Billstrong,
the most accurate way to measure a motor's velocity is using a data acquistion board that provides a high-performance counter, like an M-Series board or an NI 660x counter/timer board in combination with a quadrature encoder.
There are several methods to determine your motor's speed:
- Buffered Period Measurement
This method allows you to determine the period of each cycle of either phase A or B of your encoder signal with a resolution of 12.5 ns. Here is some more detailed information. The drawback of this method is the fact, that you don't get information about direction changes of the motor. The benefit is the very high timing resolution.
- Buffered Position Measurement
You can use a counter/timer to generate a clock signal, that latches position readings of a second counter/timer to the PCs memory. This method is less accurate than the first one, as it requires several position changes between each rising edge of the clock signal, but on the other hand you can detect direction changes and the amount of data generated is much lower than with the first method.
Please refer also to the shipping examples in LabVIEW that install with the NI-DAQmx driver.
I hope this helps,
Jochen Klier
National Instruments
