Elan Microelectronics Corporation v. Apple, Inc.

Filing 377

EXHIBITS 2-8 to the Declaration of Dr. Ravin Balakrishnan In Support of Apple Inc.'s Motion for Summary Judgment of Noninfringement filed byApple, Inc.. (Attachments: # 1 Exhibit 3, # 2 Exhibit 4, # 3 Exhibit 5, # 4 Exhibit 6, # 5 Exhibit 7, # 6 Exhibit 8)(Greenblatt, Nathan) (Filed on 8/4/2011)

EXHIBIT 4

111111111111111111111111111111111111111111111111111111111111111111111111111
USOO5149919A

United States Patent

[19]

[11]

Patent Number:
Date of Patent:

Greanias et al.

[45]

[54]

STYLUS SENSING SYSTEM

[75]

Inventors: Evon C. Greanias, Chevy Chase,
Md.; Frank L. Stein, Vienna, Va.;
Robert Donaldson; Michael Gray,
both of Annapolis, Md.

4,650,926
4,678,869
4,686,332
4,695,680
4,740,660
4,931,782

[73]

Assignee:

[ .] Notice:

International Business Machines
Corporation, Armonk, N.Y.

Filed:

The portion of the term of this patent
subsequent to May 26, 2009 has been
disclaimed.
Nov. 26, 1991

[51]
[52]
[58]

Division of Ser. No. 608,062, Oct. 31, 1990, Pat. No.
5,117,071.

Int. CI.'
U.S. Cl
Field of Search

[56]

G08C 21/00
178/19; 340/706
178/18, 19; 340/706,
340/709

References Cited
U.S. PATENT DOCUMENTS
3,818,133 6/1974 Cotter
3,886,311 5/1975 Rodgers et al.
3,999,012 12/1976 Dym et al.
4,009,338 2/1977 Dym et al.
4,423,229 12/1983 Guml et al.
4,571,454 2/1986 Tamaru et al.

Primary Examiner-Stafford D. Schreyer
Attorney, Agent, or Firm-Jeffrey S. LaBaw

ABSTRACf

178/18
178/18
178/18
178/18
210/159 B
178/18

An improved stylus detection system for use on the
surface of a display device. An overlay having horizontal and vertical transparent conductors is coupled to
other elements of the systems through a bus having a
minimal number of bus wires. A control processor issues command signal which selectively couple transparent conductors to a radiative measuring device to determine stylus position accurately. The system includes a
radiative pickup stylus having a spherical antenna
which receive the overlay signal independent of the
angle at which it is held. Further, a contact detecting
mode has been added to eliminate spurious contact
position measured between strokes, when the stylus is
proximate to but not in contact with the overlay.
16 Claims, 13 Drawing Sheets

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178/18
178/19
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178/19
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3401706

IBM TDB Article, entitled "Dual-Level Pen for Capacitive Sensing of Tablet Signals" vol. 17, No.2, Jul. 1974,
pp. 572-574.

[57]

Related U.S. Application Data
[62]

Nakamura et al.
Kable
Greanias et al.
Kable
Kimura
Jackson

OTHER PUBLICATIONS

[21] Appl. No.: 798,471
[22]

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5,149,919

STYLUS SENSING SYSTEM
This application is a divisional of copending application Ser. No. 07/608,062 filed Oct. 31, 1990, now U.S.
Pat. No. 5,117,091.
1. Field of the Invention
This invention relates generally to input devices for a
data processing system. More particularly, it relates to
an improved stylus sensing system for use with an interactive input device disposed on a display surface which
permits either finger touch input or stylus input.
2. Background of the Invention
In the past, computers were used only by scientists,
mathematicians, and other high-level, sophisticated
computer users. As computer technology progressed,
and particularly with the advent of the personal computer, data processing has reached every level of society, and every level of user. The trend is for fewer
computer users to be computer professionals or sophisticated in data processing techniques. Access to computers will increase even further in the future as computer hardware and software increase in power and
efficiency.
It has therefore become necessary to design what
have become known in the art as "user friendly" input
devices. Such "user friendly" devices are designed to
allow an unsophisticated user to perform desired tasks
without extensive training. Human factor studies have
shown that a device which allows the user to input data
directly on the visual display screen of a computer,
generally known in the art as a touch input device,
achieves greatest immediacy and accuracy between
man and machine. One of the first input devices for use
at the display surface was the light pen. The light pen is
an optical detector in a hand held stylus, which is
placed against the face of a cathode ray tube. The location of the light pen is determined by detecting the
coordinates of the dot of light which is the scanning
faster of the display. A second type of touch input device is a mechanical deformation membrane which is
placed over the display screen. The membrane is a
transparent overlay which consists of two transparent
conductor planes disposed on a flexible surface. When a
selection is made, the user mechanically displaces one of
the conductor planes to touch the other by a finger or
stylus touch, thereby bringing the conductors into electrical contact with each other. Appropriate electronics
and software translate the electrical signals generated
by the finger or stylus touch to the position on the
display surface. Another touch input device is a capacitive transparent overlay placed over the display screen,
which includes transparent conductors driven by an
electromagnetic signal. The input device can detect the
location of a fmger touch by the change in capacitance
of the overlay or, alternately, a stylus is used to return
the electromagnetic signals from the overlay back to the
computer to determine the stylus position. Yet another
touch input device uses a frame which fits around the
display screen having a number of infrared or visible
light transmitters and receptors arranged in parallel
horizontal and vertical directions. When the user's finger blocks the light beams, the horizontal and vertical
receptors note the absence of the signals, thereby locating the position of the action desired by the user.
As such touch input devices have proliferated, there
have been many efforts to write user friendly software
as well. Recently, graphical user interfaces which have

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25

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30

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40

45

50

55

60

65

2

the user point to the screen to select objects and initiate
actions have become popular. These graphical user
interfaces typically present choices to the user in menu
or window form. In addition, recent computer applications use stylus devices for freehand drawing, gesture
recognition, and handwriting capture. These new software applications utilize the capabilities of the touch
input devices to emulate the familiar ergonomics of a
paper and pen to input data into a data processing systern. These stylus applications require more precise
detection of stylus contact with the screen than do other
applications.
A particularly versatile touch input system is described in U.S. Pat. No. 4,686,332, to E. Greanias, et aI.,
entitled "Combined Finger Touch and Stylus Detection
System for Use on the Viewing Surface of a Visual
Display Device" filed Jun. 26, 1986 which is hereby
incorporated by reference. For certain applications,
such as selecting items from a list, finger sensing methods have been found more convenient. Where greater
precision is required, such as applications with a high
information density, or where freehand drawing or
handwriting is recognized, the use of a stylus has been
found more effective. The touch input system described
in the above referenced, U.S. Pat. No. 4,686,332 allows
for both finger touch and stylus detection. The system
includes a touch overlay sensor which comprises an
array of horizontal and vertical transparent conductors
arranged on the viewing surface of the visual display
device. The conductor array emits electromagnetic
signals into the region above the display surface under
the control ofa microprocessor. The magnitude of these
signals is greatest near the surface and grows smaller at
greater distances. A stylus "antenna" is connected to an
input of the detector control system and senses the
signals emitted by the array. The signal amplitude seen
by the stylus is related to the position of the stylus on
and above the display. Radiative signal measuring
means coupled to the stylus measures the electromagnetic or electrostatic signal received by the stylus. Stylus contact with the display surface is indicated when
the electromagnetic signal exceeds a prescribed threshold. The accuracy of contact determination depends on
the uniformity of the radiated signal across the touch
overlay surface.
The system includes a means for connecting the output of an electromagnetic or electrostatic radiation
source to selected patterns of horizontal and vertical
conductors in the array. A switchable path connected
to the I/O terminals of the array selects the plurality of
horizontal and vertical conductors that are connected
to the radiative source. Control signals applied to the
control input of the switchable path determine the conductors that are connected at different intervals of the
sensing procedure. The control signal timing is used to
interpret the stylus signal amplitude and determine
where the stylus is located with respect to the conductor array i: the plane of the display surface. The fmger
sensing system in U.S. Pat. No. 4,686,332 is also a capacitive measurement means which measures the capacitance of selected conductors and determines where and
when a finger touch occurs. The same switchable path
is also used to connect the capacitance measurement
means to pluralities of horizontal and vertical conductors to the capacitance sensing means in response to
control signals applied to the control input.
However, the system as described in U.S. Pat. No.
4,686,332 has a number of drawbacks, particularly with

