Measuring device

Abstract

To cancel the influence of stray capacitance by adopting a measuring system using an impedance conversion circuit.SOLUTION: The positive-side low-impedance voltage shift circuit 132 raises the voltage Vout outputted from the operational amplifier 131 by the zener voltage VZ1 and lowers the voltage Vout by the base-emitter voltage VBE, and applies the resultant voltage as the positive-side voltage (+ V) to the positive-side node of the operational amplifier 131. The voltage Vout outputted from the operational amplifier 131 is decreased by the zener voltage VZ2 and increased by the base-emitter voltage VBE by the negative-side low-impedance voltage shift circuit 133, and is applied to the negative-side node of the operational amplifier 131 as the negative-side voltage (- V). Since the positive side voltage (+ V) and the negative side voltage (- V) have the same amplitude and the same phase, the potential difference between the positive side voltage (+ V) and the negative side voltage (- V) is constant. As a result, an AC current does not flow through the stray capacitance C0 existing in the operational amplifier 131, so that the influence of the stray capacitance is canceled.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an impedance conversion circuit, a measurement device, and a distance measurement device. [Background technology]

[0002] Conventionally, as a non-contact distance sensor for measuring minute gaps or displacements, there is an electrostatic capacitance sensor that measures electrostatic capacitance according to the distance between a measurement probe and a workpiece.

[0003] Fig. 10(A) shows a distance measurement device 900A, which is a basic configuration of a capacitance type sensor. As shown in Fig. 10(A), the distance measurement device 900A is equipped with an AC constant current source 910. The angular frequency of the constant current I generated by the AC constant current source 910 is defined as ω, and the capacitance of the capacitance body between the electrode EL and the workpiece WK is defined as C. X Then, the voltage V applied to the capacitance m is given by the following equation (1) from Ohm's law of AC. V m =I / (ωC X ) …(1)

[0004] Here, for an ideal parallel plate capacitor, the area of ​​the electrode EL is A, the vacuum dielectric constant is ε0, and the relative dielectric constant of the measurement gap g is ε S Then, the capacitance C X is C X =(ε0ε S A) / g, the measurement gap g is given by the following equation (2): g=ε0ε S Air conditioning X =[(ω×ε0ε S A) / I]×V m …(2)

[0005] However, in the distance measurement device 900A, as shown in Fig. 10(A), the electric field lines are disturbed at the end of the electrode of finite size facing the grounded workpiece WK, and the electric field is no longer parallel. Therefore, in the distance measurement device 900A, the electric field lines are disturbed, and CX =(ε0ε S A) / g relationship cannot be obtained, and an accurate measurement gap g cannot be measured.

[0006] A distance measurement device 900B shown in Fig. 10(B) can eliminate such disturbances in the electric field lines. As shown in Fig. 10(B), the distance measurement device 900B has a center electrode EL c and guard electrode EL g The distance measurement device 900B also includes an AC constant current source 910 and an impedance conversion circuit 920, which is a voltage buffer circuit.

[0007] The center electrode EL of the distance measuring device 900B c The guard electrode EL performs sensing to measure the measurement gap g between the workpiece WK. g is the center electrode EL c Guard electrode EL g As will be described later, the potential of the central electrode EL c Since the potential of the central electrode EL c and guard electrode EL g As a result, the disturbance of the electric field lines at the end of the electrode of finite size facing the grounded workpiece WK is g occurs only on the outer periphery of the center electrode EL c The electric field lines are parallel just below the capacitance C X The reciprocal of is proportional to the measurement gap g, i.e., 1 / C X =g / (ε0ε S The relationship A) is obtained.

[0008] Therefore, equation (1) and the capacitance C X The inverse of is proportional to the measurement gap g, and the following equation (3) is obtained. V m =I / (ωC X ) =[I / (ωε0ε S A)]×g …(3) Thus, the voltage Vm is proportional to the measurement gap g, and the voltage V m It is convenient to use the following to measure the measurement gap g.

[0009] In the distance measurement device 900B, the input of the impedance conversion circuit 920 is the center electrode EL c The output of the impedance conversion circuit 920 is connected to the guard electrode EL g Therefore, the center electrode EL c and guard electrode EL g are at the same potential in terms of AC, so the central electrode EL c Guard electrode EL g As a result, no current flows to the center electrode EL c and guard electrode EL g The effect of the stray capacitance between

[0010] In the distance measurement device 900B, the center electrode EL c is connected to the inner conductor of the shielded braided coaxial cable CV and to the output of the impedance transformation circuit 920. g is connected to the outer conductor of the coaxial cable CV, which is insulated from the inner conductor. Therefore, the impedance transformation circuit 920 acts as a driven shield, and the effect of the capacitance between the inner and outer conductors in the coaxial cable CV is canceled.

[0011] In this way, in the distance measurement device 900B, the guard electrode EL g The central electrode EL c This eliminates the disturbance of the electric field lines at the center electrode EL c and guard electrode EL g However, in the impedance conversion circuit 920, stray capacitance exists between the input terminal and the ground potential.

