I am using CDAQ with a NI9229 4 Channel analgoue input card.
I have a sensor that outputs an analogue 5v square wave signal. The pulses are triggered from eithe a north or south pole of a magnet. I have an aluminium wheel with 30 magnets, all magnets are in opposition to each other. when the wheel is rotated the output is a square wave.
My magnet wheel is rotating at 182° per second. Over 360 degrees there is 30 magnets. Thus making the sensor switch 30 times.
The time to revolve the wheel one revolution is 1.978 seconds.
I have set up the NI9229 to the following
- sampling rate is set to 25kS/s So to record the data for one revolution it works out as 49450 bits.
- Have set the trigger levels for recording a hi or lo pulse to 2.7 and 0.7 volts.
50000 samples would be two seconds worth of data which would be more than 360° of rotation of the magnet wheel.
The problem I am having is that my pulse count changes between 30 and 31. But as I am only recording 1 revolution of data there is physically only 30 switches. Do you know why I am getting a change of 30 and 31.