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When I turn, the car hops up and down.

by PrudensOptimus
Tags: hops, turn
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PrudensOptimus
#1
Mar21-05, 10:14 PM
P: 640
Hello,


I have a 3 wheel autonomous small car... built out of legos.

And umm... the problem is when I turn the back two wheels in opposite directions... to produce left or right turns... the autonomous vehicle tends to "up and down"... ... what causes that?
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faust9
#2
Mar21-05, 10:50 PM
P: 997
Why are you turning the back wheels in opposite directions?
hypatia
#3
Mar22-05, 05:50 AM
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is it a solid axel? if it is both wheels are turning at the same speed, this makes the inner wheel hop. Because the inner is trying to turn as fast as the outer wheel.
you need a limited slip axel.

DaveC426913
#4
Mar22-05, 08:25 AM
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When I turn, the car hops up and down.

One question: your front wheel, is it on an axle, or can it freely rotate like a caster (like on a shopping cart)?

If your front wheel has an axle, then here's the problem:

By turning the rear wheels in opposite directions, you are rotating the vehicle about a vertical axis that is centred on the rear axle - the front wheel gets dragged sideways, causing the hopping.

Instead of trying to turn "on a dime", try turning while moving forward slightly. Rotate only one wheel - keep the other one wheel fixed or even moving forward slightly while turning the other forward. This will allow the front wheel to move forward as it pivots.
minger
#5
Mar22-05, 09:36 AM
Sci Advisor
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Yes, his front wheel is on an axle.
PrudensOptimus
#6
Mar22-05, 06:07 PM
P: 640
Yea on an axle...

Also... do i even need the front wheel?

What is the front wheel there for anyways? lol... i'm kinda confused myself lol.

Any benefits if i keep it there?
brewnog
#7
Mar25-05, 09:19 AM
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Quote Quote by PrudensOptimus
Also... do i even need the front wheel?
Have I missed something with your posts about 3 wheeled vehicles? Surely the purpose of having more than two wheels is so that your vehicle doesn't fall over?
PrudensOptimus
#8
Mar30-05, 10:11 PM
P: 640
What if my vehicle is balanced without the front wheel? Will there be any downside to remove the front wheel?
physicsCU
#9
Mar30-05, 11:27 PM
P: 202
No vehicle is balanced on two wheels. Even a bike, your body is constantly adjusting to maintain balance.

You need the third wheel for balance and stability.
PrudensOptimus
#10
Mar30-05, 11:33 PM
P: 640
What if the "third" wheel causes turning difficulties? What can i do about that.
faust9
#11
Mar30-05, 11:48 PM
P: 997
Quote Quote by PrudensOptimus
What if the "third" wheel causes turning difficulties? What can i do about that.
You could use a smooth plastic "nub". A little nylon sphere on a stick would work well(I've seen a lot of sumo-bots with these). Or you could go crazy and install a handfull of 3-axis motion sensors and a complicated algorythm to continually balance your bot(like the Segway).
brewnog
#12
Mar31-05, 07:32 AM
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Or you could have a caster (or did we discount that in another thread?)
DaveC426913
#13
Mar31-05, 08:58 AM
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Quote Quote by physicsCU
No vehicle is balanced on two wheels.
This is not true.

A vehicle whose centre of gravity is below the axle balances just fine thank you.

(Of course, in practice, you'll want to accelerate slowly, lest the wheels remain motionless and the vehicle does the rotating...)
physicsCU
#14
Mar31-05, 10:30 AM
P: 202
Yes, that is true, but I think in practice, it is very difficult to get the CG below the axle, unless you know what you are doing.

It is a hard mix of balance and math and design to get it though. Most people who do it end up there not on purpose
brewnog
#15
Mar31-05, 10:41 AM
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Quote Quote by physicsCU
Yes, that is true, but I think in practice, it is very difficult to get the CG below the axle, unless you know what you are doing.

It is a hard mix of balance and math and design to get it though. Most people who do it end up there not on purpose
I wouldn't say getting the centre of gravity below the axle would be an accident, nor would I say it's difficult. The difficulty is controlling such a two-wheeled vehicle.
PrudensOptimus
#16
Mar31-05, 11:19 AM
P: 640
Is controlling the movements of a humanoid difficult? LIke Asimo, is it hard to control something like that?
minger
#17
Mar31-05, 11:46 AM
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I think you should stick to your three wheeled car.

The movements of a humanoid, I would think, would be very difficult to reproduce.
Cliff_J
#18
Mar31-05, 11:56 AM
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Quote Quote by PrudensOptimus
Is controlling the movements of a humanoid difficult? LIke Asimo, is it hard to control something like that?
Consider this: Honda has over a decade and many many millions of dollars in development costs in that robot, its about the only one like it (or even close) that has been publically announced, each version improves over the previous but battery life is still measured in minutes. Pretty easy to conclude, its incredibly difficult.

Mimicing the Segway's ability to balance on two wheels is far easier and can be done with off-the-shelf parts for around $1000-1500 with only minimal code development. Some guy built one and put up a website on how he did it and you could probably google him.

Your weight distribution will make a huge difference here too. Go to a home center and push around a lumber cart, the six wheel kind. It has two fixed wheels in the center that carry almost all the load. The four casters are at the corners but mounted slightly higher then the center wheels. Viewed from the side, the cart needs to "tip" to one set of the casters or the other because of the mounting height difference. But using a balanced load it is very easy to track straight and make corners with the cart because the load is over the two fixed center wheels, the casters are more just a stop to keep the cart from tipping too far.

You could mimic that - put all your weight so its balanced over the two center wheels, now the front wheel is more like an outrigger that prevents the vehicle from tipping, probably need to add a back wheel too to keep it from tipping that direction as well. But now when the two drive motors want to turn they carry all the load and get pretty much all the traction so the outrigger is just along for the ride.

If you look at the combat robots for "Battlebots" or "Robot Warriors" and so on you'll see some two wheel designs setup like this expect they just slide the front/back edge of the machine on the ground. Since that edge is for stability but carries no significant load it doesn't even need a wheel.

If your robot is the one trying to lift something in a claw, this is going to be more difficult as now you have the weight of the load to factor in. A caster wheel sounds like an even better idea.

Look at this lawn mower - its rear wheels do all the steering, the front wheels are casters that just follow whatever the rear wants to do...
http://www.dixiechopper.com/flatlanders_.php


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