It appears as though Starstruck will be a game with a lot with of turning in it, so I was wondering what type of drive is most efficient at turning. I’ve heard that Tank drives are able to turn faster than X-Drives, but I was wondering if someone could explain to me why/if that is mathematically the case. And further, just in regards to angular motion, could an H-drive by considered equivalent to a Tank drive?
@logikai my team is using mecanum wheels so we will be able to go in all directions making it easier to obtain a object that is in a challing spot.
(forward, backward, diagonal, sideways.)
Well, I know that X-Drives have been experimentally proven to be faster than tank at going straight in any direction (don’t ask me for the math, it just works). As for turning, however, I would think maybe the larger turn radius or something makes a tank drive faster.
Having used an X drive last year (and the year before), I would say that holonomic capabilities often eliminate the need for turning entirely. While you would still need to turn occasionally, good driver practice will allow the driver to make subtle adjustments while driving, sometimes even going diagonally, and sometimes even turning while driving forward, all of which is both fun to do and more efficient (IMO).
As for H-drive/tank drive comparison, I would think (absolutely no experience with H here so might be wrong) that the two would be essentially the same. Assuming you have an omniwheel for the central wheel, you are basically just adding a point of rotation where the robot’s COG should be already, so I don’t think there will be much of an effect. However, I will say that personally I dislike the H-drive, simply because you have one motor wasted while driving forward, and 4 motors wasted while driving sideways. If you value holonomic capabilities, try an X drive; if you think you can get a lot of driver practice and not need holonomic, try a tank drive.
@Vex Dudes 8558B also mentioned the mecanum drive, and while theoretically that would be the absolute best, my biggest problem with that drive is the friction that leads to super slow speeds. If anything, the back and forth action of Starstruck requires speed and agility to effectively return points over and over again, and I just think that will be difficult with a mecanum drive.
The x drive is very good at turning, and turning is actually the best thing an x drive does. However, as you said, x drives can many times be quite slow at turning. In general, the wider your drive is, the slower it will turn, given the same gear ratios and wheel sizes.
If you were to make a tank drive with two powered traction wheels that is 16" wide, the turning radius would be roughly 6" (distance between the center of the two traction wheels divided by 2). However, if you were to make an x drive that is 16" wide, the turning radius would be roughly 7.5" (distance between the center of two opposing omni wheels [for example, front left and back right] divided by 2). So in general, when talking about similarly-sized drives, the x drive will turn more slowly. The important factor is the turning radius.
Think about how you would place a circle over your drive such that the circle is essentially the path that the wheels will follow when your robot is turning:
However, if you were to create a four wheel omni drive (which has become rather standard in the VEX Robotics Competition), there would be an even larger turning radius and the circle would look like this in comparison:
Another thing to note with this type of drive, notice that the wheels are no longer tangent to the circle. The rollers on the omni wheels will be utilized a lot while turning, which could mean you are losing efficiency when turning with this style of drive.
The smaller your turning radius is, the faster your drive will turn. Just know how to measure the turning radius of each drive type.
You also have to take friction into account, in this case, wheel scrub as the robot turns. With any kind of skid steer (that’s also known for the most part as tank drive) you will encounter more scrub or friction then with an X drive. Let’s look at those diagrams that @LegoMindstormsmaniac posted:
So looking at both the drive on the right, and the drive in the middle, these will both have low wheel scrub. This would be because the wheels in these drives are tangent to the turning radius/path. Now looking at the drive on the right, you’ll notice that the wheels are not tangent to the turning radius. This means that as the robot turns, the traction on the wheels will scrub against the ground some as the wheel does not spin exactly in the direction that it is actually traveling. This creates friction which of course slows things down.
While the drive on the right is considered a “tank drive”, it will turn extremely well because it only has 2 wheels, which results in both always being tangent to the turning radius. However a drive like this will have other setbacks. This type of drive can be seen a lot in battlebots for example (it tends to be popular as it takes up less of their weight limit, and allows for very fast maneuvers).
One thing VEX teams often do with their tank drives is they make some (or sometimes all) of the wheels omni-directional wheels. This greatly reduces the friction that occurs while turning, as the small idler wheels on them help to roll along the tangency on the turning radius. Of course, this isn’t perfect (you would need an infinite number of idler wheels around the outside diameter of the wheel to achieve that) but it drastically improves scrub. Also, even drive like the first two in the picture have some friction build up, as the wheel doesn’t have exactly one tangent point that contacts the ground in reality, there’s a small surface due to compression of the wheel, compression of the floor (in this case foam tile), etc. Using the omni-wheels however it does come at the cost of lack of traction for when you take a side hit for example.
@pwnageNick Did you mean left instead of right? Your logic was really sound but I think you were talking about the drive on the left with only two wheels, not the one on the right (or maybe the pictures are showing differently for you?)
During which part of my post?
I was a little confused because on my screen, the order of the pictures from left to right is: 2 wheel “tank”, 4 wheel X drive, 4 wheel tank. I am assuming that you were referring to the 2 wheel “tank” as having low wheel scrub and turning extremely well?
You are correct. I wrote that at like 3am so I was probably more tired then I should have been writing that post. Your interpretation is correct.
How does the geometry of an inline 6 wheel design where the outside four are omni wheels and the inside two are traction wheels?
Technically, if a Quad encoder or Gyroscope is used, every base could be the best at turning.