Casters vs. Unpowered Omni wheels

[nnfuller]

So I have been looking at a lot of older robots for inspiration for a new chassis design before build season and I was wondering if any veterans could tell me why we don't see casters anymore. I have seen a large number of robots with unpowered omnis, but no casters... Why do you think this is, becase from where I'm standing...

Casters
Pros:

• Self Contained
• Pre Assembled
• Bolt and Go Simple
• Smooth Ride

Cons:

• Have a "Direction" that must be overcome
• Cannot be Powered

[Andrew Schreiber]

Originally Posted by nnfuller

So I have been looking at a lot of older robots for inspiration for a new chassis design before build season and I was wondering if any veterans could tell me why we don't see casters anymore. I have seen a large number of robots with unpowered omnis, but no casters... Why do you think this is, becase from where I'm standing...

Casters
Pros:

• Self Contained
• Pre Assembled
• Bolt and Go Simple
• Smooth Ride

Cons:

• Have a "Direction" that must be overcome
• Cannot be Powered

Because casters tend to change the direction randomly depending on how they start. Ask any FLL team.

[ratdude747]

casters tip easily. no matter what direction they are pushed, they always pu the pivot point (the axle) in a bad position in terms of stability.

not that casters never work, but that i have only seen one robot ever that used them well , and that was in a picture (some veterans around here would be able to guess it). however, I have seen many examples where casters were a complete failure.

[Koko Ed]

We used casters on our 2008 robot for the front wheels. They worked pretty well.

[davepowers]

GUS' very first year we used casters, they worked for us, sort of. I do remember the robot did a very good job of not going straight, that might not have been the casters though. There were a few problems with turning, sometimes the casters would turn a way they shouldn't, but it would always push through.

-Dave

[IndySam]

We used casters early on until 2005. We now have a hard and fast rule, no casters allowed on the robot, ever.

[Billfred]

1293 used casters its first two years. The first robot (whose autonomous was just timing and held breath) would be thrown off-course if the casters weren't right but was otherwise underpowered enough that it wasn't a huge issue. The second one (powered by CIMs) verged on uncontrollable on the field, easily the weakest link in its design. They haven't looked back, going with treads ('07, '10), N-wheel-drive ('06, '09, '11), or mecanum ('08).

2815's always driven all the wheels on the floor, so no experience there. (I would want to know why someone wouldn't power an omniwheel; there are certainly some scenarios, but I can't see those scenarios being anything but a teensy minority.)

[SenorZ]

We used four casters on the corners, and two powered wheels centered on the sides. Using a tank drive program it worked quite well.

Pros:
1. Cheap!
2. They work!

Cons:
1. Wobble while they orient themselves
2. Look lame

[EricH]

It depends somewhat on your application and your specific type of caster. For example, 330 uses a lot of pneumatic caster wheels (modified to be driving wheels, that is!)

In general, omni wheel > ball caster > other caster types. But, that does have a few caveats.

First, you hit a major con of casters: You cannot power them. Well, not easily. With an unpowered omni wheel, powering is as simple as adding a sprocket and chain to the rest of your drivetrain, and you gain some forwards traction. Every time you add an unpowered wheel to your drivetrain, you lose normal force on the powered wheels, which loses you frictional forces, which just so happen to provide traction. Long story short, loss of traction. By placing casters, you can lose traction.

A fairly large advantage of omnis/con of casters is that omnis can almost literally drop into a drivetrain with one bolt (the axle) and no frame modifications besides a hole for said axle. (OK, so bearings in the omnis... but close enough.) A caster needs to have multiple mounting points matching the base pattern, which is a little bit more involved and takes some minor planning.

And now we get to the other con listed by the OP. For a pre-assembled, cheap (one he forgot in the pros), smooth-riding, easy-to-mount (well... see previous paragraph) solution, you get: Unpredictable driving motion. You can insert your own scenario where you need predictable driving motion, I'm sure! But with omnis, you know what's going to happen for sure, whenever you apply power in a given manner.

As a point of comparison:

Omni Wheel Pros:
-Can add power
-Predictable motion
-Can be used as a sensor wheel when unpowered (casters can't do this as well)
-Drop-in

Omni Wheel Cons:
-Cost
-Fewer sources
-Bumpy ride (depending on type)

Caster Pros:
-Self Contained
-Pre Assembled
-Cheap
-Smooth ride
-Easy to mount with planning

Caster Cons:
-Unpredictable motion
-Extremely difficult to power
-Tougher to mount without planning
-Harder to use as a distance sensor due to said unpredictable motion

FWIW, I've seen it said on CD that a peg leg is preferable to a caster wheel.

[Ninja_Bait]

Casters probably make sense for a drive like this.

Not that we have experience with casters - we've used omnis when we had to, and we powered them like 2815. Why do anything else?

[Marc S.]

This was a Fully Omni-Directional robot that successfully worked during competition.

It was very successful and arguable the best robot in 2000. Still we have to remember that the competition has since changed, and drive capabilities have greatly improved.

[Chris is me]

Originally Posted by Marc S.

This was the first FIRST Fully Omni-Directional robot to successfully work during competition.

Slight correction: 47 had an omni-directional robot in 1998 that successfully worked quite well. They won three regionals that year - I don't think there were more than a dozen.

[Marc S.]

Originally Posted by Chris is me

Slight correction: 47 had an omni-directional robot in 1998 that successfully worked quite well. They won three regionals that year - I don't think there were more than a dozen.

Here's one of there matches. Oh though your right, upon further inspection they did build one in 1998(back when there were only 5 regionals).

[Al Skierkiewicz]

Casters don't work for autonomous since they have a mind of their own. They are simply a bad idea for any game that has ramps or some other drive over hindrance. They generally do not turn within their own radius so the robot will veer of course. And...have you never driven a cart at a store where the fronts wobble?

[Ninja_Bait]

Originally Posted by Al Skierkiewicz

Casters don't work for autonomous since they have a mind of their own.

That wouldn't be a problem if they were also intelligent.