Question about collimated display systems.

Quote:

---

Here's a link that shows what we got done this past Saturday:

http://www.diy-cockpits.org/coll/col...lay_page2.html

Hopefully the server won't fall over again today. (motherboard is fading I think)

It turns out the only active patent we have to worry about covers the "ears" that help form the mirror. I don't know that there is a practical way around that one, but we'll look into it further at some point.

---

WOW Gene , nice work and a great approach ..
I liked the picture of the mirror in the previous link , it's so pure & really can be called a mirror ..
Thank you so much Gene for your work ..

Quote:

---

Originally Posted by geneb

Just a quick update - this is what the mirror "solid" looks like;
http://www.diy-cockpits.org/coll/ima...rame_solid.jpg

This will be cut apart into the bits needed to make it into a framework. The top is 4" wide and the gap is about 68" wide. The mirror frame covers 225 degrees.

g.

---

I'm working now on a 180 degrees frame , But do you think by your experience that the 225 degrees frame can shape the Mylar easy ?? I think it ll lost its horizontal tension .. Plz correct me if I'm wrong ..

another thing , If we minimized the frame from its down-sides , the Mylar would require a less vacuum power to get shape , because by this way we minimize the total space that the vacuum has to work in ..

http://www14.0zz0.com/thumbs/2010/11.../680130421.jpg

( Sorry for editing your Picture ) ..

This shows a wireframe of the mirror at it's correct 190 degree layout.
http://www.diy-cockpits.org/coll/ima..._wireframe.jpg

g.

Quote:

---

Originally Posted by Ricardo Carvalho

I saw this yesterday on discovery, did not know that the projector had a mirror between it and the screen that reflects to the vacum mirror.

http://science.discovery.com/videos/...simulator.html

---

I tried to view the video, but after the commercial for a very expensive car which I'm not at all interested in, all I got to see was a message stating "this video is not available in your area"!

(Booh!)

Quote:

---

Originally Posted by HansJansen

I tried to view the video, but after the commercial for a very expensive car which I'm not at all interested in, all I got to see was a message stating "this video is not available in your area"!

(Booh!)

---

I've got the same problem. Probably only North America. But I think I have seen the program that the video is probably from.

Quote:

---

Originally Posted by HansJansen

I tried to view the video, but after the commercial for a very expensive car which I'm not at all interested in, all I got to see was a message stating "this video is not available in your area"!

(Booh!)

---

http://videos.howstuffworks.com/scie...ator-video.htm

As it turns out, there are 2 versions of this video, one with a female voice, the one that is here, and one that i have just watched on tv with a british accented male voice that shows an animation of how the projection works just before the scene where the guy is working on the throttle.

This version here does not have that scene... don't know why.

http://www.youtube.com/watch?v=_1889jWwHUc

From where I can buy the "Mylar" ( Packaging paper or what ! ) ??? and what are the features that's recommended required ??

Hi (sorry don't know your name),

Search Google for Polyethylene terephtalate
or try: http://en.wikipedia.org/wiki/Polyethylene_terephtalate

It most found on the net in rolls of ~150 cm (or slightly more) width.
Search in the agriculture sector, it's also used in greenhouses to reflect the light (hmm, not necessary in Egypt I presume...).

As you did read before .., it's "coated" with aluminium for the mirror effect.
In Europe about 10 euro per 100 cm (width above).

But before searching too much ...:

Wait for more results of Wayne and Gene.
Without hopefully coming specific information / education (calculation method) it's not doable by trial and error.

Hessel Oosten

Quote:

---

Originally Posted by Ricardo Carvalho

http://videos.howstuffworks.com/scie...ator-video.htm

As it turns out, there are 2 versions of this video, one with a female voice, the one that is here, and one that i have just watched on tv with a british accented male voice that shows an animation of how the projection works just before the scene where the guy is working on the throttle.

This version here does not have that scene... don't know why.

