Build a cheap pen-printer with R

Buy

1. Raspberry Pi (RPi)
2. PCA9685 controller with three SG90 servos
3. 5V power supply for both RPi (2A) and PCA9685 (1A)
4. Popsicle sticks
5. Glue
6. Clip
7. Pen

Ad 1. I have cat /proc/device-tree/model Raspberry Pi Model B Plus Rev 1.2 with 'Linux rpi 5.4.51+ #1333' as operating system.

Prerequisites

Clone this repository to both your RPi and your PC or Mac. Also install R and required libraries on both RPi and PC or Mac. You can go through the code to find out which libraries you need to install. Alternatively, you can just run the code and find out by trial and error.

On your RPi install:

• R; I have R version 3.5.2 (2018-12-20) -- "Eggshell Igloo"
• pca9685servodaemon in the same folder as this repository; kudos to Tim Rowledge

Download this repository to both PC and RPi.

Hardware

Fix (glue) the first servo on two popsicle sticks (top one sticking over the paper) to e.g. some wooden plate. Connect the other servos to the first one as in the figure. First try them out following the code below to get the angles right :-) Then connect the servos to the PCA9685 controller and connect the controller to your RPi:

1. Step 1
2. Step 2
3. Step 3

The image shows the popsicle sticks below the first servo are under an angle of base_angle = 60° = pi / 3 (small angle with paper). After some simulations, this angle seems optimal to me.

Start R on your PC

Run pc-plot-girl.R. This will

• load constants
  • please take a look and update the constants where needed. E.g., the servos are now on channels (s0, first arm) 13, (s1, second arm) 14 and (s2, head with pen) 15; please update these values if you put the servos on different channels on the pca controller
• load calibration details of your servos
  • for the most accurate results, you should calibrate your servos by hand and update servo-calibration.R accordingly
• determine the positions within reach of the pen
• load img/girl.jpg and convert to lines using an edge detection library
• maximally upscale these lines to just fit within reachable area
• plot a4 with reachable area (green) and lines
• save lines as tasks.RData

Now scp tasks.RData to this repositories folder on your RPi.

The tasks data.frame looks like this:

> head(tasks)
       x        y   angle0    angle1      us0      us1 pen_down      error
50.00000 148.5000 1.756192 0.5922108 1660.427 2274.649    FALSE 0.00000000
50.58450 148.2865 1.748927 0.5993667 1655.316 2269.496    FALSE 0.10211238
51.75221 148.2227 1.741662 0.6136783 1650.205 2259.191    FALSE 0.05027103
52.33360 148.0006 1.734397 0.6208342 1645.094 2254.038    FALSE 0.06092845
53.49704 147.9321 1.727132 0.6351459 1639.983 2243.732    FALSE 0.08616331
54.07519 147.7014 1.719867 0.6423017 1634.872 2238.580    FALSE 0.03410046

The tasks data.frame consists of interpollated short lines that describe the major lines resulting from the edge detection. First two columsn (x, y) are the coordinates (mm), followed by angles of arms (rad) and corresponding pulsewidth of first two servos. pen_down indicates whether the short line should be plotted. error is a pre-calculated deviation from a straight line between start point and begin point of major line. In practice, this error will larger due to the imprecision of the cheap SG90 servos. Better servos will certainly result in a more accurate drawing.

Start R on your RPi

... in the folder of this repository. source("pi-plot-girl.R") should plot the image that is shown above.

Have fun!