Grinding burn depth estimation apparatus

Abstract

To provide a grinding burn depth estimation apparatus capable of accurately estimating the grinding burn depth of a workpiece.SOLUTION: In a grinding burn depth estimation apparatus 1, an eddy current sensor 20 outputs an output signal according to a magnetic field of eddy currents generated by inducing the eddy currents in a workpiece W by an excitation current. A first feature information acquisition unit 41 acquires first feature information by associating at least one of a value on the imaginary axis and a value on the real axis in a case where the AC output signal from the eddy current sensor 20 is represented on a complex plane in the orthogonal coordinate format with time information. An inflection point calculation unit 44 calculates an inflection point of a trajectory of an output signal P represented on the complex plane from the first feature information. An inflection point time extraction unit 45 extracts the time of the inflection point. A rotation count acquisition unit 46 acquires the number of rotations of the workpiece W from a rotation count acquisition start time to the inflection point time. A grinding burn depth estimation unit 49 estimates the grinding burn depth of a processing object part on the basis of the number of rotations acquired by the rotation count acquisition unit and the cutting amount per one rotation of the workpiece W.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a grinding burn depth estimating device. [Background technology]

[0002] Conventionally, when grinding a workpiece, the temperature of the workpiece at the processed portion tends to become high, and grinding burns may occur on the surface of the workpiece depending on the processing conditions. Grinding burns are undesirable because they may cause a decrease in the mechanical strength of the workpiece. As a method for detecting such grinding burns, the presence or absence of grinding burns is evaluated by utilizing the fact that the change in the magnetic field caused by eddy currents generated in the workpiece by an excitation current differs depending on the presence or absence of grinding burns.

[0003] For example, in the configuration disclosed in Patent Document 1, the presence or absence of grinding burns is detected with high accuracy by comparing the output signals of eddy currents generated by two types of excitation currents with different frequencies using an eddy current sensor. In addition, in the configuration disclosed in Patent Document 2, the output signals of eddy currents generated by two types of excitation currents with different frequencies using an eddy current sensor are corrected to remove the components of residual magnetic flux density remaining in the workpiece, thereby improving the accuracy of determining the presence or absence of grinding burns. In addition to these methods, the grinding burn condition on the workpiece surface is also evaluated by etching the workpiece after grinding with a corrosive solution such as nital and observing the surface condition. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2011-106932 A [Patent Document 2] JP 2018-189603 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, the configurations disclosed in Patent Documents 1 and 2 and the evaluation by etching are used to evaluate the state of grinding burns after the grinding process is completed, but the depth of grinding burns cannot be estimated with high accuracy.

[0006] The present invention provides a grinding burn depth estimating device that can estimate the grinding burn depth of a workpiece with high accuracy. [Means for solving the problem]

[0007] One aspect of the present invention is a grinding burn depth estimation device that estimates the grinding burn depth of a processed portion of a workpiece that is machined into a final target shape by grinding a surface of the workpiece using a cylindrical grinding machine, an eddy current sensor disposed opposite to a portion to be machined of the workpiece, which uses an excitation current to induce an eddy current inside the workpiece and outputs an AC output signal corresponding to a magnetic field generated by the eddy current; a first characteristic information acquisition unit that acquires first characteristic information obtained by linking at least one of an imaginary axis value and a real axis value when an output signal output by the eddy current sensor is represented on a complex plane in a Cartesian coordinate system with time information that is a time when the workpiece is ground; an inflection point calculation unit that calculates an inflection point in a locus of the output signal represented on the complex plane from the first feature information; an inflection point time extraction unit that extracts a time at which the inflection point occurs as an inflection point time; a rotation count acquisition unit that acquires the number of rotations of the workpiece from a rotation count acquisition start time to the inflection point time; a grinding burn depth estimation unit that estimates a grinding burn depth of the workpiece based on the number of rotations acquired by the number of rotations acquisition unit and a cutting depth per rotation of the workpiece; The present invention relates to a grinding burn depth estimation device comprising: Effect of the Invention

[0008] According to the above aspect, an eddy current generated inside a workpiece being machined by an excitation current is detected by an eddy current sensor, and first feature information is obtained in which at least one of the imaginary axis value and the real axis value when the output signal output by the eddy current sensor is represented on a complex plane in a Cartesian coordinate format and time information is linked. Then, from the first feature information, an inflection point in the trajectory of the output signal represented on the complex plane is calculated, and the time at which the inflection point is presented is extracted as the inflection point time. The inventors of the present application have newly found that the inflection point time corresponds to the time when a work-affected layer (also called a softened layer) indicating grinding burn generated on the workpiece by grinding is removed by the subsequent grinding process. Therefore, by acquiring the cutting depth up to the inflection point time, the grinding burn depth can be estimated. And, the cutting depth up to the inflection point time after the occurrence of grinding burn is calculated by the product of the number of rotations of the workpiece on the cylindrical grinding machine and the cutting depth per rotation, so there is no need to use data from a sizing device. In this way, by using the time of the inflection point in the trajectory of the output signal of the eddy current sensor displayed on the complex plane, the number of rotations of the workpiece, and the cutting depth, it is possible to estimate the grinding burn depth with high accuracy.

