WO2014101943A1 - Driver circuit for capacitive gap sensor - Google Patents
Driver circuit for capacitive gap sensor Download PDFInfo
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- WO2014101943A1 WO2014101943A1 PCT/EP2012/076957 EP2012076957W WO2014101943A1 WO 2014101943 A1 WO2014101943 A1 WO 2014101943A1 EP 2012076957 W EP2012076957 W EP 2012076957W WO 2014101943 A1 WO2014101943 A1 WO 2014101943A1
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- sensor
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- sensor electrode
- measurement device
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
- G01B7/14—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring distance or clearance between spaced objects or spaced apertures
Definitions
- the present invention relates to the field of contactless distance measurement, and in particular to sensors for detecting variations in a small separation distance by measuring changes in a capacitance parameter.
- the invention has application for example in the monitoring of vibrations in systems with large rotating elements, such as engines, motors, turbines or generators.
- Contactless distance sensors are regularly used to monitor the position of elements, or the separation distance between elements in machinery such as motors, generators, turbines or machine tools. Such sensors may be required to monitor slowly or rapidly moving parts with a high resolution. For example, a rotor of a generator may exhibit high-frequency vibrations, and a sensor mounted in the stator of the generator may be required to monitor any deformations of the rotor by measuring the rapidly varying gap distance between the sensor surface and the rotor pole surfaces.
- Such sensors may be placed deep inside machinery, in inaccessible locations where there is no room to include circuitry, or where local intense magnetic fields make it difficult to include circuitry for signal processing, filtering or amplification. For this reason, it is advantageous to connect the sensor to the processing circuitry via a cable which has its own capacitance, depending on its length, and its capacitance may vary significantly as a result of changes in temperature, humidity etc. It is desirable therefore to find ways of reducing or eliminating the effects of the sensor cable capacitance from the sensor signal.
- Granted Swiss patent CH696895A5 filed by the same applicant as the present application, describes a sensor arrangement which measures a gap distance by measuring the varying capacitance between a sensor electrode and the target electrode (which may be the rotor surface in the example mentioned).
- the capacitance thus formed by the sensor and target electrodes is incorporated in an RC resonant circuit, such that the resonance characteristics of the circuit at any one time are dependent on the size of the gap distance at that time.
- the capacitance to be measured may typically be significantly smaller than capacitances inherent in the elements of the sensor circuitry (referred to hereinafter as parasitic capacitances), in particular the capacitance of the cable which connects the sensor to the external circuit elements.
- the sensor might be required to measure variations in a capacitance of 1 pF or less, for example, while the capacitance of the cable might be as much as 500pF.
- the effects of varying parasitic capacitance can be mitigated by enclosing or surrounding the sensor electrode in a grounded shielding envelope.
- a further improvement is proposed using a double shielding arrangement, in which an intermediate sensor-shielding envelope, known as the guard electrode, is placed around the sensor electrode and within the grounded shielding envelope.
- the sensor electrode and guard electrode are connected to a driver circuit which is designed to maintain both the sensor and guard electrodes at the same potential.
- any capacitance between the sensor and guard electrodes and/or between their connecting wires should have little or no effect on the resonance parameters of the circuit.
- the sensor electrode is excited by a high frequency alternating voltage, and the sensor electrode circuit behaves effectively as an RC passfilter circuit, whose transfer function depends on the capacitance between the sensor electrode and the target, the parasitic capacitance associated with the connection cable being more or less
- the circuit of CH696895 must be tuned to the particular physical and electrical characteristics of the cable/sensor combination (value of R, length of cable, sensor dimensions etc), to ensure that the desired
- the small variations in output voltage from the passfilter are then detected from the guard electrode connector and amplified for transmission via another cable to remote signal processing circuitry which converts the measured voltage into a measure of the distance between the sensor electrode and the target.
- the voltage on the guard electrode (and hence the inner shield of the triaxial cable, is modulated by variations in the voltage on the sensor electrode.