[0012] Here, the capacitance between the center electrode and the workpiece is C XSince this capacitance depends on the measurement gap, it becomes smaller as the measurement gap is increased, typically about 0.1 pF to 2 pF. On the other hand, the stray capacitance of the impedance conversion circuit is typically about 5 pF. Thus, the capacitance C X is relatively small compared to the stray capacitance. Therefore, due to the influence of the stray capacitance, the capacitance C X As a result, if the influence of the stray capacitance remains, it is not possible to measure the measurement gap g accurately.

[0013] As with distance measurement device 900B, a proposed technique for a capacitance type sensor using an impedance conversion circuit is a distance sensor using an operational amplifier (see Patent Document 1: hereinafter referred to as "prior art example").

[0014] This conventional technology uses a non-inverting operational amplifier, with the detection electrode and one end of a first resistor connected to the non-inverting input terminal (+ input terminal) of the operational amplifier, and the output of the operational amplifier and one end of a capacitor connected to the inverting input terminal (- input terminal) of the operational amplifier. Also, one end of a second resistor, the other end of the capacitor, and the other end of the first resistor are connected, and an AC signal source is connected to the other end of the second resistor. The output of the non-inverting amplifier is connected to a shield electrode that is placed so as to surround the detection electrode. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-094598 Summary of the Invention [Problem to be solved by the invention]

[0016] In the above-mentioned conventional technology, a voltage follower is used in which the input is connected to the non-inverting input terminal (+ input terminal) and the output is connected to the inverting input terminal (- input terminal), and the output of the non-inverting amplifier is connected to the shield electrode. By adopting this configuration, the gain of the output voltage relative to the input voltage can be set to 1, making it possible to apply the same potential as the detection electrode to the shield electrode and to separate the detection electrode and shield electrode in terms of impedance, thereby improving shielding properties and detection accuracy.

[0017] However, as described above, there is a stray capacitance in the differential input stage of the operational amplifier, which is an impedance conversion circuit. For example, as shown in FIG. 11, in an operational amplifier 810 with a junction FET (Field Effect Transistor) input, there is a stray capacitance C between the gate and drain of the differential input stage 811. gd1 ,C gd2 , stray capacitance between gate and source C gs1 ,C gs2 In addition, for example, in a bipolar input operational amplifier, stray capacitance exists between the base and emitter, between the base and collector, etc. In the following, these stray capacitances that exist inside the operational amplifier will be referred to as stray capacitance C0.

[0018] Therefore, the conventional distance sensor is affected by the stray capacitance C0 and is unable to measure the measurement gap accurately.

[0019] The present invention has been made in consideration of the above circumstances, and aims to provide a new impedance conversion circuit that employs a measurement method using an impedance conversion circuit, cancels the effect of stray capacitance present in the impedance conversion circuit, and can accurately measure the capacitance between an electrode and a measurement object. It also aims to provide a new measurement device and distance measurement device that incorporates the impedance conversion circuit and can measure the displacement between an electrode and a measurement object using the accurately measured capacitance. [Means for solving the problem]

[0020] From a first viewpoint, the present invention provides an impedance conversion circuit used in a distance measurement device having an electrode head including a first electrode and at least one second electrode arranged to surround the first electrode, and measuring a distance between the first electrode and a measurement object by a change in electrostatic capacitance, the impedance conversion circuit comprising: an operational amplifier having a non-inverting input terminal to which the first electrode is connected, and an output terminal, the operational amplifier reducing an output impedance relative to an input impedance; a positive-side low-impedance voltage shift circuit generating a positive-side voltage to be applied to a positive terminal of the operational amplifier based on an output voltage of the operational amplifier; and a negative-side low-impedance voltage shift circuit generating a negative-side voltage to be applied to a negative terminal of the operational amplifier based on the output voltage, wherein the positive-side low-impedance voltage shift circuit and the negative-side voltage generate the positive-side voltage and the negative-side voltage so that the positive-side voltage and the negative-side voltage have the same phase and amplitude.

[0021] From a second viewpoint, the present invention is an impedance conversion circuit used in a distance measurement device having an electrode head including a first electrode and at least one second electrode arranged to surround the first electrode, and measuring a distance between the first electrode and a measurement object by a change in electrostatic capacitance, the impedance conversion circuit comprising: an operational amplifier having a non-inverting input terminal to which the first electrode is connected, and an output terminal, the operational amplifier lowering an output impedance relative to an input impedance; a elimination unit removing a DC component from the output voltage of the operational amplifier; a subsequent operational amplifier lowering the output impedance relative to the input impedance based on the elimination result of the elimination unit; a positive-side low-impedance voltage shift circuit generating a positive-side voltage to be applied to a positive terminal of the operational amplifier based on a subsequent-stage output voltage of the subsequent-stage operational amplifier; and a negative-side low-impedance voltage shift circuit generating a negative-side voltage to be applied to a negative terminal of the operational amplifier based on the subsequent-stage output voltage,

[0022] From a third viewpoint, the present invention is a measuring device used in a distance measurement device having an electrode head including a first electrode and at least one second electrode arranged to surround the first electrode, and measuring the distance between the first electrode and an object to be measured by a change in capacitance, characterized in that the measuring device comprises: the impedance conversion circuit of the present invention; a voltage measurement unit that measures the voltage applied to a capacitance between the first electrode and the object to be measured when a current is passed through the capacitance as a voltage corresponding to the output voltage of an operational amplifier of the impedance conversion circuit; and a calculation unit that calculates the distance based on the measurement result by the voltage measurement unit.

