United States Patent (19)
`Philipp
`
`54
`
`76
`
`TIME DOMAN CAPACTWE FIELD
`DETECTOR
`Inventor: Harald Philipp, 4812 Scott Rd., Lutz,
`Fla. 33549
`
`21
`22
`(51)
`
`52)
`
`58)
`
`56
`
`Appl. No.: 578,464
`Filed:
`Dec. 26, 1995
`Int. Cl. ............ F16K 31/02; E03C 1/05:
`GOR 27/26
`U.S. Cl. ........................... 137/1; 251/129.04; 4/623;
`239/24; 324/677; 324/678
`Field of Search ........................... 251/129.04; 4/623;
`239/24; 222/52; 324/677. 678; 137/1
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`3,333,160 7/1967 Gorski ..................................... 4/623 X
`3,453,535 7/1969 Anglin .................................... 324/677
`3,551,919
`1/1971 Forbes .......
`... 251/129.04X
`3,575,640 4/1971 Ishikawa
`as a
`... 4/623 X
`3,588,038
`6/1971 Tanaka .
`3,761,805 9/1973 Dornberger ............................. 324/677
`4,149.231
`4/1979 Bukosky et al.
`324/678 X
`4,558.274 12/1985 Carusillo ................................. 324/677
`4,743,837 5/1988 Herzog.
`324/678 X
`... 324/678
`4,806,846 2/1989 Kerber ......
`4,872,435 10/1989 Laverty, Jr. ...
`137/624.1
`4,972,070 1 1/1990 Laverty, Jr.
`5.025.56 6/1991 Wilson ....................................... 4/623
`5,033,508 7/1991 Laverty, Jr.
`... 137/624.
`5.159,276 10/1992 Reddy, III ......
`...... 324/678
`5,294,889 3/1994 Heep et al. ........
`324/68
`5,329,239 7/1994 Kindermann et al. .
`324/68
`5,461,321 10/1995 Sanders ................................... 324/678
`5,570,869 11/1996 Diaz et al. ......................... 251/129.04
`
`
`
`III US005730.165A
`
`Patent Number:
`11
`45 Date of Patent:
`
`5,730,165
`Mar. 24, 1998
`
`OTHER PUBLICATIONS
`G.J. Teh, I. Dendo. W. H. Ko; "Switched Capacitor Interface
`Circuit for Capacitive Transducers". Transducers '85. 1985
`International Conference on Solid-State Sensors and Actua
`tors. Digest of Technical Papers (Cat. No.85CH2127-9).
`Philadelphia, PA, USA. pp. 60-63. IEEE. Electrochem. Soc.
`NBS. US Nat. Inst. Health. et al. 11-14 Jun, 1985.
`M. Yamada et al. "A Switched Capacitor Interface for
`Capacitive Pressure Sensors". IEEE Transactions on Instru
`mentation & Measurement, vol. 41. No. 1. Feb. 1992, pp.
`81-86.
`Linear Technology, Inc., Application Note 3, Jul. 1985.
`Primary Examiner-John Rivell
`Attorney, Agent, or Firm-David Kiewit
`57
`ABSTRACT
`A capacitive field sensor, which may be used for the control
`of a water supply valve in a basin or fountain, employs a
`single coupling plate to detect a change in capacitance to
`ground. The apparatus comprises a circuit for charging a
`sensing electrode and a switching element acting to remove
`charge from the sensing electrode and to transfer it to a
`charge detection circuit. The time interval employed for the
`charging and discharging steps can vary widely. Usually at
`least one of the charge or discharge pulses is on the order of
`a hundred nanoseconds, and is shorter in duration than a
`characteristic conduction time for a body of water disposed
`about the sensing plate. Thus, the sensor can detect the
`presence of a user near a controlled faucet without being
`subject to measurement artifacts arising from standing
`water. In a controller for a water basin, a short charge or
`discharge pulse duration may be used when the controlled
`valve is closed, and a longer duration, which allows con
`duction through the water, may be used when the valve is
`open. The long duration measurement can detect the con
`tinued presence of the user as long as the user's hand
`remains in the stream of water.
`63 Claims, 7 Drawing Sheets
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 1 of 18
`
`

`

`U.S. Patent
`US. Patent
`
`Mar. 24, 1993
`Mar. 24, 1998
`
`Sheet 1 of 7
`Sheet 1 of 7
`
`5,730,165
`5,730,165
`
`
`
`
`
`Petitioner STMICROELECTRONICS, INC,
`
`EX. 1026, IPR2021-01161 Page 2 of 18
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 2 of 18
`
`

`

`U.S. Patent
`
`Mar. 24, 1998
`
`Sheet 2 of 7
`
`5,730,165
`
`I c.
`
`Apparent
`Capacitance
`
`
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 3 of 18
`
`

`

`US. Patent
`U.S. Patent
`
`Mar. 24, 1998
`Mar. 24, 1998
`
`Sheet 3 of 7
`Sheet 3 of 7
`
`5,730,165
`5,730,165
`
`
`
`
`
`Petitioner STMICROELECTRONICS, INC,
`
`EX. 1026, IPR2021-01161 Page 4 of 18
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 4 of 18
`
`

`

`U.S. Patent
`US. Patent
`
`Mar. 24, 1998
`Mar. 24, 1998
`
`Sheet 4 of 7
`Sheet 4 of 7
`
`5,730,165
`5,730,165
`
`16
`
`N
`a:
`
`g
`
`a
`o
`Q
`g ..
`
`1
`.
`i
`{1:1 8"
`\
`1
`..
`
`'5\
`8 :3 ll
`
`l
`
`/" \I
`.\1
`I \\
`
`\14
`
`J110
`
`56
`
`119
`
`‘
`
`,1
`1
`
`/
`
`1
`H?
`I
`I
`1 “HI,
`/
`‘x\ .“ "“””"”;““““
`;
`“
`xi
`1
`>
`u
`I
`1
`“)1
`
`52
`
`I118
`
`\
`
`Petitioner STMICROELECTRONICS, INC,
`
`EX. 1026, IPR2021-01161 Page 5 of 18
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 5 of 18
`
`

`

`U.S. Patent
`US. Patent
`
`Mar. 24, 1998
`Mar. 24, 1998
`
`Sheet 5 of 7
`Sheet S of 7
`
`5,730,165
`5,730,165
`
`
`
`
`
`Petitioner STMICROELECTRONICS, INC,
`
`EX. 1026, IPR2021-01161 Page 6 of 18
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 6 of 18
`
`

`

`US. Patent
`U.S. Patent
`
`Mar. 24, 1998
`Mar. 24, 1998
`
`Sheet 6 of 7
`Sheet 6 of 7
`
`5,730,165
`5,730,165
`
` 24
`
`2s
`
`16
`16
`
`&
`41
`41H\\‘:II '
`‘57;
`u I
`22
`144,
`22
`
`144
`
`150
`15°
`
`=
`
`156
`156
`
`43
`43
`
`136
`" 136
`I
`
`22
`22
`
`14
`14 I.
`
`Yis
`154
`
`152
`152
`
`Petitioner STMICROELECTRONICS, INC,
`
`EX. 1026, IPR2021-01161 Page 7 of 18
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 7 of 18
`
`

`

`U.S. Patent
`US. Patent
`
`Mar. 24, 1998
`Mar. 24, 1998
`
`Sheet 7 of 7
`Sheet 7 of 7
`
`5,730,165
`5,730,165
`
`flow /0
`
`
`
`52
`52
`
`1 00
`
`64
`
`W N46
`
`46
`
`62
`62
`
`w
`
`16
`
`—_I!-
`
`160
`
`160
`
`-
`
`102
`
`50
`50
`
`54
`
`118
`118
`
`56
`56
`
`12
`12
`
`
`
`82
`82
`
`I I
`
`8O
`80
`
`Petitioner STMICROELECTRONICS, INC,
`
`EX. 1026, IPR2021-01161 Page 8 of 18
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 8 of 18
`
`

`

`5,730,165
`
`1
`TME DOMAN CAPACTIVE FIELD
`DETECTOR
`
`BACKGROUND OF THE INVENTION
`
`10
`
`15
`
`25
`
`30
`
`35
`
`1. Field of the Invention
`The present invention deals with capacitive field sensors
`employing a single coupling plate to emit and detect a field
`disturbance.
`2. Description of Prior Art
`Many methods of measuring capacitance are known to the
`art. Of relevance to the present disclosure are those inferring
`the value of a capacitor under test from measurements of the
`time required to charge or discharge the circuit element
`under controlled conditions. Notable among these are:
`U.S. Pat. No. 5,329,239, wherein Kindermann et al.
`disclose a microprocessor controlled multimeter charg
`ing an unknown capacitance from a constant voltage
`source when a first switch is closed and discharging it
`through a selected resistor when a second switch is
`closed.
`U.S. Pat. No. 5.294.889, wherein Heep et al. disclose a
`measurement circuit using a constant current source to
`discharge the unknown capacitor.
`U.S. Pat. No. 5,159.276, wherein Reddy describes a
`circuit for detecting leaks by measuring the capacitance
`of a permeable coaxial cable with a switched constant
`current source DC coupled to the cable
`Capacitive field sensors are commonly used in a wide
`variety of applications, such as security systems, door safety
`systems, human interfaces such as keypads, material han
`ding controls, and the like. Such sensors can be divided into
`three broad classes: 1) those that emit and sense an electric
`field using separate coupling plates; and 2) those that
`employ a single coupling plate to emit and detect field
`disturbance; and 3) those that passively detect electric fields
`generated by, or present on, or ambient fields disturbed by,
`the object sensed.
`Many existing sensors employing a single coupling plate
`employ AC field techniques, and connect the coupling plate
`to an AC source, such as an RF signal source. Fluctuations
`of the signal level at the coupling plate are monitored to
`detect the proximity of an object that absorbs the electric
`field. Various known sensors of this sort include:
`A sensor employing a capacitive bridge circuit to detect
`the signal fluctuations. In this case the bridge is used to
`suppress background capacitance and to allow for high
`gain amplification of the relatively small changes in the
`signals on the plate.
`A sensor placing the plate in a tuned circuit, so that
`changes in plate capacitance caused by moving proxi
`mate objects slightly alters the tuned circuit's resonant
`frequency, which may be monitored by various means.
`55
`A sensor in which the plate is connected to an RC
`network, a time constant of which changes responsive
`to a change in capacitance on the plate. A variation of
`this type of sensor uses a fixed current source to charge
`the plate; and determines capacitance changes by mea
`suring the changes in the charging rate from a reference
`slope. Commonly the rate is determined with the help
`of a voltage comparator and a reference voltage.
`It is well known that sinusoidal AC signals are not a
`prerequisite for such sensors, and that other wave shapes can
`also be conveniently used, e.g. square waves or pulses, with
`the same essential effect.
`
`45
`
`65
`
`2
`Problems with existing designs include:
`emission of radio frequency interference-particularly
`with either pulsed or CW RF designs;
`susceptibility to external non-capacitive coupling, such as
`purely resistive paths from the plate to earth, which
`disturb the measurement process;
`high susceptibility to moisture in the region of the plate;
`inability to monitor small changes in the capacitance of
`large objects, or of objects providing large background
`capacitances;
`inability to automatically adapt to variations in the initial
`plate capacitance, especially if the overall possible
`range of such capacitance is large; and
`inability to tolerate adjacent sensor crosstalk
`Of the above cited problems, that of susceptibility to
`external non-capacitive coupling deserves special mention.
`While resistive paths to earth can sometimes be overcome
`by appropriate insulation or by a change in physical con
`figuration of the sensing environment, in many instances this
`is simply not possible. For example, in controlling a water
`faucet with a sensor using the entire faucet as the plate,
`ambient water will cause an unpredictable and time-varying
`capacitance to earth. Because of water splashing around the
`base of the spout and because of conduction through the
`water in the pipe, this varying impedance will be present
`even if the pipe is plastic. In any prior art sensor such
`conduction paths-even though non-capacitive in nature
`will effect the sensing circuit adversely. For example, if the
`sensor employs an RC circuit or variation thereof, the stray
`conduction path will rob the plate of charging current and
`will thus alter its apparent time constant. In tuned detection
`or bridge circuits, capacitive coupling to the plate will in and
`of itself become a fluctuating reactance in the presence of an
`external fluctuating conductance path, and will render the
`circuit worthless. Moreover, it is clear that there is no value
`of coupling capacitance for which this is not so.
`The shortcomings of prior art capacitive sensors have led
`many designers of proximity control systems that need to
`function in the presence of water, or other weakly conduct
`ing liquid media, to employ sensors projecting an energetic
`beam into a sensing zone and measuring the reflection of that
`beam as an indicator of a user's presence. Such systems have
`employed visible and near-infrared light, microwaves, and
`ultrasonic acoustic projected beams. Notable among such
`prior an in the area of controlling water-supply equipment
`are:
`U.S. Pat. No. 5,033.508 U.S. Pat. No. 4972,070 and U.S.
`Pat. No. 4,872.485, wherein Laverty teaches various
`aspects of infra-red control systems for the control of a
`water fountain;
`U.S. Pat. No. 5025,516, wherein Wilson teaches a con
`vergent optical beam arrangement for the control of a
`wash-basin water fountain.
`U.S. Pat. No. 5566,702, issued Oct. 22, 1996, wherein the
`applicant in the present case teaches an adaptive infra
`red faucet controller responsive to both proximity and
`motion. The teachings of U.S. Pat. No. 5.566,702 are
`herein incorporated by reference.
`SUMMARY OF THE INVENTION
`The present invention cures the above defects in the prior
`sensing art, and provides a sensor operable with a wide
`variety of sensing plate configurations. Such a sensor can be
`connected to a wide variety of objects and is not limited to
`the use of a prefabricated plate. Such objects might include
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 9 of 18
`
`

`

`5,730,165
`
`4
`charging or discharging current, the pulse source supplying
`pulses of a different duration (or pulsewidth) when the valve
`is opened than when the valve is closed.
`It is a still further object of the invention to provide
`capacitive sensing means for the control of a water delivery
`valve, the sensing means insensitive to the presence of
`standing water within a supply pipe or external to, but
`adjacent, the piping through which the water is delivered.
`DESCRIPTION OF THE DRAWING
`FIG. 1 of the drawing is an electrical schematic repre
`sentation of a sensing plate surrounded by a water film
`providing an electrically conducting path from the plate to
`earth.
`FIG. 2 of the drawing is a simplified electrical schematic
`view of the water film of FIG. 1.
`FIG. 3 of the drawing shows a curve characterizing the
`temporal response characteristics of a shunting conductor of
`interest.
`FIG. 4 of the drawing is a block diagram of a sensor
`having a single switch or Switching element, a plate
`charging circuit (which may be a resistor or other current
`source).
`FIG. 5 of the drawing is a block diagram of a circuit
`similar to that of FIG. 3, but having a second switch to
`provide charging. An optional charge subtraction circuit is
`shown in phantom in FIG. 4.
`FIG. 6 of the drawing is a timing diagram showing control
`of the two switches of FIG. 5.
`FIG. 7 of the drawing is a detailed circuit schematic of a
`sensor conforming to FIG. 5
`FIG. 8 of the drawing is a partially cut away elevational
`view of a drinking fountain controlled by apparatus of the
`invention.
`FIG. 9 of the drawing is a cut-away view of a sensor of
`the invention controlling water flow through a wash-basin
`faucet.
`FIG. 10 of the drawing is a schematic block diagram of an
`embodiment of the sensor of the invention that includes
`circuits to allow the modification of pulse durations, which
`is useful in the control of water-basin faucets.
`
`O
`
`15
`
`20
`
`25
`
`30
`
`35
`
`3
`door mounted safety sensing strips, safety zone floor mats
`and strips, automatic faucets and water fountains, valuable
`fixed objects that are to be protected from theft or tampering,
`moving or flowing industrial materials, commodities having
`a variable level within a hopper or tank, etc.
`The present invention employs the measurement of elec
`tric charge imposed upon, and shortly thereafter removed
`from, a sensing electrode (conventionally referred to as a
`"plate"). The sensing electrode may be an actual metal plate
`having a predetermined size and shape, or may be an entire
`conductive object, such as a faucet or a metal door. The time
`interval employed for the charge/discharge cycle can vary
`according to specific requirements. For example, it is known
`from experiment that sensing intervals of less than several
`hundred nanoseconds (ns) or less act to suppress the detec
`tion of localized amounts of moisture or standing water (the
`pulse width selected for this purpose will vary with the
`environment of the measurement, and is often less than one
`hundred nanoseconds). Larger measurement intervals will
`increasingly make such a sensor 'reach through' moisture
`and standing water (or "through' internal water content in an
`object sensed), to detect what appears to be higher and
`higher levels of apparent capacitance. A 100 nsec duration is
`approximately optimal when sensing a user's hand proxi
`mate the spout of a wash-basin that may have water standing
`thereabout. Other objects may require different durations.
`Apparatus of the invention comprises a circuit for charg
`ing a sensing electrode, and a switching element acting to
`remove charge from the sensing electrode and to transfer it
`to a charge detection circuit. Although the charging circuit
`may be as simple as a resistor or other type of current source,
`a better implementation uses a second switching element to
`charge the plate to a known voltage.
`A preferred embodiment of the invention comprises a
`holding capacitor to measure the charge drained from the
`plate. In this embodiment, a microprocessor can collect a
`number of readings and perform signal averaging and non
`linear filtering to effectively compensate for both impulse
`and stochastic noise, thereby allowing an increased effective
`gain of the sensor.
`A charge subtractor is optionally employed to subtract
`charge from the holding capacitor, thereby increasing
`dynamic range and canceling offset effects that may be
`introduced from charge injection by the switch(es) or from
`background levels of plate capacitance and the wiring
`thereto.
`In a preferred embodiment, algorithms stored in a com
`puter memory are employed to provide for automatic cali
`bration of the sensor, to track circuit drift, to track environ
`mental changes, and to provide output processing as may be
`required for a particular application.
`It is an object of the invention to provide capacitive
`sensing means for the control of a water delivery valve. the
`sensing means acting, when the valve is closed, to determine
`when the valve should be opened responsive to a user's
`approach, the sensing means acting, when the valve is open,
`to determine the duration during which the valve should be
`held open responsive to the user's continued presence proxi
`mate the valve.
`It is an additional object of the invention to provide
`capacitive sensing means for the control of a water delivery
`valve, the sensing means comprising a capacitor plate
`DC-coupled to a charge measurement circuit.
`It is yet a further object of the invention to provide
`capacitive sensing means for the control of a water delivery
`valve, the sensing means comprising a pulsed source of
`
`45
`
`50
`
`55
`
`65
`
`DESCRIPTION OF THE PREFERRED
`EMBODMENT
`Turning initially to FIG. 1 of the drawing, one finds an
`electrical schematic of a sensing system of the invention 10
`wherein a sensing plate 12 connected to a sensor circuit 14
`by a wire or cable 16 is surrounded by a conductive water
`film 18 shown with a dash-dot line. The water film 18 is
`shown in contact with both a spout 21 of a water faucet and
`another metallic object 20 connected to an earth ground 22.
`That is, regardless of the physical details of the conduction
`processes, the sensing plate 12 is connected to a shunting
`conductor 18 having time-dependent conduction properties
`to be discussed in greater detail hereinafter. Also shown is an
`object 24 (which may be a person intending to use the spout
`21), the object 24 approaching the plate 12 along a path
`indicated with the arrow 26 in FIG.1. The plate 12, the water
`film 18, and the metal object 20 may be atop a nonconduc
`tive surface 28, depicted with a dashed line in FIG. 1. The
`schematic of FIG. 1 is representative of a water faucet being
`used as a bulk proximity sensor, wherein the pipe connecting
`the spout 21 to the water supply comprises a short piece of
`plastic tubing for electrical isolation, and where water
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 10 of 18
`
`

`

`25
`
`S
`splashes have accumulated as a film 18 disposed on a
`counter-top 28 around the base of the spout 21.
`A slightly different model of the electrical conduction and
`reactance paths around plate 12 and object 24 of FIG. 1 is
`shown in FIG. 2 of the drawing. As is well known in the
`electrochemical arts, a body 18 of water shunting the plate
`12 to ground can be modeled as a two dimensional array of
`infinite series of resistors 30 and capacitors 32 connected
`between the plate 12 and earth 22. As shown in FIG. 2 of the
`drawing, the appropriate sensing model also includes a
`second conductive path 33 connecting the sensing plate 12
`to earth 22. This second path comprises a parallel combi
`nation of a resistance 34 and a capacitance to earth 36. For
`modeling purposes, the infinite series can be conveniently
`reduced to an approximation shown in FIG. 2, where a finite
`series of resistors and capacitors 30 and 32 is shown. It
`should not be assumed from the numbering scheme that all
`resistors 30 or capacitors 32 are equal in value; they assur
`edly are not, and indeed vary considerably. The capacitance
`between the plate 12 and the object 24 (i.e., the electrical
`quantity to be detected by the sensor circuit 14) is repre
`sented in FIG. 2 with the reference numeral 40. It is also
`noted that the object 24 has a capacitance to earth 42, at least
`part of which is free space capacitance.
`From this model it can be easily seen that there is a strong
`frequency dependence of the effective total capacitance
`measurable on plate 12. If an ac voltage is applied to the
`plate 12, at very low frequencies, the capacitors 32 can all
`charge and discharge fully on each sinusoidal cycle with
`little phase delay. At increasing frequencies, the capacitors
`32 become increasingly difficult to charge through the
`resistances 30; that is, the RC network acts as a low-pass
`filter having an upper cut-off at a characteristic frequency
`or, equivalently, there is a characteristic time constant for an
`ionic conductor such that the conductor will appear to not
`respond to pulsed signals having a duration significantly less
`than the time constant. Furthermore, the degree to which the
`capacitors 32 contribute to the measurable capacitance value
`of 12 is graduated from one extreme to the other. Only the
`fixed capacitances 36, 42, and 40 remain constant with
`respect to frequency. Thus, it appears that a capacitive
`proximity sensing approach that would achieve a desired
`independence from the effects of the incidental presence of
`an ionic conductor could be based on the use of short pulse
`durations in the detection circuit, the pulse durations being
`45
`chosen to be short enough that the ionic species present do
`not contribute to the measurement (which, as noted supra,
`has been found to require pulses having a duration generally
`less than several hundred nanoseconds or so, and often less
`than one hundred nanoseconds). Moreover, as will be dis
`closed in greater detail hereinafter, operating the sensor at a
`plurality of frequencies or pulsewidths is advantageous in
`situations in which the amount of ambient water changes in
`a foreseeable way--e.g., a different charge and/or discharge
`duration may be preferred for sensing a user's approach to
`a faucet than for sensing the user's continued presence
`proximate the faucet.
`Turning now to FIG. 3 of the drawing, one finds a curve
`illustrating the temporal response of a shunting conductor 18
`of interest-e.g. a sheet or film of water spilled about a spout
`21 or a portion of water contained in a pipe 41 intermediate
`a valve 43 and a spout 21. This temporal response was
`elucidated with pulsed capacitance measurements (using
`apparatus disclosed in detail hereinafter) and shows the
`apparent capacitance measured with pulses having a variety
`of durations. When the pulses used in the measurement are
`shorter than a first predetermined value, indicated as t in
`
`50
`
`55
`
`65
`
`5,730,165
`
`O
`
`15
`
