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#1
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Encoders (What's up with those?)
So our official season is done, but we are looking forward to some off-season events. In our attempt to remedy some weaknesses from the season we are looking to add an encoder to our shooting system. It is a miniCIM connected by ~.9 Gear ratio to a pneumatic wheel.
We have no idea what kind of sensors to look into, where to purchase them, or how to install them. Any help would be great. Thanks! |
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#2
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Re: Encoders (What's up with those?)
We have never had much luck mounting US Digital encoders to flywheels. We also haven't had luck with using another motor as a tachometer. This year and last year for flywheels we used photoelectric sensors pointed at retro-reflective tape strips stuck to the wheels. We have 6 strips of the tape stuck evenly around the wheel this year. 6 ticks = 1 revolution. Our speed control has had quick responses and accuracy.
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#3
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Re: Encoders (What's up with those?)
Photoswitches are definitely the way to go. At the wheel speeds teams are using, finding encoders with shafts is difficult. Shaftless encoders like the US digital are a little touchy, and require decent tolerances too.
Optical sensors, on the other hand, will give you excellent speed control while allowing just about any alignment you feel like. There are a number of threads on this, however, so I suggest you search some of them out. http://www.chiefdelphi.com/forums/sh...ht=photoswitch |
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#4
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Re: Encoders (What's up with those?)
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#5
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Re: Encoders (What's up with those?)
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#6
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Re: Encoders (What's up with those?)
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Fortunately, I just made a video this weekend explaining how it all works. (It was for the SpaceX FIRST application, hence the title). http://www.youtube.com/watch?v=zzNLlAUbbcU Edit: Might only leave this up for a little while. Watch it quick! Last edited by propionate : 17-04-2013 at 23:23. |
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#7
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Re: Encoders (What's up with those?)
How did you measure how accurate you were?
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I've used both of the following with no problem http://www.digikey.com/product-searc...256&vendor=136 http://www.usdigital.com/products/en...otary/shaft/S1 |
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#8
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Re: Encoders (What's up with those?)
No in-depth analysis. 0 to shooting speed is about one second. Recovery time between shots is about 300ms, accurate to within .2% speed (plenty close for what it's being used for). Barring human error, we can hit around 95% accuracy just from tests in the shop. On the field we're closer to 80 or 85% because of human error/flywheel wear and tear. |
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#9
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Re: Encoders (What's up with those?)
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Any reason you use retroreflective tape? From what I could tell, you weren't shining a light at it. |
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#10
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Re: Encoders (What's up with those?)
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#11
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Re: Encoders (What's up with those?)
We've used both this shaftless encoder and this shafted encoder from USDigital on our shooter this year, with a fair amount of success with each. The female encoders are definitely a bit more finicky. If you need help with mounting any kind of encoder, ask.
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#12
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Re: Encoders (What's up with those?)
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And we just found retroreflective to give the best response (in terms of distance the sensor could be from the wheel). Gave us more wiggle room, so to speak. |
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#13
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Re: Encoders (What's up with those?)
Sure, go ahead. The video length had to be 2 minutes or less so I had to skip a lot of details unfortunately.
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#14
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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? Why did you use 6 pieces of tape, instead of just one? 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? Are your motors directly connected to the wheel, or are there gears or belts or chains between them? What motor controllers are you using for the shooter motors? What is the range of wheel speeds you are controlling? Why did you need a custom PID? Did you try other methods (LabVIEW PID, TBH, bang-bang) and they didn't work? |
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#15
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Re: Encoders (What's up with those?)
I can answer some of these. We have CTRE Talons controlling BaneBots 550s which drive 4:1 gearboxes.
Last edited by Ernst : 18-04-2013 at 00:04. Reason: spelling |
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