How to use a counter to control sampling of analog signal

[ijg@att.net]

We are trying to obtain plots of the amplitude of a analogue signal versus the position of a device in our measurement apparatus. The device moves first in one direction, then in reverse. We want to determine the difference between the amplitudes of the analog signal for the two directions of motion at each position within the range of motion. Position is determined by a Heidenhain linear encoder. 

[margasan]

Hello ijg@att.net

You can use a TTL pulse to trigger a finite data acquisition, just open the example finder --> Hardware Input and Output --> DAQmx ---> Analog Measurements --- > Voltage and open the highlighted example in the image below. You can use this example as a reference to start building your application.

Regards

Mart G

[margasan]

Hello again ijg@att,

I just noticed you are using signalexpress so the steps to do the same in Signalexpres are different

First, to get it to trigger on every iteration (I assume that you are using the "run" mode, rather than the "run once" mode)you have to change the acquisition mode from "continuous" to "n samples." Then for example set

the sample rate at 1000 Samples/second, and increased the Samples to Read to 5000 Samples. The set the trigger parameters as seen in the image below.
Configured this way, the step will wait for the trigger, then acquire 5000 samples over a period of five seconds, then wait for another trigger, and so on. I'm attaching an image that illustrates the changes that I made. Does this do what you are trying to do? Let me know if this works for you.

Regards

Mart G

[ijg@att.net]

Hi Mark,

Thank you for considering our problem. We have not finished evaluating your suggestions since we must also consider how to configure our counting step. A typical range of motion is 1.6 inches. The encoder supplies its first rising edge every 0.00015748 inches (40 microns). Thus there are 10,160 position locations. We would like to sample the analog signal at each such location. The speed of travel is neither precisely fixed, nor precisely constant, but typically the 1.6 inches of motion will take about 25 seconds. We have tried one arrangement where we set the duration of a run to 60 seconds, start the motion a second or two after clicking on the run button. at the end of the travel, we stop the motion, hold it still for a second or two, and then restart the motion in the reverse direction. It reaches the end of its travel a few seconds before the run shuts off (at 60 seconds). Since the counter increments from a starting position of zero during forward motion, remains unchanged during the pause, and decrements during the return motion, we do get the position information that we need. While we understand your suggestions on how to configure the triggering of the analog signal, we don't quite understand how to configure the Acquisition Mode to ensure that sampling of the analog signal will occur in synchronism with the count signal for however number of samples are required and at the apprpriate rate. We are novices at this business, so your help is both vital and very much appreciated. Thank you.

Regards,

IvanG