WCD vs. Swerve

[Ether]

Originally Posted by JamesTerm

There is much more to the idea that I cannot seem to put into words...

It would be good practice to try.

An inability to communicate your ideas effectively is a handicap you would be well served to strive to overcome.

[JamesTerm]

Originally Posted by Ether

It would be good practice to try.
An inability to communicate your ideas effectively is a handicap you would be well served to strive to overcome.

Agreed... but not today. I need some time to prepare.

[Gdeaver]

Driver presentation. May be some who have actually driven a swerve will comment. We have a 4 wheel drive 4 wheel steering bot. We have used a x-box and 2 kop joy sticks. Also, we tried a joystick with twist but, the drivers hatted the twist for chassis orientation. So how do you control the 4 degrees of freedom required for swerve driving. X, Y, Chassis orientation, and velocity.
We have always used the left joy stick for x-y and extrapolate velocity from it. The right joy stick x mixes in chassis rotation. This is what the programers and drivers ended up with. I feel that after watching our driving the last 2 years there is a major problem with this choice. Our drivers can make the bot dance on our practice field with no pressure. Under pressure at a comp I see the driving deteriorate. I believe their left hand or thumb coordination is being overloaded. What have other teams used. I believe the extrapolated velocity is the problem. For a short time in the 2010 off season we had the X - Y on the left X-box controller joy stick. Velocity on the right joystick x and chassis orientation on the analog triggers. I liked it. The programmer graduated and the code disappeared. We went back to the above described method. So what is the best driver presentation. Arguments welcomed.

[JamesTerm]

Originally Posted by Gdeaver

So what is the best driver presentation. Arguments welcomed.

I think the most important design decision is to fundementally separate control and implementation in such a way where it becomes trivial to switch things around for trial and error. Also make it where there is a desired movement lead what is physically possible. I have solved these problems already where I fundementally work with a 2D vec of desired velocity and a float for heading. I have a class that figures out how to make that work. With this approach I can achieve the feel (e.g. elastic bell curve on rotation) of what the ships do in the game demo link I sent.

Me personally I think one arcade drive joystick just like tank with some strafe buttons elsewhere... but I want to customize to what the driver wishes.

[JamesTerm]

Originally Posted by Ether

It would be good practice to try.
An inability to communicate your ideas effectively is a handicap you would be well served to strive to overcome.

Ok I will take a stab at this today... with a question. What makes a swerve drive so hard to drive vs. what makes a WCD easy to drive (both tank steering and arcade configurations)?

[Ether]

Originally Posted by JamesTerm

Ok I will take a stab at this today... with a question. What makes a swerve drive so hard to drive vs. what makes a WCD easy to drive (both tank steering and arcade configurations)?

I will assume that, in this context, by "tank steering" a swerve you are referring to the driver interface (e.g. using Y axis of left and right joysticks) and not to the inverse kinematics used (i.e. not skid-steer).

Instead I assume you mean something like this:

FWD = (Y_L+Y_R)/2

RCW = (Y_L-Y_R)/2

STR = 0

... where Y_L and Y_R are the (inverted) joystick commands, and FWD, RCW, and STR are as defined here.

In that case, I will answer your question with a question: is a swerve with that driver interface "so hard to drive" ?

[JamesTerm]

Originally Posted by Ether

In that case, I will answer your question with a question: is a swerve with that driver interface "so hard to drive" ?

Thanks so much for that link! That is half the problem I was trying to figure out. The other half will be the reverse of that where given the wheel angles and speeds, what is the current x, y and rotational (i.e. angular) velocities.

For now let's ditch the tank steering 2 joysticks except to say that it can be done. I think FWD and RCW can be on one joystick where left and right perform the rotation (I believe this is called arcade drive)... Just this much is what we had this season on a WCD, and it felt intuitive (we played defense). Now add to this some strafe buttons (and not another axis). I think for me personally I'd like this because this is similar to how games like ut2004, quake etc... work. Except they use a mouse for the orientation. The strafe buttons work where they inject more strafe the longer they are held down, and then release it in the same manner. This way if the driver doesn't want to do it... it is easy to focus on the basics.

One good way to really answer this question is to create a simulation and give it to a real student driver and let him decide if it is easy or not. I *hope* to do this... next summer.

[Taylor]

Originally Posted by JamesTerm

Now add to this some strafe buttons (and not another axis).

But then the strafe would be digital, not analog. Not saying this is good or bad; it's just something to be considered.

