JP7794432B2 - Thickness Measuring Device - Google Patents

Description

The technology disclosed herein relates to detection devices and thickness measurement devices.

Devices that use a detection coil to detect eddy currents generated in an object have been known for some time. 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 coil detects the eddy currents induced in the object.

In this device, the induced voltage detected by the coil section is input to the detection section, which detects changes in the output waveform.

International Publication No. 2013/047521

When 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 to which the detection coil is connected.

It is possible to install a protection circuit to protect the receiving amplifier from large voltages. However, this protection circuit may affect measurements using eddy currents.

The technology disclosed here was developed in light of these issues, and its purpose is to protect the receiving amplifier while reducing the impact on measurements using eddy currents.

The detection device disclosed herein comprises a first detection coil and a second detection coil connected in series to detect eddy currents 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 the non-inverting input terminal of the receiving amplifier, a second end of the first detection coil is connected to the first end of the second detection coil, and a second end of the second detection coil is connected to the inverting input terminal of the receiving amplifier, and the protection circuit includes a first resistance element and a first diode connected in series and is connected in parallel with the first detection coil between the first and second ends of the first detection coil, and a second protection circuit includes a second resistance element and a second diode connected in series and is connected in parallel with the second detection coil between the first and second ends of the second detection coil.

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 resistor and a diode connected in series.

The thickness measurement device disclosed herein further includes a thickness derivation unit that determines the thickness of the object based on the detection device and the output voltage of the receiving amplifier.

This detection device can protect the receiving amplifier while reducing the impact on measurements using eddy currents.

The thickness measurement device described above can protect the receiving amplifier while reducing the impact on measurements using eddy currents.

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

An exemplary embodiment is described in detail below with reference to the drawings.

An exemplary embodiment will now be described in detail with reference to the drawings. Figure 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 using pulsed eddy current (PEC). The thickness measurement device 10 includes a processing device 6 that controls the probe 1 and a computing device 8 that calculates the thickness of the object 9. For example, the object 9 is a metal pipe through which steam or condensate flows. The pipe is formed in a cylindrical shape.

Probe 1 is used to generate eddy currents in object 9 and detect the generated eddy currents. Probe 1 is a non-contact probe and is placed in close proximity to object 9. Note that "non-contact" means that it can be used without contact, and does not exclude use in a contact state. Probe 1 is placed so as to face the surface of object 9. For example, probe 1 is placed on object 9 via a thermally insulating spacer (not shown).

The probe 1 generates eddy currents in the object 9 by forming a fluctuating 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 from 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 detection coil 12.

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 14.

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 positioned 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. Note that 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."

The probe 1 may further include a core 15 inserted into the excitation coil 11 and detection coil 12. The core 15 is generally U-shaped overall. More specifically, the core 15 is formed by stacking multiple generally U-shaped thin plates made of permalloy. The linear portion at one end of the core 15 is inserted into the first excitation coil 11A and the first detection coil 13. The 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.

When a current is applied to the excitation coil 11, it generates 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 generate magnetic fields in opposite directions in the direction of the axis, i.e., magnetic fields in opposite directions relative to the object 9. As a result, a magnetic field is generated in the core 15 along the longitudinal direction of the core 15. That is, when one end of the core 15 is a north pole, the other end of the core 15 is a south pole. Conversely, when one end of the core 15 is a south pole, the other end of the core 15 is a north 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-like shape, and then 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, generating eddy currents in the object 9.

Meanwhile, eddy currents generated in the portion of the object 9 near the detection coil 12 form a magnetic flux that penetrates 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 current in the object 9. In other words, detecting induced electromotive force with the detection coil 12 is also referred to as detecting eddy currents. Note that the magnetic flux penetrating the first detection coil 13 and the magnetic flux penetrating the second detection coil 14 are in opposite directions relative to the object 9.

The processing device 6 uses the probe 1 to generate eddy currents in the object 9 and detect the generated eddy currents. 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 currents detected by the processing device 6 (the time it takes for the eddy currents to rapidly decay, as will be described in more detail below).

The processing device 6 is placed, for example, close to the object 9. For example, the processing device 6 is installed on the object 9 via a spacer. The processing device 6 has a transmitter 61, a receiver 62, a communication unit 65, a control unit 66, and a memory unit 67.

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 commands 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.

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

The communication unit 65 communicates wirelessly with external devices. For example, the communication unit 65 transmits the voltage signal (i.e., the detection signal) detected by the receiving unit 62 to the calculation device 8.

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 by a processor such as a CPU (Central Processing Unit). The control unit 66 may also be formed by an MCU (Micro Controller Unit), MPU (Micro Processor Unit), FPGA (Field Programmable Gate Array), PLC (Programmable Logic Controller), system LSI, etc.

For example, the control unit 66 causes the transmission 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 reception unit 62. The control unit 66 stores the detection signal from the reception 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.

