Laser Scanner NTSC FREQ. camera && projector - v2
---- WHAT IS THIS THING? ---
POISED is an object which captures images using one laser and in real time another laser renders those images. It is a laser scanner, both a camera and a projector.
Presented is a prototype for a larger laser system, one which scans on a human scale and projects on an architectural one. The final form of POISED will be a unique laser projection system for use as an interactive piece of emergent technology. POISED is an object which captures images using one laser and in real time another laser renders those images. It is a laser scanner, both a camera and a projector.
The final form of POISED will be a unique laser projection system for use as an interactive piece of emergent technology.
---- HOW IT WORKS. --- (in short)
A circuit receives reflected light off the camera raster from a photomultiplier tube, a highly photon sensitive component.
This analog circuit sends those readings to an amplifier which increases the gain of that signal. That amplified signal is then sent to the projector raster, altering the brightness of the projection laser. Both the input and output lasers are synchronized in generating their raster scans, thereby scanning and projecting simultaneously.
--- SYNOPSIS of COMPONENTS
WHAT'S INSIDE THE BOX----
532nm INPUT LASER
532nm OUTPUT LASER
RESONANT SCANNER - 15Khz
GALVOS - 30hz
PMT - photomultiplier tube
---SYNOPSIS OF KEY CONCEPTS ---
MATH - NTSC FREQUENCY:
This scanning system scans at NTSC frequencies. As such I have the Resonant Scanner oscillates at 15.75khz - in keeping with the NTSC frequency. The Resonant Scanner reflects both the Input Laser's and Output Laser's Collimated light in opposite angles to their origin Trajectory.
Precisely placed Galvos reflect these horizontal lines vertically. In keeping with NTSC frequencies, the Jones Divider receives the scanner frequency (in tandem with the resonant scanner) and divides the 15.75khz signal from the scanner by 329.97hz.
KiloHertz are units comprised of 1000 Hertz.
To convert hertz to milliseconds, first determine the duration or period of one vibration (in this case - 15750hz) by dividing one second by the frequency.
1 divided by X Hz times 1000 = X Ms
To Check our sanity check the math here.
Basically all of this means:
MECHANICS of RASTERING:
In scientific applications of this kind of imaging, most scientists would use a pre-fabricated mount for these components. I have fabricated mounts for these components as I need to adjust X, Y and Z of these components when setting up for scanning.
I designed a 3 axis gimbal-esque system machined from 7075 aluminum.
With this system I am able to calibrate the angles of the lasers and galvos laterally and horizontally: allowing a fine adjustment of the components in the optical field.
OPTICS & GLASS:
Both Galvos and the Resonant Scanner have first surface mirrors.
The PMT has a 532nm optical bandpass filter which filters all wavelengths except for the 532nm laser light, allowing the PMT to translate into voltages, only the reflected light from the Input Laser raster scan.
PHOTON TO ELECTRON - SIGNAL PROCESSING:
When the Input Laser's light, while scanning - hits a surface, the 532nm collimated lightwaves bounce off this surface. Some photons are absorbed by the surface, but some bounce off - the amount of photons which bounce off from any given surface will vary based on the quality of that surface (i.e. shiny surfaces will reflect more and absorb less whereas a matte surfaces will absorb more and reflect less). These photons are received by the PMT through precise placement of the PMT and by filtering what the PMT receives through optical components.
Photomultiplier Tubes are one of the few vacuum tubes to yet be replaced by silicone versions. They are ultra sensitive devices which when photons enter through the front face of the tube, move through a surface which gives the photon an electron-philic behavior. The tube has little fingers of metal preloaded with high voltage (i.e. tons of electrons hanging out on these fingers). As the photon bounces from electron-filled finger to electron-filled finger, it collects electrons, essentially translating photons into electrons.
Once these photons have become an electronic signal, the signal is still weak, and for the signal to be significant enough to translate small variations in surfaces, this signal must be amplified. The Amplifier Circuit then increases the amplitude of the signal from the PMT and translates this to voltages which can modulate the Output Laser.