[0009] As described above, according to the above aspect, it is possible to provide a grinding burn depth estimating device that can estimate the grinding burn depth of a workpiece with high accuracy. [Brief description of the drawings]

[0010] [Figure 1] 1 is a conceptual diagram showing a configuration of a grinding system including a grinding burn depth estimation device according to a first embodiment. [Diagram 2] FIG. 2 is a functional block diagram showing the configuration of a grinding burn depth estimation device in the first embodiment. [Diagram 3] FIG. 2 is a diagram for explaining an outline of synchronous detection in the eddy current sensor in the first embodiment. [Figure 4] FIG. 1A is a diagram showing the relationship between the output signal of an eddy current sensor serving as the first inflection point determination criterion and time in embodiment 1; and FIG. 1B is a diagram showing the output signal of an eddy current sensor serving as the first inflection point determination criterion on a complex plane. [Diagram 5]FIG. 2 is a flowchart of a grinding process in the first embodiment. [Figure 6] FIG. 4 is a diagram showing a change in workpiece radius over time in the first embodiment. [Figure 7] FIG. 4 is a flowchart of a first grinding burn state estimation process in the first embodiment. [Figure 8] 4 is a diagram showing an output signal of an eddy current sensor on a complex plane when no inflection point is drawn in the first embodiment. FIG. [Figure 9] FIG. 6 is a flowchart of a second grinding burn state estimation process in the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] (Embodiment 1) The grinding burn depth estimation device 1 of the first embodiment evaluates the grinding burn state of the workpiece W ground by a processing device. Each component will be described in detail below.

[0012] 1. Processing equipment configuration In the first embodiment, a processing device includes a cylindrical grinding machine 2 shown in Fig. 1. The cylindrical grinding machine 2 rotates a workpiece W about a center line C, rotates a grinding wheel 16 as a tool that is a rotating body, and moves the grinding wheel 16 relatively close to the workpiece W in a direction intersecting the axis of the workpiece W, thereby grinding the outer peripheral surface or inner peripheral surface of the workpiece W. The cylindrical grinding machine 2 can be a table traverse type grinding machine, a wheel head traverse type grinding machine, or the like.

[0013] In this embodiment, as shown in Fig. 1, the workpiece W is, for example, a member formed in a shaft shape, and the outer circumferential surface of the workpiece W is the part to be processed. However, the shape of the workpiece W is not limited to a shaft shape, and it can be any shape, such as a cylindrical shape having an inner circumferential surface. When the workpiece W is cylindrical, the inner circumferential surface of the workpiece W can be the part to be processed.

[0014] In this embodiment, the workpiece W is substantially rod-shaped and is supported at both ends by workpiece support members. However, the workpiece W shown in Fig. 1 is only an example, and the cylindrical grinding machine 2 can grind workpieces having various shapes.

[0015] The processing unit 3 includes a grinding burn depth estimation device 1 and a control device 31 that controls the cylindrical grinding machine 2. As will be described later, the grinding burn depth estimation device 1 evaluates the state of grinding burn caused by grinding on the workpiece W, and adjusts the grinding conditions for the workpiece W based on the evaluation result. The control device 31 controls the cylindrical grinding machine 2, thereby controlling the grinding process.

[0016] The grinding burn depth estimation device 1 can function as a simulation device independent of the cylindrical grinding machine 2 and the control device 31, or can function as a simulation device that operates in conjunction with the cylindrical grinding machine 2 and the control device 31. In the former case, the grinding burn depth estimation device 1 can determine optimal grinding conditions, for example, without grinding the actual workpiece W. In the latter case, the grinding burn depth estimation device 1 can determine the presence or absence of grinding burns, adjust grinding conditions, and operate to affect various controls, for example, by processing in parallel with the grinding of the workpiece W by the cylindrical grinding machine 2. The grinding burn depth estimation device 1 can also be an embedded system of the cylindrical grinding machine 2 and the control device 31.

[0017] 2. Configuration of Cylindrical Grinding Machine 2 and Control Device 31 The configuration of the cylindrical grinding machine 2 will be described with reference to Fig. 1. In this embodiment 1, a wheel head traverse type cylindrical grinding machine is taken as an example of the cylindrical grinding machine 2. However, a table traverse type can also be applied to the cylindrical grinding machine 2. The cylindrical grinding machine 2 mainly includes a bed 11, a headstock 12, a tailstock 13, a traverse base 14, a wheel head 15, a grinding wheel 16, a grinding wheel dressing device 18, a coolant device 19, and an eddy current sensor 20.

