"KPPS" technical definition/question

Hi All,

I'm totally new to lasers, but I'm a pretty technical sort otherwise-- I wandered into the laser side of things after working with CRT based vector display systems (ie, arcade games) for way too long...

The question I have is related to galvo "speed" ratings. What is actually the 'working' (for lack of a better word) definition for "KPPS" as associated with galvo deflection systems?

To be a bit more clear, is a "point" considered to be an endpoint of a vector (two "points" make a line)? Or, is a "point" more of a single step along a vector with multiple points comprising a line?

Simple illustration: with some arbitrary coordinate grid, if we have one line with endpoints at (0,0) and (100,0) is that two "points" or is that possibly 100 "points"? (It seems to me like the definition would have to be relative to total beam distance traveled rather than arbitrary 'endpoints'-- but if that were the case it would seem to have to be related to the output resolution of the device feeding the scanner as well, so I've over-thought it now and have myself confused.)

I didn't have any luck 'search'ing for an answer on the forum, so apologies in advance if there's a simple and obvious definition posted somewhere and I missed it.

I'm wanting to drive the ILDA input directly from a little microcontroller/DAC setup (just because I think it'd be neat ;-) and I just want to be sure that I'm understanding the slew rates involved.

Thanks in advance for any wisdom on the subject!

-Clay

This usually depends on your software setup. Most softwares add "optimizations", which includes adding more points to a line/shape.

For example, if you had a square, and 1 point in each corner, your galvo's would be told to go to that point, but then they'd be told to go to the next one, and you'd end up with more of a circle than a square. Extra points are added at the corners and along lines to make sure they don't distort too much.

KPPS is just that, how many of those points the galvo can get to in a second. If you had an image with 20,000 points, it would scan the image once every second, which is wayy too slow for your persistence of vision to make an image from.

No, an ILDA kPPS rating has a direct relationship to the response speed of the scanners. Scanners are said to be 30kPPS if they can scan the ILDA test pattern (with no points added or subtracted) correctly at a DAC point rate of 30,000 points per second, typically at 8 degrees full scan angle. This scanner rating determines how complex an image you can scan without experiencing flicker or distortion.

See this excellent expansion on the ILDA test pattern and scanner kpps ratings here:
http://www.laserist.org/Laserist/tech_tips_5.html
and here:
http://www.pangolin.com/ILDAtest.html

Quote:

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Originally Posted by Things

For example, if you had a square, and 1 point in each corner, your galvo's would be told to go to that point, but then they'd be told to go to the next one, and you'd end up with more of a circle than a square. Extra points are added at the corners and along lines to make sure they don't distort too much.

KPPS is just that, how many of those points the galvo can get to in a second. If you had an image with 20,000 points, it would scan the image once every second, which is wayy too slow for your persistence of vision to make an image from.

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(First off, thanks for the reply!)

Is a "point" defined as a move of a certain percentage of the display angle or something? (If there isn't a distance that a "point" implies I'm having a hard time understanding how the galvo could be rated in "point" units? Are these "20KPPS" points close together or far apart or what? ;-)

If we had, say, a 'random' starfield-- just random dots scattered in the display area-- the order that the dots are visited would affect the distance the galvos had to move and the time taken to draw the frame. An optimal path (shortest distance) to all the dots could dramatically reduce that time and increase the frame rate even though the number of displayed "points" didn't change? (just the order that they were drawn changed-- presumably something like Dijkstra's Algorithm or another path finder being applied by the software to reduce the total 'distance/cost' of the positioning)

I guess I'm basically saying that conceptually you could have a frame with 1000 'points', that with an optimal path could maybe be displayed at 20fps with a 20KPPS galvo setup... But that same set of points with a less efficient path could potentially not be displayed at 20fps on the same scanners due to all the 'extra' time spent positioning the mirrors to the new location?

Or is it more of an empirical metric where it's given a standard frame and how fast it can complete it accurately defines the speed of the galvo?

