Capacitive sensor

Abstract

To provide a capacitive sensor capable of detecting capacitance with high accuracy. [Solution] A capacitance sensor comprising a package, an oscillation circuit provided inside the package, and a first electrode and a second electrode provided outside the package, wherein the oscillation circuit has an amplifier and a resonator provided inside the package and connected between the input node and the output node of the amplifier, the first electrode is a sensing electrode connected to one of the input node and the output node of the amplifier, the second electrode is an electrode with a fixed potential, and the oscillation frequency of the oscillation circuit changes according to a first capacitance between the first electrode and the second electrode.

Description

[Technical Field]

[0001] This invention relates to a capacitive sensor. [Background technology]

[0002] Patent Document 1 describes a liquid level detector that uses capacitance and resistance, which change in response to changes in the liquid level being measured, as oscillation time constants to cause an oscillation circuit to oscillate, and outputs a digital signal corresponding to the change in the liquid level based on the output signal of the oscillation circuit. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2003-57095 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, in the liquid level detector described in Patent Document 1, the oscillation circuit oscillates by charging and discharging capacitance, making it susceptible to fluctuations in the oscillation frequency of the oscillation circuit due to external amplitude noise, which makes it difficult to achieve high detection accuracy. [Means for solving the problem]

[0005] One embodiment of the capacitive sensor according to the present invention is: The package and An oscillator circuit provided inside the package, The first electrode and the second electrode are provided outside the package, Equipped with, The aforementioned oscillator circuit is Amplifier and An oscillator provided inside the package and connected between the input node and output node of the amplifier, It has, The first electrode is a sensing electrode connected to one of the input node and the output node of the amplifier. The second electrode is an electrode with a fixed potential. According to the first capacitance between the first electrode and the second electrode, the oscillation frequency of the oscillation circuit changes.

[0006] Another aspect of the capacitance sensor according to the present invention is an oscillation circuit, a first electrode and a second electrode, and is provided with The oscillation circuit has an amplifier, and a vibrator connected between the input node and the output node of the amplifier, and has The first electrode is a sensing electrode connected to one of the input node and the output node of the amplifier. The second electrode is an electrode with a fixed potential. According to the state of the detection target, the capacitance between the first electrode and the second electrode changes. According to the first capacitance between the first electrode and the second electrode, the oscillation frequency of the oscillation circuit changes.

Brief Description of Drawings

[0007] [Figure 1] A diagram showing the appearance of the capacitance sensor of the first embodiment. [Figure 2] A plan view showing an example of the internal structure of the oscillator. [Figure 3] A plan view showing another example of the internal structure of the oscillator. [Figure 4] A diagram showing an example of the use of the capacitance sensor. [Figure 5] A functional block diagram of the capacitance sensor of the first embodiment. [Figure 6] A diagram showing a configuration example of the drive circuit and the buffer circuit. [Figure 7] A diagram showing an equivalent circuit of the vibrator. [Figure 8]A diagram showing an example of the relationship between the load capacity CL and the normalized frequency Δf / f0. [Figure 9] A timing chart diagram of various signals. [Figure 10] A diagram showing a comparison of the frequency variable characteristics with respect to the load capacity CL between an LC oscillation circuit and an oscillation circuit using an oscillator. [Figure 11] A diagram showing the appearance of the capacitance sensor of the second embodiment. [Figure 12] A functional block diagram of the capacitance sensor of the second embodiment. [Figure 13] A diagram showing a configuration example of a drive circuit and a buffer circuit.

Embodiments for Carrying Out the Invention

[0008] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. Note that the embodiments described below do not unduly limit the content of the present invention described in the claims. Also, not all of the configurations described below are essential constituent elements of the present invention.

[0009] 1. First Embodiment 1-1. Structure of the Capacitance Sensor FIG. 1 is a diagram showing the appearance of the capacitance sensor 1 of the first embodiment. As shown in FIG. 1, the capacitance sensor 1 of the first embodiment includes an oscillator 10, a sensing unit 100, and a cable 15 that connects the oscillator 10 and the sensing unit 100.

[0010] FIG. 2 is a plan view showing an example of the internal structure of the oscillator 10. As shown in FIGS. 1 and 2, the oscillator 10 includes a circuit device 2, an oscillator 3, a package 4, and a lid 5. In FIG. 2, the illustration of the lid 5 is omitted.

[0011] The oscillator 10 is, for example, a single-seal oscillator, and the package 4 is a container that houses the circuit device 2 and the resonator 3 in the same space. That is, the circuit device 2 and the resonator 3 are located inside the package 4. Specifically, the package 4 is provided with a recess, and the circuit device 2 and the resonator 3 are housed by covering the recess with a lid 5. Note that the oscillator 10 does not have to be a single-seal structure; for example, the package 4 may be a container that houses the circuit device 2 and the resonator 3 in separate spaces. Specifically, the package 4 may be provided with two recesses on opposing surfaces, and the resonator 3 may be housed by covering one recess with a lid 5, and the circuit device 2 may be housed by covering the other recess with a sealing member.

[0012] In this embodiment, the circuit device 2 is implemented as a single-chip integrated circuit. However, the circuit device 2 may be composed of discrete components in some respects. In the example shown in Figure 2, the circuit device 2 is mounted on the inner bottom surface of the package 4 via adhesive or the like, with the side on which the multiple pads 6 are formed facing upwards. Each of the multiple pads 6 is connected by a bonding wire 7 to one of the multiple electrodes 8 formed on the surface of the recess of the package 4.

