Klemen - You'll no doubt shortly get questions about how fast you want to scan, and that EMC may not be appropriately fast enough for both axes. Since graphical laser scanning is a persistence-of-vision process, the graphical image must be continuously rescanned. How fast? In the laser graphics industry (oh, right - 10 year+ veteran here - 20 watt mixed gas, 150watt YAGs - big side-of-the-mountain type shows - lots of 480volt camlock cable!)....we measured vector graphics in points per second. When I started, a LaserMedia Z80 processor could do about 12000 pps with closed loop galvos (General Scanning's G120 were the defacto standard). Now a Pangolin or similar PC-driven system can do over ten times that. You absolutely must have closed loop scanning to do anything better than 20,000 pps with mirrors of mass more than 1.25 grams. The inertial mass of the mirrors and the rotor doesn't stop on a dime, and closed-loop scanning provides a reverse-kick-in-the-pants to prevent the rotor from overshooting the target. You will also not get sharp corners open-loop at faster than 1000 pps on those units. Physics just sucks that way. Finally, the MFE galvos (old workhorses that are still in use currently) are slow devices; check out the resonant frequency - 90-100 Hz - that's really as fast as you can accurately go before the rotor mass affects performance. This rating is exclusive of any mirror you attach - and the mirrors are small for a reason - a 1" square glass will shatter on its own from internal stresses if you wiggle it too much. That 100 Hz freq will go down as you increase the mirror mass. A high-performance scanning mirror is about 3-5mm square and 0.25grams mounted. Even the drop of acrylate glue was precision dispensed for balance. I should probably mention that points per second does not necessarily equal Hz; if you have a graphic with lots of anchors, corners, etc - you have a lot of points, thus to try and retain those corners, you have to go through the loop slower; if you have a 2 point line, you can get really zippy.
This however should not discourage you from your development - I started out the same way and would recommend everyone experiment that way too - in fact, most of the major players in the laser entertainment industry started out the same way, less than 30 years ago. The eye will maintain vision on a 30 degree field with about a 20Hz refresh - so a 20-30Hz circle is a good graphic to start scanning. It's a good test and will let you get the phasing tuned in on your PID loop. Having a known "envelope" to work with is important - you may not get text and corporate logos (you didn't mention blanking) - but you'll easily accomplish lissajous patterns and good beam effects (with a powerful enough laser and atmospheric haze) that will be a good compliment to any show. The EMC challenge is how comfortable are you to interrupting the HAL translation to push in the vector data? I expect that the gcode parser may not behave quickly enough to continuously refresh without getting bogged down. A step/direction setup will be of no value here, and straight PWM will work, but you'll severely restrict your beam/projection angle. The early "el-cheapo" pattern scanners of yesteryear used +/-15v supplies and drivers, and not always variable drive signals - not PWM either - just -15, 0 and +15 with an RC network and clever timing. The RC network locked in your scanning speed, unfortunately, but it was cheap, sturdy and reproducible. Even recent closed-loop scanners still use bipolar supplies (your MFE unit can too - the torsion bar will self-center with zero-current applied). Your R2R ladder will work - and will be faster than a serially-commanded DAC. If you can take a PWM from EMC and make it bipolar (a few opamps here, an H-bridge there...) you'll have the basis for a good driver. The top speed of EMC processing on the HAL-side (PWM generation, etc) will likely not be a factor - again your top scanning speed with the galvos you have will be around 1000-5000 pps, once you mount mirrors. I would recommend highly, especially for the instant gratification aspect, to build the servo amps and get the galvos moving first (just put in a line-level audio signal - it's fun) so you have a known working baseline instead of trying to hunt down unknown peculiarities. Cheers, Ted. >>>Original Msg>>>>>>>> Date: Sat, 1 Dec 2007 06:13:17 -0800 (PST) From: Klemen Dovrtel <[EMAIL PROTECTED]> Subject: [Emc-users] open loop galvanometer control To: [email protected] Message-ID: <[EMAIL PROTECTED]> Content-Type: text/plain; charset=iso-8859-1 I have two open loop galvanometers ( http://www.laserfx.com/Backstage.LaserFX.com/Systems/Pinouts/MFEdataLG.jpg ) which i would like to use emc controlled laser show (similar to this one: http://elm-chan.org/works/vlp/report_e.html ). I will use a hardware encoder for measuring the angle of galvanometer. I will configure hal using hal-encoder and hal-pid. I think i will manage to do this (i checked the etch-servo sample configuration). <snip> No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.503 / Virus Database: 269.16.12/1162 - Release Date: 11/30/2007 9:26 PM ------------------------------------------------------------------------- SF.Net email is sponsored by: The Future of Linux Business White Paper from Novell. From the desktop to the data center, Linux is going mainstream. Let it simplify your IT future. http://altfarm.mediaplex.com/ad/ck/8857-50307-18918-4 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
