Note: Descriptions are shown in the official language in which they were submitted.
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TO~CH POSITION SENSITIVE DEVICE
1. Pield of the Invention
This invention relates to a device for
determining the position of a surface contact and more
particularly to a touch sensitive device for use with a
synchronized signal source, such as a cathode ray tube
(CRT).
Background of the Invention
There are many applications where it is desired
to provide feedback information for information displayed
on a CRl screen. For example, it has become common
practice with the use of computers to display on the screen
a choice for the user to select from. The user is
typically instructed to operate specific keys, on a
keyboard or similar device, to select from among a menu of
possible choices. In response to the user operating the
selected key the menu is changed and the user is given a
new choice, again making the choice by operating a
particular key~ Such an arrangement is tedious since a
user must ~irst look at the screen and then go to a
separate keyboard to find the proper key. This is time
consuming and requires costly separate equipment.
One possible solution to the problem has been to
arrange the menu of choices along a side of the viewing
screen and to arrange next to the screen a series of
buttons. As the labels on the screen change the buttons
become dynamically relabeled. While this solves some of
the problems it does not allow for complete flexibility of
the visual display and still requires an artificial
arrangement of the display.
Several attempts have been made to solve the
problem, one such being the use of a light pen which is
held over the point on the CRT screen correspondin~ to the
desired response. Light from the CRT raster then enters
the pen and the position of the raster is determined by
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coordinating the si~nal output from the pen with the
position on the raster at the time of the signal. This
arrangement, while performing properly, has the
disadvantage of requiring the user to hold a pen and to
properly direct the pen to the proper place on the screen.
Other touch sensitive screens used crossed wires,
crossed beams of infra red light, reflection of acoustic
surface waves, current division in resistive sheets, force
balancing, or mechanical buttons on which a display image
was superimposed by a half silvered mirror. ~hen used with
a CRT display, the foregoing methods require careful
calibration to establish correspondence between points on
the touch screen and points on the display. The need for
special transducers or many electrical connections increase
complexity and cost. Also, most of the methods only allow
activation of one point at a time.
Thus, it is desired to solve these problerns in a
manner which allows the visual display to be touched
directly at any location on a dynamically changing basis
with the position of the touch being easily determinableO
These problems must also be solved in a manner which allows
for fingers of varying degrees of dampness and for the use
of screens in ambient light conditions which vary
considerably from place to place.
Summary of the Invention
-
Advantaye is taken of the fact tha-t various
signal sources, such as a CRT, generate a synchronized
signal, e.g., a raster scanning electron beam, the position
of which is known at any point in time. I have overlaid
the CRT screen with a device having parallel surfaces, such
as a glass plate, through which light generated by the
raster is visible to the user. The edges of the device are
fitted with photodiodes which respond to the entrapment of
light between the parallel surfaces to provide an output.
By touching the top surface of the device (i.e., changing
the medium bounding the surface) at a point, the light from
the CRT screen surface becomes entrapped within the device
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by total internal reflection.
One problem which must be addressed in designing
such a touch sensitive screen is the matter of reflectivity
as it affects output light levels. The problem is maniEest
in situations where the photodiode coupling at the side of
the screen is incapable oE distinguishing between a touch
and a non-touch in the presence of background noise or
build-up of oils on the screen surface. I have solved this
problem by overlaying the top surface of the device with a
flexible membrane separated from the device by a small air
gap~ The membrane has a half tone pattern of small white
dots on its upper surface so as to partially reflect the
light from the CRT. (Other patterns such as strips could
also be used). rlowever, as long as the air gap remains
between the device and the overlay membrane, the refraction
angle in the glass is such as to prevent total internal
reflection from occurring between the surfaces of the
device.
When a user presses the membrane it is deflected
to make intimate contact with the top surface of the device
thereby removing the air gap and allowing the reflective
rays of light to reenter the device and become entrapped
therein by total internal reflection. This trapped light
then travels to the sides of the device where the
photodiodes detect the entrapment. By coordinating the
time of the changed photodiode output with the CRT raster
position the exact surface position of the touch is
determinable.
Another use of this device is to visually verify
the proper placement of a movable member. In such an
environment a fixed light source would be used and the
member brought into contact with the membrane. When the
member was positioned properly total internal reflection
would occur and light would be seen emerging from the edge
of the device
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In accordance with one aspect of the invention
there is provided a touch position sensitive device for
use in conjunction with a signal source, said device
comprising spaced apart surfaces arranged such that
signals are introduced by total internal reflection
between said surfaces as a result of a change in medium
bounding at least one of said surfaces, and a flexible
overlay adjacent said one surface, said overlay adapted to
be deflected into contact wi.th said one surface in response
to a depression of said overlay so as to cause said medium
change thereby providing a distinctive signal level change
between the surfaces of said device when said signals are
introduced between said surfaces.
In accordance with another aspect of the invention
there is provided deflection detection apparatus for use
with a signal source, said apparatus comprising a structure
having spaced apart surfaces such that signals from said
signal source are introduced by total internal reflection
between said surfaces, and means adjacent one of said
surfaces adapted for deflection into contact with said one
surface so as to cause said introduction of signals between
said surfaces by total internal reflectionO
Brief Description of the Drawing
These attributes of my invention as well as
others will be more fully appreciated from a review of the
drawing in which:
FIG. 1 shows a pictorial view of a CRT screen
overlaid by my device;
FIG. 2 is a schematic representation showing the
device with the flexible overlay membrane, the overlay
being in the relaxed position;
FIG. 3 shows my device with the membrane being
deflected into contact with the top parallel surface; and
FIGS. 4 and 5 show the principles on which my
invention is based.
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Detailed Description
As shown in FIG. 1, CRT screen 20 is arranged in
the well known manner such that electrons from the electron
gun (not shown) impinge upon the phosphorescent screen of
the CRT in a sequential pattern, line by line, from top to
S bottom. As the electrons hit the phosphorescent surface
the surface glows. Phosphorescent images can thus be
formed on the screen under control of the electron beam.
This phenomenon, of course, is now well known and forms
the basis of television and other CRT systems.
By properly programming the system it is possible
to have any type of image displayed at any position on the
screen for any length of time. Thus ! it is possible to
create images representative of numbers, sets of numbers,
letters, or signals in any position on the face of CRT 20,
as shown. Using my device, which is a combination of
elements 10 and 11 of system 60, it is possible to allow a
user to touch any position on the device and to determine
electronically the position of the touch. In order to
accomplish this, I have overlaid the CRT screen with a
device, e.g., a glass plate 10 having parallel surfaces
through which light from the phosphorescent screen may
pass. I have overlaid device 10 with flexible membrane 30
which advantageously may be transparent silicone rubber.
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1/
embrane ~ is separated from top surface of
device 10 by any one of several means as, for example,
stretching between supports or resting against ridges,
protrusions, or flexible tabs dispersed about the surface.
The flexible membrane is constructed with a half tone white
dot pattern on lts outer surface (other patterns such as
strips could also be used). This construction allows light
from the CRT screen to pass through the membrane to be
viewed by a user as well as being reflected back towards
the CRT screen. When the CRT screen projects an image
calling for user response, a finger or other device is
placed against the outer surface of the membrane at the
position selected (the number 6 in FIG. 1). When this
occurs, as will be explained from that which will follow,
light becomes trapped within device 10. This trapped light
travels to the edge of the device and is detected by
photodiodes 101 thereby providing an output signal useable
for determining the position of the touch. The actual
determination of the touch position is accomplished by
coordinating the position of the CRT raster beam witll the
time of the output signal. Means for doing this are
generally known, as in systems employing light pens.
Turning to FIG. 2, CRT raster beam 13 is shown
impinging on the front surface of CRT 20 with light rays 21
from the phosphorescent surface passing through the
~arallel surfaces of device 10 and into membrane 11. Some
light rays (no-t shown) are transmitted outward toward the
user and some are reflected back toward the CRT screen.
Because of the air ga-~ between the lower surface of
flexible membrane 11 and outer surface 1~ of device 10 the
reflected light rays (as will be discussed) have an angle
of refraction less than the critical angle needed for total
internal reflection and thus pass through device 10. These
light rays, as they approach the edge of device 10, can
never assume an angle sufficient to become trapped between
surfaces 14 and 15 and thus whatever light approaches the
edge of the device, passes into a light absorbing surface
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such as surface 102 which may be black plastic matte. Very
little additional light impinges upon photodiode lQl and
thus the output signal reflects internally scattered light.
In FIG. 3, a finger is shown applying pressure at
a point on the top surface of membrane 11 thereby flexing
the membrane into contact with surface 14 of device 10.
Membrane 11 has coated thereon a surface 12 which is made
up, in one embodiment, of half-tone white dots to have
increased reflectance and to scatter the light rays.
With the membrane depressed light generated on
the surface of CRT 20 near the depression passes through
device 10 and into membrane 11 and is then reflected back
into screen 10. These reflected light rays, since they do
not now pass through air, do not refract as they did in
FIG. 2 (as will be discussed) and thus some of -th~ese rays
become trapped between surfaces 1~ and 15 of ~ ~ 10 by
total internal reflection.
This trapped light then travels, as shown, within
screen 10 and impinges upon photodiode 101. Note that
light absorbers 102 are ineffective to absorb this light
since the light rays do not pass through surfaces 1~ and
15. Thus, the light rays which impinge upon photodiode 101
cause an output signal which is different from the output
signal generated when trapped light does not impinge upon
the photodiode.
It is important to note that photodiode 101 may
be any type oE transducing device for converting optical or
other signals to electrical energy and may be a single
device or may comprise a number of inclividual devices. In
some applications a transducer at one surface would be
sufficient while in other applications it would be
advantayeous to surround device 10 on all sides with such a
transducer which, of course, may have a single output or
multiple outputs.
To make the device more useful, the sides of the
white dots facing away from the CR1' should be made matte
black. This increases the contrast of the ~RT image as
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viewed by the user, which would otherwise be degraded by
reflection of ambient light from the dots. The darkening
may be done by a variety of meanso for example oxidizing
the exposed surfaces, or by photoetching the dots from a
combined layer of white and dark material. Such contrast
enhancement by overlayinq matte black dots, would be useful
for any CRT, even without the touch screen described here.
Total Internal Reflec-tion Criteria
Refraction at a single surface between media of
refraction index Nl and N2 is shown in FIG. 4. Light ray
is ~erpendicular to the boundary and does not undergo
refraction. Light ray B enters the boundary with an angle
~1 and is refracted according to Snell's law which states
Nl sin 31 = N2 sin 2 (1)
Light ray C approaches the boundary angle ~C which is the
critical angle for total internal reflection (~2 =90 ).
This critical angle, when ~2 = 1~ which is the case for
air, is shown by the formula
C N2/Nl 1/N1w~enN2 = 1 (air) (2)
Total internal reflection takes place when ~ is larger than
the critical angle such that~ D is greater than ~C Since
sin of ~ is less than 1 it follows that N2 must be less
than Nl for total internal reflection to take place.
Turning now to FI~. 5, the conditions for total
internal reflection (TIF) will be reviewed with respect to
a device of refractive index M with air (refractive
index = 1) at the surfaces of the device. When light ray A
enters device 10 from air total internal refraction cannot
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take place because the index of refraction at the lower
surface bends the light ray to an internal angle smaller
than the critical angle necessary for total internal
refraction which is 1 . This follows from use of
geometry and Snell's law since
sin 1 2 o
and sin ~ is less than 1 for all ~ .
In the case of light ray B (FIG~ 5) the air space
is eliminated when the light ray is assumed to enter from a
medium wi-th an index of refraction No > 1 which occurs when
another body is in contact with the bottom surface of
device 10. 1'otal internal reflection can now take place
(where air borders the device) because the light ray is no
longer bent to an angle smaller than the critical angle at
the lower surface. This follows from the fact that
sin ~1 = sin 32 = (NO/N)sin ~O (~)
which is greater than the critical angle 1 when
o o
When the air space is eliminated on the top surface of
device 10 as shown by light ray C hitting diffusely
reflecting mediurn 51 light enters the device from diffuse
reflection at top surface 14. In this case, there is no
refraction to bend the light ray below the critical angle,
so total internal reflection can occur.
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As previously noted, because the inventive system
lnvolves the detection of light generated at the position
on the CRT touched by the ~Iser~ arrancJementS similar to
those used with light pens can be used to determine the
~osition touched. Problems exist however, in that
imperfections in the surface of the CRT screen, dirt,
grease build-up, and geometrical aberrations all cornbine to
cause some total internatal reflection to occur at all
times, even when no touch has occurrecl. The result is that
the photodetectors produce appreciable output at all
ins-tants during which the raster beam is c]rawing bright
areas on the CRT screen.
These probleMs are compounded in that for eacn
cycle of the raster, the output wave form has a rlon-
constant value resulting in a signal which, for a givenpoint, fnay be greater in magnitude wi-thout a touch than is
a portion of the signal output in the presence of a touch.
Thus, it is not always practical to simply measure the
signal level of the photodiode output in order to detect a
touch signal. One means to solve these problems is by use
of an arrangement, called the sample spot systern, which is
founded, in part, on the understandiny that a meaningful
touch can only occur at one of a set o'c specified touch
locations according to the patternf ormed on the face of
the CRT screen. Thus, the CRT screen i~ divided into
nonpermanent locations desigrlated touch areas and con-touch
areas. The areas are changeable, as required, and may
consist, at any point in time, as a single touch spot, or
there may be several touch spots or areas spaced around the
screen. The coordinate positions of the touch areas are
stored in a memory and only signals generated while the
raster beam is within the defined touch areas are
considered by the sample spot system.
The sample spot system, during the times when the
raster beam is within a defined touch area, samples the
output from the photodiodes on a time definecl basis. Ihe
values of the signal at each defined time position are
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stored in a memory. The sampling and storing is timed to
the CRT raster beam so that drifts in the video image will
cause no problems.
The sample spot system operates to sample the
photodioide si~nal durin~ all times that the raster beam
passes through CRI locations which are within the defined
touch regions. This sample is made for several successive
position is stored in a separate memory location. This
occurs at a speed fast enough that all the samples are
comple-te before the first touch could possibly occur.
Thereafter, every time a frame is drawn on -the CRT face by
the raster beam, the photodiode output corresponding to
each touch area is sampled and the sample for each time is
compared to -the priorly stored untouched average sample for
that time. h!hen one of the compared samples exhibits an
increase (mismatch) in magni~ude over the priorly stored
sample~ a touch signal is generated. Since the location of
the mismatch in memory corresponds to a known position on
the CI~T screen the position of the touch on the screen is
known exactly.
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Conclusion
.
While the focus o~ the disclosure is on a CRT
type light signal my invention may also Eind use in
situations where it is desired to position a part in a
particular location. In such an arrangement a fixed light
source may be used at the desired loca-tion and the part
moved mechanically or otherwise to make contact with the
flexible membrane. When the contact is at the location
where the light is focused total internal reflection will
occur. This total internal reflection will become visible
to a person observing the device. Thus my device may be
useful for determining a surface condition of a screen or
other device. The screen can also be constructed using the
flexible membrane alone, with photodiodes coupled to its
edge. If this sheet is positioned near the CRT face, but
separated by an air gap, light from the CRT will pass
through the sheet without reaching the diodes. ~ut if the
membrane is flexed into contact with the CRT face at a
point, some light rays from the CRT will become entrapped,
impinge upon the photodiode, and cause an output signal
which can be used as before for determining position. To
improve this device, a second membrane having a smaller
refractive index and partially light absorbent can be
overlaid on the first membrane. Light will still be
entrapped in the first membrane but the effect of oil and
other contamination on the outer surface of the device will
be reduced.
Also it is understood that the signals may be
light or may be electronic so long as they obey the
physical phenomenon described. It, of course, is to be
understood that those skilled in the art may find many
applications and modifications using my invention and it
may be built as a separate device for mating with an
existing CRT or it may be manufactured as a part of the
implosion screen itself. Also, the trapped light may be
removed from the device by any light utilization device,
such as, for example, Eiber optics or light pipes.
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Using my invention in graphics and taking
advantage o~ the fact that multiple positions can be
detected, a user could rotate a shape by touc~ing two
points and rotating them around each other. A user could
position a line by simultaneously positioning its end-
points; or could specify a quadratic curve by indicating
three points along its length. Areas could be colored or
shaded by touching them while pads indicating these
attributes were simultaneously touched.
In text processing, a screen with relabelable
keys could provide a shift button that could be pressed
simultaneously with other keys~ A text editor could
combine cursor control and touch sensitive buttons on the
same screen; and the buttons could be touched while the
cursor was moved (to change the text font for example).
My screen can also be made to discriminate
different levels of force. In graphics, this Eorce
discrimination could indicate a degree of shading, or could
be translated into linear or rotational velocity.
Force discrimination could also be used to
eliminate the effect of parallax; as cursor position could
be indicated on the screen as the user moved his or her
finger across the display, and the user could simply press
harder when the desired position was obtained.
It would also be advantageous to make the
flexible overlay translucent, and to focus an i~age upon it
by means of projection television from the rear. ThiS
would give a large area screen and the focusing would lead
to the finest spatial resolution.
