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Unread 07-13-2012, 07:26 AM
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Re: Low Res Optical Encoders for Speed Control

Quote:
Originally Posted by KrazyCarl92 View Post
This is the sort of workaround I'm interested in if we can't do it with a 360 count disc.
The point is, you can do it with a 360 CPR encoder.

As with any sensor, make sure you're getting a proper signal before trying to tune the control algorithm.


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Unread 07-13-2012, 07:38 AM
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Re: Low Res Optical Encoders for Speed Control

Ether,

we had 2 wheels and hence 2 encoders. Would that cause any problems?

also, I am not sure how much to be worried about the singal integrity over the 7 feet of wire that the students had wired next to all other wires. including the 20kHz jaguar output wires going to the motors.

Quote:
Originally Posted by Ether View Post
The point is, you can do it with a 360 CPR encoder.

As with any sensor, make sure you're getting a proper signal before trying to tune the control algorithm.


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Unread 07-13-2012, 07:41 AM
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Re: Low Res Optical Encoders for Speed Control

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Originally Posted by KrazyCarl92 View Post
This is the sort of workaround I'm interested in if we can't do it with a 360 count disc. The only thing that would usually concern me about this approach is a layer of tape (which if I remember correctly is thicker than the plastic disc) could change the tolerance on the encoder for the distance from the sensor to the disc. Was this even an issue, or could it be easily resolved if it does present a problem?
Since you only have 2 transitions, it would be better to space the disc off the actual distance so the image is not focused. When the encoder dist was spaced correctly, it was picking up an extra tick from a scratch on the aluminum. We put the disc closer to the sensor to make it blurry so it does not pick up small scratches.
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Unread 07-13-2012, 07:50 AM
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Re: Low Res Optical Encoders for Speed Control

Quote:
Originally Posted by Ether View Post
The point is, you can do it with a 360 CPR encoder.

I'm not doubting that its possible, just doubting our ability to realize that possibility based on our past results. I've also seen so many successful teams with single digit CPR sensors that it got me thinking that if we weren't having success with the 360 count, we could try something else.

We will try it though, and thanks for the help!
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Last edited by KrazyCarl92 : 07-13-2012 at 07:53 AM.
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Unread 07-13-2012, 07:52 AM
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Re: Low Res Optical Encoders for Speed Control

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Originally Posted by KrazyCarl92 View Post

Be very careful when you look at encoders. the operating RPM for that is 120 which is far below the needed rotation rate.
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Unread 07-13-2012, 08:04 AM
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Re: Low Res Optical Encoders for Speed Control

Quote:
we had 2 wheels and hence 2 encoders. Would that cause any problems?
No. (But see the following note about wiring).

Quote:
also, I am not sure how much to be worried about the singal integrity over the 7 feet of wire that the students had wired next to all other wires.
You should be worried enough to test the sensor signals thoroughly before trying to tune the control algorithm. But then again, it's proper practice to do that anyway.

Quote:
including the 20kHz jaguar output wires going to the motors
For the record, Jag PWM frequency is 15KHz.


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Unread 07-13-2012, 08:22 AM
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Re: Low Res Optical Encoders for Speed Control

Quote:
Originally Posted by BornaE View Post
also, I am not sure how much to be worried about the singal integrity over the 7 feet of wire that the students had wired next to all other wires. including the 20kHz jaguar output wires going to the motors.
I know we found a way to insolate the wire from interference and we were +/- close to 100 rpm before. After, we got it to +/- 1 rpm. I'll talk to the guys who did that and see what they actually did; I think it had some metal foil involved, so perhaps there was a faraday cage effect?
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Unread 07-13-2012, 09:32 AM
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Re: Low Res Optical Encoders for Speed Control

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Originally Posted by KrazyCarl92 View Post
I'm not doubting that its possible, just doubting our ability to realize that possibility based on our past results.
I'll bet you can do it

You said in your original post that you had the 360CPR encoders working with the Jag. So that indicates that you have the encoders properly mounted and aligned, which is half the battle.

Something to consider: the fewer the counts per revolution, the "staler" your sensor signal will be (more phase lag). If your wheel has enough inertia that probably won't matter. Just something to keep in mind from a control theory point-of-view.


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Unread 07-13-2012, 09:34 AM
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Re: Low Res Optical Encoders for Speed Control

Quote:
I know we found a way to insolate the wire from interference and we were +/- close to 100 rpm before. After, we got it to +/- 1 rpm. I'll talk to the guys who did that and see what they actually did
Please do. I think this would be of substantial interest.


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Unread 07-13-2012, 07:57 PM
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Re: Low Res Optical Encoders for Speed Control

We were worried about the little shaft of a US Didital encoder spinning at such a high rate for so long as well as the high number of counts flying around. I guess many teams did this it and it worked okay so our worries were misplaced.

We used a small 14 tooth steel sprocket and a Hall effect gear tooth sensor for a no-worry solution. Our wheel speed control was rock solid but that may have also been because we had a high mass shooter wheel.
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Unread 07-24-2012, 05:54 PM
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Re: Low Res Optical Encoders for Speed Control

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Originally Posted by peterr99 View Post
.
reported

Last edited by Ether : 07-24-2012 at 07:01 PM.
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Unread 07-24-2012, 05:56 PM
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Re: Low Res Optical Encoders for Speed Control

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Originally Posted by KrazyCarl92 View Post
I'll talk to the guys who did that and see what they actually did
Can you share what you found out?


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Unread 07-24-2012, 07:09 PM
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Re: Low Res Optical Encoders for Speed Control

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Originally Posted by peterr99 View Post
We have a 2 axle shooter, so if we had 2 axles reading that fast, could the FPGA handle it? I.E. would that then become 48,000 counts/sec?
Yes, it could handle it. Each input is independent.
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Unread 07-25-2012, 01:07 PM
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Re: Low Res Optical Encoders for Speed Control

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Originally Posted by Ether View Post
Can you share what you found out?


Our meeting last week was cancelled, but we are meeting tonight to work on this exactly. I will post it here when I find out. Thanks for the reminder.
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Unread 07-26-2012, 05:17 PM
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Re: Low Res Optical Encoders for Speed Control

So it turns out the wire we used is 22 AWG shielded cable which the mentor says is available from Digikey. It has five 22-gauge wires (we only used 4) and these are wrapped with aluminum foil. Around the aluminum is more insolation. The aluminum foil is twisted at one of the ends of the cable, and grounded to the source ground (the jaguar 5 pin connector's ground in this case). It is important to only ground one side of the cable and not both, otherwise you could create a closed circuit, which may cause EMI if there is current going through the foil. My sources say you could also get away with grounding to the frame, but I don't think this is legal and it is usually better practice to ground it to a source anyway.

"[R38]
All wiring and electrical devices, including all control system components, shall be electrically isolated from the Robot frame. The Robot frame must not be used to carry electrical current."

I could see how both sides could be argued. I don't quite know if the aluminum is considered part of the "wiring", or if this would be considered carrying electrical current by the frame, but this is a tangent anyway since it can be grounded to a source.

I wouldn't usually think about the EMI on our robots, but looking at the number of sensors, motors, and other electrical components on that robot in comparison with our past robots, it's not surprising that this would be the one out of all of them to benefit from shielded sensor wires. I could post pictures if anyone is interested.
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