4 wheels vs 3 wheels

[alucard]

Hi all,
I am designing a high speed line-following robot, which needs to turn quite often using differential drive.

Can anyone enlighten me the advantages and disadvantages of using 3 wheels (2 back driven wheels + 1 front wheel) vs 4 wheels (2 back & 2 front) ?

Thanks!

 

[jkerns]

4 wheels - skid steering by differential powering of both front and back or 2 driven wheels and 2 castors?

4 wheels will make it less likely to tip over than 3. Skid steering will be less responsive and waste some of your battery energy compared to using casters.

[alucard]

In both the 3-wheel and 4-wheel cases, the back 2 wheels are motor-driven. The front wheels are rigid castors wheels (they can rotate freely but not turn sideways)

I think for 3 wheels, it shouldn't be easy to tip over , if the robot is wide and has low centre of gravity.

I cannot use a ball castor because the robot must be able to travel on concrete pavement (with small cracks)

Are you saying I should go for  3 wheels since less skid steering (friction)?

[Soeren]

Hi,

In both the 3-wheel and 4-wheel cases, the back 2 wheels are motor-driven. The front wheels are rigid castors wheels (they can rotate freely but not turn sideways)
[...]
Are you saying I should go for  3 wheels since less skid steering (friction)?

If each front wheel is mounted on a separate (good quality) castor, it won't make much difference with 3 or 4.

However... Did you consider the driven wheels at the approximate center, with a single castor at each end?
That would keep the majority of the weight on, or close to, the driven wheels (thus it takes less power to turn) and the smaller (I assume) castor wheels won't dig into holes as easy, as it doesn't carry much load, just supporting the unit from tipping over.

[jkerns]

In both the 3-wheel and 4-wheel cases, the back 2 wheels are motor-driven. The front wheels are rigid castors wheels (they can rotate freely but not turn sideways)

I think for 3 wheels, it shouldn't be easy to tip over , if the robot is wide and has low centre of gravity.

I cannot use a ball castor because the robot must be able to travel on concrete pavement (with small cracks)

Are you saying I should go for  3 wheels since less skid steering (friction)?

So you plan to have the front wheels mounted so they can't turn (like this:  http://www.harborfreight.com/8-inch-cushion-tire-fixed-caster-46817.html and not like this: http://www.harborfreight.com/8-inch-cushion-tire-swivel-caster-46818.html ) to line up with the direction the robot is turning?  So you have to skid the wheels sideways to turn?

I would think that will not help the steering because you will have a mix of skidding the front wheels and slipping the drive wheels.

If you get reasonably large diameter castering (swiveling) wheels, they should roll OK even on pavement.

[alucard]

I am afraid that swivel wheels would get stuck at cracks in the concrete. For e.g., one part of the ground could be slightly higher, and if the wheel 'bangs' flat against the higher part, then it might get stuck

How about using an omni-wheel like http://dl.dropbox.com/u/17243832/omni.PNG

[Soeren]

Hi,

I am afraid that swivel wheels would get stuck at cracks in the concrete. For e.g., one part of the ground could be slightly higher, and if the wheel 'bangs' flat against the higher part, then it might get stuck

How about using an omni-wheel like http://dl.dropbox.com/u/17243832/omni.PNG

Omni or not, any wheel that meets a vertical obstacle higher than or equal to the radius of the wheel will be a dead stop.

[gerard]

true ^^^^
but not if it's powered

having 3 or 4 wheel drive would solve that problem, but i don't think that's necessary just for cracks
and cracks aren't anything a wheel can't deal with unless the cracks are wide
= ]

[Gertlex]

true ^^^^
but not if it's powered

but not if it's going fast. Then it's an exciting crash