Catching a number

I am using a Micrologix 1200 with analog module to a 300 micro panel view.
I need to measure the breakpoint or shearpoint of a bolt. I am driving a hyraulic cylinder with a proportional valve by a 0-10v output.
As the pressure builds a 4-20mA pressure sensor sends a feedback value.
When the bolt breaks I need to record it on the panel view.
My question is how do I catch the exact point when the bolt breaks/shears so I can display it.
Any suggestions would be appreciated.
Thanks

Is there anything (sensors or limit switches)that stops the process once you shear a pin? If so, you can move your feedback signal to a tag "current shear pin break point"until you get a signal the you have sheared a pin. I would then copy this value to another tag like "previous break point" once this condition happens. This way you can track/monitor what your system is doing.

As the pressure builds a 4-20mA pressure sensor sends a feedback value.

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Set up a routine where (after processs is started) you save the pressure every PLC scan. One the next scan, first compare the old saved pressure to new pressure. If new pressure is lower, then stop the routine because the last saved pressure was the highest before the bolt broke and the pressure dropped.

How long does the pressure stay at the break point. If it drops immediately your PLC may not be fast enough to capture it.

Thanks for your help.
I found the old PLC program they used.
It was an old Omron, been obsolete for about 20 years.
Hard to understand but I think they used 2 compare hexdec
blocks, so I'll try to use that idea.
Once again thank you for your help.

I think they used 2 compare hexdec blocks, so I'll try to use that idea.

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I can't see why they needed two compare instructions. Maybe they used two time periods based on the typical times wehn the bolt has broken in the past. Then they could compare both time (before predicted time and after predicted time) and pressure (old versus new) to determine the breaking point.

Not exactly sure what they were doing but I got mine working by using a 'Greater than' with the 4-20 signal being (A)N7:1 and (B)N7:10 then on the same rung "Move" with (A) N7:1 and (B)N7:10 This gave me a"peak" value as the pressure will increase each scan. I sent this number to the panel view after moving it to an F8 file. Then I used a "Less than" with (A)N7:1 and (B)N7:10 to turn on a B3 bit after the pressure dropped due to the shear. I'm sorry I don't have RS500 on this computer so I can't send you the ladder.

Good job. As always, the pressure will increase - until it suddenly doesn't! If you have already saved the last pressure, then the next that the PLC reads (no matter how slow it is) will be the one that shows that the bolt broke and the pressure dropped.

Do we really care how slow or fast the PLC was about seeing the pressure drop? I don't think so because we had already recorded the peak (maximum) pressure on the previous scan! There is always a possibility that the pressure really peaked in-between scans (in between recordings), but if your pressure gage is sensitive, the peak should be close enough for most purposes. If you keep your PLC program as short as possible (keep the logic short- doesn't matter if you use lots of comments because those don't go into the logic scan anyway) and as fast as possible, it will reduce the error in seeing the peak pressure.

If possible do a few test runs where all the pressures are recorded in a table. You might be able to span multiple files with indexing if needed.

Just doing a little Googling shows graphs where the first fall in pressure is not the failure point. It may be different depending on how the test apparatus is set up.

Some metals stretch or deform before they break, so that could cause a pressure drop followed shortly by the break.