Where can I find DAC info

[jack]

Hi,

I'm doing a laser display as part of a university project. I'm looking at getting a 100mW O-Like 532nm laser (here) and a pair of ScanPro LSP 20's (here).

The next step is to wire them up using a microcontroller or FPGA as a controller. The first question I have is where I can find some info on the galvo drivers? I need to know what voltages to drive them at (+/- 5V I think), and if there's anything else I need to know (is there a maximum ramp speed I should be aware of exceeding, etc). I guess they don't really come with a datasheet?

Thanks for any help you can give! These forums have proven invaluable so far!
J.

[sugeek]

Hi,

I'm doing a laser display as part of a university project. I'm looking at getting a 100mW O-Like 532nm laser (here) and a pair of ScanPro LSP 20's (here).

The next step is to wire them up using a microcontroller or FPGA as a controller. The first question I have is where I can find some info on the galvo drivers? I need to know what voltages to drive them at (+/- 5V I think), and if there's anything else I need to know (is there a maximum ramp speed I should be aware of exceeding, etc). I guess they don't really come with a datasheet?

Thanks for any help you can give! These forums have proven invaluable so far!
J.

If you are using an FPGA why are you bothering with a DAC?

[jack]

I'm not sure I understand your question entirely, sorry. I'm not planning on using an external DAC - I'm planning on using the DAC output of the microcontroller or an FPGA with some external DAC wizardry (as far as I know, they don't usually come built in to an FPGA).

But for that I need to know the signals the galvo's are expecting to see!

Cheers,
J.

[weartronics]

I need to know what voltages to drive them at (+/- 5V I think)

Most galvo amps are designed for 10V peak-to-peak differential signals. Of course, if you tie one side to ground, single-ended signals can be used too. You can read a detailed specification in the "required signals" section here:
http://www.laserfx.com/Backstage.Las.../ISP-DB25.html

is there a maximum ramp speed I should be aware of exceeding, etc

Because the frequency response of servo systems depends on so many parameters, it's difficult to characterise absolutely. Some professional galvo systems have specified frequency response (or at least small and large angle step response times). However, the majority of light show galvo systems are only specified with a "points per second" (PPS) value, which is derived from the display of a particular test frame at a particular optical angle. The PPS value can be a rough performance indication, but it's not useful for modeling the servo system because a) it's a subjective measurement, and b) the frequency response can't be described with a single value anyway.

In short, if your application is artistic, it's easy to get good results, even without an accurate model - try it! If your application is technical, you can design an experiment to characterise the frequency response of your particular galvo system.

Best regards,

weartronics

[jack]

Right on, thanks for the response weartronics. So I don't have to worry too much about ruining the galvo's or drivers, provided i'm within the 10Vpp (or other specified value)?

Does anyone have anything negative to say about my choice of scanner and laser, particularly as I'll be designing a custom controller? I'm about to buy them, but being my first foray into lasers I don't want to be making an expensive mistake

[weartronics]

Right on, thanks for the response weartronics. So I don't have to worry too much about ruining the galvo's or drivers, provided i'm within the 10Vpp (or other specified value)?

Start with slow scan rate (or small scan angle) and gradually increase the scan rate (or scan angle). When the result starts to look distorted, that is the limit of your galvo system. Driving it faster or wider than this will cause unnecessary heating.

Does anyone have anything negative to say about my choice of scanner and laser, particularly as I'll be designing a custom controller? I'm about to buy them, but being my first foray into lasers I don't want to be making an expensive mistake

The choice of laser and scanners is pretty independent of your controller design. The performance of those scanners is at the low end of the scale, but if it's sufficient for your application, then the price/performance ratio is very good (i.e. they are cheap). I know the guys at lasershowparts, and I'm sure you'll get good service if you buy from them, wherever you are located.

[mixedgas]

Elm Chan has a pretty good breakdown from the "building galvos" and controller side.
Building your own galvos is not advisable, the learning curve is steep and takes longer then a semester.

Buying galvos and doing software and D/A conversion is recomended!

http://elm-chan.org/works/vlp/report_e.html

We at PL have made or have found some weird low cost tools over the years, so if you hit a specific snag please ask.

While not appropriate for a actual university hardware project, there are some things that can be done with modified 25$ USB sound cards if you need a sanity check.

Also we have two or three guys here that are highly skilled in P.I.D loops and can help you do some measurements once you get your galvos.


Would a all analog pattern generator be a useful option for your project?

Steve

[mixedgas]

Elm Chan has a pretty good breakdown from the "building galvos" and controller side.
Building your own galvos is not advisable, the learning curve is steep and takes longer then a semester.

Buying galvos and doing software and D/A conversion is recomended!

http://elm-chan.org/works/vlp/report_e.html

We at PL have made or have found some weird low cost tools over the years, so if you hit a specific snag please ask.

While not appropriate for a actual university hardware project, there are some things that can be done with modified 25$ USB sound cards if you need a sanity check.

Also we have two or three guys here that are highly skilled in P.I.D loops and can help you do some measurements once you get your galvos.


Would a all analog pattern generator be a useful option for your project?

You might find a PSOC or two that come with a FPGA inside, but I'd start with the FPGA and Glue Logic / DAC chips.

Steve[/QUOTE]

[mikkojay]

Hey Jack, it sounds like you are getting yourself into an interesting project-
One other thing you can do while feeding experimental signals into a pair of galvos is listen to them. Signals that look fine on a PC screen or scope can wreak havoc on physically moving parts. Start with minimal amplitude first. If the galvos start to sing, you may need to do some tweaking. If they start to scream, hit the e-stop button! (One last thing- make an emergency stop button or something that will cease the offending bad signal immediately to minimize galvo wear during those "oh crap" moments) An emergency cut off for the laser or a shutter would be good for your final product as well.
Good luck and have fun!
-Mike

[jack]

That write up by Elm Chan was one of the first things I ran across when I first started looking into lasers. It's a pretty great achievement!

I've got access to function generators here at uni, so they should be pretty good to set up a slow scan for each of the galvos, to get the initial set up working. I'll keep the soundcards in mind though if (when) it's not turning out how I expect

Would a all analog pattern generator be a useful option for your project?

You mean using a bunch of opamps to control the mirrors?
Yeah it's definitely an option, but it's going to have to come later because my supervisor has suggested I do some image processing on a PC, and then use that data for the laser display (i.e. take an image on a PC, and try to redraw the edges using the laser). That's going to be a fairly time consuming task : (

I've been trying to decide on the best way for the microcontroller to store the image information (I've opted for the uC to start with, hopefully the FPGA can come later). Does anyone have any recommendations?
Currently I'm leaning towards storing the image as a series of splines (Bezier curves or something). The uC might not be fast enough to recalculate them on the fly though.

J.
p.s. Sorry for the late reply. Holidays are awesome.