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Types of Drives?
Hello Everyone,
I am currently looking at different types of gearboxes and how to optimize them. This has led to my looking at the various types of overall drive systems. I believe I understand what skid and crab drive are, but I am clueless about what a swerve drive is. Can someone please explain what these (and any unnamed) drive types are? Thanks, indieFan |
"crab" and "swerve" are two names for the same drive type
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The term Crab, comes from... well, a crab. If you have ever seen a crab walk, you know that crabs walk sideways. This makes sense since omni-directional drives can go sideways like a crab. The term swerve is attributed to the agile motion of omni-directional drives. You never really know where they are going next, they just swerve all over the place very gracefully... most of the time anyway :yikes: |
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a killough(sp?) platform or 'holonomic drive' has three degrees of freedom, and therefore is a 'true' omni-directional drive. old thread on the subject the first FIRST killough platform (i believe) was built by team 857 in 2002, and was called the 'kiwi-drive'. |
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Its been worked on, and should actually be mechanically simpler and much more robust than last year's drive. Of course, we will only go ahead if it is consistant with our strategy, which will be decided come January. The programming will be tough, but thats not my deptartment (i'm just the lead designer). Control should be a breeze. If we manage to get the solid-state gyro online enough to give us a consistant heading so that pushing the joystick north, regardless of the facing of the robot, will cause the robot to move north. and same with all the other directions. With a twist-stick joystick, we can add translational and rotational movement all into one hand . . . it is a good plan, I think. We will see. The biggest problem will be to make the chips and drills run at the same speed not only at 100% throttle, but at all the various percentages in between. Like I said, we will see how things go. |
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-Pat |
Alright... after thinking about it, I guess my definition of an omni-directional drive would be:
A driving mechanism that allows 360 degree directional movement independent from the frame. My thoughts are that you NEVER can or need to go in more than one direction at a time... so being able to do them all at the same time is a pointless criteria. Therefore, under this definition, I consider Swerve Drive an omni-directional drive :yikes: |
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That being said, however, swerve is more than enough mobility for just about anybody. There are advantages to swerve over Killough, and vice-versa, but you can do well with either. |
when I say that i will be tough to make the chips and drills to run at the same speed . . . I do not mean at full power. However, as I understand these things, if you were to take a chip and a drill, gear them so their free RPM's are equal, and then increase the voltage from zero to twelve at the same rate, the different motors will have different rates of increasing . . . ness . . . . osity . . . . or something. I mean, that, while you can sync the motors at 12 volts, they won't nescessarily be synced when you are giving them half power. Does this make sense? So, our base will run a straight line at full speed, but will pull to one side at half power and stuff. This isn't that large a problem, but its something to think about.
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Speed, Power, And, Maneuvarubalty
SPAM seems to do well with Tank treds Start, With, Attacking, My, Pproblems Speed, Width, Agility, Manurvibilty, Power And i just made these up for Swamp A Crab drive seems Great because you can point all your power in any direction, But omni wheels only a fraction at the most half in any given direction |
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The best options is to gear the motors such that the torque-speed curve lines are parallel for both motors. Then, for the motor with more power (ie the one further away on the torque-speed graph), you want to multiply the voltage applied by a constant less than one to bring the curve inward to the weaker motor. For example, the weaker motors would be used fully (PWM range -127 to 127) while the stronger motors would not be used fully (something like -108 to 108). This does waste power, but believe me, with four powered motors geared to an appropriate gear ratio and with high-traction wheels (see below), you will not have a power problem. Ensuring the same torque-speed curve using my method above will also help a LOT with control problems. There will still be some variance from motor to motor and over time and with varying heat conditions: If I were to do it, I would actually place encoders on each wheel, and use feedback control to ensure the robot is