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  #16   Spotlight this post!  
Unread 12-09-2012, 16:53
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Re: Swerve Drive Rotation Control

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Originally Posted by EricH View Post
Not necessarily. If the drive power is tied 2-and-2 on the sides, or each module is individually powered, it shouldn't be terribly difficult to program a mode where the swerve drive turns like a tank drive. As a matter of fact, the OP was asking about this very power setup.
I was referring to Al's statement about chaining the steering of all the wheels together. Unless you're saying they don't power one half and do a skid-ish turn of some sort, which is something I've never heard of. Could you please elaborate?

EDIT: As I stated in my post after this, I was over-thinking the concept and making it out to be something far more complicated. I am now embarrassed.

Last edited by Katie_UPS : 12-09-2012 at 17:42.
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Unread 12-09-2012, 17:09
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Re: Swerve Drive Rotation Control

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Originally Posted by Katie_UPS View Post
I was referring to Al's statement about chaining the steering of all the wheels together. Unless you're saying they don't power one half and do a skid-ish turn of some sort, which is something I've never heard of. Could you please elaborate?
Exactly what I'm talking about. You can use a 2-and-2 power arrangement to simulate a skid-steer. You'd only do it as one mode of driving; normal operation would be to drive like a swerve. But for a quick reorientation, a skid motion can help. (Turrets are also an option, but that's another dose of mechanical complexity.)
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Unread 12-09-2012, 17:11
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Re: Swerve Drive Rotation Control

EDIT: It was pointed out to me that I knew exactly what you mean, and was instead imagining some completely different method of turning. Original content removed for pride's sake.

Last edited by Katie_UPS : 12-09-2012 at 17:44.
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Unread 12-09-2012, 17:12
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Re: Swerve Drive Rotation Control

It'd be nice to list exactly which maneuvers you give up as you tie more and more degrees of freedom together.

[EDIT: The paragraph that was originally here was all kinds of wrong-- I'm going home.]

Last edited by Nate Laverdure : 12-09-2012 at 17:40.
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Unread 12-09-2012, 21:29
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Re: Swerve Drive Rotation Control

Katie,
Thanks for reminding me. We have used a descending foot, pneumatically operated to lift two wheels off the floor to make orientation changes. In some designs the foot became an AndyMark omni wheel(s).
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  #21   Spotlight this post!  
Unread 12-09-2012, 23:17
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Re: Swerve Drive Rotation Control

Quote:
Originally Posted by Nate Laverdure View Post
It'd be nice to list exactly which maneuvers you give up as you tie more and more degrees of freedom together.

[EDIT: The paragraph that was originally here was all kinds of wrong-- I'm going home.]
I'll take a rough empirical stab, and then let the Ethers of the world codify the Unified Field Theory of Swerves.

First off, some assumptions. Most types of swerves can pull off turn in place/turn while translating by scrubbing wheels ala a tank drive. A "scrub-less" turn is in theory more efficient (though in practice you might find more overshoot makes it harder to control without software help). In many cases (long robots + sticky wheels), scrubby turns turn into hoppy/breaker popping turns. You might need to rotate your wheels to be along the "wide" axis for scrubby turn in place to work well. So there is a very gray line separating maneuvers an arrangement can vs. cannot do.

Here are some common swerve steering arrangements:
A. All four wheels independently
B. Front/rear or left/right wheels driven together
C. Diagonal pairs of wheels steered together
D. All wheels steered together

Here are some common swerve driving arrangements:
1. All four wheels driven independently
2. Front/rear or left/right wheels EDIT:steered together
3. All wheels driven together

(Note: You could end up driving diagonal pairs of wheels together, but I cannot think of a single arrangement where this has ANY advantages over another arrangement with a similar motor allocation...hence I omit it for readability)

All possible combinations can do strafing in any arbitrary axis (f/b, l/r, or anywhere in between).

Now let's play bingo!

A1: You can pull off any maneuver - you are holonomic (assuming unlimited pod rotation and unlimited rotation speed). The "Unicorn" drive.