3

5,149,919

regard to the detection of stylus contact with the sensor
screen. For handwriting applications, only the positions
of the stylus as it touches the sensor screen should be
recorded. For example, the stylus motion after completing the stem of a "t", and before the crossing is begun,
must not be recorded, even if the stylus is moved very
near the surface of the sensor. Similarly, the stylus motion between the horizontal lines of an equal sign must
not be recorded even if the stylus moves very near the
surface. With the sensing method as described in U.S.
Pat. No. 4,686,332, the stylus would frequently be in
sufficient proximity to the touch overlay for positions
between strokes to be recorded in the tracking mode.
Thus handwriting recognition would have a great number of unintended strokes.
As discussed in U.S. Pat. No. 4,686,332, the conductors in the touch overlay are approximately 0.025 wide
and are spaced approximately 0.125 inches center-tocenter. When compared to the resolution desired for
handwriting, on the order of 250 points per inch, this is
a relatively wide spacing. To determine the position of
the stylus when it was between adjacent conductors, an
interpolation technique was used. This technique assumed that the field varied linearly with position between the two conductors where the second set of three
conductors was connected to ground. While this assumption was to determine the stylus position with
more than a fair degree of accuracy, it is not true. As the
electrical field strength from an individual conductor
varies with the distance from the individual conductor,
dielectric properties of the materials surrounding the
conductor and the location of nearby grounded conductors, the electrical field strength from the multiple
driven wires exhibited non-linear characteristics. This
effect is more pronounced where the layers in the touch
overlay are thin, and thus, the stylus is closer to the
individual conductors interpolation technique used.
Lastly, it was found that the attitude of the stylus
itself as it was held in the hand of the user had an effect
on the location sensed by the system. As mentioned
above, the stylus acts as an antenna to pick up the electromagnetic signals radiated by the touch sensor overlay. Depending on the writing angle preferred by an
individual user, the signal strengths measured by the
system could vary considerably, thus creating errors in
the accurate locating of the stylus. Contrary to the
assertions of U.S. Pat. No. 4,686,332 in column 6, lines
56-60 the signal strengths can not always be normalized
by calculation, as the stylus orientation can change
during the stroke across the overlay.

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45

50

OBJECTS OF THE INVENTION
It is therefore an object of the invention to provide an
improved interactive stylus input device for freehand
drawing, handwriting, and gestures, that can also be 55
used to select items by fmger as well as stylus.
It is another object of the invention to provide very
precise contact detection when using the stylus.
It is another object of the invention to provide a
stylus and touch sensor display system which is reliable 60
and inexpensive to manufacture.
It is a further object of the invention that to accurately determine stylus location independent of the
angle at which the stylus is held.
SUMMARY OF THE INVENTION
These and other objects are achieved by an improved
touch input system. The system includes many of the

65

4

features described in U.S. Patent No. 4,686,332 namely
a touch overlay with an array of horizontal and vertical
conductors, a control microprocessor, a stylus antenna,
a radiative signal measuring means to measure stylus
input, a capacitive measurement means to measure finger input, a radiative source to drive the conductors and
a switchable path to connect the conductors to the
radiative source, the radiative signal measuring means
and the capacitive measuring means in response to control commands by the control microprocessor. The
present invention provides several· improvements over
the prior art system. Among the more important are:
a) The stylus sensing procedure has been modified to
include a phase that is dedicated to contact detection.
In the preferred embodiment, the contact phase connects all the horizontal (x) and vertical (y) conductors
to the radiative source. Since all of the surface conductors are connected to the same electrical potential, the electric field above the surface is more uniform in the x and y directions of the display. There
are smaller perturbations in the regions directly
above conductors versus the regions between the
conductors, and the stylus signals show smaller variations with lateral motion. During this phase, signal
amplitude becomes an accurate indication of distance
above the surface and provides a more precise criterion for contact detection.
b) The perturbations of the signal amplitude at the sensor surface between conductors are measured during
the design of a particular overlay structure. These
values are stored in two tables, a tracking calibration
table and a contact detection calibration table and are
used to correct the signal amplitudes obtained during
tracking and the contact cycles. The signal deviation
at finite displacements within the square formed by
horizontal and vertical conductor pairs are recorded
at appropriate locations in the table. During the
contact phase, the most recently determined x and y
locations are used to query the table for the deviation
value at that location. It has been found that perturbation patterns in different regions of the sensor are
similar enough to allow use of one perturbation correction pattern for the entire surface. Alternatively,
the correction magnitude within a square could be
determined by using an analytical function that computes the value from a recorded deviation pattern.
The signal deviations at finite displacements between
two energized conductors are recorded in the tracking calibration table and used during tracking phase
to normalize the measured x and y locations.
c) The design of the stylus has been modified to improve the uniformity of signals that occur in use.
Users may hold the stylus at different angles with the
surface when using it, but they focus on the stylus tip
as they write and make selections. Contact and position must be determined independent of writing angle. To meet this need, a sphere is used in the tip of
the stylus as the antenna and the sphere is placed
centrally with the outside tip diameter. Thus, the
stylus angle can change considerably without changing the signal seen by the stylus. To achieve the desired angular independence, proper electrical shielding must be applied to the connection between the
sphere and the cable that conveys the signal to the
electronics, as well as the cable itself.
d) The x and y deviations of that predicted by the tracking calculations are corrected through the use of a
compensation table. The deviations can be deter-

