External source for timed while loop

[user0110]

Hello everyone, 

I have a timed while loop that is currently governed by the default KHz sample clock (on Windows). I need better resolution than this sample clock can give me, and am looking into external timing sources. I am currently using a PCIe-6320 for data acquisition, and am reading that it has a 1MHz onboard clock. I'm hoping that I can use this to get the timing accuracy I need out of the while loop, but I'm having trouble setting up the external source through DAQmx. What would be the most straight-forward way to implement a simple, fixed-interval timing source for the while loop using the 6320, or is it even possible? Thanks for your help!

[crossrulz]

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user0110 wrote:

I have a timed while loop that is currently governed by the default KHz sample clock (on Windows). I need better resolution than this sample clock can give me

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On Windows?  Not a chance.  If you need something to be more deterministic than a few ms, then you need to go to a Real Time system.

[Yamaeda]

What you usually do it set up a sample speed with the DAQ and grab a bunch at a time. You can e.g. set 10kHz sample speed and grab 1k samples at a time for a UI update of 10 Hz.

What are you trying to achieve?

/Y

[user0110]

I'm polling a rotary encoder every loop iteration to calculate the speed of a spinning cylinder. In this case, a millisecond in either direction is a huge difference in number of encoder ticks used to determine the speed. 

[crossrulz]

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@user0110 wrote:

I'm polling a rotary encoder every loop iteration to calculate the speed of a spinning cylinder. In this case, a millisecond in either direction is a huge difference in number of encoder ticks used to determine the speed. 

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You could just use a Counter for that.  Alternatively, use an FPGA.

[Yamaeda]

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@user0110 wrote:

I'm polling a rotary encoder every loop iteration to calculate the speed of a spinning cylinder. In this case, a millisecond in either direction is a huge difference in number of encoder ticks used to determine the speed. 

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That's why you dont use software timing.

As mentioned above a Counter is the best solution, but in case you dont have any, setup a sample rate in hardware and extract and calculate an array at a time.

/Y

[user0110]

How does a counter get me timing information? I'm using one already to read in the ticks of the rotary encoder, but how does that give me loop timing?

EDIT: can you also elaborate on setting up the sample rate to be read in arrays? Thanks for the help!

[crossrulz]

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@user0110 wrote:

How does a counter get me timing information? I'm using one already to read in the ticks of the rotary encoder, but how does that give me loop timing?

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Depending on the card/counter you are using, you can set it up to give you frequency.

[user0110]

Are you saying that I can generate frequency with a counter that could be used as the loop timing source? Could you direct me to an example of what you're describing?

[thoult]

No, you'd use a Counter input to measure the frequency of the encoder output pulse. If you know the spatial resolution of the encoder (via counts per rev, or distance/angle per count etc) you can convert from frequency to speed.

Alternatively, you can use a Counter input and the outputs of the encoder to measure position, then numerically differentiate the position array/waveform to get speed. The downside is that numerically differentiating a noisy (read: jittery) signal tends to amplify noise.

Some simple links:

Use Counter Frequency to obtain an Encoder Rotational Speed

Encoder Measurements - How To Guide