[0018] The bed 11 is fixed on a mounting surface. The headstock 12 is provided on the upper surface of the bed 11, on the near side in the X-axis direction (lower side in FIG. 1) and on one end side in the Z-axis direction (left side in FIG. 1). The headstock 12 supports the workpiece W rotatably around the Z-axis centered on the center line C of the workpiece W. The workpiece W is rotated by driving a motor 12a provided on the headstock 12. The tailstock 13 is provided on the upper surface of the bed 11 at a position facing the headstock 12 in the Z-axis direction, that is, on the near side in the X-axis direction (lower side in FIG. 1) and on the other end side in the Z-axis direction (right side in FIG. 1). In other words, the headstock 12 and the tailstock 13 rotatably support both ends of the workpiece W.

[0019] The traverse base 14 is provided on the upper surface of the bed 11 so as to be movable in the Z-axis direction. The traverse base 14 is moved by driving a motor 14a provided on the bed 11. The grinding wheel head 15 is provided on the upper surface of the traverse base 14 so as to be movable in the X-axis direction. The grinding wheel head 15 is moved by driving a motor 15a provided on the traverse base 14. The grinding wheel 16 is rotatably supported by the grinding wheel head 15. The grinding wheel 16 is rotated by driving a motor 16a provided on the grinding wheel head 15. The grinding wheel 16 is configured by fixing a plurality of abrasive grains with a bond material.

[0020] The eddy current sensor 20 is provided so that its sensor head faces the machined portion of the workpiece W. The eddy current sensor 20 constitutes a part of a grinding burn depth estimation device 1 described later. In this embodiment 1, the sensor head of the eddy current sensor 20 is located on the opposite side of the grinding wheel 16 with respect to the workpiece W. In FIG. 1, the reference numeral 20 indicates the position of the sensor head, and the sensor main body is not shown.

[0021] The eddy current sensor 20 induces an eddy current inside the workpiece W by an excitation current, and outputs an AC signal corresponding to a magnetic field generated by the eddy current. The eddy current sensor 20 may be configured to be able to change the phase of the output signal. In this embodiment, as shown in FIG. 2, the eddy current sensor 20 has a coil as an oscillator 21, and by supplying an excitation current to the coil, an output signal is applied to a probe 22 provided at the tip of a sensor head of the eddy current sensor 20, thereby applying a magnetic field to the workpiece W and inducing an eddy current inside the workpiece W. The eddy current sensor 20 amplifies the change in impedance acquired via the probe 22 in an amplifier 23 and inputs it to a detector 24, extracts only the frequency component of the oscillator 21, and processes the signal in a signal processor 25.

[0022] The detection unit 24 detects the signal by synchronous detection. As shown in Fig. 3, the input signal v(t) to the detection unit 24 is a signal obtained by amplifying the output of the probe 22 by the amplifier unit 23, and v(t) is defined by the following equation (1). Note that t is time, A is amplitude, f is frequency of the oscillator 21 inside the eddy current sensor 20, and θ is phase difference with respect to the oscillator 21 inside the eddy current sensor 20.

[0023]

number

[0024] Then, in detection section 24, v(t) that has passed through amplification section 23 is multiplied by a sine wave sin2πft of the same frequency as frequency f of oscillation section 21. This is called x(t). Similarly, the result of multiplying by cos2πft is called y(t). x(t) and y(t) can be expressed as the following equations (2) and (3).

[0025]

number

[0026]

number

[0027] Next, x(t) and y(t) are passed through a low-pass filter having a sufficiently low cut-off frequency fc. Generally, the relationship fc << f holds. The first terms in equations (2) and (3) are alternating currents because they contain t, while the second terms are direct currents because they do not contain t. Therefore, equations (2) and (3) after passing through the low-pass filter become the real-axis signal X and the imaginary-axis signal Y shown in the following equations (4) and (5), respectively.

[0028]

Number

[0029]

Number

[0030] Then, A and θ of v(t) can be obtained as shown in the following equations (6) and (7) from equations (4) and (5).

[0031]

Number

[0032]

Number

[0033] In the signal processing unit 25, operations such as gain and phase are performed on the real-axis signal X and the imaginary-axis signal Y of the detection unit 24. Therefore, the output signal of the eddy current sensor 20 is output through the signal processing unit 25 from the detection unit 24, and the frequency characteristics of the eddy current sensor 20 are the product of the frequency characteristics of the detection unit 24 and the signal processing unit 25. Generally, the frequency characteristics of the signal processing unit 25 are equal to or lower than the frequency characteristics of the detection unit 24. In the output signal output from the signal processing unit 25, X represents the real-axis signal and Y represents the imaginary-axis signal. The real-axis signal X represents the real-axis value in the output signal P, and the imaginary-axis signal Y represents the imaginary-axis value in the output signal P.