- the RC arrangement described in CH696895A5 has a resonant frequency which depends on the in-circuit resistor and the (varying)
- the RC resonant circuit can be arranged such that the variations in the capacitance result in an output signal which lies in a part of the frequency/amplitude transfer function which has a steep gradient, thereby offering a large amplification of the signal, which can be further amplified for transmission along a cable. Further linearization of the output signal is then required, and is performed by additional processing circuitry.
- US2002/0140440 describes a capacitive sensor arrangement in which a sensor-to-target capacitance is measured as a relaxation period or oscillation frequency of a relaxation oscillator, which varies depending on the value of the sensor-target capacitance. In order to ensure an easily-measurable time-period, the resistor is made as large as possible. US2002/01240440 does not relate to the problem of locating the driver circuitry remotely from the sensor electrode, and its oscillator components would need to be integrated with the sensor electrode. This presents difficulties in providing sufficient space at the measurement location. It also restricts the access to the oscillator components in the case where a component must be replaced or adjusted.
- the invention described in this application seeks to overcome some of the above and other difficulties inherent in the prior art.
- One problem which the invention aims to overcome is that of simplifying the construction of the driver circuit of CH696895, while keeping the sensor electrode separate from the driver circuitry, and while reducing the need for careful tuning of the driver components to the particular sensor environment.
- the invention aims to provide a driver circuit for a capacitive small-distance measurement sensor, the driver circuit comprising an astable oscillator circuit, wherein the relaxation time-constant of the astable oscillator circuit is determined by a resistance, referred to hereafter as the time-constant resistance, and by a capacitance of the measurement sensor, wherein the astable oscillator circuit comprises a bistable switching element having an input and an output, wherein the driver circuit is configured to be connected to the measurement sensor via a shielded sensor connector cable comprising at least a first inner connector, a second inner connector and an outer shielding layer, the sensor connector cable comprising a sensor end and a driver end, and wherein the time-constant resistance and the bistable switching element are both arranged at the driver end of the sensor connector cable.
- the driver circuit comprising an astable oscillator circuit, wherein the relaxation time-constant of the astable oscillator circuit is determined by a resistance, referred to hereafter as the time-constant resistance, and by a capac
- the bistable switching element is a Schmitt trigger.
- the measurement sensor is a capacitive sensor having a sensor electrode and a guard electrode
- the driver circuit is configured to be connected to the sensor electrode via the first inner connector and to the guard electrode via the second inner connector.
- the input of the bistable switching circuit is connected to the first inner connector and thereby to the sensor electrode.
- the output of the bistable switching element is connected via the time-constant resistance to the second inner connector and thereby to the guard electrode.
- a follower circuit is interposed between the time-constant resistance and the second inner connector.
- the time- constant resistance is connected between the second inner connector and a common ground.
- one of the first and second inner connectors is formed as an intermediate shielding layer, disposed between the outer shielding layer and the other one of the first and second inner connectors.
- the second inner connector is formed as the intermediate shielding layer.
- the driver circuit further comprises a demodulator circuit, and the demodulator circuit is arranged to demodulate an output of the astable oscillator circuit so as to generate a voltage which varies with the frequency of the output of the astable oscillator.
- the demodulator circuit comprises a phase-locked loop.
- the astable oscillator circuit and the demodulator circuit are galvanically isolated from one another.
- the astable oscillator circuit and the demodulator circuit are connected to each other by a shielded cable.
- Figure 1 shows an example of a sensor arrangement and driver circuit according to a first embodiment of the invention.
- a capacitive sensor electrode 1 and its associated guard electrode 4 are shown isolated from each other by electrode dielectric 3.
- the electrode assembly 1 , 3, 4 may for example be incorporated into the inner surface of the stator of a turbine or motor, which will normally be grounded. In this case a further dielectric (not shown) would insulate the guard electrode from the surrounding machinery (not shown).