Thanks for putting up with the technical-newbieness. ;-)

-Clay

Quote:

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Originally Posted by drlava

No, an ILDA kPPS rating has a direct relationship to the response speed of the scanners. [...]

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Ahhh! Excellent. Had my reply written before seeing yours posted! I'm going to read up (and probably be back with another question or two). Thanks for the pointers!

-Clay

I read up some more and have a follow-up question. :-)

Assume someone has an ILDA 30KPPS capable scanner tuned for 30K.

Let's say my hardware has a DAC update rate of 30KHz (new value output on the DAC 30,000 times/sec) and I have an 8 degree scan angle. For the sake of discussion, I want a 30Hz refresh rate. 30K/30Hz = 1000 points per frame.

Are those 1000 "points" *all* potentially worst case full deflection? (ie, could you have 1000 points alternating between opposite corners and expect the scanner to keep up?) To rephrase that another way; are the points *any* possible positions within the deflection angle?

(sorry if my terminology isn't quite right-- feel free to correct me. I'm used to 'deflection' of CRT electron beams from the center of the screen.)

Thanks for your time playing Q&A with me...

-Clay

No. The ILDA pattern is far from a worst case scenario. It's just an industry standard tuning pattern. If your 1000 points are too random or far apart, the scanners will not be able to scan them in a pleasing manner.

In fact, you may potentially be able to get better performance out of your scanners by NOT tuning them to the ILDA pattern. The ILDA pattern can dumb down a pair of scanners to allow point pulling to work. But, if your scanners can handle it and you don't want to use point pulling, you can set the gain and feedback higher so that they actually track truer to what they are told to do. It really all depends on whether you are shooting for interoperability or you want true tracking. By not tuning to the ILDA spec, you may have a better chance of scanning those worst case 1000 points that you asked about.

...If you hook a laser display system to a vector monitor (I do this for show programming) you see a bunch of bright dots with thin, dim, connecting lines, as we rely on the galvo's slow speed to "connect the dots".


...Because of the inertia of the scanner mirrors, vector laser is a bit different from stroke writing on a CRT. We tend to use more of the "travel time" between points to create a image, instead of stacking the points 1 bit apart like on a crt. . We also have a different "writing speed" between small jumps and large jumps. On a crt, you just see a thin trail between long jumps, unless you have one of the stroke writing systems that charges a capacitor to create a linear ramp waveform between points. On a laser system, the inertia of the mirror causes a gentle, even, line on short to medium jumps. Long jumps need various techniques to make the brightness even.


...To use the mirrors inertia to our advantage, the goal is to maximally, linearly accelerate the mirror for 1/2 the travel distance and then maximally, linearly, break for the other half. In reality, there is ringing, overshoot and undershoot, so the amplifier has a PID loop tuned to a given mirror/shaft inertia and there is a position feedback sensor on the shaft.

Steve



Other techniques such as adding anchor, corner, and blanking points are used to smooth out and sharpen up a image.

Anchor point, points added during long, visible jumps to guide the scanner to the next major change in the artwork.

Corner point. To get a sharp corner, often 2-3 points are added at the junction of the angle.

Blanking point: Guide point added on long jumps when the laser is off to ensure a even brightness in a image, by insuring the speed between all jumps is fairly constant.

In laser, if you have a huge 3D image or a image that is blanked at first and "revealed", ie the "write out" effect, we would tend to scan the whole point cluster even when its blanked, to keep the image brightness even.

Newer software has a lot of optimization tricks far beyond what I mention, but its a start.

Steve

Quote:

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Originally Posted by havok1919

Are those 1000 "points" *all* potentially worst case full deflection? (ie, could you have 1000 points alternating between opposite corners and expect the scanner to keep up?) To rephrase that another way; are the points *any* possible positions within the deflection angle?

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No, 'ILDA 30k' scanners have a response bandwidth of DC to somewhere between 2kHz and 3kHz. So you have to stay within those limitations, often by adding guiding points and anchor points and pulled points (as Steve says). Open the ILDA test pattern in laserboy and trace around it to see what I mean.