[0023] From a fourth perspective, the present invention is a distance measurement device comprising: the measurement device of the present invention; and an electrode head including a first electrode and at least one second electrode arranged to surround the first electrode. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a block diagram illustrating a schematic configuration of a distance measurement device according to an embodiment of the present invention. [Figure 2] 2 is a diagram for explaining the configuration of a positive-side low-impedance voltage shift circuit of FIG. 1. FIG. [Figure 3] 2 is a diagram for explaining the configuration of the negative-side low-impedance voltage shift circuit of FIG. 1. FIG. [Figure 4] 2 is a diagram for explaining positive-side voltage adjustment and negative-side voltage adjustment of the impedance conversion circuit of FIG. 1. FIG. [Figure 5] 5 is a diagram for explaining the relationship between the output voltage of the operational amplifier 131 in FIG. 4 and the positive voltage (+V) and negative voltage (−V) applied to the operational amplifier 131. FIG. [Figure 6] 10 is a diagram for explaining the relationship between a measured voltage value Vm and a measurement gap g. FIG. [Figure 7] FIG. 10 is a diagram illustrating a first modification of the impedance conversion circuit. [Figure 8]FIG. 10 is a diagram illustrating a second modification of the impedance conversion circuit. [Figure 9] FIG. 10 is a diagram illustrating a third modification of the impedance conversion circuit. [Figure 10] FIG. 1 is a diagram for explaining distance measurement using a conventional capacitance sensor. [Figure 11] FIG. 1 is a diagram for explaining stray capacitance present inside an operational amplifier. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description and drawings, the same or equivalent elements are designated by the same reference numerals, and redundant description will be omitted.

[0026] [composition] Fig. 1 shows a schematic configuration of a distance measurement device 100 according to one embodiment. As shown in Fig. 1, the distance measurement device 100 includes an attachment unit 110, an electrode head 115, and an AC constant current source circuit 120 including an impedance conversion circuit 130. The distance measurement device 100 also includes a voltage measurement unit 140 and a calculation unit 150. Here, the AC constant current source circuit 120, the voltage measurement unit 140, and the calculation unit 150 constitute a measurement device 101.

[0027] The mounting portion 110 is a cylindrical housing. An electrode head 115 is attached to the vertically downward (−Z direction) surface of the mounting portion 110.

[0028] The electrode head 115 includes a center electrode E1 corresponding to the first electrode and a guard electrode E2 corresponding to the second electrode. In this embodiment, the center electrode E1 is disk-shaped, and the guard electrode E2 is cylindrical with a bottom. The inner diameter of the cylindrical guard electrode E2 is longer than the diameter of the disk portion of the center electrode E1. The center electrode E1 is disposed inside the cylindrical guard electrode E2. An insulator is interposed between the center electrode E1 and the guard electrode E2.

[0029] The center electrode E1 has a capacitance C of the capacitance body between the center electrode E1 and the workpiece WK. X The workpiece WK is a conductor and corresponds to the object to be measured.

[0030] The guard electrode E2 assists the sensing performed by the center electrode E1. As will be described later, the potential of the guard electrode E2 is equal to the potential of the center electrode E1, so the center electrode E1 and the guard electrode E2 behave as if they were a single electrode. As a result, the disturbance of the electric field lines occurs only on the outer periphery of the guard electrode E2, and parallel electric field lines are maintained directly below the center electrode E1. For this reason, C in a parallel plate capacitor X =(ε0ε S A) / g, and the voltage V m The measurement gap g can be measured using

[0031] In this embodiment, the center electrode E1 is connected to the inner conductor (center wire) of the shield braided coaxial cable CV, and the guard electrode E2 is connected to the outer conductor (braided wire) of the coaxial cable CV, which is insulated from the inner conductor.

[0032] The AC constant current source circuit 120 includes a constant voltage AC signal generator 121, an operational amplifier 122, and a resistor R S The AC constant current source circuit 120 includes a resistor element R A ,R B ,R C ,R D Furthermore, the AC constant current source circuit 120 includes an impedance conversion circuit 130. Here, the resistor element R S The resistance value of the resistive element R A ,R B ,R C ,R D The resistance value of the resistor element R S and resistive element R A ,R B ,R C,R D The resistance value is determined in advance based on experiments, simulations, etc.

[0033] The constant voltage AC signal generating unit (AC constant voltage source) 121 generates a reference AC voltage V having a predetermined frequency and a predetermined amplitude. i Generates a resistor element R A has one end connected to the constant voltage AC signal generator 121, and the other end connected to the inverting input terminal (-input terminal) of the operational amplifier 122 and the resistor element R B is connected to one end of the

[0034] The inverting input terminal (-input terminal) of the operational amplifier 122 is connected to a resistor R A The other end of the resistor element R B The non-inverting input terminal (+ input terminal) of the operational amplifier 122 is connected to one end of the resistor element R C and one end of the resistor R D is connected to one end of the resistor element R C The other end of the resistor element R is connected to the output side of the impedance conversion circuit 130. C The other end is grounded.