[0034] In the eddy current sensor 20, the coil is configured to be supplied with a plurality of excitation currents with different frequencies. In the first embodiment, the frequency of the excitation current can be set by a frequency setting unit 52 described later. Since the penetration depth of the eddy current varies depending on the frequency of the excitation current, the frequency of the excitation current can be set according to the target penetration depth of the eddy current. The target penetration depth of the eddy current is set to match the surface layer where grinding burn may occur, and can be, for example, 1 to 100 μm from the processed surface, preferably 1 to 50 μm, and more preferably 10 to 30 μm. When the penetration depth is shallow, the sensitivity of the eddy current sensor becomes too high, the SN ratio decreases, and the detection accuracy decreases. On the other hand, when the penetration depth is deep, the detection level of the magnetic property change characteristic decreases in a shallow area from the processed surface, and the detection accuracy decreases.

[0035] The frequency of the excitation current according to the above-mentioned penetration depth can be set in a frequency band of 20 kHz to 200 MHz, preferably 100 kHz to 200 MHz, and more preferably 250 to 2500 kHz. In this embodiment, the frequency of the excitation current is set to 250 kHz so that the penetration depth of the eddy current is 30 μm.

[0036] The grinding wheel dressing device 18 shown in Fig. 1 dresses the shape of the grinding wheel 16. The grinding wheel dressing device 18 is a device that performs truing of the grinding wheel 16. The grinding wheel dressing device 18 may be a device that performs dressing of the grinding wheel 16 in addition to or instead of truing. Furthermore, the grinding wheel dressing device 18 also has a function of measuring the dimension (diameter) of the grinding wheel 16.

[0037] Truing here refers to a shaping operation, such as shaping the grinding wheel 16 to fit the shape of the workpiece W when the grinding wheel 16 has worn down due to grinding, or removing runout of the grinding wheel 16 due to one-sided wear. Dressing refers to a dressing (sharpening) operation, such as adjusting the amount of protrusion of the abrasive grains and creating the cutting edges of the abrasive grains. Dressing is an operation to correct dulling, clogging, missing grains, etc., and is usually performed after truing.

[0038] The coolant device 19 supplies coolant from a coolant nozzle to the point where the workpiece W is ground by the grinding wheel 16. The coolant device 19 cools the collected coolant to a predetermined temperature and supplies it to the grinding point again. The coolant device 19 is capable of adjusting the flow rate and supply timing of the coolant. In FIG. 1, reference numeral 19 indicates the position of the coolant nozzle. Although not shown, a temperature sensor for acquiring the temperature of the collected coolant may be provided as a detector.

[0039] The control device 31 grinds the workpiece W by controlling the driving of the grinding wheel 16, the coolant device 19, etc. in the cylindrical grinding machine 2 based on an NC program generated based on operation command data such as the shape of the workpiece W, grinding conditions, the shape of the grinding wheel 16, and coolant flow rate or supply timing information. In particular, the control device 31 grinds the workpiece W until it reaches a finished shape (target shape). Furthermore, the control device 31 performs correction (truing and dressing) of the grinding wheel 16 by controlling the grinding wheel correction device 18, etc. at the timing to correct the grinding wheel 16.

[0040] 3. Configuration of Grinding Burn Depth Estimation Device 1 2, the grinding burn depth estimation device 1 includes an eddy current sensor 20, a signal acquisition unit 40, a first characteristic information acquisition unit 41, a time storage unit 42, an inflection point judgment criterion storage unit 43, an inflection point calculation unit 44, an inflection point time extraction unit 45, a rotation count acquisition unit 46, a rotation angle acquisition unit 47, an average data minimum value time acquisition unit 48, a grinding burn depth estimation unit 49, a grinding condition adjustment unit 50, a margin calculation unit 51, and a frequency setting unit 52, which are configured by a storage device or a calculation device. The grinding burn depth estimation device 1 also includes a grinding burn depth display unit 53 configured by a predetermined display device.

[0041] The signal acquiring unit 40 acquires the output signal P output from the eddy current sensor 20. The output signal P has a value on the imaginary axis Y and a value on the real axis X when expressed on a complex plane in a Cartesian coordinate format, and can be expressed on the complex plane as shown in FIG.

[0042] The first characteristic information acquisition unit 41 acquires the first characteristic information. The first characteristic information is information in which at least one of the imaginary axis value and the real axis value when the output signal P acquired by the signal acquisition unit 40 is represented on a complex plane in a Cartesian coordinate format is linked to time information that is the time when the workpiece W is ground. In this embodiment, both the imaginary axis value and the real axis value of the output signal P represented on the complex plane shown in Fig. 4(b) are linked to the elapsed time from the start of the grinding process measured by the time memory unit 42 described later.

[0043] The time memory unit 42 measures the elapsed time from the start time T0 of the grinding process. In this embodiment, the grinding process S1 of the workpiece W includes a rough grinding process S11, a fine grinding process S12, a fine grinding process S13, and a spark-out process S14, as shown in Fig. 5. Each process will be described later.

[0044] The inflection point judgment criterion storage unit 43 shown in Fig. 2 stores in advance an inflection point judgment criterion for judging whether or not the output signal shown in the complex plane reaches an inflection point described later for each frequency of the excitation current. In this embodiment, the inflection point judgment criterion can be created based on a correspondence relationship between the frequency of the excitation current, the first characteristic information acquired by applying an excitation current having the frequency to a workpiece for creating an inflection point judgment criterion during machining, and the grinding burn state of the workpiece for creating an inflection point judgment criterion. The workpiece for creating an inflection point judgment criterion has the same shape and is made of the same material as the workpiece W to be evaluated shown in Fig. 1.

[0045] As the inflection point judgment criteria, for example, the correspondence relationship between the first feature information and the grinding burn state when the frequency of the excitation current is 250 kHz is shown in Figures 4(a) and (b). When the frequency of the excitation current is different, the correspondence relationship between the first feature information and the grinding burn state changes accordingly. For example, although not shown, the correspondence relationship shown on the complex plane is expressed as a shape rotated by -90 degrees for the case of 250 kHz for 1000 kHz.

[0046] Next, the locus of the output signal P on the complex plane will be described with reference to Fig. 4(b). On the complex plane shown in Fig. 4(b), the output signal P output from the eddy current sensor 20 is plotted at the position indicated by reference symbol A1 at the start time of the grinding process. Thereafter, in the rough grinding process S11, when grinding burn occurs on the workpiece W, the magnetic characteristics change accordingly, so that the output signal P decreases in the real axis value on the complex plane and increases slightly in the imaginary axis value over time. Thereafter, the real axis value decreases and the imaginary axis value decreases, and then the real axis value increases and the imaginary axis value decreases, until the position indicated by reference symbol A2 is reached and the rough grinding process S11 is completed.

[0047] After that, when the precision grinding step S12 starts, the grinding burn (process-affected layer, softened layer) that occurred on the workpiece W is gradually removed, and as a result, the output signal P in the complex plane has a smaller real axis value and a gradually larger imaginary axis value. That is, near the start point B1 of the precision grinding step S12, the vector direction in the imaginary axis direction in the locus of the output signal P in the complex plane is upward. After that, when grinding further progresses and the position indicated by the symbol B2 is reached, the real axis value increases, but the imaginary axis value continues to gradually increase. Then, at the position indicated by the symbol B3, the real axis value continues to increase, but the imaginary axis value decreases. That is, at the symbol B3, the output signal P becomes the maximum value in the precision grinding step S12, and thereafter, the vector direction in the imaginary axis direction in the locus of the output signal P in the complex plane changes to a downward direction. After that, the real axis value increases, while the imaginary axis value decreases, and the position indicated by the symbol B4 is reached, and the precision grinding step S12 ends.

[0048] The inflection point calculation unit 44 calculates an inflection point based on an inflection point judgment criterion. The inflection point indicates a point where the direction of the vector of the output signal P in the complex plane changes in the imaginary axis direction or the real axis direction. Therefore, in the case where the frequency of the excitation current shown in FIG. 4(b) is 250 KHz, the inflection point is represented by point B3 where the direction of the vector of the output signal P in the imaginary axis direction changes from upward to downward.

[0049] Since the locus of the output signal P in the complex plane changes depending on the frequency of the excitation current, the inflection point judgment criterion of the inflection point also changes. In this embodiment, the inflection point judgment criterion storage unit 43 stores the locus of the output signal P in the complex plane for each frequency of the excitation current for the workpiece for creating the inflection point judgment criterion (master), and based on this, the inflection point judgment criterion is stored for each frequency of the excitation current. For example, as described above, the inflection point judgment criterion when the frequency of the excitation current is 250 KHz is set to the inflection point at the point where the direction of the vector of the locus of the output signal P stored in the inflection point judgment criterion storage unit 43 shown in FIG. 4(b) changes from upward to downward in the imaginary axis direction in the fine grinding process S12.

[0050] Furthermore, the inflection point judgment criterion storage unit 43 also stores the correspondence between the first characteristic information and the grinding burn state for the workpiece for creating the inflection point judgment criterion. In the case where the frequency of the excitation current shown in FIG. 4(b) is 250 KHz, in the fine grinding process S12, the region from the fine grinding process start point B1 to the inflection point B3 indicates a state in which a softened layer that will cause grinding burn exists on the workpiece W. And, in the region from the inflection point B3 to the vicinity of the processing end point B4, the workpiece W does not have a softened layer, but the retained austenite reduced layer that was formed on the deeper side of the softened layer remains on the workpiece W. Note that the retained austenite reduced layer is not included in the grinding burn because its mechanical properties are closer to the base material than the softened layer.

[0051] The inflection point time extraction unit 45 extracts, from the first feature information, a time at which an inflection point B3 occurs in the locus of the output signal represented on the complex plane as the inflection point time. In the example shown in Fig. 4(a), the time Ta of the inflection point B3 is extracted as the inflection point time.

[0052] The rotation count acquisition unit 46 acquires the number of rotations of the workpiece W from the start time of acquiring the number of rotations to the inflection point time Ta. The start time of acquiring the number of rotations can be, for example, the end of the rough grinding process, that is, the start time T1 of the fine grinding process. Note that the number of rotations at the start time of acquiring the number of rotations is set to 0.

