difference in gear tooth counted while changing RPM

[balajikasthuri]

I have connected a photoeye to counter0 of PCI6014 to find the RPM of a (60 tooth) gear drive. When the speed of the gear drive increases the counter reads lesser than 60 edges per revolution. I later connected a digital tacometer to the output of photoeye to cross check the measured value and found that the tacometer indicates the exact value.Thus I concluded that there was no problem with my photoeye output.
What could be the reason for this?
I have modified the DAQ_STC Count Edge.VI example to measure the number of edges.

[JoshuaP]

The counters on the PCI 6014 are designed for TTL signals only. It is possible your signal is gathering a lot of noise as it increases in RPM and falls out of the TTL specs. Many tachs will respond to any type of signal, so I would not use this as a basis. Here is a KB that has some useful information.

http://digital.ni.com/public.nsf/websearch/E84361EAAF6C092686256AAA007ADA7B?OpenDocument

You should check specs for your photoeye to check against the PCI 6014 specs (http://www.ni.com/pdf/manuals/370636a.pdf) and the TTL specs to make sure it is compatable. I would also suggest getting an o-scope to look at the signal as it enters the 6014 to make sure it does not have a lot of noise on the lines. The 660x counter device have built in filters that are gre
at for reducing noise or you could build you own filter to eliminate high frequency noise.

I hope this helps.

Joshua