[0013] The oscillator 3 is a piezoelectric oscillator that uses piezoelectric materials as substrate materials, such as piezoelectric single crystals like quartz, lithium tantalate, or lithium niobate, or piezoelectric ceramics like lead zirconate titanate. Alternatively, it is a MEMS (Micro Electro Mechanical Systems) oscillator that uses a silicon substrate as the substrate material and is excited by electrostatic attraction. For example, oscillator 3 is a quartz oscillator using quartz as the substrate material, and in the example in Figure 2, it is a tuning fork type quartz oscillator. Each of the two support arms of oscillator 3 is joined to each of the two electrodes 9 formed on the surface of the recess of package 4 by a conductive bonding member 11. That is, each support arm of oscillator 3 is fixed to and electrically connected to each electrode 9. The two electrodes 9 are electrically connected to the two electrodes 8 and the two pads 6 of the circuit device 2, specifically the XD terminal and XG terminal in Figure 5, which will be described later, by wiring provided in package 4.

[0014] Electrode patterns (not shown) are formed on each of the two support arms and two vibrating arms of the vibrator 3. A signal generated on one electrode pattern is supplied from one electrode 8 to the XG terminal of the circuit device 2. The amplifier in the circuit device 2 amplifies this signal, and the amplified signal is supplied from the XD terminal to the other electrode 9 and then to the vibrator 3, causing the two vibrating arms of the vibrator 3 to continue vibrating like a tuning fork. As a result, the oscillation circuit, including the vibrator 3 and the amplifier, oscillates.

[0015] Figure 3 is a plan view showing another example of the internal structure of the oscillator 10. In the example shown in Figure 3, the circuit device 2 is mounted on the bottom surface of the recess of the package 4, and the resonator 3 is mounted above the circuit device 2 with a gap in between.

[0016] In the example shown in Figure 3, the oscillator 3 is an AT-cut quartz crystal oscillator. The oscillator 3 has metal excitation electrodes 3a and 3b on its front and back surfaces, respectively, and vibrates at a desired frequency depending on the shape and mass of the oscillator 3, including the excitation electrodes 3a and 3b. The excitation electrodes 3a and 3b are joined to two electrodes 12a and 12b formed on the surface of a recess in the package 4. The package 4 is provided with wiring (not shown) for electrically connecting the electrodes 12a and 12b to two terminals of the circuit device 2, specifically the XD terminal and XG terminal in Figure 5, which will be described later. The signal generated at one of the excitation electrodes 3a and 3b is supplied to the XG terminal of the circuit device 2, the amplifier in the circuit device 2 amplifies the signal, and the amplified signal is supplied from the XD terminal to the oscillator 3 via the other excitation electrode 3a and 3b, causing the front and back surfaces of the oscillator 3 to continue thickness-sliding vibrations moving in opposite directions. As a result, the oscillation circuit including the oscillator 3 and the amplifier oscillates.

[0017] The oscillator 10 has multiple external connection terminals (not shown) on the back surface of the package 4, which is its bottom surface. The package 4 also has wiring (not shown) for electrically connecting each terminal of the circuit device 2 to each external connection terminal provided on the bottom surface of the package 4.

[0018] As shown in Figure 1, the sensing unit 100 is located outside the package 4 of the oscillator 10 and is connected to the oscillator 10 by a cable 15. The cable 15 may be, for example, a coaxial cable, a flexible flat cable, or the like.

[0019] The sensing unit 100 comprises a substrate 110 having a surface 110a and a surface 110b which is the back surface of surface 110a. Electrodes 101, 102, and 103 are provided on surface 110a of the substrate 110. Each of the electrodes 101, 102, and 103 is elongated rectangular in shape, and electrode 102 is located between electrodes 101 and 103. On surface 110b of the substrate 110, electrode 104 is provided at a position opposite to the area on surface 110a where electrodes 101, 102, and 103 are located. For example, electrode 104 is provided over almost the entire surface of surface 110b.

[0020] Electrodes 101, 102, 103, and 104 are each connected to the external connection terminals of the oscillator 10 by wiring included in cable 15. Therefore, electrodes 101, 102, 103, and 104 are each connected to the terminals of the circuit device 2 via the external connection terminals of the oscillator 10.

[0021] In this embodiment, electrode 101 is a sensing electrode connected to one of the XD terminal and XG terminal of the circuit device 2, and electrode 103 is a sensing electrode connected to the other of the XD terminal and XG terminal of the circuit device 2. Electrode 102 is an electrode with a fixed potential. For example, electrode 102 is connected to the ground terminal of the circuit device 2 and its potential is fixed to the ground potential. Electrode 104 is connected to the ground terminal of the circuit device 2 and its potential is fixed to the ground potential.

[0022] The sensing unit 100 is arranged so that electrodes 101, 102, and 103 face the capacitance to be detected. The capacitance CD between electrode 101 and electrode 102 and the capacitance CG between electrode 103 and electrode 102 change according to the dielectric constant of the detection target.

[0023] As will be described later, when capacitance CD and CG change, the load capacitance C of oscillator 3 changes. L As the oscillation frequency f of the oscillation circuit changes, the circuit device 2 measures the oscillation frequency f and outputs the measured value of the oscillation frequency f to the outside of the oscillator 10. Based on the measured value of the oscillation frequency f, the external device can detect changes in capacitance CD and CG and determine the state of the object to be detected. If an object that is not the object to be detected is located opposite the surface 110b of the substrate 110, the electrode 104 functions as a shielding member to reduce the influence of that object on capacitance CD and CG.

[0024] Figure 4 shows an example of the use of the capacitance sensor 1. In the example in Figure 4, the detection target 300 for which capacitance is detected is a container that holds liquid LQ. The internal space of the detection target 300 is filled with liquid LQ and air AR. When the amount of liquid LQ decreases, the amount of air AR increases, and when the amount of liquid LQ increases, the amount of air AR decreases. The dielectric constant of liquid LQ is several tens of times that of air AR, and the more liquid LQ there is, the larger the effective dielectric constant inside the detection target 300 becomes. On the other hand, the capacitance CD,CG increases or decreases in proportion to the dielectric constant of the detection target 300. Therefore, the larger the dielectric constant of the detection target 300, that is, the more liquid LQ there is, the larger the capacitance CD,CG becomes, and the load capacitance C of the oscillator 3 becomes. L This increases. Therefore, the external device can calculate the amount of liquid LQ contained in the detection target 300 based on the measured value of the oscillation frequency f output from the oscillator 10.