5

5,149,919

6

mined empirically during the test of the product or
ture including several plastic substrate layers laminated
can be calculated from theoretical equations which
together by means of adhesive layers also including a
account for the materials in the overlay, thicknesses,
first plurality of transparent conductors 16A disposed in
conductor spacing and other factors.
the horizontal direction and a second plurality of transThe improvements described in this invention do not 5 parent conductors 16B disposed in the vertical direcaffect the finger touch sensing capabilities of U.S. Pat.
tion. Several of the conductors in both vertical and
No. 4,686,332, however, a scanning phase has been
horizontal directions are positioned beyond the readded to the finger sense mode. The scanning phase
cessed window 14 to allow more accurate location
does not make finger position any more accurate, but it
determination of the stylus 20 or a finger on the LCD 18
does narrow the area of the overlay in which the finger 10 at the edges of the display window 14.
position is likely to occur, thus speeding overall process
A stylus 20 is connected by cable 22 to the touch
time in the sense mode.
workpad. The stylus 20 acts as an antenna to pick up the
signals radiated by the overlay 16, and provides much
BRIEF DESCRIPTIONS OF THE DRAWINGS
greater resolution than can be provided by a finger
These objects, features and improvements will be 15 touch. The stylus 20 is discussed in greater detail with
better understood with reference to the following figreference to FIG. 4. Also on the bezel of the housing are
ures.
four button switches 24-27 which can used to change
FIG. 1 shows the front view of the overlay used for
the mode in which the data from the workpad 10 is
the detection of touch and stylus position disposed over
received. Workpad cable 28 is the connector between
a flat panel display, the entire assembly being called a 20 the workpad 10 and the computer with which the user
touch workpad.
is communicating. The workpad cable 28 provides
FIG. 2 shows a cross section view of the overlay
power to the workpad 10 as well as display signals to
attached to a flat panel display.
operate the LCD 18 and touch signals to operate the
FIG. 3, shows a cross sectional view of an alternate
overlay in both finger and touch and stylus modes. In
embodiment of the overlay in the system.
25 addition, the cable 28 is also the conduit to the comFIG. 4 shows a cross sectional view of the stylus.
puter of the measure of the signal received by the stylus
FIG. 5 illustrates the overlay as it is used for stylus
20 and the frequency change due to changes in capacidetection.
tance due to a finger touch.
FIG. 6 is a schematic view of the overlay as it is used
FIG. 2 shows a cross-sectional view of the overlay 16
for stylus detection during one of the stylus tracking 30 positioned on the display surface of the LCD 18. The
measurements.
overlay 16 shown in FIG. 2 is essentially the same as
FIG. 7 shows the radiative signal amplitude during
that shown in FIG. 12 of U.S. Pat. No. 4,686,332 and is
the PO measurement of conductor pair X6 and X7of the
attached to the glass LCD surface 18 by means of an
overlay for stylus detection.
adhesive 30 to provide a smooth, tight and well supFIG. 8 illustrates the radiative signal amplitude dur- 35 ported surface for finger touch and stylus detection. In
ing P2 measurement of conductor pair X6 and X7 for
an alternate embodiment, the overlay 16 would be atstylus detection.
tached to another piece of glass (not shown) by an adheFIG. 9 shows the radiative signal amplitude during
sive and then mounted to the LCD 18. The overlay
the Px measurement of conductor pair X6 and X7of the
consists of the inner substrate 32 which is a sheet of
overlay.
40 polyethylene terephthalate which is transparent, electriFIG. 10 depicts the radiative signal from the overlay
cally insulative, and has a thickness of approximately
during the contact detection phase of the stylus detec0.002 inches. An array of horizontal transparent contion.
ductors is deposited on the surface of the inner substrate
FIG. 11 show the relationship between radiative
32 and are designated as Yl, Y2, Y3, etc., with the Y3
signal amplitude and distance from the overlay surface 45 wire being shown in FIG. 2. The transparent conducduring the contact determination phase.
tors can be composed of indium tin oxide, for example,
FIG. 12 is a three dimensional representation of the
which is a well-known transparent conductor material.
variation in signal amplitude within a square of conducThe thickness of the transparent conductor can be aptors where all four conductors are driven on both axes.
proximately 1000 angstroms. The conductors are apFIGS. 13A and 13B depicts an architectural diagram 50 proximately 0.025 inches wide and are spaced approxiof the improved stylus and finger touch sensing system
mately 0.125 inches on a center-to-center spacing.
There are 112 transparent, vertical conductors Xl, Xl,
FIG. 14 is a flow diagram of the improved stylus and
... X112 and 112 horizontal conductors Yl, Y2, ... Y112.
finger touch sensing method which includes the dediThe horizontal Y conductors on the inner substrate 32
55 are oriented at right angles with respect to the vertical
cated contact detect cycle.
X conductors deposited on the outer substrate 34. A
DETAILED DESCRIPTION OF THE
combined insulation and adhesive layer 36 covers the
INVENTION
horizontal Y wires and joins outer substrate 34 and the
Referring to FIG. 1, a touch workpad, substantially
vertical conductors X and the inner substrate 32 and the
similar to that described in copending application Ser. 60 horizontal wires Y together. The adhesive insulation
No. 351,227 to Arbeitman, entitled "Flat Touch Screen
layer 36 can be composed of a transparent adhesive
Workpad for a Data Processing System", filed May 15,
such as ultraviolet initiated vinyl acrylic polymer having a thickness of approximately 0.002 inches. The
1989, and hereby incorporated by reference, is shown.
The workpad 10 comprises a housing 12 having a rectupper portion of the overlay 16 shown in FIG. 2 conangular recessed window 14 which surrounds the edges 65 sists of the outer substrate 34 which is a sheet of polyof a rectangular touch overlay 16. The overlay 16 is
ethylene terephthalate which is optically transparent,
electrically insulative and has a thickness of approxitransparent and is disposed on a liquid crystal display
(LCD) 18. The overlay 16 consists of a laminate stmcmately 0.002 inches. Deposited on the surface of the

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outer substrate 34 is a vertical array of transparent conductors designated Xl, X2, X3 ... X6 .... The conductors
Xl, X2, etc. are also composed of indium tin oxide and
have a thickness of approximately 1000 angstroms, 'a
width of approximately 0.025 inches and a spacing of
approximately 0.125 inches, center-to-center. The X
and the Y transparent conductors can also be composed
of gold and silver or other suitable materials. The thickness of the conductors is adjusted to provide resistance
below 50 ohms per square and an optical transmission
which is greater than 80 percent.
An anti-newton ring coating 38 may be applied to the
display side of the overlay 16 to eliminate newton rings
when the inner substrate 32 comes into contact with the
LCD 18. An electrostatic shield layer 39 consists of a
full panel coating of indium tin oxide which is
grounded. This coating shields the vertical X conductors and horizontal Y conductors from electrostatic
noise generated by the LCD 18. The electrostatic shield
layer 39 must be less than 100 ohms per square and must
exceed an optical transmissivity of 80 percent. As the
LCD 18 is much quieter electrically than the CRT in
the prior patent the electrostatic shield layer 39 can
easily bc omitted. The laminated structure 16 has an
overall thickness in the window area 14 of approximately 0.010 inches, has a high optical transparency,
and has a durable mechanical quality. In an alternate
embodiment, the Y and X array conductors could be
deposited on the outer laminate 34 and the inner laminate 32, respectively.
FIG. 3 depicts a cross-section view of a preferred
embodiment of the touch overlay used in the system of
the present invention. The overlay 16' is quite similar to
that depicted in FIG. 2, the major difference being that
the vertical conductors Xl, X2 ... are on the top side of
the upper substrate 34 facing away from the LCD display 18. A top sheet 40 was added to prevent stylus
contact with the vertical conductors. An antiglare top
coat 42 prevents operator fatigue.
The advantage of an upward facing conductor has to
do with the brittle nature of indium tin oxide (ITO) and
ITO's relative resistance to compressive forces as opposed to tensile forces. In FIG. 2, the vertical conductors Xl, X2, ... face downward toward the display surface 18. When the stylus 20 comes down on the overlay
16, it has the effect of stretching the upper conductors at
the point of contact. After repeated contacts, the vertical conductors tend to crack, thereby disrupting electrical conductivity and signal transmission in the overlay
16. In FIG. 3, the vertical conductors Xl, X2 ... face
upward and when the stylus 20 comes down on the
overlay 16', the conductors are compressed. ITO is
known to be relatively immune to compression, and
therefore the overlay 16' in FIG. 3 is not likely to suffer
reliability problems from ITO cracking. In both embodiments of the overlay in FIGS. 2 and 3, the horizontal conductors face upwards and are subjected to compressive forces. However, due to the relative thicknesses of the layers between the stylus and the lower
horizontal conductors and the stylus and the vertical
conductors, it is less critical that the lower conductors
face upwards. In yet another embodiment of the overlay, the upper conductors could face upwards and the
lower conductors could face downwards, the lower
conductors additionally being protected by the inner
substrate. The display surface 18 of the LCD is also
more compliant than the hard glass of the CRT disclosed in U.S. Pat. No. 4,686,332. It is thought that the

