Blanking delay

Hi! I'm just a happy hobbyist building a laser projector for fun. I'm an unemployed electronics engineer and I need too be a little creative :) I made everything from scratch including my own closed loop (optical PD) galvos and drivers. I have one question about blanking delay. My (homemade) laser driver has TTL modulation and I control blanking delay through the microcontroller that is a part of my DAC. But how is this achieved when you are using analog modulation? Then blanking delay would be more like a phase delay from the XY-signal..? Can blanking delay be configured in the settings of commercial software or is this controlled by the projector?

In LaserBoy it is done by reading and rewriting all of the data in the multiple tracks of its own generated wave file. The X and Y information usually stays where it is and the color channels are shifted 5 to 7 samples behind (at 48KHz). A chunk of information, stored in the header of the wave records the sample offset numbers so that it can be undone and redone again.

Another possibility for making a stand alone outboard analog time delay is to use chips called serial analog delays. They are the core of most time related audio effects, like flanging, phase shifting, chorus, echo, etc. They are a strange combination of ideas. They are clocked, but they are truly analog.

James. :)

Many software packages add extra points to begininning and ends of segments to allow the galvos and lasers to settle. This works most of the time. Side effects include longer times needed to draw the frames (since more points are added) and sometimes there are hot spots if too many points are added and the galvos momentarily stop (or slow down a lot).

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Originally Posted by Danny

My (homemade) laser driver has TTL modulation and I control blanking delay through the microcontroller that is a part of my DAC. But how is this achieved when you are using analog modulation?

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Basically the same way; it's controlled by the software, which modifies the signal timing coming out of the DAC. The controller that maintains the timing between the X and Y scan signal outputs also controls the timing of the blanking outputs. Usually there is a setting in the software that allows you to adjust the speed of the blanking (often measured in points of offset).

The extra points added to corners and so fourth have more to do with settling the galvo in position than with color. Color timing offset is a constant setting that is normally never changed once it's been adjusted correctly.

Adam

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Originally Posted by buffo

Color timing offset is a constant setting that is normally never changed once it's been adjusted correctly.

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This would require a constant scan rate, right? I'm still trying to increase maximum scan rate so I have to make the delay adjustable (just a few lines of C code and 2 buttons). Need better cooling of the galvos... Did a really bad design on my galvos when totally neglecting heat losses. Soooo stupid of me. I really should now better. I'm limited to 500mA (rms) drive current and that gives me about 12kpps @ 20 degrees. My next galvo version (if ever built) will handle a lot more current... =)

If your time delay and your sample clock are totally unrelated, then time delay is constant.

If not, then changing your sample rate will obviously change your time delay.

Can you share any of your efforts with us in pictures or more description?

I see you like C !!! :D

Delightful!

What are you up to?

James. :)

Sorry for blabbering on about my project and not posting any photos. I dug up some for you to look at. The projector is going to be fully portable and runs of a 9-18VDC source (12V lead-acid battery will be built in soon). Will also do some more C-programming :) to make it read images and animations of a SD memory card. I did not have any photo of the ILDA test image, sorry. The photos show projections before I had any blanking delay. Need to do some more tuning but I think the image quality is better than I expected to get from my homemade galvos. The laser is an 20mW Uniphase uGreen 533nm DPSS running at low intesity for safe testing.

WOW.

I'm impressed! :D

James. :)

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Originally Posted by Danny

I'm limited to 500mA (rms) drive current and that gives me about 12kpps @ 20 degrees.

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Very nice value. My current 7K-8K homemade galvos with their drivers reqiure about 3A X 22V (bipolar).

That is a AWESOME home build job! I read the guide on building galvo's, and just said to myself, "I think i'll just wait till I get some money!" :D

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Originally Posted by Danny

This would require a constant scan rate, right?

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The setting will scale with scan rate, but yes, you may need to alter it slightly if you make large changes in scan speed.

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I'm still trying to increase maximum scan rate so I have to make the delay adjustable

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Well, it needs to be adjustable anyway, so you can set it the first time! :D

But yeah, once you get it set, you normally don't change it. Just like scan speed - once you set it, you leave it there. (Because if you scan at different speeds, you really need to re-tune your scanners to get optimal performance.)

I'm impressed with the image quality you're getting. You've obviously done a great job building those scanners!

Adam

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Originally Posted by Sergey

Very nice value. My current 7K-8K homemade galvos with their drivers reqiure about 3A X 22V (bipolar).

