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Intermittent trigger failure

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Intermittent trigger failure

Postby pwright » Mon Sep 16, 2013 11:32 am

Hi,

I'm capturing load data from impacts with a load cell on the face of a mallet. About half of the time everything works perfectly, but the other half (despite the impacts being similar) it either fails to trigger or triggers part way through the impact trace. I think all the pertinent details are below. Could you please suggest how I could make this reliable.

Thanks
Phil

2205
Picoscope6
Ch A only
500us/div
100kS
Repeat trigger
rising
5% pretrig
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Re: Intermittent trigger failure

Postby Martyn » Mon Sep 16, 2013 12:01 pm

For both a good and bad capture can you use File->SaveCurrentWaveform as a psdata file and post these here then we can take a look.
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Re: Intermittent trigger failure

Postby pwright » Mon Sep 16, 2013 12:45 pm

Thanks for getting back. The attached file has 2 traces. The second is OK, clearly some zero data at the start. The first has triggered too late on a secondary feature. Obviously there is no record of the ones that didn't trigger at all.

Thanks
Phil
Attachments
20130912-0001.psdata
(15.01 KiB) Downloaded 58 times
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Re: Intermittent trigger failure

Postby Martyn » Mon Sep 16, 2013 1:16 pm

Can you try with a pre trigger of 20% to see what is happening on the trace before the false trigger condition.
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Re: Intermittent trigger failure

Postby pwright » Mon Sep 16, 2013 1:23 pm

Martyn
I don't have it setup at present, but all the saved files I have are consistant with an impact i.e some zero data while nothing is happening followed by a sharp rise to the initial peak, then some oscillation and secondary features.
Phil
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Re: Intermittent trigger failure

Postby Martyn » Mon Sep 16, 2013 1:32 pm

Is it possible that the true trigger point has happened during the re-arm time of the scope that occurs between the blocks and so it picks up on the second peak ? The dead time will be anywhere between 1ms and 60ms, after each block, depending upon PC and USB use.

What is the frequency of the hits ?
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Re: Intermittent trigger failure

Postby pwright » Mon Sep 16, 2013 1:37 pm

They are applied by hand, so the time between can't much less than half a second.

When it seemed to be having an issue with not triggering during the test, I did some trial single blows that had 2 or 3 seconds between and the intermittent triggering carried on through these.

Phil
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Re: Intermittent trigger failure

Postby Martyn » Mon Sep 16, 2013 1:51 pm

I would suggest changing to an Advanced Edge Trigger, add in some hysteresis, move the level to around 3kN and possibly increase the amount of pre trigger.
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Re: Intermittent trigger failure

Postby pwright » Mon Sep 16, 2013 2:36 pm

Thanks I'll try that.
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Re: Intermittent trigger failure

Postby pwright » Wed Sep 18, 2013 10:12 am

Martyn,
That seems to have cured it. Triggering has been reliable so far with 400N and 0.5% hysteresis and 10% pre trig.
The threshold can't be raised as some impact peaks are less than 1 kN.
Thanks
Phil
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