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Unread 18-04-2013, 00:18
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Re: Encoders (What's up with those?)

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Is there a reason why you used the encoder VI, instead of the counter VI, for this non-quadrature sensor?
This is actually just a habit now. When I made my first feedback control loop for a flywheel last year, I had some trouble getting it to work using the counter. On a whim, I tried inputting them as encoders and it worked great. I've since figured out how to do it with the counter VI (some simple problem with the math I had), but never switched back.

Quote:
Why did you use 6 pieces of tape, instead of just one?
This was the recommendation of a mentor. His explanation was that it allowed for a 'finer' count. I have tried it with just one piece of tape and didn't notice any significant different (just different scaling).

Quote:
In an earlier post you said that your speed control is accurate to within 0.2% of point. What test equipment did you use to independently measure the wheel speed to make this determination?
I should have said, the PID controller keeps the flywheel within a .2% error based on the readings from the photoelectric sensors. If those readings themselves are incorrect, then I'd have to rescind that statement.

Quote:
Are your motors directly connected to the wheel, or are there gears or belts or chains between them?
See post by team member above. Directly connect w/ 4:1 gearboxes.

Quote:
What motor controllers are you using for the shooter motors?
See above - talons.

Quote:
What is the range of wheel speeds you are controlling?
Do you mean in RPM? I'd have to check my documentation at the shop, I can't remember off the top of my memory.

Quote:
Why did you need a custom PID? Did you try other methods (LabVIEW PID, TBH, bang-bang) and they didn't work?
The LabVIEW PID has worked fine for us in the past, but it's always more fun to write the code yourself, to a reasonable extent. Better learning opportunity for rookie programmers. I don't have much experience with the half-back or bang-bang controllers, so I can't comment on those. Would be interesting to try them though and see how they compare. Sorry I don't have a good response for this one!
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Unread 18-04-2013, 00:45
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Re: Encoders (What's up with those?)

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Originally Posted by propionate View Post
...
Thanks for the detailed answers.

I don't have access to the LabVIEW help files here, so one more question:

Can you tell me what the "12" input is for in the "Configure Timer" vi, and why did you chose that particular number.



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Unread 18-04-2013, 00:54
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Re: Encoders (What's up with those?)

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Originally Posted by Ether View Post

Can you tell me what the "12" input is for in the "Configure Timer" vi, and why did you chose that particular number.



Number of samples to average. I think it was a bit of trial and error until we got a smooth reading from the photoelectrics.
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Unread 18-04-2013, 01:05
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Re: Encoders (What's up with those?)

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Originally Posted by propionate View Post
Number of samples to average. I think it was a bit of trial and error until we got a smooth reading from the photoelectrics.
Can you tell me, the way you have the encoder configured, is it sampling both rising and falling edges?


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Unread 18-04-2013, 01:18
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Re: Encoders (What's up with those?)

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Originally Posted by Ether View Post
Can you tell me, the way you have the encoder configured, is it sampling both rising and falling edges?


I believe it's configured to only count on the rising edge. I can check this tomorrow if you'd like.
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Unread 18-04-2013, 01:21
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Re: Encoders (What's up with those?)

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Originally Posted by propionate View Post
I believe it's configured to only count on the rising edge. I can check this tomorrow if you'd like.
Yes, please do. I'm going somewhere with this...


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Unread 18-04-2013, 01:43
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Re: Encoders (What's up with those?)

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Originally Posted by Ether View Post
Yes, please do. I'm going somewhere with this...


Now that you're making me rethink this, it may not make a lot of sense to be averaging every 12 samples. It looks like the default is actually 1. I'm not sure if this is an error or not... I will definitely investigate this tomorrow.

It works wonderfully right now, but this could be the path to making it better.
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Unread 18-04-2013, 08:59
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Re: Encoders (What's up with those?)

Quote:
Originally Posted by propionate View Post
Now that you're making me rethink this, it may not make a lot of sense to be averaging every 12 samples. It looks like the default is actually 1. I'm not sure if this is an error or not... I will definitely investigate this tomorrow.

It works wonderfully right now, but this could be the path to making it better.
You probably don't want the default of 1 with six pieces of tape, especially if it's sampling both rising and falling edges.

Try this: one piece of tape with the default sample size of 1 and setup to sample rising edges only. That way, it's always sampling the same edge of the same piece of tape, so the spacing is perfect... and with your 20ms update rate, you'll be getting a fresh reading every execution or your controller loop at wheel speeds of 3000 RPM or higher.


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Unread 18-04-2013, 17:14
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Re: Encoders (What's up with those?)

Quote:
Originally Posted by Ether View Post
You probably don't want the default of 1 with six pieces of tape, especially if it's sampling both rising and falling edges.

Try this: one piece of tape with the default sample size of 1 and setup to sample rising edges only. That way, it's always sampling the same edge of the same piece of tape, so the spacing is perfect... and with your 20ms update rate, you'll be getting a fresh reading every execution or your controller loop at wheel speeds of 3000 RPM or higher.


I will give this a go tonight and let you know the results. Thanks for the recommendations.
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Unread 18-04-2013, 23:22
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Re: Encoders (What's up with those?)

Sorry, didn't have any opportunity to do the tests this evening. I will send you a private message with the results when I do have a moment with the robot so as to not clutter up this thread any more.
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Unread 22-04-2013, 23:20
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Re: Encoders (What's up with those?)

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Originally Posted by asteb27 View Post
Any reason you use retroreflective tape? From what I could tell, you weren't shining a light at it.
The Banner photoeyes they used are retroreflective units - they have an emitter and detector in the same housing.
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