Due to Monday being Martin Luther King Jr day and the team not having school, we decided to have a longer work day to help make progress toward our Alpha robot. We started the morning but splitting the students into groups, one group of students making the driverails, another group of students making the bellypan, and another group of students preparing the electronics to go onto the bellyplan. Once the bellypan and driverails were made, we assembled our very first chassis of 2023.
After we mounted all of those together, we then added the arm to this assembly. It did require some disassembly and reassembly due to having to drill all the bolt holes by hand but we did this not because we wanted to finish the job but because we wanted to finish it correctly.
Then after all these were put together, we printed some 1:1 drawings of sections of the electronic mounting which helped ensure all of the holes on the electronic lined up so it can be clamped down securely. The image below shows all of this put together and as seen next to the cone, this robot is very small. (The battery isn’t in the correct state right now because the crossbar was mounted incorrectly but essentially the battery would lay horizontally flat against the bellypan in that position)
Here is a video of one of our beloved mentors showing the tipping point of the bot in its current state. Currently, it is missing a battery and a claw and our bellypan would be made out of aluminum so our actual tipping point won’t match 100%.
Having our tipping point at such a steep angle and not falling over is very helpful because when the bot is on the field we know that tipping won’t be as much of a problem for us. We aren’t saying it won’t be a problem, it will just be less of a problem than previous years.
Sorry for the late response, but as Brendan mentioned the documentation of this wasn’t the best initially when prototyping but we ended up finally getting a video today to help show the intake grabbing both game pieces.
So out of curiosity what kind of ratios are you running on that first joint on the arm and the second? Our JVN calculator is showing about 250-1 to keep the amp load under 30 amps with two falcons. Are you guys seeing the same thing? Thanks!!!
On the current configuration of our VEX arm, we are using a 200:1 ratio on both of the joints. For the second version of the arm, we plan to run either a 200 or 250:1 depending on what we see as more beneficial. The JVN calculator parameters we used are shown here to help compare with what yours,
When you extend to reach the top row nodes, wouldn’t the arm length for the lower joint be much longer than 30 inches? A single Falcon at 200:1 will very likely move the lower joint when making that max reach, but seems unlikely to do it at 15A.
You may wind up with a serious contact stress where the round tube contacts the angled charger plate. The line-to-circle contact isn’t super bad, but it’s not ideal. A square tube or a flat striker feature (like a 3D printed doober) will relieve that extra stress.
And/or amazon for less-expensive CF tubes.
How does the combined CG of the two robots affect the charger’s ability to be balanced?
I tend to be very very conservative on the JVN calc. I don’t want to be under powered on our PID. Plus I’m bouncing in my head without knowing for sure, should I be putting in the distance of the arm CG or the end of the arm? (This is end of the arm to be conservative)
An assumption we’ve made regarding these ratios - the arm length is set to 30 because we’re assuming the CoM of the links is approximately 30" from the driven pivot at worst case scenario.
Here’s the first joint CoM in our CAD:
Now the center of mass will change positions as the arms move, but we need to gear for the worst-case scenario, hence the 200:1.
Currently the whole arm weights ~18 lbs in CAD, so the second joint is geared very conservatively and the first joint is geared somewhat conservatively. But despite being geared so conservatively, the time to move 90 degrees is still about half a second.
We’ll see if we need the arms to move any faster but right now we think that’s plenty fast.
Your force diagram is incorrect. There are a few different ways to think about the system. One would be similar the way you showed it, with a pivot in the middle, and both robots at the ends. However, both robots would apply a downward force (In equilibrium, the rod is not rotating, therefore the net moment must be zero). Also, the force that the other robot applies is not 120lbs, if it was then it would be being lifted, this is actually 90lbs for your model
The other way to think about it is from the rod’s perspective, in which it is fixed at one end (to the robot), and then you have the “pivot point” applying a large force upwards, and the other robot at the other end weighing it down. In your model, the pivot would apply an upwards force of 210lbs, and the other robot would still apply a downward force of 90lbs. The diagram left out that it is Fixed on the left side.
Because the force diagram is actually a little more complicated, there is not an entirely obvious maximum bending stress location along the beam. Using a beam calculator (SkyCiv Platform), and the forces in the diagram above, the maximum bending stress is 120 ft-lbs, or 1440 in-lbs.
This should probably be the first point, because it is way more impactful than either of the other points. You used the diameter instead of the radius in the moment of inertia calculation (and I believe you added a 2 to the numerator, c should just be r)
If we include all of these changes (M_max = 1440 in-lbs, c=r, radius in moment of inertia), then we arrive at 44 ksi. This is less than half the listed strength of the carbon. If you added a second rod, this number would effectively cut in half, to 22 ksi, or about a 4x safety factor.
If we use a slightly less idealized version, with 150lb robots and a slightly longer rod (and the actual dimensions from McMaster), I get a stress of 85 ksi (so 42.5 ksi/tube). This is about 50% of the strength, which seems fairly reasonable to me. Could increase the wall thickness if you feel like it, up to you.
One thing I have heard is that the compressive strength of Carbon Fiber is somehwere around 20% lower than the tensile strength, which means the bottom could fail early, decreasing the ultimate bending stress it can take by 20%. Not sure how accurate this is though.
Also, think about what could happen if the robot you are under moves, or you move your arm, or the charge station tilts. Or, if you climb quickly. During these kinds of dynamic situations, loads can be much higher, and could be concentrated on one of the two rods. Heck, if one of them catches on a different part of the robot you are climbing under, then the forces are no longer evenly distributed (or you tilt a little).
Oh my goodness the r-for-D swap is a pretty silly mistake. I also wasn’t sure if c was r or D in this case, thanks.
Part of me is pretty embarrassed to have put this out and to have been so wrong, but also if I never put it out, I wouldn’t have known I was wrong!
Jacob clearly has a better background in beam bending and strength of materials (or at least has had occasion to use it since graduating , so hopefully this is educational to people, even if the initial post might have been wrong.
After a fast paced sprint of week 1 of build season, week 2 felt more like a casual walk. Our students had midterms, and while plenty got done, it was smaller or at least less flashy items being worked on.
Our first iteration of the end effector is solid, but could be better in a number of ways. It’s going to be a long season of claw iterations, trying to optimize the claw to be touch-it-own-it with game pieces from the shelf, and eventually the floor.
We’ve designed a few different claws to hopefully manufacture and try out this weekend. They all share a similar mounting pattern so they can all mount to the same pneumatic wrist.
Spinny No Pinchy
Single motor drives both sides of the claw. Inner rollers for cones, outer rollers for cubes.
Pinchy & Spinny
Single Motor drives both sides, pneumatics open and close the claw.
Single motor drives both rollers for cones, only bottom roller contacts cubes and pulls them onto lexan