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

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

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 from the transmission amplifier 61b to the excitation coil 11.

The detection unit 72 detects a voltage signal corresponding to 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 the time-dependent change (i.e., 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 memory 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 memory unit 67 will also be simply referred to as the "eddy current."

The computing device 8 is formed by a computer or a computer network (so-called cloud). As shown in FIG. 1, the computing device 8 has a communication unit 81, a control unit 82, and a memory unit 83.

The communication unit 81 communicates wirelessly with external devices. For example, the communication unit 81 receives signals from the processing device 6.

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

The memory unit 83 stores programs executed by the control unit 82 and various data. For example, the memory unit 83 stores a control program. The memory unit 83 is formed of 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. Specifically, the memory unit 83 stores eddy currents of the object 9 acquired by the processing device 6.

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

The thickness derivation unit 84 determines the thickness of the object 9 based on the duration of the eddy currents in the object 9. As will be described in more detail, the eddy currents induced in the object 9 by the excitation coil 11 penetrate from the front surface of the object 9 (the surface facing the probe 1) to the back surface, and rapidly attenuate upon reaching the back surface. The duration of the eddy currents in the object 9 is the time from when the eddy currents are induced in the object 9 to when they rapidly attenuate. The duration of the eddy currents in the object 9 is correlated with the thickness of the object 9.

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.

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

The eddy current decays as it penetrates into the object 9. The eddy current decays gradually from the 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 changes similar to those of the eddy current. In other words, the transient changes in the voltage signal V(t) correspond to the transient changes 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 equation (1) below.

where A is the amplification factor of the receiving amplifier 51, n is a constant related to the degree of attenuation of the voltage signal V(t), and −n represents the slope of the voltage signal V(t) on a log-log graph.

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

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 .

In other words, the duration τ varies 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 varies depending on the thickness d of the object 9. Furthermore, even if the duration τ and the thickness d vary, τ/ ^d2 remains 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.

For example, comparing voltage signals V0(t) and V1(t) in Figure 4, the voltage signal V0(t) of object 9 with thickness d0 continues for duration τ0. As the thickness d of object 9 decreases from d0 to d1, the duration τ decreases from τ0 to τ1. Note that the change in the linear portion of voltage signal V(t) on the log-log graph does not depend on thickness d, as can be seen from equation (1), and therefore is substantially the same for voltage signals V0(t) and V1(t). The thickness d1 can be determined by substituting thickness d0 and durations τ0 and τ1 into equation (3).

This type of thickness measurement will be explained in more detail using a flowchart. Figure 5 is a flowchart for thickness measurement.

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 the period during which the thickness of the object 9 is measured. If the measurement period has not arrived, the thickness derivation unit 84 repeats the determination in step S101 and waits for the measurement period to arrive.

When the measurement cycle arrives, the thickness derivation unit 84 outputs a command to the processing device 6, causing 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 generates a magnetic field in the axial direction by applying the excitation current. One excitation coil 11 and the other excitation coil 11 generate 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.

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

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 memory unit 83.

Next, in step S104, the thickness derivation unit 84 calculates the duration τ of the eddy current stored in the memory unit 83. Furthermore, in step S105, the thickness derivation unit 84 substitutes the duration τ into equation (3) to calculate the thickness d of the object 9.

Then, in step S106, the thickness derivation unit 84 determines whether to end the thickness measurement. For example, the thickness derivation unit 84 determines whether a command to end the thickness measurement has been input. For example, the user operates the calculation 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 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 calculated based on the measurement data. In other words, the calculation device 8 repeats the acquisition of measurement data and the derivation of the thickness d of the object 9 for each measurement cycle.

If an end command is input in step S106, thickness measurement ends.

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

Next, we will explain the configuration of the receiving unit 62 in more detail. Figure 6 is a circuit diagram of the receiving unit 62.

The receiver 62 has a receiver amplifier 51 and a protection circuit 52. The receiver 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 receiver amplifier 51.

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

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

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

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.

The protection circuit 52 has a first protection circuit 52A and a second protection circuit 52B.

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, i.e., so that the first diode 54a is forward biased.

The second protection circuit 52B is connected in parallel with the second detection coil 14 between the first end 14a and 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, i.e., so that the second diode 54b is forward biased.

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.

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

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. The excitation unit 71 then stops applying the excitation current to the excitation coil 11, causing the detection unit 72 to begin detecting the eddy currents. 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.

As shown in Figure 8, an induced voltage of a large absolute value is generated in the first detector coil 13 and the second detector coil 14 immediately after the excitation current is stopped. 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.

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 Figure 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 voltage Va in the first detection coil 13 and the induced voltage 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.

On the other hand, as shown in Figure 9, the output voltage Vo of the receiving amplifier 51 rises after the excitation current is stopped and remains 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 has decayed to the 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."

