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Multi-touch Table


Oni
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Hi all. I watched the episode today and it was certainly quite interesting. I've been working on a table myself and doing quite lot of research. It's nice to see it all rounded up on my fave show. I thought I'd share my project so far and the problems and trials I've had with it. All this is up at blog.section9.co.uk but I'll add to it here.

Over the Xmas holidays, I decided to make a start on the FTIR Table I've been talking about. Since playing with the prototype, I've made some progress towards a fully working table with the help of various family members and guys from the NUIGroup forums.

The general idea is to build a pedestal with a top layer formed from an LCD panel, a layer of perspex and potentially, a compliant layer over the top. Inside the pedestal there will be a strong light to illuminate the LCD and a camera to detect infrared. Around the rim of the perspex, there will be a strip of infrared LEDs projecting into the perspex. The idea is to use computer vision techniques to detect touches and gestures.

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The first thing to do was to dismantle the LCD. This proved quite tricky as I had chosen a particular Dell LCD that I had lying around. Dell do make good screens and this one had a few problems. In the first image you can see that the back of the screen has 3 boards. One is the power, one is the controller and the other is the power for the light. Originally, we had the idea to keep all of these mounted on the metal bracket in order to save messing around. Sadly, to save space, Dell have used rather short cables between these boards; the most limiting being the major blue ribbon cable at the bottom, connecting the LCD itself to the controller.

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The power connectors were quite simple to detach and didn't really cause any problems. There were 4 in total. Later, when we took all the boards apart, I decided not to relink the light board to see how the LCD looked with no backlight. Oddly enough, it still worked and one could just about make out the picture, so good news there.

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The LCD panel itself however is a different matter altogether. There are some nasty ribbon cables and plastic connectors along with a rather annoying metal sticker over the vertical connectors. This panel has both vertical and horizontal connectors which makes the job of dis-assembly even more tricky. The horizontal bar has a sticker over the top which requires care to remove.

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The most annoying feature of the monitor is the small, L-Shaped cable shown on the diagram. Because of this cable, the two controller boards attached to the LCD panel cannot hang down from the LCD (I.e rest at 90 degrees to the LCD Panel). This is necessary when using a backlight that is some distance from the panel. This means that until that cable is replaced, there will be a shadow on the LCD where the boards are attached.

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The lower half of the LCD panel has 3 layers: a Fresnel, a diffuser and the light itself. This is all mounted inside a simple plastic frame. It should be noted that a similar plastic frame holds the other side of the panel (the section containing the LCD and the controller boards). It is important to keep these panels as they can be used to "offer-up" the other materials without ruining the sophisticated electronics. Also, the actual frames can be kept to remount the electronics easily.

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On to the actual box. The construction is from 15mm MDF bought from a place called Bury's in Leyland. The supplies are quite cheap and cut to size. Space is provided for a small PC towards the bottom. The design is limited by the power of the camera being used (most likely a PS3 Eyetoy); the height of the box must be sufficient enough to allow the camera to "see" the entire top of the box. The standard field of view equation for optics can be used here. The box is actually taller than it needs to be so that users can touch the box whilst standing up, making it useful for public spaces. Doors and vents will be added later. The critical area of the box is inside the top section, where there is a shelf with a hole cut in the centre. The shelf is designed to support the plastic frame that houses the LCD, without blocking the LCD itself. Within the shelf there will be further supports for the LED rails and perspex sheet.

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Possibly the trickiest bit of the entire project is the FTIR section itself. 4 rails of LEDs are required, linked together to a power supply and presented up to the perspex in order to create the FTIR effect. The LEDs in question are 880nm Infrared LEDs from RS components. 54 in total are used on this project. To mount these consistently, 15mm Aluminum track is drilled, using a pillar drill, with 5mm holes at 25mm spacings to accommodate the LEDs. The track has enough space for wiring. The wiring schematic uses 1 resistor per 3 LEDs and wires these groups in parallel. This draws less current through each resistor than wiring the entire track in parallel.

