Task Properties Monitor

Rslogix/Compaclogix system

While monitoring a liquid transfer from a flow meter and using RSLogix on-line to watch what was happening, I noticed periodically on the "Task Properties"/Monitor tab, the Scan Time is normally running about 5 to 7 ms per scan and the totalizer updates at each scan as it should. Periodically during the 20 min transfer, the scan monitor scan seems to freeze for 2 to 3 cycles. The totalizer freezes during this time and when the scan starts back up again, the totalizer does not seem to update to what would have transferred during this time. It just starts up where it left off. This seems to be causing too much product to the transferred.

Can someone explain what might be causing the scans to stop and how to fix?

Thanks

Where is the totalizer? Is it part of the PLC-external flow meter, or is the CompactLogix program adding some scaled value of the last-measured flow to a running total?

CompactLogix program adding some scaled value of the last-measured flow to a running total

Typically, a totalizer where the input is a snapshot of a rate value that gets accumulated, is done in a periodic task so that the timing is very consistent.

My first thought reading your post is that what you are seeing is an artifact apparent on a windows PC that is experiencing some delay in updating the display or communicating with the PLC, but that there is not a real problem in the PLC.

But, you describe a symptom of the machinery that indicates an overrun, so maybe there is some unique style of programming being done that is having an effect. Or, perhaps it's both... you are studying values on a PC or HMI that appear to be freezing, and there is some coincidental unrelated issue causing the overrun.

More info is needed.

Blown99 said:

CompactLogix program adding some scaled value of the last-measured flow to a running total

Click to expand...

Then shouldn't the last-measured flowrate be multiplied by the last scan time (5-7ms typical, but sometimes 10-20ms)?


A continuous (free-running) task on a PLC will run at a typical rate (5-7ms in your case), but every now and then the PLC will do some extra housekeeping (network comms (ARP?), etc.) between scans, and which lengthens the times between scans.


So my model of your situation is that your Logix program scales the latest flowrate measurement for the average scan time (5-7ms), but when one of the long inter-scan times occurs, it adds a number that is too small to the running total.


One solution is to use the last scan time in the scaling for the value added to the running total during each scan.

Another solution would be to increment the running total in a periodic task, so the scan time is (more?) consistent.

Another solution would be to do an experiment to find (calibrate) the mean scan time over a longer period i.e. many 5-7ms values and a few 15-20ms values; depending how frequently the longer inter-scan periods occur, this may be accurate enough, but if the program changes then it would need to be re-calibrated.

I agree with OkiPC. The customer has complained of over pumps, but I think I have that pinned down to a bad power supply feeding the meter and the AI of the PLC. It is flakey and will stop providing power for several mins to a day. (Have had a replacement on order that was supposed to be delivered this past Friday before noon and it has not shown up yet, It is "On the Truck for Delivery":angr

But while waiting on the power supply, I have been watching the system (They have load cells on the tanks and can pump manually). I did not know if it was a issue with the PLC or the RSlogix running on the PC not being able to keep up.