pulsefrequency

[Dogface]

I think I'm missing something.  I'm trying to display the speed of a slow moving drum using pulsefrequency but am getting very strange results.  By strange, I mean unuseable.
T100MD24
pmon 1 in 1st scan.
npn proximity sensor connected to input 3.

The sensor is picking up the teeth on a gear which has 39 teeth and ranges from 1 to 15 RPM.

The results I get from pulse frequency go from 500 to 1600...I would be expecting something closer to 1 at slow speed and as high as 585 at maximum speed.

Any ideas?  Pulse width and Pulse period are equally unstable.

Thanks,
Sean

[support]

You may want to check if the proximity sensor is receiving multiple pulse inputs from the gear head. One quick way to test is to define HSC channel 1, keep input #4 open circuit and pulses entering input #3 will increment the HSC. Check how many pulses are received when the gear turns one revolution. If you are receiving much more than the number of teeth on the gear then there is a problem with the setup.

[Dogface]

Right you are!  The input is registering many pulses when the circuit closes.
I know that this is Tech Support for the PLC and not really meant as a forum for Electronics 101, but if someone could tell me what I've done wrong.....

Thanks,
Sean

[support]

One possibility may be that the sensor distance to the gear tooth is too far and that put it at the thresold of sensing. Is it sensing the metal or is it an optical proximity sensor? Adding a capacitor of suitable value across the sensor signal and ground may reduce its sensitivity and help even out glitches although I would leave this method as the last resort if all other solutions won't give you a steady count.

[Dogface]

Thanks for the help.  It turns out that the distance from the sensor to the PLC is the problem.  Or more correctly, either the impedance or capacitance of the wire between the sensor and the plc is not correct.  
I've set it up so that the sensor triggers a relay which activates the input.  I'll just have to change the relay every 4 months or so.

Thanks again,
Sean

[Dogface]

I finally found time to re-evaluate this issue.  Turns out that the cable between the PLC and the NPN Prox Sensor (approx. 40') was/is picking up noise caused by my Frequency Drive.  A simple 1 microfarad capacitor, between the signal and ground, located at a terminal strip near the PLC has proven to be sufficient filter.

Thanks for the help.

Dogface

[support]

Thank you for the update. We are glad you have found the cause and the solution to tackle this problem.