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compressive forces would be distributed more evenly
with the LCD 18 and the dqwnward facing lower conductors, although under mild tensile forces, would be
less likely to crack.
In FIG. 3, the inner substrate 32, the outer substrate
34 and the top sheet 40 are sheets of biaxially oriented
polyethylene terephthalate laminated together with a
thermoplastic polyester adhesive 36, 44. In a preferred
embodiment, the miner substrate 32 is 0.005 inches
thick, the outer substrate 34 is 0.002 inches thick and the
topsheet 40 is 0.001 inches thick. Adhesive layers 36, 44
measure approximately 0.0005 to 0.001 inches in thickness. As in FIG. 2, the horizontal and vertical ITO
conductors are approximately 1000 Angstroms in thickness, with a width of approximately 0.025 inches and a
spacing of approximately 0.125 inches, center to center.
The overlay 16' is attached 30. Anti-newton ring and
electrostatic shield coatings may also be placed between
the inner substrate 32 and the LCD 18.
As discussed in U.S. Pat. No. 4,686,332, the overlay X
and Y conductors are electrically connected to the
other elements of the sensing system by means of several bus wire located at the periphery of the overlay.
The requirement of a separate bus wire for each X and
Y conductor would rapidly become unwieldy and increase the size, complexity and cost of the overlay.
Therefore, several widely spaced conductors are
connected· to the same bus wire. In the preferred embodiment above in which there are 112 X conductors
and 112 Y conductors, there are 16 X bus wires and 16
Y bus wires. Consequently, when a partiCular bus wire
is energized, electromagnetic signals are entitled by
seven different conductors in the overlay.
FIG. 4 depicts the improved stylus structure of the
present invention. In U.S. Pat. No. 4,686,332, it was
recognized the stylus orientation to the overlay affected
the radiative signal amplitude picked up by the stylus.
Signal amplitude variation is accounted for in stylus
position determination by normalizing the signal
strength by calculation. Nonetheless, it is desirable to
have the signal amplitude received by the stylus at a
given location on or near the overlay to be independent
of stylus orientation, particularly for the contact determination measurement discussed below. Signal variation with stylus orientation has an adverse impact to the
contact detection phase of the improved stylus detection method. Unlike the position determination calculation, the magnitude of the signal is used without normalization, as the position calculation is the ratio of several
signal measurements. Finally, it is easier and ultimately
more accurate, not to be required to account by calculation or other means for signal strength variation due to
stylus angle.
Referring to FIG. 4, a cross section of the stylus 20 is
shown. A sphere 50 acts as the antenna for the stylus.
Because of the spherical geometry, the user is free to
hold and change the writing angle of the stylUS 20 considerably without changing the radiative signal seen by
the stylus 20. This arrangement can be made by pressing
a ball bearing 50 into a molded plastic tip cover 52. The
inner diameter of the hemispherical tip cover 52 is approximately equal to the diameter of the ball bearing 50
(approximately 0.062"). The L outer diameter of the tip
cover 52 (approximately 0.095") is a convenient size for
pointing and writing. The sphere diameter and tip cover
outer diameter were chosen to work with the geometry
of the transparent conductors in the touch overlay 16'.
Other touch overlay geometries may lead to somewhat

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different antenna and stylus tip dimensions. As depicted, the ball bearing 50 provides ample signals for
position sensing and contact detection. It would be
preferred to go to a very small diameter ball to approach a theoretical point source, however, the stylus
will not receive a sufficient signal. It is important that
both the ball bearing 50 and the tip cover 52 be substantially spherical and concentric with each other for the
stylus to be used at angles between normal and 45 degrees with little effect on the detected signal strength.
The stylus tip cover 52 fits into a hollow tube 54
which is similar in dimension to a conventional ball
point pen. The inner conductor of stylus cable 22 passes
through this tube 54 from the sphere 50 to the electronics in the workpad 10. In a preferred embodiment, a
helical spring 56 is used to press the ball bearing SO into
the tip cover 52 and to make the inner conductor of the
cable 22 make electrical contact with the ball SO. Those
skilled in the art can conceive of other methods of electrically connecting the wire to the ball. Cylindrical
shielding 58 is an electrically conductive tube within the
tube 54 connects to the spring 56 and confines the signal
sensitivity to the sphere antenna 50. Between the spring
56 and the ball 50 is a plastic insulator 57 used to isolate
the ball SO from the shield 58 and to press the ball 50
into the tip cover 52. The plastic insulator 57 should be
of a relatively hard plastic which will not allow the wire
of the spring 56 or ball 50 to be embedded therein. The
construction cost of the depicted stylus should be a
minimum as no soldering or other costly assembly
methods are required.
The method for determining stylus position is improved over that disclosed in U.S. Pat. No. 4,686,332. In
the stylus mode, the X and/or Y conductors are driven
by a 40 KHz oscillator driver so that the X and/or Y
conductors act as a transmitter of electromagnetic radiation. The stylus 20 acts as a receiver of the signal. The
signal amplitude is digitized and analyzed by the control
microprocessor, the stylus detection process consists of
several operational modes which are discussed below.
The differences between the improved procedure and
U.S. Pat. No. 4,686,332 are discussed below.
FIG. 5 depicts the arrangement for detection of the
stylus 20 when it is closer than the locate threshold
distance 60. Initially, the touch system is in idle mode.
The idle mode is used to determine whether the stylus is
close enough to the overlay 16 for location determination. In the preferred embodiment, all of the conductors
in the overlay, on both axes, are driven, and the signal
strength received by the stylus is compared to a threshold. If the signal exceeds the threshold, the Locate
Mode discussed below is invoked. In addition, the Sense
Mode, which detects finger location in U.S. Pat. No.
4,686,332 is excluded. This exclusion allows users to put
their fmgers and hand directly on the overlay 16 for
better support and control while using the stylus. The
overlay 16"depicted in FIG. 5 is similar to the overlay
16' depicted in FIG. 3, except that the adhesive layers
are not shown for the sake of clarity.
In the locate mode, the system first tries to determine
the X and Y conductors nearest to the stylus location.
The locate operation starts by driving individual sets of
wire on each axis separately and collecting the associated signal strengths from the stylus. The exact driver
pattern for the locate mode depends on the structure of
the overlay 16 and whether the overlay 16 is disposed
over a CRT or an LCD. If the overlay 16 is on a CRT,
an electrostatic shield layer will be used to damp the

