Our Team Needs Help on the Drive!

[Matt H.]

1) Which drive looks better for this competition?

The crab module will be better for the competition provided it is constructed properly and the programming is done correctly. However, the crab drive may result in disaster if things go wrong.

2) What are the pros and cons of A?

A is more easily constructed and will provide more time for driver training testing etc. It should perform almost as well as B.

3) What are the pros and cons of B?

B will be more maneuverable and better able to push in all directions; however it will be much harder to build, design, program.

4) Suggestions that we should take under advisement, and any other comments.

If you have never constructed a crab drive I would recommend you build a tank drive as in A and try to design a crab module in the off season. Too much can go wrong during the build if you attempt to try something completely new in a single build season. Try to make what ever you decide the best it can possibly be. Oh and have fun.

[writchie]

Originally Posted by ExarKun666

<snip>

1) Which drive looks better for this competition?
2) What are the pros and cons of A?
3) What are the pros and cons of B?
4) Suggestions that we should take under advisement, and any other comments.

TY to all who respond!

1. It depends on the location of the cg and whether the wheels are coplaner or some set is dropped. The trailer is effectively part of the robot.
2. See (1). What do you expect the doubled wheels to do for you?
3. See (1). What do you expect the crab wheels to do for you?

4. I believe that what the GDC is trying to do with this years game is to force teams to do the physics and math and understand all of the forces at work. They have effectively created an environment for which past rules of thumb and past experiences and intuition may not apply. But the laws of physics still apply. Use them to engineer your robot. As you get to understand the physics insights will start to appear.

OK, you have a couple of potentially viable configurations. Put the trailer in the picture. Stick the cg in some realistically attainable envelope and start calculating with the cg in the center of that envelope. What net force will the robot have? What turning moments will the robot have? Move the cg to the corners of that envelope and recalculate. Does it still work?

The traditional steps in evaluating design concepts are:
1) Does it work well on paper. If not, it still might work in the real world but it's usually not worth the risk. Engineers don't build things that don't work on paper (unless the're software engineers )
2) If it works well on paper, confirm that your assumptions and calculations are valid and that it is likely to work in the real world as well.
3) Is it feasible to complete in the allotted time, i.e. detail design, build, test, etc.
4) Does it meet all the requirements of your application (i.e.) your selected strategy?
5) Is it likely to be competitive? (in the commercial world this is often a life or death parameter. In government nobody cares. In FIRST everybody is a winner.)

It's still early. Do the math. If you need help with the physics or math just ask.

[Pjohn1959]

[quote=dtengineering;794539]And we're thinking of something like:

__          __
|[]\_______/[]|
|[]         []|
|[]         []|
---------------

With the center wheels dropped a bit, but all wheels driven. Depending on where the centre of gravity is located it begins to approximate a two wheel drive as the majority of the mass will act through the centre axles. As one approaches this state the transverse friction on the wheels preventing skid steering should begin to approach zero.

Of course, building in the wide direction does present other challenges, but also presents other opportunities. We've never built a "wide" robot before, but this year's game has inspried us to try something new.

Jason
QUOTE]

This is the exact design that we are thinking of. We used this design on our 2006 'bot, and it worked great. Though the wheel layout was different in '06, I think that the new wheel layout will prove worthy. Now to see how it "slides" on the new floor...

[comphappy]

Originally Posted by Teamcap.2395

My team (as of the last meeting) is considering a drivetrain similar to this...

___.........___
|==\....../==|
|....|.....|....|
|[]...........[]|
|[]...........[]|
|................|
|................|
|==.........==|

The == are non powered horizontal wheels that don't normally touch the ground.
They are only there to keep the chassis from hitting the ground during impacts and abrupt stops.
The [] are powered wheels.
This is essentially just a 4WD, but the wheels are as close together as possible for better turning.
They are slightly towards the front because that is where the majority of our weight is expected to be. That way we keep as much weight directly over the wheels as possible. It also helps balance things, so it has as little rock back as possible.
This (or something like it) may be a good fit for what your team is trying to do as well.

Might as well just use 2 rather then 4, you have the two on the trailer, so having them close like that does not do much for you in my opinion.

[swamp_child]

Originally Posted by ExarKun666

Okay, today our team decided on a drive for, what is to be our chaise drive train. the layout looks like:
|\-------/|
|-|-----|-| Front
|o|---- |o|
|--|----|--|
|--\___/--|
|oo-----oo| Back
________

(This is not to scale, the little - just is a space, so ignore them))

Okay the circles are the wheels, the back four wheels in the back will be powered on a chain, and the front two wheels will be none powered and will be held in a straight position, not free on a caster mechanism. Before our team dives into creating this, I was hoping for some feed back, this is essentially 2 back wheel tank drive. So here are my questions?

1) What advantages do you see with this drive?
2) Should the front and back wheels be powered or just the back ones?
3) is there a thread, or some specifics to way a tank drive is bad for this competition?
4) With the layout above, where would you put the wheels, which ones would be powered, and what kind of drive would you recommend for this base layout?

TY so much to all who respond, are team is rly in a fix on this, and can't rly decide, and answering these questions, would help us SOOO much, ty again!

1.Not to be crazy harsh, but i see just about no advantage to this drive
2. You need to power every wheel touching the ground this year. Depending on how your weight is displaced, you will be giving up tons of traction by not powering wheels. For example if you have around 20% of your weight over the middle of the bot, you could be losing upwards of 20% of your traction, which is a lot in this game.
3.Ill tell you right now why. The latteral coefficient of friction is nearly twice that of the normal CoF.

[ExarKun666]

Originally Posted by swamp_child

1.Not to be crazy harsh, but i see just about no advantage to this drive
2. You need to power every wheel touching the ground this year. Depending on how your weight is displaced, you will be giving up tons of traction by not powering wheels. For example if you have around 20% of your weight over the middle of the bot, you could be losing upwards of 20% of your traction, which is a lot in this game.
3.Ill tell you right now why. The latteral coefficient of friction is nearly twice that of the normal CoF.

Question for you, or anyone who can answer, for the above layout, elt's say the scale from the very, very front of the robot is (the two points you see in my pic) let's say that's 28 inch apart and the U cut out in the center of the bot was 14 inches wide.
5) How would you then get power to the 2 front wheels?
6) Is there another way to model this layout to have that U cutout, but still have a set of front wheels, that are powered?
7) Still not sure as to what drive our team should consider

[writchie]

I would first stop thinking about half the robot. You will need to think about the trailer joined to the robot. It doesn't matter much what your half a robot can do. It will matter a lot how your complete robot (w/trailer) behaves on the regolith.

The two halves of the robot pivot horizontally on the trailer hinge pin. Study this carefully.

You are friction limited. The force available to move forward or backward or change orientation depends only on the Normal force (weight) on your driven wheels.

You've decided on an 8 wheel vehicle (2 on the trailer). You should have an explanation of why need each and every one of them.

The 6 wheel drive you illustrate may not even be able to turn with normal wheels on carpet. It depends where the center of gravity is.

Until you are prepared to do the math and physics, your best bet is to stick with the wide kit bot configuration and drive both wheels on each side. The longer wheelbase is your friend. You may want to buy another pair of wheels and build at least a dummy trailer that you can test with.

The last thing you want this year is a toaster that doesn't move because it will end up being a scoring disaster. I would suggest that you get a simple and reliable drive working so that your drivers can practice while you work on mechanisms and software. Don't bite off more than can chew.

Good Luck!