Capacitance sensor

Abstract

Provided is an electrostatic capacitance sensor capable of detecting electrostatic capacitance with high accuracy. The electrostatic capacitance sensor includes a package, an oscillation circuit, a switching circuit, a first reference capacitor, a first electrode, and a second electrode. The oscillation circuit includes: an amplifier; and an oscillator connected between an input node and an output node of the amplifier, the first electrode is an electrode for sensing, the second electrode is an electrode with a fixed potential, and the switching circuit switches whether to connect the first electrode to a first node or to connect one end of the first reference capacitor to the first node. The first node is one of the input node and the output node of the amplifier, and when the first electrode is connected to the first node, the oscillation frequency of the oscillation circuit changes in accordance with a first capacitance between the first electrode and the second electrode.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrostatic capacitance sensor. BACKGROUND

[0002] In Patent Literature 1, a liquid level detector is described which makes an oscillation circuit oscillate with an electrostatic capacitance and a resistance which vary in accordance with a change in a measured liquid level as an oscillation time constant, and outputs a digital signal corresponding to the change in the liquid level based on an output signal of the oscillation circuit.

[0003] Patent Literature 1: Japanese Patent Application Laid-Open No. 2003-57095

[0004] However, in the liquid level detector described in Patent Literature 1, the oscillation circuit is oscillated by charging and discharging of the electrostatic capacitance, so the oscillation frequency of the oscillation circuit is easily varied by external amplitude noise, and in addition, the oscillation frequency is varied by temperature and secular change of the oscillation circuit, so it is difficult to achieve high detection accuracy. SUMMARY

[0005] One embodiment of the electrostatic capacitance sensor according to the present application includes a package, an oscillation circuit provided inside the package, a switching circuit provided inside the package, a first reference capacitance provided inside the package, and a first electrode and a second electrode provided outside the package, the oscillation circuit has an amplifier and a vibrator provided inside the package and connected between an input node and an output node of the amplifier, the first electrode is a sensing electrode, the second electrode is a potential-fixed electrode, the switching circuit switches whether the first electrode is connected to a first node or one end of the first reference capacitance is connected to the first node, the first node is one of the input node and the output node of the amplifier, and when the first electrode is connected to the first node, an oscillation frequency of the oscillation circuit is varied in accordance with a first electrostatic capacitance between the first electrode and the second electrode.

[0006] Another electrostatic capacitance sensor according to the present application includes an oscillation circuit, a switching circuit, a first reference capacitor, and a first electrode and a second electrode. The oscillation circuit includes an amplifier and a vibrator connected between an input node and an output node of the amplifier. The first electrode is a sensing electrode, and the second electrode is a potential-fixed electrode. The switching circuit switches whether the first electrode is connected to a first node or one end of the first reference capacitor is connected to the first node. The first node is one of the input node and the output node of the amplifier. A first electrostatic capacitance between the first electrode and the second electrode varies according to a state of an object to be detected. When the first electrode is connected to the first node, an oscillation frequency of the oscillation circuit varies according to the first electrostatic capacitance. BRIEF DESCRIPTION OF DRAWINGS

[0007] Figure 1 FIG. 1 is a diagram showing an appearance of an electrostatic capacitance sensor according to a first embodiment.

[0008] Figure 2 FIG. 2 is a plan view showing an example of an internal structure of an oscillator.

[0009] Figure 3 FIG. 3 is a plan view showing another example of an internal structure of an oscillator.

[0010] Figure 4 FIG. 4 is a diagram showing an example of use of the electrostatic capacitance sensor.

[0011] Figure 5 FIG. 5 is a functional block diagram of the electrostatic capacitance sensor according to the first embodiment.

[0012] Figure 6 FIG. 6 is a diagram showing an example of a structure of a drive circuit and a buffer circuit.

[0013] Figure 7 FIG. 7 is a diagram showing an equivalent circuit of the vibrator.

[0014] Figure 8 FIG. 8 is a diagram showing a relationship between a load capacitance C L and a reference frequency Δf / f0.

[0015] Figure 9 FIG. 9 is a timing chart of various signals.

[0016] Figure 10 FIG. 10 is a timing chart when a plurality of measurements are performed.

[0017] Figure 11 FIG. 11 is a diagram showing a comparison of frequency variable characteristics of an LC oscillation circuit and an oscillation circuit using the vibrator with respect to a load capacitance C L .

[0018] Figure 12 Fig. 1 is a diagram showing an appearance of an electrostatic capacitance sensor according to a first embodiment.

[0019] Figure 13 Fig. 2 is a functional block diagram of the electrostatic capacitance sensor according to the first embodiment.

[0020] Figure 14 Fig. 3 is a diagram showing a structure example of a drive circuit and a buffer circuit.

[0021] Explanation of Reference Numerals

[0022] 1: electrostatic capacitance sensor; 2: circuit device; 3: vibrator; 3a: excitation electrode; 3b: excitation electrode; 4: package; 5: lid; 6: land; 7: bonding wire; 8: electrode; 9: electrode; 10: oscillator; 11: conductive bonding member; 12a, 12b: electrode; 15: cable; 20: oscillation circuit; 21: drive circuit; 30: buffer circuit; 31: capacitor; 32: CMOS inverter circuit; 33: resistor; 40: measurement circuit; 41: frequency division circuit; 42: counter; 43: divider; 50: clock generation circuit; 60: control circuit; 70: register; 80: interface circuit; 90: switching circuit; 91, 92, 93, 94: switch; 95, 96: reference capacitor; 97: capacitor; 100: sensing portion; 101, 102, 103, 104: electrode; 110: substrate; 200: MCU; 211: amplifier; 212, 213: resistor; 300: detection object. DETAILED DESCRIPTION

[0023] Hereinafter, a preferred embodiment of the present application will be described in detail using the drawings. Furthermore, the embodiment described below does not unduly limit the content of the present application recited in the claims. In addition, the structures described below are not necessarily all essential structural elements of the present application.

[0024] 1. First Embodiment

[0025] 1-1. Structure of Electrostatic Capacitance Sensor

[0026] Figure 1 Fig. 1 is a diagram showing an appearance of an electrostatic capacitance sensor according to a first embodiment. As shown in Fig. 1, the electrostatic capacitance sensor 1 according to the first embodiment includes an oscillator 10, a sensing portion 100, and a cable 15 connecting the oscillator 10 and the sensing portion 100. Figure 1

[0027] Fig. 2 is a functional block diagram of the electrostatic capacitance sensor according to the first embodiment. As shown in Fig. 2, the electrostatic capacitance sensor 1 according to the first embodiment includes the oscillator 10, the sensing portion 100, and the cable 15 connecting the oscillator 10 and the sensing portion 100. Figure 2 Figure 1 Figure 2 ​​As shown, the oscillator 10 includes a circuit assembly 2, an oscillator 3, a package 4, and a cover 5. Furthermore, in Figure 2 The illustration of cover 5 is omitted in the text.

[0028] The oscillator 10 is, for example, a single-sealed oscillator, and the package 4 is a container that houses the circuit device 2 and the oscillator 3 in the same space. That is, the circuit device 2 and the oscillator 3 are disposed inside the package 4. Specifically, the package 4 has recesses, and the circuit device 2 and the oscillator 3 are housed by covering the recesses with a cover 5. Alternatively, the oscillator 10 may not be a single-sealed structure; for example, the package 4 may be a container that houses the circuit device 2 and the oscillator 3 in different spaces. Specifically, the package 4 may have two recesses on opposite sides, and the oscillator 3 is housed by covering one recess with a cover 5, and the circuit device 2 is housed by covering the other recess with a sealing member.

[0029] In this embodiment, circuit device 2 is implemented using a single-chip integrated circuit. However, at least a portion of circuit device 2 can also be composed of discrete components. Figure 2 In this example, the circuit device 2 is mounted to the inner bottom surface of the package 4 via an adhesive or the like, with the surface having multiple pads 6 forming the upper surface. Each of the multiple pads 6 is connected to any one of the multiple electrodes 8 formed on the surface of the recess in the package 4 via bonding wires 7.

[0030] Oscillator 3 is a piezoelectric oscillator that uses piezoelectric materials such as quartz, lithium tantalate, lithium niobate, and piezoelectric ceramics such as lead zirconate titanate as the substrate material. Alternatively, it can be 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 that uses quartz as the substrate material. Figure 2 In this example, a tuning fork type quartz oscillator is used. The two support arms of the oscillator 3 are respectively connected to two electrodes 9 formed on the surface of the recess in the package 4 via conductive bonding members 11. That is, each support arm of the oscillator 3 is fixed to and electrically connected to each electrode 9. The two electrodes 9 are connected to the two electrodes 8 and the two pads 6 of the circuit device 2 via wiring provided in the package 4, specifically to the pads described later. Figure 5 The XD and XG terminals are electrically connected.

