Touch sensitive screens first appeared on industrial control panels over 20 years ago, but they are now common on personal computers and smart phones.
There are two problems to be solved when designing the system: sensing when the screen is touched and working out where it was touched.
The most common technology is the resistive one, which uses the simple idea of a switch. The outer surface of the screen is covered with three thin layers of different materials.
The first one is metallic, the second a fine gauze made of an insulator and the third a flexible conducting plastic. Finally, the top is coated with a scratch-proof film to prevent damage.
All these layers are so thin that they are almost transparent in ordinary light. That way, it is possible to see the graphics on the screen.
Terminals are connected at one corner of the screen and a small voltage applied between the conducting plastic layer and the metallic film.
Since the two layers are separated by the insulating gauze, no current will flow, but when the screen is touched, the flexible plastic bends and makes contact with the metallic layer below through the open spaces in the gauze.
When the two layers make contact, current flows -- just like in a switch -- and the computer senses that the screen has been touched.
To determine the point that was touched, voltage is measured at the other three corners of the screen.
Since the plastic layer is not a good conductor, the voltage will be proportional to the distance from the respective corner to the point of contact.
It is possible to work out the coordinates of the point of contact by applying triangulation geometry.
Imagine a drawing compass is mounted at the first corner of the screen and then opened to draw an arch whose radius is equal to the distance calculated from the voltage.
Repeat the same drawing from the second and third corners. The three arches will meet at one point on the screen -- the point of contact. This is how the system evaluates the coordinates.
Another common type of touch screen technology applies the principle of capacitance. An electrical capacitor is simply two layers of conductors separated by an insulator.
Thus, the basic design is also in the form of layers. However, the middle layer is made of a continuous insulating material instead of the gauze.
When voltage is applied to the two conducting layers, a uniform electric field is formed between them. Touching the screen distorts this field, thereby changing the voltage.
The change in voltage depends on the location of the distortion. Therefore, by measuring from three corners of the screen, it is possible to triangulate the coordinates of the point of touch.
The major advantage of the capacitance touch screen is that it has no moving parts, making it more durable than the resistive type.
For this reason, it is the preferred design for industrial control panels, touch-sensitive ATM screens and laptop touchpads.
However, while the resistive type responds when touched by any material, the capacitive design can only be operated by bare fingers. Even wearing gloves will inhibit its operation.
2011年3月13日星期日
Gary Bogue: Reader responses to past columns
Regarding birds lined up on power wires (March 10 column), I have also noticed that for many
years in many other places.
Here's my take on it: The birds are usually lined up adjacent to the wire support points,
i.e. the insulators. Since there is no absolutely "perfect" insulator there will be a minute
voltage drop at the attachment. A voltage drop means there will be a minute current, which
results in a bit of heat. The birds are getting their bony feet warm so that the warmth can
travel up their bony legs to keep their bods warm. By the way, I'm not an electrical
engineer, although I have had some E.E. training. (Claude Benedix in cyberspace)
There are many power lines around the county favored by flocks of birds. Magnetic fields flow
around those lines and maybe a bit of vibration and maybe just enough warmth to tickle and
warm their little clawed toes. Just a thought! (Pat Voth, Martinez)
I, too, have taken the Monument offramp many, many times and notice the birds on the lines
and I have never seen them fly off or on. It seems they just sit there all day long
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without moving! My guess is that there is some heat from the lines and it keeps their little
footsies warm. (Bill of Orinda)
Any power company engineers out there like to comment on why flocks of birds only like to sit
on certain spots on certain power lines? (Gary)
Cats that don't play
In your column of March 6 (about the cat that runs and hides from its toys), you told Ruby to
try to have her cat play with some cotton string. Please tell her to put the string out of
her cat's reach when she is not playing with it.
Our cat Cougar got hold of some string and decided to eat it. We ended up spending a bundle
(would do it again if ever needed as he was my baby) to have the string removed from his
intestines.