[0035] The output terminal of the operational amplifier 122 is connected to a resistor R B The other end of the resistor element R S is connected to one end of the resistor element R S The other end of the resistor element R is connected to the center electrode E1 and the input side of the impedance conversion circuit 130. S When the resistance value is Rs, the AC constant current source circuit 120 generates an AC constant current I (hereinafter also referred to as "constant current I") given by the following equation (4). I=V i / Rs …(4) The constant current I thus generated flows through the electrostatic capacitance (electrostatic capacitance C X ) flows.

[0036] Here, the voltage input to the non-inverting input terminal (+ input terminal) of the operational amplifier 122 in the AC constant current source circuit 120 isS The voltage across the output of the operational amplifier 122 and the resistor R B The other end of the resistor R S A voltage across the

[0037] <Configuration of Impedance Conversion Circuit 130> Next, the configuration of the impedance conversion circuit 130 will be described.

[0038] The impedance conversion circuit 130 includes an operational amplifier 131. The impedance conversion circuit 130 also includes a positive-side low-impedance voltage shift circuit 132 and a negative-side low-impedance voltage shift circuit 133.

[0039] The impedance conversion circuit 130 is a bootstrap circuit up to the power supply voltage. CC The power supply line and negative power supply voltage V EE The power supply lines of the power supplies are connected to one end of a large-capacity capacitor (not shown) whose other end is grounded. As a result, the positive power supply voltage V CC and negative power supply voltage V EE The AC component of flows to the ground through the capacitor. CC and negative power supply voltage V EE is at ground level for AC.

[0040] The operational amplifier 131 is a non-inverting amplifier. A stray capacitance exists inside the operational amplifier 131. The non-inverting input terminal (+ input terminal) of the operational amplifier 131 is connected to the center electrode E1 and the resistor element R S The inverting input terminal (negative input terminal) of the operational amplifier 131 is connected to the guard electrode E2 and the output terminal of the operational amplifier 131.

[0041] That is, the operational amplifier 131 is a voltage follower in which the input is connected to the non-inverting input terminal (+ input terminal) and the output is connected to the inverting input terminal (- input terminal). Therefore, the operational amplifier 131 makes the output impedance lower than the input impedance, and the input voltage V in and the output voltage V out and the amplifier 131 functions as a voltage buffer circuit with an amplification factor of 1. As a result, the operational amplifier 131 provides an output with low impedance and in phase with the input signal.

[0042] In addition, in the operational amplifier 131, the output of the voltage follower acts as a driven shield connected to the outer braid of the shield-braided coaxial cable CV, so the effect of stray capacitance between the center line of the coaxial cable CV connected to the center electrode E1 and the braided wire of the coaxial cable CV connected to the guard electrode E2 can be ignored.

[0043] A positive voltage (+V) generated by a positive-side low-impedance voltage shift circuit 132 is applied to a positive terminal of the operational amplifier 131. A negative voltage (-V) generated by a negative-side low-impedance voltage shift circuit 133 is applied to a negative terminal of the operational amplifier 131.

[0044] 2, the positive-side low-impedance voltage shift circuit 132 includes an NPN transistor Tr1, a resistor R1, and a Zener diode ZD1. The positive-side low-impedance voltage shift circuit 132 is inserted between the positive terminal of the operational amplifier 131 and the positive power supply line. The output of the operational amplifier 131 is input to the positive-side low-impedance voltage shift circuit 132.

[0045] The emitter terminal of the transistor Tr1 is connected to the positive terminal of the operational amplifier 131. The base terminal of the transistor Tr1 is connected to one end of the resistor R1 and the cathode terminal of the Zener diode ZD1. The positive power supply voltage V CCThe positive power supply line connected to the power supply terminal of the Zener diode ZD1 is connected to the anode terminal of the Zener diode ZD1 and the output terminal of the operational amplifier 131.

[0046] 3, the negative-side low-impedance voltage shift circuit 133 includes a PNP transistor Tr2, a resistor R2, and a Zener diode ZD2. The positive-side low-impedance voltage shift circuit 132 is inserted between the negative terminal of the operational amplifier 131 and the negative power supply line. The output of the operational amplifier 131 is input to the negative-side low-impedance voltage shift circuit 133.

[0047] The emitter terminal of the transistor Tr2 is connected to the negative terminal of the operational amplifier 131. The base terminal of the transistor Tr2 is connected to one end of the resistor R2 and the anode terminal of the Zener diode ZD2. The collector terminal of the transistor Tr2 and the other end of the resistor R2 are connected to the negative power supply voltage V EE The cathode terminal of the Zener diode ZD2 is connected to the output terminal of the operational amplifier 131.

[0048] In the positive-side low-impedance voltage shift circuit 132, as shown in FIG. 4, the output voltage V out is input, and the Zener voltage V Z1 The base-emitter voltage V BE As a result, the output voltage V out is shifted to generate a positive voltage (+V = (V out +V Z1 -V BE )) is applied to the positive terminal of the operational amplifier 131.

[0049] In addition, in the negative-side low-impedance voltage shift circuit 133, the output voltage V outis input, and the Zener voltage V Z2 The base-emitter voltage V BE As a result, the output voltage V out is shifted to generate a negative voltage (-V = (V out -V Z2 +V BE )) is applied to the negative terminal of the operational amplifier 131.