[0031] Electrode patterns (not shown) are formed on the two support arms and two vibrating arms of the oscillator 3. Furthermore, a signal generated by one of the electrode patterns is supplied from one electrode 9 to the XG terminal of the circuit device 2. The amplifier in the circuit device 2 amplifies this signal and supplies the amplified signal from the XD terminal to the oscillator 3 via the other electrode 9. Thus, the two vibrating arms of the oscillator 3 continuously vibrate like a tuning fork. Therefore, the oscillation circuit including the oscillator 3 and the amplifier oscillates.

[0032] Figure 3 This is a top view showing another example of the internal structure of the oscillator 10. Figure 3 In the example, the circuit device 2 is mounted on the bottom surface of the recess of the package 4, and the oscillator 3 is mounted on the upper part of the circuit device 2 with a gap.

[0033] exist Figure 3 In this example, the oscillator 3 is an AT-cut quartz oscillator. The oscillator 3 has metallic excitation electrodes 3a and 3b on its surface and back side, respectively, to vibrate at a desired frequency corresponding to the shape and mass of the oscillator 3 containing the excitation electrodes 3a and 3b. The excitation electrodes 3a and 3b are respectively engaged with two electrodes 12a and 12b formed on the surface of a recess in the package 4. The package 4 is provided with two terminals for connecting the circuit device 2, specifically described later. Figure 5 The XD and XG terminals are electrically connected to electrodes 12a and 12b, respectively, via wiring not shown. Furthermore, a signal generated by 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 this signal, and the amplified signal is supplied from the XD terminal via the oscillator 3, which is in the opposite direction of the excitation electrodes 3a and 3b. This causes continuous thickness shear vibration, in which the surface and back surface of the oscillator 3 move in opposite directions. Thus, the oscillation circuit, including the oscillator 3 and the amplifier, oscillates.

[0034] The oscillator 10 has a plurality of external connection terminals (not shown) on the back side of the package 4, which serves as the bottom surface. Additionally, the package 4 has wiring (not shown) for electrically connecting the terminals of the circuit device 2 to the external connection terminals on the bottom surface of the package 4.

[0035] like Figure 1 As shown, the sensing unit 100 is disposed outside the package 4 of the oscillator 10 and is connected to the oscillator 10 via a cable 15. The cable 15 may be, for example, a coaxial cable, a flexible flat cable, etc.

[0036] The sensing unit 100 includes a substrate 110, which has a surface 110a and a back surface 110b of the surface 110a. Electrodes 101, 102, and 103 are provided on surface 110a of the substrate 110. Electrodes 101, 102, and 103 are each elongated rectangular in shape, with electrode 102 located between electrodes 101 and 103. On surface 110b of the substrate 110, an electrode 104 is provided at a position opposite to the arrangement area of ​​electrodes 101, 102, and 103 in surface 110a. For example, electrode 104 is provided on approximately the entire surface of surface 110b.

[0037] Electrodes 101, 102, 103, and 104 are connected to the external connection terminals of the oscillator 10 via wiring included in cable 15. Therefore, electrodes 101, 102, 103, and 104 are connected to the terminals of the circuit device 2 via the external connection terminals of the oscillator 10.

[0038] In this embodiment, electrode 101 is a sensing electrode that can be connected to one of the XD terminal and XG terminal of the circuit device 2, and electrode 103 is a sensing electrode that can be 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 at ground potential. Electrode 104 is connected to the ground terminal of the circuit device 2, and its potential is fixed at ground potential.

[0039] The sensing unit 100 is arranged such that electrodes 101, 102, and 103 are opposite to the object to be detected by electrostatic capacitance. The electrostatic capacitance CD between electrodes 101 and 102 and the electrostatic capacitance CG between electrodes 103 and 102 vary according to the dielectric constant of the object to be detected.

[0040] As described later, when electrode 101 is connected to the XD terminal of circuit device 2 and electrode 103 is connected to the XG terminal of circuit device 2, when the electrostatic capacitances CD and CG change, the load capacitance C of oscillator 3... L The oscillation frequency f of the oscillation circuit also changes. Therefore, 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. The external device can detect changes in the electrostatic capacitances CD and CG based on the measured value of the oscillation frequency f and determine the state of the object being detected. In addition, when an object that is not the object being detected is located opposite the surface 110b of the substrate 110, the electrode 104 functions as a shielding component to reduce the influence of the object on the electrostatic capacitances CD and CG.

[0041] Figure 4 This is a diagram illustrating an example of the use of the electrostatic capacitive sensor 1. In Figure 4In the example, the object 300 for detecting electrostatic capacitance is a container holding liquid LQ. The internal space of the object 300 is filled with liquid LQ and air AR. When the liquid LQ decreases, the air AR increases, and when the liquid LQ increases, the air AR decreases. The dielectric constant of liquid LQ is tens of times that of air AR. The more liquid LQ there is, the greater the effective dielectric constant inside the object 300. On the other hand, the electrostatic capacitances CD and CG increase or decrease proportionally with the dielectric constant of the object 300. Therefore, the greater the dielectric constant of the object 300, i.e., the more liquid LQ there is, the greater the electrostatic capacitances CD and CG, and thus the greater the load capacitance C of the oscillator 3. L The larger the value, the greater the amount of liquid LQ contained in the object to be detected. Therefore, the external device can calculate the amount of liquid LQ contained in the object to be detected based on the measured value of the oscillation frequency f output from the oscillator 10.

[0042] 1-2. Functional Structure of Electrostatic Capacitive Sensor

[0043] Figure 5 This is a functional block diagram of the electrostatic capacitive sensor 1 according to the first embodiment. For example... Figure 5 As shown, the electrostatic capacitive sensor 1 of the first embodiment includes a circuit device 2 and a sensing unit 100.

[0044] The circuit device 2 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, an interface circuit 80, a switching circuit 90, and reference capacitors 95 and 96. Furthermore, the circuit device 2 may be configured to omit or modify some of these elements, or to add other elements.

[0045] Circuit device 2 has a VDD terminal as a power supply terminal and a VSS terminal as a ground terminal. The potential of the VDD terminal is set to the power supply potential, and the potential of the VSS terminal is set to the ground potential, causing each circuit to operate. Additionally, circuit device 2 has XD and XG terminals, which are connected to the two ends of the oscillator 3. Furthermore, circuit device 2 also has a terminal (not shown) for data communication with the MCU 200, which is an external device of circuit device 2.

[0046] The drive circuit 21 is connected to the XD and XG terminals, causing the oscillator 3 to vibrate and generate an oscillation signal OSCO. The drive circuit 21 amplifies the signal input from the oscillator 3 via the XG terminal and outputs the amplified signal back to the oscillator 3 via the XD terminal. Thus, the two vibrating arms of the oscillator 3 vibrate, and the drive circuit 21 outputs the signal input from the oscillator 3 via the XG terminal as the oscillation signal OSCO. The oscillator 3 and the drive circuit 21 constitute the oscillation circuit 20.

[0047] The buffer circuit 30 buffers the oscillation signal OSCO output from the drive circuit 21 and outputs a rectangular wave signal BFO. In addition, the rectangular wave of the signal BFO ​​includes not only strictly rectangular waves, but also waveforms that are close to rectangular waves.

[0048] Figure 6 This is a diagram showing an example of the structure of the drive circuit 21 and the buffer circuit 30. (See diagram for example.) Figure 6 As shown, the driving circuit 21 includes an amplifier 211 and resistors 212 and 213.

[0049] Amplifier 211 amplifies the signal output from oscillator 3, and outputs the amplified signal back to oscillator 3 via resistor 213. Figure 6 In the example, amplifier 211 is a CMOS inverter circuit, but it can also be a bipolar transistor.

[0050] 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 through 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 terminal XG, and node ND is connected to terminal XD.

[0051] The signal output from amplifier 211 is a rectangular wave signal, which is input to oscillator 3. For example... Figure 2 and Figure 3 As shown, oscillator 3 is, for example, a tuning fork type quartz oscillator or an AT-cut quartz oscillator, which has a very high Q value. Therefore, the signal output from oscillator 3 is a low-noise signal that is close to a sine wave. The signal output from oscillator 3 to node NG is input to buffer circuit 30 as an oscillation signal OSCO. Furthermore, the rectangular wave of the signal output from amplifier 211 includes not only strictly rectangular waves but also waveforms that are close to rectangular waves.

