Square wave frequency & trigger

[danieleaston]

Dear developers,

I am fairly new to Labview programming so forgive me if this question or something similiar has already been asked.  I am using Labview 7.1 with a USB 6009 DAQ.  I am using a light beam break sensor to monitor the rotational position of a shaft via a slotted disk.  The slotted disk has 3 adjacent slots joined together as a larger hole, one edge of which denotes an important rotational position (BDC).   The output signal is therefore a square wave (0v to 12v) with a larger (0v) gap.

It would be brilliant if I could capture and record the rising edge of the gap as it would give me a reference point and would allow me to calculate a revolution time and hence frequency.  As well as this I would like to be able to calculate the rotational position of the shaft using each rising edge with reference to BDC.  So for example 1 rising edge after BDC equals 6 degree rotation, 2nd rising edge is 12 degree rotation etc..  I would then need to reset back to 0 at each BDC.

I hope that is clear.

Any help will be much appreciated

Many thanks

Daniel Easton 

[Stuart_R]

Daniel,

I'm not 100% sure what you question is?

Are you able to acquire the signal already? If not, I would recommend looking at some of the DAQmx examples available by going to Help>>Find Examples within LV.

Something like Cont Acq&Graph - Int Clk.vi should allow you to start reading data.

[danieleaston]

I am acquiring the square wave signal on the DAQ6009 analog input channel at 2kHz.  I can plot a graph of the data so I am sure the incoming data is correct.

I have 2 problems:

1) How do I get the frequency?
2) How do I count the rising edges of the wave?

Many thanks

[Stuart_R]

A nice function that will allow you to find the rising edge would be the peak detector.vi or the Threshold Peak Detecor.vi, try doing a palette search for this.

Frequency measurements can be done in a varierty of ways depending on the accuracy needed. For example, you could use the time stamp information to find out the period of a signal, and then do a simple 1/t to find out the frequency. The are hardware methods that would give you an accurate reading, but this can invlove using two counters/analog triggering, which isn't available on you r particular low cost device.