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Author Topic: Wheel Placement on Robot  (Read 4085 times)
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A lazy=susan like below could be used in the center of the bot to allow the flexing of the front and rear sections for turning. For skid steering keep the wheels close together for easier pivoting and turning.

http://www.homedepot.com/p/Shepherd-6-in-Lazy-Susan-Turntable-9548/100180572#.UiDGFynD_ix
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The best orientation… wheels are at the extreme corners of a square base

and

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keep the wheels close together for easier pivoting and turning


Now I'm confused... keep the wheels as close as possible or as close to the corners as possible? I have a feeling there is only one right answer for this. Personally I would think as close together would be best, but I really don't know.

Also, were should I put most of the load (12lb of batteries) over the wheels?


As far as having the robot pivot it would just be to much for this robot. I know it is possible, but time wise I don't have time to redesign the frame of the robot which is probably already assembled. (i'm having somebody weld the frame together). However, that Lazy Susan Idea looks cool so I will add that to my "when I get some extra time and money" folder.
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weight wise best placement of the load is in the center
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Now I'm confused... keep the wheels as close as possible or as close to the corners as possible? I have a feeling there is only one right answer for this. Personally I would think as close together would be best, but I really don't know.

A lot may depend on the size of the wheels compared to the size of the platform. Might be worthwhile to start by attaching the motors/wheels to a square piece of plywood and test the turning performance. Larger bots might benefit with some type of suspension if the bot will travel over uneven ground.
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Now I'm confused... keep the wheels as close as possible or as close to the corners as possible?

Think about it a bit: We are talking 4 wheels here. If you moved the wheels in the front back, and the ones in back forward, until they were near the center (but still spaced opposite each other), then yes, it would be easier to turn. Heck - if the wheels "merged", then you have the classic differential steered robot (add some casters on the front and back to balance).

Regardless, though, when the robot pivots about its center, those wheels will describe a circle. The closer the wheels are to the line that runs thru the center of the circle, the less drag/skid they will exhibit (and in the case of the classic design, little drag at all - but there will always be some drag, as any wheel that is following a curve, the inner edge /must/ move slower than the outer - and so you have wear; which is why when you car doesn't have proper alignment, your tires are messed up in all sorts of different ways - which is why, even with perfect alignment, you must rotate your tires, to even the wear).

Therefore - if the wheels are going to describe a circle anyhow - with a 4-wheel arrangement, the configuration with the least amount of drag (but more drag than the classic differential two-wheel design, of course) will be where those wheels contact the circle they are describing - which has to be a square (and, again, ideally the wheels could pivot - but as you noted, this would drive up complexity and cost, which I also realized when I posted that originally; I was merely suggesting the ideal way to minimize drag on such a 4-wheel arrangement, irrespective of complexity or cost).

Now - you may wonder why the various skid-steer equipment makers don't make their machines square? Well, it mainly has to do with the purpose of those machines and where they are operated. They are meant to be operated on surfaces with "give" (indeed, the manuals for these machines generally explicitly say not to operate them on concrete, asphalt, or other hard surfaces), so that they can easily slide without taxing the drive and other mechanical components. They are also generally designed to be small, while still carrying a good sized load, and be maneuverable, which limits making them square. Engine layout and other design needs probably also factor into the reason why they aren't square.
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Thank you for the useful insight. I will post some photos of the robot once the frame is finished.

Also, what would happen if you had 2 motors in the back that were higher torque but slower and 2 motors in the front that will lower torque but faster. Would the robot have better torque then if I just used 4 of the slower ones? (the wheel size would be the same)

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Thank you for the useful insight. I will post some photos of the robot once the frame is finished.

Also, what would happen if you had 2 motors in the back that were higher torque but slower and 2 motors in the front that will lower torque but faster. Would the robot have better torque then if I just used 4 of the slower ones? (the wheel size would be the same)


I would assume you would burn the higher speed motors as they will try to drag the lower speed motors forward.
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I would assume you would burn the higher speed motors as they will try to drag the lower speed motors forward.


I was able to get 4 of the same motors. It took some time as the motors are not officially for sale, but parallax worked with me and sold me 4 new ones. I had 2 older ones, but they are slightly slower and you said I could not use two fast motors and two slow motors. So it took some time for me to convince them that I need 4 sets as they don't have many ready. 

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Think about it a bit: We are talking 4 wheels here. If you moved the wheels in the front back, and the ones in back forward, until they were near the center (but still spaced opposite each other), then yes, it would be easier to turn. Heck - if the wheels "merged", then you have the classic differential steered robot (add some casters on the front and back to balance).


I was thinking about putting one wheel/motor on each corner of the robot but you explained how that would screw up the turning.
What if I slid the wheels from the corner down towards the middle of the robot so that they were almost touching and then put a caster wheel on each end of the robot. It would be like putting two, three wheel robots back to back.  That way the driver wheels are like .5in apart making the turning easy yet the caster wheels will help support the front/end.

http://www.geology.smu.edu/dpa-www/robo/trux/n06.jpg

Would that work? I think it would yet I have never seen a robot like that.
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I was thinking about putting one wheel/motor on each corner of the robot but you explained how that would screw up the turning.