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electrical noise from the CRT. The electrostatic shield
layer has the additional effect of driving the ITO conductors to ground and reduces the signal strength of the
electrical signal emitted by the overlay. With the electrostaticshield, only signals from the very nearest conductors will be sensed at all by the stylus. These signals
will be used to determine the conductors driven in the
following mode. If the overlay 16 is on an LCD, an
electrostatic shield layer is generally not used. The
stylus will pick up signals from many more conductors.
Where there is no electrostatic shield layer, it has been
determined that the three highest signal amplitudes
observed when the sixteen X bus wires are driven on
the X-axis will be sufficient to identify the X conductor
pair nearest the stylus pOsition. Likewise, the three
highest signal values for the Y-bus identify the nearest Y
conductor pair to the stylus position. If both axes successfully locate the "nearest" conductors, the system
will initiate the track mode of operation to determine
the stylus position more precisely. Track mode requires
larger signal amplitudes and if after successive tries, the
signal conditions are not adequate, the system returns to
locate mode, described above. The procedures for
transferring between modes are some of the changes
from U.S. Pat. No. 4,686,332.
If more than prescribed number of successive, successful locates occur and all the locate coordinates are
proximate to each other, the locations of these points
are reported to the computer. These indicate the general X and Y locations of the stylus when it is some
distance above the surface and not in contact with the
sensor. By determining this "base" location and reporting it to the system, the computer can create a pointer
image (Le. a cursor) on the display. This feedback to the
user provides significant help to the user doing fine
drawings and handwriting.
The previous art disclosed in the referenced patent
utilized a single stylus signal threshold for entering into
locate and tracking modes. Because both the locate and
track modes are expected to succeed at this point, the
threshold had to be set very conservatively. The present design allows the reporting of proximity points
when Locates have succeeded but Tracking has not,
thus providing points while the stylus is considerable
higher off the display. It also allows the system to exclude finger sense operation and concentrate on stylus
location determination much sooner.
The method disclosed in the referenced patent utilized a fIXed threshold for determining whether the
track mode measurement was successful and should be
continued. This had two detrimental effects. First, a
threshold value had to be manually selected based upon
an a prior analysis of the stylus signals. Secondly, the
threshold was not adaptive to changes in the equipment,
such as the user replacement of a stylus. The present
design compares the various stylus signals detected
while in track mode, and depending on the ratio of the
PX measurement to the PO and P2 measurements determines whether the measures were valid.
The track mode is initiated when the current nearest
conductors or "base position" has been determined in
locate mode. Only groups of conductors immediately
adjacent to the base position are driven in track mode.
In the improved method of the present invention, each
track mode cycle includes a separate contact detect
operation as well as wire drive operations and offset
calculations from the base position.

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Seven different overlay wire drive patterns are generated during track mode and seven corresponding signal
amplitudes are collected. The first three drive patterns
are applied to the X axis and are used to determine the
offset of the pen from the X base position. The fourth
pattern is a simultaneous drive of all X and Y conductors, and is used to evaluate the altitude of the stylus
above the overlay. The last three drives are applied to
the Y axis and are used to determine the offset of the pen
from the Y base position.
The first drive pattern for the X conductors for determining the stylus position in track mode is schematically shown in FIG. 6. A base wire pair is defined as
two adjacent conductors between which the stylus is
believed to lie. In FIGS. 6 to 9, adjacent conductors X6
and X7 are driven in three different patterns. When
conductor X6 and the five conductors to the left are
connected to ground, and right conductor X7 and the
five conductors to the right are connected to the oscillator, as in FIGS. 6 and 7, the drive pattern is defined as
PO. When left conductor X6 and the five conductors to
the left are driven by the oscillator and right conductor
X7 and the five conductors to the right are grounded,
the drive pattern is identified as P2. When both of the
two conductors are driven, the pattern is identified as
PX. The drive patterns PO, P2 and PX are shifted to the
current position as the stylus is moved In the improved
method, twelve conductors are in the drive patterns (6
grounded, 6 driven) rather than six in the method described in U.S. Pat. No. 4,686,332. The PI pattern of the
U.S. Pat. No. 4,686,332 has been replaced by pattern
px. Drive pattern PX is used to normalize the PO and
P2 signal measurements to reduce the impact of the
stylus altitude or positions determination. Experimentally, PX has been shown to provide better normalization than the previously used method. In general, the
more conductors driven, in this case those to the right
of X7, the more uniform the electrical field over the
driven conductors. There is a point of diminishing returns, however, in that ever greater numbers of driven
conductors require the multiplexing circuitry which
connects the connectors to the electronic circuitry to be
increasing complicated.
FIG. 7 shows the amplitude of the signal received by
the stylus 20 as it would pass from left to right from
above the conductor X/to a position above the conductor X\2. Note that within and around the wire pair X6
and X7, the stylus signal varies somewhat linearly with
position. The degree to which the signal strength curve
approximates linearity depends in part on how far the
stylus 20 is from the conductors. For this overlay, the
signal strength curve is much less linear and any calculations based on linearity must be compensated. Also
shown in FIG. 7 is the slight ripple in the signal strength
curve over the driven conductors X7, Xs, X9, XlO, XII
and X\2. The signal strength is slightly stronger directly
over a driven conductor than it is between two driven
conductors. The first phase in the locate mode is measuring the signal amplitude using drive pattern PO. Assuming that the stylus 20, is located somewhere between X6and X7, the signal amplitude measured by the
stylus 20 will be a single value along the sharp rise of the
curve depicted in FIG. 7.
The second pattern in the operation of tracking the
position of the stylus 20 is shown in FIG. 8, where the
drive pattern P2 is the inverse of the wire pair PO. That
is, the conductors XI, X2, X3, )4, Xs and X6 are driven
with the oscillator driver, whereas the conductors X7,

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XS, X9, XlO, Xll and X12 are connected to ground or
reference potential. The signal amplitude as the stylus
20 moves from left to right across the conductors is
shown for the drive pattern P2 in FIG. 7. The stylus 20
is in the same position as for FIGS. 6 and 7, therefore,
the magnitude of the signal for the wire pair P2 will be
measured somewhere along the sharp fall between conductors X6 and X7 as shown in FIG. 8. Similar to FIG.
7, there is a ripple in signal strength over the driven
conductors XI, X2, X3, )4, Xs and X6. As the stylus 20
gets closer to the conductors in the overlay 16, the
ripple over the driven conductors becomes more accentuated and the rise or fall in the signal strength between
conductors X6 and X7 becomes less linear.
The third phase of track mode is depicted in FIG. 9.
This phase is unlike any of those described in the prior
art patent. During this phase, both conductors X6 and
X7 are driven by the oscillator and all other conductors
are grounded. FIG. 9 depicts the signal strength curve
which would be measured as the stylus 20 is moved
across the overlay 16 from XI to X\2. The curve is
bimodal with the two modes occurring directly over
the driven conductors and the closer the stylus is to the
overlay the more pronounced the bimodal nature becomes. Since the PX patterns only drive two conductors, the electric field above the surface more nonuniform in the X and Y directions of the sensor and the
stylus signals show larger variations with lateral motion.
FIG. 10 depicts the fourth drive pattern in the track
mode where all the X and Y conductors in the overlay
are driven simultaneously for contact detection. While
other drive patterns could be used for stylus detection,
driving all the conductors creates the most uniform
signal across the overlay. This pattern is used to evaluate the height of the stylus above the display surface and
is distinct from the other patterns of the track mode in
that conductors far from the base position are driven.
By driving all of the conductors, very accurate height
determination is possible as the electrical field varies
very slightly with lateral position" as can be seen in FIG.
10.
FIG. 11 depicts the relationship between signal amplitude and distance from the overlay surface. If the
signal amplitude is less than predicted for contact with
the touch overlay surface, the sensed positions are not
recorded as contact points, but as proximity points by
the improved stylus sensing system. The figure depicts
signal amplitude over a single point in the overlay as the
signal amplitude is strongest where the conductors are
closest to the electrical bus and weakest at the conductor termination furthest away from the bus. The next
three drive patterns are essentially the same as depicted
in FIGS. 7,8 and 9, but for the conductors, for example,
Y I, Y2, ... Y12· Y6 and Y7 are the two conductors between which the stylus is located and around which
drive patterns PO, P2 and PX for the Y conductors.
. By this point, the locate and tracking mode of stylus
has determined the base position of the stylus. The base
position is defmed as the central point between the two
X conductors and the two Y conductors between which
the stylus is located. In the preceding FIGS., the base
position is the central point between X conductors X6
and X7 and Y conductors Y6 and Y7 on the overlay.
Next, a series of compensation or calibration steps are
taken to account the nonlinear nature of the signal
strengths for the various drive patterns across the overlay. These calibration steps are derived empirically for

