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-   -   Encoders (What's up with those?) (http://www.chiefdelphi.com/forums/showthread.php?t=116172)

matthewdenny 17-04-2013 17:57

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!

Ernst 17-04-2013 18:04

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.

Tom Line 17-04-2013 18:11

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

Ether 17-04-2013 18:52

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by ZehP (Post 1263651)
We have 6 strips of the tape stuck evenly around the wheel this year. 6 ticks = 1 revolution.

How are you decoding the signal from the photosensor? i.e. Are you reading the number of counts, and dividing by elapsed time, or are you using getPeriod() to read the elapsed time between pulses?


Quote:

Our speed control has had quick responses and accuracy.
What speed control algorithm are you using? PID, TBH, bang-bang, something else?



Ernst 17-04-2013 23:12

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by Ether (Post 1263725)
How are you decoding the signal from the photosensor? i.e. Are you reading the number of counts, and dividing by elapsed time, or are you using getPeriod() to read the elapsed time between pulses?

What speed control algorithm are you using? PID, TBH, bang-bang, something else?



I wasn't involved with the programming. Sorry. I'll try to direct my team's main programmer to this thread.

propionate 17-04-2013 23:19

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by Ether (Post 1263725)
How are you decoding the signal from the photosensor? i.e. Are you reading the number of counts, and dividing by elapsed time, or are you using getPeriod() to read the elapsed time between pulses?

What speed control algorithm are you using? PID, TBH, bang-bang, something else?




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!

asteb27 17-04-2013 23:27

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by ZehP (Post 1263651)
Our speed control has had quick responses and accuracy.

How did you measure how accurate you were?

Quote:

Originally Posted by Tom Line (Post 1263663)
Photoswitches are definitely the way to go. At the wheel speeds teams are using, finding encoders with shafts is difficult. http://www.chiefdelphi.com/forums/sh...ht=photoswitch

We have encoders in the 10,000 rpm range. I know some teams are running faster than 10k rmp, but plenty (like us) are running much slower than that.

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

propionate 17-04-2013 23:34

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by asteb27 (Post 1263995)
How did you measure how accurate you were?


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.

asteb27 17-04-2013 23:37

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by propionate (Post 1264003)
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.

I couldn't tell from the code (we use java), but do you actually measure rpm with this system? if you do, I'd be interested to see how it compares to a tachometer reading.

Any reason you use retroreflective tape? From what I could tell, you weren't shining a light at it.

Ether 17-04-2013 23:39

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by propionate (Post 1263986)
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!

Would you be interested in discussing this? If so, I have a few observations to make, and a few questions.



T^2 17-04-2013 23:40

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.

propionate 17-04-2013 23:41

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by asteb27 (Post 1264009)
I couldn't tell from the code (we use java), but do you actually measure rpm with this system? if you do, I'd be interested to see how it compares to a tachometer reading.

Any reason you use retroreflective tape? From what I could tell, you weren't shining a light at it.

The photoelectric sensors are pulled into LabVIEW as encoders. The encoder VI then takes the counts/second and calculates the rate. So it isn't precisely RPM - would have to be scaled to account for the fact that there are six pieces of tape.

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.

propionate 17-04-2013 23:42

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by Ether (Post 1264014)
Would you be interested in discussing this? If so, I have a few observations to make, and a few questions.



Sure, go ahead. The video length had to be 2 minutes or less so I had to skip a lot of details unfortunately.

Ether 17-04-2013 23:59

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by propionate (Post 1264018)
Sure, go ahead. The video length had to be 2 minutes or less so I had to skip a lot of details unfortunately.

OK here goes :-)

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?




Ernst 18-04-2013 00:03

Re: Encoders (What's up with those?)
 
Quote:

Originally Posted by Ether (Post 1264028)
OK here goes :-)


I can answer some of these. We have CTRE Talons controlling BaneBots 550s which drive 4:1 gearboxes.


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