Using DAQmx to measure time simultaneously with encoder position

[R.jon8s]

Hello,

I am trying to read the output of two encoders at high speed in parallel (rotating at up to 300 & 75 Hz respectively). I would like to measure the speed and acceleration of these encoders.

The current VI (attached) that I am using measures the position data from both encoders as I would like. However, I need the time of each position measurement to calculate the speed and acceleration. (I also need the time for other things afterwards too).

I am currently using a producer-consumer loop to save data to .TDMS file with the Z pulse on the disc encoder being used to release the queue of data acquired, so the acquisition rate is not constant.

I have tried using the high-resolution seconds VI in the producer loop the output in the VI main window appears to read the time as I would like it, but this saving into the TDMS file, I may be wrong on this (screenshot attached).

I have tried initialising the array to the correct length in the producer loop, but this doesn't work (screenshot attached).

Is there a simple way to acquire the time data at the same rates as the encoder position data?

(Using LabVIEW 24 (64bit) CRio 9055 with a NI-9401 Digital input module)

[NIquist]

Many of us can't open brand new 2024 LabVIEW code.  You will get more help if you back save (File > Save for Previous...) and choose 2019 or so.  😉

[R.jon8s]

Updated VI attatched, thanks for letting me know 👍

[ZYOng · Accepted solution]

Since you are using the Z-signal as the sample clock source, unfortunately there is no way to determine the timing accurately since it is not captured in the first place. You should use a sample clock source with a known timing, usually this can be generated using another counter.

[R.jon8s]

By another counter do you mean the output of another encoder rotating at a known speed?

I have tried using the A pulse but the acquisition can't keep up with the hardware (see attached screenshot), I have also tried using the internal 20MHz clock, but this doesn't seem to work for me (I'm not entirely sure why) - this would solve my issues if the data was acquired at a constant rate.

[Kevin_Price · Accepted solution]

No, you can use another of the counters built into your 9055 chassis to generate a pulse train at a known and constant rate.  Your chassis has 4 and you're only using 2 of them so far.

You'd probably refer to it as something like "ChassisName/_ctr2" when configuring the continuous pulse train.  Note the leading underscore character when using a chassis counter internally rather than accessing it via a DIO terminal on a module.  Then also your encoder tasks would specify their sample clock source to be something like "/ChassisName/_ctr2InternalOutput".   Finally, you should make sure to start the encoder sampling tasks *before* starting the pulse train task.

-Kevin P

[R.jon8s]

Thanks, Kevin

I have updated my VI (attached) to use the internal pulse source.

For anyone looking to replicate this, you need to include internal channels, under the "I/O name filtering option" for the internal counter source and include advanced terminals for the encoder clock source.

This appears to work broadly as I would like it to!