Encoder Speed with NI 9215 module and cDAQ-9172

Question

I've a NI cDAQ-9172 with the NI9215 analog input module, and I want to estimate the speed with an optical encoder (OMRON E6A2-CW3C). My approach was, making a comparison with a point in the midle of the high of the signal, ie 4V, making this way a perfect square signal with 1of high. After that I use the Tone Measurements and the Timing and Transition Measurements modules to calculate the frequency of the wave form, and this way I'm able to calculate the RPM speed.
The problem with this solution, is that even with the engine at a constant speed of 746RPM, measured with a stroboscope, Labview give me a high variable values between 730RPM to 760RPM, and I can't solve this problem because I really need an accurate measurement.
Should I buy for example the NI 9401 module to make this measurement? If so, does any one knows how to build a VI to get the encoder speed?
In attach I send the VI that I'm using to calculate the speed.
Thank you for help.
Attachments:
Encoder Speed.vi ‏112 KB

Answer 1

Hi,
The best way to measure a speed through a quadrature encoder is to use the counters that are inside the cDAQ chassis.
With the 9172 chassis, you need a digital I/O module (9401) in order to access the counters.
With the new generation of chassis (9178), you don't need the 9401 because you can connect your encoder signals to the BNC.
Best regards,
Thomas B. | CLAD
National Instruments France
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