[0052] 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 rectangular wave signal BFO.

[0053] Here, Figure 7 The diagram shows the equivalent circuit of oscillator 3 as a quartz oscillator. The equivalent constants of oscillator 3 can be represented by the series inductor L1, series capacitor C1, series resistor R1, and parallel capacitor C0. At this time, the series resonant frequency f0 of oscillator 3 is represented by equation (1).

[0054]

[0055] The oscillation frequency f of the oscillation circuit 20, which consists of oscillator 3 and drive circuit 21, is determined by the load capacitance C. L The reference frequency Δf / f0 of the oscillator circuit 20 changes as expressed by equation (2).

[0056]

[0057] In equation (2), γ is the ratio of parallel capacitor C0 to series capacitor C1, as expressed by equation (3).

[0058]

[0059] Figure 8 This shows the load capacitance C. L A graph illustrating an example of the relationship between the reference frequency Δf / f0 and the reference frequency. Figure 8 In the diagram, the solid line represents the curve when oscillator 3 is a tuning fork type quartz oscillator, and the dashed line represents the curve when oscillator 3 is an AT-cut quartz oscillator. According to... Figure 8 When the load capacitance C L When the frequency varies within the range of 0 to 30 pF, the reference frequency Δf / f0 also changes, and the rate of change is even greater for tuning fork quartz oscillators.

[0060] The switching circuit 90 switches between two states: connecting electrode 101 of the sensing unit 100 to one of the input node NG and output node ND of the amplifier 211 and connecting electrode 103 of the sensing unit 100 to the other of the input node NG and output node ND of the amplifier 211; or connecting one end of the reference capacitor 95 to one of the input node NG and output node ND of the amplifier 211 and connecting one end of the reference capacitor 96 to the other of the input node NG and output node ND of the amplifier 211. Figure 5 and Figure 6 In this circuit, the switching circuit 90 switches between two states: either connecting electrode 101 to the output node ND of amplifier 211 and connecting electrode 103 to the input node NG of amplifier 211, or connecting one end of reference capacitor 95 to the output node ND of amplifier 211 and connecting one end of reference capacitor 96 to the input node NG of amplifier 211. In other words, the node connecting electrode 101 or reference capacitor 95 is the output node ND, and the node connecting electrode 103 or reference capacitor 96 is the input node NG.

[0061] Specifically, the switching circuit 90 includes switches 91, 92, 93, and 94. One end of each of switches 91 and 93 is connected to the output node ND, and one end of each of switches 92 and 94 is connected to the input node NG. The other end of switch 91 is connected to electrode 101, and the other end of switch 92 is connected to electrode 103. The electrode 102 of the sensing unit 100 is connected to ground. The other end of switch 93 is connected to one end of reference capacitor 95, and the other end of switch 94 is connected to one end of reference capacitor 96. The other ends of reference capacitors 95 and 96 are connected to ground. For example, reference capacitors 95 and 96 can also be capacitors. Furthermore, to ensure integration into the circuit device 2, reference capacitors 95 and 96 are preferably MOS capacitors, capacitors using comb-shaped electrodes, or capacitors with parallel planar electrodes, but chip capacitors separate from the circuit device 2 can also be used.

[0062] Switches 91 and 92 are turned on when the switch control signal ASW is low and turned off when the switch control signal ASW is high. Switches 93 and 94 are turned on when the switch control signal ASW is high and turned off when the switch control signal ASW is low. That is, switches 91 and 92 are turned on / off exclusively with switches 93 and 94. Moreover, when switches 91 and 92 are turned on, electrode 101 is connected to the output node ND and electrode 103 is connected to the input node NG. When switches 93 and 94 are turned on, one end of the reference capacitor 95 is connected to the output node ND and one end of the reference capacitor 96 is connected to the input node NG.

[0063] Therefore, when switches 91 and 92 are turned on, the load capacitance C of oscillator 3 is... L It consists of the electrostatic capacitance CD between electrodes 101 and 102 and the electrostatic capacitance CG between electrodes 103 and 102, and is represented by equation (4). In equation (4), CS is the parasitic capacitance, which is about pF.

[0064]

[0065] According to equations (1) to (4), when electrode 101 is connected to output node ND and electrode 103 is connected to input node NG, the oscillation frequency f of oscillation circuit 20 varies according to the electrostatic capacitors CD and CG. On the other hand, when reference capacitor 95 is connected to output node ND and reference capacitor 96 is connected to input node NG, the oscillation frequency f of oscillation circuit 20 varies according to reference capacitors 95 and 96. The frequency of the oscillation signal OSCO output from drive circuit 21 is the oscillation frequency f, and the frequency of the signal BFO ​​output from buffer circuit 30 is also consistent with the oscillation frequency f. Furthermore, the switch control signal ASW from control circuit 60 can control switches 91, 92, 93, and 94 to be turned on / off exclusively, and can also control each of the four switches 91, 92, 93, and 94 independently.

[0066] like Figure 5 As shown, the measurement circuit 40 measures the first oscillation frequency f1 of the oscillation circuit 20 when electrode 101 is connected to the output node ND and electrode 103 is connected to the input node NG, and the second oscillation frequency f2 of the oscillation circuit 20 when one end of reference capacitor 95 is connected to the output node ND and one end of reference capacitor 96 is connected to the input node NG, based on the signal BFO ​​output from the buffer circuit 30. Furthermore, the measurement circuit 40 can also calculate the ratio of the first oscillation frequency f1 to the second oscillation frequency f2. Both the first oscillation frequency f1 and the second oscillation frequency f2 are similarly affected by the temperature characteristics and aging characteristics of the oscillator 3 and the drive circuit 21; therefore, the influence of the temperature characteristics and aging characteristics of the oscillator 3 and the drive circuit 21 is reduced relative to the ratio of the first oscillation frequency f1 to the second oscillation frequency f2.

[0067] For example, the measurement circuit 40 includes a frequency divider circuit 41, a counter 42, and a divider 43. The frequency divider circuit 41 divides the signal based on the oscillation of the oscillation circuit 20, i.e., the signal BFO ​​output from the buffer circuit 30, and outputs a gating time signal GT.

[0068] Counter 42 counts the number of clock signal CK pulses contained within the specified logic level period (i.e., the gating time) of the gating time output from frequency divider circuit 41, and outputs the count value CNT. For example, Figure 9 As shown in the timing diagram, the frequency divider circuit 41 outputs a high-level gating signal GT during a specified period of the output signal BFO. The counter 42 outputs a count value CNT representing the number of pulses of the clock signal CK contained during the period when the gating signal GT is high. In this case, the time during which the gating signal GT is high corresponds to the gating time.

[0069] The count value CNT is equivalent to the ratio of the frequency of the clock signal CK to the frequency of the signal BFO. The higher the frequency of the signal BFO, the smaller the count value CNT. That is, the frequency of the signal BFO ​​and the count value CNT have a one-to-one relationship, and the count value CNT is equivalent to the measured value of the frequency of the signal BFO, i.e., the oscillation frequency f.

[0070] A longer gating time results in higher measurement resolution based on measurement circuit 40, but also a longer measurement time. Therefore, the gating time is appropriately set according to the upper limit of the allowable measurement time, for example, set to several hundred milliseconds.

[0071] When the switch control signal ASW is low (i.e., when switches 91 and 92 are on), counter 42 counts the number of pulses of the clock signal CK during the selection time and outputs a count value CNT1. Conversely, when the switch control signal ASW is high (i.e., when switches 93 and 94 are on), counter 42 counts the number of pulses of the clock signal CK during the selection time and outputs a count value CNT2. The count value CNT1 corresponds to the measured value of the first oscillation frequency f1, and the count value CNT2 corresponds to the measured value of the second oscillation frequency f2.

[0072] Divider 43 divides the count value CNT1 output from counter 42 by the count value CNT2 to calculate the ratio of the first oscillation frequency f1 to the second oscillation frequency f2. The ratio of the first oscillation frequency f1 to the second oscillation frequency f2 is stored in register 70.

[0073] Additionally, the measurement circuit 40 can measure the first oscillation frequency f1 and the second oscillation frequency f2 multiple times, and calculate the ratio of the average value of the first oscillation frequency f1 to the average value of the second oscillation frequency f2. The counter 42 performs the following processing multiple times: when the switch control signal ASW is low, it counts the number of pulses of the clock signal CK included in the gating time and outputs a count value CNT1. Furthermore, the counter 42 performs the following processing multiple times: when the switch control signal ASW is high, it counts the number of pulses of the clock signal CK included in the gating time and outputs a count value CNT2. Then, the divider 43 calculates the average value of the multiple count values ​​CNT1 and the average value of the multiple count values ​​CNT2, and divides the average value of count values ​​CNT1 by the average value of count values ​​CNT2 to calculate the ratio of the average value of the first oscillation frequency f1 to the average value of the second oscillation frequency f2. The ratio of the average value of the first oscillation frequency f1 to the average value of the second oscillation frequency f2 is stored in the register 70.

