WO1990014604A1 - Proximity sensor - Google Patents

Proximity sensor Download PDF

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Publication number
WO1990014604A1
WO1990014604A1 PCT/GB1990/000746 GB9000746W WO9014604A1 WO 1990014604 A1 WO1990014604 A1 WO 1990014604A1 GB 9000746 W GB9000746 W GB 9000746W WO 9014604 A1 WO9014604 A1 WO 9014604A1
Authority
WO
WIPO (PCT)
Prior art keywords
buffer
proximity sensor
sensing
plate
sensor
Prior art date
Application number
PCT/GB1990/000746
Other languages
French (fr)
Inventor
Thomas William Bach
Original Assignee
Moonstone Designs Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Moonstone Designs Limited filed Critical Moonstone Designs Limited
Publication of WO1990014604A1 publication Critical patent/WO1990014604A1/en
Priority to GB9124383A priority Critical patent/GB2250822B/en

Links

Classifications

    • GPHYSICS
    • G01MEASURING; TESTING
    • G01VGEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G01V3/08Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
    • G01V3/088Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices operating with electric fields
    • HELECTRICITY
    • H03ELECTRONIC CIRCUITRY
    • H03KPULSE TECHNIQUE
    • H03K17/00Electronic switching or gating, i.e. not by contact-making and –breaking
    • H03K17/94Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the way in which the control signals are generated
    • H03K17/945Proximity switches
    • H03K17/955Proximity switches using a capacitive detector