- the target 2, which is at a small distance (much exaggerated in the schematic illustration) away from the sensor electrode 1 may for example be a part of the rotor of the generator, turbine etc.
- the separation distance may be of the order of 0.1 mm, for example, or in other cases as much as 50mm, and a variation in the separation distance (due to thermal expansion or contraction of the rotor and/or the stator, for example) will result in a change in the capacitance between the sensor electrode 1 and the (grounded) target 2.
- the illustrated circuit comprises an astable oscillator (components 1 , 2, 5, 7, 9, 10), which in turn comprises a capacitance (between sensor electrode 1 and target 2) connected via a shielded connecting cable 5, 7 to remotely located oscillator/driver components 9, 10 which include a resistance 9 and a switching circuit 10 (an inverting Schmitt trigger is illustrated).
- the driver circuit also includes a parasitic capacitance suppression circuit 3, 4, 6, 8, comprising high-precision voltage follower 8, intermediate cable shielding 6, guard electrode 4 and dielectric 3.
- the voltage follower 8 having a very high input impedance but very low output impedance, serves to drive the
- the free-running oscillation frequency of the astable oscillator circuit is determined by the value of the (constant) resistance 9 and the (possibly varying) capacitance between the sensor electrode 1 and the target 2.
- All the components which make up the astable oscillator circuit are located at one end (the driver end) of the sensor connector cable 5, 6, 7 - all except the capacitive sensor electrode 1 , that is. Similarly, all the components which make up the parasitic capacitance suppression circuit - all except the guard electrode 4 and dielectric 3, that is - are located at the driver end of the sensor connector cable 5, 6, 7. This allows the sensor electrode assembly 1 , 3, 4 to be made small and/or thin, and remote from the driver electronics, so that it can be used in inaccessible areas of machinery etc.
- the sensor connector cable 5, 6, 7 may in principle be any type of shielded cable, but it has been found advantageous to use a triaxial cable, for example with the sensor electrode 1 connected to the inner connector 7, the guard electrode 4 connected to the inner shielding layer 6, and the outer shielding layer 5 connected to ground (for example grounded to the surrounding machinery).
- the target 2 may be a metallic surface which is substantially parallel to the electrode surface, and it may be grounded (as illustrated) with a ground plane common to the ground of the sensor driver circuit.
- the capacitive sensor may also be used in conditions where the target is non-metallic, non-conducting or not grounded, or grounded to a different ground plane than the driver circuit. Under such conditions, local changes in the local dielectric, or the close presence of a conductor, can nevertheless result in a measurable change in capacitive parameters of the sensor 1 , 3, 4.
- the astable oscillator is shown in the example of figure 1 as comprising a bistable switching element (inverting Schmitt trigger 10), a resistance 9 and the capacitive sensor electrode assembly 1 , 3, 4.
- Other suitable components could be used for implementing the switching part of the oscillator, but the inverting Schmitt trigger has been chosen in this example because it is a simple and easily-available component with stable operating characteristics.
- a high-precision voltage follower circuit 8 allows the voltage on the resistance 9 to be applied accurately to the guard electrode 4 via the inner shielding connector 6 of the triaxial cable, thereby compensating for any impedance change affecting the phase of the guard signal.
- Galvanic separation may be achieved for example by means of an optical signal connection to processing circuit 12.
- an inductive sensor could be used, arranged as part of an LR resonant circuit, with appropriate amendments to the remaining circuitry to take account of the different voltage/current phase relationship which pertain in an inductive sensor.
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Abstract
A small-distance measurement device is described for contactless gap measurement. The device comprises a capacitive sensor electrode assembly (1, 3, 4) for sensing variations in capacitance between a sensor electrode (1) and a target (2). The device comprises an astable oscillator (1, 2, 5, 7, 9, 10) comprising a Schmitt trigger (10), where the oscillation frequency is determined by a capacitance at the sensor electrode (1) and a time-constant resistance (9). The components of the astable oscillator (1, 2, 5, 7, 9, 10), apart from the sensor electrode, are located remote from the sensor electrode assembly (1, 3, 4) and connected to the sensor electrode assembly (1, 3, 4) by a triaxial cable (5, 6, 7). The output of the astable oscillator may be galvanically separated from subsequent processing circuits, such as a frequency demodulator circuit (12). Galvanic separation can be achieved using an optical connector (11).
