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: Science

If you measure something often enough, it becomes science

HLP-200B Laser Power Meter: First Measurements

The HLP-200B Laser Power Meter arrives without much in the way of specifications:

The HLP-200B Laser Power Meter Handheld comes fully calibrated at 10.6 μm (CO2). Each laser power meter we calibrate is directly traceable to NIST absolute standards because we use GOLD standards as a reference for each calibration. You will obtain the most accurate result possible

A line in the description says “+/- 3% within the central section”, but that’s not much help. Back in the day, any error percentage referred to the meter’s full-scale value, which would be ±6 W for a 200 W meter.

So I plunked the meter in the middle of the laser platform:

HLP-200B Laser Power Meter - platform center — HLP-200B Laser Power Meter – platform center

Then took five measurements at each of ten power levels:

PWM %	10	20	30	40	50	60	70	80	90	99
°C	17.2	17.9	18.4	19.0	19.4	20.3	20.0	20.0	20.5	19.4
Tube Current	3	4	7	10	14	16	18	20	22	24
W	7.1	21.0	42.0	51.8	59.1	63.0	67.8	69.6	74.7	64.0
	6.0	19.8	37.2	48.9	52.7	56.0	65.1	69.6	72.4	71.8
	6.4	21.1	39.3	45.6	56.5	53.2	61.1	60.7	74.6	75.2
	5.6	17.8	37.1	40.4	55.3	53.2	55.1	64.2	74.9	73.5
	6.0	17.7	36.9	45.1	54.5	53.1	62.2	69.9	72.2	70.9
Avg Power	6.2	19.5	38.5	46.4	55.6	55.7	62.3	66.8	73.8	71.1
std dev	0.57	1.66	2.19	4.29	2.39	4.26	4.78	4.16	1.34	4.29

That’s easier to digest from a graph:

HLP-200B Laser Power Meter - 60 W platform center measurements — HLP-200B Laser Power Meter – 60 W platform center measurements

The absurdity of computing the sample standard deviation from five measurements taken at each power level does not escape me, but this just surveys the situation.

Earlier measurements of the tube current vs. PWM setting, using an RMS value computed by the oscilloscope’s firmware, produced a plot resembling the brown points (read the mA scale on the right) at the high end and differing greatly on the low end. These values come from the power supply’s digital meter, but the straight-line fit doesn’t look absurdly forced and the zero intercept seems plausible. I *assume* it’s actually measuring the tube current, rather than displaying a value computed from the PWM input, but I don’t know for sure.

The rather sketchy paperwork accompanying the laser had one handwritten “21 mA” seemingly corresponding to 60 W output, which looks approximately correct. The instruction manual has a table of power vs. current suggesting that 65-ish W corresponds to 18 mA, with 100 W at 23 mA; it’s unclear whether that is for the 60 W tube in the machine or applies to the entire range of available tubes. The manual recommends not using more than 95% PWM, with which I heartily agree.

Because my meter stand holds the target in the same position relative to the beam during successive measurements much better than I could by hand, I think the pulse-to-pulse variation comes from meter and tube repeatability.

Earlier measurements with a grossly abused Gentec ED-200 joulemeter suggested the laser has some pulse-to-pulse timing variation, down in the millisecond range, but produced roughly the right power for middle-of-the-range PWM settings. This meter integrates the beam power over about ten seconds, so I think variations will be due to (possible) tube power changes and meter repeatability, rather than timing errors.

Obviously, you must not depend on any single-shot measurement to fall within maybe 10% or several watts of the right answer.

With all that in mind and assuming the meter is delivering approximately the right numbers on average, the power supply overcooks the tube at any PWM setting above 50%. I’ve noticed some beam instability / defocusing over 80% while cutting recalcitrant materials, which is surely due to the tube not lasing properly. I generally avoid doing that.

The log fit to the measured power looks better than I expected, although I’m unprepared to compute natural logs in my head.

Hey, it’s my idea of a good Christmas present …

2024-12-25

HLP-200B Laser Power Meter: Holder / Stand

The overall measurement process for the HLP-200B laser power meter requires more coordination than I can muster on a dependable basis, so a third hand seemed in order:

HLP-200B Power Meter – target setup

In actual use, a pair of finger-crushingly strong magnets laid on the base hold it firmly to the honeycomb.

Because a CO₂ laser beam is invisible, the only way to know where it hits is to char a bit of paper:

HLP-200B Power Meter – target detail

With that evidence, I can jog the platform up-and-down and the gantry front-and-back to center the beam on the paper target and, thus, on the sensor behind it. That process happens at each test position across the platform:

HLP-200B Power Meter – targets

The meter shuts down a mere six seconds after completing each measurement, which means I must keep the lid open, listen carefully, and react quickly. Firing the laser thus requires defeating the lid interlock specifically wired to prevent that from happening:

Laser lid interlock sensor

Rather than install a switch to bypass the interlock, I taped a steel cover harvested from defunct electronics over the sensor:

Laser lid interlock sensor – bypassed

Which has the useful side effect of preventing me from closing the lid with the interlock defeated.

The holder is just slightly larger than the meter’s handle and some clamps produced a snug fit while the glue cured:

HLP-200B Power Meter – holder gluing

The holder keeps the meter sensor at the same position vertically and within about a millimeter horizontally. The laser beam seems to be around 5 mm in diameter (the scorches above come from the hottest central part), so the beam should hit the same position on the sensor during successive measurements, making them far more repeatable than my waving it around by hand.

The LightBurn SVG layout as a GitHub Gist:

Loading
Sorry, something went wrong. Reload?

Sorry, we cannot display this file.

Sorry, this file is invalid so it cannot be displayed.
Viewer requires iframe.

view raw HLP-200B Laser Power Meter – holder.svg hosted with ❤ by GitHub

2024-12-23
HLP-200B CO₂ Laser Power Meter
What with Christmas approaching, I got myself an HLP-200B CO₂ Laser Power Meter:

HLP-200B – front panel

It’s a hefty chunk of aluminum, as befits a device intended to soak up a 200 W CO₂ laser beam, and both sides have a relentless simplicity:

HLP-200B – back panel – redacted

Having not found an online version of the manual:

HLP-200B Laser Power Meter – Manual Download

You’re welcome.

[Edit: A slightly different version of the manual is online at https://www.ccbluetimes.net/pages/support ]

The manual does not exactly match the hardware. In particular, “so users won’t need any tools to replace the battery” is incorrect:

HLP-200B – battery lid screw

Until you loosen the M2 setscrew below the finger notch a couple of turns, “Use just fingers to remove the battery cover” will merely scuff your fingerprints. Apply a 1.5 mm or 1/16 inch straight screwdriver bit with no more than finger torque and, after two or three turns, the lid comes free.

The meter arrives without a battery, so you passed the first test.

Despite the “another screw hold (M4) is added”, there’s only one tapped hole in the case, as visible in the back panel photo. Seen from the front, it’s above the four digit LCD.

Operation is at best awkward and at worst hazardous:
- Press the blue button to turn it on and hear a beep
- It’s ready to measure within three seconds
- Hit it with the laser beam until it beeps
- The LCD shows the power for six seconds
- It shuts off with a beep
- Bonus: If the meter doesn’t detect any energy, it shuts off 20-ish seconds after the button press
Minus my power ears, the beeps are completely inaudible.

The meter is sensitive enough to respond to weak heat sources like LED bulbs and even fingertips, so you can test it without firing the laser. The numeric value shows the power from a CO₂ laser beam dumping an equivalent amount of energy into the sensor:

HLP-200B – finger heat response

The sensor target is 20 mm OD, although the instructions remind you to “Ensure the laser is emitted to the center of the sensor”. I suspect hitting the sensor with a focused laser spot will eventually damage the surface.

Making a real measurement requires:
- Set the Pulse button for continuous output
- Set the power level
- Defeat the lid interlock switch on the laser cabinet
- Push the blue button on the HLP-200B
- Quickly position the meter target accurately in the beam path
- Hold down the laser Pulse button
- Freeze in that position until the meter beeps
- Release the Pulse button
- Quickly reorient the meter and read the display
I have a visceral reluctance concerning safety interlock overrides, misgivings about poking my head inside the cabinet, and no yearning to put one hand near the beam line with the other on the console. Yes, I have known-good laser safety glasses.

The meter generates plausible results for the (claimed) 60 W tube in my machine, but further tests await conjuring fixtures to keep various irreplaceable body parts out of harm’s way.
2024-12-20
Prusa MK4 Input Shaper vs. Resonance Test Box

Although the laser ramp test fixture looked good, Brent wondered what a real test box would reveal about the Prusa MK4’s Input Shaper resonance control.

Loading the STL into PrusaSlicer, adding a text label to remind me which way it printed, then slicing with my PETG-CF profile shows the “Actual Speed”, which seems to take acceleration into consideration:

PrusaSlicer preview – actual speed

The colors in the legend don’t quite match the colors on the model, but the greenish layers with the jolts trundle along in the mid-20 mm/s range and the blue-ish straight-through layers at 30-ish mm/s.

Eryone PETG-CF has a somewhat fuzzy appearance that seems not characteristic of other brands, so I’ll try something else when these spools run out:

MK4 Resonance Test Box – overview

The right side of the box (as oriented on the platform) got all the layer retractions and came out festooned with PETG hairs:

MK4 Resonance Test Box – right side

You can check my labels by tracking the small retraction zit sticking up from the top layer; I got it wrong the first time. Open the images in a new tab to see more pixels.

The front:

MK4 Resonance Test Box – front side

The left:

MK4 Resonance Test Box – left side

And the rear:

MK4 Resonance Test Box – rear side

You can barely see the shadow of the “Rear” text on the surface, even though the wall is two threads thick and the text is indented by 0.2 mm, about half the thread width.

As far as I can tell, the MK4 Input Shaper compensation does a great job of suppressing resonance or wobble in all directions.

Looks good to me!

2024-11-29
Prusa MK4 Input Shaper: Accelerometer Tuneup
After adding bling to the Prusa MK4, I touched up the belt tensions and re-measured the axis resonances with the Prusa Accelerometer gadget to update the Input Shaper settings.