[0050] Here, the Zener voltage V Z1 value and Zener voltage V Z2 In this embodiment, when measuring the displacement, the base-emitter voltage V BE The impedance conversion circuit 130 is set so that the value of is constant. Therefore, the positive voltage (+V) applied to the positive terminal of the operational amplifier 131 and the negative voltage (-V) applied to the negative terminal of the operational amplifier 131 are equal to the output voltage V of the operational amplifier 131, as shown in FIG. out 5, even if the positive side voltage (+V) and the negative side voltage (-V) fluctuate, the positive side voltage (+V) and the negative side voltage (-V) are within the operating power supply voltage range of the operational amplifier 131.

[0051] That is, the voltage difference between the positive voltage (+V) applied to the positive terminal of the operational amplifier 131 and the negative voltage (-V) applied to the negative terminal of the operational amplifier 131 is kept constant. As a result, no AC current flows through the stray capacitance C0 present inside the operational amplifier 131. This makes it possible to reduce the influence of the stray capacitance C0 when measuring the measurement gap g.

[0052] The voltage measurement unit 140 measures the output voltage V out Here, the operational amplifier 131 functions as a voltage buffer circuit, so the output voltage V out is the input voltage V inThe measurement result by the voltage measurement unit 140 is equal to the measured voltage value V m The measured voltage value V m is the voltage applied to the capacitance body between the center electrode E1 and the workpiece WK when a current is passed through the capacitance body.

[0053] The calculation unit 150 calculates the measured voltage value V m Then, the calculation unit 150 receives the measured voltage value V m Based on this, the measurement gap g (distance) between the center electrode E1 and the workpiece WK is measured.

[0054] Here, when the influence of the stray capacitance C0 is not reduced, if the angular frequency of the constant current I generated by the AC constant current source circuit 120 is ω, the measured voltage value V m is given by the following equation (5). V m =I / [ω(C X +C0)] …(5) Here, the capacitance C of the capacitance body between the center electrode E1 and the workpiece WK is X As mentioned above, C X =(ε0ε S A) / g. Therefore, equation (5) can be transformed into the following equation (6). V m =[I / [ω(ε0ε S A+C0g)]]×g …(6) As can be seen from equation (6), in this case, the measured voltage value V m As the measurement gap g increases, V m = I / (ωC0) (constant value) and is not proportional to the measurement gap g (see Figure 6). As a result, when the measurement gap g increases, the measured voltage value V m Based on this, it becomes impossible to measure the measurement gap g.

[0055] In this embodiment, the impedance conversion circuit 130 reduces the influence of the stray capacitance C0 present inside the operational amplifier 131, so that "C0 = 0" can be satisfied. Therefore, equation (6) can be changed to the following equation (7). V m =[I / [ω(ε0ε S A)]]×g …(7) As can be seen from equation (7), in this case, even if the measurement gap g increases, the measured voltage value V m is proportional to the measurement gap g (see Figure 6). As a result, even when the measurement gap g is large, the measured voltage value V m Based on this, the measurement gap g can be measured by the following equation (8) (which is the same as the above-mentioned equation (2)). g=[(ω×ε0ε S A) / I]×V m …(8) This measurement gap g is such that the stray capacitance C0 present inside the operational amplifier 131 is reduced.

[0056] <Operation> The operation of the distance measurement device 100 configured as above will be described, focusing mainly on the measurement process of the measurement gap g and the cancellation process of the stray capacitance C0 present inside the operational amplifier 131.

[0057] When measuring the measurement gap g, the AC constant current source circuit 120 generates a constant current I. The AC constant current I thus generated flows through a capacitance (capacitance C X ) and the output voltage V of the operational amplifier 131, which functions as a voltage buffer circuit with its input connected to the non-inverting input terminal (+ input terminal) and its output connected to the inverting input terminal (- input terminal), out By adopting such a configuration, when measuring the voltage when the AC constant current I flows in the capacitance body, the output impedance is made lower than the input impedance by the operational amplifier 131, and the output voltage V out is the input voltage V inis set equal to

[0058] The output voltage V out When measuring, an impedance conversion circuit 130 including an operational amplifier 131 cancels the influence of stray capacitance C0 present inside the operational amplifier 131. In the impedance conversion circuit 130, a positive-side low-impedance voltage shift circuit 132 shifts the output voltage V out The Zener voltage V Z1 The base-emitter voltage V BE The positive voltage (+V = (V out +V Z1 -V BE )) is applied to the positive terminal of the operational amplifier 131. In addition, the negative-side low-impedance voltage shift circuit 133 shifts the output voltage V out The Zener voltage V Z2 The base-emitter voltage V BE The negative voltage (-V = (V out -V Z2 +V BE )) is applied to the negative terminal of the operational amplifier 131.

[0059] In this way, the positive voltage (+V) applied to the positive terminal of the operational amplifier 131 and the negative voltage (-V) applied to the negative terminal of the operational amplifier 131 are expressed as the output voltage V out and input voltage V in Therefore, the potential difference between the positive voltage (+V) and the negative voltage (-V) applied to the operational amplifier 131 is kept constant. As a result, no AC current flows through the stray capacitance C0 present inside the operational amplifier 131, and the measured voltage value V measured by the voltage measuring unit 140 is m (see equation (4)), the influence of the stray capacitance C0 is cancelled. mBased on this, the measurement gap g(=[(ω×ε0ε S A) / I]×V m ) is measured.

