Quote:
Originally Posted by Nate Laverdure
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.]
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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.