3.3 Design Process
Mechanism Design Process
The first iteration of the design involved creating a low-resolution prototype out of cardboard. This prototype was based on the MotionGen design and served as a proof of concept before moving on to a more detailed assembly in Fusion360.
The initial idea behind the mechanism was to use a four-bar linkage to amplify the small drop caused by the snail shell cam. The goal was to translate this minor motion into a more dramatic drop at the end of the hammer arm where it would strike the key.
After validating the concept we wanted for our project, we moved on to making a wooden prototype to improve durability and precision during testing. The wooden base plate and arms featured multiple holes to allow for adjustment of joint locations. This flexibility enabled us to test different configurations and optimize the motion of our mechanism.
To improve the smoothness of the mechanism’s motion, we modified the cam to have an offset center. This design change helped the cam follower better track the rise and fall of the cam profile.
In addition, we added rubber bands to enhance the downward force of the hammer arm. This supplemental force ensured a stronger impact on the key, beyond what gravity provided.
To further enhance the mechanism, we added a curve to the long arm (L3), which increased its range of motion. We also redesigned the follower arm as a single triangular piece to prevent slippage at joint (L2), as illustrated in the drawings over the wooden prototype.
Additionally, we began testing various cam shapes. The yellow cam was designed to be a simple prototype that would trigger a single extension and retraction of the arm during each full rotation.
Having incorporated the curve into the long arm (L3) and making a ternary link (L2), we began testing cams of different radii based on insights from the motion generation process and CAD assembly. We ended up with the pink cam that not only altered the motion profile but also enabled the mechanism to trigger multiple notes with a single rotation. We noticed there were issues with slippage and sticking of the follower along the cam edge.
We modified the follower design so that it would maintain contact with the cam through a freely rotating bearing. This reduced the friction and ensured a smoother interaction between the follower and the cam surface.
The first full implementation of the complete assembly included all the new cams and laser-cut versions of each link for improved precision and durability. We also used transparent acrylic baseplates to make the internal motion of the mechanism clearly visible during operation.
To address the playing of different notes, we created custom-designed gears that coordinated the actuation of different arms to play multiple notes in time with the song, ensuring synchronized musical output.
Cam Design Process
We first created a simple offset center cam to establish a baseline for radius change and to observe how the mechanism behaved when actuated by a cam follower.
After observing the initial design, we realized that the radial change was not ideal, so we shifted out focus to the change in length of link 5 to guide the development of a more accurate second cam.
For the next iteration, we designed an oblong cam with a more appropriate radius, which also allowed us to test whether the cam would be able to drive two extensions of the mechanism within a single rotation.
We felt that the displacement of the oblong cam was great so we went on to design a cam with the same change in radius that plates 8 notes in one rotation, which was our goal for the number of notes we wanted our mechanism to play. And before printing this model of the cam, we tested that it would function as intended in the CAD with a basic cam follower assembly
We then transitioned into designing cams that incorporated a musical pattern into their structure. Each cam was divided into eight sections, with each divot corresponding to an extension of the hammer arm to strike a note. Moments where no notes were intended were represented on the cam by filled-in divots, effectively turning the cam into an intermittent motion device.
Each note (C, D, and E) has a unique cam designed to follow its unique pattern of notes