[0060] As described above, in this embodiment, the distance measurement device 100 is driven with a constant AC current to measure the measurement gap g. When measuring the measurement gap g, the constant AC current source circuit 120 generates a constant current I. The generated constant AC current I flows through the electrostatic capacitance body (electrostatic capacitance C X When measuring the voltage at this time, the input is connected to the non-inverting input terminal (+ input terminal) and the output is connected to the inverting input terminal (- input terminal) of the operational amplifier 131, which is a voltage follower. The output voltage V of the operational amplifier 131, which has the center electrode E1 connected to the non-inverting input terminal (+ input terminal), out The voltage measurement unit 140 measures the voltage.

[0061] The output voltage V out When measuring, in the impedance conversion circuit 130, the output voltage V of the operational amplifier 131 is out The Zener voltage V Z1 The base-emitter voltage V BE The positive voltage (+V) thus generated is applied to the positive terminal of the operational amplifier 131. The negative-side low-impedance voltage shift circuit 133 also reduces the output voltage V out The Zener voltage V Z2 The base-emitter voltage V BE The negative voltage (−V) thus generated is applied to the negative terminal of the operational amplifier 131.

[0062] In this way, the potential difference between the positive voltage (+V) applied to the positive terminal of the operational amplifier 131 and the negative voltage (-V) applied to the negative terminal of the operational amplifier 131 is kept constant. As a result, no AC current flows through the stray capacitance C0 present inside the operational amplifier 131, and the measured voltage value V measured by the voltage measuring unit 140 is m The influence of the stray capacitance C0 is small. m Based on this, the measurement gap g (displacement) between the center electrode E1 and the workpiece WK is measured.

[0063] Therefore, in this embodiment, the influence of the stray capacitance C0 existing between the input terminal of the operational amplifier 131 and the ground potential can be reduced, and the measurement gap g between the center electrode E1 and the workpiece WK can be measured.

[0064] In this embodiment, the operational amplifier 131 is a voltage follower in which the input is connected to the non-inverting input terminal (+ input terminal) and the output is connected to the inverting input terminal (- input terminal). Therefore, the operational amplifier 131 has an output impedance lower than the input impedance, and the input voltage V in and the output voltage V out It can function as a voltage buffer circuit with an amplification factor of 1, where

[0065] In this embodiment, the output of the voltage follower is connected to the outer braid of the shield braided coaxial cable CV, which acts as a driven shield. Therefore, the effect of the capacitance between the center line of the coaxial cable CV connected to the center electrode E1 and the braided wire of the coaxial cable CV connected to the guard electrode E2 (between the inner conductor and the outer conductor) can be ignored.

[0066] Therefore, in this embodiment, a measurement method using an impedance conversion circuit is adopted, which cancels the influence of stray capacitance present inside the operational amplifier, making it possible to accurately measure the capacitance between the center electrode and the object to be measured. Furthermore, the accurately measured capacitance can be used to measure the displacement between the center electrode and the object to be measured.

[0067] [Modification of the embodiment] The present invention is not limited to the above-described embodiment, and various modifications are possible.

[0068] For example, in the above embodiment, the measurement gap was measured using constant AC current drive, but it goes without saying that the impedance conversion circuit of the present invention can be applied to a capacitance type sensor that uses another impedance conversion circuit.

[0069] Furthermore, in the above embodiment, the positive-side low-impedance voltage shift circuit 132 and the negative-side low-impedance voltage shift circuit 133 each include a Zener diode, a transistor, and a resistive element, and generate positive-side voltages and negative-side voltages of the same phase and amplitude by utilizing the Zener voltage of the Zener diode and the base-emitter voltage of the transistor. However, the impedance conversion circuit may have a different configuration as long as positive-side voltages and negative-side voltages of the same phase and amplitude can be obtained.

[0070] <Variation 1> For example, the impedance conversion circuit 130B may have the configuration shown in Fig. 7. As shown in Fig. 7, the impedance conversion circuit 130B has a capacitance element C H and the resistive element R H 1, and an operational amplifier 134. H corresponds to the removal section, and the operational amplifier 134 corresponds to the post-stage operational amplifier.

[0071] The above capacitance element C H is connected at one end to the output terminal of the operational amplifier 131 that constitutes a voltage follower, and the other end to the resistor element R H and the non-inverting input terminal (+ input terminal) of the operational amplifier 134. H is connected to a capacitance element C Hand the non-inverting input terminal (+ input terminal) of the operational amplifier 134, and the other end is grounded. H and resistive element R H The DC component of the output of the operational amplifier 131 is input to the capacitance element C H and the result of the elimination is input to the operational amplifier 134.

[0072] The operational amplifier 134 is a non-inverting amplifier. The non-inverting input terminal (+ input terminal) of the operational amplifier 134 is connected to a capacitance element C H The other end of the resistor element R H The inverting input terminal (-input terminal) of the operational amplifier 131 is connected to one end of the output terminal of the operational amplifier 134.

[0073] That is, operational amplifier 134 is a voltage follower in which the input is connected to the non-inverting input terminal (+ input terminal) and the output is connected to the inverting input terminal (- input terminal). Therefore, operational amplifier 134 functions as a voltage buffer circuit with an amplification factor of 1, which lowers the output impedance relative to the input impedance and makes the input voltage equal to the output voltage (output voltage at the subsequent stage). As a result, operational amplifier 134 produces a low-impedance output in phase with the input signal whose impedance has increased. This output is sent to voltage measurement unit 140. This configuration makes it possible to eliminate voltage offset elements.