---

"How Stuff Works" (like lots of Discovery programmes) is made with different voiceovers for different regional markets. Quite often the US TV version is not the 'original' voiceover (which can often to be found in the Canadian broadcast instead) because quoted measurements generally need to be converted to Imperial (aka 'English') from metric for US consumption. The UK usually gets a version dubbed with a British speaker and metric units.

So the version you saw was probably the UK version. The US version most likely had a few seconds edited out to fit the slightly shorter US half-hour slot (which has more commercials than you usually get on UK channels).

NH

BTW, if you happen to have access to a VPN service which has an US-based endpoint, you can get around the geo-ip restrictions and watch the video. Not that I would ever condone doing anything so evil and wrong, of course <g>.

NH

How about one of these space blankets? If you don't know them see http://en.wikipedia.org/wiki/Space_blanket.

I don't know if they are actually made of mylar, but they are often called 'mylar blankets'. And they are pretty cheap, and you can get them in a lot of outdoor stores and on the internet.

The downsides I can see is that they are folded very tightly because you need to be able to put it in your First Aid kit, so they will have some pretty sharp folds of witch I don't know if you can get them out with the vacuum or any other way. And it might be a problem with the size, but I think they should be big enough to cover a person's body so a small display should work.

I don't know if it works, but is nearly too cheap to not try this. You can buy these blankets for about 4 euros I think.

Just an Idea

Babeloe

How about one of these 'space blankets' they're made of mylar and pretty cheap. http://en.wikipedia.org/wiki/Space_blanket The only downside i see is that they are folded.

Hi Hessel ..

It's "Mohammed" you can call me Medo if you like :)

I appreciate for you your reply .. Thank you so much !
The wikipedia link about "Polyethylene terephtalate" that you provided me is so useful , I don't like just following steps without knowledge , I like to understand everything about what i plan to do ..

Thanks Hessel

ricardos link worked for me here in oz.
Great vid but she makes it sound so simple..lol
Cheers..

Sorry to butt in with noobish questions but I've got 2:

1.)

Quote:

---

Originally Posted by Mike.Powell

Metalized Mylar can be used to make film collimating mirrors. Up to about 40 degrees of vertical field of view, building one is merely difficult. Building one with a 60 degree vertical field of view apparently requires magic. SEOS figured it out a few years ago. Rockwell Collins was so impressed they acquired SEOS in 2008.

---

What makes 60 degrees so much harder than 40 degrees?

2.) lets talk resolutions or maybe more appropriately aspect ratios? But I haven't really started thinking about the displays of my simulator yet so I don't quite know how to talk about in a way other than layman terms. So, lets take 800x600 resolution and let's take the 40 degree by 60 degree 48" section that geneb has taken for his display. First, how can you display a square image on something that isn't of the same circumference at the top and bottom? Second, the 40 degree (height? wise) and 60 degree (horizontally? wise) gives us a viewing area of approximately 3:2 ratio? The closest one that I see to this would be 16:10 i.e. something like 1280x800 so essentially widescreen? As I said, I don't quite know how displays work so I could be totally wrong in what I'm saying but I hope I've but my question across with enough clarity.

EDIT: Just some additional information, when I tired the SAAB and Airbus simulators, I distinctly remember distortions in the image but they were generally down the bottom and off to the sides. You had to strain/be stand upish to see them so professional displays aren't as perfect as you guys might think (for those who haven't been in the real thing that is). In fact, when you find out the price, you're almost disappointed!

The Mylar initially is a flat sheet and must be stretched into a spherical-section shape. Beyond about 40 degrees vertical arc the stretching exceeds the plastic range of the Mylar. Because its properties change, it's more difficult to make the sheet form a spherical-section surface of a quality suitable needed for the application.

When projecting onto a spherical-section surface there will be geometical distortion. It's compensated for using image warping software like Nthusim.

Agreed, collimated displays are not perfect. The designs tradeoff collimation effectiveness, view box size, distortion, and so on.