Description
DRIVER CIRCUIT FOR CAPACITIVE GAP SENSOR
The present invention relates to the field of contactless distance measurement, and in particular to sensors for detecting variations in a small separation distance by measuring changes in a capacitance parameter. The invention has application for example in the monitoring of vibrations in systems with large rotating elements, such as engines, motors, turbines or generators.
Background of the invention
Contactless distance sensors are regularly used to monitor the position of elements, or the separation distance between elements in machinery such as motors, generators, turbines or machine tools. Such sensors may be required to monitor slowly or rapidly moving parts with a high resolution. For example, a rotor of a generator may exhibit high-frequency vibrations, and a sensor mounted in the stator of the generator may be required to monitor any deformations of the rotor by measuring the rapidly varying gap distance between the sensor surface and the rotor pole surfaces.
Such sensors may be placed deep inside machinery, in inaccessible locations where there is no room to include circuitry, or where local intense magnetic fields make it difficult to include circuitry for signal processing, filtering or amplification. For this reason, it is advantageous to connect the sensor to the processing circuitry via a cable which has its own capacitance, depending on its length, and its capacitance may vary significantly as a result of changes in temperature, humidity etc. It is desirable therefore to find ways of reducing or eliminating the effects of the sensor cable capacitance from the sensor signal.
Granted Swiss patent CH696895A5, filed by the same applicant as the present application, describes a sensor arrangement which measures a gap distance by measuring the varying capacitance between a sensor electrode and the target electrode (which may be the rotor surface in the example mentioned). The capacitance thus formed by the sensor and target electrodes is incorporated in an RC resonant circuit, such that the resonance
characteristics of the circuit at any one time are dependent on the size of the gap distance at that time.
The capacitance to be measured may typically be significantly smaller than capacitances inherent in the elements of the sensor circuitry (referred to hereinafter as parasitic capacitances), in particular the capacitance of the cable which connects the sensor to the external circuit elements. The sensor might be required to measure variations in a capacitance of 1 pF or less, for example, while the capacitance of the cable might be as much as 500pF.
Small changes in the parasitic capacitances can thus have a large effect on the measured capacitance. Even small amounts of thermal expansion or contraction of the connector cable or of other circuit components may result in changes in parasitic capacitance which are as large as, or greater than, the capacitance to be measured. Parasitic capacitance may also vary as a result of other changes, such as variations in ambient humidity.
As described in CH696895, the effects of varying parasitic capacitance can be mitigated by enclosing or surrounding the sensor electrode in a grounded shielding envelope. A further improvement is proposed using a double shielding arrangement, in which an intermediate sensor-shielding envelope, known as the guard electrode, is placed around the sensor electrode and within the grounded shielding envelope. The sensor electrode and guard electrode are connected to a driver circuit which is designed to maintain both the sensor and guard electrodes at the same potential. In principle, as long as the sensor and guard electrodes and their connecting wires remain at the same potential, then any capacitance between the sensor and guard electrodes and/or between their connecting wires should have little or no effect on the resonance parameters of the circuit. The sensor electrode is excited by a high frequency alternating voltage, and the sensor electrode circuit behaves effectively as an RC passfilter circuit, whose transfer function depends on the capacitance between the sensor electrode and the target, the parasitic capacitance associated with the connection cable being more or less
neutralized by keeping the sensor and guard electrode connections at the same potential. The circuit of CH696895 must be tuned to the particular
physical and electrical characteristics of the cable/sensor combination (value of R, length of cable, sensor dimensions etc), to ensure that the desired
measurement range of the output voltage falls within a part of the passfilter function which gives as large a fluctuation in output voltage as possible for small fluctuations in sensor-target capacitance. Even though the effects of the parasitic capacitance of the sensor connection cable can be greatly diminished by this arrangement, they cannot be eliminated, since the voltages on the sensor and guard connectors are not identical. For this reason, it is important to keep the length of the cable between the electrodes and the driver/filter circuitry as short as possible.
The small variations in output voltage from the passfilter are then detected from the guard electrode connector and amplified for transmission via another cable to remote signal processing circuitry which converts the measured voltage into a measure of the distance between the sensor electrode and the target.
Because of the capacitance between the sensor electrode and the guard electrode, the voltage on the guard electrode (and hence the inner shield of the triaxial cable, is modulated by variations in the voltage on the sensor electrode. The RC arrangement described in CH696895A5 has a resonant frequency which depends on the in-circuit resistor and the (varying)
capacitance between the sensor electrode and the target. By judicious choice of excitation frequency and resonant behaviour, the RC resonant circuit can be arranged such that the variations in the capacitance result in an output signal which lies in a part of the frequency/amplitude transfer function which has a steep gradient, thereby offering a large amplification of the signal, which can be further amplified for transmission along a cable. Further linearization of the output signal is then required, and is performed by additional processing circuitry.
American patent application US2002/0140440 describes a capacitive sensor arrangement in which a sensor-to-target capacitance is measured as a relaxation period or oscillation frequency of a relaxation oscillator, which varies depending on the value of the sensor-target
capacitance. In order to ensure an easily-measurable time-period, the resistor is made as large as possible. US2002/01240440 does not relate to the problem of locating the driver circuitry remotely from the sensor electrode, and its oscillator components would need to be integrated with the sensor electrode. This presents difficulties in providing sufficient space at the measurement location. It also restricts the access to the oscillator components in the case where a component must be replaced or adjusted.
Brief description of the invention
The invention described in this application seeks to overcome some of the above and other difficulties inherent in the prior art. One problem which the invention aims to overcome is that of simplifying the construction of the driver circuit of CH696895, while keeping the sensor electrode separate from the driver circuitry, and while reducing the need for careful tuning of the driver components to the particular sensor environment. In particular, the invention aims to provide a driver circuit for a capacitive small-distance measurement sensor, the driver circuit comprising an astable oscillator circuit, wherein the relaxation time-constant of the astable oscillator circuit is determined by a resistance, referred to hereafter as the time-constant resistance, and by a capacitance of the measurement sensor, wherein the astable oscillator circuit comprises a bistable switching element having an input and an output, wherein the driver circuit is configured to be connected to the measurement sensor via a shielded sensor connector cable comprising at least a first inner connector, a second inner connector and an outer shielding layer, the sensor connector cable comprising a sensor end and a driver end, and wherein the time-constant resistance and the bistable switching element are both arranged at the driver end of the sensor connector cable.
According to a variant of the driver circuit of the invention, the bistable switching element is a Schmitt trigger.
According to a variant of the driver circuit of the invention, the measurement sensor is a capacitive sensor having a sensor electrode and a guard electrode, and the driver circuit is configured to be connected to the
sensor electrode via the first inner connector and to the guard electrode via the second inner connector.
According to a variant of the driver circuit of the invention, the input of the bistable switching circuit is connected to the first inner connector and thereby to the sensor electrode.
According to a variant of the driver circuit of the invention, the output of the bistable switching element is connected via the time-constant resistance to the second inner connector and thereby to the guard electrode.
According to a variant of the driver circuit of the invention, a follower circuit is interposed between the time-constant resistance and the second inner connector.
According to a variant of the driver circuit of the invention, the time- constant resistance is connected between the second inner connector and a common ground.
According to a variant of the driver circuit of the invention, one of the first and second inner connectors is formed as an intermediate shielding layer, disposed between the outer shielding layer and the other one of the first and second inner connectors.
According to a variant of the driver circuit of the invention, the second inner connector is formed as the intermediate shielding layer.
According to a variant of the driver circuit of the invention, the driver circuit further comprises a demodulator circuit, and the demodulator circuit is arranged to demodulate an output of the astable oscillator circuit so as to generate a voltage which varies with the frequency of the output of the astable oscillator.
According to a variant of the driver circuit of the invention, the demodulator circuit comprises a phase-locked loop.
According to a variant of the driver circuit of the invention, the astable oscillator circuit and the demodulator circuit are galvanically isolated from one another.
According to a variant of the driver circuit of the invention, the astable oscillator circuit and the demodulator circuit are connected to each other by a shielded cable.
Detailed description of the invention
The invention and its advantages are explained in the following detailed description with reference to the attached figure, which illustrates in schematic form a driver circuit according to an example embodiment of the invention.
Note that the drawing is intended merely as an example illustration of how the principles underlying the invention can be implemented. The drawing should not to be construed as limiting the scope of the invention, which is set out in the accompanying claims.
Figure 1 shows an example of a sensor arrangement and driver circuit according to a first embodiment of the invention. A capacitive sensor electrode 1 and its associated guard electrode 4 are shown isolated from each other by electrode dielectric 3. As discussed above, the electrode assembly 1 , 3, 4 may for example be incorporated into the inner surface of the stator of a turbine or motor, which will normally be grounded. In this case a further dielectric (not shown) would insulate the guard electrode from the surrounding machinery (not shown). The target 2, which is at a small distance (much exaggerated in the schematic illustration) away from the sensor electrode 1 , may for example be a part of the rotor of the generator, turbine etc. The separation distance may be of the order of 0.1 mm, for example, or in other cases as much as 50mm, and a variation in the separation distance (due to thermal expansion or contraction of the rotor and/or the stator, for example) will result in a change in the capacitance between the sensor electrode 1 and the (grounded) target 2.
The illustrated circuit comprises an astable oscillator (components 1 , 2, 5, 7, 9, 10), which in turn comprises a capacitance (between sensor electrode 1 and target 2) connected via a shielded connecting cable 5, 7 to remotely located oscillator/driver components 9, 10 which include a resistance 9 and a switching circuit 10 (an inverting Schmitt trigger is illustrated).
In addition to the oscillator components 1 , 2, 5, 7, 9 and 10, the driver circuit also includes a parasitic capacitance suppression circuit 3, 4, 6, 8, comprising high-precision voltage follower 8, intermediate cable shielding 6, guard electrode 4 and dielectric 3. The voltage follower 8, having a very high input impedance but very low output impedance, serves to drive the
intermediate cable shielding 6 and the guard electrode 4 with the same high- frequency alternative voltage which is present on the sensor electrode connector 7 as a result of the free-running oscillation of the astable oscillator circuit 1 , 2, 5, 7, 9, 10. Therefore, because the sensor electrode 1 and the guard electrode 4, and the sensor connector 7 and the guard electrode connector 6 are maintained at essentially the same potential, the parasitic capacitances associated with the connector cable 5, 6, 7 and the guard electrode 4 are effectively neutralized.
The free-running oscillation frequency of the astable oscillator circuit is determined by the value of the (constant) resistance 9 and the (possibly varying) capacitance between the sensor electrode 1 and the target 2.
Changes in the capacitance between the sensor electrode 1 and the target 2 result in a change in frequency of the oscillator circuit, whose output can be connected via a galvanically isolating connection 1 1 , to a processing unit such as frequency demodulating output circuit 12.
All the components which make up the astable oscillator circuit are located at one end (the driver end) of the sensor connector cable 5, 6, 7 - all except the capacitive sensor electrode 1 , that is. Similarly, all the components which make up the parasitic capacitance suppression circuit - all except the guard electrode 4 and dielectric 3, that is - are located at the driver end of the sensor connector cable 5, 6, 7.
