Having trouble getting RPM from prox sensor

Question

I've been trying to use a proximity sensor to find the RPM on a dyno, but am having some trouble with the LV coding (I'm new to LV).  Using a cRIO 9012 with NI 9205 module.  The prox sensor basically gives about an 8V signal when a tooth on a gear passes in front of it (theres 15 teeth).  While I can get the counter part going, I can't get anything after that part in the code to work, in order to find the RPM.  I think one problem might be with timing, where the prox sensor will be going at about 500 Hz, so I'm not sure if thats giving the differential function problems?  Is a timing loop even the right thing to use?  Eventually I'd like this to run standalone on the cRIO and write to a USB stick but I know how to do that part.  If anyone could help me solve this or let me know a better way to find the RPMs it would be much appreciated.  I've tried everything I can think of, I couldn't get functions that find the frequency or period to work when I made the signal into a waveform (square wave) even though I thought that would be the simplest method.  Thanks! 

Answer 1

c ryan, 
You should take a look at the LabVIEW examples specific to cRIOs (LabVIEW Help > Find Examples > Hardware Input and Output > CompactRIO)? Under the Basic IO folder, there are a few good examples for reading analog input, and there is even an example in the Module Specific folder designed for a 9205. For that latter example, you would need to change your chassis to be the 9111 (add a new chassis, and move the module under that chassis).
After you have the signal, you can just count the number of time the signal is greater than 7V (or some threshold that will be sure to catch the 'highs' indicative of a tooth passing the sensor). Every time this count is a multiple of 15 (15 teeth per revolution), you could log the time. This would give you an effective RPM.
~kgarrett
Applications Engineer
National Instruments