II.) Design Process (WnF)

Mechanism Dissection:

Our prototype's mechanisms can be classified under two sections with 3 total working components, which are listed below:

• Walking-Beam Mechanism and Sliding Chute

• Lazy Susan Reloader

Powered by a 12V DC motor, the walking beam serves as the driving mechanism of our assembly. This mechanism has a D-shaped motion that exerts a cocking and releasing motion on the chute slider (which is placed on a linear shaft and is tensed with a rubber band) that is very similar in functionality to a slingshot. As the slider is being pushed back, it will hit the lower fan of the Lazy Susan, which in turn rotates the top fan and pushes a marble into the cutout that makes it fall into the chute slider. As the linear portion of the "D" motion ends, the pushing arm (attachment to the walking beam) will no longer be in contact with the slider which will release the tension in the rubber band and will send the slider forward at a fast speed. Lastly, the linear shaft mounts will act as a stopper and the inertia from this launches the ball forward.

Success Criteria

• Automatically launches and reloads a projectile (preferably a marble) in repeatable cycles (at least 6 per minute or 1 in 10
  seconds)
• We can plot the motion of the walking beam in MATLAB to an accurate resemblance. Ideally, a letter D or semicircle is traced and is confirmed by the swept path
• Track the housing chamber with a linear traversal of at least 100mm. This should ideally be a sinusoidal shape, but some delay is expected.
• Marbles successfully hits target from the specified range above (6 to 10 ft), with consistent probability (3 out of 5 shots at a time, binomial distribution profile).
• Complete restoration to equilibrium state per cycle; we can visually check that machine is in the same starting position after firing

Design Considerations:

• Strong links to withstand high tension
• Adjustable z-axis position for a simpler assembly
• Low friction for better performance
• Reduced weight to avoid cantilevering the walking beam 
• Optimize link lengths to find a satisfactory stroke length and obtain linear motion
• Reduce z-axis misalignment

The considerations above are crucial for a proper build of our design. The mechanism is exposed to a high tensile load from the rubber band, so having a good structure and strong build for our links is a fundamental concept. Additionally, having a model that has z-axis adjustability is very important because cantilevering will affect positioning during assembly if it is not correctly accounted for, and cantilevering can be addressed with lightweight spacers. Having low joint friction and weight on our walking beams is also significant, which is why we used press-fit ball bearings for all of our revolute joints and made spacers out of acrylic to reduce weight and help with z-axis alignment. These considerations are all implementable once an optimal stroke length is found for the cocking and releasing motion which was done by optimizing link lengths in our MATLAB code and obtaining a motion profile.  

Our project requires simple electrical powering to rotate the input link 2, which can also be done manually with a crank arm. A major concern is finding a motor that has the right torque output in order to draw back the tensed slider, which is why we ordered a high torque brushless motor. The angular velocity of the motor is not as important as long as there is a constant force that can withstand the tensile load. We designed a crank input with a handle in order to manually test our prototype, and it can even serve as a replacement for the motor in case it fails to carry the high loads.