Eggbots and Inverse Panoramas

Projects with the Raspberry Pi.

Eggbots

These generically are computer controlled drawing machines. Give them a Cartesian, flat, 2D drawing and they draw it onto objects with an axis of cylindrical symmetry, such as ping-pong balls, eggs, light bulbs and wine glasses. Basically x/y coordinates are re-purposed as latitude/longitude. I got 'into' eggbots some years ago. They are a fantastic introduction to CAM, and can be made easily from stepper motors stripped out of old printers, scanners et al. Plus hot-melt glue, BluTak, etc! In the 'old days' we used parallel ports ( remember them- LPT1, etc) and made motor drivers via software logic and discrete transistors.

Inverse Panoramas

These are the inverse. Take an object with cylindrical symmetry and from it create a 2D flat 'un-wrapped' view. I thought this up as a way to create photographic-quality records of an antique vase our family has- but in the event it is too valuable to 'play' with, hence this Poole pottery jug! There are many objects, like vases or jugs, which have decoration all round their surface. A photo can only show one direction, and the rest is either foreshortened and distorted, or the back aand sides are invisible.

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By mounting the object on a stepper-motor turntable, it is easy to get the rotate-and-step sequence. I'm still experimenting whether it is better to use a webcam or digital camera. Things will change once the Pi camera is released. At present I use an external camera since I have only lo-res webcams available, and use a servo to operate it. This means using an SD card to transfer the images.

I can automate, via ImageMagick, the merging of the images. At present it is easy to do the cropping and joining. Just needs choosing suitable strip size for a given object. Any necessary scaling and colour /contrast adjustment could also be automated. It may be a better idea to feather the edges and overlap them a bit.

After I'd got this working I discovered others have played with this, particularly in the context of recording say a human head from every direction, to be used for texture-mapping onto a 3D model of a head. But then it's very difficult to invent something really new!

The software side

It is incredibly easy to drive eggbots, since a free add-on to Inkscape, the Linux vector drawing package, sends its output to the two stepper motors and pen-control servo. You just download an extra file and unzip into a particular directory.

WARNING. On my ( early) PI Inkscape takes AGES to load and render. But it happily runs an eggbot as fast as my mechanics allow...

I do however often use my main PC instead, and I also drive the eggbots and other machines in Liberty BASIC or Python.

You require only a single interface board called an EiBotBoard, available from SparkFun for about 50 dollars ( and from SK Pang and other agents) which plugs in to a USB port, and is seen by Linux as a serial port. It has two microstepping stepper-drivers and umpteen digital I/O and servo outputs. You just need a motor power supply for the motors- I use a 12V 1A power brick. The board logic and servo draw power via the usb.

Now you can first test the setup with a serial terminal such as Minicom. Typical commands are

sm,5000,0,1536

tp

#!/usr/bin/env python

import time
from time import sleep

import serial

ser = serial.Serial( "/dev/ttyACM0", 115200)

#   check EBB & version number
ser.write( 'v' +"\r\n")
time.sleep( 1)
print ser.readline()

while 1:
    print "Rotate & take photo.."
    print "Step motor #1 120 steps, taking 1 seconds, then wait 1s more."
    #   1000ms, motor 1 120 steps, motor 2 none.
    ser.write( "sm,1000,120,0\r\n")
    s = ser.readline()
    #print s
    time.sleep( 2)

    print "Pen servo cw  on pin 4.  Delay 2 second."
    #   channel 1, duration in 12-millionths of a second, pin 4, not at slowed rate
    ser.write( "tp\r\n")
    s = ser.readline()
    #print s
    time.sleep( 2)

    print "Pen servo acw on pin 4.  Delay 2 second."
    ser.write( "tp\r\n")
    s = ser.readline()
    #print s
    time.sleep( 2)

    print ""

"C:\IM>convert +matte -resize 260x170 *.jpg animn.gif"