CO₂ Laser Tube Current: Variable Power

A test pattern with a grayscale of 1 mm bars:

Gray bars
Gray bars

There is a 1 mm white bar to the left of the leftmost black bar as a scope trace marker and 2 mm white bar to the right of the rightmost black bar for direction confirmation.

Setting the image to 254 dpi = 10 pix/mm makes the bars exactly 10 pixels wide and scanning at 100 mm/s makes them 10 ms wide. They’re tall enough to simplify scope triggering and capture.

Although using a black bar for 0% PWM = 0 mA and a white bar for 100% PWM makes numerical sense, at least to me, it’s the other way around for laser cutting / engraving: black = 100% and white = 0%. With the layer set to Fill in LightBurn, turn the layer’s Negative Image switch on, and everything comes out right.

Engraving a good grayscale or 3D image is a can of worms, so I just fired the beam into a shallow pan of water.

With signals and traces arranged as before, the beam current shows the same huge spikes during the 10% and 20% PWM bars and at the start of the 100% PWM bars:

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

At 100 mA/div, those spikes look to be 400 mA tall.

A closer look with the current scaled to 10 mA/div:

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

The controller sets L-ON high whenever the beam current should be zero, so the power supply is disabled during the 0% PWM bars. Note the descending glitch at the start of the 10% PWM bar: perhaps the power supply stayed all charged up from the 100% white bar on the left edge and took a few milliseconds to begin tracking the lower current setting.

Each step of what should be a stairway from 10% to 100% PWM has about 2 ms of good old single-pole response. The steps from 70% upward have enough ripple to obscure the steps; the rightmost 100% PWM bar show the ripple doesn’t damp down for 20 ms.

Eyeballometrically, the ramp compresses on the high-current end: equal PWM steps produce less current per step. The current spikes make PWM values of 10% to 20% look awful, PWM between 30% and 50% seem more linear, and increments beyond 60% are rather compressed. The slight nonlinearity makes no practical difference, particularly because the usual recommendation is to not exceed 70%-ish PWM to prolong the tube life.

A continuous grayscale gradient:

Gray gradient
Gray gradient

As before, there’s a 1 mm white bar on the left and a 2 mm white bar on the right, with the image inverted to make the white bars 100% PWM.

Apparently the power supply can’t regulate the current down from the 100% PWM bar fast enough to match the 0% PWM start of the ramp:

Tube Current - gray ramp - 100mm-s 10ma-div
Tube Current – gray ramp – 100mm-s 10ma-div

The compressed relation between PWM and current shows there’s definitely not much benefit in driving the tube beyond about 60% PWM.

There are no high-current spikes in that screenshot, despite having a 0% to 100% PWM gradient.

Unlike the gray bars in the first test image up top, this is a continuous ramp and shouldn’t have any discontinuities. The vertical cursors span eight ripples and sit 66 ms apart, which works out to 8.25 ms/ripple. Flip it upside down and you’re looking at 120 Hz ripple from the full-wave bridge rectifier feeding the high-voltage converter. You’d expect solid low-pass filtering after the high-frequency flyback transformer, so the input filter must have the smallest possible caps the designers could possibly use.

Another smooth gradient preceded by a 10% PWM bar and bracketed by the same white bars:

Gray gradient - 10 pct bar
Gray gradient – 10 pct bar

The current waveform is … odd:

Tube Current - gray ramp - 10 PWM bar - 100mm-s 10ma-div
Tube Current – gray ramp – 10 PWM bar – 100mm-s 10ma-div

The high-current spikes following the 100% PWM bar on the left occupy the 10% PWM bar and the start of the gradient, up to about 20% PWM. Apparently the spikes happen while the power supply attempts to produce more-or-less continuous current at PWM values below about 25%

And I thought this was going to be simple …

CO₂ Laser Tube Current: Constant Power

The test pattern consists of 1 mm blocks:

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

I set the layer speed at 250 mm/s = 4 ms / mm, then set PWM (a.k.a. “power”) for each test, and measured the results, which look like this for three power levels on corrugated cardboard:

Pulse Timing Pattern - cardboard - 10 20 30 pct
Pulse Timing Pattern – cardboard – 10 20 30 pct

The scan interval of 0.2 mm produces distinct lines at 10% PWM, the lower limit of the laser’s range. The lines remain separate at 30%, although their width is definitely increasing.