Definitions

  • the present invention relates to proximity sensors and in particular to capacitive proximity sensors for use for example in touch-sensitive key ⁇ boards or pressure-sensitive pads.
  • sensors of this type measure a change in capacitance of a capacitor, caused by changes in ambient dielectric conditions, either by detecting the change in frequency of an associated oscillator, the change in phase when the capacitor is connected into a bridge arrangement, or the change in source voltage when a gate capacitance is changed with constant charge. Some of these sensors detect the absolute change in capacitance and others detect the relative change.
  • One example includes a configuration of wires or conducting strips to define the sensing field of the capacitor.
  • a proximity sensor comprising capacitive means sensitive to changes in ambient dielectric conditions and means for detecting the changes sensed by the capacitive means, the sensor being character ⁇ ized in that it also includes buffer means associated with the capacitive means for directionally control ⁇ ling the sensitivity of said capacitive means.
  • the change detecting means comprising an oscillator circuit, the oscillat ⁇ ing frequency of which varies in dependence on the change in capacitance of the capacitive means .
  • the change detecting means may also include means for generating an analogue output indicative of the oscillating frequency, and preferably also means for displaying the analogue output.
  • the capacitive means preferably includes a sensing electrode such as a plate
  • the buffer means includes a buffer member forming the other electrode of the capacitive means, the buffer member being driven by a buffer amplifier in phase with the sensing electrode.
  • the buffer member is suitably in the form of a plate and the buffer means may include one or more further buffer plates or buffer rings also driven by the buffer amplifier and being posi ⁇ tioned relative to the sensing plate so that the 'sensing plate only senses dielectric changes in the required direction.
  • the sensing electrode, buffer electrode and other buffer means may comprise wires positioned substantially parallel to each other, or the buffer electrode and other buffer means may comprise half-cylinders positioned co-axially around the sensing wire.
  • the proximity sensor in accordance with the present invention, can be used in many applications.
  • a gas or damp meter in which case earth or ground means, such as a plate, ring or wire, may be utilised in addition to the buffer means .
  • the sensor may also be used in pressure pads or weighing mats; in a sensitive microswitch which can be acti ⁇ vated when the oscillating frequency reaches a pre ⁇ set threshold value; or as a replacement for a mer ⁇ cury switch.
  • the dis ⁇ tance between the sensing electrode and the ground means may be varied so as to control the depth into a material, such as a wall, at which dielectric changes caused by moisture are sensed by the sensor.
  • a layer of material the dielectric characteristics of which change with pressure applied thereto may overlay one or more of the sensors such that said sensor or sensors can sense the dielectric changes and generate an output indicative of the pressure.
  • FIG. 1 shows schematically a simple proximity sensor in accordance with one embodiment of the invention
  • Figures 2 to 4 show schematically-three further embodiments each suitable for use in a damp meter or dielectric tester for commercial or domestic use
  • Figure 5 shows schematically another embodiment of the proximity sensor which can be used as a re ⁇ placement for a mercury switch
  • Figures 6 and 7 show schematically two other embodiments suitable as replacements for microswitch- es
  • Figure 8 shows schematically a further embodiment suitable for use in a hydrophone or micro ⁇ phone
  • Figures 9 and 10 show schematically parts of two further embodiments of the proximity sensor.
  • Figure 1 shows a proximity sensor comprising a ⁇ circular sensing plate 1, a circular buffer plate 2 positioned beneath the sensing plate 1 and forming, together with the sensing plate / the electrodes of a capacitor, and a buffer ring 3 located concentrically with the two plates 1 and 2.
  • the sensing plate 1 senses a change in ambient dielectric conditions, which may for example be due to the proximity of a hand or other object or to the presence of moisture in a wall or of a particular gas.
  • the sensing plate 1 is connected to an oscillator 4, which consists of a unity-gain buffer amplifier 5, an inverting Schmitt trigger 6 with hysteresis, and a large value resistor 7.
  • the output of the Schmitt trigger 6 charges and dis ⁇ charges the capacitor, and any change in potential at the sensing plate due to changes in the ambient dielectric conditions causes a corresponding change in the oscillator frequency.
  • the output of the sensing plate 1 is fed to the buffer amplifier 5, the output of which drives the buffer plates 2,3 connect ⁇ ed together in phase with the sensing plate 1.
  • Oscil ⁇ lator output frequency f which is thus dependent on the changes to the ambient dielectric, is passed to a frequency-to-voltage converter 8 and the output vol -age is passed to a display 9, which is preferably an LED display.
  • the sensing plate 1 is only sensitive to dielectric changes in a general direction in front of it, rather than to its side or behind. Furthermore, the buffer plate and ring render the sensor substantially immune to r.f. interference. All the component parts of the sensor can be constructed on a rigid or flexible PCB.
  • the resistor 7 may be capable of allowing a stable oscil ⁇ lator frequency of as low as approximately 4 Hz and a low current consumption.
  • Figure 2 shows an embodiment for use in a damp meter or dielectric tester for commercial or domestic use.
  • the sensing plate 1, buffer plates 2 and 3 and a ground or earth plate 10 are all mounted on one side of a PCB.
  • the sensing field of the plate 1 detects changes in the dielectric constant of the wall caused by the presence of moisture.
  • the effective sensing field penetrates further into the wall and the decrease in the oscillator frequency f is determined by the sensed moisture of the wall.
  • a more complicated embodiment includes a non-inverting amplifier (typi ⁇ cal gain of +10) (not shown in Fig.2) to drive plate 10, which in this case is employed as an additional buffer plate, to enhance the decrease in the oscil ⁇ lator frequency as the moisture content, and thus dielectric constant of the wall, increases.
  • a non-inverting amplifier typi ⁇ cal gain of +10
  • the plate 10 can be driven by an inverting amplifier (typical gain of -10), which causes the oscillator frequency to increase as the moisture content increases .