All commercially produced collimated displays have a distortion band around the perimeter edges of the mirror. Those bands are typically masked by physical structure. That's why the only way you'll ever see them is if you move your head to see over/around those masking elements. The only time you'd ever do that is if you're doing it on purpose, which is your own fault at that point. :)

The 60x40 mirror was a proof of concept device. I was using an 800x600 projector because that's all I had available. The full system we're working on now will cover 190 degrees of "intentional" mirror, and out to about 225 when you include the "ears". The video will be provided by three Epson 705HD projectors that run at 1280x800 (and thanks to the kindness of Matt O., I'll be able to run them at that res instead of 1280x768 due to the limitations of the analog TH2G.)

Constructing a 60 degree vertical FOV mirror is mechanically tricky I suspect. Note that the current FAA Level D certification requirement is only 36 degrees. :)

g.

Quote:

---

Originally Posted by Mike.Powell

The Mylar initially is a flat sheet and must be stretched into a spherical-section shape. Beyond about 40 degrees vertical arc the stretching exceeds the plastic range of the Mylar. Because its properties change, it's more difficult to make the sheet form a spherical-section surface of a quality suitable needed for the application.

When projecting onto a spherical-section surface there will be geometical distortion. It's compensated for using image warping software like Nthusim.

Agreed, collimated displays are not perfect. The designs tradeoff collimation effectiveness, view box size, distortion, and so on.

---

Just wondering, can you apply a little "slack" when installing the mylar; you know the radius of the desired shape, compute the arc distance say for 40 degrees at 6 feet requires (40/57.29) * 6.0 = ~4.188 linear feet of mylar in the vertical when the mirrot is formed. The chord distance between the upper and lower frame is something less, but say the mylar is attached with a 4 foot width, will it stretch uniformily to form the mirror, and if that is the case would reducing the amount that the mylar has to stretch allow for a greater vertical FOV.

JW

Quote:

---

Originally Posted by castle

Just wondering, can you apply a little "slack" when installing the mylar; you know the radius of the desired shape, compute the arc distance say for 40 degrees at 6 feet requires (40/57.29) * 6.0 = ~4.188 linear feet of mylar in the vertical when the mirrot is formed. The chord distance between the upper and lower frame is something less, but say the mylar is attached with a 4 foot width, will it stretch uniformily to form the mirror, and if that is the case would reducing the amount that the mylar has to stretch allow for a greater vertical FOV.

JW

---

I don't know, but I'm inclined to think doing so would introduce unacceptable distortions.

When flat, the Mylar does not have straight sides. The full sheet is actually a broad arc. If slack is added, the edge dimensions must change somehow. Somewhere you'll end up with extra material and wrinkles when fastening the Mylar to the frame.

Build a scale model and experiment with poly drop cloths.

Anyone tried Chrome monocote? When heated it becomes drum tight and is fairly strong. Also can be attached to itself to make bigger sheets.

Hi Geneb, I didn't really pay attention to the resolution of the projector that you were using so I wasn't criticising your setup, I was just trying to figure out what aspect ratios were and how they fit on a spherical display. I just read the wiki page on aspect ratios and my wild stab in the dark in relation to my assumptions as to the relationship between viewing angle and resolutions proved correct or at least I think so. The question that I'm still mulling over in my head though is the whole issue of different circumferences at the top and at the bottom and how an image which is 800 pixels (or 853.3 to be exact) can fit on something that is lets say 100cm up top and 80cm down the bottom or does it simply get scaled due to the ?refraction?/?defraction?

