Previously, we examined the infrared encoder on a free-spinning wheel. That’s all well and good, but the encoder wheel and emitter/detector need to be attached to something solid in order to operate reliably.
I tried to eliminate having a body on this robot, but the encoder wheel and PCB required some sort of steady mount. The holder began as a block of Fortal aluminum where I roughly sketched the location of the axle.
Rough sizing encoder wheel holder on block of aluminum
After squaring up the piece, an axle hole was drilled straight through.
Drilling the axle hole
Next, a wide slot is milled out for bushings. The bushings prevent the sides of the wheel from rubbing against the aluminum block.
Milling slot for Lego hub and half bushings
Finally, the remainder of the wheel slot is cut all the way through the block. We'll see that in a minute.
Machining the wheel slot with a narrower end mill
In the previous photographs, you may have noticed that the outer corners of the aluminum block have been ground away. It is especially apparent in the photograph below. Although I apologize for the unevenness, the purpose is to avoid cutting myself on the jagged edges and to allow the block to rest flat in the vise.
The edges of any material tend to deform slightly more during machining and bounce back when the cutting tool passes. Between this phenomenon and burrs, had the sharp corners had been left intact, the imperfections would cause the piece to sit slightly crookedly on the tippy-toe edges, causing loss of precision. It is a minor distinction of an improving machinist.
Free spinning encoder wheel mount
Note the gray half bushings between the wheel and the aluminum block. As noted earlier, the bushings prevent the wheel from rubbing against the block. They also present a smaller surface area when contact is made -- reducing friction.
Although you can’t really see it in the photo, a gray cross axle has been inserted into the hole that was drilled in one of the initial steps. The axle holds the bushing and wheel in place.
A couple of holes are drilled and then tapped to make 4-40 screw threads. The pushbutton and encoder breadboard screws onto the aluminum block. The motherboard also screws onto the block, but dangles up higher using a solid 18 AWG wire.
The front stabilizer wire also attaches to the aluminum block. Even though aluminum is a lightweight metal, the block is still heavy enough to lean the robot onto the encoder wheel. Not only does this ensure reliable measurement of distance regardless of direction of travel, but prevents the robot from resting on the bendable front wire.
Lastly, the motors also mount onto the aluminum block (indirectly). It turns out there is an alignment problem with the specific motors chosen for this robot. This required a lot of thought and precise measurement to evenly attach the motors to the block, as you will see next.