[0074] In addition, the count values ​​CNT1 and CNT2 can also be stored in register 70, as can the average value of count value CNT1 and the average value of count value CNT2.

[0075] likeFigure 10 As shown, for example, the measurement circuit 40 can measure the second oscillation frequency f2 once, then measure the first oscillation frequency f1 twice, then measure the second oscillation frequency f2 once, and calculate the ratio of the average value of the first oscillation frequency f1 to the average value of the second oscillation frequency f2.

[0076] If the temperature rises or falls during the four measurements, each measurement contains errors caused by temperature changes, but when calculated according to... Figure 10 When measurements are performed in the following order, the sequence of the first measurement at the second oscillation frequency f2, the first measurement at the first oscillation frequency f1, the second measurement at the first oscillation frequency f1, and the second measurement at the second oscillation frequency f2 tends to lead to an increase or decrease in measurement error. Therefore, the difference between the error contained in the average value of the first oscillation frequency f1 and the error contained in the average value of the second oscillation frequency f2 decreases, thus improving measurement accuracy.

[0077] Furthermore, when switches 91 and 92 switch from on to off or from off to on, the oscillation of the oscillation circuit 20 may become unstable or stop oscillating. Therefore, as Figure 10 As shown, preferably, the measurement circuit 40 sets a waiting time between one measurement at the first oscillation frequency f1 or the second oscillation frequency f2 and the next measurement at the first oscillation frequency f1 or the second oscillation frequency f2. By setting such a waiting time, even if the oscillation of the oscillation circuit 20 becomes unstable or stops, it will not adversely affect the measurement.

[0078] In addition, Figure 10 During the period from the end of the first measurement of the first oscillation frequency f1 to the start of the second measurement of the first oscillation frequency f1, switches 91 and 92 remain on, and the oscillation of the oscillation circuit 20 will not become unstable. Therefore, a waiting time can be omitted.

[0079] 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 based on the measurement circuit 40. Therefore, for example, the clock generation circuit 50 can also be a ring oscillator capable of outputting signals from tens to hundreds of MHz. Furthermore, the clock signal CK from the clock generation circuit 50 can be output during the gating time period, i.e., when the gating time signal GT is at a predetermined logic level; for example, it can be output during the period when the gating time signal GT is high. Therefore, although the clock signal CK can be continuously output from the clock generation circuit 50 during the operation of the electrostatic capacitance sensor 1, a period outside the gating time can also be set where the output of the clock signal CK from the clock generation circuit 50 is stopped. In this case, the power consumption of the electrostatic capacitance sensor 1 can be reduced.

[0080] Control circuit 60 controls the operation of oscillation circuit 20. For example, control circuit 60 outputs an enable signal to oscillation circuit 20, and oscillation circuit 20 oscillates when the enable signal is high and stops oscillating when the enable signal is low. Additionally, control circuit 60 controls the operation of measurement circuit 40. For example, control circuit 60 outputs a signal indicating the start of measurement to measurement circuit 40, and measurement circuit 40 performs measurement processing according to the signal. Furthermore, control circuit 60 generates a switch control signal ASW and outputs it to switch circuit 90 and measurement circuit 40.

[0081] Interface circuit 80 is used for data communication with MCU 200. For example, when interface circuit 80 receives a measurement request from MCU 200, it changes a specified bit in register 70 from 0 to 1. When control circuit 60 detects that this bit has been changed, it causes oscillation circuit 20 to start oscillating. After a specified waiting time, it outputs a signal indicating the start of measurement to measurement circuit 40.

[0082] Additionally, for example, when the interface circuit 80 receives a readout request for a measured value from the MCU 200, it reads the ratio of the first oscillation frequency f1 to the second oscillation frequency f2, and the ratio of the average value of the first oscillation frequency f1 to the average value of the second oscillation frequency f2, stored in the register 70, and sends these ratios to the MCU 200. The MCU 200 can then calculate the load capacitance C based on these ratios. L It can also determine the state of the detected object. Alternatively, the interface circuit 80 reads the count values ​​CNT1, CNT2, and their average values ​​stored in register 70, and sends them to the MCU 200. The MCU 200 can then calculate the load capacitance C based on these values. L It can also determine the state of the object being detected.

[0083] The interface circuit 80 can be, for example, an interface circuit for an SPI bus or an I... 2 The interface circuit of the C bus. SPI is an abbreviation for Serial Peripheral Interface, and I2C is an abbreviation for Inter-Integrated Circuit.

[0084] Furthermore, electrode 101 is an example of "first electrode", electrode 102 is an example of "second electrode", electrode 103 is an example of "third electrode", and electrode 104 is an example of "ground electrode". Additionally, electrostatic capacitor CD is an example of "first electrostatic capacitor", and electrostatic capacitor CG is an example of "second electrostatic capacitor". Furthermore, reference capacitor 95 is an example of "first reference capacitor", and reference capacitor 96 is an example of "second reference capacitor". The output node ND of amplifier 211 is an example of "first node", and the input node NG of amplifier 211 is an example of "second node". Furthermore, surface 110a of substrate 110 is an example of "first surface", and surface 110b of substrate 110 is an example of "second surface".

[0085] 1-3. Effects

[0086] As explained above, in the electrostatic capacitance sensor 1 of the first embodiment, when electrodes 101 and 103 are connected to nodes ND and NG respectively, the oscillation circuit 20 does not oscillate based on the CR oscillation of the charging and discharging of electrostatic capacitors CD and CG, but rather oscillates based on the resonance of the oscillator 3 and the electrostatic capacitors CD and CG. Therefore, it is less susceptible to external amplitude noise. Thus, the electrostatic capacitance sensor 1 according to the first embodiment can detect electrostatic capacitance with high accuracy.

[0087] Furthermore, in the electrostatic capacitance sensor 1 of the first embodiment, if the interval between the period during which electrodes 101 and 103 are connected to nodes ND and NG, respectively, and the period during which one end of each of the reference capacitors 95 and 96 is connected to nodes ND and NG, respectively, is short, then the first oscillation frequency f1 measured during the former period and the second oscillation frequency f2 measured during the latter period will be similarly affected by the temperature characteristics and aging characteristics of the oscillation circuit 20. Therefore, according to the electrostatic capacitance sensor 1 of the first embodiment, the measurement circuit 40 calculates the ratio of the first oscillation frequency f1 to the second oscillation frequency f2, thereby obtaining a high-precision oscillation frequency that reduces the influence of temperature and aging changes in the oscillation circuit 20. Therefore, the MCU 200 can calculate the electrostatic capacitance with high precision based on this oscillation frequency. Furthermore, the measurement circuit 40 calculates the ratio of the average value of the first oscillation frequency f1 measured multiple times to the average value of the second oscillation frequency f2 measured multiple times, thereby obtaining a high-precision oscillation frequency that not only reduces the influence of temperature and annual changes in the oscillation circuit 20, but also reduces the influence of noise and instantaneous temperature changes. Therefore, the MCU 200 can calculate the electrostatic capacitance with higher precision based on this oscillation frequency.

[0088] Furthermore, in the electrostatic capacitance sensor 1 of the first embodiment, when electrodes 101 and 103 are connected to nodes ND and NG respectively, the oscillation frequency of the oscillation circuit 20 varies according to the electrostatic capacitances CD and CG. Therefore, compared to the case where there is no electrostatic capacitance CG, i.e., when there is no electrode 103 in the sensing unit 100, the variable amplitude of the oscillation frequency can be expanded. Therefore, the electrostatic capacitance sensor 1 according to the first embodiment can improve the detection sensitivity of electrostatic capacitance.

[0089] Furthermore, in the electrostatic 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 is a low-noise signal that is close to a sine wave. Therefore, according to the electrostatic capacitance sensor 1 of the first embodiment, since no noise-induced spikes are generated in the output signal of the buffer circuit 30, the possibility of the measurement circuit 40 making erroneous measurements can be reduced.

[0090] Furthermore, in the electrostatic capacitance sensor 1 of the first embodiment, an 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 at the position opposite surface 110b, the influence of that object on the electrostatic capacitances CD and CG can be reduced. Thus, the electrostatic capacitance sensor 1 according to the first embodiment can improve the detection accuracy of electrostatic capacitance.

[0091] In addition, such as Figure 11 As shown, compared with the oscillating circuit that utilizes an oscillator and a capacitor, the LC oscillating circuit that utilizes LC resonance based on an inductor and a capacitor has a wider variable range of oscillation frequency. Figure 11 This shows the load capacitance C. L A graph illustrating an example of the relationship between the reference frequency Δf / f0 and the reference frequency. The solid line represents the curve when oscillator 3 is a tuning fork type quartz oscillator, equivalent to... Figure 8 The solid line represents the curve. The dashed line represents the curve in the case of an LC oscillator circuit. According to... Figure 11 It can be seen that, compared with the variable amplitude of the oscillation frequency of the LC oscillation circuit, the variable amplitude of the oscillation circuit utilizing the oscillator and electrostatic capacitor is extremely narrow.

[0092] Therefore, constructing a high-sensitivity electrostatic capacitance sensor using an LC oscillation circuit is also considered. However, when using an electrostatic capacitance sensor with an LC oscillation circuit to detect, for example, pF-level electrostatic capacitance, if a small inductor in the nH range is used to achieve cost reduction, the oscillation frequency becomes GHz-level, leading to various problems such as increased circuit size and power consumption in the electrostatic capacitance measurement circuit. Conversely, to achieve a MHz-level oscillation frequency, a large inductor in the μH range is required, hindering the miniaturization and cost reduction of the electrostatic capacitance sensor.

[0093] In contrast, in the electrostatic capacitive sensor 1 of this embodiment, although the oscillation circuit 20 utilizing the oscillator 3 and electrostatic capacitors CD and CG has a relatively narrow variable amplitude of oscillation frequency compared to an LC oscillation circuit, it is possible to easily achieve kHz-level and MHz-level oscillation frequencies using a small oscillator 3. Furthermore, for example, it is also possible to achieve pF-level electrostatic capacitors. Figure 8 The variable amplitude of the oscillation frequency required for practical application is shown. Furthermore, the inductance value of the inductor is determined by its size; therefore, it is difficult to reduce the size of the inductor without changing the inductance value. However, the oscillator 3 can be further miniaturized and reduced in cost due to future advancements in manufacturing processes. Therefore, the electrostatic capacitive sensor 1 according to the first embodiment can achieve smaller size and lower cost compared to electrostatic capacitive sensors using an LC oscillation circuit, and thus can be easily used even when the detected object is a small object.

[0094] 2. Second Implementation Method

[0095] Hereinafter, regarding the second embodiment, the same reference numerals will be used for the same structures as in the first embodiment, and the descriptions that are the same as in the first embodiment will be omitted or simplified. The descriptions will mainly focus on the contents that are different from the first embodiment.

[0096] Figure 12 This is a diagram showing the appearance of the electrostatic capacitive sensor 1 according to the second embodiment. Figure 12 As shown, the electrostatic capacitive sensor 1 of the second embodiment is the same as that of the electrostatic capacitive sensor 1 of the first embodiment, including 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 that of the first embodiment, so its illustration and description are omitted.

[0097] like Figure 12 As shown, the sensing unit 100 is disposed outside the package 4 of the oscillator 10 and is connected to the oscillator 10 via a cable 15. The cable 15 may be, for example, a coaxial cable, a flexible flat cable, etc.

[0098] The sensing unit 100 includes a substrate 110, which has a surface 110a and a back surface 110b of the surface 110a. Electrodes 101 and 102 are provided on the surface 110a of the substrate 110. On the surface 110b of the substrate 110, an electrode 104 is provided at a position opposite to the arrangement area of ​​the electrodes 101 and 102 in the surface 110a. For example, the electrode 104 is provided on approximately the entire surface of the surface 110b.

[0099] Electrodes 101, 102, and 104 are connected to the external connection terminals of the oscillator 10 via wiring included in cable 15. Therefore, electrodes 101, 102, and 104 are connected to the terminals of the circuit device 2 via the external connection terminals of the oscillator 10.

[0100] In this embodiment, electrode 101 is a sensing electrode that can be connected to the XD terminal of circuit device 2, and electrode 102 is an electrode with a fixed potential. For example, electrode 102 is connected to the ground terminal, i.e., the VSS terminal, of circuit device 2, and its potential is fixed at ground potential. Electrode 104 is connected to the VSS terminal of circuit device 2, and its potential is fixed at ground potential.

[0101] The sensing unit 100 is arranged with electrodes 101 and 102 facing the object to be detected by electrostatic capacitance. The electrostatic capacitance CD between electrodes 101 and 102 varies according to the dielectric constant of the object to be detected.

[0102] When electrode 101 is connected to terminal XD of circuit device 2, when the electrostatic capacitance CD changes, the load capacitance C of oscillator 3... L The oscillation frequency f of the oscillation circuit 20 also changes. Therefore, 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. The external device can detect the change in electrostatic capacitance CD based on the measured value of the oscillation frequency f and determine the state of the object being detected. In addition, when an object that is not the object being detected is located opposite the surface 110b of the substrate 110, the electrode 104 functions as a shielding component to reduce the influence of the object on the electrostatic capacitance CD.

[0103] Figure 13 This is a functional block diagram of the electrostatic capacitive sensor 1 according to the second embodiment. (As shown...) Figure 13 As shown, the electrostatic capacitive sensor 1 of the second embodiment is the same as the electrostatic capacitive sensor 1 of the first embodiment, and includes a circuit device 2 and a sensing unit 100.

[0104] The 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, an interface circuit 80, a switching circuit 90, and a reference capacitor 95. Furthermore, instead of the reference capacitor 96, it includes a capacitor 97. Additionally, the circuit device 2 may be configured to omit or modify some of these elements, or to add other elements.

[0105] Figure 14 This is a diagram illustrating an example of the structure of the drive circuit 21 and the buffer circuit 30 in the second embodiment. Figure 14 As shown, the driving circuit 21 is the same as in the first embodiment, including an amplifier 211 and resistors 212 and 213. Similarly, the buffer circuit 30 is also the same as in the first embodiment, including a capacitor 31, a CMOS inverter circuit 32, and a resistor 33. The structure and function of the driving circuit 21, buffer circuit 30, clock generation circuit 50, control circuit 60, register 70, and interface circuit 80 are the same as in the first embodiment, therefore their description is omitted.

[0106] The switching circuit 90 switches between two states: connecting the electrode 101 of the sensing unit 100 to one of the input node NG and the output node ND of the amplifier 211, or connecting one end of the reference capacitor 95 to one of the input node NG and the output node ND of the amplifier 211. Additionally, one end of the capacitor 97 is connected to the other of the input node NG and the output node ND of the amplifier 211, and the other end of the capacitor 97 is connected to ground. Figure 13 and Figure 14 In this circuit, the switching circuit 90 switches between connecting electrode 101 to the output node ND of amplifier 211 and connecting one end of reference capacitor 95 to the output node ND of amplifier 211. One end of capacitor 97 is connected to the input node NG of amplifier 211. That is, the node connecting electrode 101 or reference capacitor 95 is the output node ND.

[0107] The capacitor 97 is built into the circuit device 2 and is not formed in the sensing unit 100, therefore the electrostatic capacitance CG of the capacitor 97 is a fixed value. The electrostatic capacitance CG is, for example, a few pF to tens of pF. In addition, the capacitor 97 can be disposed inside the package 4 or outside the circuit device 2.

[0108] The switching circuit 90 includes switches 91 and 93, one end of each of which is connected to the output node ND. The other end of switch 91 is connected to electrode 101, and the other end of switch 93 is connected to one end of a reference capacitor 95. The other end of the reference capacitor 95 is connected to ground. The electrode 102 of the sensing unit 100 is also connected to ground. For example, the reference capacitor 95 can also be a capacitor.

[0109] Switch 91 is turned on when the switch control signal ASW is low and turned off when the switch control signal ASW is high. Switch 93 is turned on when the switch control signal ASW is high and turned off when the switch control signal ASW is low. That is, switches 91 and 93 are turned on / off exclusively. Moreover, when switch 91 is turned on, electrode 101 is connected to the output node ND, and when switch 93 is turned on, one end of the reference capacitor 95 is connected to the output node ND.

[0110] Therefore, when switch 91 is turned on, the load capacitance C of oscillator 3... L The electrostatic capacitance CD between electrodes 101 and 102 and the electrostatic capacitance CG of capacitor 97 are constituted by the above-described equation (4). Moreover, according to the above-described equations (1) to (4), the oscillation frequency f of the oscillation circuit 20 varies according to the electrostatic capacitance CD. Furthermore, the switch control signal ASW from the control circuit 60 can control switches 91 and 93 to be turned on / off exclusively, and can also control switches 91 and 93 independently.

