Drive train questions

[Bjenks548]

Thanks everyone for your help. Just so i make sure I have this right...

1) Just weight, cost and complexity

2) Doesn't work

3) Attach 2 pistons together and it works great

Thanks again

[JamesCH95]

^^ You got it.

[Jon Stratis]

Originally Posted by Bjenks548

This is why I'm confused. Let's say that you do have the design I mentioned, if you drive both forward you drive forward because the right wheel is pulling forward and left, and the left wheel is pulling forward and right. Therefor left and right cancel out (assuming same speed). Same can be said for driving both backwards. Now if you drive the right wheel forward it pulls forward and left, and the left wheel backwards, backwards and left. Shouldn't you go left? And running the right one back and the left one forward shouldn't you go right?

As I see it the only reason i would need 4 mecanum wheels is to turn, not to strafe.

Someone explain to me how I'm wrong considering no one else sees this happening.

There's a good way to visualize how this would happen too. Imagine a merry go round you might have had in a playground growing up. Now, put a person on the east (right) and west (left) side of it. If both people push north, it doesn't spin (ie the robot would go straight forward). However, if the guy on the east pushes towards the North West, and the guy on the West pushes towards the South West, they can spin the merry go round (Because one is pushing towards the north and the other towards the south). It's not a perfect spin, however, because they aren't pushing tangentially to the merry go round - some of the force goes into translation.

This analogy isn't perfect, as it's hard to visualize that translation with it. But it does a great job with rotation. For a normal Mecanum drive train (with 4 Mecanum wheels in the corners), you can stick people on the North East, North West, South East and South West corners and have them push in different combination's. You'll find that when they're all pushing tangent to the merry go round, in the same direction (clockwise or counter clockwise), the merry go round will turn without translation. And when you have them working in opposing pairs there won't be any rotation, but the force has to go somewhere - you should be able to figure out what the translation would be.


In physics, this is called a free body diagram. You imagine your forces acting on a pivot arm coming from the center of mass of your object (arguably the center of your robot, for all practical purposes... although your mileage may vary based on specific robot designs). If the forces line up properly, they'll cause the object to rotate or translate (or both!). It's one of the more important concepts in physics, and one that students seem to have the hardest time grasping.

[Ether]

Originally Posted by Bjenks548

Thanks everyone for your help. Just so i make sure I have this right...

3) Attach 2 pistons together and it works great

A (perhaps obvious) caveat: if you mean the above literally, it only works if the pistons are powered in the retract direction (and the load force acts to extend the pistons).

If you are powering the pistons in the extend direction and you just attach two pistons together, they will buckle unless there is some sort of supporting structure at the point of attachment.




~

[Ether]

And when you have them working in opposing pairs there won't be any rotation, but the force has to go somewhere - you should be able to figure out what the translation would be.

One minor quibble with this explanation. The phrase "the force has to go somewhere" could be misleading to a new student. For example, if the front wheels are being torqued forward and the rear wheels being torqued backward, the "force goes nowhere". There is no motion. No translation, no turning.


~

[Jon Stratis]

Good catch... When we worked on implementing Mecanum, we only looked at the productive scenarios (since the one you point out would only serve to waste battery power), so any unproductive ones like that just didn't come to mind when i made my post. Thanks for helping to clarify that

[JamesCH95]

Originally Posted by Ether

A (perhaps obvious) caveat: if you mean the above literally, it only works if the pistons are powered in the retract direction (and the load force acts to extend the pistons).

If you are powering the pistons in the extend direction and you just attach two pistons together, they will buckle unless there is some sort of supporting structure at the point of attachment.

If the pistons are joined shaft-to-shaft with a rigid coupler (like I described) one should probably check the design design using critical buckling load calculations, especially if the pistons are of a long stroke and small bore. Fortunately the actuation pressure can be reduced to help prevent the assembly from reaching its critical buckling load.

[Siri]

Originally Posted by Bjenks548

Thanks everyone for your help. Just so i make sure I have this right...

1) Just weight, cost and complexity
...

While that's all true, you run into another advantage of the 2 gearbox method when one of your wheels lifts off the ground. (I think this has been mentioned, but since you didn't summarize it...)

With 4 independent gearboxes+motors, you're effectively just losing/not using that drive power when the wheel lifts. With the wheels coupled (1 gearbox for 2 wheels), if you lift one wheel, the power can go to the other. This is doubly beneficial because that wheel is probably overweighted, since the other is lifted. So this is good for a traction standpoint.

