Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The mirror 3 + lens tube has a distinct tilt when seen from the +X direction (the right side of the machine, so it’s in the YZ plane, I suppose).
This is not matched when seen from the -Y direction (front):
OMTech 60W beam alignment – head Y plane
The tilt does not seen to correct for a misalignment in the X axis rail or the brackets attaching the head to the linear bearing.
The beam seems properly centered on the lens and nozzle, so I’m loath to twiddle the alignment just to see what happens. One of these days, for sure, that must happen …
A bit of tinkering suggested I needed a way to repeatably position stock sheets on the honeycomb, so I conjured stops that would be slightly taller than the magnetic spikes:
Improved MDF Honeycomb Spikes – first pass
Three of those form a corner into which you can tuck victims of the same general size:
Improved MDF Honeycomb Spikes – stock alignment
Those pointy MDF spikes should start with slightly rounded tops, because that’s what they’ll look like after a few uses:
Improved MDF Honeycomb Spikes – alignment stops
I also made a low-profile stop for victims lying directly on the honeycomb for engraving:
Please Close The Gate – engraved
The SVG images include a nested version to tile across random MDF leftovers.
The 12 mm neodymium magnet is slightly larger than a single honeycomb cell, so it wants to center itself atop a cell. The stainless steel button head screw sits in the magnet’s countersunk hole and protrudes just enough to make sure the spike doesn’t slide sideways unless you want it to:
Magnetic Honeycomb Spikes – parts detail
A cloud of combustible gas doesn’t pose a threat under there:
Magnetic Honeycomb Spikes – MDF
The thin red beam comes from the targeting laser on the back of the nozzle.
Storage is easy: just smush a handful of the things against the side of the laser cabinet:
Magnetic Honeycomb Spikes – storage
Buy all the parts in lots of 100 to have supplies for other adventures!
The little DSO-150 oscilloscope has a 1 MΩ || 20 pF input with a 200 kHz bandwidth that should be entirely adequate for the OMTech laser’s millisecond-scale modulation signals from the Gentec ED-200 Optical Joulemeter. There is, however, only one way to be sure:
Gentec ED-200 – scope test setup
The two scope inputs are in parallel, so the joulemeter over on the far right sees a 500 kΩ load, half of the specified 1 MΩ load, with at least twice the capacitance. If the two scopes display pretty much the same result, then it’s good enough.
A 50 ms pulse at half power looks the same on both scopes:
Gentec ED-200 – 50 ms – DSO-150
Gentec ED-200 – 50 ms – Siglent
A 50 ms pulse at full power doesn’t quite top out:
Gentec ED-200 – 11V 50ms – DSO-150
Gentec ED-200 – 11V 50ms – Siglent
Given that the pulse duration should be less than the detector’s 1.5 ms risetime, using a 50 ms pulse is absurd. Right now I’m just looking at the overall waveform and detector range, not trying to get useful numbers out of the poor thing.
The Gentec ED-200 Joulemeter is severely underqualified to measure the OMTech 60 W laser’s beam power, because the laser’s 1 ms minimum manual pulse width isn’t much shorter than the sensor’s 1.5 ms risetime and the maximum beam power is far too high for the sensor’s health. With that in mind, I set the PWM power to 50% = 30 W (grossly too high) and looked at the peak output voltage for a series of (far too long) pulse widths:
Rounding the detector sensitivity to 11 V/J says the 1.3 V peak at 5 ms corresponds to 120 mJ and 24 W:
Gentec ED-200 – 60W 50pct 5ms
The 3.3 V peak at 10 ms is 300 mJ and 30 W:
Gentec ED-200 – 60W 50pct 10ms
The 3.4 V peak at 15 ms is 310 mJ and 21 W suggests the PWM power output is not nearly as constant as one might expect, although the pulse width looks fine:
Gentec ED-200 – 60W 50pct 15ms
The 6 V peak at 20 ms is 550 mJ and 27 W, although the on-screen display obscures the top:
Gentec ED-200 – 60W 50pct 20ms OSD
Another 20 ms pulse without the OSD produces a peak eyballometrically close to 6.4 V for 580 mJ and 29 W:
Gentec ED-200 – 60W 50pct 20ms
The KT332N controller in the OMTech 60 W laser has a pulse duration setting showing tenths of a millisecond, but (based on some additional measurements) the beam power can vary by 25% for successive pulses in the low millisecond range, so the pulse width resolution doesn’t seem to provide useful control.
Despite the over-long pulses, the calculated power corresponds surprisingly well with the nominal laser output power.
The 1 ms pulses used in LightBurn’s Dot Mode are consistent enough to punch essentially identical 0.2(-ish) mm holes in manila paper to mark the graticule:
They’re on 0.25 mm centers, with slight variations showing the difference between stepper resolution and positioning accuracy. The shorter graticule lines have three holes on one side of the center lines and four on the other, despite the design’s 1 mm length on both sides; I think there’s a missing dot on the side where the head starts the line, perhaps due to a picket-fence error.
The large beam hole came from two 10 ms pulses, one at the focal point and another 10 mm lower.
The Max Energy Density spec suggests longer pulses are allowed to deposit more energy, probably because more time gives thermal diffusion an opportunity to spread the heat across the target; at CO₂ laser wavelengths that may not apply.
With the platform lowered as far as it goes, the ED-200 is 130 mm below the laser nozzle where the beam diameter is about 6 mm for an area of 0.3 cm². Ignoring the ideal Gaussian beam profile by smearing 60 W uniformly across the circle gives a power density of 200 W/cm², which means the laser pulse must be less than 0.5 W·s / 200 W = 2.5 ms to stay inside the power density limit.
I sincerely hope Gentec overbuilt and underspecified their detector.
The Voltage Response The result is a voltage pulse that rises quickly with the response time of the device to a level proportional to the laser energy (Figure 2). It then decays exponentially over a longer period of time that is a function of the pyroelectric device and load impedance. Figure 2 also shows that there is a longer recovery time to return to the initial state of the detector. This is a function of thermal phenomena and is not affected by the load impedance as are the rise and decay times. The integrated pulse energy over this period is proportional to the peak voltage.
Pulse Width Versus Rise Time
Usually the applied laser pulse must be shorter than the rise time of the detector for all of its energy to be represented by the peak voltage. Pulse energy received after the detector voltage has peaked will not be fully integrated into that value. For very long pulses, the peak voltage will actually represent peak power rather than pulse energy.
Gentec Energy Detectors, page 2
Figure 2 shows the overall waveform:
Gentec Energy Detectors – Figure 2
Which looks a lot like this 10 ms pulse at 50% duty cycle:
Gentec ED-200 – 60W 50pct 10ms
The pulse was 10 ms long, much longer than the 1.5 ms ED-200 risetime spec, but the overall shape looks right. Dividing the 3.3 V peak by the detector’s 10.78 J/V calibration value (11 J/V works for me) says the pulse delivered 300 mJ = 300 mW·s. Dividing 300 mJ by 10 ms gives 30 W, a beam power astonishingly close to the expected value.
The OMTech laser has a nominal 60 W output, although the tube life drops dramatically with regular use over 70% = 40 W. Power does not scale linearly with the laser tube current displayed on the power supply milliammeter, with the maximum value presumably preset to the tube’s 20 mA limit producing 60 W. The 20 kHz PWM duty-cycle chopping applied by the controller should linearly scale the average power downward from there.
It looks like the ED-200 might deliver reasonable results for millisecond-scale pulses at low PWM duty cycles, but it was obviously intended for much milder lasers.
The Box o’ Optical Stuff disgorged an ancient Gentec ED-200 Joulemeter:
Gentec ED-200 – measurement setup
It’s an optical pyrometer producing, sayeth the dataplate, an output of 10.78 V per joule of energy applied to its matte black absorber. Whether it’s accurate or not, I have no way of knowing, but aiming the business end toward the sun and waving my fingers over it produced a varying voltage, so there was hope.
It has a 1/4-20 socket on one side and my spare magnetic mount expects a 3/8 inch rod, so I drilled a suitable hole in a suitable aluminum rod and cut the head off a suitable bolt:
Gentec ED-200 mounting rod – parts
A dab of Loctite intended to secure bushings completed the assembly:
Gentec ED-200 mounting rod – assembled
I later replaced the nut with a finger-friendly nylon wingnut.
Which allows a measurement setup along these lines:
Gentec ED-200 – measurement setup
The white disk atop the sensor is a homebrewed target to indicate the active sensor area and its center point:
Gentec ED-200 target – scorches
The 1 mm graticule lines give a jogging suggestion to hit the center, assuming you (well, I) manage to hit anywhere on the target at the first shot. The beam is supposed to fill most of the central region, which is obviously not going to happen here, and it must not be focused to a pinpoint. The previous owner (or his minions) put a few scars on the surface and I expect to make similar mistakes.
The Smell of Molten Projects in the Morning 
