20K/30Kpps = N /2500 = around 1.667 KHz small signal bandwidth while scanning the ILDA 30K test pattern at eight degrees. . Some of us who experiment with these things have determined the small signal bandwidth with 30K galvos at 8 degrees optical angle to be about 2.5 KHz for small angle jumps, based on the circle in the square in the ILDA test pattern. Some engineers who design galvos have quietly confirmed that for the 1990s technology that is in most Chinese copies, 2.5 KHz is a good number at small angles. Eight degrees is the ILDA specified test angle for the main test pattern.
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Scanner bandwidth falls off rapidly as scan angle increases. The wider the angle, the slower the scanners, and the model is not simple. Its also dependent a bit on the previous waveform thru the scanner.
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On a modern system, the cloned galvos use coils are about three ohms, and are just a few turns of magnet wire within say 100 microns of the permanent magnet rotor.
The main danger is driving the coils too hard, the wire expands, the rotor scrapes the coils, and usually the rotor is grounded thru the bearings, unless you have a good scanner with ceramic bearings. The classical commercial amps that your clones are copied from had an log-antilog multiplier chip as a "coil temperature calculator", that drove a fet to reduce the gain of the scanner amplifier if the user pushed the input signal hard. That was the first thing the makers of low cost galvos removed from the circuit board on the copies. That low cost chip is was out of production by the mid 2000s anyways. T
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The good news is test point two on the amplifiers is nearly always the high end of a 0.1 Ohm resistor that goes to ground and carries the coil current as part of the feedback loop. You can monitor the coil current there with an oscilloscope. You can also look at the tail end of the differential amp that processes the optical feedback signals.
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The second issue is resonance frequencies. trying to "push" the rotor hard results in vibrations trying to tear the shaft apart or out of the bearings.
The galvos will visibly protest in the projected image and you'll hear clicking and oscillation at the base of the mirror as you get close to destruction.
You can watch for that in the feedback signal, but you would have to qualify that by testing a scan pair to destruction for a given model.
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With the low cost, low performance clones of the Cambridge 6800 made with inferior magnetics (hence the 0.707 times the performance of the Prototype at 20K)
It would be cheaper to just keep a spare pair of 100$ galvos and amps laying around then to spend graduate student and professor time working on a model.
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The ILDA scanner tuning standard, was designed by a team of artists and engineers to insure the PII or PID loops on the driver amplifiers from any two given users were tuned the same, so that projected graphics looked the same when interchanged by users. It is a more a data transfer standard based on a test pattern's visible appearance then a measure of the galvo's performance. The whole driver is the X-Y waveform's repetition rate. It was never envisioned as a means to make a very specific measurement of galvo performance. There are 12K, 20K, 30K, 60K, and 90K galvos in the world.
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The team that designed the ILDA Test Pattern used to measure "PPS" never intended the pattern to make detailed measurements, and no engineering relationships have ever been published for it. It was based off a hybrid of test patterns used by a few laser show companies at the founding of ILDA. its designed so that an artist who is not an engineer can tune his or her galvos to a common image performance standard, not quantify them as an engineer would. An engineer would look at small and large angle jump time, and settling time.
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On classical laser show software, we scan the pattern at a known angle, and vary the waveform update rate, which is calibrated in DAC updates per second on older software, while modern software calculates an optimized vector. The "circle in the square" potion of the pattern is designed to be a rough measure of 3 dB rolloff, everything else is for measuring overshoot or undershoot. So you change the scan rate while projecting the standard pattern at the specified angle until the sides of the circle just touch the inside walls of the square, and when they just touch, you look at the PPS readout in software and you know the "speed" your tuned for. It is very useful for laser shows when the galvos are driven in their ballistic region, it does not directly translate to a specified engineering performance measure.
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Think of it as a TV test pattern for setting up a broadcast camera. It really just ensures your complying with a loosely defined set of conditions. It lets a non-engineer with no test gear qualify the performance of their hardware, in respect to image interchange in a laser show vector graphics environment.
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The ILDA test pattern is a series of X-Y pairs. Each time you update one pair of data, you have sent the signal to the scanner to update one point. If you do 1000 updates in one second, you have scanned 1 KPPS.
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I can send you a private message with links to papers on galvo performance in a imaging environment, however there is not a well known set of conditions to avoid destruction. There are plenty of papers on optimizing the drive waveform, many by Florian Duma. Some early papers by Pierre Brosens go into the mechanical limits on speed.
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So:
1. Do expect the galvo bandwidth to decrease with increasing scan angle.
2. Avoid shaft resonances.
3. The issue is waveform and mirror inertia dependent.
4. Galvo manufactures specify a small angle seek time and a large angle seek time on their datasheets, and some will provide you with a frequency vs scan angle roll-off curve, but only if asked. Try not to force the galvo to exceed the seek times.
5. There is no easy answer to your problem.
6. Stator housing temperature matters, try to keep your galvos cool.
7. Galvo coils are a thin wire with an upper instantaneous current limit.
8. Listen and watch for distortion in the image as a clue to galvo stress.
9. Monitor your test points for waveform compliance with an oscilloscope.
10. Don't push the coil so hard as to partially demagnetize the rotor.
11. For a given set of Chinese copies with a given mirror size, you'd have to test to destruction to learn, the model is not published.
12. IF you need really high performance, buy better scanner amplifiers with notch filters in the feedback loops to remove the resonances.
13. Some galvo datasheets have a maximum specified RMS current, but not the clones/copies.
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If your an EE, you'll learn much by looking at the waveforms. If your interested, send me a PM, and I can send you some info on what to look for at the test points. I can't host the PDF here, it is copyrighted information by a major manufacturer of Galvos.
Steve
Last edited by mixedgas; 06-30-2017 at 10:47.
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