04/11/2021
PLOTTER PEN MECHANISM
Goal
“Design a (1-DOF) linkage mechanism that positions a plotter’s pen for tracing on a flat, horizontal surface and afterwards stores it inside a fixed cap that prevents it from drying.”
Constraints
The pen must begin on a surface 1 cm below the cap
The pen must be positioned vertically at the two extreme positions
The mechanism will be actuated by a servo or stepper motor
Must be one independent structure
Initial Design
During initial sketching and development, I knew I wanted my pen to follow a path to move from one location to the other, using the stepper motor to convert rotational motion to translational motion that will pull the pen along the path. The below image is an initial design sketch to formulate my idea and decide some rough dimensions.
In order to hold the pen in place during the transitional motion I decided to use a cone shape to hold the top of the pen in place while the pen moves from the paper to the pen cap. In further iteration of this design I would like to develop a better way to hold the pen so that it easily releases when the pen is inserted into the pen cap.
Digital Design Setbacks
While developing this mechanism I ran into a few technical issues using Solidworks as a design tool. It is clear through the design and development of this mechanism that you cannot solely rely on a digital motion analysis to develop a mechanism. I was able to get my Solidworks motion analysis to move similar to how it would move in real life, however, the arm that holds the pen had to be moved using its own motor, while in real life the motion of the rotational motor transformed into transitional motion would allow the arm to glide around the path while the triangle piece moves. There would not be a need for 2 motors. As you can see below this resulted in an animation that does not accurately represent my design idea, however, for the time constraint of this assignment and my current level of Solidworks knowledge I do believe that the below video gives the basic idea of how the mechanism would function given more time to properly constrain the assembly.
When completing the motion analysis within Solidworks I had difficulty eliminating the extra mate redundancies. I believe that due to how I had to use two motors to show the motion of the mechanism this created an inaccurate representation of the motion within the analysis. According to the analysis there are 6 moving parts with 36 DOF, after eliminating each degree of freedom from the constraints you are left with an actual DOF of 2. As mentioned above, I believe this discrepancy occurred due to the way I formulated my Solidworks model, because in real life this would only have one DOF, translational motion in the x direction. The current state of my model is counting the pen holder and triangle piece as two separate moving entities, therefore it is considered 2 DOF. Similarly, to the actual motion of the model, with more time and a bit more experience in Solidworks I am confident that I would be able to develop this mechanism further to represent the actual DOF.
Due to the technical difficulties I was unable to determine the correct motor speed for this mechanism when using AISI 1020. It is unfortunate that I was unable to determine this, but I do think with a bit more adjusting this design could work.
Prototype
Throughout the prototype process I learned a few critical things. It is very important to account for the weight of your material when choosing a motor. For example, the reason my prototype did not easily spin on its own is due to the weight of the plastic material I made the two bar linkage out of. In order to better optimize this design, it would have been a wiser decision to use foam core as the links as well in order to allow for the motor to spin easily.
While I am disappointed that I was not able to get to the point where it worked fully on its own, I do think that the combination of the Solidworks motion study and the prototype video shows the design intent.
Conclusion
I now see that sometimes a simpler design is better than something more complicated, maybe a four bar linkage would have been a better design solution for the given constraints and project parameters. This was a good lesson in combining digital and physical design tools in order to design a mechanism and with further exploration this could be a very unique solution to the problem.