Hello!
My name is Daniel and I am new to the forums. I am the mentor/coach for team 323X (partner team of 323Z).
At our first competition (Jay County), we went 4-2-1. we were selected by 323Z, and won the competition. We are also second in Robot and Programming skills (both behind 323Z).
Have you guys tried hanging at all with your chain bar? We started the season with a chain bar design to try and reduce the weight but because the chain slipped so often when hanging we had to switch to a six bar.
Definitely not bad for a rookie team, especially a sister team of 323z
I like the clean construction, muscly drivetrain and intake. I understand the rationale between the fast lift system with elastic support. You understand the necessity of an autonomous program, and because of that, you can get a solid 10 point advantage, which is critical in these early to mid season game.
It’s a traditional, yet well executed robot
However, there are a few things I find interesting. You have an elevated drivetrain, and 3 wheels. That kinda takes the worst of both worlds (atleast, that’s what I think). Through what i’ve seen, it makes you slower to get over the bump, and it increases your center of gravity unnecessarily. I also noticed your intake motors are exposed. Though the intake up have their advantages with better grabbing position, you make the weakest part of the robot even weaker, making you vulnerable to defensive play. Inserting a standoff or coating the motor in non slip pad could add minor protection. I’ve never really thought about it much, but maybe it’s in your best idea to make your front wheels high traction as well. With your lift up often, and the addition of large ball intakes, hanging systems, etc, your robot’s center of gravity may move forward, meaning that your current layout would be less useful. From the video, it also looked like you had a single driver, like my team. If funding is a problem, then I suggest looking towards potentiometer programming or bumper switch programming to lighten the load on the single driver.
What went into this reasoning? Was this something you saw in your testing? My team is using a 6 wheel, 6 motor drive with the c channel frame elevated to about the same height (our wheels are in the second row from the bottom), and we have virtually no issues with CoG and go over the bump as quickly as any other robot I’ve seen. I think if done right, there should be no problem with their style of drive.
We have not tried to hang yet, though when we first built it someone grabbed the arm and it was able to lift itself an inch or two off the ground. We didn’t have any problems with the chain slipping when we did that.
We actually have two drivers. I’m sure in the next few weeks the motor placement on the intake will change to accommodate a big ball intake. This should protect it a little better.
I don’t believe that having a raised drive makes all that much of a difference, though it does change the CG slightly. Thanks for your input though!
you can see the 6 wheel drive always has a delay as the center wheel hits the bump. As for the 4 wheel drive, theres a delay when the robot doesn’t cross the bump straight. However, with the 6 wheel, drive, you can see that it never passes the bump diagonally. It’s forced to drive straight, then pass
at the start of robot skills, you can see a 2 delays when getting over the bump, the first from the back wheel, and one from the center wheel. this becomes more evident at 0:38
I believe DracoTheDragon was saying that having the axle go through the lowest hole in the C-channel raises the CG (you can see in the first picture), not the number of wheels. Correct me if I’m wrong.
Daniel’s interpretation is correct to an extent. Raising the drivetrain would be significant in changing the center of gravity compared to a lowered drivetrain, but center of gravity is only half of the larger picture: Tipping.
Technically though, adding anything to the robot would change the center of gravity. In this case, adding a 5th and 6th wheel would lower the center of gravity because it adds mass to the bottom of the robot. Since the center of gravity, or center of mass, is literally the average position of all the masses of a selected whole, adding more mass towards the bottom would thus decrease the average.
In a more mathematical parallel, it would be like averaging out numbers like [10,10, 40, 60, 100]. By adding a lot more low numbers such as 10, you decrease the overall average.
This is an oversimplification though. If the wheels weren’t directly under the center of gravity, say slightly to the left or to the right, the center of gravity would also move slightly to the left or to the right. This then becomes important with the polygon of support.
In my case however, I didn’t really consider the number of wheels in affecting the COG. But it shouldn’t be too significant in the grand scheme of things. After all, his drivetrain should be around 3 pounds, not considering motors, and his intake should counterbalance his tower, making his gravity well centered, but high. This may or may not be an important factor in push fights, probably during de-scoring large balls or hanging attempts
Don’t do this (never have 4 high traction wheels on your robot). It’ll turn extremely poorly. If you think about it, turning with 4 high traction wheels doesn’t make sense physically because traction wheels only go forwards and backwards (2 are ok because the centre of turning is always the midpoint of the two).
As said previously, this is a VERY bad idea. To extend off of that, be very conscious of where you place your 2 traction wheels. They should be placed as close to the center of rotation as possible so that you put the least amount of stress on your motors as possible. At worlds I had to switch to an all omni drive because I had problems turning with a heavy load (COG shifted forward, so the drive had to work harder to keep the original center of rotation), so learn from my mistake =P
Turning is affected by your points of contact and your center of gravity. I reasoned that in cases you would turn, your arm would be up, making your center of gravity slightly forward. The 2 major cases you would need turning is after scoring on a goal, and before/after you cross the bump. After these scenarios, your robots should already be in the direction you want to drive to.
