Lab 0 - Rapid Prototyping Principles Lab

Objective
The objective of this lab was to gut a toy to understand its use of a motor and sensors. These electronics were to guide our creation of a mechanism that launches 10 ping pong balls into buckets placed around the X50 lab.

Specifications and Limitations
The specifications and limitations given to us were as follows:

• The ping pong ball must be the activation of at least one electrical part of the system
• The competition involves aiming only once, dropping 10 ping pong balls into the system and targeting for a desired bucket all within 40 seconds
• The footprint of the mechanism must be able to fit on one of the tables along the outside wall in the lab
  

Approach
We divided this lab into 5 steps:

• Brainstorming
  
• Determining best option via Pugh chart
  
• Design
• Manufacturing and assembly
  
• Testing

We took the candy machine toy apart and found the switches for the translation motion to stop at the ends. There was also a photo gate that could detect when the toy or candy dropped into the shoot to turn sound on for celebration. The other electronics were wires and manual triggers for controlling the crane, which were not useful in our design. We ended up using one of the limit switches from our toy for our control circuit. The motor we needed was found in the lab in our provided "extras" box.

Brainstorming worked well for our team. We spent a half hour to brainstorm individually during our first meeting. After this we explained our sketches and grouped similar ideas so that we could make Pugh charts to decide on the overall best design. Some of the ideas that seemed silly actually were ranked higher than others that seemed rather basic. A couple ideas were lumped together to finalize the project starting point. Please see the figures and explanations below.

1. Spring Catapult– For this idea, a bucket would be mounted at the end of a long lever. The other end of the lever would be connected to the base by a hinge. A compression spring would be mounted between the base and a point between the two ends of the lever. By pulling back and down on the bucket, the spring would be compressed. Releasing the bucket would allow the spring to relax, transforming the potential energy stored in the spring into kinetic energy of the ping-pong ball. By adjusting how much the spring was compressed, the range of the ball could be adjusted.

2. Motorized Catapult– For this concept, a catapult arm would have a motor connected as the hinge between arm and base. The motor would be used to drive the arm forward or backward. A movable stop bar would be mounted to the
base, allowing the release angle to be controlled. Contact switches would be mounted so that the system would know when the arm was in the home position or contacting the stop bar. A photo-gate sensor could be used to determine whether or not a ball was in the bucket. When a ball was placed in the bucket, the system would drive the motor forward, throwing the ball. Upon contacting the stop arm, the lever would be driven in reverse, resetting the system.

3. Compression-Spring Launcher– Main components of this concept include a barrel, piston, and spring. The piston would be drawn back against the spring, storing energy. When released, the piston would accelerate a ping-pong ball along the barrel. Angle of the barrel could be adjusted to control range. A mechanism would be
required to draw back the piston.

4. Spinning Wheel Launcher– Another concept involved a mechanism to accelerate the ball similar to that used for hot-wheels tracks. Spinning rollers or belts would be placed along the sides of a c-shaped chute. As a ping-pong ball passed between the rollers, they would grip it, pull it along, and throw it along the chute. Range could be adjusted by changing the angle of the chute or adjusting the speed of the rollers. A photo-gate sensor could be used to determine when a ball was at the mouth of the chute and cause an actuator push it into the rollers.

5. Track-Lever Launcher– This concept involved a cart with an arm mounted on top by a hinge. The cart would be accelerated along a track and brought to an abrupt stop at the end, causing the lever to throw the ball it was carrying. After throwing the ball, the cart would need to be brought back to the beginning of the track.

6. Crossbow– A cross-bow could be used to throw the ball. By adjusting how far back the string was pulled, the initial velocity could be controlled. Angle of the launch-track could also be adjusted to control range. An actuator would be needed to pull back and release the string.


7. Blower– A hair-dryer, leaf-blower, or other fan could be used to push a ping-pong ball through a tube. The launch tube would be mounted on a pivot so that the exit angle could be adjusted. Something would be needed to insert the ball into the tube without the ball being simply pushed back out the entrance hole. One idea was to place a flexible membrane with a slit in it over a hole in the launch tube. A sensor would determine when a ball was just outside the launch tube. An actuator would then be used to force the ball into the tube through the membrane. If output velocity needs to be adjusted, a mechanism would need to be added which would allow the output from the blower to be partially or fully blocked.

8. CO2-Canister Launcher– This design would be similar to the previous concept, but would use a CO2 canister to supply pressure to propel the ball. A solenoid valve or other electronically-actuated valve would be required for this system.

9. Expendable Track– Several types of expendable tracks were proposed, including ones which used telescoping segments, fold-out segments supported by suspension-bridge-like cables, or inflatable segments. Each would cause a track to extend out over or into the bucket. Once the track was in place (determined using some sort of sensor), balls would be released and allowed to roll down the track into the bucket. Range would be limited by the number/length/stiffness of segments.


Determining the best option

We eventually selected the hair-dryer blower design. Our mechanism consisted of four main components: a hair-dryer, a barrel, a loading mechanism, and an aiming/support structure. The barrel was simply a length of PVC pipe, fitted to the hair-dryer with a flexible coupling. The loading mechanism was built using a motor with a bottle cap attached to the end of the shaft. Hot-glue was used to add a grip to the bottle cap. The bottle cap was mounted so that it protruded into a hole cut in a T-fitting added between the barrel and the hair-dryer. When turned on, it forces balls to enter the barrel against the airflow leaking out from the barrel at the entry point. This motor is turned on when a ball is bumped against a touch-switch near the top of a Plinko board mounted at the top of the loading mechanism. A relay-circuit was constructed to force the motor to remain on once the switch was bumped (See the circuit below). A foam-board tower was constructed to support the front end of the barrel, allowing it to be angled up or down by moving the tower backward or forward. A pivoting platform was attached to the top of the tower so that the barrel could remain tangent to its support, regardless of aiming angle. The back end of the barrel (with the hair dryer) was supported with a block of Styrofoam with a hole cut in it.


Design Considerations
The most robust structural part of the project is the connection between the hair dryer and the barrel. This was a purchased part, but necessary to decrease the diameter from the hair dryer to the barrel and efficient in minimizing pressure loss at this connection.

The least robust part of the design is the tubing after it is cut. The PVC deforms when drilled through or slotted. We need to drill or slot the tube so that a light ping pong ball does not get blown in the wrong direction due to the hair dryer. However, in letting the excess air to flow out the drilled holes and slots, the deformed PVC becomes too small to allow the ball to fall through.

The most entertaining part of our project was the Plinko board. We used the toy walls for this and cut up a clothes hanger for the pegs. This was a design idea that came from our brainstorming session.

Conclusion
It was immediately apparent that there was a disparity of mechatronics knowledge among the team members. Two team members are experienced with robotics through internships and extracurricular activities. The other two members have only had structural and manufacturing experience to aid in the project. The team functioned well this way, however, since the work and time was still possible to split up evenly.

It was fun taking the toy apart and learning how the coin slot worked and how the traveling crane knew when to stop at its limits. The most enjoyment was found during testing because flinging ping pong balls in a school building is a rather unique experience. We enjoyed the brainstorming process because we tried to stick with the rules of the game. There were no bad ideas and its fun to let the mind wander. The process seemed to work well since one of the more random ideas became our final project.