Thickness Measuring Device

Abstract

To protect a receiving amplifier while reducing an influence on measurements using eddy currents.SOLUTION: A processing device 6 includes: a first detection coil 13 and a second detection coil 14 connected in series; a receiving amplifier 51 to which the first detection coil 13 and the second detection coil 14 are connected; and a protection circuit 52 connected in parallel to the first detection coil 13 and the second detection coil 14. The protection circuit 52 includes: a first protection circuit 52A that includes a first resistance element 53a and a first diode 54a connected in series, and is connected in parallel to the first detection coil 13 between a first end 13a and a second end 13b of the first detection coil 13; and a second protection circuit 52B that includes a second resistance element 53b and a second diode 54b connected in series, and is connected in parallel to the second detection coil 14 between a first end 14a and a second end 14b of the second detection coil 14.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The technology disclosed herein relates to detection devices and thickness measurement devices. [Background technology]

[0002] Conventionally, there have been known devices that use a detection coil to detect eddy currents generated in an object. For example, Patent Document 1 discloses a device in which a coil generates an AC magnetic field to induce eddy currents in the object, and the eddy currents induced in the object are detected by the coil.

[0003] In this device, the induced voltage detected by the coil section is input to the detection section, which detects changes in the output waveform. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2013 / 047521 Summary of the Invention [Problem to be solved by the invention]

[0005] In detecting eddy currents as described above, the induced electromotive force generated in the detection coil is detected. Immediately after a particularly large eddy current is generated, a large induced electromotive force is generated in the detection coil. In this case, a large voltage is also input to the receiving amplifier connected to the detection coil.

[0006] It is conceivable to provide a protection circuit to protect the receiving amplifier from large voltages, but such a protection circuit may affect measurements using eddy currents.

[0007] The technology disclosed herein has been made in consideration of these points, and its purpose is to protect the receiving amplifier while reducing the influence on measurements using eddy currents. [Means for solving the problem]

[0008] The detection device disclosed herein includes a first detection coil and a second detection coil connected in series to detect an eddy current induced in an object by an excitation coil, a receiving amplifier to which the first detection coil and the second detection coil are connected, and a protection circuit connected in parallel to the first detection coil and the second detection coil, wherein a first end of the first detection coil is connected to a non-inverting input terminal of the receiving amplifier, a second end of the first detection coil is connected to a first end of the second detection coil, and a second end of the second detection coil is connected to an inverting input terminal of the receiving amplifier, and the protection circuit includes a first resistance element and a first diode connected in series and connected in parallel to the first detection coil between the first end and the second end of the first detection coil, and a second protection circuit includes a second resistance element and a second diode connected in series and connected in parallel to the second detection coil between the first end and the second end of the second detection coil.

[0009] The detection device disclosed herein also includes a detection coil that detects eddy currents induced in an object by an excitation coil, a receiving amplifier having a non-inverting input terminal connected to a first end of the detection coil and an inverting input terminal connected to a second end of the detection coil, and a protection circuit connected in parallel with the detection coil between the first and second ends of the detection coil, the protection circuit including a resistance element and a diode connected in series.

[0010] The thickness measuring device disclosed herein further includes the detection device and a thickness deriving unit that determines the thickness of the object based on the output voltage of the receiving amplifier. [Effects of the Invention]

[0011] According to the detection device, it is possible to protect the receiving amplifier while reducing the influence on the measurement using eddy currents.

[0012] According to the thickness measuring device, it is possible to protect the receiving amplifier while reducing the influence on the measurement using eddy currents. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a block diagram of a thickness measurement device. [Figure 2] FIG. 2 is a block diagram showing the configuration of a control system of the control unit of the processing device. [Figure 3] FIG. 3 is a block diagram showing the configuration of a control system of the control unit of the arithmetic device. [Figure 4] FIG. 4 is a graph showing the change over time of a voltage signal V(t) corresponding to an eddy current. [Figure 5] FIG. 5 is a flowchart of the thickness measurement. [Figure 6] FIG. 6 is a circuit diagram of the receiving section. [Figure 7] FIG. 7 is a graph showing an excitation signal input to the excitation unit. [Figure 8] FIG. 8 is a graph showing the induced voltages generated in the first and second detection coils. [Figure 9] FIG. 9 is a graph showing the output voltage of the receiving amplifier. [Figure 10] FIG. 10 is a circuit diagram of a receiving unit according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0014] Exemplary embodiments will now be described in detail with reference to the accompanying drawings.

