at the AI815 card then you need to troubleshoot this.