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Trigger based on "missing encoder pulses"?

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Trigger based on "missing encoder pulses"?

Postby swandave » Wed Aug 26, 2015 10:03 pm

Hi,

I am using a 2000 series scope to try and capture missing encoder pulses on a motor.

The motor has variable speed so fixed triggers are proving a challenge.

Has anyone seen a way to trigger based on pulse width but rather than a fixed threshold I really need a trigger that acts when the measured pulse is more that 150% of the previous pulse.

Does this sound achievable?

Hope someone has cracked this before.

Thanks
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Re: Trigger based on "missing encoder pulses"?

Postby alan » Fri Aug 28, 2015 4:50 am

Normally you would use a pulse width trigger for this but if the speed of the pulses changes to much that might not be an option.

How fast are the pulses? If they are quite slow then try capturing them in streaming mode (>200ms/div by default but you can change this). In this mode when the trace is drawn live you can increase the memory to 100,000,000 samples. For most people thats so much data that you can capture the event, but the problem is then finding it in all that data.

To find events in such large amounts of data one option is to plot the frequency or duty cycle of the waveform - see the section on maths functions here
https://www.picotech.com/library/oscill ... st-updates
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Re: Trigger based on "missing encoder pulses"?

Postby Mark_O » Sat Aug 29, 2015 1:07 am

alan wrote:In this mode when the trace is drawn live you can increase the memory to 100,000,000 samples. For most people thats so much data that you can capture the event, but the problem is then finding it in all that data.

To find events in such large amounts of data one option is to plot the frequency or duty cycle of the waveform - see the section on maths functions...

Alan, that's a slick capability for being able to highlight a needle in a haystack. Thanks for pointing it out. That's not an uncommon situation at all (acquiring a boatload of data, and not being able to isolate a specific section of interest... even when you're sure it's in there).

Thanks too for the pointer to the detailed descriptions on updated features in PS6. Somehow, I've never noticed a reference to it anywhere before.

https://www.picotech.com/library/oscilloscopes/picoscope-latest-updates

- Mark
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Re: Trigger based on "missing encoder pulses"?

Postby swandave » Wed Sep 09, 2015 12:17 pm

Hi,

Thanks for that advice. The new maths functionality is awesome and has helped me see into the problem in a way I hadn't even considered.

Couple of questions come out from my playing,

1) how do set the "streaming mode" I am just using long samples into the buffer and setting the sample to 5MS to capture enough points, is this what you meant.

2) the maths lines sometimes do not appear, like they fail to decide to compute. How do I get them to recompute if they miss.

Thanks
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Re: Trigger based on "missing encoder pulses"?

Postby Hitesh » Thu Sep 10, 2015 12:04 pm

Hi swandave,

As alan mentioned, to switch to streaming mode data collection, please set your timebase to 200ms/div or greater (or 2s or greater if this is configured as a collection time.

PicoScope6_Timebase_setting.PNG
Example Timebase setting for streaming mode


With respect to your Maths Channels, in the 'Math Channel Wizard' in PicoScope 6, please edit the range to 'Override automatic range selection' and set a 'Min' and 'Max' value.

Math_Channel_Wizard_Range.PNG
Range and Unit setting in Math Channel Wizard
Math_Channel_Wizard_Range.PNG (9.58 KiB) Viewed 472 times


Hope this helps.
Hitesh

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