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  #31   Spotlight this post!  
Unread 28-09-2011, 17:16
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Re: pic: FRC973 Presents Emperor Swerve

I've been wanting to build this drivetrain for years, and then you come along and beat me to it. So it's back to the drawing board for me...maybe I'll sketch out a flying robot this time.

(Nice work; send CAD files!)
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Unread 28-09-2011, 17:51
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Re: pic: FRC973 Presents Emperor Swerve

It's absolutely gorgeous. Well done!
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Unread 28-09-2011, 19:22
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Re: pic: FRC973 Presents Emperor Swerve

Adam,
you guys have certainly stepped it up doing great things with your team.
Wish I could witness it at the off-season event.

Glenn
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Unread 28-09-2011, 19:26
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Re: pic: FRC973 Presents Emperor Swerve

Can someone explain in simple terms what you mean by translation and rotation?
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Unread 28-09-2011, 19:30
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Re: pic: FRC973 Presents Emperor Swerve

Quote:
Originally Posted by rsisk View Post
Can someone explain in simple terms what you mean by translation and rotation?
Rotation: Turning about a single point, usually (but not always) the center of the robot, or somewhere close to that area.
Translation: Motion in a single direction (either front/back or side-side, or in the case of an omnidirectional drivetrain, some line in between).

If the point of rotation happens to be translating as you rotate, you are pulling off a very tricky move--that sort of motion is usually reserved for things like frisbees, not stuff that stays on the ground.
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Unread 28-09-2011, 19:38
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Re: pic: FRC973 Presents Emperor Swerve

Richard, our robot can drive any direction it wants to, and at the same time rotate. Much like the videos Madison posted.

Quote:
Originally Posted by EricH View Post
Rotation: Turning about a single point, usually (but not always) the center of the robot, or somewhere close to that area.
Translation: Motion in a single direction (either front/back or side-side, or in the case of an omnidirectional drivetrain, some line in between).

If the point of rotation happens to be translating as you rotate, you are pulling off a very tricky move--that sort of motion is usually reserved for things like frisbees, not stuff that stays on the ground.
The point of rotation is not a point we directly control, it is determined by the combination of x, y and rotation inputs. It is only ever on the robot when our translation speed is very slow or nonexistant (which I guess is true of all FRC robots).
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Unread 28-09-2011, 21:13
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Re: pic: FRC973 Presents Emperor Swerve

very nice
mike d
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Unread 28-09-2011, 21:19
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Re: pic: FRC973 Presents Emperor Swerve

Quote:
Originally Posted by rsisk View Post
Can someone explain in simple terms what you mean by translation and rotation?
You pick a point, usually the center of geometry of the wheels1, and then any motion of the vehicle can be described in terms of these 3 "degrees of freedom":

1) rotation of the vehicle around that point, and

2) translation (motion) of that point in the forward/reverse direction (with respect to the vehicle), and

3) translation (motion) of that point in the left/right direction (with respect to the vehicle).


Some examples:

a) a car driving forward in a straight line. The forward translation is non-zero, and the rotation and sideways translation are both zero.

b) the moon going around the Earth. Call the "front" of the moon the part that is facing the Earth. The forward translation is zero, the sideways translation is non-zero, and the rotation is non-zero (it is equal, in radians per second, to the sideways translation speed divided by the length of the radius of the moon's orbit).

Here are some example diagrams of how Ackermann, rotary, "moon", and "dosado" motions can be described in terms of the 3 degrees of freedom.


1this usually makes the inverse kinematics easier

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Unread 28-09-2011, 21:21
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Re: pic: FRC973 Presents Emperor Swerve

Quote:
Emperor Swerve is a fully omnidirectional swerve that can translate and rotate simultaneously through all orientations.
How easy is it for you to do this? (how much driver input is required?)

I remember accomplishing this on our robot, but it was a very tough maneuver.
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Unread 28-09-2011, 21:22
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Re: pic: FRC973 Presents Emperor Swerve

Quote:
Originally Posted by Ether View Post
You pick a point, usually the center of geometry of the wheels1, and then any motion of the vehicle can be described in terms of these 3 "degrees of freedom":

1) rotation of the vehicle around that point, and

2) translation (motion) of that point in the forward/reverse direction (with respect to the vehicle), and

3) translation (motion) of that point in the left/right direction (with respect to the vehicle).


