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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to touch control systems of the type which
include at least one touch sensitive device which capacitively transmits a
relatively large electrical drive signal when not touched and transmits a
relatively small electrical signal when touched and means for detecting
the touch and no-touch conditions of the touch sensitive devices. Such
touch control systems are commonly used in conjunction with other control
circuitry to activate and/or deactivate functions of an apparatus.
Generally speaking, the improved detection means of the present invention
utilizes a high gain voltage comparator having positive feedback
associated therewith which allows the multiplexing of a plurality of touch
sensitive devices to such detection means at low drive voltages and which
may be operable either asynchronously or synchronously with other control
circuitry.
2. Description of the Prior Art
In substantially all touch control systems it is necessary that the touch
and no-touch conditions of touch sensitive devices be distinguishable and
detectable to control a desired function. Accordingly, the detection means
must distinguish between the voltage amplitude of an electrical drive
signal where there is a no-touch condition and the voltage amplitude where
there is a touch condition. It is therefore desirable that the detection
means include devices with precisely defined thresholds which will clearly
and discretely detect the difference between large and small electrical
signals. Furthermore, it would be desirable that the detection means have
a high voltage gain so that low voltage drive signals may be utilized and
a plurality of touch sensitive devices may be multiplexed to a single
detection means without requiring a significantly high drive voltage.
Previous touch control systems have utilized P-channel metal oxide
semiconductors (PMOS) and complementary metal oxide semiconductors (CMOS)
as threshold detection devices (See Walter R. Spofford's copending
application, Ser. No. 762,779 entitled "An Implemental Means For A Touch
Control System"). However, PMOS devices do not exhibit clean threshold
characteristics, they have a low voltage gain, and in general they vary
widely from device to device making reproducibility of such devices on a
large scale with uniform characteristics very difficult. Accordingly, PMOS
devices when utilized in touch control systems perform inadequately and
inefficiently and thereby result in decreased reliability and increased
cost of the overall touch control system. Furthermore, because of the low
voltage gain, very high drive voltages are required when multiplexing a
plurality of touch sensitive devices to a single detection means including
PMOS devices.
As disclosed by Walter R. Spofford in his aforementioned copending
application, by utilizing multiple input inverting logic gates which
include CMOS devices three touch sensitive devices may be detected
utilizing a detection means which is a single integrated circuit package.
The integrated circuit package is capable of being manufactured such that
each of the CMOS devices exhibits uniform characteristics and therefore at
least for the detection of three touch sensitive devices the
aforementioned problems associated with PMOS devices are overcome.
However, CMOS devices exhibit low voltage gain, similar to PMOS devices,
which limits their use in a multiplexed detection scheme.
When adapting a touch control system to a larger control system requiring
asynchronous operation the detected touch condition must be remembered.
When adapting a touch control system to a larger control system requiring
synchronous operation the proper detector state, i.e. touch or no-touch,
must be maintained for some minimum pulse width. For either or both of
these operating conditions, MOS devices require additional circuitry
and/or the imposition of greater restrictions on their thresholds to make
them adaptable to the desired mode of operation.
SUMMARY OF THE INVENTION
In accordance with the present invention in its broadest concept, there is
provided an improved means for detecting the touch and no-touch conditions
of at least one touch sensitive device of a touch control system which is
reproducible, provides solutions to both the asynchronous and synchronous
timing problems described hereinabove, and allows multiplexing of a
plurality of touch sensitive switching devices utilizing a relatively low
drive voltage.
Accordingly, it is a feature of the present invention to provide a touch
control system of the type which includes at least one touch sensitive
device which capacitively transmits a relatively large electrical signal
when not touched and transmits a relatively small electrical signal when
touched and an improved means for detecting the touch and no-touch
conditions of the touch sensitive device wherein the detection means has a
high voltage gain associated therewith which allows for the multiplexing
of a plurality of touch sensitive devices by requiring a relatively low
drive voltage.
It is another feature of the present invention to provide an improved means
for detecting the touch and no-touch conditions of touch sensitive devices
of a touch control system wherein the detection means includes a
comparator having a precisely defined threshold which will clearly and
discretely detect touch and no-touch conditions of the touch sensitive
devices.