Here, the current flowing through the first protection circuit 52A and the second protection circuit 52B is fed back to the first detection coil 13 and the second detection coil 14. If the feedback current to the first detection coil 13 and the second detection coil 14 is 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 shown 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 current 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.

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. As a result, 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 determined. If the thickness of the object 9 is thin, the duration τ will be short. If the duration τ is shorter than the measurement start time T, the duration τ cannot be determined, and as a result, the thickness of the object 9 cannot be determined. In other words, shortening the measurement start time T can accelerate the time at which the voltage signal corresponding to the eddy current can be properly observed, making it possible to determine the thickness of the thin object 9.

One possible method for reducing the input voltage to the receiving amplifier 51 is to connect a diode between the first end 13a of the first detection coil 13 and the positive power supply, and a diode between the second end 14b of the second detection coil 14 and the negative power supply, instead of the first protection circuit 52A and the second protection circuit 52B. When a large induced electromotive force is generated in the first detection coil 13 and the second detection coil 14, 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, reducing the input voltage to the receiving amplifier 51. However, in this configuration, the diode leakage current is superimposed on the induced voltage signal, which increases the temperature dependence of the output voltage Vo of the receiving amplifier 51.

In contrast, a configuration that includes a first protection circuit 52A and a second protection circuit 52B reduces the effects of leakage current and reduces the temperature dependence of the output voltage Vo of the receiving amplifier 51.

As a result, the receiving unit 62 can 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.

As described above, the processing device 6 (detection device) includes a first detection coil 13 and a second detection coil 14 connected in series to detect eddy currents induced in the object 9 by the excitation coil 11, 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 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 includes a first protection circuit 52A that includes a first resistor element 53a and a first diode 54a connected in series and is connected in parallel with the first detection coil 13 between the first end 13a and second end 13b of the first detection coil 13, and a second protection circuit 52B that includes a second resistor element 53b and a second diode 54b connected in series and is connected in parallel with the second detection coil 14 between the first end 14a and second end 14b of the second detection coil 14.

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, the duration of the eddy current.

With these configurations, when a large induced electromotive force is generated in the first detection coil 13 and the second detection 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 end 13a of the first detection coil 13 and the positive power supply and a diode is connected between the second end 14b of the second detection 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 detection coil 13 and the second detection coil 14, thereby shortening the measurement start time T. As a result, the receiving amplifier 51 can be protected while reducing the impact on measurements using eddy currents, i.e., on thickness measurements of the object 9.

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

With 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.

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.

The above embodiment may also be configured as follows:

For example, the configuration of the thickness measurement device 10 is merely one example. The processing device 6 and the calculation device 8 may be configured as an integrated unit. That is, one device may have the functions of the processing device 6 and the calculation device 8. The processing device 6 and the calculation device 8 may also be connected by a wire. Multiple processing devices 6 may also be connected to one calculation device 8. The calculation device 8 may also transmit data related to the calculated thickness to another device connected wirelessly or by wire.

The probe 1 is not limited to the configuration described above. For example, while the probe 1 includes two sets of excitation coils 11 and detection coils 12, the number of excitation coils 11 and detection coils 12 may be one set, or three or more sets. The excitation coils 11 and detection coils 12 do not have to be arranged so that their respective axes are aligned. If 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.

Furthermore, thickness measurement using the thickness measurement device 10 is merely one example. There are various thickness measurement methods using PEC, so any measurement method can be used.

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. The second end 13b of the first detection coil 13 is also grounded. Specifically, the processing device 6 includes a first detection coil 13 (detection coil) that detects eddy currents 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 includes a first resistor 53a (resistive element) and a first diode 54a (diode) connected in series. Even with this configuration, the input voltage to the receiving amplifier 51 can be reduced, the measurement start time T can be shortened, and the temperature dependence of the output voltage Vo of the receiving amplifier 51 can be reduced.

The flowchart is merely an example. Steps in the flowchart may be modified, replaced, added, omitted, etc. as appropriate. The order of steps in the flowchart may also be changed, and serial processing may be performed in parallel.

The functions provided 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), CPUs (Central Processing Units), conventional circuits, and/or combinations thereof, programmed to provide 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.

In this specification, a circuit, unit, or means is hardware that is programmed to realize or performs the described functions. The hardware may be any hardware disclosed in this specification or any hardware known to be programmed to realize or perform the described functions.

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

10 Thickness measuring device 11 Excitation coil 12 Detection coil 13 First detection coil 13a First end 13b Second end 14 Second detection coil 14a First end 14b Second end 51 Receiving amplifier 52 Protection circuit 52A First protection circuit 52B Second protection circuit 53a First resistance element 53b Second resistance element 54a First diode 54b Second diode 84 Thickness derivation unit 9 Object

Claims (3)

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. 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. 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 .