With the 2014 perimeter change from a fixed dimension to a perimeter specification a tribot is no long such a disadvantage. So we made a tribot swerve. Last meeting the mechanical students put the robot together and last night the programmers got code on the robot. Its crab only now. Need to work on the chassis rotation part. Here’s a video. https://www.youtube.com/watch?v=Cs8KWuuUWJY
So what do you think? What are the advantages and disadvantages of tribots? Are we foolish? Anyway I proud of the effort the students put into this. They really worked together well and accomplished this fast.
In my opinion all teams with a swerve history should be seriously considering this. 16 did it with success for a long time even before the new frame perimeter rules since 2013, and now I feel like there’s even less reason to go with 4 wheels. The flexibility in number of wheels and wheel placement inherent to swerve means you’re not limited to an approximately rectangular drive base (this can be stretched a la 971). I think one could find a lot of benefits to navigating the field using a frame more closely approximating a circle. Or you can go for a triangle to maximize stability. 3 is also a factor of 6, which happens to be the number of CIMs available for the last two years… (no such luck with 4 drive modules).
I’ve been entertaining lots of different possibilities for a drive and frame setup like this, considering the possible advantages. 2175 is in no position to go swerve, but different frame perimeter tweaks could still be possible with a WCD.
Well, based on the video, the tribot on swerve allows some very advanced maneuvering that could easily get around defense any day. However, defense wise I feel that a tribot would be a push over. Also, it does minimize the amount of space for a shooter, intake, etc. I could see this totally being awesome for a team like 1717 in the 2012 game, Rebound Rumble. With a 3 sided intake and an on top, rotatable shooter I think it would have been even better then it already was (which in my opinion is saying something). This year with a large game piece I could see it being slightly impractical, but in other seasons with smaller game pieces it could work. Teams would also need to focus on weight because an unbalanced weight on a tribot would definitely doom it when hit right. However, super cool video and thanks for sharing!
https://www.youtube.com/watch?v=vHe4MU3x3As I think there is always advantages to different types of drive trains each year. It’s just a matter of getting it to work. Your swerve iteration is coming along very well though.
When a 3 wheel drive robot locks up in a pushing match isn’t a large amount of the force on the single back wheel? In which case it could have significantly less pushing power than a 4 wheel swerve driven with the same number of CIMs per wheel. (I realize swerve robots generally don’t need to get into pushing matches, just curious as to how it would work)
Well, depending on how you’re angled, it would be ideal to have one wheel closer to the front for steering/direction instead of two, with the back two acting as speed and stability. With one wheel upfront you should be able to turn smoother/faster.
Well, having a 3 sided bot would expose more area for a defensive bot/offensive bot to hit the bot in the corner to knock a shot off target as a triangle is essentially based off of 3 corners, not 4 corners with “flat space” in between that would do very little when hit. In short, the tribot is more exposed when it comes to being hit effectively.
Looks like the side length would be ~37 in. for an equilateral triangle. This could make passing through a 36" door difficult if the bumpers are on (we have a hard time doing this with 28" robots with bumpers as is)
You have ~23% less total volume available to you with the triangle configuration than the square configuration (with maxed out dimensions and an equilateral triangle).
These two items would dissuade me from pushing it in most cases, but in a game where space constraints are not an issue and you need wide pickups, I could see this configuration being useful.
Again, three wheels doesn’t mean 3 sides. But for a demo there’s no real reason to build more structure than is required to mount the modules.
One thing to be aware of is that you DO have a smaller contact patch. So for years where your CG is higher you may want to avoid 3 wheel swerves. see BBS 2010, the last of their 3 wheel swerves. Was actually retrofited in the off season to have 4 modules.
The thing I found interesting was the post about how you want 1 steering wheel up front. Every 3 wheel bot I’ve seen that has a denoted front (meaning, not 148 in 2008) has had 2 steering wheels up front and a steering wheel in back (16 various years, 67 in 2005). Id be willing to bet that was a function of not wanting to intake over a module? Not sure.
This sort of depends on what you mean by “smaller.” Turns out the the forward back or side to side direction (versus diagonally) is actually less stable than any direction for 3 wheels. I crunched some numbers on this a bit ago: suppose you have [strike]a unit circle[/strike]circle of diameter 1 available as your frame perimeter, and you need to fit your contact points on this circle. To simplify things (and because it’s almost always at least approximately true), the contacts are evenly spaced around the circle. How many do you want? We start by quantifying “stability,” as you have, by the distance across the contact polygon in a given direction. In some directions this will be maximized (corner to corner) and in others it will be minimized (side to side). Below is a plot of these maximum and minimum moments for 3 wheels all the way up to 10 (could easily go higher, but honestly?).
The odd numbers have the smallest spread between max and min, so they are maybe more stable when looking at all directions combined. But the even numbers actually achieve the max in a particular direction, since you can actually go from corner to corner on a diameter. So while in certain directions, 4 wheels is less stable (side to side), in others it is the most stable (corner to corner). But when do you ever push or accelerate in the corner to corner direction?
Remember that this is comparing 3 vs. 4 vs 5 wheels (etc.) on the same circle (for the sake of having some sort of control). You could possibly get more out of a 3 wheel since you can make a triangle with longer legs instead of keeping the entire frame inside the circle, but then the shape of the frame is getting more awkward.
For fun: here’s the same figure, but this time instead of keeping the contact points on a circle of diameter 1 (with the idea of having a circular frame perimeter), we make the convex contact polygon have perimeter π (the perimeter of a circle of diameter 1). This way, the frame is no bigger than it needs to be to house the wheels, and gets the wheels as far apart as possible.
Edit: oops, my spreadsheet was sort of a mess, so there was an error in this calculation. I’ve updated the figure below.
Now that we are taking full advantage of the contact shape and getting the wheels further apart with the same frame perimeter, the 3 wheel configuration is actually better than the 4 wheel, with much lower spread between min and max, and much much higher minimum moment. (The max moment for 4 wheel is still the best, but is pretty unusual that anyone is ever tipping in that direction.) It actually gets worse as you approach a continuum of contact points all along a circle (n=infinity).
However, the circular (or near circular) frame would arguably be better than a triangle when you are worried about getting out of t-bones, pins, tight spots, etc., so the other figure should not be forgotten.
I know this is off topic, but look in the thread in the quote, watch the video first, then proceed to read the comments of the thread talk about how aggressive and uncalled-for the defense was, then fast forward to this year. Kinda funny I must say
IMO, a well-designed swerve drive will beat a well-designed mecanum drive. However, mecanum wheels have only been widely available for a few years in the FIRST community, so you have to give them some time.
The door point is what would get me. I never have even considered this as a factor in robot dimensions but if we can’t get it out of our work room then we will have a tough time competing…
AndyMark first introduced their mecanum wheels in 2006, iirc. It’s been quite a bit longer than a “few years” at this point. Granted, the first robot with steered wheel modules to win a championship was 67 in 2005, iirc.