[0025] 1-2. Constitutive Configuration of Capacitive Sensors Figure 5 is a functional block diagram of the capacitance sensor 1 of the first embodiment. As shown in Figure 5, the capacitance sensor 1 of the first embodiment comprises a circuit device 2 and a sensing unit 100.

[0026] The circuit device 2 comprises a drive circuit 21, a buffer circuit 30, a measurement circuit 40, a clock generation circuit 50, a control circuit 60, a register 70, and an interface circuit 80. Note that the circuit device 2 may also have a configuration in which some of these elements are omitted or modified, or other elements are added.

[0027] Circuit device 2 has a VDD terminal, which is a power terminal, and a VSS terminal, which is a ground terminal. Each circuit operates with the potential of the VDD terminal as the power potential and the potential of the VSS terminal as the ground potential. Circuit device 2 also has an XD terminal and an XG terminal, which are connected to both ends of the oscillator 3. Furthermore, circuit device 2 has terminals (not shown) for data communication with an external device, MCU 200.

[0028] The drive circuit 21 is connected to the XD terminal and the XG terminal, and vibrates the transducer 3 to generate the oscillation signal OSCO. The drive circuit 21 amplifies the signal input from the transducer 3 via the XG terminal and outputs the amplified signal to the transducer 3 via the XD terminal. As a result, the two vibrating arms of the transducer 3 vibrate, and the drive circuit 21 outputs the signal input from the transducer 3 via the XG terminal as the oscillation signal OSCO. The transducer 3 and the drive circuit 21 constitute the oscillation circuit 20.

[0029] The buffer circuit 30 buffers the oscillation signal OSCO output from the drive circuit 21 and outputs a square wave signal BFO. Note that the square wave of the signal BFO ​​includes not only strictly square waves but also waveforms that are close to square waves.

[0030] Figure 6 shows an example configuration of the drive circuit 21 and the buffer circuit 30. As shown in Figure 6, the drive circuit 21 includes an amplifier 211 and resistors 212 and 213.

[0031] The amplifier 211 amplifies the signal output from the oscillator 3 and outputs the amplified signal to the oscillator 3 via the resistor 213. In the example in Figure 6, the amplifier 211 is a CMOS inverter circuit, but it may also be a bipolar transistor.

[0032] The oscillator 3 is connected between node NG and node ND. Node NG is the input node of amplifier 211, and the signal output from oscillator 3 is input to amplifier 211 from node NG. Node ND is the output node of amplifier 211, and the signal output from amplifier 211 is output to node ND via resistor 213. Node NG is connected to the XG terminal, and node ND is connected to the XD terminal.

[0033] The signal output from amplifier 211 is a square wave signal, and this square wave signal is input to oscillator 3. As shown in Figures 2 and 3, oscillator 3 is, for example, a tuning fork type quartz oscillator or an AT-cut quartz oscillator, and has a very high Q value, so the signal output from oscillator 3 is low noise and close to a sine wave. The signal output from oscillator 3 to node NG is input to buffer circuit 30 as the oscillation signal OSCO. Note that the square wave signal output from amplifier 211 includes not only strictly square waves but also waveforms that are close to square waves.

[0034] The buffer circuit 30 includes a capacitor 31, a CMOS inverter circuit 32, and a resistor 33. The oscillation signal OSCO is input to the CMOS inverter circuit 32 via the capacitor 31, and the CMOS inverter circuit 32 outputs a square wave signal BFO.

[0035] Here, Figure 7 shows the equivalent circuit of oscillator 3, which is a quartz crystal oscillator. The equivalent constants of oscillator 3 are the series inductance L1, series capacitance C1, series resistance R1, and parallel capacitance C0. In this case, the series resonant frequency f0 of oscillator 3 is expressed by equation (1).

[0036]

number

[0037] The oscillation frequency f of the oscillation circuit 20, which is composed of the oscillator 3 and the drive circuit 21, is equal to the load capacitance C. L The reference frequency Δf / f0 of the oscillation circuit 20 changes accordingly and is expressed by equation (2).

[0038]

number

[0039] In equation (2), γ is the ratio of the parallel capacitance C0 to the series capacitance C1, and is expressed by equation (3).

[0040]

number

[0041] Figure 8 shows the load capacity C L This figure shows an example of the relationship between the load capacitance C and the reference frequency Δf / f0. In Figure 8, the solid line is the graph when oscillator 3 is a tuning fork type quartz oscillator, and the dashed line is the graph when oscillator 3 is an AT cut quartz oscillator. From Figure 8, the load capacitance C L When the capacitance changes within the range of 0 to 30 pF, the reference frequency Δf / f0 also changes, and the rate of change is greater for the tuning fork type quartz oscillator.

[0042] Electrode 101 of the sensing unit 100 is connected to one of the input node NG and output node ND of the amplifier 211, and electrode 103 of the sensing unit 100 is connected to the other of the input node NG and output node ND of the amplifier 211. In Figures 5 and 6, electrode 101 is connected to the output node ND of the amplifier 211, and electrode 103 is connected to the input node NG of the amplifier 211. Also, electrode 102 of the sensing unit 100 is connected to ground. Therefore, the load capacitance C of the oscillator 3 L The capacitance CD between electrode 101 and electrode 102 and the capacitance CG between electrode 103 and electrode 102 are composed of these and are represented by equation (4). In equation (4), CS is the stray capacitance and is on the order of a few pF.

[0043]

number

[0044] From equations (1) to (4), the oscillation frequency f of the oscillation circuit 20 changes according to the capacitances CD and CG. The frequency of the oscillation signal OSCO output from the drive circuit 21 is the oscillation frequency f, and the frequency of the signal BFO ​​output from the buffer circuit 30 also matches the oscillation frequency f.

