16.4 Mechanical Design
As outlined in the final results form the kinematic analysis, the 8-bar linkage configuration provides the most optimal gait profiles for our needs and was chosen to be implemented for the final build in place of the Klann linkage used during the initial prototype.
Based on this, we started our design process by modeling all of our components using CAD through SolidWorks. The CAD model comprehensively incorporates all structural components, including the build plates, 8-bar linkage elements, spacers, marker mount, motors with their respective mounting brackets, as well as the pulley and belt system. The images below provide detailed views of the key system components.
System Components
The entire assembly of the Morse Code Encoder was composed of three main sub assemblies: the linkage mechanism, the paper feeder, and the base structure and mounts. Each of these three sub assemblies required unique design considerations.
Linkage Mechanism
The 8-bar linkage mechanism was a critical system for our project. From our kinematic analysis, we utilized the link lengths and angles to accurately model the entire mechanism in CAD. The linkages were designed to be 1/4” laser cut acrylic with a ~0.1mm hole tolerance. The tolerances used were determined by test cuts with hardware. Due to the fairly small size of our links, we chose to use bolts for the joint axes over bearings. This was due to size constraints and allowed us to keep the thickness and width of our links proportional to the gait profile we needed.
Some key design features of the linkage mechanism include the marker mount. Similar to the prototype, the mount for the marker we will use to inscribe the “dot” and “dash” is a compression design fitted with two bolts and nuts to allow for axial variability. This mount is fastened to Link 7 with M4 bolts. In addition, this mount was designed to have the marker tip point parallel to the back plate.
The primary driver of the linkage mechanism is a DC motor with a D-shaft. To allow for this shaft to effectively interface and drive our crank linkage, we used a flanged shaft coupler. This allowed us to set screw into the D-shaft of the motor and use the bolts to thread into the flange with our crank Link.
Lastly, the mechanism that drove the movement of the ground joint C through the slotted arc was a servo motor. To effectively interface between the servo motor shaft and the rest of the 8-bar mechanism the “Servo Link” employed two different slotted holes. This allowed for more flexibility where we needed to bolt the linkage, and allowed for some give during the servo motor’s rotation.
Base Structure
The base of our final assembly consists primarily of three laser cut 1/4” board of plywood. These are the main back plate, the front plate, and the bottom plate. This simple design allows us to hide all the electronics and wiring in the back while the movement components of the mechanism are easily displayed in the front.
The back plate is the most complex structure of this sub assembly. It has holing patterns for various features including mounting holes for both DC motor faces, the servo motor mount, the paper base, and the bearings. This piece also includes a radial slot that is designed based on calculations from our kinematic analysis. To avoid any interference, we increased the width and length of the slot. Similarly, the front and bottom plate also have holing patterns for bearings and mounts.
To keep these plates together and secured, we have various 3D printed L brackets that allow the plates to be held perpendicular to each other with M5 hardware. The back plate and the front plate are further held together with the paper base, which is a large 3D printed piece that serves as a base for the marker to write on when inscribing on the paper. This base uses heat set inserts to allow for fastening into both the front and back plates, and includes cut out for the timing pulleys as well.
Paper Feeder
The purpose of this sub assembly was to move the paper along its surface to allow the encoder to inscribe Morse Code onto different clean parts of the paper. For this mechanism, we used two belt-pulleys, powered by a DC motor, to feed the paper down the encoder counter clockwise. For the specific design considerations, we used 6mm ID bearings that press fit into our back plate and front plate. Then, we used 4 timing pulleys, 2 in the front and 2 in the rear, that set screwed onto the shaft. The DC motor that ran the entire system was wall mounted to the back plate and a shaft extender was used to allow a longer interfacing shaft. To tension the pulley properly, we also did some calculations that allowed us to obtain the optimal center to center distance to ensure a taut and tensioned pulley system.
To fasten the paper, the top edge of the paper is taped onto the right side of the belt and the end of the paper hangs off the non-actuated side. In efforts to prevent any paper wrinkling, we designed a “flattener”, shown in white, to level the paper when getting on plane. The paper base, shown in orange, serves to be a sturdy, flat plane for the marker to imprint the dot or dash. With parallel cuts along the top and bottom of the part, the top of the belts are flush with the paper base’s top surface to reduce any possible compression on the paper itself.
Complete Design
The completed CAD assembly, shown below, incorporates all the subsystems. This assembly includes the custom fabricated pieces, that are primarily made to be either laser cut or 3D printed, along with some hardware and electronics.