MECHANICAL DESIGN
The mechanical design of the arcade system is broken down into the following subsystems:
1. Structural enclosure and control panel
2. Laser turret
3. Static and moving targets
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ENCLOSURE
The Enclosure serves as the structural frame, houses the game arena and the electronic components. Due to COVID-19, the only tools available to our disposal are a small circular saw, a jigsaw and a drill so unfortunately no dove-tails can be made. Thus, it will be built using ~0.5” plywood to supply ample rigidity and robustness. Fasteners available include nails and screws. The structure consists of a control panel, two side panes, a back panel, a ceiling panel, a front panel and a bottom panel. The structure is designed to decrease the amount of environmental light as possible to mimic outer space. The video gaming arcade systems control panel standard is generally 25 in wide. Our team will use a 24 in control panel.
Note: The enclosure was not finalized due to COVID-19.
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THE CONTROL PANEL
Basic design layout:
- PS2 Joystick
- 16x2 character LCD display
- Momentary Push Button
- IR sensor Card Switch Module
The control panel is one of the most important elements in the game since it's the main point of interaction with the user. Here they can swipe to start the game, user the joystick to control the turret, press the button to activate the laser and access available game statistics on the LCD screen.
The swipe reader, the joystick and button mounts are 3D printed in PLA.
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THE LASER TURRET
Want to test your hand eye coordination?
You can aim this turret with the joystick and fire the laser!
Basic design layout:
The turret motion is provided by two 28BYJ-48 stepper motors that are driven by the two ULN2003 NPN ICs that we already had at hand. Due to preliminary uncertainties caused by COVID-19 we decided to go ahead and design around this "starter kit" motor after rigging the motors and testing its ability to carry a payload twice its own weight (100 grams) to verify motor capacity assumptions.
CADs were designed to be 3D printed and sized to be light and small due to the limited motor capabilities. The mechanical design attempts to center the weight at the pivots to minimize unwanted moments to reduce motor strain since motor and part analysis has not been performed. These parts include a "pan" and "tilt" motor mounts and a laser and barrel mount.
To activate our targets, a common KY-008 laser emitter is used.
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THE TARGETS
So anyone can hit a static target. Can you hit the moving target? Easy, medium and hard mode target speeds will keep you guessing.
Moving Target
The Target arm is mounted to the gaming enclosure enclosure base. Random motion is provided by two hefty 55 gram SunFounder simple servos when activated. Arm A links the motors. The Target is set up to freely rotate or pivot about it's bolting barrel-screw to maintain orientation normal to ground position, and the motion of the servo motors are limited to a range to prevent it from moving outside of the game arena. the motion of servo A is limited from 60 to 150 degrees and servo B from 90 to 180 degrees to avoid collisions.
* analysis summary in the next section.
Static Target
The Target is raised and lowered by a tiny 9 gram SG90 SunFounder servo depending on the game timing. The target's motion range is limited to 0 to 90 degrees.
*The motor did not supply enough torque to use the designed lever and the motor mount broke in the testing phase. Due to the lack of time, we improvised with hot glue and a Popsicle stick.
CADs were designed with the building material in mind (3/4" x 7/32" stock of balsa wood) since no 3-D printers, laser cutters or machine shops were available due to COVID-19.
To activate our targets, a common photo resistor is used to sense the incoming light of a diode laser.
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ANALYSIS AND BILL OF MATERIALS
Because the moving target is the most complex and demanding mechanical subsystem in the build, its' most important components are analyzed.
Motor Requirements
The analysis is simplified as a modified inverted pendulum. The moment of inertia are obtained and calculated using the parallel-axis theorem and operational assumptions are made to arrive at the following key findings are:
Total structure mass (m) : 0.101 Kg
Moment of inertia of load (J): 0.000351 Kg*m^2
Required Angular Acceleration: 0.942 rad/s
Required Peak Torque: 0.309 N*m
Effective Continuous Torque: 0.269 N*m
Peak Acceleration: 881 rad/s
Maximum Required Power : 0.3 W
Approx Running Motor Current @ 5V: 50 mA
These parameters are important for motor sizing and gear selection.
* Spreadsheet of analysis can be seen in figure bellow.
** See electrical section for selected motor vs load comparison.
Motor Mount
The analysis of the motor mount was performed using SolidWorks 2019 FEA analysis tool simulated with load of 20 Newton. The part is anchored at screw countersinks and force is applied laterally on the face of one of its sides.
* Analysis assumes the piece is machined as one piece from blank material (not an assembly) which means the yield strength is grossly over estimated. Material date used was pre-loaded software values for balsa wood.
Simulation concludes:
- FOS always greater than 1 thoughout structure.
- Max Deflection: 6.85E-2 mm
- Max Von Mises Stress: 2 N/mm^2
- Yield Strength: 2 MPa
With the different subsystems in the build, it's hard to keep track of all the parts. A bill of materials for the mechanical components is necessary. This way we can see if we are missing any screws.
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