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Tri-Wheel Project

 
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linuxguy



Joined: 16 Oct 2006
Posts: 120
Location: Beaverton, OR

PostPosted: Fri Nov 03, 2006 4:31 am    Post subject: Tri-Wheel Project Reply with quote

I have designed a tri-wheel assembly I plan to use on a rover design. However, I have one problem - how do I get power and i2c through the axel to the wheels? The problem comes about because the entire wheel assembly can rotate freely at any time for any number of rotations. Each of the three drive wheels is independently driven when they are on the ground, and will be powered down otherwise.

This wheel assembly is inspired by the Landmaster from the movie ""Damnation Alley"" with George Peppard. It's a classic post nuclear war survival movie. if you have never seen it, just go rent it and watch it. Very Happy

Below are a couple pictures of my rendition of the Landmaster Tri-Wheel. I am moving the ball bearing hub to the wheel side of the axel where it will be closer to the actual point of rotation. Those are 3 3/4"" diameter x 1 1/4"" wide wheels shown in the pictures - the same wheels I have on my Octabot rover now.

Tri-Wheel from the side:


Alternate view:


Does anyone have any ideas on how I could get power and i2c through the axel to the wheels?

8-Dale
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ginge
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Joined: 14 Jan 2006
Posts: 1031
Location: Manchester, UK

PostPosted: Fri Nov 03, 2006 12:35 pm    Post subject: Reply with quote

Thats a tough one!

The only thing I can think of is to use a slip ring. This might cause noise problems though.

Anyone else done something similar to this?

Barry
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linuxguy



Joined: 16 Oct 2006
Posts: 120
Location: Beaverton, OR

PostPosted: Fri Nov 03, 2006 4:48 pm    Post subject: Reply with quote

"ginge" wrote:
Thats a tough one!

The only thing I can think of is to use a slip ring. This might cause noise problems though.

Anyone else done something similar to this?

Barry


I was thinking if I leave the ball bearing hub where it is, I could hang a battery on the axel. That would require a battery for each wheel assembly, which would add extra weight I am not sure I want to deal with. I would rather move the ball bearing hub closer to the wheels and have it on the other end of the axle.

That still leaves me without a way to get power past the hub to the wheels. I want to use Open Servos on this project.

I just ordered a catalog/cd about slip rings from http://www.polysci.com/

8-Dale
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DavidCary



Joined: 24 Oct 2007
Posts: 12
Location: Oklahoma and Kansas

PostPosted: Thu Nov 01, 2007 5:37 pm    Post subject: Reply with quote

Quote:
... a way to get power past the hub to the wheels. ...

Some ways to get electrical power through a joint that continuously rotates:
* slip ring
* rotary capacitor
* rotary transformer

Getting signals across a joint that continuously rotates is much easier: the above methods, or also
* LEDs and photodiodes
* radio transmitter and reciever

Rather than getting electrical power to the wheels,
perhaps it would be simpler/better to get mechanical power to the wheels.

Two motors, one turning the entire assembly, the other one driving a shaft through a central hole in the rotating hub to independently drive the wheels:
* P'titgneugneu steering carriage (scroll almost all the way down)

One motor, turning both the individual wheels and (when they got stuck, or when swimming) the entire assembly (My understanding is that the "Damnation Alley" Landmaster did this):
* tri-wheel
* lynxmotion tri-wheel
* Landmaster LEGO model
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linuxguy



Joined: 16 Oct 2006
Posts: 120
Location: Beaverton, OR

PostPosted: Sat Nov 03, 2007 5:58 pm    Post subject: Reply with quote

DavidCary wrote:
Some ways to get electrical power through a joint that continuously rotates:
* slip ring
* rotary capacitor
* rotary transformer

Yes, indeed. I am revisiting this project after shelving for several months. I have moved the bearing hub so it attaches direct to the wheel assembly, and have designed a custom bracket to allow using GHM-04 or other motors instead of CR servos, which I never planned on actually using.