Quote:

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Originally Posted by JohnYayas

In fact, you may potentially be able to get better performance out of your scanners by NOT tuning them to the ILDA pattern.

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Sometimes you can overtune your 30k scanners beyond the ILDA test pattern at 30k, but that just means you can now run the test pattern at, say, 35k and it will look right, so you have tuned your scanners to 35k. You haven't really 'not tuned them to the pattern', you've just tuned them to a faster speed of the pattern.

Thanks for the replies guys! I think I'm much more up to speed now-- really helpful posts!

On vector monitors we have sort of the opposite problem-- we actually need to keep a uniform write speed as the beam deflects along the entire vector with very little (if any) excess dwell time at endpoints (lest the beam burn the phosphor if left in one spot too long). With the lasers and mechanical scanning systems the slew rate of the mirrors essentially *are* your integration capacitor if we were to compare it to some of the older analog-ish vector gaming systems. ;-) (Ie, pretty much all the Cinematronics and later Atari vector games you see in laser MAME.)

I get the idea for acceleration/deceleration and increased dwell time and whatnot by interpolating with multiple points along lines. Makes sense!

One more question then... If the position systems in the drivers for the mirrors are closed loop, analog controllers, would there be any reason (other than data rate from the host) not to feed them with a continuous input? (ie, having the DAC system sweep the X/Y voltages between point "A" and "B" at a constant rate less than the maximum slew rate of the mirror/amp controller?)

(I'm basically wondering if I can take my constant write-speed CRT vector display controller hardware and essentially just turn the 'draw/move' speed way down and use it with the laser. I suspect that it would always result in the scanner running a bit slower than it's potentially capable of since there wouldn't ever be any "move from A to B as fast as you can" type jumps, but I assume that the PID loop is running *much* faster than the point rate and a continuous signal wouldn't cause it issues?)

Granted, I could just *try* this, but not knowing the conventional wisdom I'm trying to be a little cautious before I abuse anything *too* much. :-)

Thanks again,
-Clay

One neat thing about PROPERLY TUNED modern galvos, if not fed ultra sharp high frequency square waves, they tend to survive anything you throw at them, especially if there are size pots between the signal source and the galvo amp. A dual 10K or 100K audio taper pot makes a great XY size control. The one radio shack sells for 3$ will enable your experiment.

A few years ago before a recent price fall in some controllers, I looked long and hard at a Zektor(tm) vector controller, but would have needed some one to write some Windows code. I had a skilled person in mind who could write that code, but he turned out to be a real failure when working with others and I canceled the project. The Zektor folks were not so keen on trying,and I did not feel like loaning them a 700$ pair of galvos at the time, so the project stopped. But at the time I did not see any obvious problems using the Zektor for laser with a few minor mods.

What I'd saying is it can't hurt to try if you keep the initial angles down using the size controls.

Newer galvo amps, as my Friend Andrew often reminds me, are more PII, then PID, so they tend to not launch into oscillation with rough drive waveforms.

Start with a circle. Work up from there.

Worse comes to worse you may need to play with/remove/increase your integration cap on your generator.

As to just turning the speed down, yes, just expect to perhaps have to do some amplifier tuning on the small jumps. Its not a big deal, three 10 turn trim pots. Since you already have a oscilloscope, one of the techniques we use is to tune one axis on a square wave for the least ringing and overshoot, while watching the feedback signal. So you can match velocities if you need to, but I bet you can just lower your speed and make changes to the image. So what, if you loose 10% of the scanner's performance, no big deal. In reality everybody opens up the angle and reduces the scan speed, ILDA 30K is really a data interchange format if you think about it. You tune to 30K/8' to share images correctly, more then anything else. When you run a show you most likely open up to 15-20' and the PPS rate comes down, mathematically the angle multiplied by PPS is roughly a constant, so as you increase angle, point rate falls off. Its not directly a linear function, but it does happen.

