Totalizer In Micro850 PLC

JackSigma

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J
Join Date
Jul 2022
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Perkasie
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I have an existing system that is measuring and totaling the flow of 6 different flow meters. They are all reading high and have no idea why. This is how I programmed them in RS500 and never had an issue. Am I missing something? All 6 flow meters have 4-20ma outputs. Here is how i have them programmed.

Screenshot (16).png
 
As has been mentioned in many threads on the subject, integrating a flow signal in a PLC is not the most accurate way to calculate cumulative flow, but sometimes it's the only tool you have available.

What is the range of your flow transmitter compared to the actual flows in your system? Is your scaling of the 4 - 20 mA signal subject to any round-off error? When you use a self-resetting time to generate the pulse that triggers the accumulator, the pulses are not one second apart. That's because, 1) you lose the duration of one PLC scan while the timer is disabled and 2) you lose any fractions of a second in excess of one second stored in the timer's accumulated value because it gets zeroed when the timer is disabled.
 
have you verified that the meters are outputing a 4-20 mA signal and that it is wired correctly to the PLC and you are seeing over 4 mA at the PLC.
 
The only reason i know that the totals are high is because there is an influent flow meter reading the flow of water coming into the well that my systems pumps water out of, and the totals of my system are consistently 5-10 thousand gallons higher every day. which is definitely not the case just by looking at the process. I have verified the 4-20 ma signal.
 
5-10 thousand out of how big a total?

Is it possible one system is 4-20mA and the other is 0-20mA, or there is some other difference in the scaling that is not accounted for in the Micro8xx?
 
Using a short-term timer(or any timer really) that resets itself guarantees totals in [short] of reality. Why yours would be in excess is odd. Timers expire only when the accumulator is greater than the preset. Better to use a very long-term timer, accumulate the value every scan and divide by the counted scans when the long-term timer expires. Maybe an hour or so. Maybe 24. Best is to just get a meter with pulsed out put or integrated accumulator.
 
Probably not worth the effort. I've had best success accumulating every scan, counting the scans, dividing by the scan count at let's say 1 minute intervals. No matter how you look at it, there is going to be timer-induced error. The bigger the time, the smaller the error. I hate totalizing things by analog flow.
 
LegacyLee said:
Using a short-term timer(or any timer really) that resets itself guarantees totals in [short] of reality. Why yours would be in excess is odd. Timers expire only when the accumulator is greater than the preset. Better to use a very long-term timer, accumulate the value every scan and divide by the counted scans when the long-term timer expires. Maybe an hour or so. Maybe 24. Best is to just get a meter with pulsed out put or integrated accumulator.
Click to expand...


1) This ^. That said, 5-10k error out of 20-30k is not caused by roundoff in the repeating timer (as @LegacyLee noted)

2) 86400s/d is 16.4 bits, so the last values of the day only have 7.6 (=24-16.4)* bits of their resolution used; 7.6 bits of resolution is half a percent. 5-10k error out of 20-30k is not caused by roundoff in the 32-bit REALs.

The code, although it could be improved at the sub-percent level, is fine as is; this is is most likely a calibration and/or a scaling problem.

* 32-bit reals have 24 bits for mantissa (i.e. resolution is 1 part in 16M (=224), 8 bits for exponent, 1 bit for sign.
 
I agree that the magnitude of the error suggests something wrong with the logic or the scaling. The issues I cited aren't likely to cause an error that large.

Here's a way to reduce the error due to resetting the timer if the Micro850 permits it. Instead of resetting the timer, subtract 1000 milliseconds from the timer's accumulated value once the timer's 'done' goes true. It made a huge difference thirty years ago, the first time I tried to create a real-time clock in ladder logic.
 
Always get a flowmeter with dual (analog & digital) outputs.

Use analog for current flow rate
Use relay or transistorized digital output for flow total count.

I've not seen a Badger with only AO, usually they have at least 1AO and 2 DO standard.

What meters are you using, exactly (provide make and part number)

Tim
 

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