So, last time I had a functioning gear-train, I just needed to disassemble everything in order to replace the outer housing. As I was putting things together, I realized that several custom fixtures would likely be needed in order to disassemble various parts cleanly. Here, I made a giant 3D printed cylinder that the front housing and planet output would bolt to, and then the central shaft could be pressed out.
Last time I covered getting to the point of having the rotor installed into the gearbox. Here we’ll look at making it actually work in that configuration.
When I first got the rotor in place, it was clearly not centered properly. Although much closer than in the plastic gearbox, it did interfere with the stator during a portion of a revolution. The first obvious problem was that the primary shaft wasn’t making it all the way through the front shaft bearing. That should have been an easy fix, but for two different very annoying reasons.
Shortly after receiving the sun gear holders, I received the first iterations of the planet output and front housing.
20x of the planet output
20x front housing
Both of these seem to have actually adhered to the tolerances I requested, so thankfully it won’t be too hard to fit everything together. However, getting everything together for the first time did involve a comedy of errors – a lack of planning for assembly order, a lack of foresight into how things would be *dis-assembled*, a stubbornly stuck shaft, and plenty of broken parts.
As seen in my draft plastic assembly, the required alignment between the rotor and stator in the gearbox is relatively tight. The difference in diameter between the inner race of the rotor and the outer surface of the stator is only about 0.2mm, which gives 0.1mm of clearance in normal operating conditions. A plastic drive train was never terribly likely to succeed. My next steps have been to machine the pieces of the gearbox critical to alignment out of aluminum, so as to ensure that the rotor and stator, (and also the gears) are held within some approximation of appropriate tolerances. The path of joints between the rotor and stator looks roughly like this:
As mentioned last time, I’m working on a parallel track to accelerate my quadruped development efforts. The current plan is to try and use a BE8108 class brushless motor, with a planetary geartrain mounted mostly inside the existing bounds of the motor.
Here’s a rough exploded view of the CAD model:
Key takeaways are:
So, after applying power to the robot for the first time, I coded up some simple scripted maneuvers I was going to use to work up to a gimmick jump video. Unfortunately, I discovered that one of my assumptions was not well founded, and some more work will be necessary.
I started in on this project intending to create a semi-standard servo motor with integrated gearbox which could be used for all the joints. The brushless motors I am dealing with are only just barely capable of their task without additional gearing. Along that development path, I built some prototype integrated gearboxes for a 50 sized brushless motor, and even took some videos of it jumping.
After getting the whole robot assembled, I quickly realized that changing the firmware on each of the 12 servos was going to be a big annoyance. Doubly so, because the lateral servo programming ports were unreachable with this chassis design without disassembly. Thus, I bumped up a deferred piece of work to implement a bootloader that would allow for reflashing the primary application over the RS485 communication bus.
The moteus controller currently uses an STM32F446 controller, which has 512kb of flash memory. The memory map pre-bootloader looked like:
After getting everything wired up and mechanically assembled, next was zeroing all the encoders and doing the first position control. I discovered a number of problems that turned up by having a full 12 servos on the bus at the same time, which were resolved pretty easily, as well as many more pain points which I’ll need to address.
But, here is the pretty video (it ends when I push down enough to over-current my lab supply):
Once all the individual legs were assembled, I needed to wire them up into the chassis. Part of that was building the necessary data harnesses and actually running the power wires from the junction board.
Next up is getting everything talking.
I am a big fan of MacroFab. They’ve built a PCB + assembly + more service that is transparent, high quality, and nearly completely self service. They appear to be making money, so hopefully they will stay in business for some time.
On top of that, they offer a “quick turn” option which gives you populated boards shipped 10 business days after you order them (and I’ve even had them ship out a few days early from time to time)! The only annoyance is that the quick turn option is limited, as I’ve mentioned before, to boards that meet certain criteria, among them having 20 or fewer items on the bill of materials. To try and get this first quadruped prototype up and running quickly, I’ve been exclusively relying on quick turn boards, which means making some compromises. Even after some moderate design sacrifices, I haven’t been able to get the servo controller board to 20 parts. At the moment it is 23. Thus, when I received the first big-ish PCB order I’ve made (qty 28), I got to spend a morning populating the remaining 3 components on all 28 boards.
