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  #16   Spotlight this post!  
Unread 03-11-2014, 16:19
Adrian Clark Adrian Clark is offline
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Re: pic: MOEnarch Swerve Drive

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Originally Posted by nathannfm View Post
I am not sure if you can tell but the encoder is also on a slot so we can adjust it along with the CIM and make it just kiss the belt so there is almost no side loading. If the belt skips im pretty sure we are just gonna go to chain, we had enough headaches dealing with skipping belts this year

We did consider a bracket like you describe but it was just another part to manufacture and would also put the encoder in a more vulnerable location. If there is some unforeseen mechanical problem with it's current location we may go this route.
I don't think your encoder position will yield good values because it isn't properly engaged with the belt. With the encoder pulley tangent to the belt I don't think the encoder will rotate in perfect relation to the belt. When you have a belt wrapped around a pulley, there is always at least one tooth that is fully engaged with the pulley. This ensures smooth movement. When a pulley is placed tangent to the belt you don't have a tooth constantly engaged with the pulley, because of this there will be some sliding action as the pulley rotates and your encoder values will not reflect the true motion of the belt. This effect is exaggerated when you decrees the size of the pulley.

This is all conjecture, but I highly recommend testing this type of encoder setup or it could come back and bite you.

-Adrian
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Unread 03-11-2014, 17:00
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Re: pic: MOEnarch Swerve Drive

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Originally Posted by Adrian Clark View Post
I highly recommend testing this type of encoder setup or it could come back and bite you.
Thanks, I had not thought of this, I will make sure to keep a close eye on it.
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Unread 04-11-2014, 00:16
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Re: pic: MOEnarch Swerve Drive

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Originally Posted by Ether View Post
Here's why:




+1. That kind of reduction will result in an extremely low speed, around 5 fps according to JVN design calc. Not really worth a 4 cim 4 minicim drive IMO, as even in high gear mecanum you're only running 12fps...
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Unread 08-11-2014, 10:02
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Re: pic: MOEnarch Swerve Drive




Free speed of CIM+MiniCIM motor combination geared 1:1

= (343 + 198) / (343/5310 + 198/6200)

~ 5604 rpm


vehicle speed (with traction wheels) @ motor combo free speed =

(pi*4*5604) / (720*12.75*40/18)

~ 3.45 feet/sec





Last edited by Ether : 08-11-2014 at 14:41. Reason: added link to graph
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Unread 19-11-2014, 21:42
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Re: pic: MOEnarch Swerve Drive

Getting there!
https://www.facebook.com/video.php?v...04577359603811
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Unread 19-11-2014, 21:43
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Re: pic: MOEnarch Swerve Drive

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Originally Posted by nathannfm View Post
The diagonal cut roughtop is killing me! Straight cut (for maximum cleat effect) is the only way to go in my mind.

Congrats on getting to the movement milestone though!
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Unread 19-11-2014, 21:47
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Re: pic: MOEnarch Swerve Drive

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The diagonal cut roughtop is killing me! Straight cut (for maximum cleat effect) is the only way to go in my mind.!
You noticed that ehh?
We actually tested this last year with a force gauge, it takes ~15% more force to pull/push the robot side to side or front to back (with locked wheels) because when you put it on a diagonal more nubs are touching the carpet per contact patch area.
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Unread 20-11-2014, 07:14
Gdeaver Gdeaver is offline
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Re: pic: MOEnarch Swerve Drive

Looks like a happy happy joy joy moment. Some thing does not sound right when the module was rotating and driven. Those are wide wheels and may cause problems with amp draw on both drive motors and steering. Pay close attention to the tuning of the steering pid. You can have to much traction. For a fall back you might want to consider 1-1/2 or 1-1/4" Colsons. Next, get 4 of them on a frame and get programming. Then you will be half way there. Driver training is the last half. If you get it going before kick off, do you want to meet and do some swerve on swerve driving?

Note for the drive encoder, Do you really need quadrature? Wouldn't a tachometer solution be good enough? I do not now what the under side of the coaxial pulley looks like but, could you put some reflective tape strips on it and use a optical reflective sensor? We used a magnet solution last year and will use optical this year.

Last edited by Gdeaver : 20-11-2014 at 08:33.
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Unread 20-11-2014, 08:37
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Re: pic: MOEnarch Swerve Drive

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Originally Posted by nathannfm View Post
it takes ~15% more force to pull/push the robot side to side or front to back
Would you please post your data and test procedure?


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Unread 21-11-2014, 02:14
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Re: pic: MOEnarch Swerve Drive

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Originally Posted by Ether View Post
Would you please post your data and test procedure?

Sure, all from memory but it should be good enough.

