Re: How do artist tablet input devices for computers work?

I can imagine many ways in which a graphics tablet _might_ work.  
But when I got your question I had no knowledge of the _actual_ 
mechanism of any tablet currently being sold -- so I went looking.
I started with my favorite "pinpoint" search engine -- the one I 
use when I know exactly what I want.  I got five hits on the 
following Boolean query:

graphics tablet near ("works by" or "how it works" or "how does it work" or 
"how do they work")

But the only item I found which directly addressed your question 
contained only a picture.
(http://bruichladdich.dcs.st-and.ac.uk/hardware/tablet2.jpg) 
There seem to be relatively few makers of graphics tablets; 
Wacom is the only company that showed up quite frequently.

While I saw no explicit statement comparing "graphics 
tablets" with "pointing devices", what I read 
implied that functionally a "pointing device" may be 
strictly two-dimensional (x-y only) but a 
"graphics tablet" offers more than one bit of z-axis 
resolution.

Wacom offered this explanation of their technology:
----------------------------------------------- http://www.wacom.com/companyinfo/pdf/PL-300.PDF
-----------------------------------------------
Wacom Technology
Wacom pens are the industry standard for professional drawing, painting 
and image editing on a computer. The company is best known for having 
introduced in America in 1989 the world's first cordless, batteryless, 
and pressure-sensitive electronic pen. It helped revolutionize computer 
art by allowing electronic artists to paint and draw with natural freedom 
and creativity in the same way they do with traditional paintbrushes, 
pens and pencils.
---------------------------------------------------- http://www.wacom.com/companyinfo/pdf/HowPenWorks.pdf
----------------------------------------------------
HOW THE WACOM CORDLESS, BATTERYLESS PEN WORKS. The Wacom stylus
looks and feels like a pen yet contains no batteries or magnets. Instead it 
takes advantage of patented resonance technology developed by Wacom Co. 
Ltd. 
in which a tiny signal is sent to the stylus and returned for position 
analysis. In operation, a grid of wires below the screen alternates between 
transmit and receive modes about every 20 microseconds.
In the transmit mode (1), the tablet's signal stimulates oscillation in the 
coil-and-capacitor resonant circuit in the pen. In receive mode (2), the 
energy of the resonant circuit oscillations in the pen is detected by the 
tablet's antenna grid. This is then analyzed to determine position and 
other 
information including pressure.  
Since the grid provides the power to the pen through resonant coupling, no 
batteries are required. Thus there are no consumables inside the pen that 
will run down and need to be replaced or that would make the pen top-heavy.
A simple analogy for this patented technology is that of a piano tuner 
using 
a tuning fork to tune a piano. As the tuning fork is brought into proximity 
of the appropriate vibrating piano string (if the fork is of the same 
frequency) it will begin to borrow energy from the vibrating string and 
resonate, generating a tone. In much the same way, as the Wacom pen comes 
close to the tablet surface, it begins to resonate, generating its own 
frequency back to the tablet. When it hears the pen, it tracks the pen's 
location with unprecedented accuracy.  The tablet then sends location, 
pressure and tilt information to the computer along with a
signal indicating whether the pen point or the eraser tip is being used.
---------------------------------------------------------------------
Although not explicitly stated, the diagram implies that pressure on the 
pen's tip changes its resonance by changing its capacitance or inductance.
There are innumerable ways a circuit's capacitance or inductance might be 
changed by pressure; the article does not detail which approach Wacom 
chose.


Another, Solias, offered:
------------------------------------------ http://www.solias.com/faq/faqdevices.htm
------------------------------------------

Q:  What is a graphics tablet, and how does it work?

A:  A graphics tablet is a flat, electronic art pad with a
matching pen. The pen acts as a mouse, doing everything a
mouse can do but so much more. A single tap is a single
mouse button click, a double tap - a double click. Dragging
and dropping, editing, highlighting, cutting/pasting are all done
with ease on a graphics tablet. All graphics tablets offer 256
levels of pressure sensitivity to pick up every nuance of artistic
expression. As in our SOLiAS Studio program, a graphic tablet
allows the use of a pressure sensitive stylus as input, in effect
accessing the Z dimension in addition to the conventional x-y
axis. Changing the pressures can be used to create brush-like
strokes, variable color fills or airbrush densities and other
unique programmable effects. Simply, press more heavily and
the line thickens just as it would with a drawing pencil or brush.

While that explains _how_it's_used_, is doesn't explain _how_it_works_.


A retailer, EESOX, offered:
-------------------------------------- http://www.eesox.com/tabletinfo.html
--------------------------------------

How does it work? 
A graphics tablet is a flat board that is used with a special
tool called a 'stylus'. The board acts as your paper and the
stylus as your pen or pencil you simply have to draw on the
board with the stylus to produce an exact replication on the
computer. You can even use the stylus as a replacement for
the mouse by selecting, clicking and dragging objects on the
screen using the additional buttons of the stylus as you
would the mouse buttons.


Again, this speaks to usage, not operating principles.

You may (or may not) find more info in:

   How Computers Work, 5th Ed) ISBN: 0789721120 
   by Ron White, Timothy Downs; MacMillan Computer Pub
   Paperback - 421 pages Bk&Cd Rom edition (September 1999) 


If you want to discuss some of the other ways a tablet might work,
email me.


William Johnson
lonestarnot@earthling.net