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DA converter versus PWM for laser power

Added by reinoud over 6 years ago

Hi folks,

wouldn't it be a better idea to provide the Laos board with its own low-pass filter to convert the laser PWM into a more-or-less steady voltage level? This will also remove the interference with the pixel clock i'm seeing when engraving. Apparently my laser kit doesn't have a low-pass filter or at least one that is not working as it should.

An idea for v5 board?

With regards,
Reinoud


Replies (4)

RE: DA converter versus PWM for laser power - Added by KalleP over 6 years ago

A low-pass filter is a good idea.

The problem is if you make it too low then you cannot change the setting at the full raster speed (possibly) so one has to keep trade-offs in mind.

Does anyone know what the maximum pixel clocks are on LAOS and on other commercial raster lasers?

We must also remember a 3rd oscillator that is involved in this equation. The SMPS circuit on the LASER HV PSU unit is not tuned for grayscale operation usually. The set-point input on some does not go to the expected comparator inputs on the controller ICs and on others it may be connected to dead-time circuits and other things. The issue is that any PWM input that is not well filtered will have unknown effects on the SMPS oscillator causing extra effects.

The best way to do it is to have the highest possible PWM frequency from the MBED so that the lowpass filter can be chosen as high as possible. It will also minimise the effect of the step rate and the PWM frequency.

The Low pass filter is just a resistor and capacitor hopefully so could be added to wiring harness.

If you plan to use duty cycle control to set the power output you MUST use the correct (hopefully available on the PSU) digital on/off control and not try and turn the level on and off with the analogue input which SHOULD be less responsive.

Kalle

RE: DA converter versus PWM for laser power - Added by Springuin over 6 years ago

The output of the mainboard is an optocoupler; it does not output a voltage, it pulls down the assumed pullup resistor of the power supply. The power supply is also assumed to have a input filtering capacitor to filter some noise in the original situation where there is a potentiometer. Those two should be sufficient to filter the PWM output.

If there is no such capacitor one could be added. But for the quality of your analog signal the best place would be as close as possible to the power supply and not on the mainboard.

I recognize the need for a nice stable analog voltage to control the laser power but adding a filter to the mainboard is not the right solution in my opinion.

Reinoud, have you tried the capacitor you mentioned in the other thread yet? I'd suggest to first determine the resistance of the pullup resistor and then select a capacitor such that the cutoff frequency is at something like a quarter of the PWM frequency: f_PWM * 0.25 = 1/(2*pi*R*C).

RE: DA converter versus PWM for laser power - Added by reinoud over 6 years ago

As the PWM freq if 5kHz and the pullup resistor is 1kOhm, i've put a 0.1 uF on the output line to the ground. This had drastic effects! For one it makes clear that my lasercutter didn't have a low-pass filter at all! We had to push the output power up quite severely to even see it engrave! Apparently it was just implementing 100% power during the PWM cycles. I haven't seen the completed result nor have investigated it thoroughly it looked a lot better at first glace. I think we're on the right track!

At first glance there still seem to be two types of `pixels' engraved: shallow ones and when there are quite a lot in a line it starts to kind of 3d etch! What is the difference between 3d etching and engraving in software terms? less dense pixelations on `white' by say `darkening' the image so dithering won't produce massive amount of pixels?

With regards
Reinoud

RE: DA converter versus PWM for laser power - Added by KalleP over 6 years ago

Something I just remembered about opto couplers. The standard types have a limited switching speed so the PWM frequency cannot be increased without limit before there will be an asymmetrical roll-off.

The Analogue input on the HV PSU cannot be assumed to have a filter capacitor at all as the POT input does not need one.

The I2C or CAN bus could be used to drive a DAC IC to get a accurate voltage but perhaps not at the pixel clock rate for 3D engraving so perhaps not much better than a PWM output in the end.

The beauty of the PWM method is that it can go through a suitable opto isolator (at the goldilocks frequency) and then be filtered on the PSU side with a simple RC filter, with a try DAC voltage output you have to go to a LOT of trouble to have an isolated analogue signal and this is to be preferred for connecting to a strange chines HV PSU that has not been introduced to your father.

There are a number of reports and techniques that use the increased pulse output at the start of a pulse to gain more penetrating power. This may be a feature in some systems (RF tubes) but is likely to be a design oversight in the PSU (DC tubes) where the output overshoots after turn on because the internal feedback mechanism is too slow, this may occur if the change of voltage is too fast for the PSU design (low internal switching frequency) or there is a large storage capacitance in the PSU that has an extra moment to charge between pulses, this may be as a result of an underrated PSU. Aiming to use this 'feature' is not the most predictable solution in a random installation and it would make more sense to try and work within the linear range of the PSU/tube combination.

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