  • Figure 3 shows an embodiment which includes both an inverting amplifier 11 connected to a buffer plate 12 and a non-inverting amplifier 13 connected to another buffer plate 14.
  • the sensing field generated between the sensing plate and buffer plate 14, which are further apart senses moisture deeper within the wall and decreases the oscillator frequency. This embodiment thus provides an indication of moisture content both at the surface and deep within a material .
  • FIG. 4 shows an alternative configuration, which enables the moisture content to be sensed at depth only.
  • an electronic switch 15 is first switched so that a ground plate 16 is connected into the circuit.
  • the sensing plate 1 acts as a small sensing pad which senses the surface moisture only and a first output voltage indicative of the sensed moisture is generat ⁇ ed.
  • the electronic switch 15 switches both plates 16 and 17 into the circuit with plate 16 acting as a buffer plate and plate 17 as a ground plate.
  • the sensing plate 1 becomes a large sensing pad which senses moisture both at the surface and at depth, and a second output voltage indicative of the sensed moisture is generat- ed.
  • the first and second output voltages are then subtracted by voltage comparison circuit 18, and the difference, which is indicative of the moisture at depth only, is displayed on the display 8.
  • a low frequency oscillator 19 is used to control the electronic switch 15 and the circuit 18.
  • ground plates 16 and 17 as used respectively in the two states, could be used instead as additional buffer plates driven by a non- inverting amplifier with typical gain of +10 or an inverting amplifier with typical gain of -10.
  • sensing, buffer and ground means in figures 2 to 4 have been illustrated as rectangular plates located side by side. However, in practice, it may be pref ⁇ erable to construct the sensing plate 1 and buffer plate 2 as circular discs, as shown in Figure 1, and the other buffer and ground means as rings or arcs or parts of rings located concentrically with the discs .
  • FIG. 5 shows another application of the proximity sensor in accordance with the invention, in which it is used as a replacement for microswitch.
  • a sensor 20 is positioned to detect the proximity of a meral plate 21 hinged at 22 and weighted by weight 21a, so that the plate 21 is suspended under the effect of gravity.
  • the output of the sensor is displayed on a display 22.
  • Figure 6 shows the use of the sensor 19 to replace a microswitch, wherein the sensor senses the proximity of an object, such as a door shown partly at 23.
  • Figure 7 shows the use of the sensor to replace a slotted opto switch.
  • This form can use two sensors 24,25 with the sensing plates individually buffered but forming the two plates of a capacitor in a suit ⁇ able oscillator with both plates symmetrically driv ⁇ en. This enables a distance between plates to be used which is typically ten times the sensing plate diameter when in disc form. In this way, the proxim ⁇ ity of an object 26 placed between the sensors can be detected.
  • Figure 8 shows the sensor used in a pressure- sensitive pad or weighing mat.
  • a sensor 27 is over ⁇ laid by a layer 28 of resilient dielectric material, for example rubber, the dielectric characteristics of which change with pressure applied thereto. The change in these characteristics is sensed by the sensor and displayed on a suitable display 29. If the dielectric layer 28 has a conductive top surface 30, for example of metal, the dielectric constant of the object weighed does not effect the oscillator frequency of the sensor. This metallisation does not need to be connected in any other way to the sensor circuit.
  • the distance between the sensing plate and buffer plate below it may be varied by the use of rubber springs which compress when force is applied directly to the sensing plate. This embodiment may provide a more linear output than the embodiment using the rubber overlay.
  • the embodiment in Figure 8 forms a very thin weighing mat, pressure sensitive finger pad, a hydrophone or microphone. If multiple cells are used in the weighing situation each is calibrated using soft or hardware against the non-linearities of the resilient material. This allows a weighing plate of cells to be unaffected by an object which has not been well-centred on the mat.
  • the pad can be made of a flexible substrate allowing more portability and conformance to a sub-shell in the case of a hydrophone.
  • the individual cells allow directional microphones to be built as the phase difference can be detected. In general the sensor oscillator runs at at least twice any sensed frequen ⁇ cy.
  • the senor can be used with an overlay of a suitable chemical the dielecrric of which changes with absorption of moisture or specific gases or exposure to radiation.
  • Another security application consists of the mat being fixed to shelving in, for example, retail stores such that the removal of objects signals or sounds an appropriate alarm.
  • Another security application consists of a capacitive sensor fixed to a metal or wooden door to sense passage of objects or people through the door.
  • the sensing buffer and ground means may con ⁇ sist of substantially parallel wires 31 as shown schematically in Figure 9, or the sensing means may be a wire 32 and the buffer and ground means may be half-cylinders 33 positioned co-axially relative to each other and co-axially with and extending beneath the sensing wire as shown schematically in Figure 10.
  • the buffer means, for example in the form of a plate and ring as in figure 1, render the sensor substantially immune to r.f. interference.
  • the buffer plate and/or ring offer low impedance paths to fields approaching the sensor, which means that r.f inter ⁇ ference approaching the sensor from any direction except from the desired sensing direction is greatly attenuated as it cannot pass through the buffer plate or ring.
  • r.f. interference approaching from the desired sensing direction also has little effect on the sensor. This is because the r.f. interference field strength per metre imposes a voltage on the following buffer amplifier which is proportional to the distance between the sensing plate and the earth.
  • the buffer plate can be located between the earth plate and the sensing plate, so that the earth plate is electrically located close behind the sensing plate. In this way the distance between the earth plate and sensing plate can be made small, and thus only a small voltage is induced on the buffer amplifier.
  • a separate earth plate may not be necessary because the use of a buffer plate enables the sensor electronics and its associated earth to be located very close to the sensing plate.

Abstract

A proximity sensor comprises a sensing plate (1), a buffer plate (2) positioned beneath the sensing plate (1) and forming, together with the sensing plate, the electrodes of a capacitor, and a buffer ring (3) located concentrically with the two plates (1) and (2). The sensing plate (1) senses a change in ambient dielectric conditions, which may for example be due to the proximity of a hand or other object or to the presence of moisture in a wall or of a particular gas. The sensing plate (1) is connected to an oscillator (4), such that any change in potential at the sensing plate due to changes in the ambient dielectric conditions causes a corresponding change in the frequency of the oscillator. Due to the location of the buffer plate (2) and ring (3), the sensing plate (1) is only sensitive to dielectric changes in a general direction in front of it, rather than to its side or behind it.

Description

PROXIMITY SENSOR
The present invention relates to proximity sensors and in particular to capacitive proximity sensors for use for example in touch-sensitive key¬ boards or pressure-sensitive pads.
Most known sensors of this type measure a change in capacitance of a capacitor, caused by changes in ambient dielectric conditions, either by detecting the change in frequency of an associated oscillator, the change in phase when the capacitor is connected into a bridge arrangement, or the change in source voltage when a gate capacitance is changed with constant charge. Some of these sensors detect the absolute change in capacitance and others detect the relative change.
The geometry of such known sensors can also be very different. One example includes a configuration of wires or conducting strips to define the sensing field of the capacitor.
However, the capacitance of these known sensors
is dependent on other external factors in addition to the capacitance change caused by the proximity of an object to be detected. For example, a metal object located behind the sensor would generate a background capacitance much larger than the capacitance change caused by the sensed proximity. This large back¬ ground capacitance desensitises the capacitor to the actual capacitance changes required to be detected, so that very high gain amplifiers are generally needed to enhance these changes for detection. Furthermore, such sensors are sensitive to r.f . interference thereby further desensitising them to the actual changes to be detected.
It is an object of the present invention to provide a proximity sensor which is substantially more sensitive to the required changes to be detected than such sensors know hitherto.
According to the present invention there is provided a proximity sensor comprising capacitive means sensitive to changes in ambient dielectric conditions and means for detecting the changes sensed by the capacitive means, the sensor being character¬ ized in that it also includes buffer means associated with the capacitive means for directionally control¬ ling the sensitivity of said capacitive means.
In this way, the effective capacitance of the capacitive means , which varies in dependence on the sensed changes, can be reduced except in the desired direction in which any changes are required to be sensed. In a preferred embodiment, the change detecting means comprising an oscillator circuit, the oscillat¬ ing frequency of which varies in dependence on the change in capacitance of the capacitive means . The change detecting means may also include means for generating an analogue output indicative of the oscillating frequency, and preferably also means for displaying the analogue output.
The capacitive means preferably includes a sensing electrode such as a plate, and the buffer means includes a buffer member forming the other electrode of the capacitive means, the buffer member being driven by a buffer amplifier in phase with the sensing electrode. The buffer member is suitably in the form of a plate and the buffer means may include one or more further buffer plates or buffer rings also driven by the buffer amplifier and being posi¬ tioned relative to the sensing plate so that the 'sensing plate only senses dielectric changes in the required direction.
Alternatively, the sensing electrode, buffer electrode and other buffer means may comprise wires positioned substantially parallel to each other, or the buffer electrode and other buffer means may comprise half-cylinders positioned co-axially around the sensing wire.
The proximity sensor, in accordance with the present invention, can be used in many applications. For example, in a gas or damp meter, in which case earth or ground means, such as a plate, ring or wire, may be utilised in addition to the buffer means . The sensor may also be used in pressure pads or weighing mats; in a sensitive microswitch which can be acti¬ vated when the oscillating frequency reaches a pre¬ set threshold value; or as a replacement for a mer¬ cury switch.
In the application of a damp meter, the dis¬ tance between the sensing electrode and the ground means may be varied so as to control the depth into a material, such as a wall, at which dielectric changes caused by moisture are sensed by the sensor.
In the application of a pressure-sensitive pad or weighing mat, a layer of material the dielectric characteristics of which change with pressure applied thereto may overlay one or more of the sensors such that said sensor or sensors can sense the dielectric changes and generate an output indicative of the pressure.
The invention will now be further described by way of example with reference to the accompanying drawings, in which:-
Figure 1 shows schematically a simple proximity sensor in accordance with one embodiment of the invention,
Figures 2 to 4 show schematically-three further embodiments each suitable for use in a damp meter or dielectric tester for commercial or domestic use,
Figure 5 shows schematically another embodiment of the proximity sensor which can be used as a re¬ placement for a mercury switch,
Figures 6 and 7 show schematically two other embodiments suitable as replacements for microswitch- es,
Figure 8 shows schematically a further embodiment suitable for use in a hydrophone or micro¬ phone, and
Figures 9 and 10 show schematically parts of two further embodiments of the proximity sensor.
