How to determine the highest frequency a PLC can catch

I am working on improve the reliability of our rotation monitor for about 30 shafts in the plant. Currently, the proximity sensor is connected into Novatech D3 DCS, where the fastest speed of DI scan is 0.25 seconds and the program scan is 1 second. During operation, this combination lost a lot of pulse and I have to extend the failure time far more longer. The highest frequency in my applicaiton is about 10Hz.
My current idea is to have a CompactLogix and normal DI card to catch the pulse. But I don't know how to determine the highest frequency this comnination can catch. So that I can choose the right PLC and DI card. Any thought?

The minimum on/off filter time for embedded inputs is 0.5ms and the quickest RPI is 0.5ms. The modular IO has a 8ms filter on it.

I think you can safely say that you will be able to handle 10Hz

I have used flags on motor shafts as poor man's encoders up to 50Hz on modular IO.

Basic rule of thumb is double the scan time so you do not miss any pulses.

Nyquist theorm

AB also makes a 1769-IQ16F which is identical to a normal 1769-IQ16 input card but is made for faster signals. I don't remember there being any price difference, but if there is it is minimal.

Nyquist is the theory but would really like to know, do you use the scan time or input card speed to calculate?

When you are using proximity switches to measure shaft rotation you need to consider the length of time the signal is true and the length of time the signal is false. When you use an encoder, those times are generally equal. The OP doesn't say anything about the nature of the target that the proximity sensor detects. A frequency of 10 Hz (100 mS per shaft revolution) should be detectable by a DC input module in just about any PLC unless the target detected by the prox is so small that the prox signal is only true for a few milliseconds.

What you really want is a high speed counter module to catch everything coming from the prox switch. You could do this with a ML1100 or 1400 for less than the price of a CLX high speed module. Then use ethernet messaging back to the main PLC.

Steve Bailey said:

When you are using proximity switches to measure shaft rotation you need to consider the length of time the signal is true and the length of time the signal is false. When you use an encoder, those times are generally equal. The OP doesn't say anything about the nature of the target that the proximity sensor detects. A frequency of 10 Hz (100 mS per shaft revolution) should be detectable by a DC input module in just about any PLC unless the target detected by the prox is so small that the prox signal is only true for a few milliseconds.

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What Steve said. Ideally, the target for the prox switch should trigger the switch for one half of the full shaft rotation.

Work out the worst case time for detecting on and the worst case time for detecting off.

Then work out the shortest time that the thing you are detecting is on and shortest time it is off.

Worst case time to on = on time of prox + on time of DI card + RPI + scan time.

Worst case time to off = off time of prox + off time of DI card + RPI + scan time.

Shortest on time is interesting. you would think it would be width of flag / circumference, but it is actually smaller or larger, based on the percentage of the prox face that needs to be covered to sense your flag.

Depending what CPLX you are using, you can use a 1734-IK if you don't get the values you wanted. Note that this will not effect the on/off time of your prox, nor the time that your flag is covering enough of the prox to activate it.

Was looking at a slightly different application. Trying to detect reflections with lasers, with objects passing at about 20 metres per second (72 fps in your land!)