pic: 1675's REAL IRI Upgrade

[Tom Bottiglieri]

I don't understand this debate. What teams choose to incorporate into their robot is their choice and that choice is made under their value system. Yeah sure, 1114 has never used mecanum, but not every team wants to be 1114.

[Lil' Lavery]

Open-field movement is not the reason to pick a mecanum, or just about any, omni-directional drive. The notable advantage is analagous to how these drive systems are used in the real world. The ability to translate in multiple axes without rotating the entire robot. This is particular useful in small, tightly packed spaces.

Think of how you parallel park in a car. Or, perhaps more of a direct analogy, moving from one normal parking space to the one next to it. That's not a very efficient or easy to execute series of actions in a FRC match. Strafing saves maneuvers like that in tight spaces.

Whether or not this is enough of an advantage to avoid the numerous drawbacks of mecanum systems is up to each particular team. I know that I won't support the selection of a mecanum system for teams that I'm associated with if FRC games continue to have a similar field lay-out and style of play. On the other hand, I probably carry a bias since an alliance partners' mecanum drive was a significant factor in ending my senior season in the Galileo semi-finals in 2007.

Originally Posted by Kevin Kolodziej

Found the video I was looking for. Its not exactly driving circles around the defender, but its certainly a maneuver that would not have been possible without mecanum wheels: http://youtu.be/PDsq1sEVVKs?t=2m28s

Sure it would have. "Bumper-locked spins" happen in FRC all the time. The real key to that maneuver was the open space where it was happening allowing for ~270deg of rotation. This type of maneuver is easily possible with a tank drive in a "side-impact" scenario like that. This isn't a perfect example, but it's one that was readily available and show the same principle being executed by a 6WD being defended by an excellent defensive machine who was on their way to Einstein.
http://www.youtube.com/watch?v=Bzpo3vVeGEY#t=3m45s
e; Another example in the same video.
http://www.youtube.com/watch?v=Bzpo3vVeGEY#t=4m48s

Originally Posted by Ryan Dognaux

After watching the clip about 5 times in a row, I disagree with this statement. 1675's only goal at that point was to get on the other side of the defender and get to the scoring rack. During the two-robot spinning motion, 1675 was on the opposite side and there was really nothing to stop them from translating away from 2761, as shown below -



I think what contributed to this motion was the mecanum wheel rollers being pushed from the side by 2761. The real trick would have been timing the translation correctly.

Notice how they escape from the spin move at the end, they don't translate directly away. They "spin off" the block, similar to how a tank drive would. Everything in that maneuver could have been executed by a tank drive, and similar situations frequently happen.

[ayeckley]

We won Buckeye in 2010 (as alliance captain) using mechanums. During the balance of the season we never felt that we were at any real disadvantage against pusher-bots. In fact, we even successfully stuffed a reasonably-competitive 4WD tank opponent into the goal once. We put a lot of time, effort and $ into the control system to make driving as intuitive as possible however (closed loop on all four wheels, strategic assignment of user I/O, etc.). That, combined with a good driver and a practice bot made us successful, at least at the Regional level which was good enough for us. I don't think we would have been as successful with a traditional drive.

Just another data point...

[Ryan Dognaux]

Originally Posted by Lil' Lavery

Notice how they escape from the spin move at the end, they don't translate directly away. They "spin off" the block, similar to how a tank drive would. Everything in that maneuver could have been executed by a tank drive, and similar situations frequently happen.

Correct - and I guess that would be a question for the driver and operator of 1675's robot, why didn't they translate away? It certainly was possible at that point, but it may have been just a heat of the moment kind of thing. It could also have something to do with they way their controls are set up. I've seen teams using two joysticks with a mecanum drive system, which seems counter-intuitive to me.

[BigJ]

Originally Posted by Ryan Dognaux

Correct - and I guess that would be a question for the driver and operator of 1675's robot, why didn't they translate away? It certainly was possible at that point, but it may have been just a heat of the moment kind of thing. It could also have something to do with they way their controls are set up. I've seen teams using two joysticks with a mecanum drive system, which seems counter-intuitive to me.

We use 2 joysticks. one is translation and the other does rotation on the x axis. Yes, this could be done with a 3-axis joystick but we didn't feel like buying a different joystick. Also tests with team member's personal 3 axis joysticks usually find the twisting in the direction of the arm can be awkward sometimes, especially if you needed to twist while translating in some directions.

[Taylor]

Originally Posted by BigJ

We use 2 joysticks. one is translation and the other does rotation on the x axis. Yes, this could be done with a 3-axis joystick but we didn't feel like buying a different joystick. Also tests with team member's personal 3 axis joysticks usually find the twisting in the direction of the arm can be awkward sometimes, especially if you needed to twist while translating in some directions.

We found the same to be true. We also found that it is very difficult to do a pure forward/backward, left/right, or cw/ccw twist and the driver would often do multiple moves at once unwittingly. For that reason, we have a 2-stick setup - one for forward and turning, one for strafing. Intuitive for laypeople, and workable in competition.

[Duke461]

Originally Posted by Taylor

We found the same to be true. We also found that it is very difficult to do a pure forward/backward, left/right, or cw/ccw twist and the driver would often do multiple moves at once unwittingly. For that reason, we have a 2-stick setup - one for forward and turning, one for strafing. Intuitive for laypeople, and workable in competition.