`35
`
`6
`FIG. 3, the shunting conductor 18 does not contribute to the
`measurement and the apparent capacitance value, C, is
`relatively low. When pulses longer than a second predeter
`mined value, t, are used, the time-dependent shunting
`conductor 18 contributes to the measurement and a higher
`apparent capacitance, C is observed. Both the values oft
`and t and the exact shape of the curve 45 are expected to
`depend on a variety of factors including the choice of
`geometry of the shunting conductor 18, the composition of
`the conductor (e.g., the salinity of water standing in a pipe),
`and the ambient temperature. Moreover, because the gener
`ally smooth and continuous nature of the response curve 45.
`a wide range of values for the pulse width can be selected for
`a given measurement. For a case of particular interest, that
`of water spilled about a spout 21, t is on the order of 100
`insec, while t is on the order of 1 usec.
`As noted supra, the resistance 34 between the sensing
`plate and earth 22 may be highly variable, depending on the
`purity and size of the water film 18 splashed about the plate
`12 (which may be the spout 21) as well as depending on the
`degree of contact between the film 18 and a grounded
`conductor 20. This value may change from one moment to
`the next, and will also change with variations in ambient
`temperature. Even the bulk capacitance 36 between the
`sensing plate 12 and ground 22 may vary with time-e.g. if
`an additional object such as a paper towel is left draped over
`the spout 21.
`Turning now to FIG. 4 of the drawing, one finds a block
`diagram of one embodiment of the sensor 14 of the inven
`tion. In this embodiment a voltage-limited current source 44
`(which in the simplest variation is simply a resistor con
`nected to a fixed voltage source 46) feeds a charging current
`to the plate 12. The current supplied by the source 44 is
`selected so that the plate 12 is charged to a predetermined
`fraction of the supply voltage V+during a first interval
`during which a discharging switch 50 is open. At the end of
`the charging interval the discharging switch 50, which is
`preferably controlled by a microprocessor 52 via a control
`line 54, closes briefly. This rapidly discharges the sensing
`plate 12 into a charge detector 56, the amount of charge so
`transferred being representative of the capacitance of the
`sensing plate 12. The charge-discharge process can be
`repeated numerous times, in which case the charge mea
`surement means 56 aggregates the charge from the plate 12
`over several operating cycles. After a predetermined number
`of cycles of charge and discharge, the charge detector 56 is
`examined for total final charge by the controller 52, and as
`a result the controller 52 may generate an output control
`signal on an output line 58-e.g. which may be used to
`cause a faucet 21 to open. As is common in the control arts,
`the controller 52 may also comprise one or more control
`inputs 60, which may include sensitivity settings and the
`like. After each reading, the controller 52 resets the charge
`detector 56 to allow it to accumulate a fresh set of charges
`from the plate 12. Alternatively, the controller 52 can take a
`reading after each individual cycle of the discharging switch
`50, and then integrate (or otherwise filter) the readings over
`a number of cycles prior to making a logical decision
`resulting in a control output. Also, as will be understood by
`those skilled in the art, various combinations of signal
`integration cycles by the charge detector 56 and by internal
`algorithmic processes in the controller 52 may be used.
`The choice of time periods over which changes in capaci
`tance are measured distinguishes between "proximity" and
`"motion" sensing methods. Proximity sensors are ideally
`those measuring a change in capacitance with respect to an
`invariant reference level. To avoid problems with compo
`
`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1026, IPR2021-01161 Page 11 of 18
`
`