[0045] As shown in Figure 5, the measurement circuit 40 measures the frequency of the BFO signal output from the buffer circuit 30. That is, the measurement circuit 40 measures the oscillation frequency f. For example, the measurement circuit 40 includes a frequency divider circuit 41 and a counter 42.

[0046] The frequency divider circuit 41 outputs a gate time signal GT obtained by dividing the frequency of the signal BFO. The counter 42 counts the number of pulses of the clock signal CK included in the gate time, which is the time defined by the gate time signal GT, and outputs a count value CNT. For example, as shown in the timing chart of Figure 9, the frequency divider circuit 41 sets the high level of the signal BFO ​​for a predetermined period and The counter 42 outputs a gate time signal GT, and the counter 42 outputs a count value CNT of the number of pulses of the clock signal CK that are included in the period when the gate time signal GT is high. In this case, the time when the gate time signal GT is high corresponds to the gate time.

[0047] The count value CNT corresponds to the ratio of the frequency of the clock signal CK to the frequency of the signal BFO, and the higher the frequency of the signal BFO, the smaller the count value CNT. In other words, there is a one-to-one relationship between the frequency of the signal BFO ​​and the count value CNT, and the count value CNT corresponds to the measured value of the frequency of the signal BFO, i.e., the oscillation frequency f. The count value CNT is stored in register 70.

[0048] The longer the gate time, the higher the measurement resolution of the measurement circuit 40, but the longer the time required for measurement. Therefore, the gate time is set appropriately according to the upper limit of the allowable measurement time, for example, to several hundred milliseconds.

[0049] The clock signal CK is output from the clock generation circuit 50. The higher the frequency of the clock signal CK, the higher the measurement resolution of the measurement circuit 40. For example, the clock generation circuit 50 may be a ring oscillator capable of outputting signals in the range of tens of MHz to hundreds of MHz. Furthermore, the clock signal CK from the clock generation circuit 50 only needs to be output during the gate time, which is a period of a predetermined logic level for the gate time signal GT. For example, it only needs to be output during the gate time, which is a period of high level for the gate time signal GT. Therefore, the clock signal CK may continue to be output from the clock generation circuit 50 while the capacitance sensor 1 is operating, but a period may be provided outside of the gate time during which the output of the clock signal CK from the clock generation circuit 50 is stopped. In this case, the power consumption of the capacitance sensor 1 can be reduced.

[0050] The control circuit 60 controls the operation of the oscillation circuit 20. For example, the control circuit 60 outputs an enable signal to the oscillation circuit 20, and the oscillation circuit 20 oscillates when the enable signal is high level and stops oscillating when the enable signal is low level. The control circuit 60 also controls the operation of the measurement circuit 40. For example, the control circuit 60 outputs a signal to the measurement circuit 40 instructing it to start measurement, and the measurement circuit 40 performs measurement processing in response to the signal instructing it to start measurement.

[0051] The interface circuit 80 is a circuit for data communication with the MCU 200. For example, when the interface circuit 80 receives a measurement request from the MCU 200, it rewrites a predetermined bit in the register 70 from 0 to 1. When the control circuit 60 detects that the bit has been rewritten, it causes the oscillation circuit 20 to start oscillating, and after a predetermined waiting time has elapsed, it outputs a signal to the measurement circuit 40 instructing it to start measurement.

[0052] Also, for example, when the interface circuit 80 receives a read request for measurement values from the MCU 200, it reads the count value CNT stored in the register 70 and transmits the count value CNT to the MCU 200. The MCU 200 receives the count value CNT and may calculate the load capacitance C L based on the count value CNT, or may determine the state of the detection target.

[0053] The interface circuit 80 may be, for example, an interface circuit of an SPI bus, or an I 2 C bus interface circuit. SPI is an abbreviation for Serial Peripheral Interface, and I 2 C is an abbreviation for Inter-Integrated Circuit.

[0054] Note that the electrode 101 is an example of the "first electrode", the electrode 102 is an example of the "second electrode", the electrode 103 is an example of the "third electrode", and the electrode 104 is an example of the "ground electrode" There is. Also, the capacitance CD is an example of the "first capacitance", and the capacitance CG is an example of the "second capacitance". Also, the surface 110a of the substrate 110 is an example of the "first surface", and the surface 110b of the substrate 110 is an example of the "second surface".

[0055] 1-3. Operational Effects As described above, in the capacitance sensor 1 of the first embodiment, the oscillation circuit 20 oscillates based on the resonance between the vibrator 3 and the capacitances CD and CG, rather than CR oscillation based on the charge and discharge of the capacitances CD and CG. Therefore, it is less affected by external amplitude noise. Thus, according to the capacitance sensor 1 of the first embodiment, the capacitance can be detected with high accuracy.

[0056] Furthermore, in the capacitance sensor 1 of the first embodiment, the oscillation frequency of the oscillation circuit 20 changes according to the capacitances CD and CG, so the variable range of the oscillation frequency can be widened compared to the case where there is no capacitance CG, i.e., when there is no electrode 103 on the sensing unit 100. Therefore, the capacitance sensor 1 of the first embodiment can improve the detection sensitivity of capacitance.

[0057] Furthermore, in the capacitance sensor 1 of the first embodiment, the oscillator 3 has a very high Q value and therefore also functions as a noise filter. The signal output from the oscillator 3 to the input node NG of the amplifier 211 has little noise and is close to a sine wave. Therefore, according to the capacitance sensor 1 of the first embodiment, no noise spikes occur in the output signal of the buffer circuit 30, thus reducing the risk of the measurement circuit 40 performing incorrect measurements.