Also agree. Our joysticks at BMR got screwed up and i was forced to use a 3-axis joystick. granted, i had no experience with one and it was still a little screwy (shavings in jaguars we assume), but i absolutely hated it. I would love to hear as to why 2 joysticks is "counter intuitive"; i see no reason as to why it would/should be. In fact, i find a 3-axis counter intuitive.
Eventually we switched back, after much pleading, to using controllers like we did in 2010. Way better.

P.S. Although i prefer one for turning, one for strafing and forward/backward.

[ratdude747]

Originally Posted by Duke461

P.S. Although i prefer one for turning, one for strafing and forward/backward.

sounds like halo style controls... left does all of your translations, right rotates. pretty standard.

3 axis style mecanum/crab is a mistake waiting to happen... when you try to translate left or right, you often rotate yourself as well...

some teams use tank-style controls where to strafe, you have to move both sticks left or right... never made sense to me... I am bad enough at tank controls, let alone that kind of horror.

[Molten]

Since nobody has commented on this yet, what if they tried to strafe with the wheels like this? What would happen? Would they slowly drive out of the treads? or would it just sit there and put tension on the drive? Anyone know?

Just curious,
Jason

[Duke461]

Originally Posted by Molten

Since nobody has commented on this yet, what if they tried to strafe with the wheels like this? What would happen? Would they slowly drive out of the treads? or would it just sit there and put tension on the drive? Anyone know?

Just curious,
Jason

Well first off, the treads would probably slide off because they're not secured too well....but ignoring that, i would guess that it would barely strafe, and skip around like popcorn popping. What i want to try is to individually wrap each roller with tread. Nobody steal my idea unless 461 doesn't do that in 2012

[BigJ]

Originally Posted by Molten

Since nobody has commented on this yet, what if they tried to strafe with the wheels like this? What would happen? Would they slowly drive out of the treads? or would it just sit there and put tension on the drive? Anyone know?

Just curious,
Jason

Like I said, the kids didn't test it on the practice field like I wanted them to!

My guess is that without securing the treads to the mecanum hubs the diagonal force of the rollers would pop the treads off or tangle them in the wheel if it popped off diagonally.

If secured to the wheel it would just act like a lumpy traction wheel.

If secured to the wheel AND strafing was tried, the robot would be trying to pull itself apart and go nowhere.

[Laaba 80]

Originally Posted by ratdude747

sounds like halo style controls... left does all of your translations, right rotates. pretty standard.

3 axis style mecanum/crab is a mistake waiting to happen... when you try to translate left or right, you often rotate yourself as well...

some teams use tank-style controls where to strafe, you have to move both sticks left or right... never made sense to me... I am bad enough at tank controls, let alone that kind of horror.

Its all about what the driver is comfortable with, it doesnt matter what anyone else thinks about it.

[Dave Scheck]

Originally Posted by Laaba 80

I believe wildstang used a 3 axis joystick in 2009, I think it worked out pretty well for them. Its all about what the driver is comfortable with, it doesnt matter what anyone else thinks about it.

Nope, not us. 2009 was a Logitech gamepad. The left stick was the crab strick. You could think of it as a top view of the crab modules. Which ever way you deflected the stick, the module would go there. If you let go of the stick, the module would center. The right stick had multiple purposes. When the crab stick was deflected, only the Y axis was used, and it acted as the throttle. When the crab stick was not deflected, the right stick acted as a single stick tank drive. There were also a few gimmicks that we used such as having a button to angle the crab slightly (helped with picking up against the wall) and some special logic to tank turn while the crab wheels were not centered (standard tank turn logic doesn't hold true when the wheels are turned because the robot will try to pull itself apart )

[Laaba 80]

Originally Posted by Dave Scheck

Nope, not us. 2009 was a Logitech gamepad. The left stick was the crab strick. You could think of it as a top view of the crab modules. Which ever way you deflected the stick, the module would go there. If you let go of the stick, the module would center. The right stick had multiple purposes. When the crab stick was deflected, only the Y axis was used, and it acted as the throttle. When the crab stick was not deflected, the right stick acted as a single stick tank drive. There were also a few gimmicks that we used such as having a button to angle the crab slightly (helped with picking up against the wall) and some special logic to tank turn while the crab wheels were not centered (standard tank turn logic doesn't hold true when the wheels are turned because the robot will try to pull itself apart )

Sorry about that. One of our mentors spoke with someone on your team about your crab drive, to get information about building our crab drive. Have you guys ever used, or tested out a 3 axis joystick for your crab drive? He was pretty convinced that a 3 axis joystick was the way to go after talking to you guys. The way he told us, the twist did the function that your left stick on the game pad did. Either he talked to the wrong person, or he confused himself.

[Dave Scheck]

I don't think we ever experimented with a 3 axis joystick. We've always gone with the two stick approach. Back in the day, we just used the X-axis of the left stick for crab angle. I think 2008 was the first time that we used both X and Y to use the full circle of the stick for wheel angle.

I would think that the control that you described, where the Z axis of the stick determined the crab angle would be really hard to work with. I feel like you would lose a ton of resolution between the range of motion of the stick, and the physical dexterity required to twist the stick (as opposed to deflecting it). Like I said, I've never tried it, so if someone has had good success with that setup, more power to you.