CO₂ Laser Tube Current: Analog RiseTime Target

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:

Gray bars 10-90
Gray bars 10-90

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

LightBurn - bandwidth test pattern setup
LightBurn – bandwidth test pattern setup

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:

Tube Current - analog - gray bars 10-90 - 100mm-s - 10 ma-div
Tube Current – analog – gray bars 10-90 – 100mm-s – 10 ma-div

[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 …

CO₂ Laser Tube Current vs. Analog Control

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

Ruida KT332 - PWM laser control wiring
Ruida KT332 – PWM laser control wiring

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:

Ruida KT332 - analog laser control wiring
Ruida KT332 – analog laser control wiring

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

OMTech 60W HV power supply - terminals
OMTech 60W HV power supply – terminals

This test pattern came in handy again:

Gray bars
Gray bars

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:

Tube Current - analog - gray bars - 10 ma-div
Tube Current – analog – gray bars – 10 ma-div

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:

Analog mode acrylic engraving
Analog mode acrylic engraving

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:

Tube Current - grayscale bars - 100mm-s 10ma-div
Tube Current – grayscale bars – 100mm-s 10ma-div

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:

Pulse Timing Pattern - 1 mm blocks
Pulse Timing Pattern – 1 mm blocks

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

Tube Current - analog - 10pct 250mm-s - 10 ma-div
Tube Current – analog – 10pct 250mm-s – 10 ma-div

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:

Tube Current - analog - 50pct 250mm-s - 10 ma-div
Tube Current – analog – 50pct 250mm-s – 10 ma-div

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.

CO₂ Laser Tube Current vs. PWM Frequency

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:

Cloudray Laser Power Supply Features
Cloudray Laser Power Supply Features

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:

LightBurn Vendor Settings
LightBurn Vendor Settings

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

The default 20 kHz:

Tube Current - 50pct 20kHz PWM - 10 ma-div
Tube Current – 50pct 20kHz PWM – 10 ma-div

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:

Tube Current - 50pct 100kHz PWM - 10 ma-div
Tube Current – 50pct 100kHz PWM – 10 ma-div

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

Tube Current - 50pct 10kHz PWM - 10 ma-div
Tube Current – 50pct 10kHz PWM – 10 ma-div

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

Tube Current - 50pct 5kHz PWM - 10 ma-div
Tube Current – 50pct 5kHz PWM – 10 ma-div

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:

Tube Current - 50pct 2kHz PWM - 10 ma-div
Tube Current – 50pct 2kHz PWM – 10 ma-div

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

Tube Current - 50pct 1kHz PWM - 10 ma-div
Tube Current – 50pct 1kHz PWM – 10 ma-div

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

Tube Current - 50pct 0.5kHz PWM - 10 ma-div
Tube Current – 50pct 0.5kHz PWM – 10 ma-div

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:

Tube Current - 50pct 0.1kHz PWM - glitches - 10 ma-div
Tube Current – 50pct 0.1kHz PWM – glitches – 10 ma-div

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:

Tube Current - 30pct 0.1kHz PWM - detail - 10 ma-div
Tube Current – 30pct 0.1kHz PWM – detail – 10 ma-div

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

Tube Current - 99pct 0.1kHz PWM - 10 ma-div
Tube Current – 99pct 0.1kHz PWM – 10 ma-div

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 …

Marquetry: Veneer Test Piece

I haven’t given Mary a diamond in forever, so:

Marquetry test piece
Marquetry test piece

Straight up: this was mostly for fun, as can be determined by the hideous juxtaposition of the diamond amid a hexagon with the grain running the wrong way.

The diamond pattern was the least awful result of searching the Intertubes for diamond svg.

I didn’t expect it to work on the first try, but apart from having to calibrate the engraving depth in the scrap of plywood paneling, things went swimmingly:

Marquetry plywood engraving depth tests
Marquetry plywood engraving depth tests

I now have settings to excavate 0.2, 0.5, and 1.0 mm into that particular paneling. The veneer sheets were just over 0.5 mm thick and stuck out just enough to sand them flush.

The ideal kerf compensation turned out to be none at all, which I found after compensating the frame 0.1 mm outward on all sides, then having it not fit in the hole nor around the inner triangles.

A layer of yellow Elmer’s Wood Glue holds everything in place.

A few licks of 120 grit sandpaper, wipe it down with polyurethane finish, let it cure overnight, and it’s presentation-ready.

Got a chuckle, which is as much as I expected.

Icemaker Water Chiller: Inlet Check Valve Debris

Because the icemaker sits atop the cooling water bucket, when the pump turns off the water drains back through the laser tube into the bucket:

Silonn icemaker - installed
Silonn icemaker – installed

The bucket contained all the water to start with, so with the icemaker and laser tube empty, all the water is back in the bucket. Getting all the bubbles out of the laser tube takes a while after the pump starts running, so I stuck a check valve on the laser output tube in the icemaker’s reservoir:

Silonn icemaker - inlet check valve
Silonn icemaker – inlet check valve

Which, after a few days, developed a slow leak, once again emptying the reservoir.

There being no way to dismantle the valve for analysis and cleaning, I just cut it apart:

Silonn icemaker - inlet check disassembly
Silonn icemaker – inlet check disassembly

Lo and behold, a small tangle of thin fibers had found its way into the valve:

Silonn icemaker - check valve debris
Silonn icemaker – check valve debris

Which held the silicone disk ajar and let the water slowly leak backwards through the valve.

I have no idea where it might have come from, but a simple filter seems like a good idea. Given that the pump produces pretty nearly zero pressure, anything fancier than a coffee filter in a funnel would present too much back pressure.

Or, with three more valves in the bag, I can wait to see how long it takes for another tangle to arrive …

Humana Email Unsubscribe FAIL: Redux

Around this time last year, Humana was spamming me with emails sporting a misconfigured unsubscribe link, so that I could not get myself off their mailing list.

This year, they have the unsubscribe link set up properly, except …

Humana email unsubscribe rejection
Humana email unsubscribe rejection

Apparently my email address was good enough to get their junk email to me, but it’s not good enough for them to stop sending junk.

I was pretty sure this was deliberate last year. Now, I’m certain.

And they want me to trust them?

You can’t make this stuff up.

Jar Lid Measuring Spoon Holders

We have accumulated enough measuring spoons (typically from garage sales) to dedicate them for specific purposes, which means keeping them from wandering away:

Jar lid measuring spoon holders
Jar lid measuring spoon holders

The design is simple enough:

Jar lid measuring spoon holder - LB layout
Jar lid measuring spoon holder – LB layout

The slot is a rounded rectangle about 2 mm larger than the spoon handle in both directions, inside a rounded rectangle large enough to put the handle just clear of the jar. The curved side comes from outsetting the jar lid OD by a millimeter (for the double-sided foam tape), then subtracting that circle from the holder.

So, yeah, they’re custom-made for the spoon and jar in hand.

They come from a sheet of 1/4 inch = 6.3 mm Vintage Acrylic. The holder on the smaller jar is two stuck together with super-whoopie low-surface-energy tape before being stuck to the lid. I’m trying the tape on some non-critical projects to see how it behaves: so far, so good.