How to measure RPM In motor in plc using proximity sensor

Please help

Are you having trouble with component selection, electrical design, or programming?

Programming

Couple of ideas come to mind.....
you would need to detect a target with the prox, usually a gear, determine the number of pulses per revolution ( 1 rev equal x teeth), then count how many revolutions per minute. How fast is your application?

Two basic methods to calculate speed from a pulse train. You have to know the number of pulses from the proximity sensor per revolution of the shaft.
1. Count the number of pulses over a fixed period of time. That gives you pulses per unit of time. Then revolutions per unit of time = pulses per unit of time divided by pulses per revolution.
2. Measure the time between pulses. That gives you the inverse of pulses per unit time (unit of time per pulse). Then revolutions per unit of time = 1 / (pulses per revolution times unit of time per pulse)

RPM Measurement Techniques:
http://documents.opto22.com/1784_RPM_Measurement_Techniques_Technical_Note.pdf