Category: Electronics Workbench

Use the sine-bar bandwidth pattern:

Engrave it in grayscale mode as a negative image with 0.1 mm line spacing:

Monitor the Ruida KT332N controller’s analog laser power control output:

The traces:

• 1 X axis DIR, low = left-to-right (yellow)
• 2 L-ON laser enable, low active (magenta)
• 3 L-AN analog voltage (cyan)

The scope triggers when the top two traces go low during a left-to-right scan with the laser beam active. The trigger point lies far off-screen to the left, with the delay set to pull the interesting part of the scan into view.

Although both the controller’s L-AN output and the laser’s IN input specify a signal range of 0 V to 5 V, the analog output voltage never goes below 0.4 V, but (as will seen later) that produces 0 mA from the laser power supply.

Set the X cursors to the top and bottom of the sine wave and read off the 4.36 V peak-to-peak value.

Set the Y cursors to matching points on successive cycles and read off ΔT=33.44 ms. Because each cycle is 1 mm wide, the scan speed is set to 25 mm/s and traveling 1 mm should require 40 ms, puzzle over that number and the related fact that 1/ΔT=29.91 Hz. This seems to happen only for speeds under 50-ish mm/s, for which I have no explanation.

Repeat the exercise at various speeds up through 500 mm/s:

The analog output voltage has dropped to 1.56 Vpp.

The average voltage increases from 2.66 V at 25 (or is it 33?) Hz to 2.78 at 500 Hz, which is reasonably close to the same value.

The signal’s -3dB point would be at √½ × 4.36 Vpp = 3.1 Vpp, which happens at 200 mm/s = 200 Hz:

Which is eerily close to the “around 200 Hz” bandwidth figured from the risetime measurements.

All of the analog output measurements as a slide show:

One might now wonder whether there’s any bandwidth difference between the analog and PWM signals as measured in the laser tube current.

Data! We need more data!

Along the same lines as the grayscale bars, a grayscale sine wave pattern allows direct bandwidth measurement:

The sine wave pattern comes from a totally cargo-culted Imagemagic invocation:

magick -size 100x100 gradient: -rotate 90 -function sinusoid 10.0,-90 'Sine bars - 10 cycles.png'

The pattern gets plunked into the same white/black frame as before, using GIMP because it’s easy.

Importing the resulting PNG image into LightBurn allows configuring the laser parameters. Each sine wave is 1 mm (ten whole pixels!) wide, so engraving at 250 mm/s covers one cycle every 4 ms for a 250 Hz signal:

Changing the engraving speed will change the test signal frequency, although the laser can’t get much beyond 500 mm/s.

The sine wave pattern goes from 0% to 100%, but at 250 Hz the controller output doesn’t reach those extremes, suggesting the output filter rolloff is lower than the 200 Hz inferred from the 1.5 ms risetime and falltime values.

Because the power supply output current isn’t matching the controller voltage excursion and its waveform is much rounder, its bandwidth is even lower.

The more I measure, the more puzzling it gets …

Given that the CO₂ laser power supply seems just as happy with an analog input as a digital PWM signal, one might wonder about the bandwidth of each mode. Rather than feeding the supply with a function generator, raster-scanning a grayscale target should suffice.

For example, this would generate five square waves:

The bars are 10 pixels wide, so scaling the image at 254 dpi makes them 1 mm wide:

As before, the first and last bars are 100% (white), with 0% (black) bars just inboard. The other bars are 10% and 90% to stay a little bit away from the 0 V and 5 V limits. I set Lightburn to invert the colors so that 100% = full power and 0% = beam off.

Engraving the pattern at 100 mm/s makes each bar 10 ms wide and the risetimes and falltimes are easy to see:

[Edit: Clicked the wrong picture.]

Although it’s a bit handwavy, a 1.5-ish ms risetime suggests a single pole (ordinary RC) time constant τ = 700 µs = 1.5 ms/2.2, so the controller’s output filter cutoff would be around 200 Hz = 1/(2π τ).

The laser tube current looks a little slower than that, so there’s a definite tradeoff among engraving speed, edge crispness, and power level.

More study is definitely needed …

Up to this point, the Ruida KT332N controller has set the laser power supply current from the PWM terminal:

The blue and purple wires go off to the oscilloscope I’ve been using to measure how the controller and power supply behave.

The L-AN terminal produces an equivalent analog signal:

The power supply accepts both analog and PWM signals on its IN terminal, so no rewiring was needed on that end:

This test pattern came in handy again:

The pattern has white bars on the left and right edges as markers. I invert the pattern in LightBurn so that white produced 100% PWM and black produced 0% PWM.

The L-AN output produces 5 V for 100% power and 0 V for 0% power, with other power fractions spread out in between:

The traces:

• 1 X axis DIR, low = left-to-right (yellow)
• 2 L-ON laser enable, low active (magenta)
• 3 L-AN analog voltage (cyan)
• 4 tube current – 10 mA/div (green)

Engraving that pattern in scrap acrylic looks like you’d expect:

There’s little trace of the discrete intensity levels in the acrylic trench and the scan interval is a rather coarse 0.2 mm.

The analog-mode current looks remarkably like the PWM-mode current for the same test pattern:

The PWM signal does not appear in that scope shot, because it runs at 20 kHz and is a blur at 20 ms/div.

