Important functions of a first robot?

FIRST General Forum
1

This season might have just come to a close but i’m already planning for next year. I’ve already started drawing up plans for my teams drive train next year and we plan to build it during the summer. We went all out this year and tried to build a vector drive in the 6 weeks but it got scrapped competition day. It came down to the fact that we had no practice with any of the parts of our robot. So after looking at the past games i came up with this drive train. It is slightly different from anything i’ve seen before.

Number one… i’ve scrapped the idea of turning. Its a waste of time i think :wink:

Number two… It had to be faster and stronger than any other robot.

Number three… Use under 4 motors.

Impossible… i think not :smiley:

Basically i saw i team that used a planetary gear box with two inputs and one output. The RPM’s from the two inputs are additve so it allows two motors to put out 300 + 250 rpm for 550 rpm with all the power of both motors or the opposite 300 - 250 rpm for 50 rpm. Using that and some nice programming you can be just strong enough to push anyone around or use all your power for speed.

I have the output from that going to all four wheels. no turning because all wheels have the same input. All the wheels though are mounted on a turning bracket powered by another motor. I can’t wait to construct it. I then plan to put a turret on the robot(how thats going to work i’m not up to yet.) Two sets of chains are going to be traveling around the side of the robot in otherwords. It uses just 2 motors for powering all 4 wheels and uses just one motor for steering the wheels. I suppose one will be used for the turret.

What i wantd was input from anyone who has tried to do a system similar to this? How did it work? Pitfalls? Problems? Does that gearbox system work well, or is it terribly inefficient?

Also for the people who have seen competitions for many years? would this work generally in most years? Have teams usually needed a lot of power? Has it been usefull to be able to drive in any direction? Is it a good idea to build a working complex drivetrain like this before you know what the game is going to be?

Better ideas on what do with my time?

Any and all input would be appreciated.

2

Check out WildStang Team 111’s drive train.

They used 4 motors: 2 drills and 2 Fisher Price, with one motor for each wheel. The wheels could be rotated to any angle.

They didn’t have a gear shift but swooped around quite quickly in any direction, at about 7 feet per sec as I recall one of their members saying.

Our drive train used the 2 black motors, and we shifted by lowering larger wheels. In high gear, we could theoretically go as fast as 111, but actually we were slower to get moving and to turn. 4 motors give you faster acceleration, which is important since you are often changing direction.

3

That is a good point. My team used 4 motors this year. They were orginally designed to be on movable mounts like wildstangs. Unfortunatly we never got it to work. It made us the fastest robot at our two regionals.

Acceleration would be a problem and that is a good point. i suppose a constant gear up would aid in acceleration but that would require a lot of testing. Thanks for the tips! I’ll go look at wildstangs more closely.

The major difference between what i plan to do and with wildstangs is that the motors won’t be mounted on the wheels. Something i haven’t seen done yet. (i think it has but, i haven’t seen it).

4

Before you commit one way or another, you need to understand the ups and downs of each system. Our crab system uses motors mounted inside the transmission housing because it reduces weight and friction. It also puts all of the gears in one place so the splinters thrown off by the gears during normal operation stay in the transmission. The type of drive suggested above requires bevel gears in most cases. Raul told us the friction created by this type of drive eats motor power and that is why he never considered using it. If you want a powerful and fast drive, you must optimize your design for speed and power by analyzing the motor curves and specs. Some of it is black magic but it can be done. Let’s face it, there were robots out there who were faster than us, and/or more powerful than us. It simply what we chose to use this year. If the game requires a different drive next year we will not use crab drive.

5

We have used planetary drives for 2001, 2002. Mainly to show off machining ability and for compactness/weight reduction.

However, planetary, worm&gear, bevel gears, and the helical gears in the kit this year, have reduced efficiency for a given gear ratio than the good, old fashioned, easy to fabricate spur gear train. I forgot sprocket and chain, but that’s less efficient as well.

Given the limitations on current (due to fuses) and the motor size, gear box mechanical efficiency may be the most important consideration in drive system design.

The energy per motor limitation is the other reason why you want to get four motors into your drive system somehow. If you try to go up to six motors, you may discover that you have put too much weight and resources into your drive system and you cannot do anything else. But, I haven’t got any experience with that.

One advantage to being a torquier bot than a speedier bot is that you end up drawing less available energy out of the battery per match. Secondly, with a crowded field, such as FIRST has provided for the past few years, acceleration is more important than speed. Pushing power is far more important than speed.

We have been bracketing the “ideal” FIRST gear ratio. We started with a 15 fps bot, which was hard to control and couldn’t run very long on one battery, to a 4 fps bot which runs forever on one battery. We also have a 6 fps bot with an inefficient drive system. This guy is better than the 15 fps bot at battery life but can only handle about 15 minutes of hard driving before you can smell the motors.

This year’s bot was a 4.1 fps with four drive motors. I think we can go as high as 5 fps with the four motor version. We had no trouble with motor heating, battery drain, motor damage, etc.

I actually took a page from Wild Stang’s 2001 robot (which they were gracious enough to show me) for our gear boxes this year. So, thanks for the tip!

6

hint on our drivetrain next year: a giant lexan sphere

however, I have NO IDEA where we’re gonna get a giant lexan sphere. :ahh: