Measuring/calculating shaft speed from a counter card

I've been trying to measure shaft speed with a 120 tooth speed wheel, a bi-directional powered pickup, and a 1756-HSC/B counter card.

I am driving an eddy current coupling with a AC drive system, and trying to control the eddy current coupling output shaft speed by measuring the speed wheel and changing the drive speed command. There is a brake on the output of the eddy coupling that applies load, and the customer wants to maintain brake speed within 5rpm while the brake is applied.

I first tried the encoder x4 mode using the Totalizer field to give me frequency, but the signal only updates roughly every 1000ms.

Currently I am using encoder x4 mode on Channel 0 for direction, and Period/Rate mode on Channel 1 to measure frequency. I am getting much quicker updates in Period/Rate mode, but as I increase in speed the reported frequency starts reporting spikes of lower value.

I tried using encoder x4 mode to calculate a difference in the number of pulses over 100ms, but must have had something wrong because it never seemed to work properly.

Does anyone have any suggestions or some sample code to give me some direction on how this is usually done?

Have I got this right
1. 120 tooth gear
2. sensor input to the HSC card
3. 5 RPM
I don't see why you are using the x4 mode unless you are using an Encoder

the Maths - 120 pulses per rev.
so each pulse can be measured against time
@ 5 RPM that is 120 x 5 = 600 pulses per minute.
OR 10 Pulses per second (10 Hz)
at that rate you could easily use a standard input

Ian:

I think you might be misunderstanding the intention of the OP. I believe he means he needs to maintain the RPM +/- 5 RPM. I don't believe he ever actually states his target RPM. Which would be useful information.

Stu...

That's normal for me Stu, I always read these posts wrong
But we still need to know the RPM of his device

The target speed is operator selectable from 0-2000rpm in both directions.

I played around a bit with the different functions and ended up running Channel 0 in Encoder x4 mode to get direction, and Channel 1 in Frequency mode to get speed. I had initially hoped that the directional frequency provided by the HSC would work for my application, but proved to have too slow of an update rate. I have not found any reference to changing that update rate either. I ended up setting the frequency update to 200ms which gives me decent feedback and 2.5rpm resolution even with the 120 tooth gear.

I tried using the period rate frequency mode but it get excessively noisy as I increased rpm.