5,149,919

13

14

a particular overlay and stylus combination, although
variations are not as great with the Dual Drive signal
the values in the tables might be theoretically calcudiscussed above as it was with the prior art, further
lated. Those skilled in the art will appreciate that other
improvements in contact detection accuracy are
calibration methods may be possible. First, the offset to
achieved using the 2 dimensional compensation table.
the base position is calculated in both X and Y direc- 5 The offset compensation table is a function of the stylus
tions.
geometry and overlay geometry and is shown below.
The offset of the stylus relative to base position is
As the X and Y coordinates have been calculated. the
calculated with the signal values collected during the
relative stylus altitude is computed from the dual drive
PO, P2, PX operations for each axis. The square area
value collected in the middle of the drive sequence.
bounded by conductors X6, X 7, Y6and Y7 is called the 10 Since the measurement value has a small dependence on
base region. In the preferred embodiment, the base
the X, Y location as well as altitude, it is also corrected
region is divided into 32 sections, so an offset from -16
by using a 2 dimensional (X,Y) compensation table.
to + 16 is reported. The coordinate value for each axis
Using the adjusted signal strength value and the value in
is computed as:
the 2 dimensional compensation table, based on current
15 stylus position the final adjusted signal strength is found
coordinate value=(32'base)+offset
with the following equation:
After the coordinate value is established, it is multiplied
by the wire pitch to obtain the position in the overlay.
The offset is non-linear function of the PO, P2, and PX 20
values. However, the track algorithm assumes that the
function is linear, calculating the offset as:
offset =C'(PO-P2)/PX

and adjusts for the non-linearity by mapping the offset
through the use of a compensation table. The offset
generated by the above equation is 1/64 of a wire distance, twice the reportable density. The compensation
table for the X and Y positions is a one dimension vector
with 64 values which corrects for nonlinearity in the
offset calculation and converts to an offset from -16 to
+ 16. If the offset is calculated by the above equation to
be +28, the system will look at the 60th value in the
vector. The vector may indicate that the stylus is really
on offset 13 rather than 28, so the value of 13 is substituted into the equation to determine the coordinate
value. The referenced patent does not compensate for
the nonlinearities in the offset calculation. These nonlinearities are due to the shapes of the signal strength
curves PX, PO, and P2 as a function of offset location.
Although the compensation is performed using a table,
one could develop an equation to perform this function.
Next as the X and Y positions have already been
determined the distance from the point where the X and
Y buses meet, the point of the strongest signal, and the
current stylus position is known. This distance is substituted in the equation:
Signal strength = signal strength
adjusted
measured

+ K' (distance

X Signal
strength measured)

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to obtain the intermediate adjusted signal strength for
use with the contact detection determination.
Finally, the variable signal strength within the base 55
region bounded by the four driven conductors surrounding the current stylus position is compensated.
The nonlinearity in the dual drive mode used for
contact detection is illustrated by the three dimensional
diagram in FIG. 12. The vertical dimension depicts 60
signal amplitude and the two horizontal axes depict the
-16 to + 16 offset divisions. As shown in the figure, the
weakest signal is measured at the base position and the
strongest signal is measured where the conductors
cross. The strength of the stylus altitude signal has been 65
found to be not only a function of the altitude, but also
the X and Y delta from the base location i.e., the location between the transparent conductors. While the

S. strength = S. strength
final
adj

+ K, (S.
-

Strength. Table (x,y»)
adj

The final signal strength value is compared to a "contact"threshold. If the value exceeds the threshold, the
coordinates generated for that cycle are tagged "in
contact" coordinates. Otherwise they are tagged "proximity" coordinates. When the track cycle is completed,
the coordinates, the tag and the corrected amplitude are
passed to the computer system.
FIG. 13 shows an architectural diagram of the improved detection system. The system depicted in FIG.
13 is very similar to that disclosed in the U.S. Pat. No.
4,686,332 in FIG. 2. The major differences include: the
overlay 16 preferably has both X and Y conductor sets
facing upward away from the display, the stylus 20 is of
the improved design depicted in FIG. 4, the touch control processor 100, random access memory 102, read
only memory 104 and the I/O controller 106 are on a
touch panel adapter card 107 in a personal computer
while the rest of the touch electronics are integrated in
the touch workpad 10. As discussed in connection with
FIG. 1, the touch workpad 10 communicates with the
personal computer and touch panel adapter card via
cable 28. The vertical conductors XI-X112 are connected through the X bus 80 to the wire select multiplexer 112 and the horizontal Y conductors Yl-Y112
are connected through the Y bus 90 to the wire selection multiplexer 112. The radiative pickup stylus 20 is
connected through the gate 120 to the radiative pickup
measurement device 122. The wire selection multiplexer 112 is connected through the mode multiplexer
116 to the capacitance measurement device 128 which is
used for capacitance finger touch detection. The wire
selection multiplexer 112 is also connected through the
mode multiplexer 116 to the 40 kilohertz oscillator
driver 126 which is used to drive the X bus 80 and the
Y bus 90 for the stylus detection operation. The mode
multiplexer 116 also has an enabling output to the gate
120 to selectively connect the output of the stylus 20 to
the radiative pickup measurement device 122, for stylus
detection operations. The output of the capacitance
measurement device is connected through the analogto-digital converter 130 to the workpad bus 110. The
output of the radiative pickup measurement device 122
is connected through the analog-to-digital converter
124 to the bus 110. A control input 114 to the wire
selection multiplexer 112 is connected to the bus 110
and the control input 118 to the mode multiplexer 116 is
connected to the bus 110.

