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Building a Pressure-Sensitive, Multi-Point TouchScreen?
sonamchauhan asks: "I'd like advice on building (yes, building) my own low-res touchscreen. The reasons for 'build' instead of 'buy' are: 1) to have it sense pressure (pressure sensing is quite expensive) and 2) to have it sense multiple points of contact simultaneously (which is a useful thing). Back in 1985, researchers at U.Toronto built (PDF file) a touch-tablet (not a touch-screen) that fulfilled both requirements (pressure-sensitive and multi-touch) and used only basic electronics: lots of diodes, A/D convertors, etc. Some 17 years later, it should be possible to build a touch-screen using the same techniques (possibly using layers of transparent conductive and insulating paint for the sensor paths.) Any comments? Some other links: a Microsoft paper (PDF file) describing a touch sensor painted onto a mouse, a basic FAQ on current touchscreen technology, and a slashdot thread that discussed building touchscreens (these links don't address pressure or multi-touch though)."
The first thing you're going to need is a Pressure-Sensitive, Multi-Point TouchScreen. Once you've got that, problem solved.
Got friends?
I would think that the time and effort that went into developing something like this, basically from scratch, would outweigh the "real" cost savings of not buying one off-the-shelf. I suppose if this is just for fun, than that's not really a concern, but if your time is worth anything at all, it ought to be considered.
blackula, I hear you can buy a sense of humor, slightly used, on eBay.
IT WUZ A JOKE!! (I sincerely hope it was, anyway.)
Opinions on the Twiddler2 hand-held keyboard?
I don't know how feasible this is, but couldn't one use LEDs in the screen frame along with detectors opposite arranged so that they form a detection grid on the surface of the screen and register an interruption of the light (such as by touch or something)?
In theory, there is no difference between theory and practice. In practice, there is.
1- have the slashdot community provide plans for a pressure-sensitive multi-point touchscreen
2- ??????????
3- profit!!
Do you want the touch points to be able to detect different pressures, or just on or off? In either case, do you want to be able to detect all points on the screen at once, or just a lot?
First, is it ok to use a stylus? If so, you're problem just got easier - you simply use the same technology that wacom uses, except you'll need to track several pens instead of one.
I'm betting, though, that you want a real 'touch' screen which registers anything hitting the surface.
Here's what you do: Use a transparent metalization process to put squares of transparant, conducting (but resistive) metal on two opposing pieces of plastic. Take a wire from each square and use a microprocessor with a lot of i/o (switched A/D converters, most likely) to detect which metal plates are being pushed together. The size of the plates determines resolution.
That's the easy, no brainer way.
The cool way (with infinite resolution ) is to use the normal method modified a bit. Normally you have two sheets covered with resistive material, one which has conductors on the vertical sides and one with conductors on horizontal sides. Increase the conductors (say, four shorter conductors on each side, and make them points instead of lines).
Send a signal to one conductor, and listen to all the other conductors for the wave front of the returning signal. Through a ton of signal processing (and sending signals from the others and measuring the response) you'll be able to detect an arbitrary number of points on the screen. You'll need to do a lot more processing to measure surface area (pressure), but you'll get there eventually.
Alternately, you get rid of it entirely and make a video system that can sense the position and movement of your hand and get rid of the 'touchyfeely' altogether. This (IMHO) would be easier and faster to develop.
-Adam
From a whole lot of research I have been involved in, for a while we worked with a company, TacTex Controls Inc.
They have a pressure sensative pad and system you can use, the MTC Express (http://www.tactex.com/prodMTC.htm), however, that is not where you would be interested directly.
Their technology uses fiberoptics in a rubber pad with a small controller. There is a lot of research in integrating their technolgies into other things (a prototype of a complete pressure sensative suit was discussed at one stage).
It fufills your requirements quite well.
256 levels of pressure for each region of the pad independently.
The MTC Express pad had only a DB9 serial connection, and needed a power input as well. However, the OEM material and controllers were a lot more flexible in usage.
ICQ# : 30269588
"I used to be an idealist, but I got mugged by reality."
They go like this:
1. Get the slashdot community (which includes myself) to provide plans for a pressure-sensitive, multi-point touchscreen.