going in the commanded direction... But I don't know if FIRST rules allow purchasing of encoders. I know there are teams that make their own with the optical sensors. But to start with, ignore the encoders idea, and if things don't work well enough, you might consider it. Now, about wheels, I have a lot of experience designing omniwheels, Here is a design I worked on last year (I've also attached a picture of final product): Exploded view of wheel Engineering Drawing Engineering Drawing of Lexan "Hub" Engineering Drawing of Roller (small rollers around the wheel) CAD View of Robot w/ Wheels The wheels have lexan hubs and delrin or aluminum rollers with rubber Buta-N o-rings wrapped around the outside of the rollers. The coefficient of friction between wheel and ground of this particular design is 1.3 for a felt-like material. I suspect it is at least 1.5 (probably closer to 2.0) on a FIRST carpet. You can machine very similar wheels with manual mill & lathe, but you will need a rotary table to do the lexan pieces. It is better if you have access to CNC. You will also need to make larger wheels, probably about double the size of these wheels (which are 2" diameter). You can order O-Rings from McMaster or try and find them at a hardware store. If you are really interested, you can actually read our mechanical design documentation at: Cornell Robocup 2003 Mechanical Design Documentation There may be some things of use in the documentation that I did not include here. Good luck! And let me know if you have any questions. - Patrick |
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Keep in mind that with the 4-wheel omnidrectional platform you can have your Y-axis geared for high-speed and your X-axis geared for high-torque without motor fighting. As said above, with robots drive systems there are three variables to control. Heading, Y-axis velocity, and X-axis velocity. Tank drive gives control over heading and y-axis velocity, but you can only manipulate one of those at any given moment. 2-wheel / caster drive gives a little bit more control over the two, and you can even do both at the same time for turns up to 45 degrees. However, you cannot make a 90 degree or more turn while maintaining Y-axis velocity. Swerve/crab drive gives the illusion of omnidirectionibility due to the ability to change the robot heading without changing robot orientation. Still, you only have control over robot heading and Y-axis velocity. Although, you have the ability to manipulate both at the same time. 3-wheel/4-wheel holonomic drive systems have true capability to manipulate all 3 at once, as Jeff said above. It's possible to change robot heading/orientation while maintaining a constant velocity in a certain direction. |
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Guys, this debate is basically just about terminology. Everyone has their own.
Here is mine (and 229s): Omni-Wheel - A wheel that rotates normally and functions as a wheel, but can also be moved perpendicular to it's rotation. Like the wheel Patrick posted above. Many teams use these to aid in turning. (Hotbot 2001) Omni-Drive - A drivetrain that consists of multiple "sets" of omni wheels in different (usually at perp.) directions. This alows the robot to move in any direction simply by powering the different wheels at different speeds. (think Metal-In-Motion 2002, which was a 6 wheeled variety. A 4 wheeled variety can be seen here .) I would consider 857's kiwi-drive to fall under this category. Crab Drive - This type of drivetrain has 2 sets of drivetrains set perp. to each other. Instead of using omni-wheels and having all wheels on the ground, it somehow actuates things so that only one set is on the ground at a given time. (Good examples of this include WPI 2003, 358 2003, TRIBE 2002). Swerve Drive - The individual wheels can "spin" changing the heading of the robot. It can move in any direction. (think Wildstang 2003/2002, Chief Delphi 2001/2002) There are many variations on this. 2-Wheel Swerve- Two wheels "spin" like a normal swerve drive (See above) the other wheels are either casters, or omni-wheels (see definition above). (see Chief Delphi 1998/2000). Articulated-Steering - Some, or all of the wheels will turn like a car to aid in turning. 2 cool examples of this include Hotbot and Thunderchickens 2003. Skid-Steering, or Tank Drive - This is the most standard type of drivetrain. All the wheels/tracks/whatever are pointed in one direction. The robot turns by reversing one side, while powering the other side forward. Examples of this include... almost everyone. When a robot in this configuration is turning, some energy is lost to "side load" because the robot is essentially dragging some of it's wheels across the floor sideways. Some teams use dropdown casters/skids to aid in turning. This is just my terminology for things. It isn't always the most "technically correct" terminology, but this is what I've developed over 4 years in this program. *shrug* it works for us. Perhaps this will benefit, someone.... John |
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