It is yet another feature of the present invention to provide an improved
means for detecting the touch and no-touch conditions of touch sensitive
devices of a touch control system wherein the detection means includes
positive feedback which may be either large or small to define a time
period for a no-touch state of a comparator whereby the touch control
system is operable either asynchronously or synchronously.
It is still another feature of the present invention to provide a method of
detecting a touch condition of at least one touch sensitive device of a
touch control system which includes the steps of comparing a voltage of an
electrical signal capacitively transmitted by the touch sensitive device
to a predetermined reference voltage, assuming a state indicative of a
no-touch condition of the touch sensitive device when the voltage of the
electrical signal exceeds the predetermined reference voltage, assuming a
state indicative of a touch condition of the touch sensitive device when
the voltage of the electrical signal is within the predetermined reference
voltage, and defining a time period for the assumed state which is
indicative of the no-touch condition of the touch sensitive device.
Other features and advantages of the present invention will be apparent
from the following detailed description of a preferred embodiment thereof,
which description should be considered in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a conventional touch sensitive device.
FIG. 2 is a schematic illustration of a touch control system including a
first embodiment of the improved detection means of the present invention.
FIG. 3 and 4 are voltage waveforms taken at several locations in the touch
control system illustrated in FIG. 2.
FIG. 5 is a schematic illustration of a touch control system including a
second embodiment of the improved detection means of the present invention
.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the above described figures and more particularly to FIG. 1, a
touch sensitive device 10 includes a dielectric material 18 which may
comprise glass, such as the NESA.RTM. and NESATRON.RTM. glass produced by
Pittsburgh Plate Glass; a metallic coating of tin oxide 12 disposed on a
surface 17 of the dielectric material 18, and two metal strips 14 and 16
disposed in spaced parallel relationship to the coating of tin oxide 12 on
another surface 19 of the dielectric material 18. In operation, metal
strip 14 is capacitively coupled to the coating of tin oxide 12 and the
coating of tin oxide 12 is capacitively coupled to metal strip 16. A high
frequency electrical signal is applied to the metal strip 14 which is
capacitively transmitted to the coating of tin oxide 12 and the other
metal strip 16 in that order. Capacitances are formed via the glass
dielectric material 18 and the close proximity of the conductive plates
12, 14 and 16. When the coating of tin oxide 12 is touched, the electrical
signal will be shunted thereby substantially reducing the output signal
from touch sensitive device 10. While the above described touch sensitive
device 10 is typically of the type used in the art it is not intended that
the invention disclosed herein be limited to its use with a touch control
system which includes this type of touch sensitive device. In fact, as
will be obvious to those skilled in the art after reading this disclosure,
the present invention may be used with various other touch sensitive
devices without departing from the essence of the invention.
Referring now to FIG. 2, a touch control system 20 is shown which includes
a first embodiment of a detection means 30 constructed in accordance with
the present invention. In addition to detection means 30 touch control
system 20 further includes a means 22 for providing a series of electrical
drive pulses, an equivalent electrical circuit 10' of the touch sensitive
device 10 illustrated in FIG. 1, and a means 28 for providing a series of
electrical pulses for enabling detection means 30. Each of the means 22
and 28 for providing electrical pulses may be any conventional square wave
oscillator which may serve as a source for digital input signals to touch
control system 20.
The equivalent electrical circuit 10' of the touch sensitive switching
device 10 illustrated in FIG. 1 includes a capacitance means 24
(representative of the capacitance between the coating of the tin oxide 12
and metal strip 14 depicted in FIG. 1), a capacitance means 26
(representative of the capacitance between the coating of tin oxide 12 and
metal strip 16 depicted in FIG. 1), and a shunting means 23
(representative of the coating of tin oxide 12 depicted in FIG. 1). As
shown, capacitance means 24 is electrically coupled to square wave
oscillator 22 at junction J1 and to capacitance means 26. Capacitance
means 26 is in turn electrically coupled to detection means 30 at junction
J2. Shunting means 23 is electrically coupled to both capacitance means 24
and capacitance means 26 whereby the electrical signal provided by
oscillator 22 is effectively shunted to ground potential as it passes
through capacitance means 14 when shunting means 23 is physically touched.
The improved detection means 30 of my invention includes a conventional
high gain differential comparator 32 of the type manufactured by National
Semiconductor and identified by the number LM2901. The negative (-) input
of the comparator 32 is electrically coupled to the touch sensitive device
10' at junction J2 and therefore is responsive to the electrical signal
transmitted by capacitance means 24 and 26. The positive (+) input of the
comparator 32 is electrically coupled through a resistor 40 to a voltage
divider network consisting of a potentiometer 36 and a resistor 38. The
voltage divider network is coupled across a DC power supply source having
a positive side (+DC) and a negative side 48. The wiper 36' of the
potentiometer 36 is electrically coupled through a resistor 34 to the
touch sensitive device 10' at junction J2 such that by setting the
potentiometer 36 a threshold voltage is established for comparator 32
which allows comparator 32 to discriminate between touch and no-touch
conditions of touch sensitive device (10'). The output 33 of comparator 32
is electrically coupled at junction J3 to a means 43 for defining a time
period for at least one of the states of comparator 32. Means 43 for
defining a time period includes a resistor 42 in a positive feedback loop
which is electrically coupled to the positive (+) input of comparator 32
at junction J4. Output 33 of comparator 32 is in addition electrically
coupled at junction J3 to the output 46 of touch control system 20 and
through a resistor 44 to means 28 for providing electrical pulses for
enabling detection means 30 at junction J5. The embodiment of my detection
means 30 described hereinabove is responsive to the negative going edges
27 of the electrical drive pulses provided by means 22 in a manner which
will be described in more detail hereinafter.
Having thus described in detail a preferred embodiment of my improved
detection means 30 and the touch control system 20 for which it is
adaptable, the operation of touch control system 20 and more specifically
of detection means 30 will now be described. The theory of operation
hereinafter described is that which is at present believed properly
applicable to the embodiment described above; however, it is not intended
to be limiting in nature.
Perhaps the best way to describe the operation of touch control system 20
is to describe its operation in relation to the various voltage waveforms
which appear at junctions J1, J2, J3, J4, and J5. Therefore, referring to
FIGS. 2, 3, and 4, oscillator 22 supplies a square wave signal 50 (FIG. 3)
to junction J1 of touch control system 20. As illustrated in FIG. 3,
oscillator 28 separately supplies an enabling square wave 60 at junction
J5 which when driven low forces the signal 58 at junction J3 low thereby
clearing detection means 30. As shown in FIG. 3, once the signal 58 at
junction J3 is forced low it remains low until a no-touch condition is
detected by detection means 30 at which time a signal 58 will again appear
at junction J3. Accordingly, junction J3 remains low even though junction
J5 is high as long as a no-touch condition does not exist.
The magnitude of the signal transmitted from junction J1 to J2 through
touch sensitive device (10') will depend upon whether the touch sensitive
device (10') has been touched. When no touch contact is being made with
shunting means 23 an exponentially decaying voltage spike 51 of relatively
large magnitude appears at junction J2 coincidental with the
positive-going edge of the square wave 50 provided by oscillator 22.
Coincidental with the negative going edge 27 (FIG. 2) of square wave 50
another relatively large spike 54 of opposite polarity occurs at junction
J2. When touch contact is being made with shunting means 23 a spike 52 of
relatively small magnitude will appear at junction J2 coincidental with
the positive-going edge of a square wave 50 provided by oscillator 22 and
coincidental with the negative-going edge 27 of square wave 50 a
relatively small spike 56 of opposite polarity occurs at junction J2. As
previously indicated, the threshold potentiometer 36 is set such that
detection means 30 discriminates between the touch and no-touch conditions
of shunting means 23 based upon the magnitude of the negative spike
appearing at junction J2.
Referring specifically to FIG. 4 the manner in which detection means 30
discriminates between touch and no-touch conditions will be described.
Shown in FIG. 4 is the relationship between the spikes appearing at
junction J2 which are indicative of touch and no-touch conditions, the
threshold or reference voltage 64 of comparator 32 appearing at junction
J4, and the signal appearing at junction J3 representing both the output
33 of comparator 32 and the output 46 of touch control system 20. If the
spike 54 appearing at junction J2 coincidental with the negative going
edge 27 of a square wave signal 50 is of relatively large magnitude and
therefore crosses the threshold voltage 64 at junction J4, comparator 32
changes states thereby allowing its output 33 to be high and producing a
signal 58 at junction J3. This state of comparator 32 is indicative of a
no-touch condition of touch sensitive device (10'). As further illustrated
in FIG. 4, the period of time during which comparator 32 will indicate a
no-touch state is determined by the magnitude of the positive feedback. A
large positive feedback 62 latches detection means 30 in a no-touch state
resulting in a signal 58 at junction J3 of long duration which may be
detected at any subsequent time as an indication of no-touch. This
no-touch state of comparator 32 will be maintained indefinitely until the
voltage at junction J5 is forced low by square wave signal source 28.
Accordingly, the touch control system 20 may be sampled asynchronously
with the drive signal 50 (FIG. 3). A small amount of positive feedback 62'
results in a signal 58' at junction J3 of a longer duration than if no
positive feedback is used. No positive feedback 62" results in a signal
58" at junction J3 which is of very short duration (probably too short to
be properly recognized by an external control system which operates
synchronously). Accordingly, the touch control system 20 may also be
sampled synchronously with the drive signal 50 by using the proper amount
of positive feedback to obtain the desired pulse widths of the signal at
junction J3 which are compatible with the remainder of the control system.
Continuing to refer to FIG. 4, if the spike 56 appearing at junction J2
coincidental with the negative-going edge 27 of a square wave 50 is of a
relatively small magnitude and therefore does not cross the threshold
voltage 64 at junction J4, comparator 32 does not change states leaving
the output 33 of comparator 32 at a low voltage which appears at junction
J3. This state of comparator 32 is indicative of a touch condition of
touch sensitive device (10').
Referring now to FIG. 5, a touch control system 80 is shown which includes
a second embodiment of a detection means 90 constructed in accordance with
the present invention. In addition to detection means 90 touch control
system 80 further includes a means 22 for providing a series of electrical
drive pulses of the type previously described in FIG. 2, an equivalent
electrical circuit 10' of the touch sensitive device 10 illustrated in
FIG. 1, and a means 28 for providing a series of electrical pulses for
enabling detection means 90 also of the type previously described in FIG.
2. Each of the means 22 and 28 and the equivalent circuit 10' are
identical to the corresponding elements shown in FIG. 2; accordingly, the
same identifying numerals have been used to identify these elements in
FIG. 5.
The improved detection means 90 of my invention includes a conventional
high gain differential comparator 92 of the type previously described in
FIG. 2. The positive (+) input of the comparator 92 is electrically
coupled to the touch sensitive device (10') at junction K2 and therefore
is responsive to the electrical signal transmitted by capacitance means 24
and 26. The negative (-) input of comparator 92 is electrically coupled
through the wiper 96' of a potentiometer 96 to a voltage divider network
consisting of resistors 94 and 98 and potentiometer 96. The voltage
divider network is coupled across a DC power supply source having a
positive side (+DC) and a negative side 108. By setting the potentiometer
96 a threshold or reference voltage is established along line K4 for
comparator 92 which allows comparator 92 to discriminate between touch and
no-touch conditions of touch sensitive device (10'). The output 93 of
comparator 92 is electrically coupled at junction K3 through a resistor
104 to a means 110 for defining a time period for at least one of the
states of comparator 92. Means 110 for defining a time period includes a
resistor 100 in a positive feedback loop which is electrically coupled to
the positive (+) input of comparator 92 at junction K2. Output 93 of
comparator 92 is in addition electrically coupled at junction K3 to the
output 106 of touch control system 80 and through resistors 102 and 104 to
means 28 for providing electrical pulses for enabling detection means 90
at junction K5. As illustrated in FIG. 5, this embodiment of my detection
means 90 is responsive to the positive going edge 21 of the electrical
drive pulses provided by means 22.
With the exception of the polarities of the various waveforms appearing at
junctions K1, K2, K3, K4, and K5, the operation of touch control system 80
is identical to the operation of touch control system 20 described
hereinabove. Accordingly, for a detailed description of the operation of
touch control system 80 reference is made to the above described operation
of touch control system 20.
In view of the above description of the embodiments of my invention it will
be seen that the several objects of my invention are achieved and other
advantageous results attained and that further modifications can be made
without departing from the spirit and scope of my invention as defined in
the appended claims.
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