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Showing posts with label K40 Laser Response Tst. Show all posts
Showing posts with label K40 Laser Response Tst. Show all posts

Monday, March 6, 2017

Laser Response Charcteristics

Laser Response Characteristics

I moved the research that I have been doing into "laser response" to this post to capture it in a more focused way. 
Understanding the lasers response is an important part of getting to the best engraving control possible. The engraving control basics are described in Engraving-and-pwm-control.html. 
Understanding the relationship between the HV power (voltage and current), the tubes gas discharge characteristics and the lasing process of the C02 is integral to an understanding of how to optimize digital control of its power.

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Your donations help fund additional research, tools and parts that I will return to the community as information. For other information on the K40-S build use the  K40-S BUILD INDEX with schematics as a source of information 

Links to Related Posts

Electrical Modulation of a CO2 Laser

"Low frequency modulation can be achieved by pulsing or chopping the electrical power to the discharge. As the frequency is increased, the effect of the varying input decreases and above a few kHz, disappears entirely. The output of a DC or RF excited CO2 laser are both CW beams.
(From: David Toebaert (olx08152@online.be).)
This remark really holds for any kind of CO2 laser (the effect gets worse at higher pressure). It's just nature: it takes time for the molecules to 'meet' one another causing the delay. For a laser at 100 mbar (around 76 Torr) and a typical gas mix, the cut-off frequency is about 3 kHz. Above that the modulation of the input power is strongly damped and hardly visible anymore in the output power.

Simply think of the discharge as a *low pass filter for the input power, no matter how you excite the discharge. Of course, it's possible to modulate the input power at much higher frequencies (e.g. an RF supply can easily be modulated up to 100 kHz, that is, the Mhz signal is modulated at 100 kHz), but from the point of view of wanting to modulate output power, it makes no sense. Maybe it's beneficial for other reasons (e.g., discharge stability)."

*Whats a low pass filter?

The Effect of Modulation Cutoff Frequency 

The article above says that the laser cannot transfer input power changes that occur at a rate > than 3kHz. That means that the max time between changes that will be useful is: 3kHz  = 1/3000 = .00033 = 330 us. 
This means that in the model above the suggested pwm period (200us = 5kHz) is longer than the response time of the lasers gas ionization. 

 If that is true then:
  1. The PWM period needs to be much longer or the gantry slowed down considerably
  2. The much faster times I measured (2us) by monitoring current suggests that it is not a  indication of the speed of light output. Is the current flow during ionization much faster than the light output?
More testing needed :(.

Laser Gas Discharge Characteristics

Negative Resistance
Negative resistance of a gas discharge: the voltage and current increase as described by ohms law until the discharge point. Then rapidly the voltage decreases and the current increases.

The hypothesis used in testing (above) and modelling; that the current flowing through the laser tube can be used as a faximile of the response of its light output does not track with the above modulation information :).

Laser Power Sources

Electric Discharge


Pumping the Laser


This video shows measurements of response speeds in the 4ms region.

Laser Operation

Lots of info here: 

CO2 Info Summarized From Links

Gas mixture & its function:
13.5% N2 :    excited by gas discharge (pink glow) collides with and moves CO2 to level 3
9.5%   CO2: molecule that lases at energy level 3-2.
77%    He:    collides with CO2 at level 2 and then collides with tube walls for cooling
2%      H2:    gas discharge disassociates CO2 into CO and 02. H2 mixes with CO & O2 to regenerate CO2  

Ionization Voltage: 25 KV
Voltage at lasing:    13-15 KV
Negative resistance: 200-300K

Breakdown voltages

https://en.wikipedia.org/wiki/Dielectric_gas
https://en.wikipedia.org/wiki/Dielectric_strength

CO2: Air *.95
Air:  3,000,000 V meter

Laser Tubes

Synrad 40 W Lasers: Specifies a 100us rise time.

References To Aricles and Previous Work

https://en.wikipedia.org/wiki/Grayscale
Understanding CO2 lasers
Principles of plasma discharge
Dynamic PSpice Model of C02 Laser Tube
Gas Laser Electronics
Basic Laser Principles


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Enjoy and leave comments and discussion;

Maker Don
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Thursday, July 14, 2016

K40 Dynamic Laser Response testing

K40 Laser Response Testing:

This post tracks activities related to testing the laser light response to various control inputs to the Laser Power Supply (LPS).
This testing was prompted by the confusion surrounding how to control the laser  and it's power from aftermarket controllers, more specifically, the Smoothie.

Test equipment:

There are three artifacts of the testing environment.

1. PWM generator:

The PWM generates a 5v, low or high true PWM signal with variable frequency and duty cycle.

Mode switch PWM position:

  • This switch must be in the "PWM" position to output a pwm signal.
  • In the Pulse position the momentary button is enabled.

Signal polarity:

The polarity of the pwm signal is selected via jumpers on the breadboard. See schematic.

Freq. & Duty cycle:

The frequency and duty cycle are set using two potentiometers on the control panel.
The PWM generator

2. Interconnect: 

The cable from the breadboard connects to the LPS supplying power to the tester which in turn supplies PWM and pulse control signal's to the Laser Power Supply.

3. IR Receiver Module (IRRM):

The IR receiver module contains a reverse biased photo diode mounted below a small water reservoir made from a drilled acrylic plate. The water is expected to be an attenuator at high power.
The detectors output varies proportional to the laser light providing an electronic signal that correlates with the dynamic and static characteristics of the lasers output. This signal can be connected to an oscilloscope showing the dynamics of the lasers output and timing. A voltmeter can also be connected to the IRRM to measure static outputs.
Top "Laser: View
Perspective view
The IR sensor (from RS)

Test plan:

Potential Sources of Error:

  1. IR detector response: if the IR sensors light response is slower than the lasers output response we could be measuring the response of the sensor and not the laser [unlikely but be aware]
  2. Peak Measurements: the IR sensor module is not capable of accurately measuring the output in watts as this is a dynamic peak measurement device.[this test is not attempting to measure static power output].
  3. High Power: at various power levels the attenuation characteristics of the water drop could non-linear-ize the amount of light the IR sensor detects.[test repeat-ability]
  4. Optical reflections: the optical characteristics of the water drop could reduce the repeat-ability of the IRRM measurement. [test repeat-ability]

Improvements:

I can see where an Arduino based PWM controller might be advantageous:

  • Better isolation between DF setting and frequency (in 555 designs it is hard to isolate these controls)
  • More accurate and wider frequency range
  • Ability to control the number of PWM cycles making it safer and more predictable measurements. Not advertenlty left with PWM running.
  • DSP measurement of IRRM output in more sophisticated version
  • Automated data collection 
  • Automated control of the laser analog power.


Enjoy and comment
Don
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