In relation to building a 60 degree vertical field of view. After getting into the topic of cullimated displays just yesterday (just to give you guys an idea of how little I know and how totally not based on any experience my thinking is but)... I was thinking that the only real restriction that I could figure out that made the 60 degree version nearly impossible to make was just the nature of materials and the fact that it would be more difficult to get a nice spherical shape after applying the vacuum as there are essentially 4 points of contact... well... 2 if we're talking about the vertical plane... so in other words, with 40 degrees, it's not very far off the back plate that it needs to vacuum to but with 60 degrees, there's a much bigger space behind it so I was thinking along the following lines:

We know the properties of the Mylar (at least I'm guessing the specfications of the stuff would tell it's elasticity per square cm or inch or meter or whatever... I'm assuming that whoever designed it, knows such properties...). If that's the case, then let's assume the 40 degree version uses a stupid method of calculating the right size material i.e. we take the vertical distance between the two mounting points and the circumferences and then simply attach it, vacuum it and we're done. With the 60 degree version, due to the greater amount of stretch required (based on absolutely no experience remember)... the Mylar would stretch at a much greater range... if that makes no sense, then hopefully this will clear it up. If you stretch any material, the way I see it, it stretches unequally, in some spaces it stretches a lot, in some not so much and I'm guessing it stretches the most right in the middle of two points holding the material in any given lateral direction (I could be wrong here and it might stretch the most at the points of contact and least in the centre but I hope I've made my point clear). SO! What we need to figure out is how much stretch is happening at the different parts of the Mylar on the 40 degree one and then scale it up to the 60. In other words, we need to basically create a piece, which if vacuumed, will have similar amount of stretch at similar places to the 40 degree one.

In the image below, I'm showing my train of thought.

Attachment 4547

The smaller the vertical field of view, the more we can get away with a simple rectangular piece, the larger the vertical field of view, the more we need to make it bend. I'm not accounting for the fact that the circumferences are different at the top and bottom which would I assume, make the shapes a little bit more funky around the sides but I hope I made my point clear. Perhaps those more in the know might be able to correct what I'm saying and totally discredit it :).

EDIT: Just found this in a patent document:

"It is known for typical wide-bodied cockpits that a 40 degree vertical field of view may satisfactorily be achieved with a spherical mirror of about 3m radius and a spherical screen. A vertical field of view of 50 degrees may be achieved with the same radius mirror, but the screen must become aspheric. For a 60 degree vertical field, not only must the screen become aspheric, but also the mirror radius must increase to about 3.3m."

I don't have my raytracer handy, or I'd make some good illustrations for you. I'll include them in a later post.

If the design eyepoint is very near the center, the field of view is limited to about 40 degrees. This is partially due to material limitations - when the mylar is stretched from a conic shape developable from a flat surface into the spherical shape, the arc defined by about 40 degrees produces a stretch very close to the material limits of the Mylar. If you try to stretch more than that, and you pass over the peak of the stress-strain curve and the mylar pops.

There is also the choice in vertical positioning of the eyepoint. If you move the eyepoint downward, that same 40 degrees of arc can cover more than 40 degrees of vertical FOV, because you're physically closer to the bottom of the sphere. Because of the extreme off-axis eyepoint, spherical aberration comes into play more strongly, and the required screen shape gets weird. The volume in which the collimation effect is acceptable shrinks. The distortion band around the bottom edge also increases in width, just due to the varying amount of stretch. Since you're further down in the sphere, the entire sphere has to be enlarged in order to fit the cockpit structure. Also because you're so far down in the sphere, the shape of the unrolled film becomes more curved, leading to a requirement for an even wider sheet.

There is also the choice of horizontal eyepoint position. If you move your eyepoint towards the mirror, the usable portion of the mirror in front of you fills a larger vertical FOV, but the portions to the side fill a smaller vertical FOV. One solution for getting a big vertical FOV is to make a big mirror and sit closer to it. This works well for large commercial sims, as the cockpit structure and windows does a good job of cutting the FOV to the sides anyway. In fact, with each pilot shifted foward and outward from center, each pilot has the largest FOV just outboard of the main windows, just as in the actual aircraft. The screen design gets more challenging, but by no means impossible. Again, though, to fit the cockpit structure within the mirror, the mirror needs to be rather large. As I still haven't found a source of mylar in sizes larger than ~56" without requiring the purchase of an entire 10,000 lb production roll, that is the main limiting factor at the moment.

