Recommendations Please

Hello. I am a novice at plc programming and am trying to achieve the following goal. I have an input from a sensor to detect the rotation of the idler wheel on a bandsaw. The input switches on to off at an irregular rate. I wish to control the state of a bit to tell when the rotation has stopped. Any recommendations for the most efficient way to accomplish this task? I am working in RS500.

Are you saying that the sensor is sending a pulsed signal but the signal is not a steady frequency? When the wheel is stopped does the sensor send any signal? What type of PLC do you use a SLC or MicroLogix?

Yes, the frequency is not steady. The sensor is switching based on a light/dark detection thru a hole in the wheel. The wheel could stop in any position causing the input to be on or off. Processor is SLC505. Thank you very much for your response.

Depending on the maximum frequency you need to detect, you may need a high speed counter module.
If the duration of the on and the off state of the sensor is always greater than the PLC scan time, you can wire the sensor to a discrete input point and set up two timers in logic. One timer for the off state, the other for the on state. Set the presets for the timers longer than the normal duration of the sensor states. If either timer reaches its preset, motion has stopped.
If the duration of the on and off states could be less than the PLC's scan time you will need to use a high speed counter module. That will be a little more of a challenge for a rookie.

Use a timer that gets reset by the sensor. If the timer times out, then the wheel has stopped.

see if something like this will help ... note that depending on which processor you're using, you might have to substitute an ONS instruction for the OSR ...

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If there is a chance that the sensor can be in an on or off state when the wheel stops then perhaps a way to do this is to look at whether the signal has changed states in a certain time period. the Slc505 has a STI or selectable time interrupt meaning that you choose at what time period you jump to a selected subroutine. If you could count the number of times the signal is pulsed during a time period then you have a way to track it. For example if during a 1 second period you get 100 on to off pulses. when the number of pulses per time period is near zero then the wheel has stopped. In this way it can stop with a signal either high or low and you would determine that the wheel has not moved. at the end of each time period you could reset your counter. If you don't care so much how accurate the time period is then there is a status word in the SLC that will pulse at a known rate. The rate is determined off the processor clock if I remember correctly. If you are getting more than 32000 pulses per time period then a HSC could be a better route.