I’m a sophomore at MIT who joined PSS as an extern over Independent Activities Period (IAP). Free to choose how to spend the month of January, students can take an extended vacation, attend short, intensive classes, do research in MIT’s various labs, etc. Many like myself choose to participate in short internships with MIT alumni – the correct lingo for this type of job experience is “externship”.
I was assigned the task of 3D modeling a reaction wheel for a 25 kg satellite. Essentially, the wheel controls the orientation of the satellite in space. Comprised of a small axial flux motor and a flywheel for added inertia, the wheel sits at 40 mm tall and 80 mm wide. It must spin in both directions, and meet tight dimensional constraints. I believed I really had my work cut out for me.
Embarrassingly enough, I came into this job having absolutely no idea how a motor worked. Yes, I study mechanical engineering at MIT; no, I do not know everything there is to know about machines. So my first job was to learn how motors function, specifically the axial flux motor I’d be designing. Surprisingly, it didn’t take very long at all!
The following day, I began modeling components (the stator, rotor, flywheel, supports, base, etc) in Autodesk Inventor. The initial designs took longer to complete, since I had never used Autodesk before. More difficult than the modeling, designing the actual motor gave me the most trouble. Should I make the axle stationary and bind the stator to it? Should I make the axle rotate and attach the stator to external stationary supports? Where/how will the flywheel attach, and how will the entire spinning device sit on a stationary surface? Where exactly do bearings go? Which design will allow for ease of construction and assembly?
After much internal debate, conversations with my colleagues, and several CAD iterations, I finally decided on a design: a rotating axle, bound to two rotors and the flywheel via supports. The stator sits between the two rotors, and the entire armature rests on a base; bearings separate both the stationary base and the stator from the rotating axle.
The team decided that perhaps they wanted to try different coil windings on the stator after the wheel had already been assembled, which presented me with my next challenge: making the wheel disassemble-able. We came up with many ideas – a slotted axle that just comes apart, a threaded axle for pieces to screw onto and off of, angle-brackets connecting the rotors to the axle, the works! Well, I guess the opposite of “works”. While modeling each design in Autodesk, major flaws with each idea eventually emerged.
After scratching my head for a week, I finally came up with a solution: a removable flywheel, and a shouldered axle that the rotors can be screwed into! This mechanism allows for the removal of the flywheel and top rotor, exposing the stator coils. The motor can still spin both ways, since the rotors do not screw in along the axis of rotation. Also, the shoulder and screw holes both space and align the rotors correctly onto the axle! I had struck engineering gold!
The last big hurdle I had to jump over was finding space for an angle encoder. Not realizing how large commercial encoders can be, I had to slightly redesign the reaction wheel to fit such a device. My co-worker Gary found an angle encoder that met our tight dimensional constraints, however, the hub for the rotary scale still did not fit. So, I designed a custom hub meeting our constraints.
All in all, this externship has been a great experience. I learned much about CAD and motor design. I also had the chance to meet a great group of engineers and scientists! Many thanks to PSS for giving me this opportunity. This is one IAP I will never forget.