Referring now to the Figures, Figure 1 shows a proximity sensor comprising a ^circular sensing plate 1, a circular buffer plate 2 positioned beneath the sensing plate 1 and forming, together with the sensing plate/ the electrodes of a capacitor, and a buffer ring 3 located concentrically with the two plates 1 and 2. The sensing plate 1 senses a change in ambient dielectric conditions, which may for example be due to the proximity of a hand or other object or to the presence of moisture in a wall or of a particular gas. The sensing plate 1 is connected to an oscillator 4, which consists of a unity-gain buffer amplifier 5, an inverting Schmitt trigger 6 with hysteresis, and a large value resistor 7. The output of the Schmitt trigger 6 charges and dis¬ charges the capacitor, and any change in potential at the sensing plate due to changes in the ambient dielectric conditions causes a corresponding change in the oscillator frequency. The output of the sensing plate 1 is fed to the buffer amplifier 5, the output of which drives the buffer plates 2,3 connect¬ ed together in phase with the sensing plate 1. Oscil¬ lator output frequency f, which is thus dependent on the changes to the ambient dielectric, is passed to a frequency-to-voltage converter 8 and the output vol -age is passed to a display 9, which is preferably an LED display. Due to the location of the buffer plate 2 and the concentric buf er ring 3, the sensing plate 1 is only sensitive to dielectric changes in a general direction in front of it, rather than to its side or behind. Furthermore, the buffer plate and ring render the sensor substantially immune to r.f. interference. All the component parts of the sensor can be constructed on a rigid or flexible PCB. The resistor 7 may be capable of allowing a stable oscil¬ lator frequency of as low as approximately 4 Hz and a low current consumption.
Figure 2 shows an embodiment for use in a damp meter or dielectric tester for commercial or domestic use. The sensing plate 1, buffer plates 2 and 3 and a ground or earth plate 10 are all mounted on one side of a PCB. As the sensing plate is brought close to the surface of a wall, the sensing field of the plate 1 detects changes in the dielectric constant of the wall caused by the presence of moisture. By making the distance between sensing plate 1 and ground plate 10 greater, the effective sensing field penetrates further into the wall and the decrease in the oscillator frequency f is determined by the sensed moisture of the wall. A more complicated embodiment includes a non-inverting amplifier (typi¬ cal gain of +10) (not shown in Fig.2) to drive plate 10, which in this case is employed as an additional buffer plate, to enhance the decrease in the oscil¬ lator frequency as the moisture content, and thus dielectric constant of the wall, increases. Alterna- tively the plate 10 can be driven by an inverting amplifier (typical gain of -10), which causes the oscillator frequency to increase as the moisture content increases .
Figure 3 shows an embodiment which includes both an inverting amplifier 11 connected to a buffer plate 12 and a non-inverting amplifier 13 connected to another buffer plate 14. The sensing field 3 generated between the sensing plate 1 and buffer plate 12, which are relatively close together, senses the oscillator frequency. On the other hand, the sensing field generated between the sensing plate and buffer plate 14, which are further apart, senses moisture deeper within the wall and decreases the oscillator frequency. This embodiment thus provides an indication of moisture content both at the surface and deep within a material .
Figure 4 shows an alternative configuration, which enables the moisture content to be sensed at depth only. In this embodiment an electronic switch 15 is first switched so that a ground plate 16 is connected into the circuit. In this state, the sensing plate 1 acts as a small sensing pad which senses the surface moisture only and a first output voltage indicative of the sensed moisture is generat¬ ed.
In a second state, the electronic switch 15 switches both plates 16 and 17 into the circuit with plate 16 acting as a buffer plate and plate 17 as a ground plate. In this state, the sensing plate 1 becomes a large sensing pad which senses moisture both at the surface and at depth, and a second output voltage indicative of the sensed moisture is generat- ed. The first and second output voltages are then subtracted by voltage comparison circuit 18, and the difference, which is indicative of the moisture at depth only, is displayed on the display 8. In this arrangement, a low frequency oscillator 19 is used to control the electronic switch 15 and the circuit 18.
In practice, the ground plates 16 and 17, as used respectively in the two states, could be used instead as additional buffer plates driven by a non- inverting amplifier with typical gain of +10 or an inverting amplifier with typical gain of -10.
For reasons of clarification only, all the sensing, buffer and ground means in figures 2 to 4 have been illustrated as rectangular plates located side by side. However, in practice, it may be pref¬ erable to construct the sensing plate 1 and buffer plate 2 as circular discs, as shown in Figure 1, and the other buffer and ground means as rings or arcs or parts of rings located concentrically with the discs .
Figure 5 shows another application of the proximity sensor in accordance with the invention, in which it is used as a replacement for microswitch. In this embodiment, a sensor 20 is positioned to detect the proximity of a meral plate 21 hinged at 22 and weighted by weight 21a, so that the plate 21 is suspended under the effect of gravity. The output of the sensor is displayed on a display 22.
Figure 6 shows the use of the sensor 19 to replace a microswitch, wherein the sensor senses the proximity of an object, such as a door shown partly at 23.
Figure 7 shows the use of the sensor to replace a slotted opto switch. This form can use two sensors 24,25 with the sensing plates individually buffered but forming the two plates of a capacitor in a suit¬ able oscillator with both plates symmetrically driv¬ en. This enables a distance between plates to be used which is typically ten times the sensing plate diameter when in disc form. In this way, the proxim¬ ity of an object 26 placed between the sensors can be detected.
