Chet's Robot Reveal- CAD

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.
Step 2.png
Dumping forward.png

This is easily a great design. When I read it, I thought: “This is a good thought in theory, but the elaborate design makes it very hard to account for your alliance in autonomous.” Then I realized that that could all be fixed with different autonomous routines using 2/3 or all of the prongs.

The design is really good, but doesn’t seem too hard to build (at first glance), which is making me think that this is one of the designs that get’s copied. However, I still want to see someone prototype it.

While I think it’s a good idea, I believe there are more factors to consider in this design to make it completely viable. I think that the arm should be reinforced in order to avoid torsion. If you happen to have more stars/cubes on one side of the intake than the other side, the arm holding the intake will be vulnerable to torsion. It could work better if you had two arms (or maybe a single wide strong arm) holding the intake instead of one. I know it’s just an idea what you’re demonstrating but I think it’s nice to consider other factors in case someone goes with that design route. Just my opinion. :smiley:

That is a good point, if there was an active wrist on it I would probably make a pretty sturdy four bar. I really haven’t had time to CAD it as I would build it, there are a lot of specifics left to consider, but I think it would be pretty simple to build. You’re just making a normal dump bot, the only difference is a wrist like this, and the expandable tray. You could even make it without the wrist. So take a normal dump bot and add two trays that can fold out and back in. Pretty easy to make.

This is one of the ideas we considered way back in June when we were getting started building. We decided to go for something we felt was simpler and easier, and scrapped the idea. Now, with experience from a competition, I feel that there are a couple specific reasons why a design like that would be pretty hard to pull off well.

  1. Friction. I guess this problem alone is what causes all your difficulties. The friction of the stars and cubes against the ground is really high. The obvious solution to this would be to have the forklift slightly above the ground to avoid friction, but this contributes to problem #2

  2. Stars against fence/middle of the field. With a forklift, robots are roughly 2.5-3 feet long. This means that they can basically only intake well if the scoring objects are along the field perimeter, away from the fence. Having seen a full field with 4 robots all moving about at our recent competition, I can tell you for a fact that the majority of stars will not be in this ideal position; rather, they’ll all be along the fence side rather than the perimeter. Intaking against the fence (at least for us, maybe enough driver practice solves this :P) is pretty hard, because the forklift prongs keep bumping into the fence.

I think it’s really easy to underestimate the crowdedness of the objects and robots on a small half field (I know because I’ve done it myself :P). I am not sure this design would be optimal in a competition setting, but as always, testing would be necessary. Maybe this thread will inspire some team and we can see the triple forklift in action :slight_smile:

Why not instead expand the tray further to hold over half of the points in stars/cubes. Then dump them at the end of the match?

I think a tray this size would need to be much larger, while I’m sure it’s possible to do what you are talking about, the intent of this design is to be simple enough for most any team to add on. It’s also based off a strategy of constantly overwhelming the other team through out the whole match.

As far as friction goes, I wasn’t able to find much about foam on foam coefficients of friction, I’m sure a deeper search will yield better results. Ideally, the outer two trays would fold in when holding stars, and lift up slightly. When it comes to driving and crowding, I must admit I have not witnessed or experienced an actual match, let alone field. I think in my head what makes this not an issue is that the tray can retract into the same size as a normal forklift quickly. Sure if you left them out the whole time it would get bad, but a well practiced driver could turn it into a very fluid and dynamic system with the whole tray out only when needed. I don’t know what the solution for stars clumping to one area is, but I think it would be the same as any other forklift’s due to its ability to retract to normal size.

Was on the YouTube and I found this…

Kinda similar to what I had in mind, I liked the way they articulated the wrist a lot. To me it wouldn’t be worth the difficulties driving if you couldn’t reduce the size to make it easier. Good find though.

I was thinking about the same video as soon as I saw the CAD Design :smiley: