Stylus tilt detection apparatus for communication with a remote digitizing display

An architecture is disclosed for improved tilt detection for a radiative pickup stylus. The architecture includes a contact sensing mechanism and a coaxial conductor assembly which provides both position measurement and tilt sensing measurement features. By using the contact sensor to establish a calibration for the position and tilt detection elements, an improved, highly accurate position and tilt sensing capability is provided for radiative pickup stylus for a digitizing display.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention disclosed broadly relates to data processing systems and more 
particularly relates to improvements in input/output devices for pen-based 
computer systems. 
2. Related Patents and Patent Applications 
The following patents and patent applications are assigned to the IBM 
Corporation and are incorporated herein by reference. 
U.S. Pat. No. 4,686,332 entitled "Combined Finger Touch and Stylus 
Detection System for Use on the Viewing Surface on a Visual Display 
Device," by Evon C. Greanias, et al. 
U.S. Pat. No. 5,007,085 entitled "Remotely Sensed Personal Stylus," by Evon 
C. Greanias, et al. 
U.S. Pat. No. 5,117,071 entitled "Stylus Sensing System," by Evon C. 
Greanias, et al. 
Copending U.S. patent application Ser. No. 07/778,431, filed Oct. 16, 1991, 
entitled "Touch Overlay for Improved Touch Sensitivity," by Evon C. 
Greanias, et al. 
Copending U.S. patent application Ser. No. 07/351,227, filed May 15, 1989, 
entitled "Flat Touch Screen Workpad for a Data Processing System," by 
Gordon W. Arbeitman, et al. 
Copending U.S. patent application Ser. No. 07/969,864, filed Oct. 30, 1992, 
entitled "Architecture for Communication of Remote Devices to a Digitizing 
Display," by Guy F. Verrier, et al. 
3. Background Art 
Pen-based computer systems have been described in the prior art, for 
example in the above referenced U.S. Pat. No. 4,686,332 by Greanias, et 
al. In that system, a touch overlay membrane is positioned over the 
viewing surface of a display device such as a computer monitor, for 
interacting with a pickup stylus. The pickup stylus described in the '332 
patent is connected by means of a wire to the pen-based computer system. 
The pen-based computer system generates a radiative signal in the touch 
overlay. The radiative signal is picked up by the stylus and sent back 
over the wire to the pen-based computer. The computer then calculates the 
relative X-Y position of the stylus with respect to the overlay. The 
relative proximity Z of the stylus to the overlay is determined by the 
signal amplitude picked up by the stylus from the electromagnetic energy 
radiated from the overlay. An improvement in the stylus pickup antenna is 
described in the above cited U.S. Pat. No. 5,117,071 by Greanias, et l. In 
the '071 patent, the antenna configuration in the tip of the stylus is a 
small sphere, whose shape enables a uniform signal strength to be picked 
up without regard for the relative orientation of the stylus with respect 
to the planar surface of the overlay. A further improvement has been made 
to the stylus by eliminating the cable connection between the stylus and 
the pen-based computer system, as is described in U.S. Pat. No. 5,007,085 
by Greanias, et al. In the '085 patent, the signal detected by the stylus 
antenna which has been picked up from the electromagnetic radiation 
emitted by the overlay, is converted into a second signal which is 
transmitted either by infrared radiation, microwave radiation or radio 
frequency radiation at a different frequency, from the stylus back to an 
electromagnetic detector coupled to the pen-based computer system. Another 
improvement to pen-based computer systems is described in the above 
referenced copending U.S. patent application Ser. No. 07/351,227, filed 
May 15, 1989 by Arbeitman, et al. In the Arbeitman, et al. patent 
application, a flat touch screen workpad is substituted for the 
combination of the computer display monitor and touch overlay membrane. In 
the Arbeitman, et al. patent application, the radiative pickup stylus is 
connected by means of a wire to the flat touch screen workpad, for 
receiving the electromagnetic radiation emanated from the overlay 
membrane, the wire transferring the detected signal from the pickup stylus 
back to the electronics contained in the flat touch screen workpad. 
In the description of the invention contained herein, the term "digitizing 
display" will be used to generally refer to either the combination of a 
computer display monitor and touch overlay membrane described in the '085 
patent or alternately, the workpad display and overlay described in the 
Arbeitman, et al. patent application. 
The above cited U.S. Pat. No. 5,007,085 entitled "Remotely Sensed Personal 
Stylus," by Evon C. Greanias, et al., describes a pen-based computer 
system which has a digitizing display which radiates electromagnetic 
signals which are picked up by a remotely sensing personal stylus. The 
antenna in the tip of the stylus picks up the electromagnetic signals 
radiated from the display overlay, and converts those signals into a 
suitable form for retransmission back to the pen-based computer system. 
The relative signal strength of the signals picked up by the stylus is 
used to infer the relative separation distance Z of the tip of the stylus 
with respect to the overlay. The converted signals are transmitted back to 
the pen-based computer system by means of a transmitting circuit and 
transmitting antenna within the stylus. The technique for determining 
contact between the stylus tip and the surface of the overlay, is by 
measuring the relative signal strength of the electromagnetic signal 
picked up by the tip of the stylus. 
A problem with this prior art is that the detection of actual touchdown of 
the stylus onto the workpad surface is not very sensitive. Consider in 
normal handwriting how the writer's hand pressure varies during the course 
of writing a signature. The pressure applied by the pen to the paper 
varies over the course of inscribing the signature on the paper and 
sometimes includes a slight raising of the pen from the surface of the 
paper. The process of writing a signature on a digitizing display by means 
of a radiative pickup stylus, does not faithfully reproduce the intended 
shape on the display screen. When the stylus is raised slightly from the 
surface of the digitizing display, the separation of the stylus tip from 
the surface will not be detected in prior art systems. What results is 
spurious traces and artifacts which remain in the "inked" depiction of the 
user's signature because the system cannot detect the actual small 
separation of the stylus from the surface of the digitizing display. This 
problem has been solved by the above cited copending application Ser. No. 
07/969,864 by Guy Verrier, et al. 
Prior art stylus configurations are suitable for determining the linear 
displacement between the tip and the digitizing display surface, however 
for those applications such as signature validation, calligraphy such as 
Kanji Japanese character writing, and other applications where the angle 
of the stylus becomes important, prior art stylus structures are not 
adequate to provide the tilt information necessary. 
OBJECTS OF THE INVENTION 
It is therefore an object of the invention to improve the accuracy of 
representing handwritten characters and script in a pen-based computer 
system. 
It is another object of the invention to more accurately detect the tilt 
orientation of a stylus with respect to the surface of a workpad in a 
pen-based computer systems. 
It is a further object of the invention to provide a data processing 
architecture for transferring digital information between a stylus and a 
pen-based computer system. 
It is yet another object of the invention to provide a data processing 
system for communicating contact and tilt information between a stylus and 
a pen-based computer system. 
It is still another object of the invention to provide an improved 
technique for determining the tilt orientation of a stylus with respect to 
the plane of the digitizing display. 
SUMMARY OF THE INVENTION 
These and other objects, features and advantages are accomplished by the 
stylus tilt detection apparatus for communication with the remote 
digitizing display. A radiative pickup stylus for a digitizing display is 
disclosed, having a tilt orientation sensitivity. The assembly includes a 
stylus housing having a hollow tip portion for mechanically supporting a 
coaxial conductor assembly. A coaxial conductor assembly is mounted in the 
tip of the housing, having a central conductor surrounded by an insulating 
sleeve with an external cylindrical surface. The central conductor has an 
end extending axially beyond a terminating surface of the sleeve. The 
coaxial conductor assembly includes a first hollow, cylindrical conductor 
portion supported on the external surface of the insulating sleeve and 
electrically connected to a reference potential. The coaxial conductor 
assembly further includes a second hollow, cylindrical conductor portion 
supported on the external surface of the insulating sleeve and in spaced 
relation with the first hollow, cylindrical conductor. The second hollow, 
cylindrical conductor is nearer to the terminating surface of the sleeve 
than is the first hollow, cylindrical conductor. The coaxial conductor 
assembly further includes a spherical terminating electrode mounted to the 
end of the central conductor and in spaced relation to the second hollow, 
cylindrical conductor. 
In a first detection state, the second hollow, cylindrical conductor is 
selectively connected to the reference potential and the spherical 
terminal electrode is correspondingly connected to a radiative field 
detector, to measure a first distance of the spherical terminal electrode 
from the plane of the digitizing display while it radiates an 
electromagnetic signal. In a second detection state, the second hollow, 
cylindrical conductor is connected to the radiative field detector and the 
spherical terminal electrode is connected to the reference potential, to 
measure a tilt orientation value of the coaxial conductor assembly with 
respect to the plane of the digitizing display. In this manner, a tilt 
orientation for the radiative pickup stylus can be measured in an improved 
manner. 
Further in accordance with the invention, the coaxial conductor assembly is 
mounted within a contact detection mechanism which uses a pressure 
transducer to detect an unambiguous contact between the stylus and the 
plane of the digitizing display. Calibration adjustments for the relative 
signal strength detected by the spherical terminal electrode and the 
second hollow, cylindrical conductor when the mechanical contact is 
detected by the contact detection mechanism, provide accurate tip location 
and tilt orientation measurements for the stylus at other locations with 
respect to the plane of the digitizing display.

DISCUSSION OF THE PREFERRED EMBODIMENT 
FIG. 1 is an overall architectural diagram of the architecture for 
communications of remote devices to a digitizing display. The digitizing 
display which is described herein is described in greater detail in the 
commonly assigned U.S. Pat. No. 5,007,085 by Greanias entitled "Remotely 
Sensed Personal Stylus" and U.S. Pat. No. 4,764,885 by Greanias entitled 
"Proximity Sensing Blinking Cursor Feature " and U.S. Pat. No. 4,686,332 
by Greanias entitled "Thin Plastic Overlay for Finger and Stylus Display 
Input." 
FIG. 1B shows the pen-based computer system that includes the personal 
computer 74, the digitizing display or workpad tablet 22 and the stylus 
20. The personal computer 74 includes a memory coupled by a system bus to 
a central processing unit (CPU), a keyboard, a display monitor, a disk 
drive bulk storage, a local area network (LAN) adapter and the tablet 22. 
The memory of the personal computer 74 stores an operating system program, 
device driver programs and application programs which are sequences of 
executable instructions that are executed in the CPU. The LAN adapter of 
the personal computer 74 connects to a local area network (LAN) that can 
connect the personal computer 74 to other computers and other networks. 
The stylus 20 communicates with the tablet 22 by means of an 
electromagnetic link that can be frequency modulated radio signals, 
amplitude modulated radio, or modulated optical or infrared radiation 
signals. 
The architecture shown in FIG. 1 is divided in two principal parts. The 
first part is the stylus 20 and the second part is the tablet or 
digitizing display 22. The stylus 20 includes a first branch which can be 
for example branch 24 which is a mechanical contact detecting branch. The 
second branch can be for example branch 26 which is a position detecting 
branch. 
The contact detecting branch 24 can include a pressure detector 38, which 
is shown in greater detail in FIGS. 2A and 2B. Connected to the pressure 
detector 38 is a signal amplifier 40 which has its output connected to the 
pressure detector circuitry 42. The output of pressure detector circuitry 
42 goes to the analog-to-digital converter 44 which outputs a digital 
number representing the pressure applied by the pressure detecting 
mechanism 38 to the front surface of the electrostatic tablet 54 in FIG. 
1. The output of the analog-to-digital converter 44 is then applied to a 
first input of the multiplexer 36. 
FIGS. 2A and 2B show the stylus 20 and in particular, show the details of 
the mechanical contacting mechanism 38. A pressure transducer 10 includes 
a layer formed of a force sensitive resistant (FSR) transducer material, 
for example, as manufactured by Interlink Electronics, Santa Barbara, 
Calif. Such material changes its resistance when compressed by the 
application of a force on its surface. Electrically conductive electrodes 
on the printed circuit board 11 contact separated portions of the surface 
of the FSR transducer layer 10 such that a complete circuit is formed 
between the conductors on the printed circuit board 11, by way of the FSR 
transducer layer 10. The electrodes on the printed circuit board 11 are 
pressed against the transducer 10 to complete the electrical circuit. 
When the operational amplifier 40 shown in FIG. 1 is turned on but the 
stylus 20 is not yet in use, a voltage will be applied across the FSR 
transducer 10 by way of the stylus terminals and conductive electrodes 11. 
Then, when the tip 4 of the stylus 20 is pressed against the tablet 
surface 54, the FSR transducer layer 10 is compressed between the surfaces 
of two opposing parts. The first part is displaceable as the stylus tip 4, 
which is slightly displaced when pressure is exerted on the stylus tip 4. 
The other part of the compression mechanism is stationary and is the 
printed circuit board 11 which is held in place by the housing 2 shown in 
FIG. 2. When the FSR transducer layer 10 is compressed, its electrical 
resistance changes such that a current and/or voltage change is produced 
at the output connected to the operational amplifier 40. This change is 
used to trigger the pressure detector 42, to acquire the pressure sensing 
data. Also included within the hollow tip 4 of the stylus 20 of FIG. 2, is 
the pickup antenna 29 shown in FIG. 1, which is radiatively coupled to the 
radiating electrodes 56 and 58 of the tablet 54. 
In the position detecting branch 26 is the antenna device 29 described in 
U.S. Pat. No. 5,117,071 cited above, which picks up electromagnetic 
signals radiated from the conductors 56 and 58 in the electrostatic tablet 
54. The output of the antenna 29 is coupled through the amplifier 30 to 
the signal strength detector 32. The output of the signal strength 
detector 32 is then applied to the analog-to-digital converter 34. The 
output to the analog-to-digital converter 34 is a digital representation 
of the signal strength detected by the antenna 29 for the signals radiated 
from the conductors 56 and 58 in the electrostatic tablet 54. The signals 
radiated from the electrostatic tablet 54 represent positional information 
when detected by the antenna 29, as described in U.S. Pat. No. 5,686,332, 
cited above. 
The output of the signal strength detector 32 is applied to the 
analog-digital converter 34, which in turn outputs a digital 
representation of the relative position of the stylus 20 over the 
electrostatic tablet 54, as a number to a second input to the multiplexer 
36. 
The multiplexer 36 is shown as multiplexer 36' in the schematic diagram in 
FIG. 3. There it is seen that the multiplexer 36 or 36' generates a 
time-divided sequence of numbers in data frames as shown in FIG. 4, which 
are applied to the transmitter 46. The multiplexer can be controlled to 
change the order and content of the data stream. Returning to FIG. 1, the 
multiplexed data stream of numbers alternately output from the pressure 
detector A-to-D converter 44 and the position detecting A-to-D converter 
34 are applied to the frequency shift key (FSK) transmitter 46. The output 
of the transmitter 46 is then applied to the antenna 48 in the stylus 20. 
The stylus 20 then radiates the information which includes the applied 
pressure on the pressure detecting mechanism 38 and the X-Y position 
information from the antenna device 29. This data stream is radiated from 
the antenna 48 and is detected, in accordance with the invention, by the 
embedded antenna 62 in the tablet or digitizing display 22. 
FIGS. 2A and 2B also show the hollow, cylindrical conductor 86 which is 
shown in spaced relation to the spherical terminal electrode 29 inside the 
tip housing 4. The cylindrical conductor 86 is shown electrically 
connected to the tip switch 88 which also receives an electrical 
connection from the spherical electrode tip 29. The output of the tip 
switch 88 is then connected to the input of the amplifier 30. In 
accordance with the invention, selected combinations of the radiative 
detection by the spherical tip 29 and the cylindrical conductor 86, as 
switched through the tip switch 88, provide both linear displacement 
measurement and relative tilt orientation measurement for the stylus 20 
with respect to the plane of the digitizing display 54. 
FIG. 3 shows the multiplexer 36', which can replace the multiplexer 36 in 
FIG. 1. The multiplexer 36' in FIG. 3 has four input devices connected to 
it, the pressure detector 42, the signal strength detector 32, the 
personal identification number storage 104, and the RAM 106. The pressure 
detector 42 transfers the applied pressure signal input from the tip 4 of 
stylus 20 to the analog-to-digital converter 44, which outputs a digital 
representation of the applied pressure on input A to the multiplexer 36'. 
The signal strength detector 32 receives X-Y related position information 
and Z separation information from the antenna 29 and applies it to the 
analog-to-digital converter 34. The converter 34 outputs a digital 
representation of the X-Y position and Z separation over the input B to 
the multiplexer 36'. The signal strength detector 32 receives its signal 
from the amplifier 30, which in turn receives its signal from the output 
of the tip switch 88. The tip switch 88 is connected both to the antenna 
spherical electrode 29 and also to the cylindrical conductor 86 as shown 
in FIG. 8. The signal strength detector of FIG. 3 receives the X-Y related 
position information and Z separation information from the cylindrical 
conductor 86 and applies it to the analog-to-digital converter 34. The 
converter 34 outputs a digital representation of the X-Y position and Z 
separation for the cylindrical conductor 86 with respect to the plane of 
the digitizing display 54, over the input B to the multiplexer 36'. 
Whether the cylindrical conductor 36 or alternately the spherical terminal 
29 are connected to the amplifier 30, is a function of the detection state 
determined by the switch 88. The output J from the multiplexer 36' to the 
tip switch 88, provides the selective switching between connecting the 
spherical terminal element 29 or alternately the cylindrical conductor 
element 86 to the amplifier 30. The personal identification number storage 
104 can be a read-only storage or a writable EPROM which outputs an 
identifying digital number on the input C to the multiplexer 36'. The RAM 
106 can store information which is output as digital information to the 
input D to the multiplexer 36'. 
FIG. 4 shows a waveform diagram of the multiplexed data stream output from 
the multiplexer 36' of FIG. 3 to the transmitter 46 for transmission over 
the antenna 48 in the stylus 20. The waveform diagram of FIG. 4 shows the 
multiplexed outputs A1, B, C1 and D1 during a first time frame, which are 
respectively output over the lines A, B, C and D to the multiplexer 36' of 
FIG. 3. The waveform diagram of FIG. 4 also shows a second time frame 
during which a similar sequence of data words A2, B*, C2 and D2 are 
respectively output on the inputs A, B, C and D in FIG. 3. 
The digitizing display 22 can be, for example, a transparent overlay 
superimposed on a conventional display monitor or a workpad coupled to a 
pen-based computer system. Included in the overlay, for example, can be 
the embedded antenna 62. Alternately, the antenna 62 can be proximate to 
the transparent overlay, but sufficiently close to enable detection of the 
multiplexed data stream transmitted from the antenna 48 of the stylus 20. 
The stylus 20 transmits by means of the antenna 48 information from the 
pressure detector 38 and the X-Y detector 29 to the antenna 62 in the 
digitizing display 22. Antenna 62 in the digitizing display 22, is coupled 
through the amplifier 64 to the FSK receiver 66. The output of the 
receiver 66 is then applied to a microprocessor 68. The microprocessor 68 
can, for example, strip out the respective numbers representing the 
contact pressure detected by the detector 38 in the stylus 20 and 
separately, the number representing the X-Y position location of the 
stylus with respect to the electrostatic tablet 54. The numerical values 
can then be forwarded by the microprocessor 68 to the pen-based computing 
system 74. 
Further, the stylus 20 can include a transmit/receive switch 50 which 
alternately connects the antenna 48 to either the FSK transmitter 46 or 
alternately to the FSK receiver 52 in the stylus 20. If the receiver 52 is 
connected to the antenna 48, the the stylus 20 can receive information in 
the form of electromagnetic signals radiated from the antenna 62 in the 
digitizing display 22. In this mode of operation, the FSK transmitter 70 
transfers digital information from the microprocessor 68 to the amplifier 
72, which applies the digital information over the antenna 62 of the 
digitizing display 22. The radiated electromagnetic signals from the 
antenna 62 containing the digital information from the microprocessor 68, 
is radiated to the antenna 48 in the stylus 20. The antenna 48 is then 
connected through the transmit/receive switch 50 to the FSK receiver 52, 
which applies the received digital information from the digitizing display 
22 to the multiplexer 36. The multiplexer 36 will then apply the digital 
information to for example a storage device 76 in the stylus 20 or the RAM 
106 in FIG. 3. In this manner, digital information can be transmitted from 
the tablet or digitizing display 22 and received and stored in the stylus 
20. 
The side cross-sectional view of the stylus 20 shown in FIG. 2A and in FIG. 
2B provides a detailed illustration of the mechanical parts of the 
pressure contacting portion 38. The stylus 20 is designed to precisely 
determine when the tip 4 is in mechanical contact with the electrostatic 
tablet 54 and when it is not in mechanical contact. However, the tip 
contacting portion 38 can also output several intermediate states of 
pressure. For example, the contact portion 38 of the stylus 20 can output 
16 levels represented by four binary bits of pressure. The 16 levels of 
pressure can be used for applications such as shading of drawings, moving 
a cursor fast or slow, drawing heavy or light lines, and other drawing 
applications. The construction of the stylus 20 shown in FIG. 2A and FIG. 
2B enables very small longitudinal displacement of the probe tip 4 within 
the housing 2 to detect a range of force from 30 grams to 300 grams. The 
stylus 20 can use a transducing material such as the force sensing 
resistor made by Interlink, described above. 
The design of the contact portion 38 shown in FIG. 2A eliminates friction 
which would prevent the detection of low pressure. It also allows the 
stylus to relax back to its non-contact state, to avoid confusing readings 
when detecting the tip 4 is just coming off the surface of the display 54. 
The design shown in FIG. 2A also provides a very low pre-load pressure on 
the force sensing resistor 10 so that the dynamic range of the force 
sensing resistor 10 is maximized. In addition, the design of the contact 
portion 38 in FIG. 2A minimizes the detection of lateral rather than axial 
or longitudinal forces. 
The tip 4 of FIG. 2A floats freely between the cone 3 and a brass shaft 
which holds the ball of the electrostatic sensor 29, as described in the 
above referenced U.S. Pat. No. 5,117,071. Bushings 8 and 9 are made of 
tetrafluoroethylene to minimize friction. 
The gasket 6 is made of very low durometer rubber to establish a minimum 
amount of pre-load pressure. Without some pre-load pressure, the tip 4 
will move around inside the cone 3 and create false readings as the stylus 
20 is moved, rotated, shaken or tilted. However, if the gasket 6 is too 
stiff, this will reduce the sensitivity of the stylus to the detection of 
low forces and reduce the dynamic range of the stylus. 
The tip 4 detects axial forces, along the line of the axis of the housing 
2. The pen tip 4 contains the electrostatic sensing element 29 used for 
X-Y position determination, as is described in U.S. Pat. No. 5,117,071 and 
shown in FIG. 2B. The force sensing resistor 11 is the type used in 
membrane switch devices. The force sensing resistor 11 meets with a small 
circular printed circuit board 10 which is used to make appropriate 
contact with the force sensing resistor 11. 
The force sensing resistor 11 generates a variable resistance which depends 
on the pressure applied to it by the tip. The force sensing resistor 11 
can be biased by a reference voltage source for example 1.4 volts, so that 
a differential voltage across the device can be amplified by the 
operational amplifier 40. The output voltage can be adjusted to match the 
output voltage for the range of X-Y positioning of the amplifier 30 for 
the electrostatic pickup device 29. In this way, a common 
analog-to-digital conversion circuit could be used to digitize both the 
pressure output by the pressure detector 42 and the signal strength output 
by the signal strength detector 32 in FIG. 1. In such an alternate 
embodiment, appropriate switching between the pressure detector 42 and the 
signal detector 32 with the single analog-to-digital converter could be 
accomplished to perform an analog multiplexing of the analog signal input 
to the common analog-to-digital converter. Item 8 in FIG. 2A is a bushing 
to maintain the tip 4 in concentric alignment with the cone 3 attached to 
the tubing 2 of the stylus 20. The separator 5 shown in FIG. 2A can be 
used to separate the pressure detecting electronics 42 from the signal 
strength detecting electronics 32 in the stylus 20. The end 7 opposite 
from the tip 4 for the stylus 20 shown in FIG. 2, can optionally hold an 
optical input for optical character recognition or an optical input for 
bar code detection. Alternately, the end 7 can hold a liquid crystal 
display which will present a visual output to the user. 
In another embodiment of the invention, the storage device 76 can be a 
read-only storage such as 104 in FIG. 3, which stores a personal 
identification number or a password or other security information, which 
can be transmitted from the stylus 20 to the digitizing display 22, for 
processing in the pen-based computer 74, to validate the stylus or the 
user of the stylus. 
FIG. 5 shows a more detailed view of the workpad embodiment of the tablet 
or digitizing display 22 and the associated pickup stylus 20. FIG. 6 is a 
side view of the workpad 22. FIG. 5 and FIG. 6 show the relative location 
of the embedded antenna 62 within the housing of the workpad 22. It can be 
seen how the electrostatic tablet 54 with its horizontal and vertical 
conductors 56 and 58 are positioned in relation to the antenna 62. The 
wire 74 connects the workpad 22 to the pen-based computing system. 
FIG. 7 shows a side cross-sectional view of the tip 38 for the stylus 20, 
showing in particular the construction of the coaxial conductor assembly. 
FIG. 8 shows a side view of the coaxial conductor assembly, and its 
electrical connection to the tip switch 88. The radiative pickup stylus 20 
for the digitizing display 54, has a tilt orientation sensitivity feature, 
in accordance with the invention. The stylus housing 20 has a hollow tip 
portion 38 for mechanically supporting the coaxial conductor assembly 75 
shown in FIG. 7. 
The coaxial conductor assembly 75 mounted in the tip 38 of the housing 20, 
has a central conductor 80 surrounded by an insulating sleeve 82 with an 
external cylindrical surface. The central conductor 80 has an end 
extending axially beyond a terminating surface 85 of the sleeve 82. The 
coaxial conductor assembly 75 includes a first hollow, cylindrical 
conductor 84 which is supported on the external surface of the insulating 
sleeve 82 and which is electrically connected to a reference potential 
such as ground potential, as is shown in FIG. 8. The coaxial conductor 
assembly 75 further includes a second hollow, cylindrical conductor 86 
which is supported on the external surface of the insulating sleeve 82 as 
is shown in FIG. 7, in spaced relation, to the first hollow, cylindrical 
conductor 84. The space between the two conductors is shown as G in FIG. 7 
and can have a value of 0.005 inches for example. The diameter of the 
cylindrical insulating sleeve can be for example 0.062 inches and the wall 
thickness of the coaxial conductor 84 and the coaxial conductor 86 can be 
for example 0.006 inches. 
The second hollow, cylindrical conductor 86 is positioned nearer to the 
terminating surface 85 of the sleeve 82 than is the first hollow, 
cylindrical conductor 84. The coaxial conductor assembly 75 includes the 
spherical terminal electrode 29 mounted to the end of the central 
conductor 80 and in space relation to the second hollow, cylindrical 
conductor 86. The distance L from the proximate end of the second 
conductor 86 to the nearest portion of the spherical surface of the 
electrode 29 can be for example 0.045 inches. The axial length of the 
second hollow, cylindrical conductor 86 can be for example 0.050 inches. 
The radius of the sphere for the spherical terminal electrode 29 can be 
0.031 inches for example. The separation distance W between adjacent 
electrodes 56 in the tablet or digitizing display is 0.125 inches, in this 
example. 
In accordance with the invention, two measurement states can be provided 
for the coaxial conductor assembly 75. The first measurement state enables 
the spherical terminal electrode to detect the electromagnetic signal 
radiated from the digitizing display 54 to establish a first location for 
the tip 38. In the second state, the second hollow, cylindrical conductor 
86 will be used to measure the relative distance from the plane of the 
digitizing display 54, to provide a measure of the tilt orientation for 
the stylus 20. 
In accordance with the invention, in the first detection state, with the 
tip switch 88 shown in FIG. 8, the switch electrode 90 for the cylindrical 
electrode 86 is connected to ground potential 94 while the spherical 
terminal electrode 29 is connected through switch element 92 to the 
detection amplifier 30 terminal 96. The spherical electrode 29 thus serves 
as the pickup antenna for the electromagnetic signal radiated from the 
tablet or digitizing display 54. In this first detection state, the second 
hollow, cylindrical conductor 86 is connected to the reference potential 
of ground and the spherical terminal electrode 29 is connected to the 
radiative field detector 30, to measure a first distance H of the 
spherical terminal electrode from the plane of the digitizing display 54 
radiating an electromagnetic signal. 
Further in accordance with the invention, in the second detection state, 
the spherical terminal electrode 29 is connected to ground potential 
through the switch element 92 to the terminal 98 as shown in FIG. 8, and 
correspondingly the cylindrical conductor 86 is connected through the 
switch element 90 to the terminal 96 which connects to the detection 
amplifier 30. In this manner, the centroid of the cylindrical conductor 86 
serves as the charge center for the location which is attributed to the 
electrode 86, with respect to the electromagnetic signal being radiated 
from the digitizing display 54. In this second detection state, the second 
hollow, cylindrical conductor 86 is connected to the radiative field 
detector 30 and the spherical terminal electrode 29 is connected to the 
reference potential of ground, to measure the tilt orientation value of 
the coaxial conductor assembly with respect to the plane of the digitizing 
display 54. In this manner, an improved technique is provided for 
measuring the tilt orientation of the stylus 20 with respect to the plane 
of digitizing display 54. 
By calibrating the coaxial conductor assembly with the mechanical contact 
information provided by the pressure detector 42, an accurate position 
coordinate and tilt orientation value can be measured for the stylus at 
other locations and other orientations relative to the plane of the 
digitizing display 
FIG. 2A shows a cross-sectional view of the stylus 20 and in particular 
shows the tip 38 which is placed in proximity to the digitizing display 
54. FIG. 2B is a more detailed illustration of the assembly in FIG. 2A, 
and shows in particular the combination of the spherical antenna 29 
connected to the switch 88 and the cylindrical antenna 86 also connected 
to the switch 88. As can be seen, the output of the switch 88 is connected 
to the input of the amplifier 30. 
