Team 24 - Design Process
Mechanism Design
We originally thought of building a 4-bar crank and rocker mechanism to archive a limited curved path. We wanted to retract the dart holder to reload it, then a quick stop so the dart slides out.
This original design was too simple and created a uniform curve, but the path that a dart takes when thrown by a human has a more logarithmic curve shape.
Therefore the team looked into 6-bar systems that would create a similar shape. We wanted a continuous rotation in the input link for the DC motor, and for the links to constraints the path for the output.
Below is the Stevenson II six-bar design, with the path of the top link.
The top half curve of the output link follows the desired path and fulfills the requirement for the input link to do a full rotation. We decided to edit this setting to get the desired output curve, resulting in the following mechanism design:
The adapted Stevenson’s II mechanism edited the links' lengths and the second link's orientation. It contains the requirement for full rotation on the input link, creates the natural curve of a dart throw, and completes a quick release when the input rotates counterclockwise.
Calculating Mechanism Dimensions
Our team used the Planar Mechanism Kinematic Simulator to create the mechanism previously shown. This software returns the x and y coordinates of each of the joints. Therefore, the team had to calculate the link lengths and angles based on the provided coordinates. To do so, we wrote a MATLAB code that inputs the coordinates and maps out the mechanism.
Initializing Coordinates
Calculating link lengths
Calculating initial position angles
From the above MATLAB code, we calculated the below link lengths and angles.
| Link Names | Link Lengths (mm) | Link angles (deg) |
1 | "ground x" | "5.643" | "323.3201" |
2 | "ground y" | "-0.511" | "-29.27814" |
3 | "input" | "3.3625" | "100.6081" |
4 | "tri_top_bottom" | "7.3597" | "45.88632" |
5 | "tri_top_left" | "3.571" | "53.24242" |
6 | "tri_top_right" | "3.8454" | "39.05772" |
7 | "middle" | "2.4721" | "12.97377" |
8 | "output" | "5.045" | "150.92" |
9 | "tri_ground_top" | "5.0201" | "173.3195" |
10 | "tri_ground_left" | "7.4328" | "103.3482" |
11 | "tri_ground_right" | "7.4087" | "63.80848" |
This data was used to create a SolidWorks model of the mechanism.
SolidWorks Model
The SolidWorks model encountered a few iterations discussed in Manufacturing and Assembly section.
In order to make the model more modular for future edits, all the link length relationships from joint to joints were linked to the global equations folder.
All the original lengths from the MATLAB code previously shown were saved in the equation and divided by a gain. The gain was to make the input link 10cm, then it would scale all other links to their respective proportions. It also allowed the team to change the overall shape of the links without affecting the integrity of the geometry. As can be seen in Manufacturing and assembly, the teams switched from a high surface to a bone structure.
The team was provided a high-speed 12V Pololu motor. The input link is directly connected to the motor. The spacing between the grounded links is also crucial for the correct movement of the output. For such a design, the motor had to be elevated to exactly 1 in, and the correct distance was positioned with the holes on the base of the mechanism.
It's important to note that although the T and the top 3 link joint seem symmetric, they are not.
The sides of the top 3 link bar have a 12 mm difference, and the T-link has a 1 mm difference. The orientations for these links were also taken into account when manufacturing.