Questions about a simple scanner setup

[skibum729]

Hello all, I just registered for this site but have been reading absolutely everything I could find on here for the last couple of weeks.

I'm an engineering student in california and am doing research this summer that requires galvos. I'm only using one laser for my scanner and am trying to get it to image pretty fast.

I asume that I can use a function generator (basic sine wave from +5 to -5 volts) to provide the input signal to my galvo drivers. With the generator I can get any frequency input I want but my scanner is only 20kpps. So I guess my first question is what does the 20kpps actually mean? Will it break or overheat if I try to move it to quickly? I plan on only making small (~2 degree) movements and was hoping that meant I could increase my input frequency. Has anyone used a keyboard or music to directly drive your galvos? And if so what frequencies (notes?) did you use?

Any help is greatly appreciated. When I get this part of the project done then I can start on the 3D part. Think something like this: http://phys.org/news11251.html Hopefully when I finish I'll have something cool to show you all.

Thanks,
Chris

[Jimboy3625]

Welcome to PL Chris, please fill out your location in your profile, although I know you said you were in California it helps to know what city because a forum member may be near by and able to help you directly.

Well, to my knowledge 20kpps would mean 20k as in 20,000 and pps is points per second. So 20kpps scanners can draw 20,000 points per second.

Good luck,
Jim

[planters]

I have yet to hear a clear and logical description of the meaning of the 20,000 points per second specification. It seems to be a value like A.U. (arbitrary units) used to compare primarily entertainment scanners(galvos); 30K is better than 20K etc. How this compares to resonance frequency, degrees/second and resolution is still a mystery to me. I'll follow this with you.

[Badpip]

http://camtech.com/applications/index.html

Laser Projection Laser projection for both industrial and entertainment markets have benefited for Cambridge Technology's galvo systems. The laser entertainment marketplace was revolutionized by the low inertia, high speed, high accuracy and compact size of the 62XX product line. Capable of sinusoidal frequencies up to 1.5kHz, our scanners have pushed display rates beyond 60K pps. The precision and speed of our line of moving magnet and moving coil optical scanners allow the extreme accuracy and pattern complexity of laser CAD projection for industrial fabrication, alignment and cutting applications.

So 1.5KHz is more than 60Kpps - how that is calculated is beyond me...

/Thomas

[mixedgas]

To answer the OPs question 0-2 Khz is just fine for X-Y for Lissajous with 30K feedback scanners. 0-1 Khz is fine for 12K scanners. Size your color control oscillators to range from about 1/5th of the lowest oscillation frequency to 5x your fastest oscillation frequency and you will get great stuff. Stick to sine and triangle at first, square waves are not that useful for anything but color, as a beginner. The patterns repeat, so there is little point in exceeding 500 Hz in most cases for X-Y deflection. At some point the amount of "filligree" produced starts to match your beam size, and it gets pointless.


Every three months I find myself explaining this. In the Beginning there was man... Jump forward to the 1960s. Man refines the torsion bar ampmeter into something faster. In the late 1980s and early 1990s, Man discovers that if Laser Show companies co-operate, life can be better. In order to facilitate this co-operation, it is best if there is a means of interchanging images. Hence the ILDA standard happens.

Much of this is pre-internet, which seems to make it something that younger folks can't cognate. Since most of them get their scanners already tuned, they have no idea how hard it was to standardize images when the only major galvo company at the time charged mucho extra to tune a board at the plant, then to ship you untuned scanners. So everyone tuned their own, and they all did it different ways. Many of these ways were poor in structure, because they were devised by artists, and had little appreciation of the physics involved. I used to see some REALLY bad images when I was young.

So ILDA happened and a technical committee was formed. Some of the best minds in the business showed up with their "SEKRET" and "Proprietary" test patterns. Some of these were skilled artists, and some were highly skilled engineers. A few were a bit of both. They sat down in a room, and as committees do, reached a compromise. Some of these guys had water cooled, 60K scanners before others even knew what the test pattern would be and only had 8-10K scanning. Back then there was only one major maker of scanners, and only a few companies making scanner amps or EVEN DOING LASER SHOWS. At the time this was EARTHSHAKING co-operation between competitors.

The reason why PPS exists is a software/hardware one. The one thing that is common to every show system that is computer controlled is the point update rate sent to the dacs, to direct the scanners. Every software guy had access to the "PPS" rate in software, be it called refresh rate, scan speed, whatever...

So accepting PPS gave you a measurable, adjustable, and repeatable function that is in EVERY piece of show software, no matter what the programmers lie and say about having true "VECTOR" scanning using say WAV files. Dacs update their analog outputs in discrete steps, no matter what. It is physically unavoidable with computers. The people making a GOOD compromise saw this, and it WAS GOOD! Remember, at the time, people "pulled points" by hand when getting images from other companies.

