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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