[0015] Exemplary embodiments will be described in detail below with reference to the drawings. FIG. 1 is a block diagram of a thickness measurement system 100. The thickness measurement system 100 includes a probe 1 and a thickness measurement device 10. The thickness measurement device 10 measures the thickness of an object 9 by pulsed eddy current (PEC). The thickness measurement device 10 includes a processing device 6 that controls the probe 1 and a calculation device 8 that calculates the thickness of the object 9. For example, the object 9 is a metal pipe through which steam or drain flows. The pipe is formed in a cylindrical shape.

[0016] The probe 1 is used to generate an eddy current in the object 9 and to detect the generated eddy current. The probe 1 is a non-contact type probe and is placed in close proximity to the object 9. Note that the term "non-contact type" means that it can be used without contact, and does not exclude use in a contact state. The probe 1 is placed so as to face the surface of the object 9. For example, the probe 1 is placed on the object 9 via a spacer (not shown) having thermal insulation properties.

[0017] The probe 1 generates eddy currents in the object 9 by forming a varying magnetic field. The probe 1 also detects changes in the eddy currents generated in the object 9 as induced voltages. Specifically, the probe 1 includes an excitation coil 11 that induces eddy currents in the object 9 with magnetic flux generated by an excitation current, and a detection coil 12 that detects the eddy currents in the object 9. The probe 1 induces eddy currents in the object 9 using the excitation coil 11, and detects the induced eddy currents using the detection coil 12. The probe 1 may further include a casing that houses the excitation coil 11 and the detection coil 12.

[0018] In this example, the excitation coil 11 includes a first excitation coil 11A and a second excitation coil 11B. The detection coil 12 includes a first detection coil 13 and a second detection coil .

[0019] More specifically, the first excitation coil 11A and the first detection coil 13 are arranged so that the axis of the first excitation coil 11A and the axis of the first detection coil 13 are aligned. In this case, the first detection coil 13 is arranged closer to the object 9 than the first excitation coil 11A. The same applies to the positional relationship between the second excitation coil 11B and the second detection coil 14. When there is no need to distinguish between the first excitation coil 11A and the second excitation coil 11B, they will simply be referred to as the "excitation coil 11." When there is no need to distinguish between the first detection coil 13 and the second detection coil 14, they will simply be referred to as the "detection coil 12."

[0020] Furthermore, the probe 1 may include a core 15 inserted into the excitation coil 11 and the detection coil 12. The core 15 is formed into a generally U-shape as a whole. More specifically, the core 15 is formed by stacking a plurality of generally U-shaped thin plates made of permalloy. A linear portion at one end of the core 15 is inserted into the first excitation coil 11A and the first detection coil 13. A linear portion at the other end of the core 15 is inserted into the second excitation coil 11B and the second detection coil 14. The core 15 magnetically connects the two sets of excitation coils 11 and detection coils 12.

[0021] When a current is applied to the excitation coil 11, the excitation coil 11 forms a magnetic field in the direction of its axis. Current is applied to the first excitation coil 11A and the second excitation coil 11B so that they form magnetic fields in opposite directions in the direction of the axis, i.e., magnetic fields in opposite directions with respect to the object 9. As a result, a magnetic field is formed in the core 15 along the longitudinal direction of the core 15. That is, when one end of the core 15 is an N pole, the other end of the core 15 is an S pole. Conversely, when one end of the core 15 is an S pole, the other end of the core 15 is an N pole. For example, a magnetic flux is generated from the first excitation coil 11A toward the object 9, and a magnetic flux is generated from the object 9 toward the second excitation coil 11B. More specifically, most of the magnetic flux emitted from the first excitation coil 11A exits in the direction of the axis of the first excitation coil 11A, enters the object 9, passes through the object 9 in a substantially arc shape, heads toward the axis of the second excitation coil 11B, and enters the second excitation coil 11B. By varying the current applied to the excitation coil 11, the magnetic field generated in the object 9 varies, and eddy currents are generated in the object 9.

[0022] Meanwhile, eddy currents generated in a portion of the object 9 near the detection coil 12 form magnetic fluxes that penetrate the detection coil 12. When the magnetic flux penetrating the detection coil 12 changes, an induced electromotive force is generated in the detection coil 12. The detection coil 12 detects this induced electromotive force, thereby detecting the eddy currents in the object 9. In other words, detecting an induced electromotive force by the detection coil 12 is also referred to as detecting eddy currents. The magnetic flux penetrating the first detection coil 13 and the magnetic flux penetrating the second detection coil 14 are oriented in opposite directions relative to the object 9.

[0023] The processing device 6 uses the probe 1 to generate an eddy current in the object 9 and detect the generated eddy current. The processing device 6 is an example of a detection device. The calculation device 8 calculates the thickness of the object 9 based on the duration of the eddy current detected by the processing device 6 (the time until the eddy current suddenly decays, which will be described in detail later).

[0024] The processing device 6 is disposed, for example, in the vicinity of the object 9. For example, the processing device 6 is installed on the object 9 via a spacer. The processing device 6 has a transmitting unit 61, a receiving unit 62, a communication unit 65, a control unit 66, and a storage unit 67.

[0025] The transmitter 61 applies a pulsed excitation current to the excitation coil 11. The transmitter 61 has a pulse generator 61a and a transmission amplifier 61b. The pulse generator 61a generates a pulse signal based on a command from the control unit 66. The transmission amplifier 61b amplifies the pulse signal from the pulse generator 61a and outputs it to the excitation coil 11 as an excitation current.

[0026] The receiving unit 62 receives the induced electromotive force generated in the detection coil 12 in response to the eddy current in the object 9. The receiving unit 62 has at least a receiving amplifier 51 that receives the voltage generated in the detection coil 12 and amplifies the voltage. The receiving unit 62 may further have a filter that filters the voltage signal.

[0027] The communication unit 65 performs wireless communication with an external device. For example, the communication unit 65 transmits the voltage signal detected by the receiving unit 62 (that is, the detection signal) to the arithmetic device 8.

[0028] The control unit 66 controls the entire processing device 6. The control unit 66 performs various types of arithmetic processing. For example, the control unit 66 is formed of a processor such as a CPU (Central Processing Unit). The control unit 66 may also be formed of an MCU (Micro Controller Unit), an MPU (Micro Processor Unit), an FPGA (Field Programmable Gate Array), a PLC (Programmable Logic Controller), a system LSI, or the like.

[0029] For example, the control unit 66 causes the transmitting unit 61 to output an excitation current for a predetermined period of time. After the output of the excitation current is stopped, the control unit 66 acquires a detection signal via the receiving unit 62. The control unit 66 stores the detection signal from the receiving unit 62 in the memory unit 67, and transmits the detection signal stored in the memory unit 67 to the calculation device 8 via the communication unit 65 as appropriate.

[0030] The storage unit 67 stores various data and programs executed by the control unit 66. For example, a control program is stored in the storage unit 67. The storage unit 67 is formed of a non-volatile memory, a hard disk drive (HDD), a solid state drive (SSD), or the like.

[0031] 2 is a block diagram showing the configuration of a control system of the control unit 66 of the processing device 6. The control unit 66 realizes various functions by reading out a control program from the storage unit 67 into memory and expanding the program. Specifically, the control unit 66 functions as an excitation unit 71 that applies an excitation current to the excitation coil 11 to induce an eddy current in the object 9, and a detection unit 72 that detects the eddy current in the object 9 via the detection coil 12.

[0032] The excitation unit 71 causes the transmission unit 61 to apply an excitation current to the excitation coil 11. Specifically, the excitation unit 71 outputs a command to the pulse generator 61a, causing the pulse generator 61a to generate a pulse signal. As a result, an excitation current is applied to the excitation coil 11 from the transmission amplifier 61b.

[0033] The detection unit 72 detects a voltage signal corresponding to the eddy current as the eddy current in the object 9. Specifically, the detection unit 72 detects a voltage signal corresponding to the induced electromotive force in the detection coil 12. More specifically, the detection unit 72 continues to detect the voltage signal for a predetermined period after the application of the excitation current to the excitation coil 11 is stopped. In other words, the detection unit 72 detects a time-varying change (i.e., a transient change) in the eddy current in the object 9 after the application of the excitation current to the excitation coil 11 is stopped. The detection unit 72 stores the detected eddy current, i.e., the voltage signal, in the storage unit 67. Hereinafter, for convenience of explanation, the voltage signal detected by the detection unit 72 may be simply referred to as the "eddy current." For example, the voltage signal corresponding to the eddy current stored in the storage unit 67 will also be simply referred to as the "eddy current."

[0034] The arithmetic device 8 is formed by a computer or a computer network (so-called cloud). The arithmetic device 8 has a communication unit 81, a control unit 82, and a storage unit 83, as shown in FIG.

[0035] The communication unit 81 performs wireless communication with external devices. For example, the communication unit 81 receives signals from the processing device 6.

[0036] The control unit 82 controls the entire arithmetic device 8. The control unit 82 performs various types of arithmetic processing. For example, the control unit 82 is formed of a processor such as a CPU (Central Processing Unit). The control unit 82 may also be formed of an MCU (Micro Controller Unit), an MPU (Micro Processor Unit), an FPGA (Field Programmable Gate Array), a PLC (Programmable Logic Controller), a system LSI, or the like.

[0037] The memory unit 83 stores programs and various data executed by the control unit 82. For example, the memory unit 83 stores a control program. The memory unit 83 is formed of a non-volatile memory, a hard disk drive (HDD), a solid state drive (SSD), or the like. The memory unit 83 also stores signals transmitted from the processing device 6, etc. Specifically, the memory unit 83 stores eddy currents of the object 9 acquired by the processing device 6.