[0074] <Variation 2> Alternatively, an impedance conversion circuit 130C having the configuration shown in Fig. 8 may be used. As shown in Fig. 8, the impedance conversion circuit 130C differs from the impedance conversion circuit 130 of the above-described embodiment (see Fig. 4) in that it includes a positive-side low-impedance voltage shift circuit 132C instead of the positive-side low-impedance voltage shift circuit 132, and a negative-side low-impedance voltage shift circuit 133C instead of the negative-side low-impedance voltage shift circuit 133.

[0075] The positive-side low-impedance voltage shift circuit 132C is different from the positive-side low-impedance voltage shift circuit 132 in that forward-biased diodes D11 to D1 are used instead of the reverse-biased Zener diode ZD1. k The negative-side low-impedance voltage shift circuit 133C differs from the negative-side low-impedance voltage shift circuit 133 in that it includes forward-biased diodes D21 to D22 instead of the reverse-biased Zener diode ZD2. k The difference is that it has the following.

[0076] Diodes D11 to D1 k and diodes D21 to D2 k are connected in series, and the value of k is determined in advance based on experiments, simulations, etc. By using such a configuration, it is possible to speed up processing.

[0077] <Variation 3> Alternatively, an impedance conversion circuit 130D having a configuration shown in Fig. 9 may be used. As shown in Fig. 9, impedance conversion circuit 130D differs from impedance conversion circuit 130B of the above-described first modification (see Fig. 7) in that it includes a positive-side low-impedance voltage shift circuit 132D instead of positive-side low-impedance voltage shift circuit 132, and a negative-side low-impedance voltage shift circuit 133D instead of negative-side low-impedance voltage shift circuit 133. Furthermore, a difference is that the output of operational amplifier 134 is input to positive-side low-impedance voltage shift circuit 132D and negative-side low-impedance voltage shift circuit 133D.

[0078] The positive-side low-impedance voltage shift circuit 132D includes a resistor R T1 and Zener diode ZD T1 and the capacitance element C T1 The resistor element R T1 One end of the Zener diode ZD is connected to the positive power supply, and the other end is connected to the positive terminal of the operational amplifier 131. T1 and the capacitance element C T1are connected in parallel and inserted between the output of the operational amplifier 134 and the positive terminal of the operational amplifier 131. Also, the Zener diode ZD T1 and the capacitance element C T1 The other end of the resistor R T1 is connected to the other end of the

[0079] The negative-side low-impedance voltage shift circuit 133D includes a resistor R T2 and Zener diode ZD T2 and the capacitance element C T2 The resistor element R T2 One end of the Zener diode ZD is connected to the negative power supply, and the other end is connected to the negative terminal of the operational amplifier 131. T2 and the capacitance element C T2 are connected in parallel and inserted between the output of the operational amplifier 134 and the negative terminal of the operational amplifier 131. Also, the Zener diode ZD T2 and the capacitance element C T2 The other end of the resistor R T2 is connected to the other end of the

[0080] In this case, the output voltage (post-stage output voltage) of the AC component of the operational amplifier 134 is input to the positive-side low-impedance voltage shift circuit 132D, and the Zener diode ZD T1 The Zener voltage V T1 The voltage is shifted to a higher voltage by the capacitance element C T1 and is supplied to the positive terminal of the operational amplifier 131. The positive terminal is connected to a resistor R T1 A DC component is supplied from the positive power supply via the positive voltage. This allows the positive voltage to operate at an amplitude that exceeds the positive power supply voltage.

[0081] Similarly, the negative-side low-impedance voltage shift circuit 133D receives the AC component output voltage of the operational amplifier 134, and the Zener diode ZD T2 The Zener voltage V T2 The voltage is shifted to a lower voltage by the capacitance element C T2and is supplied to the negative terminal of the operational amplifier 131. The negative terminal is connected to a resistor R T2 A DC component is supplied from the negative power supply via the negative voltage. This allows the negative voltage to operate at an amplitude that exceeds the negative power supply voltage. This improves the S / N ratio.

[0082] Furthermore, the input of operational amplifier 134 removes the DC component from the output of operational amplifier 131. In this case, a voltage shift can be performed on the output voltage of operational amplifier 134, which has no offset, and the operating voltage range of operational amplifier 131 can be optimized.

[0083] In the above embodiment, the center electrode is disk-shaped, the guard electrode is cylindrical with a bottom, and the center electrode is disposed inside the cylindrical guard electrode with an insulator interposed therebetween. However, the center electrode and the guard electrode may have other shapes, and the number of guard electrodes may be three or more.

[0084] In the above embodiment, the AC constant current source circuit is a sense resistor (resistance element R S ), but instead of the sense resistor, a sense impedance (a capacitance element, an inductance element, a resistance element, or a complex of any combination of these elements) may be arranged. In this case, if a sense resistor is not used, there is no influence of Johnson noise due to the sense resistor, and the measurement gap can be measured with high accuracy.

