I won’t be participating in VEX this year so I decided to go ahead and release an idea I’ve been cooking up these past two days. This is a rough (very rough) CAD model to show an idea, none of my thoughts have been tested, I don’t have stars, much of this will be hypothetical.
This robot is a dump bot, and is designed to clear a large portion of the field in one sweep, picking up anywhere from 6-9 stars (in theory). Think of a normal dump fork lift, then add two more fork lift trays that are operated by two independent motors, similar to this. The idea is that you can have the advantage of a ridiculously large intake area, while not sacrificing maneuverability because you can fold the two outside trays in actively.
I believe that it could work with just a shoulder joint that allows the robot to dump backwards, but I think the addition of a wrist joint, similar to this one, would allow the robot to dump in front of it, giving the stars the proper spin to roll forward.
The design is very reliant on light, effective, forklifts. I haven’t tested any, the bars you see in the pictures are just to represent a forklift. I think a possibility is poly carbonate strips orientated so that the thickest part is upright, aka it is easy to move left and right, but rigid up and down. Again no proof that it would work though.
I know it will be heavy (in theory it could be built pretty light), but fortunately we can use all of the stored energy we want in the forms of rubber bands. I’m a huge fan of counterweight, there’s no reason for the motors to have to do the work required to lift the actual arm itself when you can use rubber bands. So now all the arm is lifting is the weight of atleast 3 cubes, 9 stars, or some combination for those.
When it comes to how many motors will be required, we can break down the physics of it. Torque= force X distance, so the force of one star is mg, m=0.226796 kg, and g=9.81 m/s/s so the force due to gravity of one star is 2.22 N. For the distance, the arm pictured is 24" long, assuming it is the hypotenuse to a triangle with two other sides that are 17.5" long. In real life I would make it shorter than this. I’m going to assume that the center of mass of a full load of stars is the center of the center forklift . This gives us 24"+ (17.5/2)=32.75"=0.83185 meters. So the torque at the shoulder joint when the lift has 9 stars in it (I don’t think this would ever happen, but this is engineering) would be 0.83185 X (9 X 2.22)=16.4706 Nm.
The stall torque for standard internal geared 393 motors is 1.67 Nm. If there are four motors we have a stall torque of 1.67 X 4=6.68 Nm. Motor torque X gear ratio= output torque. If we gear that 3:1 there is now a stall torque of 20.4 Nm. I’m a big fan of over preparing, so lets go for a ratio of 5:1, giving us a stall torque of 33 Nm. Say we want our backwards dump to go back almost 180 degrees. The driven gear needs to complete half of a rotation, therefore the driving gears only need to complete 2.5 rotations. A standard 393 spins at 100 RPM, so 100/60=1.66 revolutions per second. Now 2.5/1.66=1.49 meaning it takes 1.5 seconds to flip our arm almost a full 180.
So with a 5:1 ratio, 4 motors, and some rubberbands, we have an arm that can lift twice the maximum expected capacity of stars (9), and can fully dump in 1.5 seconds. (I don’t think you’d want to flip a full 180 to dump but we’re talking worse case scenario here)
Now let’s talk about the impact on game strategy, and I’m not just talking about how well it could double as a wall/defense bot. For the purpose of this analysis I will assume that both alliances have one robot that high hang, and this robot is not the one that does it. (Not because it couldn’t in theory, I just want to focus on stars and cubes) I’m also going to ignore the auton bonus, but I will talk about the auton capabilities. There are 28 objects, 24 stars and 4 cubes. These are what I consider to be variable points, and have a max score of 64 if all are in the far zone.
The span of my cad model’s fully expanded forks is 50". The far zone is 34.625" long, and available half of the field is 64.75" long. It is reasonable to assume that this robot can clear all of the far zone, and most of the near zone. If it can pick up 4 stars from the far zone and 2 from the near (because most of it is in the far zone), that would mean taking away 10 points from their score. Since it is a dumping robot, lets play it safe and say all of the dumped stars will fall into our opponents near zone, scoring 6 points. So in one dump the game’s score has swung 16 points. 25% of the potential score. That one dump takes roughly 2 seconds to lift (which can be done while positioning, so I will include it in that time). Say it takes 10 seconds to drive across the field in one sweep, then 5 to line up. So 15 seconds for one cycle. 120/15=8 cycles. Let’s cut this number in half because we all know how competitions go, nothing is ideal. So 4 cycles in a match, each time shifting around 25% of the total possible score. There is a lot more to talk about with match logistics, its all pretty relative and so theoretical that it would be kinda pointless. I just wanted to convey that the large capacity can really swing matches. It’s 4 a.m at this point, I’m not sure how coherent this last paragraph even is.
The real beauty of this design is that driving from one side of the field to the other completely clears (in theory) the far zone, and part of the near. It’s not extremely difficult to drive, just tiny adjustments as you drive forward. The real challenge is dealing with your alliance partner. My solution to this is either starting infront of them, or stopping before them. Point is just communicate so that they know when you’re going to sweep, and they can get out of the way. The two extra trays could be hooked up to be independent of one another, allowing you to use two thirds of max capacity, but also allowing for more room for a partner. You can also fold in the sides after you scoop up stars so that it takes up less space and you can hold onto objects easily.
The other cool thing about the large size, is that it is very easy to knock off all of the stars on your fence section if you lined up with the middle. This makes autonomous pretty easy. All you have to do is drive forward, expand, knock off stars, then raise up your arm, making essentially a wall.
Here are some pictures of the cad. I realize that there is a collision in the front where the lift arm comes down, again this is just to show an idea. There would need to be something on the main arm to allow stars to roll down it so that they don’t fall into the robot when you go to dump backwards.
Again, I won’t build this robot, and I haven’t tested anything.
TLDR: This is a different take on the dump bot, one that allows larger capacity, but doesn’t sacrifice maneuverability.
