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Unread 30-01-2003, 21:43
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I think there is some confusion about drive systems... and locomotion in general.

On a plane (i.e. the surface of the playing field), describing an object's position at any instant in time requires three coordinates. For example, a robot can have an x-location, y-location, and direction (angle) which the robot is pointing. You can not describe the robot's position correctly with less than three coordinates. It is also possible to describe position with polar coordinates and other coordinate systems.

Now, over time, a robot can alter these coordinates. Typically, a robot can move forward and backwards. In other words, it can translate along one axis (move in the direction of the front of the robot). Most robots can also turn at the same time (adjust the angle which their robot is pointing). These two "degrees of freedom" are what you get out of a tank-drive system, which most teams choose to use. The number of degrees of freedom your robot has is defined as the number of coordinates (x-translation, y-translation, and z-rotation) that your robot can adjust simultaneously. A tank drive might be able to turn and translate in another direction, but it can not translate sideways, thus it does not have the third degree of freedom.

Typically, an omni-directional drive system is defined as a drive system with three degrees of freedom. Very few (I can only think of one last year) teams ever have three degrees of freedom. Tank drive only has two. In fact, even if you can turn all your wheels in any direction you like (i.e. swerve drive) you still have only two degrees of freedom, because at any instant in time your wheels are pointed in a given direction, and your robot is restricted to that linear and angular movement, giving you only two degrees of freedom. However, the advantage of the swerve is that you have the ability to change the direction of your prismatic (translational) degree of freedom with respect to your robot. If you can change wheel angles almost instantaneously, your robot is almost as good as one that can go accelerate in any direction at any angle, thus you virtually have three degrees of freedom. Robots that have a set of wheels that drop down perpendicular to your main set also only have two degrees of freedom, since at a given instant in time they can only move in one translational direction and rotate.

Now, a crab-walking robot could be built such that it has three degrees of freedom, but it would be difficult and almost certainly very very slow. The efficiency of an electric motor is far better than the efficiency of a crab-walking mechanism.

There only two mechanical ways I know of to get three degrees of freedom... meaning at any time, you can have any x-acceleration, any y-acceleration, and any angular acceleration. One of these I have posted a brief paper on how to get started on applying it to a FIRST robot (in the white papers) and the other is a little bit abstract and not too likely to work on a FIRST robot. One team had omnidirectional last year, and I forget the number, but I think it was a first or second year team. Basically it entails having three or four omniwheels perpendicular to the center of the robot. With three wheels, each unique combination of independent torques to the three wheels results in a unique direction and angular velocity of the robot.
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Unread 30-01-2003, 22:15
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One word: POWERED CASTERS

Sure, sounds impossible (or just very hard to do in FIRST) but someone I know came up with a design and an idea and I want to go and look at it.

From what my friend says, it has 8 options of movement.
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Unread 31-01-2003, 15:57
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Quote:
Originally posted by patrickrd
One team had omnidirectional last year, and I forget the number, but I think it was a first or second year team. Basically it entails having three or four omniwheels perpendicular to the center of the robot. With three wheels, each unique combination of independent torques to the three wheels results in a unique direction and angular velocity of the robot.
You're talking about team 857, they weren't exactly rookies last year:

Quote:
Originally posted by Andy Baker
The core of this team has been in FIRST for about 7-8 years. These guys at Michigan Tech (Anthony, Justin, Adam, etc.) are some of the best robot builders in FIRST. They had one of the coolest drivetrains last year (the kiwi drive), and have been in the top 4 at the Championships before.

We can all learn a thing or two from team 857. From what I heard, they really didn't want to be rookies last year, but it was an administrative decision to do so.

Andy B.
You can see their 2002 'bot on their site: http://stuweb.ee.mtu.edu/~alkrajew/FIRST/kiwi.mpg
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Unread 01-02-2003, 05:23
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[
Quote:
Originally posted by GregT
I'm talking about a robot with a 360 degree freedom of movement, instantly - no turning. From standing still it can go any direction it feels like.

Greg
This is also known as "holonomic" motion.
The long technical name we used for our 'Kiwi' drive system was an 'omnidirectional holonomic platform with zero kinematic redundancy'.
Basically meaning it could drive in any direction (omnidirectional) like a swerve/crab drivetrain as well as do this with instant acceleration in any direction (holonomic) i.e. no need to turn the wheels like on a swerve/crab drive all while having no wheel drive in the same vector plane (zero kinematic redundancy) i.e. three wheels 120 degrees so no tow drive wheels produce motion in the same direction.

Ideally, the Kiwi (a true three degree of freedom drivetrain) can drive in any direction (x & y translational velocity) while rotating around its axis (rotational velocity). This is what a crab/swerve drive can not do. They can drive in any direction. And they can rotate on their axis. But they can't do it at the same time.

Now, what does this mean in competitions? 2002 was the perfect year for the Kiwi. We had three matched motors (the drills), a flat playing field, and a great deal of research ahead of time. A well constructed crab/swerve will give you nearly equal mobility with mechanically guaranteed straight-line motion (if done correctly, of course). Still, it won't give you the fluid motion and overall 'coolness' of holonomic motion.

Adam
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Unread 01-02-2003, 08:19
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Quote:
190s wheels aren't onmidirectional or crab steering, but we still won't need to turn in most matches.
Wait a minute, what could you do in this game with out turning -- That's crazy talk, man.
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Unread 01-02-2003, 14:48
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Some teams have strategies where they hardly move at all after the first five seconds, ut not us.
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Unread 04-02-2003, 15:04
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s

its funny because most of you completely skipped patricks post in which he explains what omnidrive is.

now you dont get it.
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Unread 04-02-2003, 15:19
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Personally, I don't see any advantage to going with a crab or omniwheel design. Unless you have a design from year's past or an excellent design and packaging team you are costing yourself valuable time. Not only will a crab system cost you weight and torque but if you lose one wheel, then you lose your entire system. Personally I would use a tank style drive system, and plain old fasioned wheels.
Strategically, a crab system will allow for some nice manouverability on the top of the hill. However, this could be countered by a single wing with suction cups. Also, unless its well designed, your crab system WILL NOT allow you to move instantly in any direction...it takes time fo rthose wheels to change directions.
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Unread 07-02-2003, 11:27
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Take a look at www.team639.org/movies/omni3.mpeg and tell me there is no good use for omni drive in this years game.

Greg
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Unread 08-02-2003, 11:57
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That omni drive is so cool. How is team 639 thinking of using it though?
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Unread 08-02-2003, 23:12
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Who said anything about us using it?

Greg
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Unread 10-02-2003, 18:48
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Are you using it?
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Unread 11-02-2003, 06:13
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The 45 degree omniwheels are great. Same concept used by AirTrax.

Adam
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