If your center of gravity centered around the 4 wheels, then your turning scrub should be minor because your drive train is wide and your wheels are close together. In turn however, it increases your traction. This will be important when scoring bucky balls or manipulating large balls
Yes, theoretical physics says that friction is dependent on your coefficient of friction and your normal force. In turn this should indicate that the number of high traction is not proportional to your traction.
However let’s say a robot hits you slightly towards your back. This would be very common (what are the chances a robot will hit you 100% dead on?). this creates torque centered around your center wheels, and the only thing to oppose it is your front wheels. Well, if your front wheels are omni wheels, you’ll slide with no resistance. However, if you have high traction wheels, there will be resistance.
Now, the other half of traction is normal force. The main factor behind that is weight. People are worried about weight, but in years past, robots were a lot heavier.
In sack attack, robots usually lifted 5 pounds worth of sacks and battled over troughs. Their drivetrain had to push or drive over sacks without stalling out, further straining their speed ratios. Robots were 15 inches tall
In gateway, teams had full 18 inch robots geared for speed with only four 393 motors. Teams usually lifted 6 game objects (which were about 2 pounds) with a full 6 bar linkage. All of these were also done for a longer period of time
In round up, teams also had full 18 inch robots, fully loaded with 4-8 rings. Pneumatic systems were often used for hanging, goal stashing, and triggering other systems like de-scorers. Teams who attempted goal stashing had to lift 10 pounds worth of weight. All of these were also done for a longer period of time
In clean sweep, teams were battling to have the largest storage capacity. Not to mention, game objects were also half pound, quarter pound monsters and teams were backdriving their motors again and again just to get past an opposing alliance partner with a full load of gamepieces. All of these were also done for a longer period of time
In comparison, toss up forces you to build a smaller robot, with more power, and manipulate lighter game pieces in a shorter period of time . Especially with the rapid gameplay of this year, teams may only score bucky balls once and manipulate large balls for the rest of the match. True, Large balls, are almost a pound each, but the chances of manipulating 3 large balls at a time are very slim.
The times you actually need to turn in place, where turning is the hardest. In all other cases, resistance is much lower.
It may prove your point, but it doesn’t nullify mine. By no means am I asking them to make a version of acme’s drivetrain with full traction wheels to fully shut down their drive. I’m trying to express why increasing the resistance wisely is enough to increase performance, but not enough to damage
As Justin (UnforseenGamer) has said, the very REASON why you want traction wheels (ie. not to be pushed sideways) means that it will be IMPOSSIBLE to turn with four traction wheels (Ok, lets not speak in absolutes, it will be extremely difficult to turn resulting in a LOT of friction). If you don’t have time to test it on a robot, try drawing a picture, square chassis with 4 wheels. Stick a pin through the piece of paper (representing the centre of turning) and try rotate it. You will never be able to find a point on the robot where you can turn it without at least one pair of wheels sliding sideways, which they are designed to RESIST doing. The position of the arm is therefore irellevant, because there is no centre of turning which results in the wheels not scrubbing.
Any more than one pair of traction wheels will always have a negative impact on performance, never a positive one. Keep in mind that the positive impact from your front high traction wheels would become bigger as they move further forwards, because it becomes harder for someone to push you and turn you. The negative impact also increases as you move the front high traction wheels further forwards, because it becomes more difficult for you to turn when you WANT to.
Ok hold up. I’m not suggesting a square chassis, or a polygon of support. Nor does my suggestion get close to that scenario. I also agree that traction is a trade off factor. More traction means it’s harder to turn, but also increase defensive resistance. Less traction means it’s easier to turn, but also decreases defensive resistance.
With his current orientation, he has a rectangular 6 wheel drivetrain, wider than it is long. His back wheels are omni, eliminating a majority of friction. My assumption is that his center of gravity will be forward, minimizing the turning scrub between the front and middle wheels
With this scenario, his wheels do not produce perfect perpendicular force to turn. However, it is still very efficient. If you draw the vectors, you’ll see that there is some power decay, but this power isn’t crippling until it reaches a square. If the wheels have a slight scrub, it should increase the resistance noticeably without trading off too much mobility
I’m under the assumption that the trade off is not a 1 to 1 ratio
It’ll be better if he tests it for himself. It should be a fairly quick change: remove a few collars, swap the wheels, replace. If he doesn’t like it, then he can ignore my suggestion.
Draco, in Gateway, my team built a 6 traction wheel chassis. After the first day of worlds, we were in the hotel building a completely new chassis with omni wheels. We would stall every single time we tried to turn. Having an all-traction wheel chassis will NOT give you ANY improved mobility.
Ok, I miss-spoke when I said square, I should have said rectangle. At least we’re sort of on the same page now. I’m still under the impression that the trade-off here is an awful one. I would argue that the extra unneeded scrub doesn’t result in a the drive train being “very efficient”. I would also argue that there is VERY little added benefit from an extra pair of traction wheels. Is it really worth sacrificing ANY drive performance for the minimal advantages of an extra pair of traction wheels???
I don’t think so, but you are right in saying that it is better for him to test it himself.