For those wondering what a Zektor is:

http://www.zektor.com/zvg/downloads.htm

Steve

Quote:

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Originally Posted by mixedgas

A few years ago before a recent price fall in some controllers, I looked long and hard at a Zektor(tm) vector controller, but would have needed some one to write some Windows code.

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Hahahah... Small world! I've known Zonn for *years* on the arcade game side of things. One of my business partners actually did the PC-side development for the ZVG back in the day. (Nasty, parallel port business such that is was. ;-)

Zonn and I had a (friendly) holy war going on for ages about analog vs. digital vector generators. Zonn implemented sort-of a modern take on the Cinematronics vector generator for the ZVG. I basically advocated a modern digital update of the basic design from Atari or Sega's digital vector generators. (ie, no integration caps, just direct high speed DAC control)

Since Zonn actually shipped the ZVG as a product though, he wins the debate by default. ;-)

Anyway... I'm still doing gaming stuff, so basically I'm just looking to find another fun use for the technology-- and I've always wanted to have a laser scanner...

Thanks for the pointer on the RS pots. There's a few parts they still sell that are useful after all. :-)

I'll take it slow and see how it goes. I'm really looking to do original projected laser games (we own an arcade and need some neat wall/ceiling "decoration") but using our own hardware platform instead of a PC based setup.

-Clay

Keep us informed. Laser Mame and LazyMame are BIG favorites around here. As is Lasertennis and Asteroids, the Pangolin and Zoof versions.

Where are you at? It is possible that a PLer could stop by with a pair of galvos.


Steve

Quote:

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Originally Posted by mixedgas

Keep us informed. Laser Mame and LazyMame are BIG favorites around here. As is Lasertennis and Asteroids, the Pangolin and Zoof versions.

Where are you at? It is possible that a PLer could stop by with a pair of galvos.


Steve

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Is there a Lasertennis for the LD2000?

Quote:

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Originally Posted by mixedgas

Where are you at? It is possible that a PLer could stop by with a pair of galvos.

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I'm in the Portland,OR area (Vancouver, WA actually-- but so close as to make no difference). Our arcade is in Portland (Ground Kontrol).

I actually bought a couple systems off eBay to mess with so I'm "dangerous" if not necessarily knowledgeable at this point.

My business partners and I design and build the hardware/software that's inside a lot of commercial arcade stuff (if you ever see the "Arcade Legends"/"Ultimate Arcade"/"Extreme Arcade"/"Golden Tee Clubhouse" machines at places like Costco, Sears, Sports Authority, etc. those run our hardware/software inside.

I just like having little low-power, low-cost dedicated hardware to run things instead of PC's, so that's pretty much what I'm doing. I've written "Vector Breakout" (ran on an Atari Tempest machine) and a few games that run on the Vectrex game system ("Moon Lander", etc.), so I want to try getting some of those going with the laser instead. ;-)

There's a pic of Vector Breakout here:
http://www.multigame.com/MG_IMAGES/vb2.GIFhttp://www.multigame.com/MG_IMAGES/VBREAKOUTs.GIF

And someone put up a video of Moon Lander running on the real Vectrex hardware here:
http://www.youtube.com/watch?v=95-pezJ37ig

...anyway-- I'll undoubtedly need to ask some more questions and muck about a bunch before I get anything worth sharing. ;-) Probably run everything on a scope just to be safe for a while!

I know that *years* ago some guys tried running Vector Breakout on a laser setup at the California Extreme (arcade gaming) show. (Might have been around 1999 or 2000?) I tweaked a lot of the code there to get rid of long jumps and extra stuff so it would hold up framerate better, but I have no doubt that things on the laser scanner side are 10x better/faster/cheaper now!

-Clay

Wouldn't it be awesome if this game would be ported to laser display ?

http://www.youtube.com/watch?v=abH1tdSVLa4

Portland/Vanc area? There are at least 4 laserists in your area.

