The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Machine Shop

Mechanical widgetry

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

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

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

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

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

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

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

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

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

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

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

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

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 …
2022-10-27
Marquetry: Veneer Test Piece

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

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

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.

2022-10-26
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

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

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

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

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 …

2022-10-25
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

The design is simple enough:

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.

2022-10-21
Hand Shower Mounting Bracket

For reasons not relevant here, a hand shower will come in mmm handy for a while in a month or two. The threads on its plastic diverter valve pretty nearly match those on the 70 year old iron pipe in the front bathroom, although the original brass shower head may have been installed by John Henry the Steel-Drivin’ Man.

In any event, you’re supposed to drill two screw holes in the wall for the holder, which is just not happening. Instead, scan the bottom of the holder and blow out the contrast for the next step:

Hand Shower bracket – scan

Yes, those holes are off-center in their molded bosses. They’re centered in their front recesses and I cannot imagine how, in this day and age of CAD everything, a designer could misalign the front and the back, but there it is.

A little cleanup produces a reasonable mask:

Hand Shower bracket – mask

The holes are centered in the outline, as you’d expect.

Import it into LightBurn, trace the perimeters, put those vectors on a tooling layer, and hand-draw a much simpler / smoother outline on the cutting layer. One of the vintage acrylic sheets is 1/4 inch thick, just enough for the shortest M4 brass inserts, so wrap the holes around the inserts:

Hand Shower bracket – LB layout

Some acrylic adhesive goops the inserts in place, although I’m not convinced it has enough pull strength in those slick holes:

Hand Shower bracket – mounting plate

~~When~~ if it fails, I’ll rebuild the plate with an engraved ring around the back of each hole, along the lines of the earrings, and epoxy the inserts in place.

Double-sided foam tape will eventually stick the holder to the tile above the tub, but finding the proper location requires UX research.

2022-10-20
Rattlecan Coasters

Rattlecan spray paint, unlike scribbled markers, produces vivid colors on white chipboard:

Coaster Assortment

Cut the painted sheets cut face-down atop magnetic spikes on the honeycomb platform, with tabs to keep the petals in place and 0.15 mm kerf compensation. A light touch with an Xacto knife severs the tabs, after which the petals press firmly into the frames. Spread yellow PVA wood glue across the bottom disk, align the perimeters and press together, lay parchment paper between the coasters, clamp the stack between plywood sheets, and they emerge perfectly flat the next day.

They’re too labor-intensive for any economic activity, but I like ’em:

Coaster assortment

The pale gray petals in a white frame looks remarkably like the washed-out color scheme on whatever device you’re reading this, doesn’t it?

2022-10-18
Dell Sound Bar Under-Shelf Mount

A bedroom rearrangement displaced the Dell Sound Bar attached to the streaming music player from its accustomed perch, so I conjured a mount from the parts bin to hang it from a shelf:

Dell sound bar mount – installed

The sound bar originally fit below any Dell monitor with the appropriate lugs under the bezel, but a bit of bandsaw work and hand filing produced a reasonable facsimile from an aluminum sheet:

Dell sound bar mount – plate installed

The bar’s plastic bits require a few millimeters of clearance above the sheet, now provided by a matching plywood shape:

Dell sound bar mount – parts

A trial fit showed all the parts would fly in formation:

Dell sound bar mount – trial fit

A laser-cut cardboard template maintained alignment and spacing while I stood on my head screwing the mount in place.

All’s well that ends well!

2022-10-17