--- PRODUCTION of Version 2.0 ---
The AD818AN video amplifier op amp will receive the signal from the PMT.
As I am not an engineer, and in prior iterations, I have attempted to use circuits from a variety of resources. These disallow the amplifier circuit from translating the PMT signal to a useable signal for the output laser.
So, I've decided to send boards to a PCB manufacturer, and are boards representing 3 examples on the datasheet. This will give me a baseline of circuits to modify the values of the components to acquire a reliable and useful amplified signal.
I included The Differential Line Receiver (above) because of:
It allows extraction of a low level signal in the presence of common-mode noise...
It requires -5V, +5V and a "commom-mode noise" signal which is 10nV divided by the square root of the Hz of the input signal.
I have severe reservations about this circuit working within my system.
The Video Line Driver
drive(s) back-terminated 75Ω video line to standard video levels
It requests a coaxial cable input and a 75Ω impedance wire at output. Again, I am not super sure that this will be the circuit for my system, but it may be able to be modified.
The Single Supply Amplifier is my strongest bet within this trifecta of example circuits.
It is designed to receive a singular input and will provide a higher current at output. The values of the capacitors and resistors in this circuit are particular to the configuration of the input signal and requirements for the output signal. Therefore, I ordered 10 versions of this board to allow for many iterations of components.
-----SIGNAL GEN FOR GALVOS----
The Jones Divider, is a pic chip programmed for me by the famous Jones Video Synthesizers -used to divide the resonant frequency of the Resonant Scanner to create the 525 lines per frame for the Galvos. As it is a pic chip, I am unable to reprogram the duty cycle (the Galvos are non-responsive because the duty cycle is too high).
Therefore I got myself a teensy prototyping board to re-create the code.
***Be sure to follow these instructions carefully before attempting to upload to the teensy**
Because I need the Galvos frequency to be synchronized with the Resonant Scanner frequency, I need to listen to the output sine wave generated by the Resonant Scanner and PID the output to the Galvos by checking in with the Resonant Scanner. The Scanner outputs a sine wave at analogRead values from 1-1023, as expected.
*Triggering is a tricky beast, so I still consider the images below to be approximate. Regardless as to whether this component drifts or not, this needs to be considered and troubleshot, even if the Resonant Scanner functioning perfectly*
The Resonant Scanner resonates, but has some variability in frequency:
This means that the two components will eventually go out of phase with one another.
---- OPTION A- TEENSY CODE ---
To compensate for this digitally, by sampling the output of the Resonant Scanner, waiting for a rising edge of a wave, and then triggering a 29.97hz at 50% duty cycle waveform. At the end of that cycle, the code should re-sample the Resonant Scanner sine wave, and adjust accordingly.
The code I am currently drafting will wait 6 times the target frequency (15.75Khz) - so that the Resonant Scanner will have enough time to begin to resonate (it takes a few seconds).
Then the code will start a frequency count using this library. Once this frequency has been read, the Galvo signal will be derived from the frequency sampled, and be divided by 29.97. Using this library, I will produce a PWM signal at a specific duty cycle which is a direct derivative of the Resonant Scanner Frequency.
Here is the Code for this:
The fallback of this logic is that I will no longer be producing a 30-frame-per-second video, but a 15 frame-per-second video.
---- OPTION B- Circuit Divider ---
J. M. De Cristofaro was kind enough to share a divider circuit he has designed using AND gates, R2R resistor ladder, after translating the SINE wave from the Resonant Scanner to a square wave with a 40106 - sending that to a counter (4040).
Here is the circuit he shared with me (his was on graph paper, I made the Eagle, so if there are errors in the circuit diagram below, rest assured, those belong to my mistranslation of his design).
--- Safety Notes ---
--THANKS AND APPRECIATION --
This is the second iteration of POISED. The first was one of learning and exploration, and there is a very very long-winded blogpost about that discovery stage here - if anything is unclear here, it will be spelled out on this blog - somewhere ... :