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PicoScope 2202 and 200ms/div Trigger Problem

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PicoScope 2202 and 200ms/div Trigger Problem

Postby RickRandom » Sat Jan 12, 2008 10:38 am

I am trying to use a PicoScope 2202 (and PicoScope 5.18.0 and PicoScope 6.0.12.2) for rpm measurement, and had some success initially but I am now having trouble measuring over the required time.

I have a test rig that should run at a constant rpm, then increase the rpm and then reduce to 0rpm, at controlled rates of change. These might be 100rpm to 130rpm to zero, all at 50rpm/sec, or 250rpm to 300rpm to 0rpm, at 150rpm/sec, or other such permutations. I need to validate that it does achieve this within certain limits. (I suspect from other results that it is not within these limits and also that it varies cyclically over a series of runs.)

The wheel has 168 teeth, which I monitor via a proximity sensor. So 300rpm with 168 teeth gives a frequency of 840Hz, which is a rough square wave, with frequency varying with rpm. I also monitor a signal that tells the motor controller to start to change from the initial constant value (it's just a step change) which I use as the trigger.

I succeeded in measuring lower rpm, from 95 to 117 to zero, in ~0.7 sec, so 100ms/div was fine to get all the data. I then calculated (on a spreadsheet) the instantaneous rpm from the time between 5 pulses. Thus gave a sensible curve, so I was quite happy with my method.

My plan was to set the test rig to run the 250-300-0 rpm profile continuously for 100 runs and use the Save On Trigger feature. The rig will do one run every 30 sec or so, giving plenty of time to store the data from one run and re-arm the trigger. Of course I needed a longer duration than before, around 2 sec, so set it to 200ms/div, and now it doesn’t trigger, it just keeps running. I read in the Help that above 100ms/div it change the mode by which it stores the data, so I guess that alters the trigger. How can I keep it triggering even with the required duration? The longest duration I will need is 4 seconds.

Because I need to compile enough data to verify the current performance, then try various motor controller changes, and retest over another 100 runs, etc, etc, I do need to automate the process.

For the 2 sec condition I can use a 1 sec time-delay (with PicoScope 6) on the trigger with 100ms/div, but this means I can only see the final phase of the rpm profile. I need to verify the whole of the rpm profile.

I’m sure I’m missing something simple, but I can’t work out what. Please Help.

I can email one of the psd files that worked over 0.7 sec.

(PS. Sorry to waffle on for so long.)
RickRandom
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Postby picojohn » Wed Jan 16, 2008 2:23 pm

Hello Rick,

I believe that you can achieve what you need by using the PicoLog application in 'Fast block mode'.

For example, a sampling interval of 205 us and 9800 samples will yield 2 s of data collection and, you may trade between sampling rate and number of samples to get the collection time that you need. At 205 us sampling interval, you can achieve up to 7 s collection times.

In general, a sampling rate of about 5X your highest frequency should prove more than adequate.

Regards
John
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Postby RickRandom » Wed Jan 16, 2008 4:21 pm

Thanks.

I'll give it a go and let you know.
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