2. Build one as a component for my project.
3. Taiwanese/Chinese/American/.... manufacturers notice the plans on slashdot.org
4. Cheap pressure-sensitive multi-point touchscreen become available.
Hi! I'm trying to avoid a stylus in favor of just using fingers (which is why the screen only needs to be low-res).
> you have two sheets covered with resistive
> material, one which has conductors on the
> vertical sides and one with conductors on
> horizontal sides. Increase the conductors (say,
> four shorter conductors on each side, and make
> them points instead of lines).
Actually, I was looking at using something similar to the circuit in the U.Toronto paper. That circuit seems to require only one layer of sensors that sense touch and pressure by measuring capacitance at the point being touched. I think this method also provides infinite resolution.
I was thinking on the lines of using this idea and painting-on the sensors to the screen with transparent conductive paint. Another layer of transparent insulating paint would cover the track upto the sensor point.
You know about these guys:
http://www.fingerworks.com/
right? They've been mentioned on Slashdot before. Their products can detect multiple fingers at once.
They are selling tablets and keyboard replacements, not touchscreens, but I think I read somewhere (on their website maybe) that they believe it would be possible some day to use this tech in a touchscreen.
The cool thing is it sees your whole hand via infrared, so it can tell which finger you're using. For the first time, you can "right click" on a touch surface, or use other finger combinations/gestures for various operations (drag, pan/scroll, rotate, etc.)
If not, you might think about using a standard touch screen system, then building (or buying, if they exist - probably don't) pressure sensitive "gloves". For the sensors (at the finger tips), buy thin ribbon plastic tubing (multiple tubes joined in a ribbon - used in many continuous ink kits for ink-jet printers, so you should be able to buy it somewhere by itself), spread one end and seal the ends of each tube, apply gentle heat and blow "bubbles" at the end of each tube (this may be difficult, but not impossible). Then, attach these bubbles to the fingertips of the gloves, and use a T-connector on the other end of the cable to hook the tubing to a very low pressure air supply and the other side of the T-connector to a air-pressure transducer. Hook the transducers up to a PIC or Stamp (or other ADC system) to measure the pressure change (the positive pressure keeps the bubbles inflated at all times, and reinflates them after being depressed - a closed system might work too, with no active pressurisation - you might have to experiment here).
Other cool things to do with the gloves:
1. Touch gestures (like a chorded keyboard)
2. Use water instead of air, then warm or cool the water with a peltier to provide feedback.
3. Pulse the pressure for other feedback needs.
Hope this helps...
Reason is the Path to God - Anon
> . A touchscreen with hundreds of tiny
> etched "wires" across the inside of two
> surfaces that "connect" when you touch can get
Just as an aside: the most efficient 'layer' to build this would be LCD display manufacturers. The way I see it, they already adress each pixel individually to output data (light); they could do the same to input data (pressure).
Here's an idea: You know that a sheet of plastic behaves somewhat like a fibre-optic channel, so that when you shine light in one side, it is easily visible on the other side, but doesn't interfere with just looking right through the plastic.
However, if you press on the plastic, deforming it, then the light can be seen around your finger, because now you have a sharper angle of incidence in the plastic. My idea is to take a thin sheet of plastic (eg. overhead transparency), put it over top of a flexible substrate (a gel or something like that), then shine LEDs in one edge of it, and look at the amount of light on the other edge (using eg. photo diodes and A/D inputs). This may give you an indication of absolute pressure as well.
When you press on the plastic, the sensors in line with your finger will have a reduced (or maybe increased?) amount of light. Anyways, you will be able to find the center of the disturbance. Do this both vertically and horizontally, and it looks like you have a touch screen!
This can't sense more than one input point simultaneously because you just get two horizontal readings and two vertical readings, but maybe you could use an idea which has already been suggested and remember which of the two locations was pressed first. This may not work in the pathological cases, but what the heck...
Another alternative way to solve the problem is with proximity sensing behind the screen (I guess this wouldn't work for CRTs). Proximity sensing is done (for body parts) by detecting the water in people's hands. This increases the effective capacitance of two plates side by side. Then you need an oscillator circuit which goes from stability to instability as you change this capacitance. You would probably need a bunch of these circuits, so this is more difficult than my other idea.
Anyone else have ideas what this could be used for?
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