(An obvious disadvantage of 2 gearboxes is if your drive requires independent steering.)

[jspatz1]

Originally Posted by Al Skierkiewicz

B,
There is nothing that prevents you from using two cylinders in series to obtain a three position mechanism. Apply air to one for the middle position and both for the fully extended.

If you make these cylinders of two different lengths, you would have four positions - both rectracted, short one extended, long one extended, both extended.

Stopping a cylinder in mid-stroke is fairly straightforward by using a 3-position solenoid valve. Center position is all ports blocked, so the cylinder is pseudo-locked in position, although with some spongyness. With the right position feedback, programming, and flow controls, you could move to any variable position.

[Bjenks548]

A few more questions for cd!

What is a good gear ratio from a duel cim transmission for

1) A high speed robot (reasonable I know you could go 1:1000 and just stall)

2) A high torque robot (again reasonable I know its possible to go 9001:1)

Thanks for the help again!

[JamesCH95]

Originally Posted by Bjenks548

A few more questions for cd!

What is a good gear ratio from a duel cim transmission for

1) A high speed robot (reasonable I know you could go 1:1000 and just stall)

2) A high torque robot (again reasonable I know its possible to go 9001:1)

Thanks for the help again!

That all depends on how big of a wheel you plan on using. As an extreme example, a 2" wheel will be 10x slower and have 10x the 'pushing force' as a 20" wheel with the same motor gearing.

Do you know how to do the calculations? They are fairly straight-forward to explain if you don't (and it's okay if you don't ).

[Al Skierkiewicz]

A rule of thumb I have heard thrown around is 8-12 feet per second for most games to make it easy to drive and minimize collisions. How you get there from the motor speed to transmission to drive coupling to wheel size is up to you.

[JamesCH95]

8-12ft/s is about right. Think 5-6ft/s for pusher robots.

[BJC]

Originally Posted by JamesCH95

8-12ft/s is about right. Think 5-6ft/s for pusher robots.

Actully, I disagree. I think that your robot should be geared to go as fast as the drivers can control it. If you look at teams such as 67, 1114, and 254 (just off the top of my head) they all have extreamly fast drivetrains that they can all control very well. This is generally because their drivers are very experienced from a combination of past seasons and practice robot driving.
Because games in FIRST are unfailingly "who can do more/faster" senerios if you want to win you have to go fast. So if you are really good but not that fast you can never hope to be as good as the robots that are both.

As for low gear make it low enough to push stuff but not low enough that other robots can run away, that aside its all preferance.

my 2 cents

[JesseK]

Originally Posted by BJC

Actully, I disagree. I think that your robot should be geared to go as fast as the drivers can control it. If you look at teams such as 67, 1114, and 254 (just off the top of my head) they all have extreamly fast drivetrains that they can all control very well. This is generally because their drivers are very experienced from a combination of past seasons and practice robot driving.
Because games in FIRST are unfailingly "who can do more/faster" senerios if you want to win you have to go fast. So if you are really good but not that fast you can never hope to be as good as the robots that are both.

As for low gear make it low enough to push stuff but not low enough that other robots can run away, that aside its all preferance.

my 2 cents

Those teams also understand the tradeoffs of having 14-16fps drive trains.

To go 14fps+, a robot that expects to have all of its drive train motors last the entire season would need all 4 CIM motors on the drive train. The robot better also have a low gear because simply turning the robot would result in extreme torque loss at any gear ratio that gives such a high speed. I suspect that's why the poof's advertised speeds are 7fps / 16fps on a couple of their recent robots rather than 4-5 / 16. I also don't ever notice the poofs trying to push another robot in a defensive manner; usually I see them pushing through a situation and then zooming away, but I've only seen a small percentage of their recent matches.

To go even faster, even more drive train motors are needed. For a 60-lb (or so) robot that tumbled around the track in 2008, 148 used 6 motors on their 3-wheel crab that (iirc, off the top of my head) went somewhere around 18fps.

Additionally, there are some slide decks on AndyMark's website that give insight as to why their SuperShifters have the ratios that they do -- anything lower than 4-5fps for low gear has a tendency to make the wheels slip on carpet.

I do agree with the "who can do more faster" concept, but that really only applies to Einstein-grade teams who are anticipating competition against other teams just like them. I say that because they also need extra practice in controlling the drive train at high speeds (as well as fast manipulators as high speeds), thus they allot time for extra practice & sensor integration before competition. Ergo, versus an average team an Einstein-grade team would (probably) win regardless of what drive train speed it chose so long as the speed was "fast enough".