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Topic: Wheel Placement on Robot (Read 5792 times) previous topic - next topic

Drew Davis

Hello,

I'm building a 4WD robot, but I'm a little confused about how to optimize the robot for turning. I need to turn the robot 90 degrees, and I need the center of the robot to stay in the same spot as it turns. Anyway, were should I place the wheels? Should I put them far from each other, or as close a possible? The Robot will be 2ft wide and 3.5ft long.


Thanks!

MillerJLee98579

are you using a servo or motor on each wheel?

Drew Davis

One 12v, 3 amp (no load) motor with 6in wheels.

MillerJLee98579

in my opinion i would say have the wheels near the corners with the transmission in the dead center of the robot if you want to keep the turning radius in the center

MillerJLee98579

but in that case you might just need a motor for each side of the robot

Drew Davis

Sorry I wrote that wrong...
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One 12v, 3 amp (no load) motor with 6in wheels.



One Motor for each 6in wheel.

MillerJLee98579

than possibly not sure if you have the access to scrap wood or not but you might want to make a couple of wood platforms with those dimensions and test different mounting positions for the wheels that way when you found a place that works with the radius you want you will have the measurements and everything (yes i learn a lot by trial and error)

zoomkat

#7
Aug 29, 2013, 05:07 am Last Edit: Aug 29, 2013, 05:09 am by zoomkat Reason: 1
Bots like below operate like skid steer equipment, which operate similar to tanks. The wheels on the same side operate together.

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cr0sh


I'm building a 4WD robot, but I'm a little confused about how to optimize the robot for turning. I need to turn the robot 90 degrees, and I need the center of the robot to stay in the same spot as it turns. Anyway, were should I place the wheels? Should I put them far from each other, or as close a possible? The Robot will be 2ft wide and 3.5ft long.


Hope those motors are fairly powerful. Hope it won't be run on carpet (unless you don't care about your carpet long-term).

What this is called is "skid-steering" (actually differential steering) - meaning the wheels skid sideways as the vehicle turns:

http://en.wikipedia.org/wiki/Skid-steer_loader

In fact, your dimensions seem really close (scaled) to that of many skid-steer loaders like depicted in that link above. Regardless, it really tears up the ground, and if used on hard surfaces (like concrete and such) that have no "give" - it can mess up bearings, transmission components, and tires in short order. Keep that in mind for your robot. If you plan on this being used on dirt or grassy surfaces only, all the better. But if being used on hard surfaces (especially with grippy tires and lots of weight), expect the components to fail quicker than normal.

The best orientation (as far as wheels are concerned - tracks are a whole 'nother matter) for the wheels are at the extreme corners of a square base (that is, at the points where, if a circle was circumscribed around the square, the vertices of the square would meet the line of the circle). Ideally, you would also be able to angle the wheels such that they tracked the circle as the vehicle turned in-place. This of course would be more complex to implement.

I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

Drew Davis

Thanks for the replies.


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Ideally, you would also be able to angle the wheels such that they tracked the circle as the vehicle turned in-place

If your talking about what is in the image below,  it would be to hard and cost to much to add that into the robot I'm working on.


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than possibly not sure if you have the access to scrap wood or not but you might want to make a couple of wood platforms with those dimensions and test different mounting positions for the wheels that way when you found a place that works with the radius you want you will have the measurements and everything


I will give that a try.

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The best orientation… wheels are at the extreme corners of a square base


I will try that out on the wooden prototype.


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Regardless, it really tears up the ground, and if used on hard surfaces



It will be running on hard surfaces (the road) but I'm not to worried about messing it up as it looks strong...


http://www.parallax.com/product/27971



Thanks!

Jantje

My robot is very similar in size, has similar sized wheels and runs on grass.
My experience:
I'm still looking for a motor that lasts longer than a couple of weeks :-( I have killed lots of motors of several types in the last years  :(
I have upgraded the rods to the wheels from 6mm steel to 6mm rust free steel to 10mm rust free steel; because they broke ( sometimes in a couple of hours) due to the turning.
I ended up using 2 ball bearings in a aluminium case (the plastic ones broke) to support each wheel rod.
The motor is tightly attached to the wheel rod but it is flexibly attached to the alu frame. This means that the motor can not turn around its axe (and as such can deliver power) but it can move when the wheel is forced up/down/left/right (so it does not take any axial or radial force except for its own weight induced force).
So from experience I'm fully with Cr0sh here.

I'd be interested to see the wheels you are planning to use as I'm not fully happy with the wheels I'm currently use and I hardly find 6 inch wheels.

Best regards
Jantje
Do not PM me a question unless you are prepared to pay for consultancy.
Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -

Drew Davis

I posted the link to them above, but I will post again.


http://www.parallax.com/product/27971

Jantje


I posted the link to them above, but I will post again.


http://www.parallax.com/product/27971


Och I wasn't aware it was a motor wheel assembly.
This looks good on paper but the device seems to be under revision which sounds bad to me.
Best regards
Jantje
Do not PM me a question unless you are prepared to pay for consultancy.
Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -

zoomkat

If turning is difficult, the bot might be hinged in the middle and rotated left/right to help with turning. Another setup below.

http://www.youtube.com/watch?v=0FGWKwIIKVM
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Drew Davis

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This looks good on paper but the device seems to be under revision which sounds bad to me.


I have been in contact with the company and they have had the kit for years, however, their supplier for the motors just stopped making that model. They found comparable motors, but the mounting is slightly different so they are working on a new mounting block.

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If turning is difficult, the bot might be hinged in the middle and rotated left/right to help with turning. Another setup below.


I think making the robot swivel like that would just be to hard and time consuming for this bot, but that could be fun to do in the future with a smaller robot!

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