[0058] Furthermore, in the capacitance sensor 1 of the first embodiment, electrode 104 is provided on surface 110b of the substrate 110 at a position opposite to the arrangement area of ​​electrodes 101, 102, and 103 on surface 110a. Therefore, when an object that is not the target of detection is located opposite surface 110b, the influence of that object on the capacitance CD and CG is reduced. Accordingly, the capacitance sensor 1 of the first embodiment can improve the detection accuracy of capacitance.

[0059] Incidentally, as shown in Figure 10, an LC oscillator circuit that utilizes LC resonance due to an inductor and capacitance has a wider range of oscillation frequency variation compared to an oscillator circuit that utilizes a resonator and capacitance. Figure 10 shows the load capacitance C. L This figure shows an example of the relationship between the oscillator and the reference frequency Δf / f0. The solid line is the graph when oscillator 3 is a tuning fork type quartz oscillator, and corresponds to the solid line graph in Figure 8. The dashed line is the graph for the LC oscillator circuit. From Figure 10, it can be seen that the variable range of the oscillator circuit using an oscillator and capacitance is extremely narrow compared to the variable range of the oscillation frequency of the LC oscillator circuit.

[0060] Therefore, it is conceivable to construct a highly sensitive capacitance sensor using an LC oscillator circuit. However, when detecting capacitance on the order of pF using a capacitance sensor with an LC oscillator circuit, for example, if a small inductor on the order of nH is used to reduce costs, the oscillation frequency will be on the order of GHz, which can lead to various problems such as an increase in the size and power consumption of the circuit that measures the capacitance. Conversely, in order to raise the oscillation frequency to the order of MHz, it is necessary to use a large inductor on the order of μH, which hinders the miniaturization and cost reduction of the capacitance sensor.

[0061] In contrast, in the capacitance sensor 1 of this embodiment, the oscillation circuit 20 utilizing the oscillator 3 and capacitances CD and CG has a considerably narrower range of oscillation frequency variation than the LC oscillation circuit. However, it is possible to easily achieve oscillation frequencies on the order of kHz or MHz using a small oscillator 3. Furthermore, for capacitances on the order of pF, for example, a practically necessary range of oscillation frequency variation can be obtained as shown in Figure 8. In addition, the inductance value of the inductor is... Since it is determined by the ischium, it is difficult to reduce the size of the inductor without changing the inductance value. However, the oscillator 3 can be further miniaturized and cost-effective through future advances in manufacturing processes. Therefore, the capacitance sensor 1 of the first embodiment can be made smaller and cost-effective compared to a capacitance sensor using an LC oscillation circuit, and can be easily used even when the object to be detected is small.

[0062] 2. Second Embodiment In the following description of the second embodiment, the same reference numerals are used for components similar to those in the first embodiment, and descriptions similar to those in the first embodiment are omitted or simplified. The description will primarily focus on the differences from the first embodiment.

[0063] Figure 11 shows the external appearance of the capacitance sensor 1 of the second embodiment. As shown in Figure 11, the capacitance sensor 1 of the second embodiment, like the capacitance sensor 1 of the first embodiment, includes an oscillator 10, a sensing unit 100, and a cable 15 connecting the oscillator 10 and the sensing unit 100. The structure of the oscillator 10 is the same as in the first embodiment, so its illustration and description are omitted.

[0064] As shown in Figure 11, the sensing unit 100 is located outside the package 4 of the oscillator 10 and is connected to the oscillator 10 by a cable 15. The cable 15 may be, for example, a coaxial cable, a flexible flat cable, or the like.

[0065] The sensing unit 100 includes a substrate 110 having a surface 110a and a surface 110b which is the back surface of surface 110a. Electrodes 101 and 102 are provided on surface 110a of the substrate 110. On surface 110b of the substrate 110, an electrode 104 is provided at a position opposite to the area on surface 110a where electrodes 101 and 102 are located. For example, the electrode 104 is provided over almost the entire surface of surface 110b.

[0066] Electrodes 101, 102, and 104 are each connected to the external connection terminals of the oscillator 10 by wiring included in cable 15. Therefore, electrodes 101, 102, and 104 are each connected to the terminals of the circuit device 2 via the external connection terminals of the oscillator 10.

[0067] In this embodiment, electrode 101 is a sensing electrode connected to the XD terminal of the circuit device 2, and electrode 102 is an electrode with a fixed potential. For example, electrode 102 is connected to the VSS terminal, which is the ground terminal of the circuit device 2, and its potential is fixed to the ground potential. Electrode 104 is connected to the VSS terminal of the circuit device 2, and its potential is fixed to the ground potential.

[0068] The sensing unit 100 is arranged such that electrodes 101 and 102 face the object whose capacitance is to be detected. The capacitance CD between electrodes 101 and 102 changes according to the dielectric constant of the object being detected.

[0069] When capacitance CD changes, the load capacitance C of oscillator 3 changes. L As the oscillation frequency f of the oscillation circuit 20 changes, the circuit device 2 measures the oscillation frequency f and outputs the measured value of the oscillation frequency f to the outside of the oscillator 10. Based on the measured value of the oscillation frequency f, the external device can detect the change in capacitance CD and determine the state of the object to be detected. The electrode 104 also functions as a shielding member to reduce the influence of an object that is not the object to be detected on capacitance CD when that object is located facing the surface 110b of the substrate 110.

[0070] Figure 12 is a functional block diagram of the capacitance sensor 1 of the second embodiment. As described above, the capacitance sensor 1 of the second embodiment includes a circuit device 2 and a sensing unit 100, similar to the capacitance sensor 1 of the first embodiment.

[0071] Circuit device 2, like the first embodiment, includes a drive circuit 21, a buffer circuit 30, a measurement circuit 40, a clock generation circuit 50, a control circuit 60, a register 70, and an interface circuit 80, and further includes a capacitor 90. Note that circuit device 2 may have some of these elements omitted or modified, or other elements added.

[0072] Figure 13 shows an example configuration of the drive circuit 21 and buffer circuit 30 in the second embodiment. As shown in Figure 13, the drive circuit 21 includes an amplifier 211 and resistors 212 and 213, similar to the first embodiment. The buffer circuit 30 also includes a capacitor 31, a CMOS inverter circuit 32, and a resistor 33, similar to the first embodiment. The configuration and function of the drive circuit 21, buffer circuit 30, measurement circuit 40, clock generation circuit 50, control circuit 60, register 70, and interface circuit 80 are the same as in the first embodiment, so their description is omitted.

[0073] The electrode 101 of the sensing unit 100 is connected to one of the input node NG and output node ND of the amplifier 211, and the electrode 102 of the sensing unit 100 is connected to ground. Also, one end of the capacitor 90 is connected to the other of the input node NG and output node ND of the amplifier 211, and the other end of the capacitor 90 is connected to ground. In Figures 12 and 13, the electrode 101 is connected to the output node ND of the amplifier 211, and one end of the capacitor 90 is connected to the input node NG of the amplifier 211.

[0074] Since the capacitor 90 is built into the circuit device 2 and is not formed in the sensing unit 100, the capacitance CG of the capacitor 90 is a constant value. The capacitance CG is, for example, several pF to several tens of pF. Note that the capacitor 90 may be located inside the package 4 or outside the circuit device 2.

[0075] Load capacitance C of oscillator 3 L This is composed of the capacitance CD between electrode 101 and electrode 102 and the capacitance CG of capacitor 90, and is represented by equation (4) above. Then, from equations (1) to (4) above, the oscillation frequency f of the oscillation circuit 20 changes according to the capacitance CD.

[0076] The other configurations and operations of the capacitance sensor 1 in the second embodiment are the same as those of the capacitance sensor 1 in the first embodiment, so their description will be omitted.

[0077] Note that electrode 101 is an example of a "first electrode," electrode 102 is an example of a "second electrode," and electrode 104 is an example of a "ground electrode." Also, capacitance CD is an example of a "first capacitance." Furthermore, surface 110a of substrate 110 is an example of a "first surface," and surface 110b of substrate 110 is an example of a "second surface."

[0078] In the second embodiment of the capacitance sensor 1 described above, the oscillation circuit 20 oscillates based on resonance between the oscillator 3 and the capacitance CD, rather than on CR oscillation based on charging and discharging of the capacitance CD. Therefore, it is less susceptible to external amplitude noise. Consequently, the capacitance sensor 1 of the second embodiment can detect capacitance with high accuracy.

[0079] Furthermore, in the second embodiment of the capacitance sensor 1, the oscillator 3 has a very high Q value and also functions as a noise filter. Noise input from the electrode 101 of the sensing unit 100 connected to the output node ND of the amplifier 211 is greatly reduced by the oscillator 3, and the signal output from the oscillator 3 to the input node NG of the amplifier 211 has less noise and is a sine wave. This signal is close to [a certain value]. Therefore, according to the capacitance sensor 1 of the second embodiment, the risk of detection accuracy being reduced by noise input from the electrode 101 is reduced.

[0080] Furthermore, in the capacitance sensor 1 of the second embodiment, electrode 104 is provided on surface 110b of the substrate 110 at a position opposite to the arrangement area of ​​electrodes 101 and 102 on surface 110a. Therefore, when an object that is not the target of detection is located opposite surface 110b, the influence of that object on capacitance CD is reduced. Accordingly, the capacitance sensor 1 of the second embodiment can improve the detection accuracy of capacitance.

[0081] Furthermore, the capacitance sensor 1 of the second embodiment can be made smaller and less expensive compared to a capacitance sensor using an LC oscillator circuit, so it can be easily used even when the object to be detected is small.

[0082] 3. Variant The present invention is not limited to this embodiment, and various modifications can be implemented within the scope of the gist of the present invention.

[0083] For example, in the second embodiment described above, the electrode 101 of the sensing unit 100 is connected to the output node ND of the amplifier 211 and the capacitor 90 is connected to the input node NG of the amplifier 211, but the electrode 101 may be connected to the input node NG and the capacitor 90 may be connected to the output node ND.

[0084] Furthermore, in each of the above embodiments, the circuit device 2 measures the oscillation frequency f based on the signal BFO ​​and outputs the measured value of the oscillation frequency f to the outside of the oscillator 10, but the signal BFO ​​may also be output to the outside. Then, the external device measures the oscillation frequency f, which is the frequency of the signal BFO, and based on the measured value of the oscillation frequency f, the load capacitance C L The value of may be calculated. In this case, the circuit device 2 does not need to be equipped with a measurement circuit 40. Alternatively, the circuit device 2 may calculate the load capacitance C based on the oscillation frequency f. L Calculate the value of load capacity C L The value may be output externally.

[0085] Furthermore, while the oscillator 10 in each of the above embodiments is a simple oscillator such as an SPXO, it may also be an oscillator with temperature compensation function such as a TCXO, or an oscillator with frequency control function such as a VCXO. SPXO stands for Simple Packaged Crystal Oscillator. TCXO stands for Temperature Compensated Crystal Oscillator. VCXO stands for Voltage Controlled Crystal Oscillator. In addition, the oscillator 10 may also be an oscillator with temperature compensation function and frequency control function such as a VC-TCXO, or an oscillator with temperature control function such as an OCXO. VC-TCXO stands for Voltage Controlled Temperature Compensated Crystal Oscillator. OCXO stands for Oven Controlled Crystal Oscillator.

[0086] Furthermore, while the embodiments described above show examples of using the capacitance sensor 1 as a sensor for detecting the amount of liquid in a container, the capacitance sensor 1 can also be used in other ways, such as a proximity sensor for detecting the approach of an object to be detected, a touch sensor for detecting contact with an object to be detected, and various sensors for detecting rain, fog, ice, snow, gas, etc. For example, it can be used as a sensor to detect the approach or contact of a finger to a car door and output an unlock signal. It can also be used as a sensor to determine not only the amount, approach, and contact of an object to be detected, but also the type of object to be detected and the liquid concentration.

[0087] Furthermore, a part of the vehicle body may be used as at least a part of the sensing unit 100. For example, a first insulating layer may be provided on one side of the front and back surfaces of a metal plate used as the vehicle body material, and at least one of the electrodes 101 and 103 of the sensing unit 100 may be made of the metal of the first insulating layer. The first insulating layer is provided on the side opposite to the metal plate. The second insulating layer is provided on the other side of the front and back surfaces of the metal plate. When a part of the human body, such as a finger or hand, comes into contact with the side of the second insulating layer opposite to the metal plate, the capacitance value of the metal plate to ground changes. This capacitance to ground and the capacitance between the metal plate and the electrode provided on the side of the first insulating layer opposite to the metal plate are connected in series between the ground potential and the XG terminal, or between the ground potential and the XD terminal. With this configuration, a touch sensor can be realized with the side of the second insulating layer opposite to the metal plate as the contact surface. That is, a capacitance sensor can be realized in which the oscillation frequency changes depending on whether or not there is contact with the contact surface. Furthermore, a third insulating layer may be provided on the side of the electrode opposite to the first insulating layer, relative to the electrode provided on the side of the first insulating layer opposite to the metal plate. This makes it possible to realize a touch sensor with the side of the third insulating layer opposite to the electrode as the contact surface. The metal plate may be an iron plate or an aluminum plate, etc. The first insulating layer and the second insulating layer may be a coating provided on the metal plate. For example, this capacitive sensor can be used as a touch sensor for vehicle theft prevention.

[0088] The embodiments and variations described above are examples only and are not limited thereto. For example, each embodiment and each variation can be combined as appropriate.

[0089] The present invention includes configurations substantially identical to those described in the embodiments, for example, configurations with the same function, method, and results, or configurations with the same purpose and effect. Furthermore, the present invention includes configurations in which non-essential parts of the configurations described in the embodiments are replaced. Furthermore, the present invention includes configurations that produce the same effects or achieve the same purpose as those described in the embodiments. Finally, the present invention includes configurations that add known technology to the configurations described in the embodiments.

[0090] The following can be derived from the embodiments and modifications described above.

[0091] One embodiment of a capacitive sensor is, The package and An oscillator circuit provided inside the package, The first electrode and the second electrode are provided outside the package, Equipped with, The aforementioned oscillator circuit is Amplifier and An oscillator provided inside the package and connected between the input node and output node of the amplifier, It has, The first electrode is a sensing electrode connected to either the input node or the output node of the amplifier, The second electrode is an electrode with a fixed potential, The oscillation frequency of the oscillation circuit changes according to the first capacitance between the first electrode and the second electrode.

[0092] In this capacitance sensor, the oscillation circuit oscillates based on resonance between the oscillator and the first capacitance, rather than on CR oscillation based on the charging and discharging of the first capacitance between the first and second electrodes. Therefore, it is less susceptible to external amplitude noise. Consequently, this capacitance sensor can detect capacitance with high accuracy.

[0093] Incidentally, an LC oscillator circuit, which utilizes LC resonance between an inductor and capacitance, has a wider range of oscillation frequency variation compared to an oscillator circuit that uses a resonator and capacitance. Therefore, it is conceivable to construct a highly sensitive capacitance sensor using an LC oscillator circuit. However, However, when detecting capacitance on the order of pF using a capacitance sensor with an LC oscillator circuit, for example, using a small inductor on the order of nH to reduce costs can lead to various problems, such as an increased oscillation frequency on the order of GHz, which increases the size and power consumption of the capacitance measurement circuit. Conversely, to achieve an oscillation frequency on the order of MHz, it is necessary to use a large inductor on the order of μH, which hinders the miniaturization and cost reduction of capacitance sensors.

[0094] In contrast, while an oscillator circuit utilizing a resonator and capacitance has a considerably narrower range of oscillation frequency variation than an LC oscillator circuit, it can easily achieve oscillation frequencies on the kHz or MHz order using a small resonator, and for capacitances on the pF order, it can also provide a practically necessary range of oscillation frequency variation. Furthermore, since the inductance value of an inductor is determined by its size, it is difficult to reduce the size of the inductor without changing the inductance value. However, resonators can be further miniaturized and reduced in cost through future advancements in manufacturing processes. Therefore, this capacitance sensor can achieve smaller size and lower cost compared to a capacitance sensor using an LC oscillator circuit.

[0095] Another embodiment of a capacitive sensor is, Oscillator circuit, First electrode and second electrode, Equipped with, The aforementioned oscillator circuit is Amplifier and A oscillator connected between the input node and output node of the amplifier, It has, The first electrode is a sensing electrode connected to either the input node or the output node of the amplifier, The second electrode is an electrode with a fixed potential, The first capacitance between the first electrode and the second electrode changes depending on the state of the object being detected. The oscillation frequency of the oscillation circuit changes according to the first capacitance.

[0096] In this capacitance sensor, the oscillation circuit oscillates based on resonance between the oscillator and the first capacitance, rather than on CR oscillation based on the charging and discharging of the first capacitance between the first and second electrodes. Therefore, it is less susceptible to external amplitude noise. Consequently, this capacitance sensor can detect the capacitance according to the state of the object being detected with high accuracy.

[0097] Furthermore, this capacitance sensor can be made smaller and less expensive compared to capacitance sensors using LC oscillator circuits, making it easy to use even when detecting small objects.

[0098] One embodiment of the aforementioned capacitance sensor is: A buffer circuit to which the signal output from the oscillator to the input node of the amplifier is input, A measurement circuit for measuring the frequency of the signal output from the buffer circuit, It may be provided.

[0099] In this capacitance sensor, the oscillator has a very high Q value, so it also functions as a noise filter. The signal output from the oscillator to the amplifier's input node is low in noise and close to a sine wave. Therefore, with this capacitance sensor, no noise spikes occur in the output signal of the buffer circuit, reducing the risk of the measurement circuit making incorrect measurements. It will be reduced.

