ME360 Exercise 3:
Pen Plotter
Design Goals:
Construct and control a 2.5 DOF Linear Drive System that is able to draw a square with a pen on paper.
Learning Objectives:
- Learn the mechanism behind a linear drive (angular to linear displacement).
- Learn how to control a linear drive system.
- Collaboration across different teams.
Design:
The concept behind linear drive is relatively simple. Using a motor that outputs shaft we can convert this rotation into linear motion in a handful of different ways. This system used a threaded rod fixed to the motor shaft, to drive the rotation of a threaded washer.
Fig. 1. Fully constructed linear drive.
At the far end of the linear drive is a Stepper Motor (NEMA17) which provides our rotation. With an Arduino we can send voltages which correspond to output RPM of the motor. With careful timing, we can control linear displacement.
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We used a program called Repetir to control all stages of the linear drive simultaneously. This synchronization is needed for precise control, which allows us to draw on paper.
Fig. 2. Rod Mount.
Fig. 3. Rod Mount and Motor Riser made of foamboard.
Bill of Materials (BOM):
Fig. 4. BOM.
Foamboard:
To mount all these components (listed above), we used foamboard. Foamboard is great for prototyping because it is structurally sound, but has enough give to not constrain the mechanism too much if problems with alignment occur.
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The main purpose of the foamboard was to align the Motor and Threaded Rod. Any excessive misalignment would strain the motor and cause the system to not drive smoothly. Seen in Fig. 2 and 3 are risers for the Threaded Rod, which spin freely in the Rod Mounts. The foamboard risers were glued to the Rod Mounts and heighted accordingly. Seen in Fig. 3 is stacks of foamboard to raise the motor to be level with the other rod mounts.
Assembly:
With one linear drive complete, we just needed to assemble three drives to create 2 axes of motion. These other two drives came from other groups. Two drives were assembled parallel to each other, and the third ran across. The ends of the third drive should be mounted to the brass threaded washer, allowing the motor to control its position.
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The drive running across controls the a motor attached to the pen. This motor allows the pen to move in the Y-Direction to contact the page and retract when needed. This motor to pen assembly is also mounted to the brass washer. See Fig. 6 for reference.
Fig. 5. Pen Plotter result.
Fig. 6. Full Assembly in motion.
Results:
The pen plotter worked as expected, and was able to draw a square. With the assembly of linear drives, combined with code, we were able to create a path for the pen and draw a simple square. See Fig. 5.
Something interesting we noticed was that the sides of the square weren't exactly straight lines, despite the code moving the motor in straight lines. This was because the pen was skipping on the page, causing slight inconsistency during drawing. This was caused by the Pen Mount not being rigid enough. The length of the pen extending down to the page caused the friction between the pen and paper to exert a moment back on the pen, causing skipping.
Takeaways:
Overall, this project was a great exercise in collaboration between teams, and taught us all the basics of linear drives. This knowledge of linear drive mechanisms directly translates into Asssignment 4 where we had to design a linear drive system from scratch, using more rigid materials.