Flow Meter On CompactLogix

I am using a flow meter with a CompactLogix L24 processor. I believe that my flow is too fast, and I am missing some counts.

For example, a count of 1410 pulses one run results in 300cc of fluid. But a count of 1410 on another run results to 290 cc of fluid.

I am trying to figure out a good way to approach if my flow meter is matched well with my PLC. Any advice on how to do this will help?

For example, my RPI is set to 20.0 ms for the embedded counter I am using, but the update time in the counter configuration is 10 ms. Is this a mismatch that I should be concerned with? I'm just not sure what I should be looking at to prevent the mismatches I am seeing..

http://www.plctalk.net/qanda/archive/index.php/t-78529.html

http://www.plctalk.net/qanda/showthread.php?t=67770

http://www.plctalk.net/qanda/archive/index.php/t-49537.html

The above previous threads may help you out.
Regards,

I will take a look at those links, thank you. I flow meter is sending around 1400 pulses per second. The scans for my continuous task are coming in around 3 ms. For 3ms, I cannot exceed 333 pulses per second right? 1/.003 is 333.333. Is that the correct way to look at it?

Yes this is the calculation on how long the input must be on in order for the PLC to read it. It is also how long the output needs to be off in order for the PLC to acknowledge that it is also off.
The duty cycle is 50%. Amount of time on vs off.
Scan times in the PLC can change depending on a number of factors. Some PLCs will allow you to fix the PLC scan to a value.
I would always divide this by 2 to get the rate of the input to the PLC. So I would use the value of 166 pulses per second.

Regards,

The L24 PLC has an embedded hi speed counter. Does this change anything? Right now my flow is over 1000 pulses a second and it works pretty good, but still has errors. It seems like reducing this down to 170 pulses per second is a pretty big reduction in flow.

Is there something in the embedded hi speed counter that lets me flow faster than the scan time limits?

http://www.plctalk.net/qanda/showthread.php?t=49549

http://www.plctalk.net/qanda/showthread.php?t=102811

The above threads will help you out with the HSC function on the PLC.

Regards,

Thanks Garry, those posts do help clarify the HSC. However, I am still unsure if the HSC allows me to defy the scan time of the PLC. Using the embedded HSC on the L24, would I be able to read 1000 pulses per second on a PLC that has a scan rate of 0.003 seconds?

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1769-um006_-en-p.pdf

The embedded HSC allows you to count independent of the PLC scan. They can also be set up to fire the outputs with different range settings.
See the above manual.

Regards,

fifty60 said:

I will take a look at those links, thank you. I flow meter is sending around 1400 pulses per second. The scans for my continuous task are coming in around 3 ms. For 3ms, I cannot exceed 333 pulses per second right? 1/.003 is 333.333. Is that the correct way to look at it?

Click to expand...

Not really. Look at the Task Properties for your continuous task. There are two sets of numbers displayed:
Scan Times (Elapsed Time)
Interval Times (Elapsed Time Between Triggers)

Scan times is how much processor time was used to execute the program.

Interval times is the real time that elapsed between one start of the program execution and the next.

The difference is from the activity going on at the same time as the continuous task (the overhead time slice and any higher priority tasks).

It is possible to have a program that executes in 100ms, but only executes once per second.

The continuous task is not the best place for tracking fast inputs. You may want to look at "event tasks" or high speed counters.

Very helpful information, thank you. I am trying to set up the filter values in the input configuration. My options are 0.2mS, 0.7mS, and 7ms. The signal i am sampling is coming in at around 1500 pulses per second.

Looking at the HSC manual from AB for the 1769-L24ER-QBFC1B embedded module, I see that this is a low pass filter. Meaning, it lets in frequencies below a certain threshold.

I understand that much, but after reading the manual a couple times i still do not know what to set the filter to. I would like to simply block out any frequencies above 2000Hz. Any help for setting this would be greately appreciated.s