CCD_Rasnik straightness monitoring system
CCD_Rasnik: Hints for designing a new mask.
Choosing a basic building block (or
B3) depends on a number of requirements.
It should be big enough to contain a sufficient amount of codebits
to encode the complete mask. The it must be small enough to ensure that
a little more than one blocksize of the mask is seen by the camera.
Or more if redundancy is required.
The size of the dots (or fields) depends on the camera used.
The size of the pixels of the CCD sensor play
an important role.
The image of the mask on the camera will not be completely sharp
due to diffraction. In the data generated by the framegrabber,
the dots must big enough to be recognized.
Recognizing flat tops for black and white in the data
all over the view seems good practice.
On the other hand, smaller dots will generate more
edges to do a precise fit on. So the choice of the size
of the mask pixels depends on the quality and aperture
of the optics, but also on the camera-framegrabber combination.
It is good practice to be able to recognize a codeline
twice or more, to compensate for dust, damage etc..
When the orientation of the mask in respect to the
camera is relatively fixed the width to height ratio
of a B3 could be about 4/3 for a standard camera.
When also the projection is very sharp, also the
fields in the B3 could have this ratio.
A square B3, with sqare fields, is
mirror symmetrical over the line X=Y.
This allows to make contact copies of the mother mask,
without the need to rotate the origin of the copy.
It is good practice to choose an odd numbers of dots
for both horizontal and vertical B3 size.
In that case the resulting pattern of the coded
locations are inverted every next B3.
This results in a more stable average illumination
of the camera when scanning over the mask.
Also completely black or white codelines are avoided.
This is illustrated below. The mask sizes 150x150 dots.
The B3 is 6x6. The 'cameraview' shows a little more
than two B3s in both directions.
Two views (next to the mask) show a clear
difference in dot density.
The view next to them is a sample of the same
size of a mask with B3s of 7x7.
A 150x150 mask with B3s with an odd number of fields in X and Y
direction gives a more stable average illumination of the
camera when scanning over the mask. Also completely black
or white codelines are avoided. This mask is shown here.
Go to the
CCD_Rasnik home page.
You can mail
suggestions for this page to:
Henk Groenstege.
august 1995