[0111] The measurement circuit 40 measures the first oscillation frequency f1 of the oscillation circuit 20 when the electrode 101 is connected to the output node ND, and the second oscillation frequency f2 of the oscillation circuit 20 when one end of the reference capacitor 95 is connected to the output node ND, based on the signal BFO ​​output from the buffer circuit 30. Furthermore, the measurement circuit 40 can also calculate the ratio of the first oscillation frequency f1 to the second oscillation frequency f2. Both the first oscillation frequency f1 and the second oscillation frequency f2 are similarly affected by the temperature characteristics and aging characteristics of the oscillator 3 and the drive circuit 21; therefore, the influence of the temperature characteristics and aging characteristics of the oscillator 3 and the drive circuit 21 is reduced relative to the ratio of the first oscillation frequency f1 to the second oscillation frequency f2.

[0112] For example, the measurement circuit 40 includes a frequency divider circuit 41, a counter 42, and a divider 43. The frequency divider circuit 41 divides the signal BFO ​​output from the buffer circuit 30 and outputs a gating time signal GT.

[0113] When the switch control signal ASW is low (i.e., when switch 91 is on), counter 42 counts the number of pulses of the clock signal CK during the period when the gating time signal GT is at a specified logic level, such as high, and outputs a count value CNT1. Conversely, when the switch control signal ASW is high (i.e., when switch 93 is on), counter 42 counts the number of pulses of the clock signal CK during the gating time and outputs a count value CNT2. The count value CNT1 corresponds to the measured value of the first oscillation frequency f1, and the count value CNT2 corresponds to the measured value of the second oscillation frequency f2.

[0114] Divider 43 divides the count value CNT1 output from counter 42 by the count value CNT2 to calculate the ratio of the first oscillation frequency f1 to the second oscillation frequency f2. The ratio of the first oscillation frequency f1 to the second oscillation frequency f2 is stored in register 70.

[0115] The measurement circuit 40 is similar to that in the first embodiment, and can also measure the first oscillation frequency f1 and the second oscillation frequency f2 multiple times, and calculate the ratio of the average value of the first oscillation frequency f1 to the average value of the second oscillation frequency f2.

[0116] In addition, the count values ​​CNT1 and CNT2 can also be stored in register 70, as can the average value of count value CNT1 and the average value of count value CNT2.

[0117] The other structures and operations of the electrostatic capacitive sensor 1 in the second embodiment are the same as those in the electrostatic capacitive sensor 1 in the first embodiment, so their description is omitted.

[0118] Furthermore, 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". Additionally, electrostatic capacitor CD is an example of a "first electrostatic capacitor", and reference capacitor 95 is an example of a "first reference capacitor". The output node ND of amplifier 211 is an example of a "first node". 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".

[0119] In the electrostatic capacitance sensor 1 of the second embodiment described above, when the electrode 101 is connected to the node ND, the oscillation circuit 20 does not oscillate based on the CR oscillation of the electrostatic capacitor CD during charging and discharging, but rather based on the resonance of the oscillator 3 and the electrostatic capacitor CD. Therefore, it is less susceptible to external amplitude noise. Thus, the electrostatic capacitance sensor 1 according to the second embodiment can detect electrostatic capacitance with high accuracy.

[0120] Furthermore, in the electrostatic capacitance sensor 1 of the second embodiment, if the interval between the period during which electrode 101 is connected to node ND and the period during which one end of reference capacitor 95 is connected to node ND is short, the first oscillation frequency f1 measured during the former period and the second oscillation frequency f2 measured during the latter period will be similarly affected by the temperature characteristics and aging characteristics of the oscillation circuit 20. Therefore, according to the electrostatic capacitance sensor 1 of the second embodiment, the measurement circuit 40 calculates the ratio of the first oscillation frequency f1 to the second oscillation frequency f2, thereby obtaining a high-precision oscillation frequency that reduces the influence of temperature and aging variations of the oscillation circuit 20. Therefore, the MCU 200 can calculate the electrostatic capacitance with high precision based on this oscillation frequency. Moreover, the measurement circuit 40 calculates the ratio of the average value of the first oscillation frequency f1 measured multiple times to the average value of the second oscillation frequency f2 measured multiple times, thereby obtaining a high-precision oscillation frequency that not only reduces the influence of temperature and aging variations of the oscillation circuit 20 but also reduces the influence of noise, instantaneous temperature changes, etc. Therefore, the MCU 200 can calculate the electrostatic capacitance with even higher precision based on this oscillation frequency.

[0121] Furthermore, in the electrostatic capacitive sensor 1 of the second embodiment, the oscillator 3 has a very high Q value, and therefore also functions as a noise filter. The noise input from the electrode 101 of the sensing unit 100, which is connected to the output node ND of the amplifier 211, is significantly reduced by the oscillator 3. The signal output from the oscillator 3 to the input node NG of the amplifier 211 is a low-noise signal that is close to a sine wave. Therefore, according to the electrostatic capacitive sensor 1 of the second embodiment, the possibility of the detection accuracy being reduced due to noise input from the electrode 101 is reduced.

[0122] Furthermore, in the electrostatic capacitance sensor 1 of the second embodiment, an 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 at the position opposite to surface 110b, the influence of that object on the electrostatic capacitance CD can be reduced. Therefore, the electrostatic capacitance sensor 1 according to the second embodiment can improve the detection accuracy of electrostatic capacitance.

[0123] Furthermore, the electrostatic capacitive sensor 1 according to the second embodiment can achieve smaller size and lower cost compared to an electrostatic capacitive sensor using an LC oscillation circuit, so it can be easily used even if the object being detected is a small object.

[0124] 3. Variations

[0125] This invention is not limited to this embodiment, and various modifications can be implemented within the scope of the spirit of this invention.

[0126] For example, in the second embodiment described above, the switching circuit 90 switches between connecting electrode 101 to the output node ND of amplifier 211 and connecting one end of reference capacitor 95 to the output node ND of amplifier 211. However, it can also switch between connecting electrode 101 to the input node NG of amplifier 211 and connecting one end of reference capacitor 95 to the input node NG of amplifier 211. That is, it is also possible that one end of each of switches 91 and 93 is connected to the input node NG, the other end of switch 91 is connected to electrode 101, the other end of switch 93 is connected to one end of reference capacitor 95, and one end of capacitor 97 is connected to the output node ND.

[0127] Furthermore, in the above embodiments, 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 oscillator 10, but it can also output the signal BFO ​​to the outside. Moreover, an external device can also measure the oscillation frequency f, which is the frequency of the signal BFO, and calculate the load capacitance C based on the measured value of the oscillation frequency f. L The value of the measurement circuit 40 may be omitted from the circuit device 2 in this case. Alternatively, the circuit device 2 may calculate the load capacitance C based on the oscillation frequency f. L The value of the load capacitor C L The value is output externally.

[0128] Furthermore, the oscillator 10 in the above embodiments can be a simple oscillator such as an SPXO, but it can also be an oscillator with temperature compensation function such as a TCXO, or an oscillator with frequency control function such as a VCXO. SPXO is an abbreviation for Simple Packaged Crystal Oscillator. TCXO is an abbreviation for Temperature Compensated Crystal Oscillator. VCXO is an abbreviation for Voltage Controlled Crystal Oscillator. Additionally, the oscillator 10 can also be an oscillator with both temperature compensation and frequency control functions such as a VC-TCXO, or an oscillator with temperature control function such as an OCXO. VC-TCXO is an abbreviation for Voltage Controlled Temperature Compensated Crystal Oscillator. OCXO is an abbreviation for Oven Controlled Crystal Oscillator.

[0129] Furthermore, while the above embodiments exemplify the use of the electrostatic capacitive sensor 1 as a sensor for detecting the amount of liquid within a container, the electrostatic capacitive sensor 1 can also be used, for example, as a proximity sensor for detecting the approach of a detected object, a touch sensor for detecting contact with a detected object, and various sensors for detecting rain, fog, ice, snow, gases, etc. For instance, it can also be used as a sensor that detects a finger approaching or touching a car door during opening and closing, and outputs a door unlocking signal. It can be used not only as a sensor for determining the amount, proximity, and contact of a detected object, but also as a sensor for determining the type of detected object, liquid concentration, etc.

[0130] The above-described embodiments and variations are examples and are not limited thereto. For example, the various embodiments and variations can be appropriately combined.

[0131] This invention includes structures that are substantially the same as those described in the embodiments, such as structures with the same function, method, and result, or structures with the same purpose and effect. Additionally, this invention includes structures that replace non-essential parts of the structures described in the embodiments. Furthermore, this invention includes structures that perform the same function and effect as those described in the embodiments, or structures that can achieve the same purpose. Additionally, this invention includes structures that incorporate known techniques into the structures described in the embodiments.

[0132] The following content is derived from the above implementation methods and variations.

[0133] One embodiment of an electrostatic capacitive sensor includes: a package; an oscillation circuit disposed inside the package; a switching circuit disposed inside the package; a first reference capacitor disposed inside the package; and a first electrode and a second electrode disposed outside the package. The oscillation circuit includes: an amplifier; and an oscillator disposed inside the package and connected between an input node and an output node of the amplifier. The first electrode is a sensing electrode, and the second electrode is a potential-fixed electrode. The switching circuit switches between connecting the first electrode to a first node and connecting one end of the first reference capacitor to the first node. The first node is one of the input node and the output node of the amplifier. When the first electrode is connected to the first node, the oscillation frequency of the oscillation circuit varies according to a first electrostatic capacitance between the first electrode and the second electrode. In this electrostatic capacitance sensor, when the first electrode is connected to the first node, the oscillation circuit does not oscillate based on the CR oscillation of the first electrostatic capacitance between the first and second electrodes, but rather on the resonance between the oscillator and the first electrostatic capacitance. Therefore, it is less susceptible to external amplitude noise. Consequently, this electrostatic capacitance sensor can detect electrostatic capacitance with high accuracy.

[0134] Furthermore, in this electrostatic capacitance sensor, if the interval between the period during which the first electrode is connected to the first node and the period during which one end of the reference capacitor is connected to the first node is short, the oscillation frequency of the oscillation circuit will be similarly affected by the temperature and aging characteristics of the oscillation circuit during both periods. Therefore, according to this electrostatic capacitance sensor, for example, an external device can correct the oscillation frequency of the former period based on the oscillation frequency of the latter period, thereby obtaining a high-precision oscillation frequency that reduces the influence of temperature and aging variations in the oscillation circuit. Thus, the electrostatic capacitance can be calculated with high precision based on this oscillation frequency.

[0135] Furthermore, compared to oscillating circuits utilizing an oscillator and a capacitor, LC oscillating circuits using LC resonance based on an inductor and a capacitor offer a wider range of variable oscillation frequencies. Therefore, constructing a high-sensitivity capacitive sensor using an LC oscillating circuit is also considered. However, when using a capacitive sensor with an LC oscillating circuit to detect, for example, pF-level capacitance, if a small inductor in the nH range is used to achieve cost reduction, the oscillation frequency becomes GHz-level, leading to various problems such as increased circuit size and power consumption for measuring capacitance. Conversely, to achieve MHz-level oscillation frequencies, a large inductor in the μH range is required, hindering the miniaturization and cost reduction of the capacitive sensor.

[0136] In contrast, while the oscillation circuit utilizing an oscillator and a capacitor has a much narrower variable frequency range compared to an LC oscillation circuit, it can easily achieve kHz and MHz-level oscillation frequencies using a small oscillator. Furthermore, for example, a practically required variable frequency range can be obtained using a pF-level capacitor. Moreover, the inductance value of an inductor is determined by its size, making it difficult to reduce the inductor's size without changing the inductance value. However, the oscillator can be further miniaturized and reduced in cost due to future advancements in manufacturing processes. Therefore, according to this capacitive sensor, compared to capacitive sensors using an LC oscillation circuit, it is possible to achieve smaller size and lower cost.

[0137] Another embodiment of the electrostatic capacitance sensor includes: an oscillation circuit; a switching circuit; a first reference capacitor; and a first electrode and a second electrode. The oscillation circuit includes: an amplifier; and an oscillator connected between an input node and an output node of the amplifier. The first electrode is a sensing electrode, and the second electrode is a potential-fixed electrode. The switching circuit switches between connecting the first electrode to a first node and connecting one end of the first reference capacitor to the first node. The first node is one of the input node and the output node of the amplifier. The first electrostatic capacitance between the first electrode and the second electrode varies according to the state of the detected object. When the first electrode is connected to the first node, the oscillation frequency of the oscillation circuit varies according to the first electrostatic capacitance.

[0138] In this electrostatic capacitance sensor, when the first electrode is connected to the first node, the oscillation circuit does not oscillate based on the CR oscillation of the first electrostatic capacitance between the first and second electrodes, but rather on the resonance between the oscillator and the first electrostatic capacitance. Therefore, it is less susceptible to external amplitude noise. Thus, this electrostatic capacitance sensor can accurately detect the electrostatic capacitance corresponding to the state of the object being detected.

[0139] Furthermore, based on this electrostatic capacitance sensor, for example, an external device can correct the oscillation frequency during the period when the first electrode is connected to the first node by using the oscillation frequency during the period when one end of the reference capacitor is connected to the first node as a reference, thereby obtaining a high-precision oscillation frequency that reduces the influence of temperature and annual changes in the oscillation circuit. Therefore, the electrostatic capacitance can be calculated with high precision based on this oscillation frequency.

[0140] Furthermore, this electrostatic capacitive sensor can achieve smaller size and lower cost compared to electrostatic capacitive sensors that use LC oscillation circuits, so it can be easily used even if the object being detected is a small object.

[0141] Alternatively, the electrostatic capacitance sensor may include a measurement circuit that measures the first oscillation frequency of the oscillation circuit when the first electrode is connected to the first node, and the second oscillation frequency of the oscillation circuit when one end of the first reference capacitor is connected to the first node of the amplifier.

[0142] Based on this electrostatic capacitance sensor, for example, an external device can calibrate the first oscillation frequency using the second oscillation frequency as a reference, thereby obtaining a high-precision oscillation frequency that reduces the influence of temperature and annual changes in the oscillation circuit. Therefore, the electrostatic capacitance can be calculated with high precision based on this oscillation frequency.

[0143] The electrostatic capacitive sensor can also be configured such that it includes a clock generation circuit for generating a clock signal, and the measurement circuit includes: a frequency divider circuit for dividing the signal oscillating based on the oscillation circuit; and a counter for counting the number of pulses of the clock signal contained during a period when the signal output from the frequency divider circuit is at a predetermined logic level.

[0144] In another embodiment, the electrostatic capacitive sensor may include a buffer circuit that receives a signal from the oscillator output to the input node of the amplifier, and the frequency divider circuit divides the signal output from the buffer circuit.

[0145] In this electrostatic capacitive sensor, the oscillator has a very high Q value, thus also functioning as a noise filter. The signal output from the oscillator to the input node of the amplifier is a low-noise, near-sine wave signal. Therefore, according to this electrostatic capacitive sensor, since no noise-induced spikes are generated in the output signal of the buffer circuit, the possibility of erroneous measurements by the measurement circuit can be reduced.

[0146] Alternatively, in the electrostatic capacitance sensor configuration, the measuring circuit may calculate the ratio of the first oscillation frequency to the second oscillation frequency.

[0147] Based on this electrostatic capacitance sensor, by calculating the ratio of the first oscillation frequency to the second oscillation frequency, a high-precision oscillation frequency is obtained, which reduces the influence of temperature and annual changes in the oscillation circuit. Therefore, for example, an external device can calculate the electrostatic capacitance with high precision based on this oscillation frequency.

[0148] Alternatively, in the electrostatic capacitance sensor configuration, the measurement circuit may measure the first oscillation frequency and the second oscillation frequency multiple times, and calculate the ratio of the average value of the first oscillation frequency to the average value of the second oscillation frequency.

[0149] Based on this electrostatic capacitance sensor, by calculating the ratio of the average of the first oscillation frequency measured multiple times to the average of the second oscillation frequency measured multiple times, a high-precision oscillation frequency is obtained, which not only reduces the influence of temperature and annual changes in the oscillation circuit, but also reduces the influence of noise and instantaneous temperature changes. Therefore, for example, an external device can calculate the electrostatic capacitance with higher precision based on this oscillation frequency.

[0150] In the electrostatic capacitance sensor configuration, the measurement circuit may measure the second oscillation frequency once, then measure the first oscillation frequency twice, then measure the second oscillation frequency once, and calculate the ratio of the average value of the first oscillation frequency to the average value of the second oscillation frequency.

[0151] In this electrostatic capacitive sensor, when the temperature changes during measurement, the measurement error tends to increase or decrease in the order of the first measurement at the second oscillation frequency, the first measurement at the first oscillation frequency, the second measurement at the first oscillation frequency, and the second measurement at the second oscillation frequency. Therefore, according to this electrostatic capacitive sensor, the difference between the error contained in the average value of the first oscillation frequency and the error contained in the average value of the second oscillation frequency decreases, thus improving the measurement accuracy.