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Yes, I think they are really effective. This is nice since I will be running the projector on batteries but I would have liked a higher scan rate. I have a theory on how they got that fast despite the low drive current. They are made of plastic(!). This meens that I have huge problems with heat dissipation but in return no eddy currents are formed to slow down the rotor. Any conducting material near the rotating neodymium magnet would cause a opposing magnetic field as the magnet moves. That was why I used plastic... The magnetic coupling between coil and rotor is also quite close. The L/R time constant of the drive coil is also important... I'm running both galvos of a +/-15V 1.5A DC/DC converter.

When I started this thread it was about blanking delay... I will shut up now or start a new thread on galvo design :) Thanks for your positive comments. I'm gonna try to solve the issues with heat dissipation in my mk2 galvo. For now I'll try to tune the galvos for a lower scan angle and hope to get better scan rates.

What are you using for a DAC? Where are the signals coming from? Is all that your design as well?

James. :)

I'm using two serial 10-bit DAC IC's fed from a microcontroller. This is connected to my PC through the printer port and the microcontroller is emulating a DAC0832. I did it this way because I had the DAC IC's laying around at home, and I also wanted to be able to add a few features later. I will add support for reading images from an SD memory card (eliminating the PC). I also thought of creating a "game mode" to play a simple pong game directly on the projector. It's ment to be fully portable too for some serious outdoor fun :D

Does it have an ILDA port? :D

James. :)

Nope! I'm using my own standards! :D

I can dig it. :D

James. :)

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Originally Posted by Danny

Nope! I'm using my own standards! :D

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dont even think! of shutting up! nice work! And keep this thread ;')

getting rid of eddy currents is nice trick!
if you have graphics/modulation questions PM with a phone number and I can walk you through the basics.


whats your jump time in microseconds on a scope for a 2.5' jump? A 10' jump?
Steve

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Originally Posted by mixedgas

getting rid of eddy currents is nice trick!
whats your jump time in microseconds on a scope for a 2.5' jump? A 10' jump?
Steve

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Yes, I think the eddy currents can be a real problem. The faster the rotor tries to move the more the opposing magnetic field will try to brake it. This force can be really strong. Try this experiment sometime: Drop a neodumium magnet between the fins of a aluminum cooler. It will slooowly slide down. Looks absolutely fascinating!

This project has been on ice for a while now. I don't remember the step respose time exactly. I think it was somewhere between 500-700us at it's best (dont remember at what angle though). I think the time constant (L/R) of the coils where somewhere near 40us wich correspended exactly to the phase lag between a square wave input signal and galvo movement.

I would like to build new galvos using a ceramic material (like in the washers used to electrically insulate power transistors from their coolers) to dissipate the heat. I'm also thinking of water cooling to really overdo it the next time :) This would mean building a completely new projector. All that stuff is NOT going to fit in my cheap flightcase. I will complete this first and then mayby start all over again... :rolleyes: I'm just dreaming about the speed I could reach if my galvos could take like ten times the current I'm feeding them today... I'm also thinking of a two or three phase coil system with corresponding drivers to increase speed. As I said: I make my own standards! :D

there are some near zero inerta galvo sized motors with 3 phase coils to drive them. Bill Benner from Pangolin brought some to a conference once, very cheap and galvo sized from Faulhaber Motor. If somebody figgured out how to do a fast lookup table from differential to three phase, and we used back EMF for position feedback, they would approach 30K,have real bearings, be swiss made, and indestructable, as well as possibly cheaper then chinese galvos. I know how to do the back EMF part.

at 400-700 uS jump, your 1/2 way there to the older professional grade galvos. That rocks. If you have access to the lathe, I am just about to throw out some macor like machinable ceramic, you lathe it then fire it in a oven to harden it. Firing oven is not that hot, like 900C, which is a off the shelf cheap tube furnace with nichrome elements. it does shink a little after firing, but that is a known quanity.

keep us informed.

Steve

My idea was that a multi-phase coil system would be faster than a single-phase design that seems to be standard among commercial manufacturers. It would complicate the driver a lot though. But what the heck. I'm a electronics engineer - I live for this kind of things :D I found the attached picture in somewhere this forum and I think that galvo looks very much like a two phase design... I think they where made by scanergy.

By the way. I have almost figured out the perfect design :D Water cooling and heat conducting ceramics. No electrically conductive materials anywhere near the moving magnet. I have even figured out how to make a flux ring to boost the magnetic field strength without creating any eddy currents. I really need to try this. I let you all know how it went in a few years when I have found the time to try these ideas :D