The counter output is not a smooth increment, it looks like a stairway

Question

I am trying to smooth out the rpm reading from my counter 0 on a PCI-MIO-16E-4 daqboard. When I increase or decrease the speed, to 0 RPM it looks like a stairway going up and down.

Answer 1

It sounds like a discretization effect that can arise if you figure RPM by counting edges and dividing by the sampling period.
For example, suppose your sampling period is 0.010 sec (100 hz sampling rate). Your edge count can only change by discrete integer amounts each sampling period. The change may be 0,1,2,3,... counts leading to calculated frequencies of 0.100,200,300,... hz. But you will only ever see these discrete values, hence the stair-stepping effect when you accel or decel your device.
Odds are you can get much more precision using the buffered period measurement technique -- though you'll also likely get substantially more data to handle. In this method, you'll use the high-frequency internal timebase (20 MHz) to measu
re the intervals between your device's pulses.
For example, suppose your device pulses at 1250 hz. Each interval will measure 16000 cycles of the 20 Mhz timebase. Now if you have an off-by-one discretization error, you'll get 16001 cycles in an interval for a measurement of 1249.92 hz.
There's a really good tutorial about frequency measurement error posted here.
-Kevin P.