A self spinning gyroscope
I created Momentum as a combination of the knowledge that I had obtained throughout my masters program at Stanford. The combination of precision machining, dynamics, electrical design and mechanical design really appealed to me, and led me to the design I ended up with. I also greatly enjoyed working with brass, as its color is very evocative of clockwork and it is a pleasure to machine.
To make the rotor of the gyroscope also a driven motor, I embedded magnets in the rim to provide something I could drive with an electromagnet. The magnets were alternated to provide an oscillating magnetic field as the motor spun. This simple prototype was cut from acrylic, to verify the dimensions and coil design before locking in a design in brass. Because stall torque was not a critical parameter for the system, and there were very little mechanical losses in the system, motor design was greatly simplified.
Due to limited space in the final design, I tried my best to minimize the required components to drive the motor. It was a sensored, single phase brushless motor where a hall effect sensor was used to drive the base of a high current PNP bipolar junction transistor. The hall effect sensor provided the phase offset for the commutation by being out of phase with the drive coil . By using a BJT instead of a MOSFET, I was able to control the power dissipation in my hand wound stator coil much more easily. This allowed me to carefully calibrate how much power I could send to the motor while avoiding overheating the drive coil.
With the motor design verified, I moved on to machining all the individual parts of the gyroscope. Each part was machined from brass on a HAAS Officemill, with additional steps taken on manual machines, such as adding threaded holes for the cone bearings or spring pins which conducted current from the outer frame to the inner frame. Magnets were seated in the rim of the rotor using retaining Loctite, as was the shaft of the rotor. In addition I wound the drive coil on a lathe, to ensure tight consistent coils.
Before finishing or adding electronics, I put all the mechanical components together to verify that each of the bearing fits were working properly, the rotor was spinning freely, and that the inner frame would rotate in a compelling way when the outer frame was played with. Even under hand power, the gyro was rather compelling and served to quell any concerns I had about the design. With the design fully settled, I soldered the spring pins that formed the outer pivots to the outer frame and then began the process of sanding and buffing all of the brass elements.
To add contrast and highlight the beauty of the brass, I added a base made from oiled walnut that also housed the batteries. A switch with an on-off indicator was added, as the motor was not guaranteed to start and I wanted to ensure that the user would know whether it was on or off, even when not spinning. In the end, I was satisfied with the result of my little machine, as it had a lot of angular momentum, ran quietly, and was visually entrancing.