Figure 8 shows the sensor used in a pressure- sensitive pad or weighing mat. A sensor 27 is over¬ laid by a layer 28 of resilient dielectric material, for example rubber, the dielectric characteristics of which change with pressure applied thereto. The change in these characteristics is sensed by the sensor and displayed on a suitable display 29. If the dielectric layer 28 has a conductive top surface 30, for example of metal, the dielectric constant of the object weighed does not effect the oscillator frequency of the sensor. This metallisation does not need to be connected in any other way to the sensor circuit. As an alternative to the rubber overlay, the distance between the sensing plate and buffer plate below it may be varied by the use of rubber springs which compress when force is applied directly to the sensing plate. This embodiment may provide a more linear output than the embodiment using the rubber overlay.
If made on flexible material the embodiment in Figure 8 forms a very thin weighing mat, pressure sensitive finger pad, a hydrophone or microphone. If multiple cells are used in the weighing situation each is calibrated using soft or hardware against the non-linearities of the resilient material. This allows a weighing plate of cells to be unaffected by an object which has not been well-centred on the mat. The pad can be made of a flexible substrate allowing more portability and conformance to a sub-shell in the case of a hydrophone. The individual cells allow directional microphones to be built as the phase difference can be detected. In general the sensor oscillator runs at at least twice any sensed frequen¬ cy.
In the case of measuring phase shift however it will suffice for the oscillator to run at the sensed frequency. As the oscillator is being frequency modulated normal R.F. theory can be used for detec¬ tion.
In another embodiment the sensor can be used with an overlay of a suitable chemical the dielecrric of which changes with absorption of moisture or specific gases or exposure to radiation.
Many other applications of the present sensor will be envisaged. Further examples of such applica¬ tions are as follows:-
(1) Multiplexing of many sensors in a line or matrix and the demodulation of information into analogue form from each sensor. This gives a three dimension¬ al output of dielectric and can be used for locating voids in concrete.
(2) Duplicating each sensor into a matrix accessed by a multiplexer and microprocessor forming a proxim¬ ity keypad. This keypad is usable in harsh environ¬ ments or through books , maps or pictures. Its sens¬ ing depth is generally at a depth through paper equal to the diameter of a single sensing disc. Through a homogenous medium its maximum sensing depth is about ten times the diameter of the sensing disc.
(3) Using many sensors at close spacing to form a drawing pad which does not need to be touched or with a rubber overlay to form a pressure-sensitive drawing pad. This can be achieved by transparent means, grids of wires -or a conductive medium, to form a superior sensing screen over a VDU which is not smudged by touching.
(4) Using the proximity keypad in a dual mode in which touching signifies one thing and proximity only signifies another.
(5) Using the keypad as a pressure sensitive musical instrument keyboard, or replacing conventional capac¬ itive sensors with the present more sensitive sensors in all keyboard applications.
(6) For security applications using large sensors serially addressed to determine location of people or objects. If weighing mats having weight read-outs are used the number of objects or people can be determined. If capacitive mats are used the location only can be determined. This can be used in stores to determine the illicit removal of objects.
(7) Another security application consists of the mat being fixed to shelving in, for example, retail stores such that the removal of objects signals or sounds an appropriate alarm.
(8) Another security application consists of a capacitive sensor fixed to a metal or wooden door to sense passage of objects or people through the door.
(9) Using the sensor as a self-contained battery powered unit fixed to an object to detect its remov¬ al.
Whilst particular embodiments of the present invention have been described, various modifications will be envisaged without departure from the scope of the invention. For example, instead of plates or rings, the sensing buffer and ground means may con¬ sist of substantially parallel wires 31 as shown schematically in Figure 9, or the sensing means may be a wire 32 and the buffer and ground means may be half-cylinders 33 positioned co-axially relative to each other and co-axially with and extending beneath the sensing wire as shown schematically in Figure 10.
As referred to hereinabove, the buffer means, for example in the form of a plate and ring as in figure 1, render the sensor substantially immune to r.f. interference. This is because the buffer plate and/or ring offer low impedance paths to fields approaching the sensor, which means that r.f inter¬ ference approaching the sensor from any direction except from the desired sensing direction is greatly attenuated as it cannot pass through the buffer plate or ring.
More advantageously, if an earth is provided adjacent the buffer plate or ring, r.f. interference approaching from the desired sensing direction also has little effect on the sensor. This is because the r.f. interference field strength per metre imposes a voltage on the following buffer amplifier which is proportional to the distance between the sensing plate and the earth. If the earth is in the form of a separate plate, the buffer plate can be located between the earth plate and the sensing plate, so that the earth plate is electrically located close behind the sensing plate. In this way the distance between the earth plate and sensing plate can be made small, and thus only a small voltage is induced on the buffer amplifier. In practice, a separate earth plate may not be necessary because the use of a buffer plate enables the sensor electronics and its associated earth to be located very close to the sensing plate.