wledzian,
Sending you a pm.
Mike

Wayne & I got a lot of really good design work done this weekend - when I've got time to re-create the model in Inventor, I'll post pics. (I should note that he DOES know how to use AutoCAD! *laughs*)

The big win this weekend was a solid grasp of the geometric limitations of the simulator cab. Because we're limited to the size of mirror we can make with a 56" wide roll of Mylar, that kind of dictates what cockpits can be built for use in the system. Right now the tiny GA 2 placers are an easy fit. The Cessna 152 has got tons of room and the 172 will fit as well. Both wouldn't be good choices because they're high-wing and the mirror goes from 0 to 40 degrees, which is far less than what you can see out the side windows. Any one of the "standard" low-wing GA aircraft should work though (Piper, Mooney, & Beechcraft are the popular ones). Wayne even managed to squeak a King Air 200 into the space available. It would require a special 3D model in order to show the wing & nacelle on each side, but that's fairly easy to do.

We also were able to cost-reduce the framework quite a bit. This is a Good Thing(tm). My original design was to use 12mm Baltic Birch throughout with 3/4" plywood partial width doubler layers in specific places.

We backed off the 190 form to 180 in order to make it break down to three sub-assemblies that would be easier to manage - the design goal is to create no component too large to navigate the interior spaces of Wayne's house. :) The new design uses 7/16" OSB for the top, bottom and end plates and 3/4" plywood for interior screwing rails. Each segment will bolt to one another using 1/4-20 bolts. The center segment and "interior" ends of the left & right segments will be under-sized along the mirror arc in order to prevent any contact with the mirror as it forms. The segments will also use two 1/32" rubber gaskets in order to prevent leaks.

The segments will have multiple coats of paint in order to make them as leak-proof as possible. (A high vacuum will actually pull air through OSB and some other materials like MDF)

When I post pics, a lot of my rambling will make more sense I think. :)

g.

Wow, Quite disappointing in regards to the 56" maximum size. I've got a got a feeling that cullimated guys should come together and purchase the big roll and then use what we need and either try to sell the rest to farmers or something, or simply keep it and sell it as other guys decide to get into cullimated displays.

Sounds good geneb, can't wait to see your progress. I'm probably going to do something similar to your desing i.e. build it in 3 parts for some flexibility if moving is needed. I've just got one question, I can't remember reading your solution to the back of the frame i.e. when you create the vacuum, do you have some sort of flat backing that you have attached to the formers to which the mylar conforms when the vacuum is created or do you simply create a vacuum and use whatever shape the mylar naturally forms? I was thinking making the whole thing out of fibreglass but I then realised that fibreglass isn't exactly flat and finishing it off to make it flat would be a nightmare. I then thought of simply creating a large number of formers such as the ones that you have and simply sticking them together side by side and creating a solid wooden spherical strucutre type backing but then I realised it would be insanely heavy and totally not appropriate for me considering I want to in the long run create a motion platform. Carbon fibre would just be too expensive so that's not really an option either. Any ideas in this regard?

When I say "full production roll", I'm talking the full, ten-thousand-pound spool.

The vacuum is used to form the mylar. It is not used to hold it to a surface. The volume behind the mirror is hollow. That being said, you can build your frame from whatever material will be strong enough to stand up to the loads without significant flexing. Fiberglass would be a good choice, if you're comfortable working with that material. The required stiffness could be achieved fairly easily by making it a foamcore structure.

Quote:

---

Originally Posted by wledzian

When I say "full production roll", I'm talking the full, ten-thousand-pound spool.

---

Yep, I understand which is why I said we needed to pool resoruces to get it. Sure, that's a lot of people! But still :P. Not totally serious about that, I'll admit.