It’s worth noting that the tube current has large startup spikes at low power levels in both PWM and analog control, so the spikes are generated internal to the power supply and have nothing to do with the PWM input signal.

Another test pattern using constant power:

At 10% power the analog output is about 0.5 V:

At 50% power the analog output is a constant 2.5 V and the tube current settles at a constant 12-ish mA, about half of the power supply’s maximum 25 mA:

Obviously, controlling the laser power to intermediate values using an analog signal does not involve switching the current between the supply’s minimum and maximum values: there are no PWM pulses involved to do the switching.

I suspect the analog output comes from the PWM signal run through an internal low-pass filter similar to the one in the power supply. Based on the PWM frequency measurements and squinting at the rise / fall times, the analog filter cutoff is probably around 1 kHz.

Other than bragging rights, I don’t see much advantage to using the analog signal in place of PWM.

Laser cutter controllers generally set the tube current (and, thus, beam power) through a digital PWM signal to the HV power supply. Confusingly, the same power supply input terminal can receive an analog signal controlling the output current. Both signals have the same 0 to 5 V range.

I have yet to see a PWM frequency spec for any HV laser power supply, although surely there must be one. The specs for the Cloudray power supply on my shelf seem typical:

I have no spec sheet for the replacement power supply OMTech sent, which is now installed in the laser and is measured below. I believe all similar HV laser power supplies, regardless of the nominal brand, are essentially the same inside and will have similar, if not identical, behavior.

Controllers descending from the GRBL codebase default to a PWM frequency of 1 kHz, a legacy of using the PWM output for spindle motor speed control. GRBL’s Smoothieware descendant has a configuration option for the period in microsecond steps with (I think) a default of 20 µs = 50 kHz. Ruida controllers similar to the (Ryxon) KT332N in my OMTech laser (seem to) default to 20 kHz frequency:

The laser frequency is used to set the pulse frequency of the control signal used by the laser. The glass tube is generally set to about 20KHZ

KT332N Manual, p 55

Knowing how a dozen measurements outweigh a thousand opinions, I recorded the power supply output current as a function of PWM frequency. The test setup is the same as for the original series of current measurements, with oscilloscope traces arranged thusly:

• 1 unused (yellow)
• 2 L-ON laser enable, low active (magenta)
• 3 PWM signal (cyan)
• 4 tube current – 10 mA/div (green)

I set the KT332N controller for a 200 ms pulse when poking the front-panel button, which is long enough to show any interesting behavior, and changed the PWM using its awkward controller interface. LightBurn provides access to the “vendor settings” which include the PWM frequency, which I set as needed:

So, we begin by varying the PWM frequency with a constant 50% PWM …

The default 20 kHz:

The upper half of the scope screen shows the entire 200 ms pulse, with the small slice near the middle appearing zoomed across the bottom half. The readout just above the buttons along the bottom gives the measured PWM percentage and frequency. The green trace shows the tube current is about 12 mA, half of the power supply’s maximum 25-ish mA.

The Tek current amplifier has plenty of thermal drift that I have not attempted to compensate, so always eyeball the average current with respect to the baseline around the pulse in the upper half of the screen.

No trace of the 20 kHz PWM signal appears in the tube current, which runs at a constant 12-ish mA for the duration of the 200 ms pulse.

Increasing the PWM frequency to 100 kHz (!) produces no change, although I cranked up the zoom timebase to better show the PWM pulses:

Reducing the PWM frequency to 10 kHz produces very small ripples in the output current corresponding to the PWM cycle:

At 5 kHz the tube current becomes sinusoidal, with an average around the same 12 mA produced at higher frequencies:

The sine wave current is about 90° out of phase with the square wave PWM, although much of that must come from delay through the entire power supply, rather than just an RC low-pass filter.

At 2 kHz the tube current takes on a decidedly lumpy look:

At 1 kHz there’s definitely something odd, perhaps a resonance, going on inside the supply, although the average current remains 12 mA:

At 500 Hz the PWM is slow enough that the tube current resembles the output of an integrator, rather than a filter:

At 100 Hz, the digital PWM signal is so far below the filter cutoff that it’s behaving as an analog input, with the tube current ramping between minimum and maximum:

The current has regular full-on glitches halfway through the “off” part of the PWM signal, so running at absurdly low PWM frequencies does not prevent them. Also note that the PWM signal does not control the current at the same speed as the L-ON enable signal, due to the low-pass filter rolling off the transitions.

Now, holding the PWM frequency constant at (the absurdly low) 100 Hz and varying the % PWM duty cycle …

At 30% PWM, the output current becomes triangular due to the low-pass filter:

At 99% PWM, the output stays at the power supply’s 24 mA maximum output, with small downward ramps marking the 1% off times:

Some observations for this HV power supply, which seems typical of similar supplies sporting other “brand names”:

• A PWM frequency below 10 kHz introduces output current variations due to the power supply interpreting the PWM waveform as a somewhat analog input, rather than a purely digital signal. This effect increases as the frequency decreases.
• An Arduino-speed digital PWM near 1 kHz will be interpreted as an analog signal, with the tube current varying significantly around the PWM signal’s average analog value. It does not control the current in an on-off digital manner.
• Due to the effect of the low-pass filter, the PWM signal cannot switch the tube current between “full off” and “full on” at any frequency. The current will always follow a ramp with a slope controlled by the filter rolloff, so low PWM inputs will have low peak currents.

I must switch to the controller’s analog output …