I need to get back in touch with Moog and see what kind of solutions they can offer me for power and signal transfer through the hub to the wheel controller and motors. I do have most of the brackets to build one of these tri-wheels right now.

DavidCary wrote:
Rather than getting electrical power to the wheels,
perhaps it would be simpler/better to get mechanical power to the wheels.

The Landmaster of the Damnation Alley movie has three independently powered motors, one for each wheel. That is what I want to create also. The wheel that is not on the ground can be shut off to save battery power, but I have not worked on a way to have the wheels go on and off at the right times, depending on which is off the ground.

8-Dale
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kbb



Joined: 01 Jun 2007
Posts: 180

PostPosted: Sat Nov 03, 2007 9:15 pm    Post subject: Reply with quote

When I first saw this, I thought that maybe you could cannibalise an electric motor for the rotating fixtures and use the commutator with power diodes for power delivery to the wheel assembly- you would be looking for one with more than two points of contact for safety.

For wheels on the ground vs. wheel in the air: Options seem to included using an accelerometers (too expensive?) to determine the position of the assembly and then assume that the gravitationally "upper most" wheel is off the ground. If each wheel had independent suspension, then maybe you could use force feed back.

Landmaster from Damnation Alley

Kevin.
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guru



Joined: 03 Jan 2006
Posts: 128
Location: St Pete Beach, FL

PostPosted: Thu Nov 08, 2007 6:53 pm    Post subject: Reply with quote

I still think the mechanical transfer would be easier. and if you use an offset gear towards earth, the wheels on the ground would always be under power and the top one idle. No programming or accellerometers. My friend and I had a conversation the other day about mechanical systems versus electro-mechanical systems and usually the simple mechanical works better.
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linuxguy



Joined: 16 Oct 2006
Posts: 120
Location: Beaverton, OR

PostPosted: Thu Nov 08, 2007 11:30 pm    Post subject: Reply with quote

guru wrote:
I still think the mechanical transfer would be easier. and if you use an offset gear towards earth, the wheels on the ground would always be under power and the top one idle. No programming or accellerometers.

I don't want to add gears to the assembly. I can tell if a wheel does not have a load on it by the load info from an Open Servo, or at least I think this would work.

I want to be able to adjust the speed/direction of each wheel independently so I can keep them all synced up for straight travel. This wheel setup could even drive as a 4WD using one wheel down on each tripod. I have not decided whether I want to add a motor for the center of each tripod to turn the whole tripod. Doing this would make for better climbing.

8-Dale
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linuxguy



Joined: 16 Oct 2006
Posts: 120
Location: Beaverton, OR

PostPosted: Fri Nov 09, 2007 12:46 am    Post subject: Reply with quote

I have to get some tubes and tubing hubs from Lynxmotion so I can meet with the Moog rep and show him what I need. I can't do that until next month.

8-Dale
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linuxguy



Joined: 16 Oct 2006
Posts: 120
Location: Beaverton, OR

PostPosted: Sat Nov 10, 2007 7:58 am    Post subject: Reply with quote

DavidCary wrote:
One motor, turning both the individual wheels and (when they got stuck, or when swimming) the entire assembly (My understanding is that the "Damnation Alley" Landmaster did this):
* tri-wheel
* lynxmotion tri-wheel
* Landmaster LEGO model

The Lynxmotion Tri-Wheel thread is mine also. Very Happy

After looking a lot closer at the various pictures of the Land Master (and I want to get the movie again), I think you are right about having one motor drive all three wheels.

However, I am not sure I really want to duplicate the Land Master tri-wheel now. The only real issues are getting power through the ball bearing hub that is between the axle and the wheel assembly, and getting I2C lines thrrough without adding noise to them. I am looking at slip rings now, and how to specify them. I can't really do much on this though, until I get some of the Lynxmotion HUB-08 tubing hubs and a tube such as the AT-01.

8-Dale
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