The Prusa belt tension guide pretty much explains that subject, with their Belt Tuner making up for my utter tone deafness. FWIW, if the Belt Tuner produces inconsistent results differing by an octave, either up or down from the correct value, the belt is way too loose: give the axis belt tension screw a turn or two to drag the results into the right time zone, then fine-tune from there.

While it is possible to reach both tensioning screws without too much trouble, they’re definitely not convenient.

The accelerometer fits on the hot end:

Prusa MK4 Accelerometer – on hot end

Then under the steel sheet, where it’s clamped by the platform magnets:

Prusa MK4 Accelerometer – on platform

The MK4 firmware measures the resonant frequencies while prompting you to put the accelerometer in the proper locations, then computes the best shaper values.

For reference, the stock OEM values:
- X = MZV 50 Hz
- Y = MZV 40 Hz
Just after I got the accelerometer and without doing anything to prep the MK4, these results popped out:
- X = MZV 56 Hz
- Y = MZV 42 Hz
Now, with bling and properly tensioned belts:
- X = MZV 59 Hz
- Y = MZV 45 Hz
The most recent values were also the most stable, once again pointing out the value of careful assembly and maintenance.

With that in mind, though, I built the laser ramp focus fixture shortly after doing the first recalibration and it has no visible ripples on any of its walls:

Ramp Test Fixture – corner detail

That’s a square corner perpendicular to the sloped top surface at the default 45 mm/s. It’s not as difficult a test as some you’ll see, but it suffices for my simple needs. The MK4 definitely behaves better around corners than the Makergear M2.
2024-11-26
Laser Cutter: Fourth Corner Fix Summary
A discussion on the LightBurn forum about a large-format machine with a misaligned beam prompted me to think through the whole “Fourth Corner” problem and come up with this summary based on my beam realignment adventure:
Here’s what I think is going on, referring to the 4×8 foot (!) machine in that discussion and lightly edited to improve readability & fix minor errors …

Mirror 1 alignment gets the beam parallel to the Y axis, averaged over the gantry travel between front and rear. The path length variation on your machine is four feet.

Mirror 2 alignment gets the beam parallel to the X axis, averaged over the laser head travel from left to right. The path length variation on your machine is eight feet.

When the laser head is in the left rear corner, the total path length is maybe a foot or two. When it’s in the front right corner, the total path length is upwards of twelve feet.

The “Fourth Corner” problem comes from a slight angular misalignment of Mirror 1, because you (and I and everybody) must set it with a maximum path length around four feet (Mirror 1 to Mirror 2 with the gantry at the front end of the machine). But with the laser head in the right front corner, the path length (Mirror 1 to Mirror 3) is three times longer, so the error due to a slightly mis-set angle at Mirror 1 is correspondingly larger.

A tiny tweak to Mirror 1 changes the spot position at Mirror 2 by very little, but moves the spot at Mirror 3 by much more due to the longer path length.

Tweaking Mirror 1 cannot compensate for a warped machine frame, but it will get the beam alignment as good as it can be made.

The next point of contention was my “middle of the mirror” suggestion. AFAICT, the spot burned into the target at each mirror marks only the useful part of the beam with stray energy in a halo around it. Centering the spot keeps that stray energy away from the mirror mounts, so it doesn’t cause unnecessary heating. This will be particularly important with a high-power laser.

Angular adjustment of each mirror puts the beam parallel to the axes, but cannot also center it on the mirrors. After it’s aligned, the path from the laser tube through the nozzle depends on the position of the tube relative to the nozzle: moving the tube up/down and front/back moves the beam position on the mirrors and through the nozzle, but (in an ideal world) doesn’t change the angular alignment.

So after aligning the beam parallel to the axes, you must move the laser tube, the mirrors (up/down left/right front/back), and maybe the laser head to center the beam in the mirrors and also in the nozzle. Because we don’t live in an ideal world, moving any of those pieces wrecks their angular alignment, so it’s an iterative process.

The goal is to reach this point:

Beam Alignment – Mirror 3 detail – 2023-09-16

Those are five separate pulses, one each at the four corners and center of the platform.

The beam then goes pretty much through the center of the laser head and lens:

Beam Alignment – Focus detail – 2023-09-16

Works for me, anyhow.
2024-11-13
Laser Test Paper: Weathering

Three months of outdoor exposure suggest that laser test paper can survive use as a plant tag for one growing season, at least when it remains flat:

Laser test paper – small plant labels – 3 month exposure

The two upper tags demonstrated the paper has no flexibility worth mentioning, so it cannot become a tag wrapped around a stem.

The two lower labels spent their time tucked into a window frame where they got plenty of sun & rain without the benefit of a backing plate. Looks good to me!

Contrary to my expectation, the craft adhesive sheet behind this label survived intact, although the label itself took some damage, perhaps from the more direct sunlight out on the deck:

Laser test paper – plant marker – 3 month exposure

In any event, they look Good Enough™ for our simple needs and next year’s plants will be properly labeled.

2024-11-04