Introduction

We all have experience of saving coins into a piggy bank as a child. A saving coin teaches a child a good saving habit to manage his/her personal asset. Japanese invented a coin stealing toy bank to make this experience even more entertaining. The bank looks like a small box. When a person places a coin above the toy bank triggering a switch, small animal paw comes out from the box and drag the coin into the box. 

I was interested in the mechanism behind this cute toy and recreated this coin bank using materials at my home and Karakuri book.  Although the shape of the scope of this project has changed because of the recent COVID-19 pandemic crisis, I did my best to successfully finish the course.

Design Concept Generation

In the beginning, I don't know what is inside the coin stealing bank. I found a Youtube video that shows the inside of the box. 

We can see the machinery inside is a combination of a few different mechanisms. 

3 mechanism should be integrated into the system:

1. Opening of the coin box door using a cam mechanism. The follower of the cam (the animal head) lifts the box door up and allows the arm to reach the coin. Then it will close when the arm is returning.
   
   
2. 4-links mechanism pushes the arm to reach it to the coin. There will be a crank attached to the shaft at point O, which drives link 2. The link 3 is the arm touching and dragging the coin. The link 4 will be a spring to help returning mechanism.
   
3. The arm return mechanism using spring need to design. I need to use a spring with enough stiffness. Thus it toggles the returning mechanism when the copular point (the end of the arm) reaches the coin.

First Prototype

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. Therefore, I decided 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.

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.