Come on WA laserists, at least one of you would like to see laser arcade games, help this guy out!

Steve

Quote:

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Originally Posted by mixedgas

Portland/Vanc area? There are at least 4 laserists in your area.

Come on WA laserists, at least one of you would like to see laser arcade games, help this guy out!

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Good to know! I'm not in a huge rush just yet (waiting on some PCB's and components and some extra time to work on stuff!), but knowing there's some people up here I might start another thread and ask for local assistance for some compatibility testing down the road. :-)

-Clay

That gravitron game would convert to laser display very well with a few minor changes. It looks great!

Quote:

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Originally Posted by havok1919

(I'm basically wondering if I can take my constant write-speed CRT vector display controller hardware and essentially just turn the 'draw/move' speed way down and use it with the laser. I suspect that it would always result in the scanner running a bit slower than it's potentially capable of since there wouldn't ever be any "move from A to B as fast as you can" type jumps, but I assume that the PID loop is running *much* faster than the point rate and a continuous signal wouldn't cause it issues?)

Granted, I could just *try* this, but not knowing the conventional wisdom I'm trying to be a little cautious before I abuse anything *too* much. :-)

Thanks again,
-Clay

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Hi Clay, good to see some more game development here !
Laser display is about showing as much detail with as few points as possible. If you use too many points your image will flicker, if you use too little points your image will be deformed by overshoots. So you will run into the challenge of optimizing the point placement along the beam's path to get as much detail out of the number of points that you can use. I might be able to help there when you arrive at that point.

looking forward to your projects.

Matthijs

Also, if you don't want to mess with point placement at all, both RIYA and QM2000 have auto-optimizing vector input libraries. All your code has to do is draw the most basic lines and the libraries take care of scanner optimization.

Quote:

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Originally Posted by drlava

Also, if you don't want to mess with point placement at all, both RIYA and QM2000 have auto-optimizing vector input libraries. All your code has to do is draw the most basic lines and the libraries take care of scanner optimization.

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I would definitely go that route if I was running on a PC. I'm going to try to adapt one of our little ARM based systems to drive the laser directly (instead of a vector monitor) though.

(Yes, I'm making this way harder than if I just used a PC to write a game, but dedicated hardware has some advantages-- and I just want to try it. ...and throwing GHz worth of PC horsepower at a line based game just isn't even sporting! ;-)

Thanks for all the pointers and tips!

-Clay

Quote:

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Originally Posted by havok1919

(Yes, I'm making this way harder than if I just used a PC to write a game, but dedicated hardware has some advantages-- -Clay

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Like REAL TIME for example! ................. Thats music to my ears! :cool:

Good on you cobber ! *DZ

Cheers :)

We r trying to build a DIY laser-show projector using BLDC motors with optical encoders + PLC for control.
We r newbies and have some doubts regarding the ILDA test pattern:

1) We have the same doubt as : havok1919/Clay.
"Is a "point" defined as a move of a certain percentage of the display angle or something? (If there isn't a distance that a "point" implies I'm having a hard time understanding how the galvo could be rated in "point" units? Are these "20KPPS" points close together or far apart or what?"
"if we have one line with endpoints at (0,0) and (100,0) is that two "points" or is that possibly 100 "points"?"


2) For example: there is a square in the ILDA pattern. Is it made of 4 points or more points?

3) I opened the ILDA pattern using LaserBoy software. It says there are ~1191 vertices in the pattern.
Now if Iam tuning for 12KPPS, does that mean my frame rate during the tuning would be : 12000/1191 ~= 100 fps?

4) Does anyone have the ILDA pattern co-ordinates as a ascii file or a DXF file?
Would like to go thru the co-ordinates and understand what kind of anchor points, guiding points, blanking points etc. has been added to the pattern.

regards

Quote:

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Originally Posted by TanveerChawla

We r trying to build a DIY laser-show projector using BLDC motors with optical encoders + PLC for control.
We r newbies and have some doubts regarding the ILDA test pattern:

1) We have the same doubt as : havok1919/Clay.
"Is a "point" defined as a move of a certain percentage of the display angle or something? (If there isn't a distance that a "point" implies I'm having a hard time understanding how the galvo could be rated in "point" units? Are these "20KPPS" points close together or far apart or what?"
"if we have one line with endpoints at (0,0) and (100,0) is that two "points" or is that possibly 100 "points"?"