[0038] 3 is a block diagram showing the configuration of a control system of the control unit 82 of the calculation device 8. The control unit 82 realizes various functions by reading out a control program from the storage unit 83 into memory and developing it. Specifically, the control unit 82 functions as a thickness derivation unit 84.

[0039] The thickness derivation unit 84 determines the thickness of the object 9 based on the duration of the eddy current in the object 9. As will be described in more detail, the eddy current induced in the object 9 by the excitation coil 11 penetrates from the front surface of the object 9 (the surface facing the probe 1) to the back surface, and rapidly attenuates when it reaches the back surface. The duration of the eddy current in the object 9 is the time from when the eddy current is induced in the object 9 to when it rapidly attenuates. The duration of the eddy current in the object 9 is correlated with the thickness of the object 9.

[0040] The thickness derivation unit 84 determines the duration of the eddy current (specifically, the voltage signal) detected by the processing device 6. The thickness derivation unit 84 determines the thickness of the object 9 from the duration based on the correlation between the duration and the thickness.

[0041] Here, the relationship between eddy currents and the thickness of the object 9 will be described in detail. FIG. 4 is a graph showing the time variation of a voltage signal V(t) corresponding to an eddy current. The graph in FIG. 4 is a double logarithmic graph. In FIG. 4, voltage signal V0(t) is the voltage signal of an object 9 having a thickness d0, and voltage signal V1(t) is the voltage signal of an object 9 having a thickness d1 that is thinner than d0.

[0042] The eddy current decays as it penetrates into the object 9. The eddy current decays gradually from the front surface of the object 9 (the surface facing the probe 1) until it reaches the back surface, and then decays rapidly once it reaches the back surface. The voltage signal V(t) also shows the same change as the eddy current. In other words, the transient change in the voltage signal V(t) corresponds to the transient change in the eddy current. The change in the voltage signal V(t) until the eddy current reaches the back surface of the object 9 is represented linearly on a double logarithmic graph. Thereafter, the voltage signal V(t) decays rapidly. This changing voltage signal V(t) is expressed as in the following equation (1).

[0043]

number

[0044] As can be seen from equation (1), the voltage signal V(t) gradually decays but continues until time τ, at which point it rapidly decays. For convenience of explanation, τ will be referred to as the "duration." The duration τ is expressed by the following equation (2):

[0045] τ=σμd 2 ···(2) where σ is the electrical conductivity of the object 9 , μ is the magnetic permeability of the object 9 , and d is the thickness of the object 9 .

[0046] That is, the duration τ changes depending on the thickness d of the object 9. Assuming that the electrical conductivity σ and magnetic permeability μ of the object 9 are constant, the duration τ changes depending on the thickness d of the object 9. Also, even if the duration τ and the thickness d change, the time τ / d 2 is constant. Therefore, if the duration τ0 for a known thickness d0 and the duration τ for an unknown thickness d are known, the unknown thickness d can be calculated based on the following equation (3). Equation (3), the known thickness d0, and the duration τ0 are stored in the memory unit 83.

[0047]

number

[0048] Such thickness measurement will be explained in more detail using a flowchart, as shown in Figure 5.

[0049] The thickness derivation unit 84 determines whether a predetermined measurement period has arrived in step S101 of the thickness measurement flowchart. The measurement period is a period for obtaining thickness measurements of the object 9. If the measurement period has not arrived, the thickness derivation unit 84 repeats the determination in step S101 and waits for the arrival of the measurement period.

[0050] When the measurement period arrives, the thickness derivation unit 84 outputs a command to the processing device 6 to cause the processing device 6 to acquire measurement data. The measurement data is data for measuring the thickness of the object 9, specifically, the eddy current of the object 9. Specifically, when the processing device 6 receives a command from the calculation device 8, the excitation unit 71 applies an excitation current to the excitation coil 11 to excite it in step S102. The excitation coil 11 forms a magnetic field in the axial direction by applying the excitation current. One excitation coil 11 and the other excitation coil 11 form magnetic fields in opposite directions in the axial direction. For example, a magnetic flux is generated from one excitation coil 11 toward the object 9, and a magnetic flux is generated from the object 9 toward the other excitation coil 11.

[0051] Subsequently, in step S103, the excitation unit 71 stops outputting the excitation current, and the detection unit 72 starts detecting the eddy current generated in the object 9. The detection unit 72 continues detecting the voltage signal for a predetermined period. In this way, the detection unit 72 detects the transient change (change over time) of the induced electromotive force of the detection coil 12, i.e., the transient change of the eddy current generated in the object 9.

[0052] The processing device 6 transmits the eddy current as measurement data to the calculation device 8. The calculation device 8 stores the received eddy current in the storage unit 83.

[0053] Next, in step S104, the thickness derivation unit 84 obtains the duration τ of the eddy current stored in the memory unit 83. Furthermore, in step S105, the thickness derivation unit 84 obtains the thickness d of the object 9 by substituting the duration τ into equation (3).

[0054] Thereafter, in step S106, the thickness derivation unit 84 determines whether or not to end the thickness measurement. For example, the thickness derivation unit 84 determines whether or not a command to end the thickness measurement has been input. For example, the user operates the arithmetic device 8 to input an end of the thickness measurement. If an end command has not been input, the thickness derivation unit 84 returns to step S101 and determines whether or not the next measurement cycle has arrived. When the next measurement cycle arrives, measurement data is acquired again, and the thickness d of the object 9 is found based on the measurement data. In other words, the arithmetic device 8 repeats acquiring measurement data and deriving the thickness d of the object 9 for each measurement cycle.

[0055] In step S106, if an end command has been input, the thickness measurement is ended.

[0056] In this way, the thickness measuring device 10 generates an eddy current in the object 9, detects the generated eddy current, and determines the thickness d of the object 9 based on the duration τ of the eddy current.

[0057] Next, a more detailed description will be given of the configuration of the receiving unit 62. FIG.

[0058] The receiving unit 62 has a receiving amplifier 51 and a protection circuit 52. The receiving amplifier 51 is connected to the first detection coil 13 and the second detection coil 14. The protection circuit 52 is connected in parallel to the first detection coil 13 and the second detection coil 14. In other words, the protection circuit 52 is connected in parallel to the receiving amplifier 51.

[0059] The receiving amplifier 51 is an operational amplifier having a non-inverting input terminal (+), an inverting input terminal (-), and an output terminal, and is connected to a positive power supply and a negative power supply.

[0060] The first detector coil 13 and the second detector coil 14 are connected in series. Specifically, the first detector coil 13 has a first end 13a and a second end 13b. The second detector coil 14 has a first end 14a and a second end 14b. The second end 13b of the first detector coil 13 is connected to the first end 14a of the second detector coil 14. In this thickness measurement example, the direction of the magnetic field generated by the excitation coil 11 is constant, so the direction of the induced magnetic field induced by the eddy current generated in the object 9 and penetrating each of the first detector coil 13 and the second detector coil 14 is also constant. Therefore, when detecting the eddy current in the object 9, the direction of the current flowing through the first detector coil 13 and the second detector coil 14 is fixed. In the first detector coil 13, the current flows from the second end 13b to the first end 13a. That is, the first end 13a has a higher potential than the second end 13b. In second detection coil 14, a current flows from second end 14b to first end 14a. That is, first end 14a has a higher potential than second end 14b. By connecting second end 13b of first detection coil 13 and first end 14a of second detection coil 14, the induced electromotive forces generated in first detection coil 13 and second detection coil 14 are added together.

[0061] A first end 13a of the first detection coil 13 is connected to a non-inverting input terminal (+) of the receiving amplifier 51. A second end 14b of the second detection coil 14 is connected to an inverting input terminal (-) of the receiving amplifier 51.

[0062] A common line Com is connected to the second end 13b of the first detection coil 13 and the first end 14a of the second detection coil 14. The common line Com is grounded.

[0063] The protection circuit 52 includes a first protection circuit 52A and a second protection circuit 52B.

[0064] The first protection circuit 52A is connected in parallel with the first detection coil 13 between the first end 13a and the second end 13b of the first detection coil 13. In other words, the first protection circuit 52A is connected between the non-inverting input terminal (+) of the receiving amplifier 51 and the common line Com. The first protection circuit 52A includes a first resistor element 53a and a first diode 54a connected in series. The first diode 54a is connected so that the anode of the first diode 54a is on the first end 13a side and the cathode of the first diode 54a is on the common line Com side, that is, so that the first diode 54a is forward biased.

[0065] The second protection circuit 52B is connected in parallel with the second detection coil 14 between the first end 14a and the second end 14b of the second detection coil 14. In other words, the second protection circuit 52B is connected between the common line Com and the inverting input terminal (-) of the receiving amplifier 51. The second protection circuit 52B includes a second resistor element 53b and a second diode 54b connected in series. The second diode 54b is connected so that the anode of the second diode 54b is on the common line Com side and the cathode of the second diode 54b is on the second end 14b side, that is, so that the second diode 54b is forward biased.

[0066] Furthermore, a resistive element 55a for impedance matching is connected between the non-inverting input terminal (+) of the receiving amplifier 51 and the common line Com. A resistive element 55b for impedance matching is connected between the common line Com and the inverting input terminal (-) of the receiving amplifier 51.

[0067] Next, the operation of the receiving unit 62 will be described. Fig. 7 is a graph showing the excitation signal input to the excitation unit 71. Fig. 8 is a graph showing the induced voltage generated in the first detection coil 13 and the second detection coil 14. Fig. 9 is a graph showing the output voltage of the receiving amplifier 51.

[0068] In thickness measurement, in step S102 described above, the excitation unit 71 applies an excitation current to the excitation coil 11 to generate eddy currents in the object 9. Thereafter, the excitation unit 71 stops applying the excitation current to the excitation coil 11, thereby starting detection of the eddy currents by the detection unit 72. The excitation unit 71 outputs the excitation current when the excitation signal is ON, and stops outputting the excitation current when the excitation signal is OFF. That is, as shown in FIG. 7, at time t1, the excitation unit 71 stops outputting the excitation current, thereby stopping the generation of a magnetic field by the excitation coil 11. Fluctuations in the magnetic field at this time generate eddy currents in the object 9, and these eddy currents generate an induced electromotive force in the detection coil 12.

[0069] 8, immediately after the excitation current is stopped, an induced voltage with a large absolute value is generated in the first detector coil 13 and the second detector coil 14. A positive induced voltage Va (solid line in the figure) is generated in the first detector coil 13, and a negative induced voltage Vb (dashed line in the figure) is generated in the second detector coil 14. The voltage difference between the induced voltage Va in the first detector coil 13 and the induced voltage Vb in the second detector coil 14 (in this example, the voltage obtained by adding the absolute value of the induced voltage in the first detector coil 13 and the absolute value of the induced voltage in the second detector coil 14) becomes the input voltage to the receiving amplifier 51.

[0070] At this time, current flows through the first protection circuit 52A and the second protection circuit 52B due to the action of the first diode 54a and the second diode 54b. In Fig. 8, the induced voltages Va and Vb when the first protection circuit 52A and the second protection circuit 52B are not present are shown by two-dot chain lines. The absolute values ​​of the induced voltages Va in the first detection coil 13 and Vb in the second detection coil 14 are reduced by the first protection circuit 52A and the second protection circuit 52B. As a result, the input voltage to the receiving amplifier 51 immediately after the excitation current is stopped is reduced.

[0071] 9, the output voltage Vo of the receiving amplifier 51 rises after the excitation current is stopped and is saturated at its upper limit for a while. Eventually, as the induced voltage Va in the first detection coil 13 and the induced voltage Vb in the second detection coil 14 decay, the output voltage Vo of the receiving amplifier 51 also begins to decay. When the output voltage Vo decays to a predetermined threshold x, that is, at time t2, measurement of the duration τ begins. The time from time t1 to time t2 is the time required from the stopping of the excitation current until measurement of the duration τ begins, and is hereinafter referred to as the "measurement start time."

[0072] Here, the currents flowing through the first protection circuit 52A and the second protection circuit 52B are fed back to the first detection coil 13 and the second detection coil 14. If the feedback currents to the first detection coil 13 and the second detection coil 14 are large, it takes time for the induced voltage Va in the first detection coil 13 and the induced voltage Vb in the second detection coil 14 to decay, and it also takes time for the output voltage Vo to decay, as indicated by the two-dot chain line in the figure. This could result in a longer measurement start time T. However, the first protection circuit 52A includes a first resistor element 53a, and the second protection circuit 52B includes a second resistor element 53b. These first resistor element 53a and second resistor element 53b reduce the feedback currents to the first detection coil 13 and the second detection coil 14. As a result, the decay of the output voltage Vo is accelerated, and the measurement start time T is shortened.

[0073] This is particularly effective when measuring the thickness of a thin object 9. The eddy current graph in Figure 4 is schematic; in reality, the voltage immediately after the generation of the induced electromotive force is large and therefore cannot be properly detected. Therefore, the monotonically decaying voltage change shown in Figure 4 cannot be observed until the measurement start time T is reached. In other words, the duration τ cannot be calculated. If the thickness of the object 9 is thin, the duration τ becomes shorter. If the duration τ is shorter than the measurement start time T, the duration τ cannot be calculated, and as a result, the thickness of the object 9 cannot be calculated. In other words, if the measurement start time T is shortened, the time when the voltage signal corresponding to the eddy current can be properly observed can be advanced, and the thickness of the thin object 9 can be calculated.

[0074] As a method for reducing the input voltage to the receiving amplifier 51, instead of the first protection circuit 52A and the second protection circuit 52B, a diode may be connected between the first end 13a of the first detection coil 13 and the positive power supply, and a diode may be connected between the second end 14b of the second detection coil 14 and the negative power supply. When a large induced electromotive force is generated in the first detection coil 13 and the second detection coil 14, a current flows from the first detection coil 13 to the positive power supply and from the negative power supply to the second detection coil 14, thereby reducing the input voltage to the receiving amplifier 51. However, in this configuration, the leakage current of the diode is superimposed on the induced voltage signal, which increases the temperature dependency of the output voltage Vo of the receiving amplifier 51.

[0075] In contrast, in the configuration in which the first protection circuit 52A and the second protection circuit 52B are provided, the influence of the leakage current is reduced, and the temperature dependency of the output voltage Vo of the receiving amplifier 51 is reduced.

[0076] As a result, the receiving section 62 can reduce the input voltage to the receiving amplifier 51, shorten the measurement start time T, and reduce the temperature dependency of the output voltage Vo of the receiving amplifier 51.

[0077] As described above, the processing device 6 (detection device) includes the first detection coil 13 and the second detection coil 14 connected in series to detect the eddy current induced in the object 9 by the excitation coil 11, the receiving amplifier 51 to which the first detection coil 13 and the second detection coil 14 are connected, and the protection circuit 52 connected in parallel to the first detection coil 13 and the second detection coil 14, and the first end 13a of the first detection coil 13 is connected to the non-inverting input terminal (+) of the receiving amplifier 51, and the second end 13b of the first detection coil 13 is connected to the first end 14a of the second detection coil 14. The second end 14b of the second detection coil 14 is connected to the inverting input terminal (-) of the receiving amplifier 51, and the protection circuit 52 has a first protection circuit 52A including a first resistance element 53a and a first diode 54a connected in series and connected in parallel with the first detection coil 13 between the first end 13a and the second end 13b of the first detection coil 13, and a second protection circuit 52B including a second resistance element 53b and a second diode 54b connected in series and connected in parallel with the second detection coil 14 between the first end 14a and the second end 14b of the second detection coil 14.

[0078] The thickness measuring device 10 also includes a processing device 6 (detection device) and a thickness derivation unit 84 that determines the thickness of the object 9 based on the output voltage Vo of the receiving amplifier 51, specifically, based on the duration of the eddy current.

[0079] According to these configurations, when a large induced electromotive force is generated in the first detector coil 13 and the second detector coil 14, a large current flows through the first protection circuit 52A and the second protection circuit 52B due to the action of the first diode 54a and the second diode 54b, thereby reducing the input voltage to the receiving amplifier 51. Unlike the previously described configuration in which a diode is connected between the first terminal 13a of the first detector coil 13 and the positive power supply and a diode is connected between the second terminal 14b of the second detector coil 14 and the negative power supply, the first diode 54a and the second diode 54b are not connected to the positive power supply and the negative power supply, respectively, thereby reducing the temperature dependence of the output voltage Vo of the receiving amplifier 51. Furthermore, the current flowing through the first protection circuit 52A and the second protection circuit 52B is reduced by the first resistor element 53a and the second resistor element 53b. This reduces the feedback current to the first detector coil 13 and the second detector coil 14, thereby shortening the measurement start time T. As a result, the receiving amplifier 51 can be protected while reducing the influence on the measurement using the eddy current, that is, on the thickness measurement of the object 9.

[0080] Further, the second end 13b of the first detection coil 13 and the first end 14a of the second detection coil 14 are connected to a common line Com.

[0081] According to this configuration, the voltage difference between the induced voltage of the first detection coil 13 and the induced voltage of the second detection coil 14 is input to the receiving amplifier 51.

[0082] Other Embodiments As described above, the above embodiment has been described as an example of the technology disclosed in this application. However, the technology of the present disclosure is not limited to this and can be applied to embodiments in which modifications, substitutions, additions, omissions, etc. are made as appropriate. Furthermore, the components described in the above embodiment can be combined to create new embodiments. Furthermore, the components described in the accompanying drawings and detailed description may include not only components essential for solving the problem, but also components that are not essential for solving the problem in order to exemplify the technology. Therefore, the fact that these non-essential components are described in the accompanying drawings or detailed description should not be interpreted as immediately determining that these non-essential components are essential.

[0083] The above embodiment may be configured as follows.

[0084] For example, the configuration of the thickness measuring device 10 is merely an example. The processing device 6 and the arithmetic device 8 may be configured integrally. That is, one device may have the functions of the processing device 6 and the arithmetic device 8. The processing device 6 and the arithmetic device 8 may be connected by a wire. A plurality of processing devices 6 may be connected to one arithmetic device 8. The arithmetic device 8 may also transmit data related to the calculated thickness to another device connected wirelessly or by a wire.