Yesterday’s post explains the test wiring setup and the signals in the scope screenshots.

The 10% PWM current waveform looks like nothing you’d expect:

Tube Current - 10pct - 250mm-s - 5ma-div
Tube Current – 10pct – 250mm-s – 5ma-div

The scope triggers at the start of a left-to-right scan line, with 50 ms devoted to ramping up the speed to 250 mm/s before the start of the vertical bar along the left edge and slowing down before reversing.

The green trace shows huge spikes in the laser current, not a well-defined DC current pulse, and they’re offscale beyond 30 mA at 5 mA/div. The baseline sits well above the 0 V line due to the AM502 amplifier’s breathtaking thermal drift; I occasionally touch it up, but the current really is zero between the pulses.

Similarly for 20% PWM:

Tube Current - 20pct - 250mm-s - 5ma-div
Tube Current – 20pct – 250mm-s – 5ma-div

Even through there’s little visible difference between the 10% and 20% current waveforms, there’s a distinct difference in the actual beam power delivered to the cardboard.

At 30% PWM the beam current looks a bit more reasonable:

Tube Current - 30pct - 250mm-s - 5ma-div
Tube Current – 30pct – 250mm-s – 5ma-div

The 2 mm = 8 ms bar on the right gives the current time to stabilize at 6 mA, but all of the pulses have at least 3 ms of spikes. The first pulse definitely looks worse, so it seems the power supply gets better as the scan line progresses.

At 40% PWM the beam current pulses look more like pulses:

Tube Current - 40pct - 250mm-s - 5ma-div
Tube Current – 40pct – 250mm-s – 5ma-div

They still have 3 ms or so of those startup spikes, as seen in this closer look at the first pulse in a line, scaled at 10 mA/div (along with the PWM drive signal):

Tube Current - 40pct PWM first detail - 250mm-s - 10ma-div
Tube Current – 40pct PWM first detail – 250mm-s – 10ma-div

The top of those spikes exceed 70 mA!

At 80% PWM, the current waveform looks like a damped tank circuit:

Tube Current - 80pct first - 250mm-s - 5ma-div
Tube Current – 80pct first – 250mm-s – 5ma-div

The 20 mA at the end of that pulse suggests the maximum tube current would be 25 mA, which is undoubtedly why OMTech recommends running at no more than 70% PWM = 17-ish mA.

The pulses start immediately after the L-ON signal goes active and stop promptly when it goes inactive, so there’s no question about the responsiveness. What baffles me is why the current looks the way it does.

I must figure out how to have the scope compute the RMS value of those spikes, with a sufficiently large mA/div setting to keep the entire range of the pulses on the screen.

Stone Cold Swerve

We’re southbound on Rt 376, ticking along at about 15 mph, with fresh string-trimmer debris littering the shoulder:

T – 50 ms

Did you notice the rock? I didn’t.

The fairing ripples as my front tire hits the left side of the rock:

T = 0

I have no memory of the next two seconds.

The offset impact turns the front wheel to the left, so the bike steers out from underneath my weight:

T + 500 ms

Because the bike frame was still aimed straight ahead, the wheel is steering further to the left and putting me even more off-balance. I am somehow trying to lean left far enough to get my weight lined up with the bike:

T + 1.0 s

One second into the event, Mary has no idea what’s going on behind her.

My memory resumes with an image of the yellow midline just beyond my left foot:

T + 2.0 s

Mary heard an odd sound and asks (over the radio) “Are you all right?”

I’m approximately balanced, turning toward the shoulder, and manage to shout “NO!”:

T + 3.0 s

I’m coasting toward the shoulder with my feet off the pedals:

T + 4.0 s

Mary is stopping and I coast past her:

T + 5.0s

Landing gear out:

T + 6.0 s

Back on the shoulder, lining up with the guide rail:

T + 7 s

Dead slow:

T + 8.0 s

Docking adapter deployed:

T + 9.0 s

And stopped:

T + 10.0 s

I sat in that exact position for nearly four minutes.

A slideshow view of the same images so you can watch it unfold:

Doesn’t look like much, does it?