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The perspex is 8mm thick. As shown in the image, the LEDs are almost central to the perspex when placed next to it. The final height will be adjusted when the layers are finally placed within the box.

The next steps are to complete the box construction and the wiring. Once the LED circuit works, the placement of the various layers will be critical in coupling the infrared and providing enough visible light to the LED.

I'm a Northern UK Boy and I worked on this with my Dad in a garage (though I like to say shed), but I live and work in London so the baby is being shipped down here for final assembly and testing this weekend.

Things to Consider

  1. FTIR Transmission of the Acrylic
  2. Brightness of the Box
  3. Circuit Design

The first is the most important of all. I tested a small piece of a Acrylic in the visual spectrum with 880nm lights. We certainly managed to get some transmission which is great. However, testing with filters was much less rewarding. However, I have just recently modified a PS3 Eyetoy with a specific filter obtained from the states. It picks up the right light with no problems so, fingers crossed, the whole thing will work! :D

The brightness of the box is important. LCD lights are close to the LCD for a reason and so... I need to test this and get a good response.

My mum, who is the best soldering ninja alive reports that my circuit may not be correct. Using good old V=IR and LED Calc I came up with a circuit that *should* work but one can never tell until its wired in properly. That's something I'll need to sort out this tuesday.

The cost of the project is around £130 so far which isnt bad considering I've convinced work to pay for it :D We have some slight modifications to make which are *hush hush* for now :D I'll post up more photos, UK suppliers and all that if people are interested :)

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I wondered what these guys in the ep are using for the perspex. They dont give details but this bit is key as generally, you take some perspex and coat it in some sort of silicon. The idea is that Frustrated Total Internal Reflection occurs where the silicon is compressed.

If, however, the perspex is a good wave guide AND has a similar refractive index to your finger, you are all set! :D However, these two properties are damn hard to get right so at this moment, im kinda praying it'll work :D

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  • 2 weeks later...

Update! We have lift off!

Taken from my blog at blog.section9.co.uk / www.section9.co.uk

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During the Xmas holidays, I began construction on the cabinet with my Dad and had a lot of fun doing it. I left it in his capable hands for painting, sanding, and the installation of the interior frame and handles. Here is the result. Looks rather awesome. The height is important as one needs to make sure the camera can see the entire top of the box. Also, there needs to be space for the various electronics that will be installed in the bottom along with a 4 Gang. the height is also just about right for the table to be used standing.

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Here is the EyeToy. I used the PS3 eyetoy as it came recommended by the guys on the NUIGroup Forums. It runs at 640x480 at 30fps, has two aperture settings and space for a filter. There is a problem with removing the infrared filter. Most cameras have one and you need to remove it. On my cheap logitech this was easy because it was simply a piece of plastic that fell out. With the Eyetoy, it's a piece of ground glass, set into the plastic. You need to gouge it out and be careful with it too as the lens is right behind it! There is a good video here> on how to do this.

You need to get a filter that matches your LEDs. In my case, I bought a small 880nm filter from eBay. I can't remember the guys name but if you search on eBay you'll find a guy in the states who sells these small, round filters for this exact purpose. It works really well. Once you've put that in, you are ready to go.

The PS3 sits on top of the light that came with the LCD panel. Since this light was obviously right next to the LCD itself, you need to extend the cables that come from this light using solder and some extra wire. It is important that when you take apart the panel, you remember which plug goes where. Take photos at every stage!

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Here are the innards of the LCD Panel. Some of the wires needed extending also as they tend to be jam packed inside the original Dell Case. They are mounted on some wood except for the light's power supply which I have yet to attach. The more recessed you can make these the better as they will show up on the screen if you don't which makes the effect a bit naff.

On the left, you can see the wires that go up to the LED arrays. You need to link these to an adaptor. Check out old mobile phone chargers as they are dead handy for this sort of thing. For my circuit, I needed a 5V supply capable of around 1.5 amps. I managed to find a charger that did 5V and up to 2 Amps which turned out to be perfect.

