Note: Descriptions are shown in the official language in which they were submitted.
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Sl~lF-OPII~IZING ~OUCII l~An SlNSOR CIRCUIT
BACKGROUND OF THE INVENTION
This invention relates to a touch pad sensor
circuit, and more specifically relates to a novel arrange-
ment for multiplexing the readout of a plurality of capacitive
touch pads using digital techniques.
Capacitive touch sensors are a well kno~n ~.eans for providing
inputs to various devices including home appliances such as
kitchen ranges which have touch pad devices arranged in a
keyboard style array. The capacitive touch sensor is useful
in this application since it isolates the user from system
control circuits an~ voltages.
A typical capacitive touch sensor of the prior art
includes a high voltage pulse generating device coup:Led to a
capacitive touch plate which is, in turn, coupled to a
receiver circuit. The touch plate is one electrode of a
capacitor, isoLatedby a dielectric such as a glass plate.
When the user touches the glass plate, the capacitance value
of the touch plate capacitor is changed since the capacitance
to ground of the one electrode of the device is changed by
the presence of the user. This capacitance change is small,
but it will attenuate the pulse signal transferred by the
capacitor from the pulse generator circuit to the receiver
circuit. However, to provide reliable detection of the
touch of the plate by a user, relatively high voltages
should be generated by the pulse source, and sensitive detection
circuits are needed which have stable, long-term operating
characteristics.
A digital capacitance measuring circuit for digitiz-
ing the output of a capacitive touch plate is described inour U.S. Patent 4,039,940~ dated August 2, 1977, entitled
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C~PACITAN~ SENS0R, and assigned to the assignee of the
present invention. In this patent, a charge transfer analog-
to-digital converter (hereafter CTAD) circuit is shown which
produces a digital output from the touch pad detector, which
digital output changes from a no-touch value to a touch
value in response to the touching of the sensor.
All touch pad systems to date, including the touch
pad system described above in our U.~. Patent ~,039,940, have
been hampered by a number of interrelated problems. Thus,
the multiplexing limit of presently known touch pad systems
is approximately two touch pads per sensor. In order to
exceed two touch pads per sensor, extremely high drive
voltages on the order of 100 to 200 volts, for example, are
r~quired. Moreover, parasitic capacitive coupling between
different drivers and the same sensor will cause differences in
~he voltage sensed for the different drivers for either touch or
no-touch conditions.
Another of the problems is that in order to reliably `
sense the touch or no-touch condition, only about two drive
lines can be associated with any given sensor. As many as
three drive lines may be used for a given sensor but extreme
care would be needed to balance the received signals from
various drivers. This extreme care extends to touch pad
layout and places undue layout restrictions on the placement
and size of the touch pads.
A further problem is that in systems where there
are a large number of touch pads, the necessity for having
different threshold values for each sensor increases the
system cost and requires a liarge number of trimming operations.
Variations in the characteristics of the individual
touch pad panels have produced additional problems where
these variations may be caused because of differences in the
thickness of the glass and in the dielectric constant of the
glass of the different tOUC]I pads. Variations of only a few
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percent between touch pads can cause significant difficulty
in making the touch or no-touch decision.
A -further source of variability is the aging of
the touch pads, over a long period, and in environmental
effects on the touch pad, such as buildup o-f grease and
cooking material films on the glass panel which can be
expected in household appliances.
In view of the above problems, good reliability
can be obtained with a multiplexing level of one but the
number of sensors required would mean additional circuit
chips and a tremendous number of connections to the touch ` .
pad panel.
Multiplexing levels of two to three might be used
with presently available technology to obtain reliable
operation but these impose significanl; constraints on the
appearance and user function aspects of the panel design.
BRIEF DESCRIPTION OF THE PRESENT INVE~TION
In accordance with the present invention, a
plurality of capacitive touch pads is- associated with
memory and logic elements in a ~anner *o allow a very high
level of multiplexing to be obtained. The arrangement of
the present invention is inherently tolerant to variations
in the actual touch pad circuitry and in the touch pad
driyer circuitry, and permits a minimum number of connections
between the touch pad panel and the touch pad sensing electronics.
The present invention also accommodates long-term drift in
circuit parameter values, and permits maximum latitude to the
system designer in the placement and configuration of the
touch pad.
In accordance with the invention, an analog-to-
digital (A/D) conve~ter, preferably a CTAD,is provided to
convert voltages associated with each of the touch pads into
digital values. The CTAD converter may utilize a circuit
identical to that disclosed in our U.S. Patent 4,039,940
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di.s(ulsst-~c] ~Il)ovt,~. 'I'he di.g:it~ L readi~ ol>tained ~f'or the no
tOUC]l co]1tlit.iol1 :for each touch pacl i.s stored iTI a memory. A '',
control logic c:irc:u:it then cycles through all touch pads and
compares the digital reading obtained from eacl1 of the tOIlCl1 ~
pads to the value of the no-touch reading for the respec1,ive ..
touch pad which has been stored in the memory. IY11en a .'
significant departure from the no-touch condition is o~)tained, .:
in the proper direction, a touch indication is given for
that particular touch ~ad and an appropriate control. functio]1
l.0 is initiated throu~h other contro1 circuitry. ,,
Two different circuits are provided for determining
~hen a touch condition exists. In the first, a fixed value
below the no-touc}1 value in memory is required to indicate a ,'
tOUC]1 condit:ion. l'n the second arrange]llt-~l1t~ a fixed percentage
of the no-touch value is de~ined, below which any received pulse
is recognized as a touch condition. ''
In accordance with an important feature of the ,:
' inventionl the touch pad sensor circuit is made to be self-
optimi.zing by periodically operating all touch pads in an
20 optimizing mode whi.ch updates the no-toucll digital output in'.
the system mellZory. ~`he updated reatding, hol~ever, can be
changed by only one count cluring any cycle of the optimizing
mode to.prevent A transiellt sigTlal from causing any subst.ll1tial
change in the no-touc}1 readings for any touch pad. The
optimizing mode is interspersed with the normal touch or no-
touch sensi.n~ mode as desired and as determinetl by the
control logi.c means. Thus, the circuit hecomes inherently
tolerant of all variations in the tOuc]l pad circuitry and is
freed Eor the need for any maTIl.lal adjuslment of trimming
of the circuit.
From the al)ove, orle object of the present inventio
is to digitize the developed vol.tages associa~etl with each
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Or a plur~ y of t:o~lch p~ s in a to~ch co11i.rol1el systeln;
to store l:h( no-~oucl~ d.i.gitl:l value fol- each tollch pad in a `:.
memory; and to compare the present digital reading for eac11 -
touch pad with the no-to~lcl1 di~ital reading prev;.ously ~
stored to determine when a touch condition has occurled. n
Anot11er object of this invention is to provide a
novel method for continuous]y up1ating the no-~ouch vo1tage
reading in a digital memory in such a way that these 1~eltlings
are not significantly changed by short-term transients.
Stili another object of ti1is invention is to obtain
an adjustment-free method for sensi.ng touch pad signals~ wh:ich
are subject to a high level of`multiplexing.
Yet another object of this invention is to provide ....
a novel tOUC]I control s~ste~ l1ic}1 provi.des the designer
with great freedom o:f layollt arld manufactllrit1g tolerance
standards. :
These and other objects of this inventi.on wil]. become
apparent upon consideration of the :Eollo~ing detailed desc1iption
taken with the drawings
BRIEF DESCRIPrION 0F 1`~iE DRAWIN(S ::
Figure l is a block di.agram of a first embodiment
of the invention wherein a touch condition is indicated ~hen
tl1e digital OUtpl~t of any of the touch pads is some fixed
value below the no-touch value of the pad wl1ich was stored
in the memory of the system.
I:igure 2 :is a block diagram of a secol1d embodiment
of the invention wherei11 -ti1e tolicll co11~ition is recognizcd ..
when the digi~al o~lt~Ut of any of the touch pad; is a fi~ed .
percentage of the no-toucl1 vallle below the no-touct1 value stored
3) i.n ~he memory; a;;d .:
~ `:igures 3a to 3h are timing diagrams for Lhc
various voltages an1 signals of Figllres l and 2.
I)lrAILII) I)L~CRIlTI()N OF TIIE INV~NI`I~N
l~eferring first to Figure 1 theIe is sho~.n in
bloc~ diagrammatic form a processing circuit for processing
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the outputs of four touch pads TPl, TP2, TP3 and TP4. While ~.
four touch pads are shown in Figure l, it should be under~
stood that any desired number of touch pads could have been ,
~ho~n. ~ t
Each of the touch pads of Figure 1 consists of a '7.,
flat plate, shown as fla~ plates lO to 13 for touch pads TPl
to TP4, respectively, where each of the flat plates -~
represents the co~non electrodes of a pair of coupling~'.
capacitors Ccl and Cc2 (shown only Eor TP3, in the interests of
~implicity).This common electrode of the coupl.ing capacitors may .
be coupled to ground, as during a touch condition, so that ~,~
the capacitance-to~-ground of the touch pads changes when an 'i~:!
operator physically touches the common electrode (or an insulator 1'
suppor~ed by the common electrode~ with his finger.
Touch pads TPl and TP2 are connected to driver :
line Do b~ the coupling capacitors C'cl~ and are similarly ;3'~
coupled to output lines SO and Sl, respectively, by the coupling "'
capacitor Cc2. Additional touch pacls of an extended array
could have been connected to the line Do. Line Dl is sia~ rl.
coupled to touch pads TP3 and TP4 and these touch pads are , `~
also capacitively coupled to the output lines SO and Sl,
respectively. Again, additional touch pads could have been
coupled to the line ~l and to other outpu~ lines.
Lines Dl and Do are output lines of a driver means `.
14 which produces output voltage pulses on lines Dl and D
(Figures 3b and 3c, respectively) which may be relatively :
low-voltage pulses of the order, for example, of lO volts as
directed by a control logic means 15, as will be described -.
more fully hereinafter.
The output lines SO and Sl are connected to the
input of a suitable analog-to-digital converter 16. The
signal on each line SO or Sl (~igures 3d and 3e, respectively)
or any other line coming into the analog-to-digital convelter
means, as selected by t}le control logic means 15, is converted
from an analog value to its di~,ital value by A/D converter ]6.
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In a preferred embodlment o~ the invention, the A/D converter
16 i5 of the CTAD type shown in our U.S. Patent ~,039,940.
The ou~put 16a o the CTAD eircuit 16 i8 applied to `.
a count input 17a of a CTAD counter 17. The A/D converter
16 also produces a "conve~sion done" output 16b (Figure 3f~
when it is has completed the conversion of the signal on line 1
SO or ~
A window-preset-word input circuit 18 is also .~.
ap~lied to a data input 17b o~ the counter 17 to establish a
preselected, fixed count in counter 17 prior to counter 17 1 3
receiving a conversion count from A/D converter means, for :
a purpo~e hereinbelow more fully described. It should be ;1
under8tood that a single "window" count may be permanently ,'l~t
wired into the window circuit (as implementing block 18 by .,.
permanent wiring at the data input of a resetable counter ,`1;
used for clrcuit 17), and that a manually or electrically .. ¦~
selectable fixed count can be implemented by means of swI,ches 1~
and the like at the counter data inputs in manner ~!'
known to the art~
A digital memory 19 which may be a semiconductor '1l:
memory or any other desired type of memory is provided to ~.^
store the no-touch digital output reading of each of the ~-
touch pads TPl to TP4 as will be described hereinafter. The ~i
memory 19 is controlled by the control logic means 15 and -,
can read out or receive data, via output 19b. or input '~.
19c, r,espectively, with respect to up/down counter` i-,.
: 20. The up/down counter 20 and the counter 17 are each
connected to inputs of comparator 21 as shown. The comparator
21 has output lines 22 and 23, which, respectively, go high
when the count in counter 20 exceeds or is less than 1
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the count in counter 17.
The original processing circuit includes AND gates 24
and 25 which are connected to the up and down inputs, re-
spectively, of the up/down counter 20 as well as AND gates
26 and 27 which are connected to the preset and clear input
terminals 17c and 17d respectively of counter 17 as shown.
One input of AND gate 26 includes an inverter 29 connected
to the optimized output line of the control logic means 15.
Finally, an AND gate 30 is provided which has an output
indicating a touch condition at whichever touch pad TPl
to TO4 has been addressed by the control logic means 15 at
the time the touch-detected output is produced.