If I could have looked over my shoulder, this is what I would have seen, starting at T = 0 with the rock impact blurring the image:

Surely scared the daylights out of that driver, perhaps confirming all the usual expectations of crazy bicyclist behavior.

Here’s what Mary would have seen over her shoulder, again starting at T = 0 with the fairing bulging from the impact:

Timing is everything.

That Benz is new enough to have automatic emergency braking, as it slowed pretty dramatically while I was busy getting out of the way, but it’s not clear whether AEB knows about small / lightweight targets like pedestrians and bicyclists.

We completed the ride as planned, although I finally realized the front fender bracket had broken a few miles later.

Every adult human male has at least one story beginning “But for that millisecond or inch, I wouldn’t be here.” Now I have one more.

I must not fear. Fear is the mind-killer. Fear is the little-death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain.

Frank Herbert, Dune

Tree Frog Season

This year brings an abundance of tree frogs:

Tree frog - on dahlia stem
Tree frog – on dahlia stem

Despite the snappy green color, they’re Gray Treefrogs:

Tree frog - on patio step
Tree frog – on patio step

Their camouflage works better in the wild than atop a trash can lid:

Tree frog - on trash can lid
Tree frog – on trash can lid

They are much smaller than you’d expect from their voices in the night:

Tree frog - on trash can lid - thumb for scale
Tree frog – on trash can lid – thumb for scale

We think the drought brings them closer to the house in search of water, as Mary collects rainwater in the trash cans where the frogs easily walk up & down the inside surfaces.

OMTech 60 W Laser: Replacement HV Power Supply Waveforms

While I had the hatch open, I thought it would be interesting to look at the HV supply’s current waveforms:

HV laser power supply - current probe setup
HV laser power supply – current probe setup

The Tek current probe over on the right measures return current through the cathode wire, the point in the circuit where you might be tempted to install an ordinary analog (moving-coil) panel milliammeter, oriented so (conventional) current returning from the tube will produce a positive voltage.

Unfortunately, an analog meter isn’t up to displaying anything meaningful for this nonsense:

HV laser power supply - 5 mA-div - 50 ms 10 pct pulse
HV laser power supply – 5 mA-div – 50 ms 10 pct pulse

Admittedly, that’s a 50 ms pulse, during which an analog meter would barely twitch. The vertical scale is 5 mA/div, so the highest peaks exceed 35 mA, more than twice the tube’s recommended “14-15 mA”.

A closer look at the pulse startup waveform:

HV laser power supply - 5 mA-div - 50 ms 10 pct pulse - detail
HV laser power supply – 5 mA-div – 50 ms 10 pct pulse – detail

It sure looks like the chaotic current through a forced neon-bulb relaxation oscillator. Remember neon bulbs?

An even closer look:

HV laser power supply - 5 mA-div - 50 ms 10 pct pulse - tight detail
HV laser power supply – 5 mA-div – 50 ms 10 pct pulse – tight detail

That’s at 10% PWM, close to the threshold below which the laser just won’t fire at all. The power supply must ramp up to produce enough voltage to fire the tube while simultaneously limiting the current to prevent the discharge from sliding down the negative resistance part of its curve.

Apparently this supply isn’t quite up to the task.

A 10 ms pulse at 50% PWM gives the supply enough time to stabilize the current:

HV laser power supply - 5 mA-div - 10 ms 50 pct pulse
HV laser power supply – 5 mA-div – 10 ms 50 pct pulse

The 14-ish mA at the tail end of the pulse (note the baseline offset) matches my previous 13 to 14 mA measurements as closely as seems reasonable. That 2 ms of hash on the leading edge suggests the start of each cut or engraving line will be a bit darker than you might expect.

Another 10 ms pulse, this time at 99% PWM:

HV laser power supply - 5 mA-div - 10 ms 99 pct pulse
HV laser power supply – 5 mA-div – 10 ms 99 pct pulse

The peak 24-ish mA matches the previous measurements. Note that the peaks in all the previous pictures exceed the 99% PWM current level.

AFAICT, all PWM values below about 25% produce equivalent results: random current spikes with unpredictable timing and amplitude. Changing the PWM value does not affect the (average) tube current or laser output power in any predictable way.