[0152] In the electrostatic capacitance sensor configuration, the measurement circuit may also set a waiting time between one measurement at the first oscillation frequency or the second oscillation frequency and a subsequent measurement at the first oscillation frequency or the second oscillation frequency.

[0153] According to this electrostatic capacitance sensor, even if the oscillation of the oscillation circuit becomes unstable or stops oscillating during the waiting period after the measurement is switched, it will not have an adverse effect on the measurement.

[0154] In another embodiment, the electrostatic capacitive sensor may include a substrate having a first surface and a second surface as the back surface of the first surface. A first electrode and a second electrode are disposed on the first surface of the substrate, and a ground electrode is disposed 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.

[0155] In this electrostatic capacitance sensor, a ground electrode is provided on the second surface of the substrate, opposite to the arrangement area of ​​the first and second electrodes on the first surface. Therefore, when an object not being detected is located opposite the second surface, the influence of that object on the first electrostatic capacitance is reduced. Thus, according to this electrostatic capacitance sensor, the detection accuracy of electrostatic capacitance can be improved.

[0156] In one embodiment of the electrostatic capacitive sensor, the first node may also be the output node of the amplifier.

[0157] In this electrostatic capacitive sensor, the oscillator has a very high Q value, thus also functioning as a noise filter. Noise input from the first electrode, which is connected to the output node of the amplifier, is significantly reduced by the oscillator, resulting in a low-noise, near-sine wave signal output from the oscillator to the input node of the amplifier. Therefore, according to this electrostatic capacitive sensor, the possibility of detection accuracy being reduced due to noise input from the first electrode is decreased.

[0158] One embodiment of the electrostatic capacitance sensor may include: a second reference capacitor disposed inside the package; and a third electrode disposed outside the package, the third electrode being a sensing electrode. The switching circuit switches between states such that the first electrode is connected to the first node and the third electrode is connected to the second node, which is the other of the input and output nodes of the amplifier, or one end of the first reference capacitor is connected to the first node and one end of the second reference capacitor is connected to the second node. When the first electrode is connected to the first node and the third electrode is connected to the second node, the oscillation frequency of the oscillation circuit varies according to the first electrostatic capacitance and the second electrostatic capacitance between the third electrode and the second electrode.

[0159] In this electrostatic capacitance sensor, since the oscillation frequency of the oscillation circuit varies according to the first and second electrostatic capacitances when the first electrode is connected to the first node and the third electrode is connected to the second node, the variable amplitude of the oscillation frequency can be expanded compared to the case without the second electrostatic capacitance. Therefore, according to this electrostatic capacitance sensor, the detection sensitivity of electrostatic capacitance can be improved.

[0160] In another embodiment, the electrostatic capacitive sensor may include a substrate having a first surface and a second surface as the back surface of the first surface. A first electrode, a second electrode, and a third electrode are disposed on the first surface of the substrate, with the second electrode located between the first electrode and the third electrode. A ground electrode is disposed on the second surface of the substrate at a position opposite to the arrangement area of ​​the first electrode, the second electrode, and the third electrode on the first surface.

[0161] In this electrostatic capacitance sensor, a ground electrode is provided on the second surface of the substrate, opposite to the arrangement area of ​​the first, second, and third electrodes on the first surface. Therefore, when an object not being detected is located opposite the second surface, the influence of that object on the first and second electrostatic capacitances is reduced. Thus, according to this electrostatic capacitance sensor, the detection accuracy of electrostatic capacitance can be improved.

Claims

1. An electrostatic capacitive sensor, comprising: Encapsulation; An oscillation circuit is disposed inside the package; A switching circuit is disposed inside the package; The first reference capacitor is disposed inside the package; as well as The first electrode and the second electrode are disposed on the outside of the package. The oscillation circuit has the following characteristics: Amplifier; as well as An oscillator, disposed inside the package, connects between the input and output nodes of the amplifier. The first electrode is a sensing electrode. The second electrode is an electrode with a fixed potential. The switching circuit switches between connecting the first electrode to the first node and connecting one end of the first reference capacitor to the first node, where the first node is either the input node or the output node of the amplifier. When the first electrode is connected to the first node, the oscillation frequency of the oscillation circuit varies according to the first electrostatic capacitance between the first electrode and the second electrode.

2. The electrostatic capacitive sensor according to claim 1, wherein, The electrostatic capacitive sensor includes: A second reference capacitor, which is disposed inside the package; and The third electrode is disposed on the outside of the package. The third electrode is a sensing electrode. The switching circuit switches between two states: connecting the first electrode to the first node and connecting the third electrode to the second node, which is the other of the input and output nodes of the amplifier; or connecting one end of the first reference capacitor to the first node and connecting one end of the second reference capacitor to the second node. When the first electrode is connected to the first node and the third electrode is connected to the second node, the oscillation frequency of the oscillation circuit varies according to the first electrostatic capacitance and the second electrostatic capacitance between the third electrode and the second electrode.

3. The electrostatic capacitive sensor according to claim 2, wherein, The electrostatic capacitive sensor includes a substrate having a first surface and a second surface that serves as the back surface of the first surface. The first electrode, the second electrode, and the third electrode are disposed on the first surface of the substrate. The second electrode is located between the first electrode and the third electrode. On the second surface of the substrate, a ground electrode is provided at a position opposite to the arrangement area of ​​the first electrode, the second electrode, and the third electrode on the first surface.

4. An electrostatic capacitive sensor, comprising: Oscillating circuit; Switching circuit; The first reference capacitor; and Electrode 1 and Electrode 2, The oscillation circuit has the following characteristics: Amplifier; and An oscillator, which is connected between the input and output nodes of the amplifier. The first electrode is a sensing electrode. The second electrode is an electrode with a fixed potential. The switching circuit switches between connecting the first electrode to the first node and connecting one end of the first reference capacitor to the first node, where the first node is either the input node or the output node of the amplifier. The first electrostatic capacitance between the first electrode and the second electrode changes according to the state of the object being detected. When the first electrode is connected to the first node, the oscillation frequency of the oscillation circuit varies according to the first electrostatic capacitance.

5. The electrostatic capacitive sensor according to claim 1 or 4, wherein, The electrostatic capacitance sensor includes a measurement circuit that measures a first oscillation frequency of the oscillation circuit when the first electrode is connected to the first node, and a second oscillation frequency of the oscillation circuit when one end of the first reference capacitor is connected to the first node of the amplifier.

6. The electrostatic capacitive sensor according to claim 5, wherein, The electrostatic capacitive sensor includes a clock generation circuit for generating clock signals. The measurement circuit includes: A frequency divider circuit that divides the frequency of a signal oscillating based on the oscillation of the said oscillation circuit; and A counter that counts the number of pulses of the clock signal during a period when the signal output from the frequency divider circuit is at a specified logic level.

7. The electrostatic capacitive sensor according to claim 6, wherein, The electrostatic capacitive sensor includes a buffer circuit that receives a signal from the oscillator output to the input node of the amplifier. The frequency divider circuit divides the signal output from the buffer circuit.

8. The electrostatic capacitive sensor according to claim 5, wherein, The measuring circuit calculates the ratio of the first oscillation frequency to the second oscillation frequency.

9. The electrostatic capacitive sensor according to claim 5, wherein, The measuring circuit measures the first oscillation frequency and the second oscillation frequency multiple times, and calculates the ratio of the average value of the first oscillation frequency to the average value of the second oscillation frequency.

10. The electrostatic capacitive sensor according to claim 9, wherein, The measuring circuit measures the second oscillation frequency once, then measures the first oscillation frequency twice, then measures the second oscillation frequency once, and calculates the ratio of the average value of the first oscillation frequency to the average value of the second oscillation frequency.

11. The electrostatic capacitive sensor according to claim 10, wherein, The measurement circuit sets a waiting time between one measurement at the first oscillation frequency or the second oscillation frequency and the next measurement at the first oscillation frequency or the second oscillation frequency.

12. The electrostatic capacitive sensor according to claim 1 or 4, wherein, The electrostatic capacitive sensor includes a substrate having a first surface and a second surface that serves as the back surface of the first surface. The first electrode and the second electrode are disposed on the first surface of the substrate. On the second surface of the substrate, a ground electrode is provided at a position opposite to the configuration area of ​​the first electrode and the second electrode on the first surface.

13. The electrostatic capacitive sensor according to claim 1 or 4, wherein, The first node is the output node of the amplifier.

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