The opera-tion of Figure 1 is now described with
reference being made to the timing diagrams of Figure 3a to
3h. As shown in Figure 3a, the control logic means 15
produces a reset pulse at the beginning of each time interval
tl, t2, t3 and t4. Each of these pulses is produced at the
same time that the memory means 19 is addressed for the no-
touch readings stored in the memory for one of the touch
pads TPl to TP4, respectively. Thus, during the time interval
tl, the no-touch reading for touch pad TPl is being addressed
and the processing of the circuit is concerned with the
condition of the touch pad TPl. In a similar manner, during
intervals t2, t3 and t4, the conditions of touch pads TP2,
TP3 and TP4, respectively, are involved.
Considering first the cycle involving touch pad TPl,
the cycle begins with a reset pulse (Figure 3a)
at reset output 15a at the beginning of cycle tl, which pulse
turns on the transistors 31 and 32 to connect lines S
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and Sl to ground during the duration of the pulse. At the
same time, the reset pulse will either clear or preset
counter 17, depending upon the o~her inputs to gates 26
and 27 as will be disclosed more fully hereinafter. The
reset pulse also fills the up/down counter 20 with the
contents of the appropriate memory location of memory 19
containing the digital value o the lat3t no-touch
reading of touch pad TPl. Note that the control logic 15
will supply the proper address to the address input l9a of
~ memory 19, corresponding to the particular touch pad being
measured. Memory 19 outputs, at l9b, the stored no-touch
value for the particular touch pad, for transfer to up/down
counter 20.
When the reset pulse o~ Fi.gure 3a in interval tl
ends, the appropriate driver line Do or Dl is turned on by
t~e control logic means 15, as shown in Figures 3b and 3c,
respectively Thus, when touch pad TPl is being measured,
line Do is turned on. Note that line Do is also turned on
in the next period t2 when touch pad TP2 is being measured.
Line Dl i8 turned on when touch pads TP3 an~ TP4 are being
measured in time intervals t3 and t4, respectively.
The AtD converter 16 then measures the voltage on
the appropriate sensed line SO or Sl (Figures 3d and 3e,
respectively) as addressed by the control logic to
correspond to the particular touch pad being measured. The
counter, prior to receipt of the reading from A/D converter 16,
has had a "window" count preset therein by the action of
turning on the preset input 17c (via AND gate 26 whenever ~he
RESET and OPTIMIZE outputs of control logic 15 are
respec~ively present and not presentj i.e. respectively
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a logic one and a logic zero in a positive logic system).
The resultant reading from the A/~ converter 16 i.s then
counted in the counter 17, in additlon to the preset "wlndow"
count already stored in çounter 17.
When the voltage conversion in converter 16 is
compl~ted, as indicated by a conversion-done signal (Figure 3f)
which is applied to gates 24, 25 and 30, a comparison is
made in the comparator 21 of the contents of counter 17 which
i5 the measured value of (a) the touch pad output plus
the preset "window" value, if any such value has been previously '
selected, and (b) the contents o counter 20 received
from memory 19 which is the last no-touch value of the touch
pad being measured. If the counter 17 count (signal A)
is less than the count of counter ~0 (signal B), a signal
(B ~A out7~ut) is applied to comparat:or output line 22.
This output, along with the conversion done output from
counter 16 and the output from inverter 29 (since there is
not an`"optini~e" output), will trip the touch-detect gate 30
indicating that a touch condition has been detected for the
touch pad under evaluation. An output from the touch-detect
gate 30 during interval tl identifies that touch pad TPl
has been touched, thereby to cause the operation of some
suitable control circuit (not shown). Thus, as illustrated for
TPl, in Figure 3d, the no-touch output NT has a magnitude
exceeding the magnitude of the touch otuput T for the
particular touch pad then being interrogated. The difference
between the no-touch and touch outputs (i.e. NT count - T count)
must be established at some non-zero value3 taking into
account variations of the signal on the output line te.g. SO)
due to noise and other transient signals. Thus a lower
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limit NT' is established as a number of counts below which
limit the touch pad output mu~t fall prior to a valid
touch condition (at magnitude T) being establlshed. The
difference W between the expected no-touch magnitude NT and
the lower no-touch limit magnitude (i.e. NT-NTI~ is the
"window"; this "window" is a preselected fixed-offset
coun~ preloaded from circuit 18 into counter 17 to bias the
count in a direction opposite that of a touch output to
assure that transients and other undesired signals
do not cause a lower, touch count (T) when the associated
touch pad has actually not been activated.
In a similar manner, the output of a touch-detect
signal during intervals t2, t3 or t4 will indicatethat touch
pads TP2, TP3 or TP4, respectively, have been touched,
as shown in Figure 3h.