[0053] If the headstock 12 has a rotation angle acquisition unit 47 that acquires the rotation angle of the rotation axis of the workpiece W, the number of rotations of the workpiece W can be acquired based on the rotation angle of the rotation axis of the workpiece W acquired by the rotation angle acquisition unit 47.

[0054] In the grinding process of the workpiece W by the cylindrical grinding machine 2, the phase in which the heat influence is deepest in one rotation of the workpiece W, i.e., the phase where the burn depth is maximum, corresponds to the time when an extreme value of the output signal of the eddy current sensor 20 appears. The extreme value is a maximum value or a minimum value, and which of them is determined by the workpiece temperature due to the grinding heat. The period in which this extreme value appears corresponds to the rotation period of the workpiece W. The method of acquiring the extreme value is to acquire a maximum value or a minimum value for each rotation period of the workpiece starting from the end of the rough grinding process, i.e., the start time T1 of the fine grinding process. Here, it has been found that when eddy current is measured for steel materials with different tempering temperatures, the eddy current value is reversed at the boundary of steel materials with a tempering temperature of about 250°C. At 250°C or higher, softened tissue is formed by tempering, so the higher the temperature, the more the value of the output signal of the eddy current sensor 20 shifts to the positive. On the other hand, at temperatures below 250°C, tempering does not occur and softened tissue is not formed, so the higher the temperature, the more the value of the output signal of the eddy current sensor 20 shifts to the negative. The temperature at which the eddy current value is reversed varies depending on the material of the workpiece W.

[0055] 4(a), this boundary is estimated to be the average data minimum value time Tb, which corresponds to the downward convex portion of the average data of the waveform of the real axis signal of the eddy current sensor 20. Since the workpiece temperature due to grinding is higher toward the surface layer, the phase with the deepest thermal effect is the time of the maximum value before this downward convex portion (average data minimum value time Tb), and the time of the minimum value after the downward convex portion.

[0056] The average data minimum value time Tb is acquired by the average data minimum value time acquisition unit 48 shown in Fig. 2. A known method can be used to calculate the average data. In this embodiment, the average data of the output signal of the eddy current sensor acquired by the signal acquisition unit 40 is represented by a solid line in Fig. 4(a), and the time when the average data becomes a minimum value is represented as time Tb.

[0057] On the basis of this, in this embodiment, the rotation count acquiring unit 46 acquires the number of rotations of the workpiece W based on the extreme value data of the output signal of the eddy current sensor 20. More specifically, the rotation count is acquired based on the maximum value data of the output signal of the eddy current sensor in a first interval F1 from the rotation count acquisition start time T1 to the average data minimum value time Tb, and the rotation count is acquired based on the minimum value data of the output signal of the eddy current sensor 20 in a second interval F2 from the average data minimum value time Tb to the inflection point time Ta.

[0058] That is, in the example shown in Fig. 4(a), the maximum value data in the first section F1 is the crest of a mountain shape that repeatedly appears in the real axis signal, and the number of occurrences is 4, and the minimum value data in the second section F2 is the bottom of a valley shape that repeatedly appears in the real axis signal, and the number of occurrences is 5. Since the inflection point time Ta is located between the last minimum value data Va and the next minimum value data Vb in the second section F2, the interpolated data obtained by linearly interpolating between the two can be set to 0.5 times. Therefore, the rotation count acquisition unit 46 can acquire the number of rotations as 9.5 times, which is the total of the number of occurrences of the crest in the first section F1 (4 times), the number of occurrences of the bottom of the valley in the second section F2 (5 times), and the interpolated data (0.5 times).

[0059] The grinding burn depth estimation unit 49 estimates the grinding burn depth of the workpiece based on the number of rotations N acquired by the number of rotations acquisition unit 46 and the amount of cut per one rotation of the workpiece W. More specifically, the grinding burn depth D can be calculated by the formula D=N·q as the product of the number of rotations N acquired by the number of rotations acquisition unit 46 and the amount of cut q per one rotation of the workpiece W. The amount of cut q per one rotation of the workpiece W can be a value preset as a grinding condition.

[0060] The grinding condition adjustment unit 50 adjusts the grinding conditions of the workpiece based on the estimation result of the grinding burn depth estimation unit 49. For example, when the grinding burn depth is estimated, the grinding efficiency in the rough grinding step S11 in the grinding process of the next workpiece can be reduced so that the grinding burn (softened layer) generated in the rough grinding step S11 is removed in the subsequent fine grinding step S12. Also, when the grinding burn generated in the rough grinding step S11 is completely removed in the fine grinding step S12, the grinding efficiency can be increased within a range in which no grinding burn remains, with reference to the margin described later.