FIG. 4 is a timing diagram showing the multiplexed sequence of information 
packets A1, B, C1 and D1 which are transmitted during a first time frame 
before the time T1. The packet B comes from the spherical antenna 29 by 
virtue of a prior setting of the tip switch 88. At the time T1, the signal 
J is received on the receiver 52 and is passed through the multiplexer 36' 
to the tip switch 88, switching its state so that the cylindrical antenna 
86 is now connected. Following the time T2 in FIG. 4, a second frame of 
digital packets are transmitted by the transmitter 46 in the sequence of 
A2, B*, C2 and D2. B* is the measurement information received from the 
cylindrical antenna 86 through the tip switch 88. 
FIG. 7 shows the digitizing display 54 in cross-sectional view and it can 
be seen that there are five radiative lines 56 embedded in the digitizing 
display 54, in particular lines L1, L2, L3, L4 and L5. The mutual 
separation distance between adjacent lines L4 and L5, for example, is the 
magnitude W. In the example lines 56 shown herein, the magnitude W is 
0.125 inches and the frequency of their radiation in 40K Hertz. 
For optimum results, the spherical terminal 29 has a diameter less than the 
magnitude W. The diameter of the cylindrical conductor 86 is also less 
than the magnitude W, for optimum results. 
FIG. 9A shows the orientation of the stylus 20 during its normal use, at a 
tilt angle. The tilt angle is defined as the angle between the axis of the 
cylindrical housing 20 and the plane of the digitizing display 54, in a 
plane which is perpendicular to the plane of the digitizing display 54 and 
through which passes the axis of the cylindrical housing 20. FIG. 9A also 
shows a typical orientation of the stylus 20 during a first phase of 
calibration as will be described in connection with FIG. 10A. 
FIG. 9B is a side cross-sectional view of the stylus 20 shown laying down 
on its side on the surface of the digitizing display 54, so that the 
cylindrical axis of the stylus 20 is substantially parallel with the plane 
of the digitizing display 54. This orientation corresponds to the second 
phase of calibration as will be described in connection with FIG. 10A. 
FIG. 9C shows an alternate arrangement for establishing calibration values 
for the stylus and in particular shows applying the tip 4 of the end 38 
against a vertical pillar 55 mounted perpendicularly to the plane of the 
digitizing display 54. 
FIG. 10A is a flow diagram of a sequence of operational steps which can be 
carried out in a computer program running in the microprocessor 68 of FIG. 
1 or in the pen-based personal computer 74 of FIG. 1B connected to the 
microprocessor 68. FIG. 10A is a calibration method for the stylus 20, 
enabling the calibration of both positional coordinates X0,Y0,Z0 and the 
tilt angle T with respect to the plane of the digitizing display 54. 
The calibration method in the flow diagram of FIG. 10A begins with step 
200, which begins the calibration of the sphere 29. Step 202 has the user 
input an alert signal, perhaps from the keyboard of the personal computer 
74, which will initialize the phase 1 of calibration. Then step 204 has 
the user touch down the tip 4 on the digitizing display 54 such as is 
shown in FIG. 9A, at a predetermined location on the digitizing display 
which has the coordinates X0,Y0,Z0. 
Since the tip 4 is in mechanical contact with the digitizing display 54, 
the pressure detector 42 and A-to-D converter 44 output a signal A to the 
multiplexer 36' which transmits it over the transmitter 46 and the antenna 
48 to the data antenna 62 of the electrostatic tablet 22. Step 206 
receives the A output from the pressure transducer 10. Then in response, 
step 207 sends the J signal to the switch 88 by virtue of transmission 
from the data antenna 62 and the tablet 22 to the antenna 48 and the 
receiver 52 in the stylus 20. The J signal passes through the multiplexer 
36' to the tip switch 88 and sets it so as to connect the sphere 29 to the 
amplifier 30. 
Then step 208 in FIG. 10A records the B output from the sphere 29 which is 
a measured location X1,Y1,Z1 for the centroid of the sphere 29 as is shown 
in FIG. 9A. This information is transmitted via the antenna 48 to the data 
antenna 62 of the tablet 22 and is received in the microprocessor 68 and 
can be passed on to the pen-based computing system 74. Then in step 210, a 
computation is made of the location correction for the sphere 29. The 
value delta X1 is the error in the location along the X coordinate, the 
value delta Y1 is the error in the location along the Y coordinate and the 
value delta Z1 is the error in the location along the Z coordinate for the 
spherical antenna 29 with respect to the predetermined location X0,Y0,Z0. 
Delta X1=X1-X0 
Delta Y1=Y1-Y0 
Delta Z1=Z1-Z0 
Then in step 212, the user lays the stylus 20 on its side on the upper 
surface of the display 54, as is shown in FIG. 9B. This will position the 
cylindrical axis for the cylindrical housing of the stylus 20 parallel 
with the plane of the digitizing display 54. Then in step 214, the user 
inputs a second alert signal, for example through the keyboard of the 
personal computer 74, and this signifies the beginning of phase 2 for the 
calibration. Then in step 215 of FIG. 10A, after the user inputs the alert 
signal in step 214, the J signal is transmitted from the data antenna of 
62 of the tablet 22 to the stylus 20 to pass through the multiplexer 36' 
to the tip switch 88, switching the tip switch into the first state where 
it connects the sphere 29 to the amplifier 30. This step is performed to 
insure that the appropriate connection from the sphere 29 to the amplifier 
30 is maintained for the beginning of the phase 2 calibration sequence. 
