CRAB!!!!!!!

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Posted by Josh.

Student on team #40, Trinity, from Trinity High School.

Posted on 9/6/2000 4:24 PM MST



I am with team 40 and was wondering if you could give us some information on your crab system from prior years. Pics and any other info would be very much appreciated! Please e-mail me at kacz100@hotmail.com with the info!

Thank you

Josh

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Posted by Joe Johnson.   [PICTURE: SAME | NEW | HELP]


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

Posted on 9/6/2000 5:14 PM MST


In Reply to: CRAB!!!!!!! posted by Josh on 9/6/2000 4:24 PM MST:



Crabbing (or , The Swerve(TM), as we have named it) is an awesome feature. I highly recommend that more teams should join the club.

BUT...

It is a great and terrible thing. It takes excellence in 4 areas to pull it off successfully:

#1 solid mechanical design -- well designed drive train and pivoting system
#2 feed back -- the computer must know the steering angle of the wheels
#3 programming -- this can be a very tricky system to get right, especially using PBASIC
#4 driver skill -- without drive time, the feature will not be worth the resources (design effort, cost, weight, etc.)

If you have the strengths necessary to make it successful, I cannot urge you enough to implement this feature on your next robot.

This is about as helpful as I what to be right now. I guess I will add that you can learn a lot from reviewing old robot photos (not just of our robot but 111 from 2 years ago, team 349 and team 48 last year, among others).

Good Luck.

Joe J.

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Posted by Lora Knepper.

Other on team #419, Rambots, from UMass Boston / BC High and NASA, Mathsoft, Solidworks, Analog Devices.

Posted on 9/7/2000 5:45 PM MST


In Reply to: Look but don't touch... posted by Joe Johnson on 9/6/2000 5:14 PM MST:



I was a driver on team 69 for the past 3 years, and although last year was the first attempt we made at replicating the 'swerve' I can say one thing....give your drivers practice!!!! You didn't see the 69 machine crab walk much becasue we were far to topheavy to move safely on the ground, but for even the small amount we did, it took practice for my co driver to get used to it (though with only a day or so, I have to say she rocked!)

If I knew the mechanical specs from the drive, I'd help you out, but on my page www.FIRST-a-holics.com I think there are some closeups from 69's drive train...especially in the BattleCry @ WPI photo album. Good Luck!

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Posted by Joe Johnson.   [PICTURE: SAME | NEW | HELP]


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

Posted on 9/7/2000 7:43 PM MST


In Reply to: 69 did too.... posted by Lora Knepper on 9/7/2000 5:45 PM MST:



Our picture gallery is packed with robot pics. Get busy looking!

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Posted by Dodd Stacy.

Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.

Posted on 9/7/2000 7:51 AM MST


In Reply to: CRAB!!!!!!! posted by Josh on 9/6/2000 4:24 PM MST:



Josh,

There are a number of different ways to accomplish 'crabbing' in a mobile platform. I agree with Joe's comments as they relate to approaches that involve steering (and powering) the driving wheels in the direction(s) you want them to go. That involves huge challenges in the four areas noted, and succeeding like Team 47 has done in the last two years (a few others, as well) is a truely amazing accomplishment.

However, an approach using 'holonomic' wheels provides equally universal platform maneuverability with no particular challenges in pivot design (there aren't any), wheel position feedback (they don't steer), or programming (well, maybe a little). Holonomic wheels develop traction and can deliver drive power in the direction of wheel rotation, but are able to slip freely (without steering) in the perpendicular direction if something else pushes the platform in that direction. There are lots of ways to do this, and a number of teams have made them in years past. Wheel design and manufacture is a chore, but nothing unusual compared to the mechanisms that many teams use elsewhere in their robots.

We used such a system last year, and it worked really well in a purely manual control mode. We were not able in time to successfully work in the use of the yaw rate sensor as a gyro-compass and transform the controls to an absolute spatial reference (go North) versus the standard robot reference system (go right). So our driver still had to 'put his brain on the robot' to consider its heading in making stick inputs, but that's what all the drivers do.

We used three holonomic wheels, each with an independent drive motor - a left and a right on a common axle line, and a middle wheel at 90 degrees to the other two. The left and right were programmed in conventional 'tank drive' style and operated off a single joystick. The middle wheel simply slips 'sideways' (its idea of sideways) when the left and right are driving the platform fore and aft, and serves as a pivot point when the left and right operate differentially to rotate the platform. The middle (or 'crab') wheel was controlled either off the side-to-side motion of the second stick or off a pot, depending on driver preference. Whenever the crab wheel drives the platform sideways, the left and right wheels slip freely in that direction. All modalities can operate simultaneously to move the bot in any compass direction while also rotating in either direction.

I will agree with Joe on the importance of driver practise with any omnidirectional system - we're not used to driving machines with this kind of mobility! In that regard, there are many different ways you can rig the control inputs without involving fancy programming, depending on what feels good to the driver(s). You can, for example, do fore/aft and crab left/right on the Y and X of one stick, with rotate left/right on another stick or on a pot. You can use the gyro chip to feedback to the rotate routine to maintain the bot's pointing direction constant in the absence of driver command inputs to rotate - I think many teams have done this in their tank drive program to compensate for left/right drive differences.

One final note - you need to insure that any and all of the driving wheels are adequately weighted so that they can deliver power to the ground under all conditions. This may mean some kind of limited suspension so the wheels can follow irregularities in the floor or accomodate some push and shove without losing traction. Good luck.

Dodd

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Posted by Joe Johnson.   [PICTURE: SAME | NEW | HELP]


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

Posted on 9/7/2000 10:44 AM MST


In Reply to: Skinning Cats posted by Dodd Stacy on 9/7/2000 7:51 AM MST:



I was remiss in not including the excellent machine from Lebanon in 1999. They had a very clever drive system.