This allows the sensor electrode assembly 1 , 3, 4 to be made small and/or thin, and remote from the driver electronics, so that it can be used in inaccessible areas of machinery etc. The sensor connector cable 5, 6, 7 may in principle be any type of shielded cable, but it has been found advantageous to use a triaxial cable, for example with the sensor electrode 1 connected to the inner connector 7, the guard electrode 4 connected to the inner shielding layer 6, and the outer shielding layer 5 connected to ground (for example grounded to the surrounding machinery).
Note that the target 2 may be a metallic surface which is substantially parallel to the electrode surface, and it may be grounded (as illustrated) with a ground plane common to the ground of the sensor driver circuit. However, the capacitive sensor may also be used in conditions where the target is non-metallic, non-conducting or not grounded, or grounded to a different ground plane than the driver circuit. Under such conditions, local changes in the local dielectric, or the close presence of a conductor, can nevertheless result in a measurable change in capacitive parameters of the sensor 1 , 3, 4.
The astable oscillator is shown in the example of figure 1 as comprising a bistable switching element (inverting Schmitt trigger 10), a resistance 9 and the capacitive sensor electrode assembly 1 , 3, 4. Other suitable components could be used for implementing the switching part of the oscillator, but the inverting Schmitt trigger has been chosen in this example because it is a simple and easily-available component with stable operating characteristics. A high-precision voltage follower circuit 8 allows the voltage on the resistance 9 to be applied accurately to the guard electrode 4 via the inner shielding connector 6 of the triaxial cable, thereby compensating for any impedance change affecting the phase of the guard signal. Inaccurate application of the sensor signal to the guard electrode would result in a significant or deleterious reduction in the sensitivity of the capacitive sensor due to the increased effect of the parasitic capacitances on the oscillation frequency of the astable oscillator circuit.
The output of the astable oscillator is galvanically isolated
(represented by 1 1 ) from subsequent processing circuitry in order that the oscillator characteristics should not be affected by the connector resistance or the input impedance of the frequency demodulator circuit. Galvanic separation may be achieved for example by means of an optical signal connection to processing circuit 12.
Note that this application describes a capacitive gap-measuring sensor. However, the inventive principles described here could be adapted for use with an inductive sensor. Instead of an RC resonant circuit for the astable oscillator, for example, an inductive sensor could be used, arranged as part of an LR resonant circuit, with appropriate amendments to the remaining circuitry to take account of the different voltage/current phase relationship which pertain in an inductive sensor.
Claims
1 . Capacitive small-distance measurement device comprising: a capacitive sensor electrode assembly (1 , 3, 4) comprising a sensor electrode (1 ) and a guard electrode (4), separated by a dielectric (3); a sensor driver circuit (8, 9, 10), locatable at a separate location from the sensor electrode assembly (1 , 3, 4); a sensor connector cable (5, 6, 7) for connecting the sensor driver circuit (8, 9, 10) to the sensor electrode assembly (1 , 3, 4), wherein the sensor connector cable (5, 6, 7) comprises a sensor electrode connector (7) for connecting the sensor electrode (1 ) to a first part (8) of the sensor driver circuit (8, 9, 10), and a guard electrode connector (6) for connecting the guard electrode (4) to a second part (10) of the sensor driver circuit (8, 9, 10); characterized by: an astable oscillator circuit (1 , 2, 5, 7, 9, 10), wherein the oscillation frequency of the astable oscillator circuit (1 , 2, 5, 7, 9, 10) is determined by a resistance component (9) of the sensor driver circuit (5, 6, 7), referred to hereafter as the time-constant resistance, and by a capacitance of the sensor electrode assembly (1 , 3, 4); and a parasitic capacitance suppression circuit (4, 6, 8) for applying an oscillating voltage of the astable oscillator circuit (1 , 2, 5, 7, 9, 10) to the sensor electrode connector (7) and to the guard electrode connector (6) so as to hold the guard electrode connector (6) at substantially the same oscillating voltage as the sensor electrode connector (7).