Do you think that the guys in the video (forgot which company they were from) who were doing the 777, do you think/know their mylar conforms to the backing material? I've still got a lot to learn about Collimated displays, I've only recently started reading about them so I don't know quite a lot of basic stuff. I remember there was mention of this in the earlier threads but I didn't find a subsequent response which I thought answered that question. I suppose the fact that there was talk of a feedback loop indicated that indeed, they were calculating some sort of optimal value for the amount of volume of air/pressure in the 'vacuum' to which the system tried to revert back to throughout the operation but I'm not sure.

The mylar does not conform to a backing material.

The mirror frame is hollow. The mylar is stretched into final form by the vacuum. Except for the the edges where it attaches to the frame, the mylar does not touch anything.

For the prototype mirror, we used an adjustable bleed valve on the back of the frame. This was adequate for our purposes, as we haven't used the mirror for any long period of time. We've had to slightly adjust the valve each time, and this would be very inconvenient once the final version is built. A feedback device that operates purely on pressure differential may not work adequately, as mylar does return to its original form somewhat, but not completely, and has the odd property of slowly stretching back to its final shape over time. That is to say, if the vacuum has been turned off for a while and turned back on, it will require a stronger vacuum to draw it down initially, but will continue to stretch a little bit and require that the vacuum be relaxed a little as it stretches. Gene's got a concept for a contact position sensor based on a fine copper tinsel. I've got a concept for a contactless optical sensor based on phototransistors. Both concepts will modulate the vacuum pump and a small bleed valve in order to maintain the mirror at the proper position.

The cool thing is that both systems could co-exist and might even be able to work together. I suspect that the robot builders have a lot of experience with using photo transistors and could be of some help in perfecting the concept you want try out.

I checked out using the little ultrasonic sonar units the robot builder types use, but the ones I found didn't appear to have the resolution we'd need. I should check with the Arduino guys and see what they'd recommend for very precise distance-to-surface measurement. I'm going to need to find a solenoid valve too - that silly thing I bought from Surplus Center.com was waaaay to tiny to use for anything. Although, if the pump has a check-valve in it, we might not need the secondary valve - the pump itself would prevent vacuum loss when it was turned off.

Lots and lots of fun stuff to think about.

g.

Hi guys, just to have my say on the vacuum side of things! Im a farmer, as you may have probably guessed, and use vacuum pumps on a daily basis for the milking machine. We have now got 2 robotic milking units, which use a variable pressure pump, which basically pumps ata much higher rate while pulling the desired vacuum, and then when it has the pressure, it slows down, until it will just maintain that pressure, its also extremely quiet, i often dont realise its going, so n a small box, it would be almost silent.

This may help you, it may not, but do look into the vacuum pump solutions that us farmers use, theres been alot of development into this area, and im sure you will benefit from it!

Jordan

That's an interesting idea Jordan. The only issue I can think of is that the required stop point would change over time, but a differential feedback mechanism might solve that.

The pump I bought is small enough to fit in the palm of my hand. It's 110v AC and doesn't have much of a volume rating, but I'm hoping it can pull at least 6 PSI.

g.

I'm no expert in physics but based on my limited knowledge, I've got a feeling that no sonar based solution will work (but I'm basing that off the fact that mylar is quite thin. Maybe this is what you were referring to when you said sonar has inadequate resolution but I'm no exper in the area)... well at least no sonar based solution that is cheap. Pretty sure optical solutions are on the right track and I think it's easier to get right also or maybe I'm just more comfrotable with this tech. Either way, this vacuum revelation has just made me realise that this collimated screen will be much harder to make than I first anticipated.

The other question that I've thought of in light of this information is, is it possible/advantageous to have mutiple vacuum sources i.e. you have pumps in various locations? The way I see it, where the mylar is attached, it will not stretch at all, so there will be differences in tension in the mylar. The only simple solution that I see to this would be to approximate where the mylar should be stretched to from the begining and then attach it in a stretched state. Based on the CAE video, I cant help but feel that this is what is happening when the guy goes over the whole screen on the ladder. You can see distinct vertical lines in the mylar that are equally spaced which to me indicates that the material has been stretched at predefined intervals. So if this is the case, then I suppose basically one vacuum source will be enough but maybe distributed sources would allow greater control over the shape of the mylar?

