Escap MU915L motors

We just learned about the machining of the aluminum mounting block that holds the encoder wheel. The encoder wheel does not drive the robot; instead, two other wheels do. In order to make the wheels turn, the robot needs some motors.

Motors are so critical to a robot that the robot is often built around the motor. That is, the shape, size, shaft orientation, and electrical requirements of the motors often dictate the shape, size, orientation, and batteries of the robot. Afterthought Cake was no exception.

I was fortunate enough to purchase a handful of beautiful Portescap MU915L rectangular prism motors when they were available at Solarbotics. Early sources of these motors were the cassette tape reel turners of Sony Walkmans.

escap MU915L rectangular motors

escap MU915L rectangular motors

Besides their unusual shape, these M915 L61 205 362 gearmotors are:

Unfortunately, there is a significant issue with the offset output shaft that doesn’t become apparent until you try to use a pair of the motors back to back.

Difficulty aligning motor body when shaft is neither centered vertically or horizontally

Difficulty aligning motor body because the shaft is neither centered vertically or horizontally

Because the motor shaft is not in the center of the motor body, either vertically or horizontally, the two bodies don’t sit at the same height when arranged to drive a pair of wheels. One solution is to flip them end-to-end, which would be fine in a long robot that lays flat and drives the wheels in the center of the robot’s body. In contrast, Afterthought Cake stands up.

If you align the bodies as opposed to the shafts, then your robot will lean to one side. I suppose this is an opportunity to make your own wheels, and have one wheel slightly smaller than the other.

Instead, I decided to make slightly different mounting brackets.

Thin brass motor mounting plates

Thin brass motor mounting plates

The brackets are identical, except that one has screw holes on one side of the motor shaft, and the other has screw holes on the other side of the motor shaft. Because the distance from the aluminum block mounting holes and the motor shaft hole is constant between the brackets, the motor shaft will appear at the same height for both motors. The motor bodies are slightly offset from each other, which makes no difference.

Aligning motor shafts with slightly different mounts

Aligning motor shafts with slightly different mounts, ignoring motor body position

The brackets were a major pain to make, not so much due to the measuring, but because machining thin metal strips is difficult. Thin material is unnatural to hold in a vise. And, because of its light weight, thin metal also has a tendency to become a flying razor blade if not held securely.

Thin metals tend to bow and bend rather than drill and machine cleanly. Also, the burrs and warped exit holes contribute significant machining errors due to the sizes of the imperfections compared to the material thickness.

Perhaps my brass was the wrong alloy -- for engraving nameplates as opposed to high machinability. In any case, I absolutely have not perfected the art of machining thin material. You can tell that I gave up on trying to square up the edges or provide an even surface pattern.

So why bother making the motor mounting brackets so thin?

Motor mount made from thin brass sheet due to short shaft and nearby mounting holes

Motor mount made from thin brass sheet due to short shaft and nearby mounting holes

The shafts of the escap motors are only 3.7 mm in length, which is very short. It is even awkward to get a 2-56 setscrew in place.

In hindsight, it might have been easier to create a motor mount that surrounds the motor body. Or, possibly epoxy nuts or other threaded mounting material to the body. A precisely-machined indirectly-mated coupler with a key in the hole would be superior to a set screw, but would complicate machining and assembly.

Everything turned out fine for this robot. But, although I enjoy the beauty, craftsmanship, and other benefits of the square-edged escap gearmotors, it is a lot less work to mount cheaper motors.

Before examining the motor driver electronics, let’s take a close look at the physical connection between the motor and the wheel.