Some examples:

a) a car driving forward in a straight line. The forward translation is non-zero, and the rotation and sideways translation are both zero.

b) the moon going around the Earth. Call the "front" of the moon the part that is facing the Earth. The forward translation is zero, the sideways translation is non-zero, and the rotation is non-zero (it is equal, in radians per second, to the sideways translation speed divided by the length of the radius of the moon's orbit).

Here are some example diagrams of how Ackermann, rotary, "moon", and "dosado" motions can be described in terms of the 3 degrees of freedom.


1this usually makes the inverse kinematics easier
It's funny. I learn more from Ether's posts than I do from my normal science class! Thank you, Ether, for your interesting yet educational posts!
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Unread 28-09-2011, 21:24
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Re: pic: FRC973 Presents Emperor Swerve

Quote:
Originally Posted by BeltSanderRocks View Post
How easy is it for you to do this? (how much driver input is required?)

I remember accomplishing this on our robot, but it was a very tough maneuver.
Without a gyro on there, it'd take some skill. With a gyro it's a trivial maneuver. You shift the coordinate inputs by the gyro's angle, it's known as "field-centric" drive. We don't plan to have the robot that way all match, just when the driver wants to spring down the field while doing high magnitude rotation.
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Unread 28-09-2011, 22:01
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Re: pic: FRC973 Presents Emperor Swerve

Is there a video of the robot performing yet? This looks really Awesome though!
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Unread 28-09-2011, 22:03
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Re: pic: FRC973 Presents Emperor Swerve

Quote:
Originally Posted by BeltSanderRocks View Post
How easy is it for you to do this? (how much driver input is required?)

I remember accomplishing this on our robot, but it was a very tough maneuver.
Adam beat me to it, but I'll go ahead and post this in case it adds anything to the explanation.

If you have a stable and reliable gyro, it can be used to convert "field-centric" translation commands into the corresponding robot-centric translation commands. Some maneuvers that would otherwise take a great deal of driver practice and skill can be greatly simplified by doing this.

The way a field-centric command works is this:
1) The driver uses the joystick (or other input) to command the vehicle to go in desired direction (with respect to the field)

2) The computer takes this command, and uses the gyro reading to figure out what the "robot-centric" commands (fore/aft and port/starboard with respect to the robot) must be to accomplish the driver's desired motion.
If the driver is commanding a constant field-centric direction and speed, and the vehicle is simultaneously rotating, then the computer is continuously converting that constant field-centric command into the time-varying fore/aft and port/starboard robot-centric commands necessary to keep the vehicle moving in the direction and speed commanded by the driver, regardless of the vehicle's angular orientation.

So the driver can command field-centric "forward" (usually defined as "downfield" - away from the driver station) and the vehicle will travel in a straight line in that commanded direction regardless of the independently commanded vehicle rotation (subject to the caveats mentioned earlier).



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Unread 29-09-2011, 08:14
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Re: pic: FRC973 Presents Emperor Swerve

Adam,

You mentioned that one of the "d'oh" problems you encountered was wheels momentarily opposing one another as the pods rotated to their commanded angles. What algorithms do you plan to use to correct for this? (I suspect I know the answer, but it would be a great topic for discussion here, as this is one of the common "gotchas" that makes swerve programming much, much more complicated than just implementing the inverse kinematics)

Thanks
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Unread 29-09-2011, 11:27
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Re: pic: FRC973 Presents Emperor Swerve

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Originally Posted by Jared341 View Post
this is one of the common "gotchas" that makes swerve programming much, much more complicated than just implementing the inverse kinematics
Exactly.

I see two general classes of solutions to this problem:

1) rate-limit the driver commands. advantage: effective, and simple to implement. disadvantage: possibly poor responsiveness

2) foresake independent control of each pod... make the control algorithm for each pod be dependent on what the other pods are doing so that their actions are coordinated. advantage: potential for better performance than option1. disadvantage: may take quite a bit of thought and debugging to get something that works reliably under all situations; and there may still be latent bugs waiting for the right Murphy moment to bite.


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