ME 14 is the one-term introductory hands-on design course offered by Caltech's Mechanical Engineering Department. It involves two team projects: a transmission design contest that kicks off the course, followed by a mechatronics design project of the team's choosing. My team of four built a two-stage spur gear transmission for the first project, and "Sunflower" --- a solar panel that follows the sun --- for the second project.
Transmission Contest
The ME 14 transmission contest involves building a transmission to transfer the rotation of a motor to a bicycle wheel, with competition points awarded based on both initial acceleration and peak speed.
Choosing the optimal gear ratio therefore required an estimation of the system's efficiency; we estimated a 0.75-0.8 efficiency, which made the optimal gear ratio, given motor specs and scoring criteria, between 6.65 and 6.7. We chose a ratio of 6.76 accomplished in two stages of spur gears (chosen for their ease of implementation, minimal space usage, and low cost).
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Faced with a choice between bushings and ball bearings, we chose the slightly pricier but more forgiving ball bearings to hold our D-shafts.
Each team was provided with a 6" x 24" sheet of 1/2" acrylic out of which to machine the transmission frame. The diagram above plans efficient usage of the material for our needs, and the assembled transmission is shown in the CAD screenshot.
Note the carefully milled slots in the acrylic base, and the identical slotted "cap" above the vertical plates giving the system extra rigidity. This rigidity served the system well
--- the transfer of rotation was very smooth.
a sun-tracking solar panel
Sunflower is a solar panel system that locates the optimal orientation to receive the most sunlight (pictured right). It moves in two degrees of freedom: a servo controls the panel's angle of inclination, and a stepper motor rotates the system's base. Electronics are controlled via an Arduino. The program scans the area for maximum sunlight, one rotational axis at a time, and then periodically adjusts to accommodate changing light conditions.
Solar Panel Inclination
Solar panel (5W 12V) reports the strength of sunlight it receives to the system to inform how it might best be positioned.
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½” thick aluminum posts mount to the rotating base. Posts hold bearings (¼” ID, â…œ” OD) for the D-shaft; one post holds the servo in a rectangular hole. Their tops are rounded so the solar panel, no matter its inclination, may rest upon them.
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Spur gears (20 degree pressure angle, 48 pitch, 60 teeth) connect the rotation of the servo and the D-shaft attached to the panel with a 1:1 ratio.
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DS3218 Servo precisely controls solar panel tilt over a large range of motion.
Base Rotation
Turntable (9.12" OD) allows Sunflower to smoothly rotate with ball bearings, and 3D-printed side pieces hide the electronics underneath. A slip ring prevents entanglement of the wires above.
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Internal/external gears waterjet from aluminum use a 3:1 gear ratio to transfer rotation from the stepper motor to the turntable.
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NEMA 17 Stepper motor enables precise control of the base rotation for solar tracking.
