Advantages/Disadvantages of a Mars Rover Style Robot (Think Taurus 2)

[Raeven]

    Lately I have been feeling the urge to build a robot which was similar to the Taurus 2 robot on this site and perhaps, enter into the mobot competition. What I was wondering is (as the subject says) what are the designs advantages/disadvantages. I understand it would consume more energy compared to your average four servo four wheeled robot however it is able to turn in place, but what I am really interested in is for energy consumption/programming difficulty issues why not just use omni-wheels instead?

Thanks

[SmAsH]

why not just use omni-wheels instead?

What happens if your on a hill with omni wheels?

[Raeven]

What does happen?

[little-c]

i think it rolls down the hill!

[Oz]

It does roll down the hill

[Raeven]

Yes I was wondering whether or not it could roll up a hill (It could right?) but for whatever reason was not considering the possibility of rolling down a hill. So omni-wheels are definitely not a good choice for the mobot course. So back to discussion other than being able to handle hills and rugged terrain better what advantages does Taurus have?

[madsci1016]

i think it rolls down the hill!

Not all omni wheels have this problem.

I can't find the smaller robotic version of these wheels, but i know they exist.

http://www.airtrax.com/

Notice the wheels are different, the outer rollers are mounted at and angle, and on the vehicle, the front wheel and back wheel, the outer rollers are angled in opposite directions. This creates a footprint like:
   ---------------
 //              \\
//                \\

so it does not roll side to side when pushed, or by gravity down a hill.

[SmAsH]

Yeah but normal onmi wheels are just bad for outdoor terrain imo. The was admin had taurus 2 was cool, 360 degree turns on the spot

[Raeven]

I know I suppose I'm a sucker for the mars rover type wheels javascript:void(0); haha. So basically omni wheels are probably better indoors because they use less servos thus less battery power and the mars rover type wheels can handle rugged terrain much better.

[madsci1016]

Yeah but normal onmi wheels are just bad for outdoor terrain imo. The was admin had taurus 2 was cool, 360 degree turns on the spot

Dirt and soil is a no go, but that company claims those wheels work fine on dirty parking lots and sidewalks no problem.

[SmAsH]

Yeah but normal onmi wheels are just bad for outdoor terrain imo. The was admin had taurus 2 was cool, 360 degree turns on the spot

Dirt and soil is a no go, but that company claims those wheels work fine on dirty parking lots and sidewalks no problem.

Meh, ive seen a fair few companies who "claim" a lot of things...
I wouldn't take onmi wheels outdoor full stop. I can see then clogging up with dirt of something and locking up.

[madsci1016]

Meh, ive seen a fair few companies who "claim" a lot of things...
I wouldn't take onmi wheels outdoor full stop. I can see then clogging up with dirt of something and locking up.

We have robots on base with omni's like these and i have seen them operate outside no problem. Since the outer wheels are mounted at an angle away from the wheel there isn't much of an area where the wheel is close to another surface; no real places for it to clog.

The reason the project fell through, was the fact omni's smaller contact area created a higher PSI on the surface it rolled on, and many surfaces were not rated to go that high (like a ship's deck).

[paulstreats]

You generally find that omni wheels are made of a hard plastic that has a bad(in this case) co - efficient, meaning that they would slip quite easily - as opposed to a soft rubber which would have a better grip. "Slippage" or "Grippage" is what can really count when building or following maps, and expecting sensor data to match with distance supposedly travelled. Also fast acceleration would cause them to slip or wheel skid on a hard surface.

 Im sure that there are some omni wheels out there with a softer and grippier surface, but you would pay a lot more for the priveledge

i think it rolls down the hill!

this would only happen if the drive motors were left unpowered and allowed to free wheel (but this would happen with normal wheels anyway - unless they were orientated horizontally on the hill).
 - if they do roll down hills, then it would be a case where if an omni wheeled robot were moving forwards and then the motors stopped, then the robot would keep on travelling through momentum. This doesnt happen so neither would they roll down a hill(unless all power was cut to the motors like said above)

[SmAsH]

What i was saying about the hill thing was that omni wheels have smaller wheels inside them, these have to brakes on them.
Say you had a four wheeled robot, what stopping it from rolling sideways down a hill?

[paulstreats]

Erm, the 2 wheels facing up the hill are propelled by motors. thats what gets it up there in the first place. If you remove all power from the motors then fair enough, it would roll back down the hill(as mentioned above, the same would happen with a robot with standard wheels). If the motors are braked, ie. locked, then those wheels connected to these motors will not move either, preventing the robot from moving. Sure the locked wheels have wheels inside too but these will be facing horizontally (across) the hill and not down, so preventing freewheel motion downwards.

[madsci1016]

What i was saying about the hill thing was that omni wheels have smaller wheels inside them, these have to brakes on them.
Say you had a four wheeled robot, what stopping it from rolling sideways down a hill?

First generation wheels had that problem, the new ones don't. That why they mount them funny at an angle. If you draw out the new force diagram, there are two y vectors pointing at each other from either wheel, preventing the outer wheel from moving side to side freely, unless the main wheels turning themselves.

[Admin]

Omni-wheels are inherently inefficient.

For example, an omni-wheel thats angled 45 degrees from the direction of travel is, under perfect idealistic conditions, only 50% efficient. Factor in friction and slip, and it'll drop even further.

Thats why I made my Fuzzy robot have four omni wheels, so that it has the option of driving straight at high efficiency. But its only efficient when driving straight, not while doing funny strafing maneuvers . . .

[z.s.tar.gz]

Mars Rover type of bots have the added advantage of climbing stairs if you make them with independent suspension and brakes.

[madsci1016]

These are the wheels i was trying to explain:

http://www.fingertechrobotics.com/proddetail.php?prod=ft-mecanum-set

They do not roll freely side to side, and are efficient on regular travel forward / back. They are arguably 'inefficient' when moving side to side, however it's not really a matter of inefficiency, but increased torque required to move. you get a gearing action when you move side to side, so it requires more torques to move at slower rpm, but this could be canceled out by a decent drive train.

They still would have issues outside in dirt, but i wanted to let show what i was trying to explain before.

[Comrade117]

I think omni-wheels are ineffective.
You would do better job with suspensions and wheels. This would allow bot to go over extreme surfaces. 
I think mars rover needs more suspensions, wheels and power, but else is ok. 

[Hertz32]

taurus 2 was cool but i would just hack a tank