Proposal Bot/Spring/2020
Pen Plotter Design
Author: Jonathan Valdez
Verification: Ethan Thiessa
Table of Contents
Introduction
The Proposal Bot is designed with the capability to write out a message in block letters. The message is to be written in pen and this pen can go up and down at the users will using this Pen Plotter design and an SG90 servo motor connected to an Arduino. This design is made up of several separate parts including the two-part pen plotter base, pen holder, and the metal rods as well as two tiny springs found at the base of the two rods. This pen plotter is designed to bolt down to the center of the base of the bot with a square cut hole in front of the pen plotter so the pen can reach the floor under the bot.
Pen Plotter Design Process
Figure 1: Preliminary sketch of pen plotter design
This is a preliminary sketch of the 3D printed pen plotter design. This design was implemented based on the limited space constraint since there are already so many things inside of the Proposal bot I wanted it to take up as least space as possible yet be thick and sturdy enough to handle the weight of the sliding mechanism. One part of this design took several iterations to develop a method to attach the sliding pen holder to the servo motor so that it can move up and down. I initially designed a hook to connect the pen holder to the servo motor lever, however, the hook was in the way and would not allow the pen holder to slide all the way up. I changed the orientation of the design so that it would not be an obstruction to the sliding movement however it was now an obstruction to the placement of the pen, so another iteration was developed. The final design iteration for this dilemma was the simplest, straight forward, and most cost-efficient. The pen itself is already designed with a hook on it, therefore simply attach a zip tie from the pens hook to the lever of the servo motor.
Figure 2: Center-to-center measurements
This sketch is the center-to-center measurements used for the placement of the four holes used to anchor the base of the Pen Plotter to the foundation of the bot. The Pen Plotter is to be located at the center of the foundation of the bot with an orientation facing in the same direction of forward motion.
Figure 3: Two-part Pen Plotter base design
I used the free 3D modeling program called 123 Design to implement this two-part Pen Plotter base design. I initially made the rectangular base with the measurements from my sketch above with what appears to be six holes however actually only four of the six holes go all the way through. The other two holes only go about halfway through since it was designed to hold in place two rods which are to be locked in place with super glue. The 3D model of the pen holder seen below will be able to slide freely up and down these rods utilizing an sg90 servo motor. The second part of this two-part Pen Plotter design is the tower-like piece with the two holes I created and attached that to the base to make it one solid piece. The two holes on the tower like piece is where you slide in the two rods all the way down till they hit the base where they will be secured with superglue, the rods should be approximately 4-5mm in diameter and 131mm in length.
Figure 4: Pen Holder
This piece is the pen holder and is simply an eight-millimeter-thick rectangular box with holes going all the way through the center. The holes are there for two rods that go through not only this piece but also the pen plotter base design. If you notice the two holes are slightly offset from the center so that when a pen is attached to this piece using a zip tie it will not collide with the base when sliding up and down the rods. The pen used is to be weighted down with two four-gram fishing weights glued to both sides of the pen. These weights are designed to give the pen a perpendicular normal force like a hand applying pressure to a pen when writing.
Motor Placement
Figure 5: Servo Motor placement
The sg90 servo motor will be used to control the z-axis up-down motion of the pen plotter. The sg90 servo motor was chosen due to some design constraints including size, weight, and cost. The servo motor is to be placed on top of the pen plotter tower as seen in the figure above which has a limited area of 1900mm2, however, the servo motor is only 23mm long and 12.2mm wide therefore it will fit on top of the pen plotter perfectly. This motor is small yet powerful enough to control the motion of the weighted pen with a torque of 1.80kg-cm at a voltage of 4.8V. This motor is not only light in weight, coming in at only 9 grams but also relatively inexpensive with a price of only $2.10 per unit which will help reduce our overall expense of the project.
Arduino Code SG90 Motor Control
#include //add '<' and '>' before and after servo.h int servoPin = 3; Servo servo; int servoAngle = 0; // servo position in degrees void setup() { Serial.begin(9600); servo.attach(servoPin); } void loop() { //control the servo's direction and the position of the motor servo.write(180); // Turn SG90 servo back to 90 degrees (center position) delay(3000); // Wait 1 second servo.write(90); // Turn SG90 servo back to 90 degrees (center position) delay(3000); //end control the servo's direction and the position of the motor //control the servo's speed //if you change the delay value (from example change 50 to 10), the speed of the servo changes //end control the servo's speed } }
This is a lower-level general Arduino Code utilizes the servo.h library that controls the SG90 servo’s direction, position of the motor and the speed. We will be using an Arduino UNO board to control this servo motor to go back and forth from 0 to 90 degrees, which is the motion needed to control the pen plotter even though it has the option for 180 degree range of motion. All code documentation used can be found in the resources section at the end of the blog.
Conclusion
The idea for this Pen Plotter design came from a youtube video however it required slight variations and several iterations to get it where it is now. Theoretically, this design will work smoothly as expected and be able to control the vertical movement of the pen with ease as desired to meet the project objective of this pen plotter. The pen is not present in these illustrations but there is a youtube video with an example found in the resources section. Unfortunately, we never got to test these 3D printed models since the library closed and we did not have access to the 3D printed parts we paid for.