Butcher 3.0
- The weapon motor pulley constantly decoupled from the weapon motor, especially on big impacts
- The drive motors were too small of in-runners. They were too fast and the pinions on the motors regularly came loose on the shaft
First, for the weapon system I had two parts that I changed out to make this issue hopefully disappear. First the weapon motor pulley in previous versions had a plane bore that was held on to the can by some small set screws and eventually just epoxied on there when the set screws failed. The core issue is that shock loads would get transfered back to the weapon motor from the disc when it hit something. The weapon motor in Butcher is big enough (4250 brushless outrunner) that it needs a secure attachment to make sure this joint doesn't fail. To combat this, I had a custom aluminum pulley CNC machined to perfectly fit the motor. You can see in the picutre below that there is not only a hole pattern on the top face of the pulley, to interface with the matching bolt pattern on the top face of the motor, but there is also a set of 5 bosses that are machined on the inside of that top face. These bosses slide into the cutouts in the top of the motor with a slip fit. These are designed to contact the machined body of the motor before the pulley can rotate enough to put this force on the tiny M2 screws in the face of the motor. The screws should be enough to hold the pulley onto the motor, and the machined bosses can take the shear loads from the sudden impacts meaning hopefully this pulley will never come apart from the motor.
A less critical but way cooler change I made for this version of the weapon system as well is the new disc I designed. Again this is detailed in the design post linked above, but instead of the large single tooth disc that I have been running, I created a 3 tooth disc of roughly the same weight that looks like a set of spinning butcher's knives as an ode to the name of the robot. This disc not only looks way cooler, it will be nice to have as a secondary option instead of the single tooth disc. In theory this will be able to deliver smaller hits while continuing to spin as well as spin up faster given the smaller MOI. Both of these traits will be really useful in fights against some of the tank like wedge robots that are commonly found in the hobby weight class.
The drivetrain upgrades started with the motors. I switched to the Turnigy Aerodrive SK3 3536 1050kV motors. These are lower kV, higher power, higher torque motors that should help with the drive issues I'd been having. They also have a 5mm output shaft that should help keep the pinion gears more securely attached. The one downside to these motors however is the fact that as outrunners, the can of the motor is spinning at a very high rpm and can wear down anything touching it, including key electrical wires that would cause you to lose a fight. To protect these cans I developed some 3D printed wire guards that bolted onto the frame that surrounded the motors entirely. I wanted to continue supporting the rear of these motors like I had in previous versions of this design as I had had the problem of the motors coming loose of the gearbox in the past and wanted to make this drivetrain bulletproof. Because these are outrunners now, that meant adding bearings into the motor guards that slipped onto the nub on the end of the brushless motors to support them and avoid this issue. I took a similar approach to the output shaft of the gearboxes by adding a bearing into the wheel guards on the other side of the wheel. I haven't done this in the past due to weight, but with some of the recent upgrades, I made weight for some of these more experimental design elements. I'm interested to see how it works out. I should prevent the gear boxes from seeing big shock loads from being thrown around the arena, but might over-constrain the output shaft if it gets warped and might cause a different failure in practice.
Drop Test 2.1
I have since bought my own 3D printer and re designed Drop Test to be more printable. You can see a PLA mockup of the new design below that shows the 4 independent frame rails that are much more printable, even without supports, and make assembly and maintenance way easier.
As you might be able to tell from my bitter retrospective tone, this didn't turn out that well for me. This project throughout its lifetime has been the most complex combat robot I've made to date. And as such, requires much more focused attention to detail than any of my other projects. Given that combat robotics is just a fun hobby for me, I don't always put the same effort into details as I would for my job. I also don't spend nearly as much time on it as I would for more professional projects, maybe a few hours a week. All of this meant two things:
- This build still had a lot of small oversights in the design mechanically that had to be fixed
- This project took a lot longer than I expected. Even more so because of point 1
All that being said, I did make some good progress on this design, it just didn't come to completion. The biggest win in my opinion is the fact that I got the motor driver working as intended. I was unable to get a good video of it as the belt that connects the motor to the trigger was too loose and couldn't handle the loads required. As such, the video below shows the auto firing of the mechanism by way of my exciting the triggering pin on the controller by hand to simulate the PWM input from my transmitter. You can hear the belt skipping in the video, but it does work.