Cost of raw materials sans motor, wheels, controller, batteries.. |
Luckily, the quest for a fast thing didn't end there. Last summer, I wound up with one of these babies:
[Vrooming Intensifies] |
A kid's pocketbike complete with things like brake (singular), belt drive, chassis mounts for electronics, and bodywork ripe for plastering with obnoxious stickers.
Stock, they come with a 250W brushed DC motor and controller, and some SLAs. However, this was soon going to change.
Womp @_____@ |
Running the show is a 72V 100A Kelly sourced from no less than Dane.
But Noel, the motor has the shaft on the wrong end!
Have no fear, italicized text: we thought of that already. We have the technology.
In search of the one true shaft, I began by removing the end-bell shaft. Unfortunately, these motors had gone swimming once upon a time, and the four fasteners keeping the shaft attached had suffered from the softening.
Ripe for stripping |
Success! |
After the liberal application of WD-40, the shaft bolt was removed and I undid the motor mount-side screws keeping the can attached. The rotor and stator assemblies then happily came apart after some pulling. Undocumented is the use of the mill to pull things apart: I clamped the motor mount in the vise and used the drill chuck attachment to grab hold of the reinstalled end-bell shaft.
Afterwards, I took the rotor assembly, removed the end-bell shaft and the bolt securing the one-true shaft and clamped the can in the vise and pulled yet again.
Pliers unrelated |
Tape was applied to the magnets promptly after removal to prevent crap from accumulating in the air gap.
With the shaft extracted, I took some measurements and turned a new, longer one that would protrude out of the mount side. The taper was completely unnecessary and may as well just be a step to make machining easier.
Left: Original. Right: New and improved. |
The tolerances on the motor are such that the bearing interfaces with the shaft are slip fits, while the interface between can and shaft is a press fit.
Whew |
The motor mounting bracket was also made out of a piece of U-Channel left over from last season's car.
U-Channel evolved into ... angle bracket ಠ_ಠ |
Almost a thing |
1 rotation = 360 mechanical degrees = (360 * N pole pairs) electrical degrees.
For this motor, we get 10 times the number of electrical cycles per mechanical cycle.
Going further, we get a conversion factor of
0.1 mechanical degrees = 1 electrical degree
So for 120 degrees of electrical offset, we need the sensors to be mechanically offset by 12 degrees.
The idea of having the hall effect sensors mounted to a board was also attractive, so I went ahead and made a board in Circuitmaker in part to get some more experience in the Altium-like environment, but also because importing funny board shapes was especially convenient.
The design is a blatant knock-off of the boards Charles has made in the past with adjustments made to fit the larger can size and pole count of the Tiramisu and the addition of a JST-XH connector on the back. You can buy Charles' stuff here.
As a Circuitmaker project, all of the files are free to access in their community vault. Just search for "Tiramisu Hall Board" in the public projects bar.
IRL |
With board made, I made up a quick mount in Solidworks that I ended up printing and bolting to some holes I drilled in the motor mount.
Rawr |
Motor on a Bench |
Whirrrrrrrrr
>Looks at current draw
>Adjusts Encoder
Whirrrrrrrrr
...
Once the position was settled, I filed down some flats on the motor shaft, mounted the pulley and everything was dumped into the frame.
Next up: batteries and wiring!