Anyway, the photoplotter has a 6.519" diameter drum (circumference = 20.480") which is 20" long. It is rotated at 10 rev/sec by a motor, and it's rotation is monitored by a 1024 line/rev shaft encoder. A digital PLL multiplies 1024 pulses / rev by 20 to get 20480 pulses / rev which equals 1 pulse per .001" of circumference (which works out to 20.480".) A stepper motor moves a carriage along the drum .001" every rotation. The optical head focuses the beams from a 5 mW 670 nM Laser diode through a 13 mm FL lens onto red-sensitive high-resolution film.
The unit exposes film at the rate of .01"/sec, or .6"/minute in the axis parallel to the drum.
General view of the Photoplotter. Drum drive motor and encoder
are at upper right. Stepper motor is at lower right. Optical
carriage is now about 1/3rd of the way to the left edge of drum.
Film is usually attached to the drum with electrical tape.
(Small board sitting out at
left can be swapped for the digital laser driver to write
gray-scale images to photographic imaging film.)
After using up all my Agfa film, I have just switched to Kodak PRD film, which is a lot more sensitive. I had to turn down the laser intensity to about half.
I just replaced the aging Windows 95 computer which had an ISA bus and a DMA card with a Beagle Bone Black. I used the Beagle's PRU processor to emulate the DMA function of the old system. Works great! The original computer was already a cast-off when I built the original photoplotter in 1996, it sure is well-past it's normal life cycle.