Measuring Motion against Frequency

[walter-donovan]

My test is measuring and plotting engine valve motion against RPMs. I need to sample valve position (analog signal) every A pulse of a rotary encoder and also sample RPM. Then later I have to extract single plots, each one revolution of positional data. Each plot must be referenced against the approx RPM at which it was sampled. This will run to about 66kHz for 8 seconds so it will build a lengthy file.

I propose to start sampling on the Z pulse signal (1 per rev) and take 1 analog position meas each A pulse (720 / rev). Store this to array. At the same time I want to run a task to capture frequency once each revolution (Z pulse) by measuring the A pulse. Store these vals to a separate array.

After measurement is done I can save the large position meas array to a measurement file and save the frequency data to a separate, much smaller file (call it an index file).

In this way, for analysis I can load the index array and count the length of it to determine the lenth (in data points) of the large meas array. This could allow me to poll the index aray for a chosen RPM and select the correct 720 positional points to plot against encoder angle (plotting displacement as y against 1/2 deg increments as x.

Does this plan have any merit? I am new to this type of measurement and if anyone knows of similar test procedures I sure would like to hear and consider them. Is there perhaps a more efficient method?

[johnsold · Accepted solution]

That seems reasonable. Look at the Threshold 1D Array function.  You can use that to search the index array for a particular speed.  Round the result to the nearest smaller integer. The index in the positional array will 720*that integer (maybe -1).  If your speed can vary up and down, you may need to use the start index or reverse the array to find the location you want.

Lynn

[walter-donovan]

Once again thanks for your time. I now have a plan. I will have questions about the freq measurement but I'll post them in a new thread since they aren't related to the original question.