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The workpad bus 110 is connected via workpad interface 111 to the cable 28 which connects to PC interface
113 in the touch panel adapter card 107 in the personal
computer. The PC interface 113 communicates to the
main system bus 115 and to the adapter card bus 117.
The I/O controller 106 has an I/O bus 108 which connects to the main 115 bus of the Personal Computer.
The I/O controller 106 is also connected to adapter
card bus 117. The bus 117 also interconnects the control
processor 100 with the read only memory (ROM) 104,
and the random access memory (RAM) 102. The personal computer includes standard devices such as a
CPU 132, ROM 134, disk storage 136, a memory 138
which stores operating system 140 and application program 142, a keyboard 144 and display 146.
The wire selection multiplexer 112 and the mode
multiplexer 116 connects selected patterns of a plurality
of the horizontal and vertical conductors in the overlay
20 to either the capacitance measurement device 128 or
the 40 kilohcrtz oscillator driver 126, in response to
control signals applied over the control inputs 114 and
118 from the bus 110 by the control processor 100.

~~~;gd~~f:: ~~~c~a~Pi~~a~~~;; ~~:;l~~t~~:~~;hm:~

mode multiplexer 116 and the wire selection multiplexer
112 to selected ones of the horizontal and vertical conductors in the overlay 16 in response to control signals
from the control processor 100. The output of the capacitance measurement device 128 is converted to digital values by the converter 130 and is supplied over the
bus 110 to the control processor 100, which executes a
sequence of stored program instructions to detect the
horizontal array conductor pair and the vertical array
conductor pair in the overlay 16 which are being
touched by the operator's finger.
In the stylus detection mode, the 40 kilohertz output
of the oscillator driver 126 is connected through the
mode multiplexer 116 and the wire selection multiplexer
112 to selected ones of the conductors in the overlay 16,
in response to control signals applied over the control
inputs 114 and 118 from the control processor 100. The
electromagnetic signals received from the overlay 16 by
the stylus 20 are passed through the gate 120 to the
radiative pickup measurement device 122, which measures those signals and provides an output which is
digitized by the converter 124 and output to the control
processor 100. The control processor 100 executes a
sequence of stored program instructions to detect the
proximity of the stylus to the overlay 16 in the proximity detection mode and then to locate and track the
horizontal and vertical position of the stylus with respect to the overlay 16 in the location and tracking
mode. The stored program instructions for carrying out
these operations can be stored in the read only memory
104 and/or the RAM 102, for execution by the control
processor 100. Positional values and other result information can be output through the I/O controller 106 on
the I/O bus 108 to the host processor for further analysis and use in applications software.
FIG. 14 is a flow diagram of a the preferred embodiment of the invention where either fmger touch operations or alternately stylus detection operations can be
carried out. It has been found that there are very few
circumstances in which a user wants to carry out finger
touch operations to the exclusion of stylus detection
when the stylus is proximate to the overlay. On the
other hand, much of the time a user will rest his hand on
the overlay while w.iting with the stylus so the capaci-

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tance changes in the overlay due to the user's hand must
be excluded while the stylus is sensed.
The flow diagram begins with all the conductors in
the overlay 16 being driven in idle mode in step 200.
Idle mode is the mode in which the sensing system is
typically placed by the controller processor 100. Appropriate control messages are sent to the wire select
multiplexer 112 to drive all X and Y conductors via the
X-bus 80 and V-bus 90 and to the mode multiplexer 116
to send all radiative signals captured by the stylus 20
back to the control processor 100 through the gate 120
radiative pickup measurement device 122 and A/D
converter 124. In idle mode, the mode multiplexer 116
also connects the 40 kilohertz oscillator 126 to the overlay 16 through the wire select multiplexer 112.
If a signal amplitude above the finger sense exclusion
threshold is sensed in step 202, stylus detection steps 204
through 218 are attempted. If the signal amplitude is
below the threshold, finger sense steps 220 through 228
are attempted.
If the threshold has been crossed, the sensing system
transfers to stylus locate mode in step 204. In locate
mode, the control processor 100 sends the appropriate
control messages to the wire select multiplexer 112 to
selectively connect each of the X and Y conductors to
the 40 kilohertz oscillator 16 in the locate driver pattern. The signals received by the stylus 20 are sent back
to the control processor 100 in an attempt to identify the
two conductors in both X and Y planes which return
the highest signal amplitude, and therefore, the conductors which are closest to the stylus 20. If the control
processor 100 is successful in locating the stylus 20 over
a predetermined number of locate scans in step 206, the
stylus sensing system passes into track mode in steps 208
through 216. If the stylus is not located in step 206, the
sensing system returns to idle mode, step 200.
In track mode, the wire select multiplexer 112 receives control messages from the control processor 100
to drive the twelve X conductors closest to the stylus 20
in the drive patterns depicted in FIGS. 7, 8 and 9 in step
208. Next, in step 209, all the conductors are coupled to
the oscillator 126 to determine whether the stylus 20 is
contact with the overlay 16 as described in conjunction
with FIGS. 10 and 11. The wire select multiplexer 112
then receives control messages to drive the twelve Y
conductors closest to the stylus 20 in drive patterns
analogous to those depicted in FIGS. 7,8, and 9 in step
210. The signal amplitudes received by the stylus 20 are
sent back to the control processor 100. In step 212, the
PO, P2, PX and Altitude signals are evaluated to determine whether the stylus 20 was in contact with overlay
16, the calculated stylus position is compensated by the
above mentioned calibration tables to correct for the
nonlinearity of the electrical field over the overlay 16 in
step 214. In addition, the measurement step 209 is compensated for nonlinearity. The calibrated pen point is
output by the control processor along bus 117 either to
RAM 102 for storage or to I/O controller 106 for output to the personal computer along the I/O bus 108.
After the stylus position is successfully tracked, the
system goes back to step 208 to track the next stylus
position. The stylus 20 was not in contact with the
overlay 16, the stylus position is reported by the control
processor 100 as a proximity point in step 218. Proximity information is of use to application programs run on
the personal computer which are aware of the workings
of the stylus sensing positions. After the proximity point

17

5,149,919

is reported, the system returns to locate mode in step
204.

If the signal amplitude received by the stylus is not
above the finger sense exclusion threshold in step 202,
the system passes into sense mode to determine whether 5
a finger has touched the overlay 16. The sense mode
described below is similar from that disclosed in U.S.
Pat. No. 4,686,332. In step 220, the wire select multiplexer 112 and the mode multiplexer 116 receive appropriate control signals from the control processor 100 to 10
begin scanning all conductors for a change in capacitance indicating the presence of a finger. In response to
these signals, the wire select multiplexer 112 begin connecting selected X and Y conductors to the capacitive
measurement device 128 which has been connected by 15
the mode multiplexer 116. The mode multiplexer 116
also disconnects the 40 KHz oscillator 126, and therefore no signals can be picked up by the stylus. If the
finger touch is located in step 222 by the change in
capacitance in a particular area of the overlay 16, the 20
system goes on to a more precise scan in step 224 of the
area the overlay 16 to locate the finger touch. The
change in capacitance values are set back to the control
processor 100 via the AID converter 130. The finger
position is output by the control processor 100 along 25
bus 117 either to the RAM 102 or 110 controller 106.
At this point, the system returns to idle mode 100. If the
finger touch was not located in the initial scan of the
conductors for'capacitance change in step 220, the average values for the ambient capacitance of the overlay 30
are updated, step 228, and the system returns to idle
mode 200. The ambient capacitance drifts somewhat
with temperature, video image and other factors. An
updated average ambient capacitance is kept to determine whether any measured difference in capacitance is 35
due to a finger touchdown on the overlay.
The term "electromagnetic" is used above to describe
the type of signals generated by the overlay and measured by the stylus. Some of those skilled in the art
might classify the signal amplitude received in the 40 40
KHz range as an electrostatic signal rather than an