2. Measurement device according to claim 1 , wherein the sensor electrode connector (7) comprises a core connector (7) of the sensor connector cable (5, 6, 7), and the guard connector (6) comprises an inner shielding
connector (6) layer, and wherein the sensor connector cable further comprises an outer shielding connector (5) layer.
3. Measurement device according to claim 1 or claim 2, wherein the astable oscillator circuit comprises a bistable switching element (10) of the driver circuit (8, 9, 10), the bistable switching element (10) having an input and an output.
4. Measurement device according to claim 3, wherein the bistable switching element (10) is a Schmitt trigger.
5. Measurement device according to claim 3 or claim 4, wherein the input of the bistable switching circuit (10) is connected to the sensor electrode connector (7).
6. Measurement device according to one of claims 3 to 5, wherein time-constant resistance (9) is connected between the output of the bistable switching element (10) and the input of the bistable switching element (10).
7. Measurement device according to claim one of claims 3 to 6, wherein the input of the bistable switching element (10) is connected to an input of a voltage follower circuit (8), and wherein an output of the voltage follower circuit is connected to the guard electrode connector (6).
8. Measurement device according to one of the preceding claims, wherein the driver circuit further comprises a demodulator circuit (12), and wherein the demodulator circuit (12) is arranged to demodulate an output of the astable oscillator circuit (1 , 2, 5, 7, 9, 10) so as to generate a voltage which varies with the frequency of the output of the astable oscillator (1 , 2, 5, 7, 9, 10).
9. Measurement device according to claim 8, wherein the astable oscillator circuit (1 , 2, 5, 7, 9, 10) and the demodulator circuit (12) are galvanically isolated from one another.
10. Measurement device according to claim 8 or claim 9, wherein the astable oscillator circuit and the demodulator circuit (12) are connected to each other by an optical connection (1 1 ).
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PCT/EP2012/076957 WO2014101943A1 (en) | 2012-12-27 | 2012-12-27 | Driver circuit for capacitive gap sensor |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PCT/EP2012/076957 WO2014101943A1 (en) | 2012-12-27 | 2012-12-27 | Driver circuit for capacitive gap sensor |
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| WO2014101943A1 true WO2014101943A1 (en) | 2014-07-03 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/EP2012/076957 WO2014101943A1 (en) | 2012-12-27 | 2012-12-27 | Driver circuit for capacitive gap sensor |
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| US9927905B2 (en) | 2015-08-19 | 2018-03-27 | Apple Inc. | Force touch button emulation |
| EP3434427A1 (en) * | 2017-07-26 | 2019-01-30 | Fogale Nanotech | Robot with capacitive detection means and referenced walls with a guard potential |
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| US9726922B1 (en) | 2013-12-20 | 2017-08-08 | Apple Inc. | Reducing display noise in an electronic device |
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| US9927905B2 (en) | 2015-08-19 | 2018-03-27 | Apple Inc. | Force touch button emulation |
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| WO2017202564A1 (en) * | 2016-05-25 | 2017-11-30 | Fogale Nanotech | Zero-guard capacitive detection device |
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| KR20190022437A (en) * | 2017-07-26 | 2019-03-06 | 포걀 나노떼끄 | The item (s) based on the guard potential and the robot equipped with the capacitive sensing means |
| WO2019020874A1 (en) * | 2017-07-26 | 2019-01-31 | Fogale Nanotech | Robot provided with capacitive sensing means and member(s) referenced at a guard potential |
| WO2019020873A1 (en) * | 2017-07-26 | 2019-01-31 | Fogale Nanotech | Robot provided with capacitive sensing means and walls referenced at a guard potential |
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| KR102058809B1 (en) | 2017-07-26 | 2020-02-07 | 포걀 나노떼끄 | Robots with capacitive sensing means and walls based on guard potential |
| KR102058808B1 (en) | 2017-07-26 | 2020-02-20 | 포걀 나노떼끄 | Robot with component (s) and capacitive sensing means based on guard potential |
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