How do I use a Counter input to acquire frequency data that is slower than my sample rate?

Question

Rick 2013-9-20

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https://ww2.mathworks.cn/matlabcentral/answers/87787-how-do-i-use-a-counter-input-to-acquire-frequency-data-that-is-slower-than-my-sample-rate

回答： Stephan Stranick 2020-10-12

I am trying to write an application with the Data Acquisition Toolbox that acquires a shaft speed via a counter input. The shaft speed sensor input to the counter channel will vary from 0 (stopped) to 1600 Hz. I will also be acquiring several standard analog inputs simultaneously.

I want to be able to acquire the data at a fixed sample rate of 2000 Hz. Any time my counter input signal frequency is less than my sample rate, I get a hardware error:

"Multiple Sample Clock pulses were detected within one period of the input signal. Use a Sample Clock rate that is slower than the input signal."

This is of course kicked because there are less than 2 and sometimes no edges counted in the interval of each sample.

It appears that I need to be able to dynamically vary my sample rate as a function of the counter input frequency such that my sample rate is always slightly less than the counter input. This doesn't work for my application since I need to acquire the other analog signals at a higher sample rate.

I could set up a secondary session that only acquires the counter input data at a slower frequency, but then I wouldn't accurately measure transients in the counter frequency that occur quickly.

I also tried using another counter output to generate the sample clock based on the current value of the shaft speed counter input, but I found that counter inputs cannot use an external sample clock.

DAQ hardware is a NI PCIe-6321.

Simple representation (in practice I'm doing this in a GUI via background acquisition)

    s = daq.createSession('ni');
    s.addAnalogInputChannel('Dev1','ai0','Voltage');
    s.addCounterInputChannel('Dev1','ctr0','Frequency');
    s.Rate = 2000;              
    s.DurationInSeconds = 10;    
    [data,time]=s.startForeground();