During the last part of any cycle associated with
any particular touch pad, the contents of the up/down
counter 20, which is the no-touch value o~ the particular
touch pad, is rewritten into the memory 19, as indicated in
Figure 3g.
This process then continues sequentially to constantly
monitor whether or not a touch has been registered on any o~
the touch pads TPl to TP4 as well as any other additional
touch pads w~ich might be added to the system.
In accordance with an important feature of the
invention, the system of Figure 1 is systematically operated
in an optimi~ation mode after some given number of measure-
ment modes of operation. During the system optimization
mode, the memory l9 will be updated with the latest values
of the no-touch condition ~or the touch pads TPl to TP4.
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When the optimization mode is entered, the control
logic cirucit 15 produces an output signal on the "optimize"
line 15b and the operation of the circuit is ldentical to its
normal operation with two exceptions. The first is that
during the reset period, the counter 17 is not preset to a
"window" value (as the output of gate 26 does not activate
counter input 17c), but instead the counter is cleared to a
zero value. That is, no offset is applied to counter 17. This
is because the two inputs of gate 27 are both high with the
optimize and reset signal, thereby activating the clear
input 17d of counter 17. Secondly, when the conversion is
completed by converter 16, a comparison is made between the
contents of the up/down counter 20 (which is the past no-touch
value) and the contents of the CTA~ counter 17 which is
the present no-touch value.
I~ the two counts are equal, then no action is
taken. However, if the present measured value is greater
than the past value, then an output signal is applied to
line 23 to trip gate 24 and apply an input signal to the up
input of counter 20 to bump counter 20 up by one count.
This revised value is thereafter routed to the data input
l9c o~ memory 19 and is then rewritten into memory 19 responsive
to a WRITE signal from control logic 15 to memory 19.
Similarly, if the present no-touch count is lower than the
count in counter 20, then line 22 is activated to trip gate 25,
25, thereby ap~lying an input signal to the down terminal of
counter 20 bu~.~ing the counter down by one count. Again,
this altered value is rewritten into memory 19 at the end of
the optimized cycle.
By permitting a change of only one count in the
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up/down counter 20 in any given cycle, the circuit accommodates
the conditions where a large voltage transient may have
occurred during optimization or the optimiza~ion mode was
entered simultaneously with a uSeT touch condition occurring
on th,e particular pad. This condition will then be corrected
during the next optimization mode.
From the abo~e, it will be seen that the novel
arrangement of Figure 1 permits the multiplexing of a large
number of touch pads while requiring relatively few con-
nections to the touch pad system and accommodating wide ,
varia~ions in the touch pad circuits. The novel circuit
also permits ~he continuous updating of the no-touch condition
for each of the touch pads, ~hich updating is not signifi-
cantly changed by short-term transients so that the circuit
is self-optimizing.
Figure 2 shows a second embodiment of the inventlon
whcrein the CTAD counter 17 is preset with a
fixed percentage of the no-touch value of any one of the
touch pads TPl th~ough TP4. Thus, the circuit differs from'
Figure 1 essentially in that the AlD counter 17 in Figure 1
was preset with a fixed percentage value. The circuit
of Figure 2 and its operation are otherwise identical to
that of Figure 1 and similar components have been given
similar identifying numerals.
In Figure 2 an inverter 40 has been added which
brings a signal from memory output 19b to a shift input 17e
~of the coun~er 17. This will enable the shift of bits in
the counter 17 to the right in order to divide by two, four,
eight or the like, thereby to preset the desired percentage
of the no-touch value which is to be read out of the counter 17
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during the comparison operation. Thus, to apply a fixed
"wlndow" of (100/2N)%, ~he previously stored no-touch
value i6 applied to counter 17 and is shifted N times
to the right, in the coun~eri an ~-right shift is equivalent
to a division by 2N. Illustratively, if N=2, the stored
value is shifted right twice, whereby 100/2N% or(25%) of the
no-touch value is preset into coun~er 17; a touch condition
now occurs only if the touch pad output value is below
` 75Z~ (i.e. lOO - 100/2N)% o the no-touch value.
: Although a preferred embodiment of this invention
has been described, many variations and modifications will
now be apparent to those skillPd in the art, and it is
therefore preferred that the instant invention be limited
not by the specific disclosure herein but only by the
appended claims.