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This photo shows the annoying, overlapping PCBs that fall on both sides of this LCD screen. This means that about an inch from two sides of the LCD panel are obscured which is very annoying. The only thing stopping this from being fixed is the current configuration of the box and the small L shaped plastic track that links the two PCBs together. If I could get a longer, flat plastic cable I'd be in business. At the moment, it doesn't affect things to much to be fair but it makes calibration with the NUIGroup tool impossible at present. I'll aim to sort it manually.

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Here is the top of the table with the lid attached. Looks good no? The plastic came from a place called Barkstons in Leeds. You need to go to their "plastic people" / yellow site as they will do decent perspex cut to any size you like for a decent price, delivered. Make sure you go for at least 8mm (I used 8mm) and have the edges polished. At first I was unsure whether or not this would make a decent wave guide but it appears that it does.

Lots of people talk about Endlighten and other fancy perspex, or a compliant surface made of silicon. I've had no problems with this cheap stuff and ordered two pieces in case one was damaged. Its still all good.

PS. Ignore the thing on the top left of the photo! :D

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With the lid off, you can see the LED arrays. They are arranged on the aluminium frames and connected together. The circuit was designed with the help of this handy calculator. Some people link the whole thing up in parallel; indeed, I did this for the first mini table. The problem with that is that it uses a lot of power and needs a heavy resistor. All you need to know is this:

V = I x R

Yup, that good old equation. Checking the RS Data sheet for my LEDs I found that each one requires 100mA and drops 1.5V. That means, with a 5V supply, we can only have a maximum of 3 LEDs in series (1.5 x 3 = 4.5V). Now, to get 100mA over each one we do some maths. V = I x R . Rearrange to get V / I = R . Substitute the numbers: 0.5 / 0.1 = 5 Ohms. What did we do here? Well, we need to go from 5V, down to 0V after going through 3 LEDs. Since each LED drops 1.5, we are left with half a volt to sort out and 100mA of current to send. So 0.5 / 0.1 = 5 Ohms. The closest rated is 5.6 Ohms and you can buy these cheap from either Farnell or RS Components.

I managed to get my Mum, who is a much better solderer than me to do this bit! :P Shrink sleeving and everything. A quality job well done.

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You can get a better view of the internals here. You can see that the LEDs line up quite well with the perspex and that is indeed the trick. With all that done, you need to power it up and grab the software from the NUIGroup and play till your hearts content.

This project was a lot of fun and it still isn't over. There are a few funky things I'd like to add, though I need to do some boring things like putting the shelf in and calibrating the screen but there are many other things in the pipeline.

Total costings:

Bandpass Filter from Ebay: £15.17

54 LEDs from RS: £22.46

Perspex Sheets from Barkston - £31.68

MDF, Aluminium Track, Hinges, Drill-bit, Handles - £23.58

Maplin Wire and Solder - £12

RS Components Resistors - £8

PS3 EyeToy from eBay - £20.51

Grand Total: £133.40p

Of course, I had to scrounge the LCD panel and there is no mac mini involved but damn, its cheap! :D

Overall, it was hard to get solid answers on certain questions like "Is this a good wave guide?", "Is the infrared leaking?", "exactly how good is the eyetoy with this?" etc etc. Generally, the best advice is just go for it with the guides and providers I've mentioned and you can't go far wrong. I guess I got lucky with the plastics and the spacings of the LEDs and the filters etc, but generally, it doesn't seem too hard to get a good result.

But of course, no tutorial would be complete without a proper video showing some funky touch software:

http://www.vimeo.com/9252405

Some Resources

The VIRTTable. If you look at no other page, you should definitely check this guy out.

Todd Vanderlin's Table - He doesnt give much away but its a good link

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  • 2 weeks later...

I don't think this one is multitouch. But I did have the privilege of installing one of these for my company at the 2009 Fort Lauderdale International Boat Show.

http://www.engadget.com/2007/09/08/hands-o...e-coffee-table/

I've got pics of it actually installed at the booth if folks want to see, but in essence the whole thing is just a MACbook pro, and the touchscreen is an overlay on top of the LCD panel.

Its just a big ass remote in this instance.

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