Quote:

---

Originally Posted by geneb

I've been thinking about this nasty problem for the last three weeks. So much so that I didn't do a thing with it today - I gave my poor IBM 5160 some quality time instead. *laughs*

First, you need one of these...

http://www.diy-cockpits.org/coll/ima...rror_frame.jpg

...but that's not all. :)

I'm going to try to get a frame that will fit into the one shown above to retain a solid skin - hopefully I'll get to it tomorrow.

One thing I noticed about the video from How It's Made - the mylar _crawls_. I've only seen that when a material is being drawn against a solid surface. I strongly suspect there's either a solid skin with a Metric Buttload(tm) of tiny holes, or there's a gas permeable solid like MDF there. Fortunately, I've got me a little gadget that's perfect for drilling holes in Metric Buttload quantities. :D

I should note that the frame in the picture above is a 60 degree wide slice of a 48" radius circle. It's 40 degrees high. The top is at zero degrees. This is a good size for a practical test. It also means that if it works, I only have to build two more to get 180 degrees. :)

g.

---

Hi Gene :)

Up until now, I've been lurking here for a long time, and I have followed the collimated topics from the very beginning. I'll introduce myself officially in the proper thread later. Great job on your prototype !

I've watched the how it's made video dozens of times, and that JUST what I was thinking too. It crawls as if it was being drawn onto a surface. However, I highly doubt that there are any permeable materials into play. IMO, even the slightest of pin holes would probably show up as some kind of distortion on the mylar surface. I think what is happening is:

The mylar has the greatest slack in the center. That would allow the center of the mylar to get drawn down onto the surface FIRST once vacuum is applied, and gradually get drawn down until it reaches the outer edges. That way, there would be no air pockets trapped underneath the mylar. Also, there may be a possibility, that the frame that holds the mylar, might be adjusting the rate of tension somehow, via an automated, or passive mechanism, during the vacuum stage, to allow said gradual contact of mylar, from the center to ouside edges. Of course, this is JUST my theory, but one that may fit the facts.

On the other hand, the mylar itself, may just be thick enough NOT to distort, if there IS some kind of perforated forming block with very tiny pinholes on it's surface. The only thing I TRULY believe after watching that video over and over, is that the mylar ultimately is being drawn onto some kind of surface. As you've said before, the properties of mylar itself, do form a great image, but in time as it is streched and released, it starts to develope more and more slack, that would cause undesired geometric changes in the image over time. To get a constant image every time, it would only make sense to me, to have a permanent forming block behind the mylar. Just my opinion though.

Quote:

---

Originally Posted by Atomic_Sheep

The other question that I've thought of in light of this information is, is it possible/advantageous to have mutiple vacuum sources i.e. you have pumps in various locations? The way I see it, where the mylar is attached, it will not stretch at all, so there will be differences in tension in the mylar. The only simple solution that I see to this would be to approximate where the mylar should be stretched to from the begining and then attach it in a stretched state. Based on the CAE video, I cant help but feel that this is what is happening when the guy goes over the whole screen on the ladder. You can see distinct vertical lines in the mylar that are equally spaced which to me indicates that the material has been stretched at predefined intervals. So if this is the case, then I suppose basically one vacuum source will be enough but maybe distributed sources would allow greater control over the shape of the mylar?

---

You are right, there are differences in tension along the edges. There is reasonably even stretch perpendicular to the edge, but almost none at the edge. In fact, this is the main cause of the edge distortion which forms the unusable "dead band".

