CamRap

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Camrap

Release status: unknown

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Description
Documentation of Camrap R&D
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CAD Models
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Overview

Camrap is intended to be cheap, safe (no high temperatures) and very high resolution.

Key Concepts

Positioning

Camrap is named for it's positioning system, which uses a webcam as a means to read a 2d image which encodes X and Y position. Z-axis positioning will likely be "stepped" and have its position inferred as with stepper motors.

The position detection works by processing a captured image of a printed grid, which is attached to the "build platform" and moves with it in X and Y.

Photogrid01-5cpiece.jpg

The horizontal and vertical line positions can be measured quite accurately by averaging subpixel line positions at each pixel along the line. This determines the image centre point in partial grid units, and the positions of each line in relation to the whole grid can be determined from the diagonal lines. The diagonal lines represent "digits" which can be either /, \ or blank, and each 4*3 sub-grid has a unique pattern of these.

Structure

The current (work-in-progress) prototype Camrap uses 10mm extruded aluminium tube, FIMO polymer clay and some balsa wood as unprinted parts.

The current design is a similar shape to Huxley, but has only single tubes as edges, two of which are also used as X-axis rails. The Y-axis travels with the X-axis carriage.

Printed parts will be some form of polymer clay if possible. "Metal" clays are available which can be fired to remove a binder and leave a sintered metal product behind.

If that doesn't work well enough though, an existing Reprap extruder design could be used.

Because positioning is measured rather than inferred from motor steps or encoder wheels, drive options are not limited to stepper motors. Currently cheap continuous rotation servos are used, but they may prove to be underpowered.

Control Systems

A PC, webcam and an Arduino. Possibly an Android mobile phone could be substituted for the PC and webcam (possibly the Arduino too).


Software

Camrap ss.png A screenshot of the software used to read 2d position.


Data from the software.

The camera was not moved during this run, so this data is a result of: camera noise, lighting changes, vibration and other errors.

The y axis is 4mm "grid units", x axis is frames.

Panning

The camera digital pan function was used to step a few mm and back again. (4mm units again.)

10 frame averages

This data is from averaging every 10 frames to reduce noise.

10 frame averages, another run


Moving an X-Y stage with continuous rotation servos.

Project log

24 JUN '11

Wiki page with short overview created.

01 MAY '12

Positioning software data and screenshot added.

02 MAY '12

Github repository created.

https://github.com/darkpaw/camrap

README stuff:

The camrap software is written for Python v2.7.

It will capture images of a printed grid with a webcam and report the camera X-Y location based on image analysis.

It needs some python libraries:

pygame numpy scipy PIL

On Ubuntu/Debian they can be installed with:

sudo apt-get install python-pygame python-numpy python-scipy python-imaging


Run it with:

python camrap.py

There are some options for resolution and averaging multiple frames at the top of mainloop.py

The "target" grid can be found in tools as 4mmA3.ps, it should print OK on A4 too.

further reading