[0061] The margin calculation unit 51 calculates the margin of grinding of the workpiece W based on the estimation result of the grinding burn depth estimation unit 49 and the cutting amount in the fine grinding process S12. After the inflection point time, which is the time of the inflection point B3, no grinding burn (softened layer) remains on the workpiece W. Therefore, as shown in Fig. 6, the depth D1 ground from the time T1 of the start of the fine grinding process B1 to the inflection point time Ta indicates the depth of the grinding burn that occurred on the workpiece W in the rough grinding process S11, and the depth D2 ground from the inflection point time Ta to the grinding end time Te indicates the margin of grinding.

[0062] As described above, the frequency setting unit 52 sets the frequency of the excitation current in the eddy current sensor 20. The setting range of the frequency of the excitation current is not limited, but can be set as described above.

[0063] 4. Grinding process S1 description The grinding process S1 will be described with reference to Fig. 5. As described above, the grinding process S1 includes the rough grinding step S11, the fine grinding step S12, the fine grinding step S13, and the spark-out step S14.

[0064] In the rough grinding step S11, the control device 31 rotates the grinding wheel 16 at a predetermined speed based on operation command data such as the shape of the workpiece W, grinding conditions, the shape of the grinding wheel 16, and coolant flow rate or supply timing information, to grind the workpiece W with a first cutting depth. In the fine grinding step S12, the control device 31 grinds the workpiece W with a second cutting depth lower than the first cutting depth. In the fine grinding step S13, the control device 31 grinds the workpiece W with a third cutting depth lower than the second cutting depth. In the spark-out step S14, the workpiece W is rotated at a preset rotation speed to grind the portion left unground in the fine grinding step S13, and the cross-sectional shape is made into a perfect circle. The cutting depth in the spark-out step S14 can be set to zero.

[0065] The cutting depth can be adjusted by controlling the cutting position of the grinding wheel 16 with the control device 31. The first to third cutting depths in each of the steps S11 to S13 are appropriately set within a range that satisfies the above-mentioned relationship, and the actual cutting depth in the spark-out step S14 is substantially zero. The first cutting depth in the rough grinding step S11 is the largest, and the rough grinding step S11 has the highest grinding efficiency among the steps S11 to S14. Therefore, grinding burn occurs substantially only in the rough grinding step S11.

[0066] 5. First grinding burn depth estimation process S2 Next, the grinding burn depth estimation process S2 performed by the grinding burn depth estimation device 1 will be described with reference to the flow diagram of Fig. 7. The grinding burn state estimation process S2 is performed in parallel with the grinding process S1.

[0067] In the grinding burn depth estimation process S2, first, in step S21, the frequency setting unit 52 sets the frequency of the excitation current in the eddy current sensor 20. Then, in step S22, the eddy current sensor 20 generates an eddy current in the workpiece W, and the signal acquisition unit 40 acquires the output signal P that is the output signal of the eddy current sensor 20. Then, in step S23, the first characteristic information acquisition unit 41 acquires first characteristic information that links the imaginary axis value and real axis value when the output signal P is represented on a complex plane in a Cartesian coordinate format with time information that has elapsed since the grinding start time. The time information is stored in the time memory unit 42.

[0068] Then, in step S24, the inflection point calculation unit 44 sets an inflection point judgment criterion. The inflection point judgment criterion is set based on the frequency of the excitation current set in step S21. In this embodiment, for example, when the frequency of the excitation current is set to 250 kHz, the judgment criterion for a frequency of 250 kHz stored in the inflection point judgment criterion storage unit 43 shown in FIG. 4(b) is set as the inflection point judgment criterion. That is, the judgment criterion is set such that the point at which the direction of the vector of the locus of the output signal P changes from upward to downward on the imaginary axis is set as the inflection point.

[0069] Thereafter, in step S25, the inflection point B3 is calculated by the inflection point calculation unit 44. If the inflection point B3 cannot be calculated, the flow proceeds to No in step S25, and in step S26, it is evaluated that no grinding burn has occurred, and this flow ends. Note that, when the inflection point B3 cannot be calculated, for example, as shown in FIG. 8, when the output signal P is represented on a complex plane in a Cartesian coordinate format, the locus of the output signal P does not draw an inflection point. Since the locus of this output signal P does not form an area indicating the existence of a softened layer from the fine grinding process start point B1 to the inflection point B3 in FIG. 4(b), it can be evaluated that no softened layer, i.e., no grinding burn has occurred.

[0070] 7, if the inflection point B3 can be calculated, the process proceeds to Yes in step S25. Then, in step S27, the inflection point time Ta of the inflection point B3 is extracted from the time storage unit .

[0071] Furthermore, in step S28, extreme value data is extracted by the rotation count acquisition unit 46. The extreme value data is extracted by acquiring maximum value data in a first range F1 from the precision grinding process start time T1 to the average data minimum value time Tb based on the average data minimum value time Tb acquired by the average data minimum value time acquisition unit 48, acquiring minimum value data in a second range F2 from the average data minimum value time Tb to the inflection point time Ta, and also acquiring linear interpolation data between the last minimum value data Va in the second range F2 and the next minimum value data Vb beyond the inflection point time Ta.