A2: You incur more wheel scrub and would need to rapidly rotate individual pods 180 degrees during compound maneuvers (such as translate + rotate). Worst case is forward/backwards + rotate if the front/back pods are driven together, and strafe + rotate if the left/right pods are driven together.

A3: A lot of scrub any time you aren't driving in a straight line or turning in place, but you can still do some low-speed rotation while translating.

B1: You give up car/snake steering during strafes and perfect turn in place. You can still turn differentially while translating if the drive geometry supports it.

B2: If the same pair of wheels is driven and steered: You get a tank drive in one axis (f/b or l/r) and scrubby "snake" steering in the other (since the inside and outside wheels of the car are being driven at the same speed). If opposite pairs of wheels are driven and steered (example, steer f/b, drive l/r): You get great snake steering in one axis, almost no ability to turn while translating in the other, and scrubby turn in place.

B3: You can in theory get differential turn in place with scrub, and some measure of snake steering at shallow wheel angles.

C1: You get perfect turn in place, but can only rotate while translating via differential steering.

C2: You get perfect turn in place (through reversing motors), but no effective rotate while translating.

C3: You can't turn. Period.

D1: You turn just like a wide or long tank drive (and can switch between the two).

D2: You are a wide OR long tank drive that can also strafe (but cannot rotate while doing so).

D3: You can't rotate at all. Time to build a turret. See: 118, 2007-2008. 148, 2008.

In conclusion, arrangements A1, A2, B2, C1, C2, and D3 seem to be the most worthwhile to me...
A1 is the Unicorn...you get everything at a high motor/sensor cost.
A2 is still pretty versatile if you want to switch between single axis "car steer", rotationless strafing, and decent turn in place.
B2 removes 4 motors from A1 (2 from A2) and sacrifices scrubless turn in place.
C1 preserves quick and easy turn in place while only requiring two turning transmissions/sensors.
C2 is great if you never want to rotate + translate at all.
D3 theoretically can be done with only 2 motors...as long as you don't need to rotate.

Feel free to add to the notes! I'm just I've missed plenty of (counter) examples to my above generalizations.

Last edited by Jared Russell : 13-09-2012 at 18:23.
  #22   Spotlight this post!  
Unread 12-09-2012, 23:31
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Re: Swerve Drive Rotation Control

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Originally Posted by Jared341 View Post
B. Front/rear or left/right wheels driven together
steered?

Nice write-up.


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Unread 13-09-2012, 08:36
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Re: Swerve Drive Rotation Control

Our team has done 4 wheel independently drive, Independently steered, continuous rotation, co-axial swerve for the last 3 years. It has been quite an experience. So many little problems to solve. Do not underestimate the difficulty of getting a swerve to the point that it gives a competitive advantage. The risk of failure is very high. Our team after allot of debate decided that if we were going to take on the challenge and invest all the time effort and MONEY to develop swerve then we did not want a design with limitations. Thus we developed the unicorn version. Our first year we completed the mechanical but the software wasn't quite right. Our first regional was a little embarrassing. The robot had some moments when it had what I refer to as a grand mal seizure. The team called it the happy dance. We did solve the software issues for our next regional. And it performed well except we ran into the locking pin and Jag issue with the window motors we used for steering. From our first year the most important thing we learned was the difficult of repairing and servicing the system. While the wheel modules were independent, they were built into the frame. For our second year, We redesigned each wheel module to be total independent. If we have a problem We remove 4 bolts, disconnect 3 pairs of wires and remove the unit. Reverse the process with a pre-calibrated spare run a test and get on to the next match. With tying wheels together you are going to have a much more complex servicing task and you will most likely miss matches. It's a service nightmare in the pits. After our experience I would highly recommend a modular approach with independent modules. Take the time to perfect it.
One advantage with unicorn drive is that you can choose the location of the center point that the robot rotates around. In 2010 we had a mode were the robot rotated around the center point of the soccer ball making aiming easier. 2011 we used the same concept to rotate around the pole base to align the mini bot. Think carefully before you go down a swerve path with linked wheels. Team 1717 has posted their solution. This is a link to our solution.
http://wiki.team1640.com/index.php?t...II_Drive_Train
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