Re: omni drive vs tank
 

hey billy goats...do you think you could show me those videos??? i'm on a second year team and we were looking at tank treads, could you tell me where i could find those???

Re: omni drive vs tank
 

Tank treads would be profitable if you can get the gear ratio adjusted so that its fast enough to keep up. Problem is, the biggest advantage of having treads is their grip and strength on the carpet. So gearing it for speed would lose the power aspect of having treads.

Re: omni drive vs tank
 

If you look at the video you will see how fast we run at. we would be one of the fastest robot on the floor. but with the most power. you are right you have to get the gearing right. we always used 4 motors for the drivetrain.


how fast do you want to go. to fast and you have no control. Now with multi gear shifting we have all we need. :)

Re: omni drive vs tank
 

Our team usually goes with 4 wheel drive tank style. We have thought about Omni Drive, but it turned out that we want maneuverability. Tank Drive is the best..

Re: omni drive vs tank
 

????? :confused:
Wouldn't omni-drive type systems give a far greater maneuverability?

Re: omni drive vs tank
 

tank drive is extremely maneurerable when done correctly. I think it is better than omni wheels becuase part of my definition of maneuverability is ease of getting it to the specific spot, well omni wheels get thrown around to easily.

just some thoughts

Re: omni drive vs tank
 

I would say tank because i see alot of pushing going on

Re: omni drive vs tank
 

The major problem I see with omni's is stability. Depending on your strategy, you might want to stay in a fixed location to shoot balls. However, Even given that you can try and stay fixed with omni's I think that you wil find your robot being bumped all day.

Just an opion though.

Re: omni drive vs tank
 

obviously. But you don't get the power that tracks get you

Re: omni drive vs tank
 

See video/thread/link below.

See note on above quote.



2005 Team195 Tank Tread Drive System+ 4 speed transmission

Guys, seriously this thread is pretty much out of control. None of you really outline good reasons why a tank drive system is not effective. The video shows a tank system being very effective in maneuverability.

Re: omni drive vs tank
 

That's because it is much easier to spout worthless generalities and platitudes than to provide an argument of any substance.

As people learn more and mature (especially as engineers) they learn that in a technical discussion phrases similar to "Never use tank treads, they can't turn." aren't worth reading.

$.02
-JV

Re: omni drive vs tank
 

Having said all that, I'll get off the soapbox and describe some of the actual physical phenomena that are being loosely described in this thread.

Robot Myth Busters - Linear Pushing Force:

Myth: Tank Treads have a significantly greater pushing force than Omni drives.

Math:
In actuality, pushing force is dependant on 2 things:
1. Traction
2. Torque

If you have lots of traction, but not enough torque, you stall your motors and/or blow your breakers.

If you have lots of torque, but no traction, you spin your wheels and can't push anyone.

The trick to designing a super-powerful pushing robot is do get as much traction as possible, then gearing your robot so that when it is under load (in a pushing match), the motors only draw 40 amps each.

Traction is dependant on:
a. Robot Weight
b. Wheel Coefficient of Friction

So your first step, is to maximize the robot weight. This is relatively easy in a game where the robots have a maximum allowed weight, and there are no heavy goals to lift up on (Refer to 2002, where teams increased pushing force by lifting 190 lb goals).

You then need to find a grippy tread. Most teams have comparable wheel-treads. The 'best' stuff is arguably the brecoflex stuff that many teams use.

Using wider wheels/tracks won't get you more pushing power, but it will distribute the load over a greater area; this means you won't burn holes in the carpet as easily, and it will take longer for your treads to wear down.

If you've *maxed* out your pushing power, now you need to figure out your gearing (to ensure you have enough torque). As I said briefly above, the main requirement for low-gear is: "Can push against a wall, without popping your breaker?" (We use pushing against the wall, as the worst-case load scenario).

What does this mean?
Two Robots both weigh the same. One robot has tank-treads, one robot has omni-wheels. Both are geared such that their wheels slip, and their motors only draw 40 amps during "wall push". Their CG is in relatively the same place. Who will win in a pushing contest?

Neither! They will stalemate.

Reality:
It is extremely difficult to get omni-wheels with a coefficient of friction equal to the brecoflex treads. So this scenario doesn't usually happen. However, it very easily could given the right design.

Followup:
Are you sure that the surface area of the tank-treads doesn't result in a greater coefficient of friction?

No. This is true for "ideal surfaces" only. In actuality, the surfaces aren't "perfectly flat, and smooth". This results in an interlocking matrix of materials (think velcro, on a smaller scale). In this case, the surface area does play a small effect. (This is measurable.)

However-- You can calculate an "overall coefficient of friction" for a given robot configuration. This is done with a simple pull test. Based on this, it would be possible to find an omni-wheel drive which has a comprable coefficient of friction to a tank-tread drive. So the above theory of a stalemate is still valid. Again-- the difficulty comes in finding such a system.

Overall:
Robots with traction wheels, will in most cases, win pushing matches against omni-drives. When someone invents a better omni-wheel; traction wheeled robots beware!

Re: omni drive vs tank
 

Having said all that, I'll get off the soapbox and describe some of the actual physical phenomena that are being loosely described in this thread.

Robot Myth Busters - Robot Turning:

Myth: Tank Tread Drives can't turn.

Math:
Read this whitepaper:
http://www.chiefdelphi.com/forums/pa...le&paperid=222

It is possible to design a tank-tread robotto spin on a dime. It is all based on the turning scrub of the drivetrain-wheelbase configuration, and the robot gearing.

Reality:
Using a reduced length-wheel base, (wide-body, or dropped-middle system) it is possible to create a spinning tank-tread bot. However, this may result in other negative factors such as the robot rocking back and forth between front and rear.

Overall:
Tank-treaded robots can be made to turn; however the nature of their design results in significant scrub, which must be overcome in some way.

Re: omni drive vs tank
 

Design is all about tradeoffs and compromise.
Understanding the physics and reality of each configuration is important; taking this reality and determining what is most important to construct the overall robot system is critical.

Isn't engineering fun? :)

-JV

Re: omni drive vs tank
 

Like John, I'm going to try to add some useful information to this thread so we can all speak the same language.

There is actually some truth to this statement.

Using a 4 wheel drive, tank steer machine without omni-wheels can cause a great deal of difficulty turning, so it's common practice to use omni-directional wheels, either in the front or the back (or occasionally front and back) of your machine.

The difficulty in turning occurs because you're trying to essentially slide your wheels in directions they do not want to turn. By adding omni wheels in the front, for instance, these wheels will slide (much more easily) in the lateral direction while you are turning, which reduces current draw and provides a much smoother motion.

However, to get back to the quote, one potential downside is that these omni wheels are ALWAYS easy to slide, and in the case of being bumped (from the side, near the front for example) you can expect your machine to be rotated more easily from side hits than otherwise. This becomes especially apparent when you use 4 omni-wheels on your machine - there's very little to stop you from being pushed all the way across the field when you are hit from the side.

That's my 2 cents. *Plink Plink*

Matt