[0100] One embodiment of the aforementioned capacitance sensor is: The substrate comprises a first surface and a second surface which is the back surface of the first surface, The first surface of the substrate is provided with the first electrode and the second electrode, A ground electrode may be provided on the second surface of the substrate at a position opposite to the arrangement area of ​​the first electrode and the second electrode on the first surface.

[0101] In this capacitance sensor, a ground electrode is provided on the second surface of the substrate at a position opposite to the placement area of ​​the first and second electrodes on the first surface. Therefore, when an object that is not the object to be detected is located opposite the second surface, the influence of that object on the first capacitance is reduced. Consequently, this capacitance sensor can improve the accuracy of capacitance detection.

[0102] In one embodiment of the aforementioned capacitance sensor, The first electrode may be connected to the output node of the amplifier.

[0103] In this capacitance sensor, the oscillator has a very high Q value and therefore also functions as a noise filter. Noise input from the first electrode connected to the amplifier's output node is greatly reduced by the oscillator, and the signal output from the oscillator to the amplifier's input node is low in noise and close to a sine wave. Therefore, with this capacitance sensor, the risk of detection accuracy being reduced by noise input from the first electrode is reduced.

[0104] One embodiment of the aforementioned capacitance sensor is: The package comprises a third electrode located outside the package, The third electrode is a sensing electrode connected to the other of the input node and the output node of the amplifier. The oscillation frequency of the oscillation circuit may change depending on the first capacitance and the second capacitance between the third electrode and the second electrode.

[0105] In this capacitance sensor, the oscillation frequency of the oscillation circuit changes according to the first and second capacitances, thus widening the variable range of the oscillation frequency compared to when there is no second capacitance. Therefore, this capacitance sensor can improve the detection sensitivity of capacitance.

[0106] One embodiment of the aforementioned capacitance sensor is: The substrate comprises a first surface and a second surface which is the back surface of the first surface, The first surface of the substrate is provided with the first electrode, the second electrode, and the third electrode. The second electrode is located between the first electrode and the third electrode. A ground electrode may be provided on the second surface of the substrate at a position opposite to the arrangement area of ​​the first electrode, second electrode, and third electrode on the first surface.

[0107] In this capacitance sensor, a ground electrode is provided on the second surface of the substrate at a position opposite to the arrangement areas of the first, second, and third electrodes on the first surface. Therefore, when an object that is not the object to be detected is located opposite the second surface, the influence of that object on the first and second capacitances is reduced. Consequently, this capacitance sensor can improve the accuracy of capacitance detection. [Explanation of Symbols]

[0108] 1...Capacitance sensor, 2...Circuit device, 3...Oscillator, 3a...Excitation electrode, 3b...Excitation electrode, 4...Package, 5...Lid, 6...Pad, 7...Bonding wire, 8...Electrode, 9...Electrode, 10...Oscillator, 11...Conductive bonding member, 12a,12b...Electrode, 15...Cable, 20...Oscillator circuit, 21...Drive circuit, 30...Buffer circuit, 31...Capacitor, 32...CMOS inverter circuit, 33...Resistor, 40...Measurement circuit, 41...Frequency divider circuit, 42...Counter, 50...Clock generation circuit, 60...Control circuit, 70...Register, 80...Interface circuit, 90...Capacitor, 100...Sensing unit, 101,102,103,104...Electrodes, 110...Substrate, 200...MCU, 211...Amplifier, 212,213...Resistor, 300...Detection target

Claims

1. The package and An oscillator circuit provided inside the package, The first electrode and the second electrode are provided outside the package, Equipped with, The aforementioned oscillator circuit is Amplifier and An oscillator provided inside the package and connected between the input node and output node of the amplifier, It has, The first electrode is a sensing electrode connected to either the input node or the output node of the amplifier. The second electrode is an electrode with a fixed potential, A capacitance sensor in which the oscillation frequency of the oscillation circuit changes according to a first capacitance between the first electrode and the second electrode.

2. Oscillator circuit, First electrode and second electrode, Equipped with, The aforementioned oscillator circuit is Amplifier and A oscillator connected between the input node and output node of the amplifier, It has, The first electrode is a sensing electrode connected to either the input node or the output node of the amplifier. The second electrode is an electrode with a fixed potential, The first capacitance between the first electrode and the second electrode changes depending on the state of the object being detected. A capacitance sensor in which the oscillation frequency of the oscillation circuit changes according to the first capacitance.

3. In claim 1 or 2, A buffer circuit to which the signal output from the oscillator to the input node of the amplifier is input, A measurement circuit for measuring the frequency of the signal output from the buffer circuit, A capacitive sensor equipped with the following features.

4. In claim 1 or 2, The substrate comprises a first surface and a second surface which is the back surface of the first surface, The first surface of the substrate is provided with the first electrode and the second electrode. A capacitive sensor wherein a ground electrode is provided on the second surface of the substrate at a position opposite to the arrangement area of ​​the first electrode and the second electrode on the first surface.

5. In claim 1 or 2, The first electrode is a capacitance sensor connected to the output node of the amplifier.

6. In claim 1, The package comprises a third electrode located outside the package, The third electrode is a sensing electrode connected to the other of the input node and the output node of the amplifier. A capacitance sensor in which the oscillation frequency of the oscillation circuit changes according to the first capacitance and the second capacitance between the third electrode and the second electrode.

7. In claim 6, The substrate comprises a first surface and a second surface which is the back surface of the first surface, The first surface of the substrate is provided with the first electrode, the second electrode, and the third electrode. The second electrode is located between the first electrode and the third electrode. A capacitive sensor wherein a ground electrode is provided on the second surface of the substrate at a position opposite to the arrangement area of ​​the first electrode, second electrode, and third electrode on the first surface.

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