2) For example: there is a square in the ILDA pattern. Is it made of 4 points or more points?

3) I opened the ILDA pattern using LaserBoy software. It says there are ~1191 vertices in the pattern.
Now if Iam tuning for 12KPPS, does that mean my frame rate during the tuning would be : 12000/1191 ~= 100 fps?

4) Does anyone have the ILDA pattern co-ordinates as a ascii file or a DXF file?
Would like to go thru the co-ordinates and understand what kind of anchor points, guiding points, blanking points etc. has been added to the pattern.

regards

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Hi, it can be confusing, be sure to read the links in my first post above. There are a few things to keep in mind:
The transfer function of control to optical deflection of traditional mechanical galvos (with drivers) can crudely modeled as a low-pass filter, with higher point rates input, past a certain frequency, giving greater and greater distortion on the output image.

1) A 'point' in itself is nothing more than a DAC sample, nothing more, nothing less.

However when used in the context of kpps rating of galvos, it's most accurately referring to the ability of the galvos to draw the 12-point circle so its edges meet the square at a certain scan angle. A scanset that can fit that requirement (typically) at 8 degrees total frame scan angle when the frame is being clocked out of the DAC at a sample rate of 30,000 smaples (points) per second is said to be 30kpps rated.

2) it is made of more points, or 'optimization' points.

3) about 10FPS, yes.

4) if you have laserboy and have opened the test frame, you can not only iterate through the points to see for yourself, but can save as a text file as well. To iterate, press k to get to the points editor, and square brackets to iterate through.

Hi,

I opened the ILDA pattern file in the laserBoy and was able to see the "points" locations in the point-editor mode.
Looks like the square has lot of extra points at the corners.
My question is : 1) Are these "optimisation" points there in the orginal ILDA file or added by some other software.

2) The co-ordinates in the ILDA file: Are these "X,Y" co-ordinates or "motor1Angle,motor2Angle" co-ordinates.
I believe we need to do some inverseKinematics calculations to get motor-angles given the XY co-ordinates.
Is this typically done for laser-shows or it is not so important?

regards

Hi,

I opened the ILDA pattern file in the laserBoy and was able to see the "points" locations in the point-editor mode.
Looks like the square has lot of extra points at the corners.
My question is : 1) Are these "optimisation" points there in the orginal ILDA file or added by some other software.

2) The co-ordinates in the ILDA file: Are these "X,Y" co-ordinates or "motor1Angle,motor2Angle" co-ordinates.
I believe we need to do some inverseKinematics calculations to get motor-angles given the XY co-ordinates.
Is this typically done for laser-shows or it is not so important?

regards

Quote:

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Originally Posted by mixedgas

Portland/Vanc area? There are at least 4 laserists in your area.

Come on WA laserists, at least one of you would like to see laser arcade games, help this guy out!

Steve

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Here! How may I be of service? ;)

-Jonathan

quote:
My question is : 1) Are these "optimisation" points there in the orginal ILDA file or added by some other software. end quote.

No, they are there as part of the design of the standard frame. The test pattern takes advantage of certain characteristics of a analog scanner amplifier PID loop, and the inertia of the galvanometer scanner rotor system.

Quote:
2) The co-ordinates in the ILDA file: Are these "X,Y" co-ordinates or "

End quote.

No, they are a X-Y plot.

Remember that galvo scanners work over small angles (8 degrees or such), and at small angles sin(x) approximately equals x and cos (x) approximately equals 1-x (where the angles are in radians) so for the small angles involved scan angle and x/y position become almost equivalent.

Regards, Dan.