Counter Measurument Problem

[Gilfoyle]

Hi

I have a CompactRIO counter module. I am using it to make period measurument. I am also using a third-party hardware that sends me RPM. The same sensor is connected to either CompactRIO or the third-part hardware.

I know that the resolution of CompactRIO module is bigger than the other device. But when I look at the measurument results, I see that at low frequencies there is big measurument difference in between the two devices.(for example -+500 RPM) When the speed of engine increases, the difference decreases (for example -+200 RPM)

That should not be, there is something wrong. But I could not figure out.

Note: Input filter of CompactRIO counter is set to 1 ns. (I did not try the other options except 1 ns)

Any idea??

[Brian.D]

Hi sefaa,

If you try to measure a known period with your cRIO setup, do you measure what you expect? How about with the third-party hardware?

[Gilfoyle]

Sir, as I said, I am sure that the resolution of CompactRIO is bigger than the other device. There is no doubt.

But when I move throttle signal %40 for example, the other device gives me 40120 RPM for example, but CompactRIO gives me 40670 RPM. I mean there is something wrong. The difference should not be so much. Because the speed should be 40000 RPM according to the throttle signal.

So if I change input filter of counter module from 1 us to 16 us for example, does the 40670 value decrease?? Or the input filter is irrelevant with this subject??

[Kevin_Price]

I'd venture that you're seeing an artifact of quantization.   I have no way of knowing which measurement is more at fault.

The fact that you see a greater discrepancy at low speeds suggests that one of the measurements may be measuring the # of encoder pulses seen in a fixed time period.  At slower speeds that count will be smaller, leading to a greater quantization effect.

Another method to measure intervals is to count the # of high-freq timebase pulses between successive encoder edges.  (Here the encoder edges serve as a variable rate sample clock.) 

Either way, the size of the quantization effect depends on the ratio of the two timing signals.  The former method is good when the encoder freq is high relative to the sample freq.  The latter method is good when the encoder freq is low relative to the timebase freq.

-Kevin P