My one issue with holonomic drive wheels (also known as omni-rollers or omnidirectional wheels) is that in the direction transverse to the 'major' direction of rolling, the effect wheel diameter gets pretty small.

I know that it can be made to work, but I still get a bit nerveous with wheels smaller than 3 inches or so. The holonomic wheels I have seen are much smaller than that when rolled against the grain.

Just my two cents worth.

Again, the idea is very clever and has some very cool advantages. I am only pointing out that there are some issues to be aware of when you are making your design decisions.

Joe J.

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Posted by Dodd Stacy.

Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.

Posted on 9/7/2000 12:42 PM MST


In Reply to: Holonomic warnings posted by Joe Johnson on 9/7/2000 10:44 AM MST:




: My one issue with holonomic drive wheels (also known as omni-rollers or omnidirectional wheels) is that in the direction transverse to the 'major' direction of rolling, the effect wheel diameter gets pretty small.

: I know that it can be made to work, but I still get a bit nerveous with wheels smaller than 3 inches or so. The holonomic wheels I have seen are much smaller than that when rolled against the grain.

: Just my two cents worth.

: Again, the idea is very clever and has some very cool advantages. I am only pointing out that there are some issues to be aware of when you are making your design decisions.

: Joe J.

Your point is apt, at least when considering the type of wheels most folks have seen, the so-called Mecanum or roller-rim type. We built this type last year, with 8 staggered rim rollers of 1 1/2' diameter on a 5' diameter wheel. We thought about the limited transverse obstacle climbing ability you note, decided the floor was pretty flat (and the rules precluded laying down tethered obstacles, etc - remember Torroid Terror?), and tried it. It worked pretty well, and was awfully handy in maneuvering the puck around the field after clamping onto the pole. Also for positioning TO clamp on the pole.

Anyway, I'm responding to your message not to be argumentative, but to point out that there are other classes of holonomic/omni-directional 'wheels' out there with different attributes, and a group of minds like the FIRST community can come up with more yet. In developing an omni-directional material handling platform at work for use on aircraft carrier decks, we came across (and developed further) a family of designs that uses a large diameter sphere as the rolling element in contact with the deck/floor. This 'wheel' confronts any obstacle with its major diameter, regardless of direction of motion. This is handy for rolling over 2' diameter arresting gear cable, tie-down chains, etc.

All of the rollers, bearings, and other complex bits for loading the weight of a bot onto the sphere, keeping the sphere corraled in the bot, enabling it to rotate in all directions, and driving the sphere in 1 or 2 directions - they all bear from above upon the nice predictable surface of the sphere (as long as you keep it swept clean). These 'wheel' systems have more parts and are more mechanically involved than the Mecanum type, but they finesse the concern you raise.

Design decisions always involve trade off judgements, and there are always more ways to do things than we already know about. I'm trying to embolden Josh and other teams like his to tackle the manageable challenges of omni-directional drive now and field creative wonders to thrill and delight us all in 2001. Actually, I'd like to see some team go Dean's IBot one better and make a bot that balanced on ONE spherical omni-directional rolling element. Kind of a unipsycle.

Dodd

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Posted by Jason Iannuzzi.

Engineer on team #11, Marauders, from Mt. Olive HS. and BASF, Rame Hart, CCM.

Posted on 9/7/2000 12:50 PM MST


In Reply to: Balls posted by Dodd Stacy on 9/7/2000 12:42 PM MST:



>>Actually, I'd like to see some team go Dean's IBot >>one better and make a bot that balanced on ONE >>spherical omni-directional rolling element. Kind of >>a unipsycle.

I'd eat my shoes if I saw a FIRST machine do that. I honestly would. Whew, we have trouble coming up with a NORMAL drive train every year...I can only imagine the control system that the IBot uses.

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Posted by Joe Johnson.   [PICTURE: SAME | NEW | HELP]


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

Posted on 9/7/2000 3:00 PM MST


In Reply to: Re: Balls posted by Jason Iannuzzi on 9/7/2000 12:50 PM MST:



I am fairly certain that this could be done with stuff in the kit.

Don't get me wrong, it would be a huge challenge, but I think it could be done.

My major concerns are with the following:
I am not sure that the control system could stand the dead zone in the midde of the Victors and I am concerned about the updated rate to the Victors (50Hz for the link from the Robot Controller to the Victor and data packets at the rate of 40Hz -- probably not fast enough).

In addition, I think that the robot could not do much else beyond balance on 2 wheels. There is not likely to be enough CPU power to do much else. Beyond this, waving arms & actuators would definitely make the system more challenging to control.

It would be an interesting machine, even if it didn't do much to play the game.

Joe J.

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Posted by Matt Leese.

Student on team #7 from Parkville High School and NASA, Black & Decker, AAI, Raytheon.

Posted on 9/8/2000 6:36 PM MST


In Reply to: I think it could be done. posted by Joe Johnson on 9/7/2000 3:00 PM MST:



Trying to get stuff to balance on a robot is real hard mainly because of the lack of a timing circuit. It's not easy to figure out where the robot is actually positioned relative to up. We tried to figure out a way to do this with the rotational rate sensor but we never got it working (we dropped work on it after we realized that it wouldn't be put on the robot). The main problem I ran into was the fact that if you simply add up the velocities every time the program loops through you don't end up centering the device but instead it off-centers. The ability to power the motors just slightly is also missing which helps to make this task much harder. Although it's a very cool idea (if anyone pulls this off I think they win the control systems award hands down), everyone please remember the I-Bot is powered by 3 PIIIs (correct me if I'm wrong), and we work with a little Basic Stamp 2 controller. I don't think the processing power exists to do this whole setup well. Best of luck to anyone who tries.....

Matt