Overview
October - December 2019
skills
Electronics | Mechanics | Physical Prototyping | Simulation | CAD/CAM
software
Solidworks | Blender | DaVinci Resolve | Adobe Lightroom | Microsoft Excel
grade
First
collaborators
Shafae Ali
Summary
GIZMO was a Second Year mechatronics project completed in pairs with a very open brief; make something electrical and mechanical with a human interaction, and keep it within 250 x 250 x 300 mm. The project is designed to give us the opportunity to develop our electronic and mechanical skills and to allow us to stretch ourselves as far as we can.
My partner and I wanted to create a remote controlled turret. We explored different projectiles, firing mechanisms, features and input methods. We ended up with a wireless, joystick controlled turret which could launch 25 mm foam balls at over 150 mph. The turret could rotate 360 degrees, tilt up and down, and featured a camera mounted on the barrel which transmitted to a wireless headset for remote use.
My role in the project
In this project pair, I took the responsibility of mechanical elements and the promotional video, while my partner took on the electronics/coding and technical pages summary. I tested projectiles and ideated firing mechanics, ran rigid body simulations to design projectile feeding mechanisms, calculated component specifications, created 3D CAD models for part fabrication, and completed the final assembly.
Promotional Video
Featuring project partner, Shafae Ali, produced by myself
Gallery
Eagle I
Trigger to drop foam balls
7.4v LiPo battery
Front view
Side profile
Adapted joystick controller
First person view camera
Tilt servo & 2.4 GHz reciever
Motor speed controller
Brushless motors & continuous rotation servo
Foam balls launched by opposed wheels
70mm wheels
Summary of my contributions to the project
Ammunition and Launch
Work began with selecting a launch projectile. We wanted a good range, accuracy, and of course safe. Discs, foam rings, foam balls, foam darts and elastic bands were shortlisted and ranked based on their flight distance, accuracy, packing density (to fit more in a magazine) and difficulty of reloading. We ended up choosing foam balls, which have the added bonus of not being dependant on their orientation when launched.
Next was to ideate launch mechanisms for the foam balls, options included: opposed rotating discs, springs, elastic, striking the ball and catapulting. One of our main goals was to make reloading fast and effortless. The rotating discs stood out as the ideal approach as projectiles could be fed into the discs continuously.
Magazine and Loading
I then explored how to feed the foam balls into such a launcher. Ideally, the turret would hold as many foam balls as possible and would therefore have multiple magazines or a hopper type of system. First I tried a hopper type of system. Unfortunately, the foam balls jammed very easily at the bottleneck where the path narrowed to a single width just before the firing wheels. Rigid body simulations were then ran to investigate how multiple magazines would interact. Like the hopper, the foam balls became jammed very easily. This led us to a single, interchangeable magazine design.
Feeding the launch wheels
The turret requires a more or less horizontal barrel to feed the launch wheels, and a vertical magazine to hold the foam balls so they can fall into the firing barrel. This proved to be one of our biggest challenges. Having any sort of pushing mechanism jammed subsequent balls as they fell through the junction between the magazine and barrel, even when curved joints were used. Instead we opted to control the dropping from the magazine with a small servo. The servo could be timed to retract and return in just the right time to drop one ball and stop the next. We then needed to move dropped foam ball along the barrel to the discs, even when the barrel is tilted upwards. The chosen solution was to mount blower fans to the rear of the barrel and blow the foam ball towards the wheels as they fell. Prototyping showed this setup to work.
Launch speed
Next I calculated certain parameters which would help us chose components. Here I look at the motors to spin the launch wheels, and the servo to rotate the entire turret 360 degrees. Based on the geometric restrictions and standardised part choices, I decided to use 70 mm wheels with rubber tires to launch the balls. I could then calculate launch speed based on the wheel rpm, assuming all tangential velocity of the wheel is transferred to the ball. The graph shows the relationship for this setup. We were then able to choose a target launch speed in mph and find the required rpm. We decided on 200 mph to also allow for speed losses from inefficiency and maintain a fast firing speed. This led to the approximately 14,000 rpm motor specification.
Turret rotation speed
To specify the 360 degree rotation servo we used a desired angular acceleration as the input variable, for which we wanted to know what torque servo would be required. Relating these two quantities is the equation for angular acceleration and inertia. I approximated the moments of inertia for the major parts on our assembly and summed to find the required torque. We chose 5 rad/s^2 as a target angular acceleration so that from rest the turret could achieve a rotation in under 2 seconds. The minimum required torque assuming no friction turned out to be 0.22 Nm. We looked for parts capable of twice this to account for frictional losses and limitations in the overly simplified assembly.
Assembly design
With the rest of the parts now also specified, I began to design the assembly. I broke the assembly down into three categories; structural elements, mounting brackets, and off the shelf parts. Our off the shelf parts have set dimensions and can be used as restrictions to the design. I then designed a structure to hold the parts in the correct places. The structure needed to be rigid and lightweight and quickly manufactured. Laser cut plywood stood out as the best choice. This structure was designed alongside the mounting brackets in Solidworks so I could ensure the parts would lock together perfectly and not interfere. Using a Solidworks assembly allows the movements to be tested to check all parts fit and don't collide. If parts collided, I went back and redesigned the parts to iterate the design forward.
In the model shown, blue parts are mounting brackets and were 3D printed to perfectly fit the off the shelf components. The wooden parts were laser cut from 6 mm plywood and use slotted connections to ensure a very rigid structure when glued together (the uprights are double layered).
