Part detection

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geo0rpo

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Nov 17, 2021, 11:27:55 AM11/17/21
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Hello all!!!
I have read and reread but cannot understand what the "difference" in part detection method does. Please help me understand.


mark maker

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Nov 17, 2021, 1:03:36 PM11/17/21
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I don't know what you mean. Please elaborate.

On 17.11.21 17:27, geo0rpo wrote:
Hello all!!!
I have read and reread but cannot understand what the "difference" in part detection method does. Please help me understand.


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geo0rpo

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Nov 17, 2021, 1:19:01 PM11/17/21
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https://github.com/openpnp/openpnp/wiki/Setup-and-Calibration:-Vacuum-Sensing

I tried the absolute measurement method, works like a charm. 
I want to try the Difference method, I read your description over and over but I do not get it.
Different compared to what? To absolute value? Difference range means a tolerance of the absolute value?  I really cannot understand the values I should enter there.
Thanks!

mark maker

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Nov 17, 2021, 1:41:16 PM11/17/21
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The Wiki says:

The difference is computed in comparison too a baseline: For the Part-on this is the established vacuum level, before the nozzle is lifted up. For Part-off this is the vacuum level before switching on the valve for a quick probing pulse.

I don't know how to formulate it better. 🙁

Maybe a native English speaker could elaborate.

_Mark

geo0rpo

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Nov 17, 2021, 2:16:50 PM11/17/21
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yes I read this many times but I did not get it. maybe you could give me example values if possible. if my absolute value is for example 100 what is the difference value? or it is irrelevant? 

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mark maker

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Nov 17, 2021, 2:45:47 PM11/17/21
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It first measures the baseline value, as described. Then later, it measures the comparison value as described. The difference between the two is now the criteria.

So for instance, while the nozzle tip is still pressed down on the part, it measures the vacuum. Then it lifts the part to Safe Z and again measures the vacuum. The difference between the two measurements then gives you the difference value to compare against your Difference Range. If the vacuum was  falling too much then probably the part was not successfully picked.

In fact, you can ignore all these technicalities: There is a field Last Reading that tells you what the value was, when you last tested a pick. You can try with a successful pick and the retry with an unsuccessful pick (perhaps remove the part before, so it is picking an empty feeder). Then you have two Last Reading values. And you can set the Difference Range accordingly, i.e. it should include the successful Last Reading with some margin, but exclude the unsuccessful Last Reading with some margin.

_Mark

geo0rpo

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Nov 18, 2021, 4:58:57 AM11/18/21
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Mark,
First of all thank you sincerely for taking the time to explain.
I understand the operation now.
So, when you select the "difference" method  openpnp also checks for the absolute value in the vacuum range above?

mark maker

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Nov 18, 2021, 5:49:36 AM11/18/21
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Yes, please read the whole Wiki again, now that you understand this aspect.

_Mark

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