Manufacturing and Prototype Iterations

Overview: 

Throughout the design and prototyping process, two major components of the project required significant iterations, those being the compliant joints and the bean-shaped one-way track. In both cases, additive manufacturing was used for a majority of the parts due to the ability to create custom parts quickly with acceptable tolerances. This allowed for more iterations to be made than if we had attempted to prototype parts using conventional processes. The printer used was a modified Voron Trident with a 300mm x 300mm bed size, as seen below.

Flexure Joints: 

The initial designs for the flexure joints were simply strips of 3D printed plastic thin enough to be bent. Several iterations of print orientation, thicknesses, and layer lines were used to yield the most resilient flexible strips possible. However, due to the lack of any geometric constraints, the flexible strips were susceptible to bending sharply at a single point and failing from fatigue shortly afterwards. 

The next iteration used compliant rolling contact joints, which enforced a constant bending radius on the compliant joint. This produced reliable motion that avoided fatiguing the flexible strips to failure. An added benefit was the high levels of out of plane stiffness and the nearly constant force profile across the range of motion. The initial design for these compliant joints was taken from the compliant mechanisms community where these joints have been around for several years. Typically, they are cut from isotropic materials with high flexibility. In 3D printing them, much of the difficulty came in maintaining the flexibility needed for the mechanisms. This was achieved through several iterations, and by altering the strip design, thickness, and print settings. The initial compliant rolling contact joint prototype is shown below.

The first iteration of the compliant rolling contacts proved their viability. The next iteration focused on improving manufacturing tolerances (tighter tolerances decreased out of plane motion) and decreasing the size of the joints to better fit the intended mechanism. Mounting features such as hex nuts and heat-set inserts were also added. These ended up being the final iteration of the compliant joints used in the final prototype.

Future Iterations: 

The use of flexible metallic strips instead of 3D printed elements could provide greater out of plane stiffness. Better assembly techniques such as using wire or self-aligning geometries could improve the quality of the joints.

Bean-shaped one-way track:

Taking inspiration from push latch mechanisms, we aimed to create a solid state (geometry driven) push-latch mechanism that converted linear motion into cyclical motion. The first iteration shown below has two plates to avoid cantilevered forces that encourage binding on the track. This initial enclosed design was used to prove that the geometry created the one way mechanism that we were looking for. A CAD image of one half of the initial design is shown below, as is the initial prototype.

After the first iteration proved that the geometry of the track could convert linear motion into cyclical motion, the next prototype integrated the nitinol actuators along with a cantilevered design so that the nitinol drive could be attached to the compliant mechanism.

The final iteration fixed some of the flaws found with the previous prototype, and optimized the position of the nitinol wires. Some points that were interfering with the mechanism's travel were removed or chamfered. Care was also taken to flatten the design to better fit with the complaint mechanism. Shown below is the final iteration attached to the final prototype.

Future iterations:

Future iterations would aim to reduce the footprint that the mechanism occupies. The use of more compact nitinol spring designs would greatly aid in this effort. A three or more phase nitinol drive may be implemented in the future, sidestepping the need for a push-latch biasing element to create the cyclical motion. 

Electronics:

The electronics only went through one iteration due to a lack of materials. The heating of the two nitinol wires was controlled using two mechanical relays connected to a power supply that heated the two wires in an alternating fashion. The nitinol wire itself was used as the heating element by running current through the wire. The controls were handled via an Arduino Uno. Iterations in the code consisted of altering heating times to allow the mechanism to work in open-loop control.

Future improvements:

The addition of temperature sensors for each of the nitinol wires as well as a means to determine the location of the driving joint would allow for closed-loop control. This would be a much more reliable method and less subjective to environmental factors.

Next Section: The Final Prototype and Demonstration