First I had to build The Bathroom Scale of Science, the "force gauge" we used to test this because the real one we had in the lab maxed out at 50lb. The bars that stick out from the sides are the handles and the X that crosses the face of the scale has 4 ropes that converge at a shackle that was placed around a 1x1 frame member on the robot in the bumper zone. The wheels were then locked by jamming a scrap between the spokes. The robot was pulled with the ropes remaining as horizontal, and the scale remaining as vertical as possible to the ground. A reading of ~120lb was observed on the scale. The wheels were then switched to those with diagonal tread, the same procedure was followed and a reading of ~140lb was observed. A ~15% increase.

We also used a similar procedure to demonstrate how ineffective trying to drive out of a T-bone is. This time the wheels were unlocked and I pulled sideways acting as an opposing robot pushing from the oppisite side as our driver floored it (and the robot started to drive in a circle around me with a radius of the rope (~5ft)) you could see the scale instantly drop from a reading of ~140lb to something like 60lb (I don't remember the exact number but it was something drastic)

We again used the test to determine that 2" wide 4" dia wheels were ~15% better than 1 inch wide wheels. Take 4 1" wide wheels, place them on the four corners of the robot, lock them, drag. Place 4 more 1" wide wheels on the same axles, lock them, drag.

*All tests performed on actual FRC carpet
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Last edited by nathannfm : 21-11-2014 at 02:17.
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Unread 21-11-2014, 02:45
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Re: pic: MOEnarch Swerve Drive

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Originally Posted by Gdeaver View Post
Looks like a happy happy joy joy moment. Some thing does not sound right when the module was rotating and driven. Those are wide wheels and may cause problems with amp draw on both drive motors and steering. Pay close attention to the tuning of the steering pid. You can have to much traction. For a fall back you might want to consider 1-1/2 or 1-1/4" Colsons. Next, get 4 of them on a frame and get programming. Then you will be half way there. Driver training is the last half. If you get it going before kick off, do you want to meet and do some swerve on swerve driving?

Note for the drive encoder, Do you really need quadrature? Wouldn't a tachometer solution be good enough? I do not now what the under side of the coaxial pulley looks like but, could you put some reflective tape strips on it and use a optical reflective sensor? We used a magnet solution last year and will use optical this year.
It was (MOE cheer right after the video cut out ) If you were referring to the pulsating of the sound I think that was because most of the noise was coming from the bevel gears and their location was changing relative to the mic, but I will look into it. Do you grease yours? We also have not played with tension of the rotation motor belt, we will look into that next meeting (we did notice it affected the speed of rotation between modules) We will definitely be keeping an eye on the amps and have Jags on every motor to monitor that, The RoboRIO should also make that easy when it arrives. We will hopefully get driving by Monday. Our programmers already have a basic crab drive program and are working on fancier moves now. I think we would love to have some swerve on swerve driving! You and your team have already been a great help this summer, I will ask the drive team about this at the next meeting and get back to you.

As for the encoders we debated this, but we haven't had very reliable results from magnetic sensors in the past, and there is not much room to mount an optical sensor under the pulley. We also had all the encoders we plan to use in stock so we just went with them.
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Unread 21-11-2014, 10:07
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Re: pic: MOEnarch Swerve Drive

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Originally Posted by nathannfm View Post
The robot was pulled with the ropes remaining as horizontal, and the scale remaining as vertical as possible to the ground. A reading of ~120lb was observed on the scale.
Was this the breakaway force, or the min force to continue motion?

Was the height of the horizontal rope above, below, or at the height of the wheel axes?


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Unread 21-11-2014, 13:41
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Re: pic: MOEnarch Swerve Drive

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Originally Posted by Ether View Post
Was this the breakaway force, or the min force to continue motion?
The force to continue motion, but surprisingly the difference was only a few pounds. Nothing like the dramatic example of the spinning wheels.

Quote:
Originally Posted by Ether View Post
Was the height of the horizontal rope above, below, or at the height of the wheel axes?
It was about 2 inches above, in the middle of our bumper (one bumper removed for test to attach shackle to frame)
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Unread 24-11-2014, 23:39
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Re: pic: MOEnarch Swerve Drive

Hey Everyone!

This is just a quick update on the progress of the MOEnarch drive system.
We have mounted the four modules onto the square chassis and it's working smoothly.

This is the Facebook link to the video:
https://www.facebook.com/video.php?v...type=2&theater

As you will notice, the robot doesn't have any encoders on the modules just yet. We are looking to set those up by the end of next Monday and have a robot-centric swerve drive ready to go before Winter Break.
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Unread 25-11-2014, 07:26
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Re: pic: MOEnarch Swerve Drive

Sounds like a swerve, kind of moves like a swerve, it must be a swerve. Welcome to the 3 degrees of freedom club.
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