electromagnetic signal. In the electrostatic case, the
flow of electrons to the stylus is measured, whereas in
the electromagnetic case the electromagnetic field
strength is measured. In either case, the device as de- 45
scribed above would be capable of sensing stylus and
finger touch position.
While the invention has been described with respect
to several illustrative examples, it would be understood
by those skilled in the art that modifications may be 50
made without parting from the spirit and scope of the
present invention. The embodiments presented above
are for purposes of example only and are not to be taken
to limit the scope of the appended claims.
We claim:
55
1. An improved stylus detection system having a
transparent overlay with a transparent array of horizontal and vertical conductors, the horizontal conductors
being spaced from the vertical conductors by an insulating material, a control processor, a radiative source for 60
stylus detection, a switchable path to couple selected
conductors to the radiative source in response to appropriate commands from the control processor and a stylus wherein:
the control processor is connected to a radiative 65
source measurement means to receive measured
radiative signal values of the conductors when the
switchable path has connected a first pattern of

18
conductors to the radiative source to detect the
vertical and horizontal location of the stylus with
respect to the overlay;
the control processor is connected to the radiative
source measurement means when the switchable
path has connected a second pattern of conductors
to the radiative source to detect the height of the
stylus with respect to the overlay; and,
whereby stylus horizontal and vertical location and
stylus height with respect to the overlay can be
determined.
2. The improved stylus detection system as recited in
claim 1 which further comprises:
first compensation means to compensate for the nonlinearities of the meaSure radiative signal values
when the first pattern of conductors is connected
to the radiative source to detect the vertical and
horizontal location of the stylus with respect to the
overlay.
3. The improved stylus detection system as recited in
claim 1 which further comprises:
second 'compensation means to compensate for the
distance of the stylus from a location on the overlay which produces a strongest radiative signal
when the second pattern of conductors is connected to the radiative source to detect the height
of the overlay with respect to the overlay.
4. The improved stylus detection system as recited in
claim 3 which further comprises:
third compensation means to compensate for the
nonlinearities of the measured radiative signal values when the second pattern of conductors is connected to the radiative source to detect the height
of the overlay with respect to the overlay.
5. The improved stylus detection system as recited in
claim 3 wherein the second pattern comprises connecting all of the horizontal and vertical conductors to the
radiative source to produce the most uniform radiative
signal across the overlay.
6. The improved stylus detection system as recited in
claim 3 wherein the system labels a particular horizontal
and vertical stylus location as a contact point if the
compensated signal strength exceeds a contact threshold and labels a particular horizontal and vertical stylus
location as a proximity point if the compensated signal
strength is below the contact threshold.
7. The improved stylus detection system as recited in
claim 1 wherein the stylus comprises:
a spherical antenna for receiving signals radiated
from the horizontal and vertical overlay conductors to maintain the cross sectional area of the antenna which receives the radiated signals independent of the angle of the stylus with respect to the
overlay;
an insulating tip cover in which the spherical antenna
is embedded having an outer surface substantially
'concentric with the spherical antenna to maintain
the distance of the spherical antenna from the overlay independent of the angle of the stylus with
.. respect to the overlay; and
an electrical shield to confine signal sensitivity to the
spherical antenna within the stylus, so that the
radiated signal value received by the stylus is independent of the angle of the stylus with respect to
the overlay.
8. The improved stylus detection system as recited in
claim 7 wherein the stylus further comprises:

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a helical spring to press the spherical antenna in the
top cover and to make electrical contact with the
spherical antenna; and,
.
an insulator to separate the spherical antenna from
the electrical shielding.
9. An improved stylus detection system having a
transparent overlay with a transparent array of horizontal and vertical conductors, the horizontal conductors
being spaced from the vertical conductors by an insulating material, a control processor, a radiative source for
stylus detection, a switchable path to couple selected
conductors to the radiative source in response to appropriate commands from the control processor and a stylus, the stylus comprising:
a spherical antenna for receiving signals radiated
from the horizontal and vertical overlay conductors to maintain the cross sectional area of the antenna which receives the radiated signals independent of the angle of the stylus with respect to the
overlay;
an insulating tip cover in which the spherical antenna
is embedded having an outer surface substantially
concentric with the spherical antenna to maintain
the distance ofthe spherical antenna from the overlay independent of the angle of the stylus with
respect to the overlay; and,
an electrical shield to confine signal sensitivity to the
spherical antenna within the stylus, so that the
radiated signal value received by the stylus is independent of the angle of the stylus with respect to
the overlay.
10. The improved stylus detection system as recited
in claim 9 wherein the stylus further comprises:
a helical spring to press the spherical antenna in the
tip cover and to make electrical contract with the
spherical antenna; and,
an insulator to separate the spherical antenna from
the electrical shielding.
11. An improved method for stylus detection in a
transparent overlay having a transparent array of horizontal and vertical conductors, the horizontal conductors being spaced from the vertical conductors by an
insulating material, a control processor, a radiative
source for stylus detection and a means for selecting and
coupling conductors to the radiative source and a stylus
to receive signals radiated by the overlay comprising
the steps of;
determining whether a threshold has been exceeded
by a radiative signal value received by the stylus;
locating a general position of the stylus with respect
to the overlay, by selecting a horizontal and a vertical conductor pair from which the stylus receives
the highest strength signals;

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determining whether the stylus is within a predetermined height above the overlay;
accurately tracking a precise position of the stylus, if
the general position of the stylus has been located,
and if the stylus is determined to be within the
predetermined height above the overlay; and,
repeating the tracking step as long as the stylus is
within the predetermined height above the overlay.
12. The improved method for stylus detection as
recited in claim 11 wherein:
the general position of the stylus is determined by
successively coupling each of the vertical and horizontal conductors to the radiative source;
the precise position of the stylus is tracked by coupling a first pattern of conductors to the radiative
source; and
the height of the stylus is determined by coupling a
second pattern of conductors to the radiative
source.
13. The improved method for stylus detection as
recited in claim 11 which further comprises the step of
compensating for the nonlinearities of the measured
radiative signal value when the first pattern is coupled
to the radiative source.
14. The improved method for stylus detection as
recited in claim 11 which further comprises the step of
compensating for the distance of the stylus from a location on the overlay which produces a strongest radiative signal when the second pattern is coupled to the
radiative source.
.
15. The improved method for stylus detection as
recited in claim 14 which further comprises the step of
compensating for the nonlinearities of the measured
radiative signal values when the second pattern is coupled to the radiative source.
16. The improved method for stylus detection as
recited in claim 11 wherein the stylus comprises:
a spherical antenna for receiving signals radiated
from the horizontal and vertical overlay conductors to maintain the cross sectional area of the antenna which receives the radiated signals independent of the angle of the stylus with respect to the
overlay;
an insulating tip cover in which the spherical antenna
is embedded having an outer surface substantially
concentric with the spherical antenna to maintain
the distance of the spherical antenna from the overlay independent of the angle of the stylus with
respect to the overlay; and
an electrical shield to confine signal sensitivity to the
spherical antenna within the stylus, so that the
radiated signal value received by the stylus is independent of the angle of the stylus with respect to
the overlay.

. . .. . ..