Digital input confusion.

I am looking for a low cost solution to count the revolutions on a shaft. I am currently using a Logo OBA6 controller on the machine. The system supply voltage is 230V ac and the required frequency/rpm is 500 Hz or rpm. I also have a VSD in the system to control the speed of the shaft/motor. Because of the VSD I cannot use a timer on the shaft as the speed is adjusted on percentage of the rated frequency of the motor. The functionblock program uses a counter that gets it's signal from input block. Thusfar my the closest I get to a solution is to use a proximity switch, but the ones with a high enough switching frequency are dc voltage. If a dc voltage proximity switch does work, how do I connect it to provide switch for the input block?

Hope you can help.

However it is an absolute neccesity that I count the revolutions of the motor as it is critical to the proccess. Are there any low cost alternatives that I can look in to? My experience with electronics and which technologies are available on the market are limited unfortunately, so I'm hoping you can help.