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In the first prototype, I tried to install a cam mechanism from the Karakuri book shown under. However, because the cover of the box was too rigid, the cam was unable to lift up the cover clearly and folded. ThusTherefore, I just use the coin grabbing arm Mechanical Analysis
Final Prototype
Conclusiondecided to use the coin grabbing arm to both opening the cover and grabbing a coin.
Also, because the four-link mechanism has 1 degree of freedom, the mechanism was not suitable for my coin bank because it needs two different motions: (1) reaching out to the coin without touching (2) grabbing the coin into the box. Therefore, I decided to reduce one constrain to increase the degree of freedom.
Mechanical Analysis
Because the 3-link freestanding arms with only one-fixed end have the degree of freedom of 2 = 3(3-1)-2(2), I need to guide the movement of the end effector with geometrical constrains.
I performed the kinematic analysis for the position of the end effector:
Final Prototype
The final design of the coin box is shown below. I reinforced the driving axis by adding a wooden chopstick into the paper core.
I was expecting the cover to close automatically when the arm is returned to the original position by its own weight. However, because the cover of the box was too light, I stayed open. So, I added a rubber band to close the cover when the arm is back to the initial position. Previously, I was planning to use a small spring, but it was too stiff to use in paper system.
The video shows the who does the coin stealing box work. If I had a motor, Arduino, and switch, I could automate the mechanism.
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Conclusion
I learned that designing a mechanism with a degree of freedom more than 1 is challenging. The system was highly dependent on the velocity input. Thus, the mechanism is not suitable for human-driving that can cause variation in the input. However, the output motion will be consistent if the input velocity is constant.