>FTIR Table Progress
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.
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.
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.
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.
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.
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.
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.
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.
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.