The vertical lines are just a result of being slightly unevenly stretched during mounting. It's got nothing to do with intentionally stretching at predefined intervals. He's going over the whole mirror with a ladder bit-by-bit to get it as even as he can. If you just attach one edge first, it is very difficult to get the film to lay right, Gene and I ran into that issue if one of us got even a couple inches ahead of the other when attaching the film. If you look at the video of the first application of vacuum on the prototype, you'll see a similar pattern of wrinkles. Distributed sources of vacuum would require separate sealed zones, and would produce more distortion.

Thanks, very helpful. I'm seriously contemplating purchasing some mylar in the very near future to start experimenting with it a little bit.

I was doing some mathematical calculations (Pythagoras) yestarday to design a frame suitable for my cockpit , and i discovered a big barrier , it's the "huge Space" .. but i think it would be a great way for the design if we all recognized the validity of it ..

I'm not sure if those calculations are correct or not , but they seems so logic .. correct me if I'm wrong ..

well , my Cockpit width is 165 cm ( Sorry for the non-US unit ) , so this is the idea of the calculations :-

1- Forward- Projection screen's radius
- I began by the base of the Forward-Projection screen , I planned to be as width as the cockpit .. plus say 5 cm , so the light rays would be free of hitting the cockpit , because it's the first task to guarantee that all light rays from the Forward-Projection screen would hit the whole of the mirror totally , and after i guarantee that task , i can plan the mirror radius / widths / etc according to those calculations .. See Picture below !

http://www.mycockpit.org/photopost/u...597/Radius.jpg


2 - Mirror's Radius
- After i calculated the Forward-Projection screen's radius , I then calculated the Radius of the Mirror .. The idea is the Screen would be in a distance from the Mirror less than the focal length , So i first assumed the screen would be 20 cm in front of the focal point , so the focal length would be [ screen's radius - 20 cm = 249 - 20 = 229 cm ] , Now I could calculate the Mirror's radius by multiplying the focal length by 2 [ 229 x 2 = 485 cm ] , See Picture below !

http://www.mycockpit.org/photopost/u...597/Mirror.jpg

3- Mirror's Lower ( Bottom ) width
- The triangle of the lower width , it's base would be the Mirror's lower width that we aim to calculate , as we know the total sum of any triangle = 180 degrees , so as the base angle = 40* , so both the other angles = [ 180 - 40 / 2 ] = 70* .... & see the Picture below !!

http://www.mycockpit.org/photopost/u...597/Bottom.jpg


4 - Mirror's Upper width
- see the picture , i guess it's now clear !!

http://www.mycockpit.org/photopost/u...1597/Upper.jpg


Remarks
- The first 30* angle , to draw a latitude of 30 South ( If we looked to the screen as the earth )
- The 2nd 40* angle , because our Field of View ( FoV ) = 40* .. drawing latitude of 70 South .
- The opposite side is the same as above .
- As the half circle = 180 degrees , so the remain angle ( Mirror's Lower width's angle ) would equal [ 180 - {(30* x 2 ) + ( 40* x 2 )} ] ...

Looking forward for your thoughts

Mohammed Sayed

For those who have been following this discussion or anyone for that matter ;-) here is a link to an excellent set of tutorials on how image warping works and techniques for projecting flat images onto curved screen surfaces

http://local.wasp.uwa.edu.au/~pbourk...hics/cylinder/

For the more adventurous, there is source code on the OpenSceneGraph website (http://www.openscenegraph.org/) detailing the mathematics.

JW

And here ....

On this site:

http://www.mod-gmbh.com/Collimated-Display.48.0.html

is an interesting paper about the collimated display principles:

http://www.mod-gmbh.com/fileadmin/us...Sale-Paper.pdf

Hessel Oosten

Hi Guys :)

Forget mylar.....
Get a load of this: http://www.youtube.com/watch?v=HIqxd...eature=related


And THIS: http://www.anishkapoor.com/works/gal...urve/index.htm

If you want even more info, just do a google search for "Anish Kapoor C-curve" This guy is a sculptor, so how did he make this? If we find out, then things get a bit simpler. ;)

Hessel, Thanks for the links. The last link is VERY interesting. :)

Matt Olieman