pic: Final 2011 Drivetrain

[JVN]

Wow. I don't know if this is a joke, in which case I just wasted my time typing up the following post. In the rare event that it isn't a joke, I'll describe some motor loading for you. If nothing else this post is a good summary of what goes on when a robot accelerates (or catches fire, whichever comes first).

Short version:
If you build this thing, it won't work at all. My spreadsheet should have some kind of "are you serious?" warning built into it...

Please review this presentation immediately:
http://www.chiefdelphi.com/media/papers/2429

JVN's "in a nutshell" description of motor loading and acceleration:
Motors have limited power, this means for a given amount of load they can only move so fast. The less load they have on them, the faster they move. At some load they won't move at all (stall), and at no load they have a maximum speed they spin at (free speed). They draw current from the battery depending on how high the applied load is. If the current drawn is too high, the breakers will trip (or the motors will catch fire, whichever comes first).

When a robot is accelerating, at the instant it starts moving, it isn't moving -- the motors output their stall torque. Drivetrains typically increase this torque with a speed reducer/torque increase geartrain. This torque is then applied on the ground as a force which is used to accelerate the robot. As the robot accelerates the robot starts moving which means the motor spins faster which means the torque output decreases (the motors speed & torque are linear, remember?). So why does this matter?

With CIM motors, the stall current is much higher than the capacity of the 40amp circuit breakers. If you try to accelerate with too little gear reduction (i.e. the drive is too fast) or with NO gear reduction as you show, the output force of the wheel on the ground will cause the robot to accelerate very slowly. If this acceleration is too slow, the motor will be very high on the torque curve for a long period of time, which means it will be drawing lots of current for a long period of time, which means it will catch fire (or pop the breakers, whichever comes first).

The moral of the story... your robot would take approximately the length of an airfield to accelerate to top speed, which it never would because it would pop the breakers or catch fire (whichever comes first).

Ohh... not to mention that if you ever try to get into a pushing match the wheels act as brakes on the motor and if your gearing doesn't reduce the torque load enough it will also cause the motor to draw too much current and cause the robot to catch fire or pop the breakers (whichever comes first).

Ohh... not to mention that you have traction wheels opposed at 90-degrees from each other. So in order for the robot to move in any given direction it needs to slide a set of high traction wheels sideways across the carpet... which of course it needs torque to do, but since it has no gearing it will probably just catch fire, or pop the breakers (whichever comes first).

Physics is such a pain in the butt when it gets in the way of innovation, isn't it? I guess true innovation is when you can actually harness physics to do what you want. Heck -- that sounds suspiciously like engineering.

-John

PS - Expert tip: robots work much better if they are 9-sided. 100% of 9-sided robots have won World Championships. Add an extra side, quick! True story.

[Cory]

This has to be a joke. Conservatively, that would be a NINE HUNDRED POUND block of aluminum. 7075 is about $7-10 per pound. That's a $6000-9000 block of aluminum. No sponsor is stupid enough to waste that kind of material. This also ignores the fact that there is no such thing as 7071 aluminum. The frame would also be substantially weaker than a properly constructed welded tube or sheet-metal chassis.

Basically...obvious troll is obvious.

[SirTasty]

In that case, I must leave a well-deserved,

[Sean Raia]

Consider the following: his name is poohbear, this was his first post, his location is "the dirty south", and he claims his robot can go 105 miles per hour... this all sounds pretty legitamate to me.

[quinxorin]

Responding to the previous few posts, here's the actual calculations:

CIM starting stall torque (the maximum amount of torque it can exert before stalling and not moving):

343.4 ounce-inches.

Converting to foot-lbs:
(343.4/16)/12 = 1.778 ft-lbs.

Force exerted by each wheel (assuming the wheel is 8in), therefore, is:
1.778 x 3 (4in, the radius, is 1/3 of 12in) = 5.366 lbs-force.

Apparently, each wheel exerts 5.366 lbs of force pushing forward.

Now, let's look at the friction properties of the wheels. Because the wheels are perpendicular, they will have to overcome each other's friction in order to move. At least 2 of the wheels will have to slide at all times.

We can assume that high-traction treads have a coefficient of friction (μ) of at least 1 on carpet, though it is probably much greater (think 1.7-2). I will use 1 as the estimate. An object (i.e. the wheel) is capable of sliding if the following inequality is true: F(force) x μ > weight. The robot, including batteries, bumpers, and the minibot, will weigh 169.2 lbs. Therefore:

5.366 x 2 (there's two wheels) x 1 > 169.2
10.731 >169.2

That inequality is definitely false. Therefore, your robot will not move. Sorry, but it's the sad truth. And that doesn't even include the actual force it would take to move the robot, just to beat the friction of your other wheels.

A lot of people have been posting like me, that the drive train simply won't work. But none have posted a recommendation. If you want traction and not speed, here's mine:

build a six-wheel drive train, wide type. Use traction wheels on one pair of opposing corners and omnis on the other (otherwise your robot won't turn well). Use a gearbox with a ratio of at least 12.75:1 (standard AndyMark Toughbox). If you REALLY, REALLY want traction, put belts on it. I don't recommend that, though.

Here's the lesson to learn from this: You can have traction or you can have speed. You can't have both.

[LLogan]

I would just like to mention that I believe this team branched off from team 1746.

Team 1746 is the "Forsyth Alliance." It unifies a handful of high schools from a district. It looks like North Forsyth split into their own team, probably consisting of many members that used to be on 1746.

While they may be a new team, they probably aren't "rookies."

I thought the troll was hilarious, anyway.

I think that aluminum frame may be worth over $700-800 market value, which is going to violate <R14>, you'll want to check that.

[ratdude747]

Not this thread again

[MaxMax161]

I just found this thread and I would like to say it's completely wonderful.

The only suggestion I would make is since we all know more motors --> better robot you clearly need an 8 motor drive train. However I don't know if there's space for a traditional gearbox, have you thought about mounting extra motors to your motors so they spin while they spin?

[asid61]

I have to say, this was one of the funniest threads I have ever seen.
6 cims drives are also good too; if the OP is still around I would add that.
There's also 4 minicims available for moar acceleration.