Drive train questions

[Bjenks548]

I'm running over some possible drive train ideas, with physics not starting till next year i have a few questions.

1) What (if any) advantages is there to having 2 gear boxes with 2 cims in each as appose to 4 gear boxes with a cim in each?

2) Just wondering if this would work. A drive train with 2 mecanum wheels in the center and an omni wheel in each corner. Only the mecanum wheels are powered. If I'm thinking of this correctly you always are crabing with no way to turn your robot. Is this correct? I'm not looking for people telling me how stupid it would be to have a robot that can't turn, i know this, just would this happen?

3) In the two years I've been on the team we have not used pneumatics. Just making sure, the cylinders can be fully extended, fully compacted, but can they be put somewhere in between? Like half extended.

I'm sure I'll come up with more questions to ask about drive trains, that's all i can think of right now.

[Brandon Holley]

Originally Posted by Bjenks548

I'm running over some possible drive train ideas, with physics not starting till next year i have a few questions.

1) What (if any) advantages is there to having 2 gear boxes with 2 cims in each as appose to 4 gear boxes with a cim in each?

2) Just wondering if this would work. A drive train with 2 mecanum wheels in the center and an omni wheel in each corner. Only the mecanum wheels are powered. If I'm thinking of this correctly you always are crabing with no way to turn your robot. Is this correct? I'm not looking for people telling me how stupid it would be to have a robot that can't turn, i know this, just would this happen?

3) In the two years I've been on the team we have not used pneumatics. Just making sure, the cylinders can be fully extended, fully compacted, but can they be put somewhere in between? Like half extended.

I'm sure I'll come up with more questions to ask about drive trains, that's all i can think of right now.

1. Aside from complexity, weight is a big savings. It is 2 less sets of reductions you need to take care of.

2. Mecanums need to be used in sets of 4. I'm not going to go into all of the details, but the physics of mecanums require 4 to achieve "crabbing". Thus, using 2 mecanums to drive your robot will not do what you are trying to achieve.

3. The short answer is yes they can. The long answer is that its fairly complicated to do so. The rules this past year on pneumatics were loosened slightly, and in some cases I saw teams that were using 3 position cylinders.


-Brando

[Ether]

Just wondering if this would work. A drive train with 2 mecanum wheels in the center and an omni wheel in each corner. Only the mecanum wheels are powered. If I'm thinking of this correctly you always are crabing with no way to turn your robot. Is this correct? I'm not looking for people telling me how stupid it would be to have a robot that can't turn, i know this, just would this happen?

You could go forward or backward, and you could do a combination strafe+turn left or right. You couldn't strafe without turning, or turn without strafing.

the cylinders can be fully extended, fully compacted, but can they be put somewhere in between?

I think the word you are looking for is "retracted". The simple pneumatic cylinders that came with the 2010 KoP (by request) are force devices, not servos. Operated open-loop, they're not designed to stop halfway and hold that position against an external load. You could, of course, add a position sensor and try to turn the cylinder into a linear servo by doing closed-loop control in software. You'd have a tough time holding the position against a varying load due to the compliance of pneumatics. Getting a robust stable solution would be challenging at best.

[Bjenks548]

Thanks for the help, I thought mecanum wheels worked by pulling at a 45 angle from the rotation and you only needed 4 to turn, guess i was wrong. Another drive train to the trash.

[Ether]

Originally Posted by Bjenks548

Thanks for the help, I thought mecanum wheels worked by pulling at a 45 angle from the rotation

That is how they work. And it's why you need 4 to strafe. You need the back wheels to cancel out the fore-aft force vector components.


~

[Bjenks548]

Originally Posted by Ether

That is how they work. And it's why you need 4 to strafe. You need the back wheels to cancel out the fore-aft force vector components.


~

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.

[Hawiian Cadder]

that would work if the two mechanum wheels were occupying the same point in space, however this method would cause the robot to twist when you tried to strafe due to the fact that they wheels being apart from each other would cause a rotational force to be applied about the center of gravity. in a perfect world this would work, but not in the real world.

[Clem1640]

These days, I live in the real world.

[Ether]

Originally Posted by Bjenks548

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?

Here's where your upcoming physics course will be helpful. Since the forward force from the right wheel and the backwards force from the left wheel are not colinear, they create a torque moment which causes the vehicle to turn left as it is strafing left.

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

If you want to strafe without turning, the rear wheels are necessary. To strafe to the right, the front left and rear right are driven forward, and the front right and rear left are driven backward. The front left and rear right generate forward and right force components; the front right and rear left generate backward and right force components. The backward force component of the front right wheel is colinear with and cancels the forward force component of the right rear wheel; the forward force component of the left front wheel is colinear with and cancels the backward force component of the left rear wheel. What remains is the right force component of each of the four wheels. So the vehicle strafes to the right, with no torque to cause turning.

[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]

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

[Jeff 801]

Originally Posted by Bjenks548

3) In the two years I've been on the team we have not used pneumatics. Just making sure, the cylinders can be fully extended, fully compacted, but can they be put somewhere in between? Like half extended.

I know it can be done because in 2008 we did it with a 12" long 1.5" bore cylinder and basically we de-pressurized the cylinder after so many milliseconds so it stopped half way but it would not hold its position.

I know Team 254 used a different type of piston seen here http://www.team254.com/media/photos?func=detail&id=3146 and here http://www.team254.com/media/photos?func=detail&id=3133 and I want to say it was used to shift their drive system into natural

[NickE]

Originally Posted by Jeff 801

I know Team 254 used a different type of piston seen here http://www.team254.com/media/photos?func=detail&id=3146 and here http://www.team254.com/media/photos?func=detail&id=3133 and I want to say it was used to shift their drive system into natural

Yes, the 2010 rules on pneumatic cylinder choice were very relaxed so we were able to use more complicated cylinders. Note that 3-position cylinders will require more than a standard single or double solenoid valve (we used two single solenoids).

Also, back in 2008, we used a center-closed pneumatic solenoid from SMC. This solenoid allowed us to hold our intake arm in the middle of its stroke.

[Clem1640]

For your first question (2 versus 4 gearboxes), Brandon's reply concerning weight covers a large portion of the correct answer, but there are nuances.

• Another clear advantage of 2 gearboxes & 4 CIMS is that if the front or rear wheels of your robot become under-weighted (and therefore lose traction, such as when climbing a ramp) the drive power becomes available to the other, over-weighted wheels. So you do not lose effective drive power when the weight distribution to your wheels shift.
  A second advantage of 2 gearboxes is that you can better afford the weight (and cost) of a shifting gearbox.
  Some drivetrains, however, require independent drive to function. Mecanum is a good example of such a drive.

I have nothing new to add to the Mecanum subject.

Regarding pneumatics, it is possible to have a position control on pneumatic devices, although the devices for FIRST are designed for binary (extended / retracted) use. In the chemical process industry (I am a Chemical Engineer), many flow control valves are pneumatic and they control flow by controlling the valve stem position. They work very well & reliably. Such analog pneumatic devices work using a controlled analog pressure signal which varies between 3 and 15 psig. I/P (current to pressure) converters convert higher pressure instrument air to this 3-15 psig signal proportional to a 4-20 mA input analog electrical signal.

In FIRST, however, our pneumatics are designed for on/off service. It is possible to stop at an intermediate point with the standard parts, but it is difficult and complicated. We've done this in test-beds, but have never incorporated this concept into a working robot.