Drive train questions
 

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.

Re: Drive train questions
 

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

Re: Drive train questions
 

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.


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.

Re: Drive train questions
 

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.

Re: Drive train questions
 

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

Re: Drive train questions
 

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.


~

Re: Drive train questions
 

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.

Re: Drive train questions
 

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.

Re: Drive train questions
 

Not necessarily a fault of multiple gearboxes, but if you have say 4 CIMs and 4 gearboxes, and each gearbox drives one of four, two of 8, etc. wheels, then when one wheel leaves the ground, you're transmitting less torque and aren't getting all of the power you could. With both wheels on one side chained together (ether with a single gearbox for both motors, or two gearboxes for two motors but all of it connected) you can still use both CIMs with a wheel lifted.

Re: Drive train questions
 

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.

Re: Drive train questions
 

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.

Re: Drive train questions
 

These days, I live in the real world.

Re: Drive train questions
 

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.


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.

Re: Drive train questions
 

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.

Re: Drive train questions
 

Darnit, Al beat me to it. We used a system like this to good effect in 2010 for our blocker. With two pistons retracted we could drop the blocker completely to go through the tunnel, with one set deployed the blocker was at 45deg for hitting balls out of the return, and with both sets deployed the blocker was vertical, good for blocking shots.

We found the best way to do this was to use a threaded coupler and jam nuts to attach two piston shafts together and use the supplied brackets to mount the 'base' of each piston. It was a very robust and reliable system, and flow-control valves can turn the pistons from essentially undamped springs into spring-dampers, which may or may not be useful to you.