Looking at the failure, I was surprised I used so little material in the region in question. For now, I just made it 4x thicker and we’ll see how long that lasts, although ultimately it may need to be a different design or machined instead of 3d printed.
Another of the tasks I’ve set for myself with regards to future Mech Warfare competitions is redesigning the turret. The previous turret I built had some novel technical features, such as active inertial gimbal stabilization and automatic optical target tracking, however it had some problems too. The biggest one for my purposes now, was that it still used the old RS485 based protocol and not the new CAN-FD based one. Second, the turret had some dynamic stability and rigidity issues. The magazine consisted of an aluminum tube sticking out of the top which made the entire thing very top heavy. The 3d printed fork is the same I one I had made at Shapeways 5 years ago. It is amazingly flexible in the lateral direction, which results in a lot of undesired oscillation if the base platform isn’t perfectly stable. I’ve learned a lot about 3d printing and mechanical design in the meantime (but of course still have a seemingly infinite amount more to learn!) and think I can do better. Finally, cable management between the top and bottom was always challenging. You want to have a large range of motion, but keeping power and data flowing between the two rotating sections was never easy.
My concept with this redesign is twofold, first make the turret be basically an entirely separate robot with no wires connecting it to the main robot and second, try to use as many of the components from the quad A1 as I could to demonstrate their, well, flexibility. Thus, this turret will have a separate battery, power distribution board, raspberry pi, pi3 hat, and a moteus controller for each axis of motion. These are certainly overkill, but hey, the quad A1 can carry a lot of weight.
The unique bits will be a standalone FPV camera, another camera attached to the raspberry PI for target tracking, a targeting laser, and the AEG mechanism, including a new board to manage the firing and loading functions.
When I first designed the full rotation leg, I didn’t fully appreciate the importance of torque in the knee joint. Despite the fact that my first force based IK showed that when the legs are immediately under the body, the knee joint carries the entire load of the robot, I still managed to not add any reduction there.
The initial design used a 1:1 ratio, because that allowed me to use the same single piece 3d printed gear design I had used before. A 28 tooth gear with 5mm pitch resulted in a gear that was larger than the output plate on the qdd100 servo, so it could just be bolted directly on. To work with a smaller number of teeth, I had to split the gear into two parts, connected by pins, as the gear is now smaller than the qdd100 output plate.
So that I could use the same belts, I extended the upper leg about 8mm, and while I was at it, extended the lower leg by 15mm to make the overall leg a bit more symmetric.
We’ll see shortly how this works out when printed and assembled.
Previously, I described the overall plan for my improved foot. To make that work, I needed to cast a 3d printed part into the squash ball such that it would likely stay attached during operation, be suitable rigid and yet damped, and do so repeatably.
To start with, I used a random single yellow dot squash ball with a hole cut in one side using a pair of side cutters. For the casting foam, I just used Smooth-On Flex Foam-IT 17, which is what Ben Katz originally used at least. Initially I just mixed up a batch, poured it in to a random level, stuck my bracket in and hoped for the best.
Well, something sure happened! But not exactly what I wanted. The foam didn’t fill in the interior cavity, nor make a great connection overall with the bracket. On top of that, the process wouldn’t exactly be described as “repeatable”. Since I just eyeballed the level of foam, there was no way to get the same amount in.
For my next runs, I decided to do everything by weight. I tried a few different foam masses, curing orientations, and venting strategies. Eventually, I got something that seems to look pretty good. We’ll see how well it works on the actual machine shortly!
Here’s a bunch of different intermediate attempts:
And here’s a cutaway of the process I’ve settled on for now. This particular one has a slight bit of overfill on one edge that is more than is typical, but the inside fill is pretty good:
As mentioned long ago in my post on failing more gracefully, it was obvious I wanted to strengthen the lower leg and foot mechanism to remove the point of failure observed there. For now, I’m attempting to basically copy the original Mini-Cheetah foot principle, although with more 3d printing and less machining.
The basic idea is to print the entire lower leg in a single go laying on its side, so that delamination is unlikely. The foot bracket will be cast into a squash ball, then epoxied onto the lower leg.
One of the parts on the original quad A0’s leg that was prone to failure was the “knee stud”, a little cylinder that acted as the mating interface between the upper leg and the lower leg. It directly attaches to the upper leg, and has bearings that ride between it and the lower leg. The entire tension of the leg belt is born in shear by this part.
In the mk1 leg, this part was 3d printed with heat set inserts used to form the threaded holes. This mostly worked, although occasionally the stud could shear along the 3d printed lamination lines. Thus, for the mk2 leg, I’m making this part out of 6061.
The first op takes a 0.875 inch cylinder, and does all the work on one of the sides. That includes roughing it down to length, getting the outer diameter that the bearing rests on accurate, and drilling and threading the holes.
At that point, the part is turned over and bolted into a 3d printed fixture.
Then, all the tool paths are repeated on the other side, as well as the middle being cut away. I didn’t really worry about surface finish on the middle section, since it will never be seen. This of course would be much easier on a CNC lathe with live tooling, but hey, you use what you’ve got!
To get ready for initial limited production of the fdcanusb I wanted to make some kind of enclosure so that you didn’t have to just grab the raw PCB and risk ESD failures. I also wanted to be able to expose the status LEDs without having to do a window or anything else with multiple materials.
So for now, I just used a translucent PETG print, with light pipes and a thin wall above each of the LEDs. The result isn’t too bad, you can clearly see the status LEDs and it feels plenty rugged for desk work.