Pulse frequency seems high

[Bill]

I am using Pulse Frequency function to measure the frequency of a rotating wheel.

My sensor is a one pulse/rev encoder attached to a 12 " wheel.  The wheel is riding on a driven shaft. I am intrigued by the large numbers.

I am storing the measurement Pulse Frequency 1 to variable (A) and reading that value with hyperterminal. My value in (A) with machine running is 4525 hz.

If sensor is one pulse per rev, then according to the value of (A) it is rotating at 4525 rev/second or 271500 RPM.

In one revolution, it advances pi*D. The 12 " diameter is 1 ft, so that is pi feet per revolution. It seems unrealistic, but if 271500 RPM is correct, that is 853 000 ft/min.

Am I doing something wrong? or maybe do not understand the function?

[support]

Do you mean you program the PLC to send the pulsefrquency output to HyperTerminal?

Did you use online monitoring to check the value of A if A is assigned with the PULSEFREQUENCY(1) data.

Also did you check the pulses from the encoder using an oscilloscope to see if there are noise signal riding on the outputs.

When defined as HSC or PMON, the input software filtering is OFF so the inputs are very sensitive and can pick up the shortest pulses. So you can try to attach a 0.1uF capacitor across the input to filter out the short pulses and check the result.

[Bill]

Support,

Yes, I have function sending raw data to serial port and am using hyperterminal to log to a file. I have a 1st scan bit to enable PMON 1 in custom function. The output from the device is connected on input 3 of PLC, and assigned PULSE FREQUENCY 1 to variable A.  When monitoring online, value in variable A matches the data I am reading from the serial port.  

I have not measured with a scope. I will do this and report my findings. Also, if I add the capacitor, is it between input 3 and +24V?

Thanks as always for your direction.

[support]

Please add a capacitor between the PLC's PMON input (the one used for pulse measurement) and 0V.