Claims

1. A proximity sensor comprising capacitive means (1) sensitive to changes in ambient dielectric condi¬ tions and means (4) for detecting the changes sensed by the capacitive means (1), the sensor being charac¬ terized in that it also includes buffer means (2,3) associated with the capacitive means (1) for direc- tionally controlling the sensitivity of said capaci¬ tive means ( 1 ) .
2. A proximity sensor as claimed in claim 1, wherein the change detecting means (4) comprises an oscillator circuit (4), the oscillating frequency of which varies in dependence on the change in capaci¬ tance of the capacitive means ( 1 ) .
3. A proximity sensor as claimed in claim 2, including means ( 8 ) for generating an analogue output indicative of the oscillating frequency.
4. A proximity sensor as claimed in claim 3, including means (9) for displaying the analogue output .
5. A proximity sensor as claimed in any preceding claim, wherein the capacitive means (1) comprises a sensing electrode (1) and the buffer means (2,3) includes a buffer electrode (2) forming the other electrode of the capacitive means ( 1 ) .
6. A proximity sensor as claimed in claim 5, wherein the buffer electrode (2) is driven in phase with the sensing electrode (1) by a buffer amplifier
(5).
7. A proximity sensor as claimed in claim 5 or 6, wherein the buffer means (2,3) includes one or more additional buffer members (3) driven in or out of phase with respect to the sensing electrode and positioned relative to the sensing electrode (1) so that the sensing electrode only senses changes in a required direction.
8. A proximity sensor as claimed in claim 7, wherein the one or more additional buffer members (3) are driven in or out of phase with a gain of greater than unity.
9. A proximity sensor as claimed in claim 5, 6, 7, or 8, wherein the sensing electrode (1) and the buffer electrode (2) are each in the form of a plate.
10. A proximity sensor as claimed in claim 9, wherein the plates are generally circular and the buffer means (2,3) includes at least one further buffer member (3) in the form of a ring (3) or part of a ring positioned concentrically with the buffer plate (2).
11. A proximity sensor as claimed in claim 5, 6, 7 or 8, wherein at least one of the sensing electrode (1) and the buffer means (2,3) is in the form of a wire.
12. A proximity sensor as claimed in any one of claims 7 to 11, including earth or ground means located adjacent the sensing electrode (1) in order to increase the immunity of the sensor to r.f. inter¬ ference approaching the sensor from the required sensing direction.
13. A proximity sensor as claimed in claim 12, wherein the buffer means (2) is located between the sensing electrode (1) and the earth or ground means.
14. A proximity sensor as claimed in claim 12 or 13, wherein the distance between the sensing elec¬ trode (1) and the earth or ground means is variable.
PCT/GB1990/000746 1989-05-17 1990-05-15 Proximity sensor WO1990014604A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB9124383A GB2250822B (en) 1989-05-17 1991-11-13 Proximity sensor

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Cited By (13)

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EP0564164A1 (en) * 1992-04-01 1993-10-06 AT&T Corp. Capacitive proximity sensors
WO1994008257A1 (en) * 1992-09-29 1994-04-14 Minnesota Mining And Manufacturing Company Circuit tracer
US5453960A (en) * 1994-03-24 1995-09-26 Asulab S.A. Watch including a manual control device
EP0723166A1 (en) * 1995-01-18 1996-07-24 Carlo Gavazzi AG Capacitive sensor
EP0911973A2 (en) * 1997-10-21 1999-04-28 STMicroelectronics, Inc. Solid state capacitive switch
US6201398B1 (en) * 1996-03-28 2001-03-13 Oht Inc. Non-contact board inspection probe
WO2007066195A2 (en) * 2005-12-05 2007-06-14 Ciro Scaramucci Device and method for switching and controlling electrical equipment comprising a proximity sensor
WO2010043936A1 (en) * 2008-10-17 2010-04-22 Faurecia Bloc Avant Sensor device for detecting an object in a detection area
JP2012506190A (en) * 2008-10-17 2012-03-08 フォルシア ブロック アバン Object detection device for an automatic vehicle
EP2607932A1 (en) * 2011-12-19 2013-06-26 Rechner Industrie-Elektronik GmbH Capacitative measuring method with controllable directional effect and corresponding capacitative sensor for industrial use
DE102012107412A1 (en) * 2012-08-13 2014-05-28 Jaromir Remes Activity sensor system for use with sensor- or actuator arrangement for detecting presence or activity or inactivity of living beings in to-be monitored space, has sensors arranged side by side, one or multiple actuators and evaluation unit
WO2015104480A1 (en) * 2014-01-09 2015-07-16 Open App Invisible, contactless switch device
WO2017009279A1 (en) * 2015-07-10 2017-01-19 Roberto Airoldi Contactless switch intended to replace a visible contact switch