`

`5,730,165
`
`O
`
`15
`
`20
`
`25
`
`30
`
`35
`
`7
`nent aging and drift effects, a practical adaptive proximity
`sensor is one measuring a change of capacitance with
`respect to a slowly varying reference level-e.g., a variation
`occurring over a time period significantly longer than the
`maximum time a user 24 would interact with a controlled
`mechanism. (A sensor of this sort is disclosed in the inven
`tor's co-pending application 08/266.814.) Motion sensors,
`on the other hand, are those measuring only a rapid change
`in capacitance-e.g., those responsive to the absolute value
`of the algebraic difference between capacitance values mea
`sured at two instants exceeding a predetermined value.
`Correspondingly, motion sensors may be configured to aver
`age several readings taken during a requisite short sensing
`interval in order to avoid problems with noise.
`It is noteworthy that there is no coupling capacitor
`between the sensor circuit 14 and the plate 12. In the
`presence of eternal conductances and reactances to earth
`such a coupling capacitor would inject its own reactance into
`the system, and the sensor would no longer merely be
`reading charge on plate 12, but would also be reading charge
`bled onto the coupling capacitor from other sources. The
`total charge measured in such an arrangement would vary
`with the values of the resistance 30 and capacitance 32 of the
`water film and with the direct resistance to ground 34 of the
`sensing plate 12.
`It may be noted that the circuit of FIG. 4 is unable to
`handle cases in which the magnitude of the direct resistance
`to earth 34 is so low as to prevent the plate 12 from
`becoming fully charged. Calculations must be made to
`ascertain that this conductance path 34, if present, cannot
`interfere with valid signal readings by loading the current
`source 44. Also, since no provision is made in the circuit of
`FIG. 4 to shut of the current source 44, when the discharg
`ing switch 50 closes it will conduct charge from the source
`44 into the charge detector 56 as well. This additional charge
`can usually be accounted for as a fixed offset.
`A preferred embodiment of the invention is schematically
`depicted in FIG. 5. Here a second, charging, switch 62 is
`employed in place of the current source 44. The charging
`switch 62, like the discharging switch 50, is preferably a low
`resistance switching element, such as a transistor, operating
`under control of the microprocessor 52 via a charging
`control line 64, to charge the plate 12 very quickly to the
`known voltage V+. Should there be a conductive path
`offered by a low direct resistance 34, the current flow
`through the resistance 34 is notable to significantly drop the
`voltage impressed on plate 12, provided that the relative
`impedances of the direct resistance 34 and of the charging
`switch 62 are highly disparate.
`Turning now to FIG. 6 of the drawing, one finds a timing
`diagram depicting a preferred mode of closing and opening
`the charging 62 and discharging 50 switches in sequence. As
`shown by the top trace 70, the charging switch 62 closes at
`a first time, indicated as t, thereby connecting the plate 12
`to the voltage source 46 so that the plate 12 charges quickly
`(as illustrated by a trace labeled with reference numeral 72)
`and reaches saturation at or before a second time t (The rate
`of rise of the waveform 72 depends on the bulk capacitance
`of the plate 12. and the internal resistance of the charging
`switch 62) at which the charging switch 62 disconnects the
`plate 12 from the source of DC voltage 46. After a brief
`delay (t-t), which may be a few nanoseconds, and which
`is chosen to prevent switch crossover conduction, the dis
`charging switch 50 (shown in waveform 74) closes at t
`thereby connecting the plate 12 to the charge measurement
`means 56 so as to rapidly discharge the plate 12. The
`waveform labeled with reference numeral 76 shows the rise
`
`8
`of charge in the charge detector 56 after the discharging
`switch 50 closes.
`Because the switches 62,50 have intrinsic internal capaci
`tances which inject charge into the charge detector 56, and
`because the plate 12 may have a very large inherent capaci
`tance 36, it is often desirable to cancel these charges as fully
`as possible to prevent saturating the charge detector 56 with
`these background signals. To this end, a charge subtractor 80
`is provided in some embodiments of the invention. When
`pulsed by a buck line 82 from the controller 52 (shown as
`waveform 84 in FIG. 6) the charge subtractor 80 subtracts
`charge from the charge detector 56. With such a circuit, the
`output of the charge detector 56 would look like waveform
`86 of FIG. 6, rather than like waveform76. Because only the
`charge detector's offset is affected by the charge subtractor
`80, there is no change in gain of the charge detector and the
`overall system sensitivity remains unaffected.
`The effect of the circuit of FIG. 5 in wet environments is
`to prevent capacito