HARDWARE DESIGN
Once we finalized our concepts, we began the design phase of our first high-fidelity prototype. First, we designed the flywheel projectile mechanism, determining the necessary electronic components and flywheel diameters. With the general idea of the flywheel design, we then designed the overall structure and ball magazine system. The structure had to withstand the torques of the motors, constrain the magazine axially, and optimize electronics placement. After completing the first high-fidelity prototype, we implemented changes and optimized the design for the final showcase.
Assembly of internal components
Top View
FEA analysis of arm straps
The base structure is meant to be purely functional, not aesthetic, and houses the electronic components, motors, magazine, flywheels and paddle wheel. An aesthetic top will be designed to cover the base and improve the appearance of the overall device. One of the key features of the base is the support walls that allow the magazine to easily be inserted and removed, allowing for simple reloading. The openings in the supports match the geometry of the magazine and restrains it from rotating and moving axially. Since there is an opening in the back of the base that matches the geometry of the magazine, it can only be inserted the correct way.
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In addition to the support walls, a barrel was added at the front of the base in order to guide the trajectory of the ball. This will allow the ball to fly straight when shot out of the NERF prosthetic. The barrel is placed as close as possible to the flywheels, without colliding with them, so that any issues in trajectory can be corrected as soon as possible. A slit through the bottom of the base will be added in the future to wrap.
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The sole purpose of two of the straps is to properly secure the gun to the user’s arm. The primary purpose of the third strap is to attach electrodes to the user through an embedded EMG sensor.4 Due to the sensitivity of EMG sensors, the sensor is placed on a separate strap to allow the sensor to work uninhibited by the load generated from securing the gun in place.
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Top View
The internal firing mechanism consists of two flywheels driven by a single motor. Once a bullet is placed between the two flywheels, the flywheels spin to accelerate the bullet out the gun. The motor and EMG sensor are powered by an easy-to-access lithium-polymer battery to allow users to quickly swap out batteries. The gun also features an easy-to-access magazine to allow the user to easily reload the gun. The magazine is designed and placed such that the bullets will be perfectly land in between the two flywheels once the bullets are pushed out of the magazine.
Paddle Wheel
Out of the two magazine designs, a hopper and a magazine, we chose to implement the spring-loaded magazine because of the ease of control. With the hopper, there were chances of balls jamming, and it would probably need more actuation, and therefore more weight. For the spring-loaded magazine, it would need something to block the balls, a nonlinear spring, and something to push the balls to the flywheel. With some research, we found a nerf ball magazine that we will modify to fit our design. Although the magazine is approximately ~13 inches, it will protrude from the overall structure and be minimally interfering with the user’s arm. In order to implement the paddle wheel mechanism, we will mill a slot in the tube.