Plotter Pen Mechanism

Challenge

For this assignment, the challenge is to use a 1 degree of freedom mechanism, controlled by either a servo or stepper motor, to move a pen from a vertical writing position to a vertical storage position with the cap on.

Constraints

• In the vertical writing position, the pen must be able to write on a horizontal surface that is 1cm from the bottom of the mounting plate

• When the pen is in the capped position, nothing may extend below the mounting plate

• The pen must be completely vertical with respect to the writing surface in both writing and capped positions

• The mechanism must be actuated through one servo or stepper motor

• The whole assembly must be independent from any surroundings and able to move freely in x & y direction

• The prototype must be able to be constructed at home with available supplies

Objectives

• Get the pen from the writing to capped position using a single mechanism

Goals

• Minimize moving parts to less than three

Ideals

• Linkage can be universal to any type of writing utensil

• Pressure of writing device is adjustable in writing position

 Conceptual Design

Necessary Function: Convert rotational motion to vertical-vertical stop-stop linear motion

Four-Bar Linkage

A variety of conceptual designs were explored to achieve the desired function as defined by problem parameters. In theory, it seems like a four bar mechanism is an ideal candidate for its simplicity and the desired motion. A four bar mechanism would be able to lift the pen and crank it up and over the edge of the floating base, then drop it onto a vertical cap. A variety of conceptual designs were explored with sketches, and estimated pros and cons were considered. However, a glaring issue arose in the detailed design stage. With a four-bar mechanism, it is easy to synthesize stop-stop motion using perpendicular bisectors to determine joint positions and link lengths. The problem is that, to cap the pen, it is desired to achieve a brief, fully vertical motion, which proved difficult, since the motion is determined by the movement of a rotating crank.

Six-Bar Linkage

It was determined based on the issues with four-bar linkages, that a six-bar linkage may be more suitable for design. Though a six bar linkage would be more well suited for achieving a good capping motion, the synthesis of the linkage proved very difficult, so it was shelved for feasibility issues. However, there is more to be explored on this subject given more time to explore methods of linkage synthesis.

Modified Four-Bar with a Pin/Slider

After determining that a six-bar linkage was too much to swallow for the nature of this project, a variation of a four-bar linkage was explored that combined a slider/crank mechanism with a four-bar linkage. This concept paired a motor-controlled crank at the lower attachment point of the pen with a pin and slot mechanism at the upper attachment point on the pen. The crank would still move the tip of the pen in a circular motion; however, the pin/slot joint at the upper part of the pen would allow for the designer to dictate the desired “capping” vertical motion at the end. Unfortunately, Solidworks was unable to simulate this type of linkage due to an unknown error. Therefore, to ensure project deliverables were able to be completed, the idea was ultimately shelved.

Vertical Four-Bar Linkage

Thus, the conceptual design has gone full circle, and the designer resorted to using a four bar linkage with special attention paid to getting the tip of the pen as close to the near side of the cap as possible for the beginning of the “capping” motion to ensure the undesired horizontal motion at the end of the movement would not affect the mechanism’s ability to cap the pen. A variety of possible arrangements were explored for the four-bar linkage, but it was determined that to achieve the desired forceful downward motion of the pen for the “capping” motion, a vertical arrangement of the linkage with the base link being fixed to the floating carriage for the plotter pen would be ideal.

 Detailed Design

Design of Mechanism in Solidworks

In order to simulate the motion and calculate required motor speeds and torques for the project deliverables, a 3-D model was constructed using Solidworks, and a motion study was performed. Using the motion study, peak torque and velocity values are obtained to achieve the desired motion. For links made out of AISI 1020 steel and a 500ms stop to stop motion, a motor with at least 2.8N*mm of torque would be required. The motor would also have to achieve an angular velocity of 393deg/s or 6.68rad/s.

Physical Model

A prototype is created using an Arduino, a servo motor, foam board, hole pins, and gorilla glue. The model is actuated using a potentiometer.

Conclusions

The physical prototype was unable to cap a pen, but was able to achieve the desired motion once or twice without breaking. The size of the mechanism was its biggest weakness. The smaller links were constructed well enough, but the method of joining between links made the foam board too weak. The pin joints were made by poking holes in the foam board with the X-acto knife and the pins in the lab kits. This construction method added a notable amount of resistance from friction and weakened the foam board.

The dimensions for the holes were accurate, but the base link was too weak where the motor was attached to remain rigid at the top. The top link had a lot of wiggle due to the weak base link, and it made the mechanism bind up toward the top of the motion in addition aforementioned resistance in the links.

I would be keen to attempt another construction of the physical model, maybe with larger overall dimensions, as this prototype may still perform well if it was structurally sound. The Arduino part of the device worked well, though. Using a stepper motor in place of a servo to allow for positional control and feedback.

 References

Gutierrez-Wing, E.(2021). Motors and Sensors [Zoom Lecture]. Retrieved from Boston University Product Design M360 A2.

Norton, R. L. (2011). Design of Machinery: A. McGraw-Hill Education.