So the next step was a ENGINEERING STEP, but no paper on it was ever published. It was hashed out in a meeting of some very good engineers and artists. They needed a way to measure the small signal,. small jump, response of the scanner, which is far DIFFERENT then the long jump speed. So like all good ELECTRICAL and MECHANICAL engineers, they came up with a way to measure the 3 Decibel Down point of the galvo. This was GOOD because the galvo amp and galvo combination is essentially a multi-pole non-linear filter from a electro-mechanical point of view. (Poles and Zeros are not a subject of this post. Go see Wiki for
that)

So the Square in the pattern was designed in such a way that any half tuned galvo amp will display a square with a constant size and good shape. The Square is loaded with LOTS of carefully placed points to make sure the square is scanned in a uniform, constant size, manner. In order to insure quality, a SCAN ANGLE for the image size was specified ensuring the Square would always work as designed.

The Circle in the test pattern has just a few points, and the points in the pattern before and after the circle are set up in such a way that the scanner is coming in at a controlled velocity before entering the circle.

So, this gives you a means to measure the small angle tuning speed in a known and constant means. It is also a means of test that still works as Galvos speed up. When the circle just kisses the inside of the square, the 3 dB down speed response under a given set of conditions is met, and you can glance at the PPS rate and know where you stand.

The exact calculations for the circle and square were never published, they were part of a trade secret of a very large company, and were made available to the committee, but not the general public. So we don't know all the answers, but it was reviewed, tested, and modified at the time by a large number of professional Laserists, both engineers and artists. Some of the people involved have left the industry, retired, or have passed on. That makes it difficult to reconstruct.

It was done by volunteers guys, it is expensive to fly a bunch of people to one place and have them bang out documents. So there is little documentation on the actual pattern engineering. But it works. At the time documents moved at 1200 bits of data per second, faxes were slow, the mail was even slower, and photocopies averaged 17-20 cents per page, and photocopiers were beyond expensive. Moving data was not cheap, so it was easier to fly some place and hash it out. That basically is what happened.

So for a 30K scanner, you have a medium angle response of about 2.5 Khz to SINE waves, and other waveforms are about 75% of that or less. This depends if the scanner is open loop, closed, loop, ballistic or large jump mode, and that is the subject of many scientific papers and about 200 pages in a book on scanner design. The SECOND best way to find out is to measure it on the bench using sine, square, and triangle waves, at many angles. How the amplifier is "TUNED" matters. Cambridge and GSI show at least 4 different tuning schemes in their manuals for applications other then display.

The relation between PPS and Scanner Response is not easy to relate, because, in the words of one Galvo company founder and chief engineer, it requires solving 23 simultaneous equations for each jump in real time. Some of those equations require hundreds of Iterations for each jump. That is why only a few companies have self tuning DSP scanner amps. It is also why there are no "ASIAN CLONE" DSP scanner amps.

It is also why the amps use simple PII and PID loops, analog computing is fast and cheap.

The Physics of scanners is NOT hidden from you, it is published. Stop at my house some time and I'll let you see the books. It is just so damn complex that the simple but humble ILDA test pattern does a good job of approximating things.

When you load up Pangolin, some of the pre-ILDA test patterns are still there. Laser Media is there, and so is the Grid used by another company.

IT Amazes me that most people don't know what the basic functions of the Laser Media test pattern are. They test for overshoot, and make sure your X and Y scanners have EQUAL velocity on long jumps, leading to STRAIGHT lines and LOW DISTORTION in graphics. The diagonal lines are scanned twice in each direction. If they BOW outward or INWARD one scanner has a different velocity then the other.

Cue some one who will say it is time to dispose of the test pattern, but will have nothing to replace it. Nor will they have any math to back up their statements.

Steve



Steve

[skibum729]

Thanks for the quick responses everyone. Thomas that source is great, just wish I knew how they got to that number.

I thought that maybe the 20k points was the speed at which the driver can process information. Since it has to get your input, the galvo's angular velocity and position, then calculate the PID control, and output the voltage maybe that calculating time is what limits the speed. Its a theory at least.

With that in mind, have any of you tried running your scanners without the driver? It would be an open loop system (no PID so less accurate) but it should be able to respond faster.

This is the galvo set I have: http://www.ebay.com/itm/20Kpps-Hight...item35b61b6733 I know its not great but its cheap and this is a prototype. Looking at the manual again it says I can get 35kpps at 5 degrees so I don't think the driver theory works.

I'm going to contact the manufacturer and see if I can get the specs on the galvos. Then... http://www.scienceteecher.com/images...product-01.jpg

Let you all know how it goes,
Chris

Edit: I posted this before I read the above post. Steve thank you that was incredibly informative. I'll give the literature a look and see if I can do some tests so see how my scanner responds.

[mixedgas]

I think you'll find "Tuning Strategies" in this document enjoyable as a Engineering Student:

http://www.camtech.com/archive/176-2...X_Manual_G.pdf

If you want a circuit diagram to see whats in a scanner amp:

http://www.skywise711.com/lasers/scanner/scanner.html

A friend of lasers hosts that for me, its a mix of a older Lasermedia 12K amp and a mid 90s Cambridge amp.

You need to heatsink that galvo well. The galvo mount pictured is intended to mount to a fair amount of 1/4" aluminum baseplate.
The right circular cylinder portion of the galvo is what needs the heat removed, not the position detector flange in the base.

What you have is a copy/clone of this product line: http://www.camtech.com/products/6800/index.html

Steve

[mixedgas]

Your amp is most likely a varient of this :

http://rtime.felk.cvut.cz/~sojka/laser/678xx_board.pdf

Steve