[0085] In the above embodiment, the present invention is applied to a gap sensor that measures the measurement gap between an electrode and a measurement object. However, the present invention may also be applied to a laser processing device, a liquid crystal inspection device, a disk runout measurement device, a device that measures wafer thickness flatness, a device that performs positioning control for measuring stage movement, and the like, that utilizes measurement gap measurement. The present invention may also be applied to various copying devices that utilize measurement gap measurement. [Explanation of symbols]

[0086] 100... Distance measuring device 101 ... Measuring equipment 110 ... Mounting part 115 ... Electrode head 120 … AC constant current source 125...AC voltage source 121 … AC signal generation section 122 ... operational amplifier 130... Impedance conversion circuit 130B, 130C, 130D ... Impedance conversion circuit 131 ... operational amplifier 132 ... Positive side low impedance voltage shift circuit 132C, 132D ... Positive side low impedance voltage shift circuit 133 ... Negative side low impedance voltage shift circuit 133C, 133D ... Negative side low impedance voltage shift circuit 134...Operational amplifier (post-stage operational amplifier) 140 ... Voltage measurement section 150 ... Calculation section 810 ... operational amplifier 811 ... Differential input stage 900A... Distance measuring device 900B … Distance measuring device 910…AC constant current source 920 ... Impedance conversion circuit CV...coaxial cable E1 … Center electrode (first electrode) E2: Guard electrode (second electrode) EL…Electrode EL c …center electrode EL g … Guard electrode R1, R2, R T1 ,R T2 ,R A ,R B ,R C ,R D ,R S ,R H ...resistive element TR1: Transistor (first transistor) TR2: Transistor (second transistor) ZD1: Zener diode (first Zener diode) ZD2: Zener diode (second Zener diode) ZD T1 ,ZD T2 … Zener diode D11~D1 k …diode D21~D2 k …diode C T1 ,C T2 …capacitance element C H … Capacitive element (removal part) WK: Workpiece (measurement object)

Claims

1. A measuring device having an impedance conversion circuit used to measure the distance between a first electrode and an object to be measured by a change in capacitance, based on the use of an electrode head including a first electrode and at least one second electrode arranged to surround the first electrode, an operational amplifier having a non-inverting input terminal, an inverting input terminal, and an output terminal; a line body connecting the first electrode and the non-inverting input terminal, the line being based on an inner conductor at the center of a coaxial cable; a wire body connecting the second electrode and the inverting input terminal, the wire being based on an outer conductor of the coaxial cable and insulated from the inner conductor; a positive-side low-impedance voltage shift circuit that generates a positive-side voltage to be applied to a positive-side terminal of the operational amplifier based on a voltage output from the operational amplifier; a negative-side low-impedance voltage shift circuit that generates a negative-side voltage to be applied to a negative-side terminal of the operational amplifier based on a voltage output from the operational amplifier; a voltage measurement unit that measures an output voltage applied to the capacitance element based on a predetermined constant current flowing through the first electrode; Equipped with The wire based on the inner conductor is By connecting the resistor to an AC constant current source circuit via the resistor, the constant current corresponding to the resistance value of the resistor can be caused to flow to the capacitance body, thereby transmitting an input impedance to the non-inverting input terminal; The positive side low impedance voltage shift circuit and the negative side low impedance voltage shift circuit are generating the positive side voltage and the negative side voltage so that the positive side voltage and the negative side voltage have the same phase and amplitude; The operational amplifier a first connection is made between a line based on the inner conductor and the non-inverting input terminal; and a second connection is made between the inverting input terminal and a line based on the outer conductor, through which an output impedance lowered relative to the input impedance is transmitted, thereby forming a voltage follower; The impedance conversion circuit includes: Based on the configuration of the voltage follower, an output with a low impedance can be sent to the second connection or the voltage measurement unit in the same phase as the input signal with a high impedance from the first connection. A measuring device characterized by:

2. A measuring device having an impedance conversion circuit used to measure the distance between a first electrode and an object to be measured by a change in capacitance, based on the use of an electrode head including a first electrode and at least one second electrode arranged to surround the first electrode, an operational amplifier having a non-inverting input terminal, an inverting input terminal, and an output terminal; a line body connecting the first electrode and the non-inverting input terminal, the line being based on an inner conductor at the center of a coaxial cable; a wire body connecting the second electrode and the inverting input terminal, the wire being based on an outer conductor of the coaxial cable and insulated from the inner conductor; a positive-side low-impedance voltage shift circuit that generates a positive-side voltage to be applied to a positive-side terminal of the operational amplifier based on a voltage output from the operational amplifier; a negative-side low-impedance voltage shift circuit that generates a negative-side voltage to be applied to a negative-side terminal of the operational amplifier based on a voltage output from the operational amplifier; Equipped with The wire based on the inner conductor is By connecting the resistor to an AC constant current source circuit via the resistor, a constant current corresponding to the resistance value of the resistor can be made to flow to the capacitance body, thereby making it possible to input impedance to the non-inverting input terminal; The positive side low impedance voltage shift circuit and the negative side low impedance voltage shift circuit are generating the positive side voltage and the negative side voltage so that the positive side voltage and the negative side voltage have the same phase and amplitude; The impedance conversion circuit includes: The wire based on the inner conductor is connected to the wire based on the outer conductor so as to transmit an output with a low impedance in the same phase as an input signal with a high impedance due to the wire based on the inner conductor, thereby forming a driven shield configuration. A measuring device characterized by:

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