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Old March 3rd, 2010, 07:21 AM   #1
rguimond
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Digital Counter "jumping"

I made a recent hardware change to a flow meter system and have come up with a strange problem. I hope someone can lead me in the right direction.

The counter - Veeder-Root (Form 797586-220)is connected to an output on a ML1100. A pulse from a flow meter is sensed by an input on the ML1100 and is used to increment an internal counter. In parallel with the internal counter is the output to the digital counter. The counter sends one pulse per litre. The flow rate is 17,000 litres/hour (approximately 4.722 pulses / second)

Originally, the counter was connected directly to the meter (no PLC in the way) and would increment every litre.

Currently, the counter sometimes increments by litre, but also skips from time-to-time.

I tried connecting the counter in parallel with the PLC input with similar results.

I should add that the internal counter updates perfectly.

I was thinking of installing a pull-down resistor between the pulse and ground. Any other suggestions?
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Old March 3rd, 2010, 01:58 PM   #2
Lare
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How you have programmed your plc program?

If your plc output is true only one plc scan when flow pulse is sensed by input, this is maybe little too fast for external counter and it won't count all pulses.

Try adding pulse "stretchening" timer for plc output (ex. tof timer 10-100ms on plc program before output)
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Old March 3rd, 2010, 03:35 PM   #3
rguimond
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Thanks. I see what you mean.

I set this up. Should hold the output closed, but the ONS may mess it up. Maybe "screenshot2" would work better?

In any case, the homogenizer is washing now, so I'll have to wait until later to try it.
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File Type: jpg Screenshot2.jpg (83.2 KB, 12 views)
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Old March 3rd, 2010, 03:55 PM   #4
Lare
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"Sreenshot2" probably works, first won't because of ons insruction. But if there is TOF-timer block on ML1100, I would use it instead of TON-timer.
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Old March 3rd, 2010, 04:17 PM   #5
Lare
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I thinked your sreenshot2 better, it won't keep output closed longer time than flow pulse duration.

Instead your output will go to closed state after ton-timer expires, if it even expires at all.

If you wan't use ton timer, I think you have to use sr-flipflop. flow pulse sets output, and if output have been closed ex. 500ms then plc program resets output. But, as I wroted before, easiest way is to use tof-timer and ons inscruction.
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