Thomas Shea
Design Portfolio
2.5 Degrees of Freedom Plotter
This project tasked a group of four of us with designing a complete 2.5-axis motion system. This would be done by first producing the mechanical portion of the product, and then implementing the electronic controls. In doing so, we would be able to learn how to devise different ways to embody joints, how to design mechanical components based on their function, accuracy, and assembly, as well as how to integrate and test a prototype with both electrical and mechanical components.
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More specifically, we were tasked with building a 2.5-axis motion system that had a work volume of 2.5 in. x 2.5 in. x 2.5 in. This system would have to be small and lightweight enough to easily carry in a backpack, and could be disassembled fully or partially to do this as long as it could be reassembled by one person in less than ten minutes. This product would be assembled using 8020 aluminum extrusions of different sizes, stepper motors, servo motors, pulleys, linear guides, and belts. Anything else that would be used for this product would be produced using 3D printing.
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To start the design process, we first had to come up with something our system would do. One of our original ideas was a spice shaker, which would act by moving a bowl or plate in two directions (for two degrees of freedom) and a spice shaker to apply salt by shaking it (the half degree of freedom) when instructed to. We decided that this might be a little bit larger than we could really work with because of the fact that the bowl would likely be pretty large. What we ended up deciding on is a plotter that would draw multiple shapes and letters, based on what was programmed. This would achieve 2.5 degrees of freedom by moving the paper in two directions (two degrees) and then applying a sharpie in either an up or down position (half degree).
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To start the design process, we had to get a solid understanding of how the motor would work to produce motion. To obtain translation motion using the motor's rotating force, we would either need a belt or a screw. We started sketching how this would be done with one motor, so as to better understand how to implement the design using two motors. When we came up with a reasonable sketch, we designed a model using styrofoam, and then hooked this motor up to our professor's computer to test it. This sketch, as well as the model, can be seen in the images below. One thing that is worth noting is that this model was created with a screw attached to the motor and carriage, but in the process we were only able to use a belt, which isn't quite as precise.
Once we figured out how to produce translation motion in one direction, we started to think about implementing this to get 2.5 full degrees of freedom. As seen in the pictures at the top of the page, we ended up using servo motors to move the paper tray in one direction, and the marker in the direction perpendicular to the paper carriage's motion. We then used a stepper motor to alternate the marker between an up and down position (the other half degree) which would depend on whether or not it is drawing at a certain point. The pieces that held these to the frame and allowed them to move smoothly along the glides were all produced by 3D printing CAD models we produced. Once connected, we could start wiring the motors and moving the parts electronically.
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To implement this motion, we programmed different letters and shapes using Arduino. Once connected, the Arduino would tell the motors what to do in order to draw the desired shape. If we were to improve this product, we would like to implement image processing, so the marker would draw an image, such as a fractal, instead of a programmed shape or letter. This was tough for us mainly because of the design space limitations. With such a small design space, as well as a thick sharpie, makes it difficult to draw a larger image with crisp, clear lines. If we were to improve the product, this is the first thing we would try to accomplish.
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In terms of design requirements, we were very satisfied with our product. The working volume was a small piece of paper, which is perfect for the 2.5 in. cube we were allowed. The vertical bars that hold up the sharpie and its motor are screwed into the base at the bottom of the product. These screws can easily be loosened, and the product can be folded up. This makes it very easy to fit into a backpack. This, as well as our product in action, can be seen in the kickstarter type video we made, which is displayed below.