Then, in step 216, the B signal output from the sphere 29 is recorded, for 
the measured location X2,Y2,Z2 of the centroid of the sphere 29. 
Then in step 218, the antenna 62 of the tablet 22 transmits the J signal to 
the tip switch 88 to change the state of the switch 88 into the second 
state, to connect the cylinder 86 to the amplifier 30. Then in step 220, 
the data packet B* is output from the cylindrical antenna 86 and is 
transmitted through the amplifier 30 and A-to-D converter 34 and through 
the multiplexer 36' and the transmitter 46 to the data antenna 62 of the 
tablet 22. The output B* is recorded for the cylindrical antenna 86 at a 
measured location X3,Y3,Z3 of the centroid of the cylinder 86. 
Then, in step 222, the microprocessor 68 or the personal computer 74 
connected thereto, computes the distance delta D from the charge center of 
the cylinder 86, which is typically its centroid, to the charge center of 
the sphere 29, which is typically its centroid. This distance delta D is 
computed as the following expression. 
EQU Delta D=[(X3-X2)**2+(Y3-Y2)**2]**(1/2)] 
Then the calibration program of FIG. 10A flows to step 224 which returns to 
the main program running in either the microprocessor 68 or in the 
pen-based personal computer 74 connected thereto. 
During normal operation of the stylus 20 in conjunction with the digitizing 
display 54, when it is desired to measure both the location of the tip 4 
and the relative tilt orientation of the stylus 20 with respect to the 
plane of the digitizing display 54, the sequence of operational steps 
shown in FIG. 10B is carried out. These steps are embodied in a sequence 
of instructions in a computer program residing in either the 
microprocessor 68 or alternately in the pen-based personal computer 74 
connected thereto. 
The flow diagram of. FIG. 10B begins with step 230 which begins the normal 
stylus use with the tilt feature. Then in step 232, the user touches down 
the tip 4 on the surface of the display 54, as is shown in FIG. 9A. Then 
in step 234, the A signal is received from the pressure detector 42 for 
the pressure transducer 10. Then in step 234, in response to receiving the 
A signal from a touch down of the tip 4, a J signal is sent from the 
tablet 22 to the stylus 20 to switch the state of the switch 88 to 
connected the spherical antenna 29 to the amplifier 30 for a tip location 
measurement. Then in step 236, the B data packet is output from the 
spherical antenna 29 and the coordinates X4,Y4,Z4 are recorded for the 
measured location of the centroid of the spherical antenna 29. 
In response to this, step 238 sends the J signal from the tablet 22 to the 
stylus 20 to switch the state of the tip switch 88 in order to connect the 
cylindrical antenna 86 to the amplifier 30. Then in step 240, the B* 
output from the cylindrical antenna 86 is received from the stylus 20 by 
the tablet 22, and the coordinates for the location of the centroid of the 
cylinder 86 of X5,Y5,Z5 are recorded. 
Then, step 242 of the flow diagram of FIG. 10B performs the computation of 
the tip location for the tip 4. The tip location coordinates are X6,Y6,Z6. 
X6=X4-delta X1 
Y6=Y4-delta Y1 
Z6=Z4-delta Z1 
This computation for the location of the tip 4 of the stylus 20 has been 
corrected for the offset of the centroid of the spherical antenna 29 with 
respect to the contact point for the tip, as determined by the calibration 
steps carried out in the flow diagram of FIG. 10A. 
Then in step 244 of FIG. 10B, the tilt angle T is computed for the 
orientation of the cylindrical axis of the stylus 20 with respect to the 
plane of the digitizing display 54. The tilt angle T is computed from the 
horizontal projection of the cylindrical axis for the stylus onto the 
plane of the digitizing display 54. The horizontal projection has a 
magnitude of delta H. 
EQU Delta H=[(X5-X4)**2+(Y5-Y4)**2]**(1/2)] 
Then the tilt angle T is computed as the arc cosine of the projection of 
the cylindrical axis onto the plane of the digitizing display, taken with 
respect to the value delta D for the computed distance between the 
centroid of the cylinder and the centroid of the sphere as was computed in 
the calibration flow diagram of FIG. 10A. The computation for the tilt 
angle T is: 
EQU T=ARCCOS(delta H/delta D) 
These computations for the coordinates of the touch down point of the tip 4 
on the surface of the digitizing display 54 and of X6,Y6,Z6, and the 
computation for the tilt angle T of the cylindrical axis for the stylus 
with respect to the plane of the digitizing display, are then utilized by 
the application program in the pen-based computer 74 for its intended 
purpose. Example applications can be for signature verification, the 
inscription of calligraphy such as Kanji painted characters, and other 
applications where the tilt orientation of the stylus is meaningful for a 
particular application. 
Although specific embodiments of the invention have been disclosed, it will 
be understood by those having skill in the art that changes can be made to 
those specific embodiments without departing from the spirit and the scope 
of the invention.