Some samples to illustrate the point, starting with a different 50 ms pulse at 10% PWM than the first one up above:

HV laser power supply – 5 mA-div – 50 ms 10 pct

A 50 ms pulse at 15% PWM:

HV laser power supply - 5 mA-div - 50 ms 15 pct
HV laser power supply – 5 mA-div – 50 ms 15 pct

A 50 ms pulse at 20% PWM:

HV laser power supply - 5 mA-div - 50 ms 20 pct
HV laser power supply – 5 mA-div – 50 ms 20 pct

A 50 ms pulse at 25% PWM:

HV laser power supply - 5 mA-div - 50 ms 25 pct
HV laser power supply – 5 mA-div – 50 ms 25 pct

Now, that last one is different. After the hash during the first 8 ms or so, the power supply actually produces a stable 5 mA beam current, which is roughly what I measured using the power supply’s meter.

However, the other three are pretty much identical: the 10% PWM pulse does not delivers half as energy as the 20% PWM pulse. The waveforms may be different, but not in a meaningful or consistent way: the two 50 ms 10% pulses are different, but you’d (well, I’d) have trouble separating them from the 20% pulse.

To summarize:

  • The first several millisconds of any pulse will consist of randomly distributed spikes with very large tube currents.
  • For PWM values greater than 25%, the tube current will settle down to the corresponding current after 5 to 10 ms. Before the current settles down, the tube will be firing those random spikes.
  • For PWM values less than 25%, the tube current never settles down: the entire pulse, no matter how long, will be short, high-intensity spikes, without a consistent DC-ish level.

No matter what an analog meter might show.

I have no way to know if this power supply is defective, but I’ll certainly ask …

Smashed Glass vs. Epoxy

Just to see what happens, I laid some smashed glass in puddles of epoxy:

Smashed Glass vs epoxy - samples
Smashed Glass vs epoxy – samples

Backlighting with the LED light pad reveals more detail:

Smashed Glass vs epoxy - backlit samples
Smashed Glass vs epoxy – backlit samples

The chunk on the left is the proof-of-concept shot glass coaster with a form-fit black acrylic mask atop a clear epoxy layer on a clear acrylic base. The chunk at the top is raw shattered glass fresh from the pile. The two chunks on teardrop acrylic scraps are bedded in transparent black and opaque black tinted epoxy.

A look through the microscope at all four, laid out in that order, with the contrast blown out to emphasize the grain boundaries:

Smashed Glass vs epoxy - magnified comparison
Smashed Glass vs epoxy – magnified comparison

You may want to open the image in a new tab for more detail.

The raw chunk has air between all its cuboids, so it’s nicely glittery. All the others have much of their air replaced by epoxy.

Clear epoxy produces an essentially transparent layer where it fills the gaps, because its refractive index comes close enough to the glass. The stretched contrast makes the gaps visible again, but the backlit image shows the unassisted eyeball view.

Transparent black dye sounds like an oxymoron, but it fills the gaps with enough contrast to remain visible. The overall chunk is not particularly glittery, but it’s OK.

Opaque black dye produces a much darker tint; the slightly tapered thin layer between the glass and acrylic (the small white circles are air bubbles) cuts down on the transmitted light. The gaps remain nearly as prominent as in the air-filled chunk, although with very little glitter.

Bedding the glass in epoxy against an acrylic sheet should reduce its tendency to fall apart at the slightest provocation, although the proof-of-concept poured coaster showed the epoxy must cover the entire edge of the glass sheet to bond all the slivers in place.

Onion Maggot Flies vs. Sticky Traps: Round 2

Mary decided the second round of sticky traps had collected enough Onion Maggot Flies (and other detritus) to warrant replacement, so this season will have three sets of cards.

The two sides of each card after about a month in the garden:

  • VCCG Onion Card A - 2022-07-17
  • VCCG Onion Card B - 2022-07-17
  • VCCG Onion Card C - 2022-07-17
  • VCCG Onion Card D - 2022-07-17
  • VCCG Onion Card E - 2022-07-17
  • VCCG Onion Card F - 2022-07-17

There are many flies that look (to me) like Onion Maggot Flies, in contrast with the first round of cards which had far fewer flies after about six weeks in the bed.

Some could be Cabbage Maggot Flies, but my fly ID hand is weak.

One of the frames screwed to a fence post suffered a non-fatal mishap, so I made and deployed a seventh trap. We’re pretty sure the garden has enough flies to go around.