Laser EMF Control Test
8/12/2023
M.L. Hembrey

QUESTION:
Is the laser output power controlled by a PWM signal or a DC voltage?

EQUIPMENT:
Beckman Industrial Corp. DM15XL digital multimeter
RIGOL DS1102E digital oscilloscope
Samsung TAB 10.5 camera mode
Sainsmart 2018 Pro CNC with 5.5w option

PROCEDURE:

All voltages are referenced to a GND pin on the GRBL controller board.

With the DM15XL, I measured 4.95 volts to the laser with the LCM (LASER CONTROLLER MODULE)test button pressed. This is noted as the maximum average design voltage applied to the laser. When released, the voltage dropped to about 2.8 volts and gradually lowered toward zero. There was no knowlege of the applied waveform.

With the DM15XL, I measured 8.65 volts to the yellow control "PWM" wire of the LCM. This was with no output of the laser beam commanded.

I then connected the scope (DC1102E) to the yellow wire to the laser. Scope settings are 2.0 volts/div and DC input. This was followed by a G01S750F50X40 command. As the head traverse the work, I snapped a series of scope pictures. The first, which is uploaded as MLH Laser EMF Test.jpg, shows the scope screen capture.

The scope did not show a PWM signal, but a DC voltage that varied a lot. The 9 pictures taken at random captured these voltages:
3.28
4.08
4.88
3.44
3.28
4.64
3.04
3.60
2.96 (I believe this one was after the laser had stopped.)

The second test was at the yellow PWM wore inpit to the LCM. The scope input scale was set to 5.0 volts/div due to the ogher inpit. I commanded another pass, at G01S500F50X40, with the laser. A series of pictures showed the control voltage to the LCM was relatively steady at about 1.4 volts, returning to about 8.65 volts at the end. I included MLH LCM PWM EMF Test.jpg to show this result.

SUMMARY:
Contrary to popular opinion, including mine, there is no PWM signal to either the laser nor the laser control module. The output of the GRBL controller is a varying DC voltage with no inherent waveform.

It appears the PWM voltage is entirely within the GRBL controller circuitry, and is used to establish an adjustable DC power source.

My particular LCM inverts the voltage swing. As the voltage goes from 8v to zero, the output goes from zero to 5v. This inverted output from the GRBL controller apparently is performed in the firmware.

CONCLUSION:
My laser control module is obviously of poor design. A 2-volt swing in the output with a constant input means the output power of the laser varies a lot.

M. Hembrey