It wouldn't screw up the turning, you would just have some (maybe a lot) of wheel drag; I also don't know how well the center would remain "centered". Is there a reason to keep the center from moving much?

Would that work? I think it would yet I have never seen a robot like that.

You could try it; it might work - but I fail to see how that would be better than just a single pair of wheels in the center, and a caster on the front and back?
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A little belated response, but for a smallish robot, like shown in reply #7, I would imagine that motor/wheel arrangement will be fairly reliable. After all, Lynxmotion has been selling those things for years.

OTOH, if you're talking about 6" wheels and a 2' x 3' or so base, that's pretty heavy-duty, and you really need to go with strong motors and probably wheels mounted on shafts riding on bearings, and connected to the motors via pulleys or chains.
http://damencnc.com/en/components/mechanical-parts/ballbearings/279

Skid steering works, but as mentioned, the carriage needs to be extremely robust for something that size and weight, and they certainly work best on surfaces where the wheels can, in fact, "skid" easily (eg, not carpets).

It's very common for people to use casters as a 3rd wheel [tripod arrangement] for differential-steered bots, but you'll notice this thing is "tiny" compared to what's being talked about,
http://www.geology.smu.edu/dpa-www/robo/trux/n06.jpg
A large, heavy frame will tip over if not properly balanced.
« Last Edit: September 13, 2013, 11:32:22 pm by oric_dan » Logged

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Is there a reason to keep the center from moving much?

I wold like to keep the turning as accurate as possible.


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you would just have some (maybe a lot) of wheel drag

that would make my turned less exact and decrease the life of the motors. (I'm mostly concerned with the turning) Right?

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I fail to see how that would be better than just a single pair of wheels in the center, and a caster on the front and back?

That was my original plan, but two motors do not have enough torque to push the robot. It was cheeper to buy another two wheels/motors  then to buy better ones. I assume that by adding two more motors/wheels I doubled my torque. Right?

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wheels mounted on shafts riding on bearings

They are.


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A large, heavy frame will tip over if not properly balanced.

If the wheels are  towards the middle of the robot It will tip over, but the caster wheels will help balance it. Maybe tip is not the right word. Think of a seesaw. If the wheels are in the middle of the robot it will pivot and the back & front could hit the ground.


Thanks! Would my idea work? I can't test it out because I don't have the two caster wheels.
« Last Edit: September 14, 2013, 10:16:10 am by Drew Davis » Logged

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What I was referring to is, a heavy robot with tripod arrangement and only 1 caster can easily tip over.
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What I was referring to is, a heavy robot with tripod arrangement and only 1 caster can easily tip over.

I agree! That's why I thought that a caster on each end would prevent it form tipping. Would my idea for a 6 wheel robot work?
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That was my original plan, but two motors do not have enough torque to push the robot. It was cheeper to buy another two wheels/motors  then to buy better ones. I assume that by adding two more motors/wheels I doubled my torque. Right?
You can add the torque indeed. But you are probably not so much interested in torque but more in "speed" and "acceleration speed"
You speed will certainly not double.
The acceleration speed will increase but it will not be double.
There are many things at play in a 4WD and engine loads in general. To be honest try and see is the best way to find out.

cr0sh his explanation about the square and powers for skit-steering is 100% correct if you only turn on the spot. If you also turn while moving forward (or backward) the square shape no longer has a benefit to the rectangle because in this case the centre of the turning circle is outside the ground plane of the robot.
The width of the contact surface of the wheel is very important. The wider the more friction the more axial forces the stronger the wheel bearing must be. In case of on the spot turning a square will make that the centre of the wheel is friction less and you have half of the friction on both sides of the wheel.

Best regards
Jantje
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What I was referring to is, a heavy robot with tripod arrangement and only 1 caster can easily tip over.

I agree! That's why I thought that a caster on each end would prevent it form tipping. Would my idea for a 6 wheel robot work?
If you mean 4WD with the drive wheels close together, and 2 casters on the ends, that sounds doable, but I don't see a situation where that design will be at all useful. IE, why even bother with 4WD?

I think a lot depends upon where this robot will operate most of the time. If outdoors on earthen terrain, I don't think having casters is a good idea at all, as they will probably dig into the dirt a lot. For that, I would go with a basic design with 4 drive wheels on the corners of the robot, and relie on a very robust motor/wheel system to keep it from breaking down.

Or else, use 2WD with 2 extra "large size" idler wheels [much larger than casters] on the opposite end from the drives. This guy has some ideas,
http://www.amazon.com/Build-All-Terrain-Robot-Robotics-ebook/dp/B001UQ5HW4

For indoor use on smooth surfaces, a caster system will work, You might check this out,
http://davidbuckley.net/FR/Cycler/Stability+Loads/Stability&LoadTest.htm
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