[0085] The probe 1 is not limited to the configuration described above. For example, the probe 1 includes two sets of excitation coils 11 and detection coils 12, but the number of sets of excitation coils 11 and detection coils 12 may be one, or three or more. The excitation coils 11 and detection coils 12 do not have to be arranged so that their respective axes are aligned. When the excitation coils 11 and detection coils 12 are arranged so that their respective axes are aligned, the excitation coil 11 may be arranged closer to the object 9 than the detection coil 12. Furthermore, the probe 1 does not have to include a core 15.

[0086] Furthermore, the thickness measurement using the thickness measurement device 10 is merely an example. There are various thickness measurement methods using PEC, so any desired measurement technique can be adopted.

[0087] The receiving unit 62 is merely an example. For example, as shown in FIG. 10, the receiving amplifier 51 of the receiving unit 262 may be a single-power operational amplifier. In the example of FIG. 10, the detection coil 12 includes only one first detection coil 13. The protection circuit 52 includes only one first protection circuit 52A. The inverting input terminal (-) of the receiving amplifier 51 is grounded. In addition, the second end 13b of the first detection coil 13 is also grounded. That is, the processing device 6 includes a first detection coil 13 (detection coil) that detects an eddy current induced in the object 9 by the excitation coil 11, a receiving amplifier 51 having a non-inverting input terminal (+) connected to a first end 13a of the first detection coil 13 and an inverting input terminal (-) connected to a second end 13b of the first detection coil 13, and a first protection circuit 52A (protection circuit) connected in parallel with the first detection coil 13 between the first end 13a and the second end 13b of the first detection coil 13, the first protection circuit 52A including a first resistor element 53a (resistance element) and a first diode 54a (diode) connected in series. Even with this configuration, it is possible to reduce the input voltage to the receiving amplifier 51, shorten the measurement start time T, and reduce the temperature dependence of the output voltage Vo of the receiving amplifier 51.

[0088] The flowcharts are merely examples. Steps in the flowcharts may be changed, replaced, added, omitted, etc. as appropriate. The order of steps in the flowcharts may also be changed, and serial processing may be performed in parallel.

[0089] The functions performed by the components described herein may be implemented in circuitry or processing circuitry, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), a CPU (a Central Processing Unit), conventional circuits, and / or combinations thereof, programmed to perform the described functions. A processor includes transistors and other circuits and is considered a circuit or processing circuit. A processor may also be a programmable processor that executes a program stored in a memory.

[0090] In this specification, a circuit, unit, or means is hardware that is programmed to realize or performs the described functions, which may be any hardware disclosed herein or any hardware known to be programmed to realize or perform the described functions.

[0091] If the hardware is a processor considered to be a type of circuitry, the circuit, means, or unit is a combination of hardware and software used to configure the hardware and / or processor. [Explanation of symbols]

[0092] 10 Thickness measuring device 11 Excitation coil 12 Detection coil 13 First detection coil 13a 1st end 13b 2nd end 14 Second detection coil 14a 1st end 14b 2nd end 51 Receiving amplifier 52 Protection circuit 52A 1st protection circuit 52B 2nd protection circuit 53a first resistor element 53b second resistor element 54a First diode 54b Second diode 84 Thickness extraction section 9 Objects

Claims

1. a first detection coil and a second detection coil connected in series to detect an eddy current induced in the object by the excitation coil; a receiving amplifier to which the first detection coil and the second detection coil are connected; a detection device including a protection circuit connected in parallel to the first detection coil and the second detection coil; a thickness derivation unit that determines the thickness of the object based on the output voltage of the receiving amplifier; a first end of the first detection coil is connected to a non-inverting input terminal of the receiving amplifier; a second end of the first detection coil connected to a first end of the second detection coil; a second end of the second detection coil is connected to an inverting input terminal of the receiving amplifier; The thickness measurement device includes: a first protection circuit including a first resistor element and a first diode connected in series and connected in parallel with the first detector coil between the first end and the second end of the first detector coil; and a second protection circuit including a second resistor element and a second diode connected in series and connected in parallel with the second detector coil between the first end and the second end of the second detector coil.

2. 2. The thickness measuring device according to claim 1, The second end of the first detection coil and the first end of the second detection coil are connected to a common line.

3. a detection coil for detecting an eddy current induced in the object by the excitation coil; a receiving amplifier having a non-inverting input terminal connected to a first end of the detection coil and an inverting input terminal connected to a second end of the detection coil; a detection device including a protection circuit connected in parallel with the detection coil between the first end and the second end of the detection coil; a thickness derivation unit that determines the thickness of the object based on the output voltage of the receiving amplifier; The protection circuit includes a resistor and a diode connected in series.

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