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GB2005422A (en) * 1977-10-05 1979-04-19 Buck R Proximity switches
US4748390A (en) * 1984-09-19 1988-05-31 Omron Tateisi Electronics Co. Capacitive-type detection device
WO1989003052A1 (en) * 1987-10-02 1989-04-06 Detection Systems Pty. Ltd. Capacitive material presence detecting apparatus
WO1989008352A1 (en) * 1988-03-03 1989-09-08 Setec Messgeräte Gesellschaft M.B.H. Capacitive proximity detector

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GB2005422A (en) * 1977-10-05 1979-04-19 Buck R Proximity switches
US4748390A (en) * 1984-09-19 1988-05-31 Omron Tateisi Electronics Co. Capacitive-type detection device
WO1989003052A1 (en) * 1987-10-02 1989-04-06 Detection Systems Pty. Ltd. Capacitive material presence detecting apparatus
WO1989008352A1 (en) * 1988-03-03 1989-09-08 Setec Messgeräte Gesellschaft M.B.H. Capacitive proximity detector

Cited By (27)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0564164A1 (en) * 1992-04-01 1993-10-06 AT&T Corp. Capacitive proximity sensors
WO1994008257A1 (en) * 1992-09-29 1994-04-14 Minnesota Mining And Manufacturing Company Circuit tracer
US5453960A (en) * 1994-03-24 1995-09-26 Asulab S.A. Watch including a manual control device
EP0674247A1 (en) * 1994-03-24 1995-09-27 Asulab S.A. Clock with manual control
FR2717917A1 (en) * 1994-03-24 1995-09-29 Asulab Sa Watch having a manual control device.
EP0723166A1 (en) * 1995-01-18 1996-07-24 Carlo Gavazzi AG Capacitive sensor
US6201398B1 (en) * 1996-03-28 2001-03-13 Oht Inc. Non-contact board inspection probe
US6373258B2 (en) 1996-03-28 2002-04-16 Naoya Takada Non-contact board inspection probe
EP0911973A2 (en) * 1997-10-21 1999-04-28 STMicroelectronics, Inc. Solid state capacitive switch
EP0911973A3 (en) * 1997-10-21 2000-08-02 STMicroelectronics, Inc. Solid state capacitive switch
WO2007066195A2 (en) * 2005-12-05 2007-06-14 Ciro Scaramucci Device and method for switching and controlling electrical equipment comprising a proximity sensor
WO2007066195A3 (en) * 2005-12-05 2007-10-04 Ciro Scaramucci Device and method for switching and controlling electrical equipment comprising a proximity sensor
WO2010043936A1 (en) * 2008-10-17 2010-04-22 Faurecia Bloc Avant Sensor device for detecting an object in a detection area
US8878550B2 (en) 2008-10-17 2014-11-04 Faurecia Bloc Avant Sensor device for detecting an object in a detection area
CN102246417A (en) * 2008-10-17 2011-11-16 佛吉亚汽车前端模块公司 Sensor device for detecting an object in a detection area
JP2012506042A (en) * 2008-10-17 2012-03-08 フォルシア ブロック アバン Sensor device for detecting an object in a detection area
JP2012506190A (en) * 2008-10-17 2012-03-08 フォルシア ブロック アバン Object detection device for an automatic vehicle
RU2489285C2 (en) * 2008-10-17 2013-08-10 Форесья Блок Аван Sensor for detection object in observation zone
US20110273188A1 (en) * 2008-10-17 2011-11-10 Faurecia Bloc Avant Sensor Device for Detecting an Object in a Detection Area
US8779782B2 (en) 2008-10-17 2014-07-15 Faurecia Bloc Avant Object detection device for an automotive vehicle
EP2607932A1 (en) * 2011-12-19 2013-06-26 Rechner Industrie-Elektronik GmbH Capacitative measuring method with controllable directional effect and corresponding capacitative sensor for industrial use
DE102012107412A1 (en) * 2012-08-13 2014-05-28 Jaromir Remes Activity sensor system for use with sensor- or actuator arrangement for detecting presence or activity or inactivity of living beings in to-be monitored space, has sensors arranged side by side, one or multiple actuators and evaluation unit
DE102012107412B4 (en) * 2012-08-13 2016-03-24 Jaromir Remes Activity sensor, floor or wall construction method and activity evaluation method
WO2015104480A1 (en) * 2014-01-09 2015-07-16 Open App Invisible, contactless switch device
AU2015205454B2 (en) * 2014-01-09 2018-02-01 Roberto Airoldi Invisible, contactless switch device
WO2017009279A1 (en) * 2015-07-10 2017-01-19 Roberto Airoldi Contactless switch intended to replace a visible contact switch
FR3040107A1 (en) * 2015-07-10 2017-02-17 Open App CONTACTLESS SWITCH FOR REPLACING A VISIBLE CONTACT SWITCH

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GB8911274D0 (en) 1989-07-05

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