This is just a word of caution so no other animal has to go through what our Cougar kitty
went through.
Thank you, Gary. (Rita and Danny Martinez, Vallejo)
One piece of advice I'll give to anyone who plans to put an open grocery bag on the floor for
kitties to play in: CUT THE HANDLES OFF! Or at least cut them. Once I gave my cats a grocery
bag to play with, and one got his head caught in one of the handles.
It was quite a task to get him to slow down long enough for me to rescue the panicked little
fellow! (Sandy in San Ramon)
Scratching car covers
This is in regard to your reader whose cat likes to claw the car cover (March 3 column).
When I purchased a classic convertible, it came with an expensive thick "waffled" car cover.
My cat immediately climbed on the roof (convertible, remember?) and started digging into the
thick cover. Not wanting the car top to be damaged, I removed the cover and replaced it with
a good quality plain cotton cover. The cat loved it and spent her outdoor time sleeping on
it, not digging! The texture seems to be the problem.
Does your cat like to use your large planter (box or cask) as a bathroom? I cut pieces of
fiberglass screening (from hardware store) and lay them on the dirt in the planter. No more
issues. In fact, I found the cat sleeping on the screening material. (Dave Pastor,
Pleasanton)
years in many other places.
Here's my take on it: The birds are usually lined up adjacent to the wire support points,
i.e. the insulators. Since there is no absolutely "perfect" insulator there will be a minute
voltage drop at the attachment. A voltage drop means there will be a minute current, which
results in a bit of heat. The birds are getting their bony feet warm so that the warmth can
travel up their bony legs to keep their bods warm. By the way, I'm not an electrical
engineer, although I have had some E.E. training. (Claude Benedix in cyberspace)
There are many power lines around the county favored by flocks of birds. Magnetic fields flow
around those lines and maybe a bit of vibration and maybe just enough warmth to tickle and
warm their little clawed toes. Just a thought! (Pat Voth, Martinez)
I, too, have taken the Monument offramp many, many times and notice the birds on the lines
and I have never seen them fly off or on. It seems they just sit there all day long
Advertisement
without moving! My guess is that there is some heat from the lines and it keeps their little
footsies warm. (Bill of Orinda)
Any power company engineers out there like to comment on why flocks of birds only like to sit
on certain spots on certain power lines? (Gary)
Cats that don't play
In your column of March 6 (about the cat that runs and hides from its toys), you told Ruby to
try to have her cat play with some cotton string. Please tell her to put the string out of
her cat's reach when she is not playing with it.
Our cat Cougar got hold of some string and decided to eat it. We ended up spending a bundle
(would do it again if ever needed as he was my baby) to have the string removed from his
intestines.
This is just a word of caution so no other animal has to go through what our Cougar kitty
went through.
Thank you, Gary. (Rita and Danny Martinez, Vallejo)
One piece of advice I'll give to anyone who plans to put an open grocery bag on the floor for
kitties to play in: CUT THE HANDLES OFF! Or at least cut them. Once I gave my cats a grocery
bag to play with, and one got his head caught in one of the handles.
It was quite a task to get him to slow down long enough for me to rescue the panicked little
fellow! (Sandy in San Ramon)
Scratching car covers
This is in regard to your reader whose cat likes to claw the car cover (March 3 column).
When I purchased a classic convertible, it came with an expensive thick "waffled" car cover.
My cat immediately climbed on the roof (convertible, remember?) and started digging into the
thick cover. Not wanting the car top to be damaged, I removed the cover and replaced it with
a good quality plain cotton cover. The cat loved it and spent her outdoor time sleeping on
it, not digging! The texture seems to be the problem.
Does your cat like to use your large planter (box or cask) as a bathroom? I cut pieces of
fiberglass screening (from hardware store) and lay them on the dirt in the planter. No more
issues. In fact, I found the cat sleeping on the screening material. (Dave Pastor,
Pleasanton)
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