[JamesTerm]

Originally Posted by Taylor

But then the strafe would be digital, not analog. Not saying this is good or bad; it's just something to be considered.

That all depends on how you look at it... It is possible to swap out a button control on functionality that uses an analog intensity parameter. The idea is that the button starts digital, but then transforms to analog given the amount of time it is held down. I hope that makes sense.... it is like taking the joystick and moving it from zero to full intensity in a given amount of time.

The most important point of this is that if designed right it is easy to swap buttons with axis controls with minimal code change... or overhead. This is why I love c++.

[Ether]

Originally Posted by JamesTerm

The other half will be the reverse of that where given the wheel angles and speeds, what is the current x, y and rotational (i.e. angular) velocities.

That is called the Forward Kinematic Problem, and for swerve it has no kinematic solution for arbitrarily chosen values of the wheel speeds and angles. See the discussion starting at the bottom of Page7 of this paper.

For now let's ditch the tank steering 2 joysticks except to say that it can be done. I think FWD and RCW can be on one joystick where left and right perform the rotation (I believe this is called arcade drive)... Just this much is what we had this season on a WCD, and it felt intuitive (we played defense). Now add to this some strafe buttons (and not another axis).

See the list of driver interface suggestions on Page 1 of this paper.

[JamesTerm]

Originally Posted by Ether

That is called the Forward Kinematic Problem, and for swerve it has no kinematic solution for arbitrarily chosen values of the wheel speeds and angles. See the discussion starting at the bottom of Page7 of this paper.

Thanks for this link as well... I am a bit surprised that there are no solution cases. In my mind the robot is going to do something for those cases, and I'd look at this from a physics standpoint... by applying a vectored force to each point and let the opposing forces cancel each other out. Perhaps this is over-simplified, but perhaps it is worth testing a solution like this against the problem presented here, and see how equal they become.

[Ether]

Originally Posted by JamesTerm

Thanks for this link as well... I am a bit surprised that there are no solution cases. In my mind the robot is going to do something for those cases

I did not say there were no solutions or that the robot would not do something. I said there were no kinematic solutions. Take a closer look at the link I posted previously; the meaning of this is explained there.

, and I'd look at this from a physics standpoint...

Kinematics is physics :-)

by applying a vectored force to each point

This is dynamics (also physics). For arbitrarily chosen values for each of the four wheel speeds and steering angles, a dynamic analysis of this sort could be done. Write a paper when you're done.

[JamesTerm]

Originally Posted by Ether

I did not say there were no solutions or that the robot would not do something. I said there were no kinematic solutions. Take a closer look at the link I posted previously; the meaning of this is explained there.

Kinematic until just now was Greek to me... thanks for pointing that out... I come from a kinectic and dynamic study of problems in my work on the game. I don't really understand these abstract terms to their fullest yet, but I do know the details of what I know just not really the scope and label of it.

I find it interesting how dropping an adjective from a sentence can really change the meaning.

FWIW I am not good at writing papers, but I can submit some code example when I get to that point.

[Siri]

Originally Posted by JamesTerm

I have a question for you... (or anyone else that can answer)... Does your robot work in a way where you can submit desired angles to each wheel independently? (I presume that is what the *independent* means in your statement).

Technologically, yes. We do not have inputs on the Driver's Station that can literally say, "right-front at 40deg", "left-rear at 60deg", etc, but in theory we have both the hardware and the programming input & feedback to move any wheel at any angle and speed (limited by motors/gearboxes) we'd like. Really we just steer in "modes", e.g. crab, snake, and some useful spins (who's radius we can control).

Originally Posted by JamesTerm

If so, how do you sense when the rotation has reached its angle? potentiometer? IIRC Bomb Squad uses windows motors on each wheel to control the swerve. From what I have heard, other teams use a rod to swerve both front wheels the same amount, and can then have a manual control setup doing it this way.

Our swerve rotation is controlled by Banebot RS-540's on 256:1 4-stage planetary gearboxes, one per module. (We used window motors last year.) We use Vishay encoders (981HE0B4WA1F16) which we can hand-adjust every once in a while as needed. This works quite well (way better than the Cherry AN8's), though the PID isn't perfect yet and the backlash is especially obvious in autonomous.

More: Team 1640's LogoMotion Drive Train

[JamesTerm]

Originally Posted by Siri

More: Team 1640's LogoMotion Drive Train

Thanks very much for this link... I hope our team will be brave enough to try something like this for off-season.

It seems like that swerve-drive teams are of a rare breed.