[0072] Then, in step S29, the rotation count acquisition unit 46 calculates the number of rotations of the workpiece W from the start time T1 of the fine grinding process to the inflection point time Ta based on the acquired data. Next, in step S30, the grinding burn depth estimation unit 49 calculates the product of the number of rotations N of the workpiece W and the cutting depth q per rotation in the fine grinding process S12 to estimate the grinding burn depth.

[0073] In this embodiment, thereafter, in step S31, the margin calculation unit 51 calculates the margin. The margin is calculated as the difference D2 between the dimensional information at the inflection point time Ta and the dimensional information at the grinding end time Te. Then, in step S32, the grinding condition adjustment unit 50 adjusts the grinding conditions for grinding the workpiece W from the next time onwards based on the calculated margin D2, and the flow ends.

[0074] 6. Second grinding burn depth estimation process S2 The second grinding burn depth estimation process S2 by the grinding burn depth estimation device 1 will be described with reference to the flow diagram of Fig. 9. Descriptions of steps equivalent to those in the flow shown in Fig. 7 will be omitted.

[0075] First, steps S21 to S27 shown in Fig. 9 are performed in the same manner as in the flow shown in Fig. 7. Then, after step S27, in step S280 shown in Fig. 9, the rotation angle of the workpiece W from the start time T1 of the precision grinding process is acquired by the rotation angle acquisition unit 47. After that, in step S290 shown in Fig. 9, the rotation number acquisition unit 46 calculates the number of rotations of the workpiece W based on the rotation angle of the workpiece W. Next, in step S30, the grinding burn depth estimation unit 49 estimates the grinding burn depth in the same manner as in the flow shown in Fig. 7. Thereafter, the same procedure as in the flow shown in Fig. 7 is followed.

[0076] 7. Effects According to the grinding burn depth estimation device 1 of the first embodiment, the eddy current sensor 20 detects an eddy current generated inside the workpiece being machined by the excitation current, and obtains first feature information in which at least one of the imaginary axis value and the real axis value when the output signal output by the eddy current sensor 20 is represented on a complex plane in a Cartesian coordinate format and time information is linked. Then, from the first feature information, an inflection point B3 in the trajectory of the output signal represented on the complex plane is calculated, and the time at which the inflection point is present is extracted as the inflection point time Ta. The inflection point time Ta corresponds to the time when a work-affected layer (also called a softened layer) indicating grinding burn generated on the workpiece W by grinding is removed by the subsequent grinding, so the grinding burn depth can be estimated by obtaining the cutting depth up to the inflection point time Ta. The cutting depth up to the inflection point time Ta after the occurrence of grinding burn is calculated by the product of the number of rotations of the workpiece W on the cylindrical grinding machine 2 and the cutting depth per rotation, so there is no need to use data from a sizing device. In this way, by using the time Ta of the inflection point B3 in the trajectory of the output signal of the eddy current sensor 20 displayed on the complex plane, the number of rotations of the workpiece W, and the cutting depth, the grinding burn depth can be estimated with high accuracy.

[0077] In the first embodiment, the grinding process of the workpiece W is configured to perform a fine grinding process S12 after a rough grinding process S11, the rotation count acquisition start time is the rough grinding process end time T1, and the grinding burn depth estimation unit 49 estimates the grinding burn depth of the processed portion based on the rotation count acquired by the rotation count acquisition unit 46 and the cutting depth per rotation of the workpiece W in the fine grinding process S12. This makes it possible to calculate the grinding burn depth at the rough grinding process end time T1 with high accuracy without waiting for the end of the grinding process.

[0078] Furthermore, in the first embodiment, the rotation count acquisition unit 46 acquires the rotation count of the workpiece W based on the extreme value data of the output signal of the eddy current sensor 20. This makes it possible to accurately acquire the rotation count of the workpiece W used to estimate the grinding burn depth, thereby improving the estimation accuracy of the grinding burn depth.

[0079] Moreover, in the first embodiment, an average data minimum value time acquiring unit 48 is further provided which acquires an average data minimum value time which is a time when the average data of the output signal of the eddy current sensor 20 becomes a minimum value. The rotation count acquiring unit 46 acquires the rotation count based on the maximum value data of the output signal of the eddy current sensor 20 in a first interval F1 from the rotation count acquisition start time T1 to the average data minimum value time Tb, and acquires the rotation count based on the minimum value data of the output signal of the eddy current sensor 20 in a second interval F2 from the average data minimum value time Tb to the inflection point time Ta. This makes it possible to acquire the rotation count of the workpiece W used to estimate the grinding burn depth more accurately, and to further improve the estimation accuracy of the grinding burn depth.

[0080] Moreover, in the first embodiment, a rotation angle acquisition unit 47 is provided that acquires the rotation angle of the rotation shaft of the workpiece W, and the rotation number acquisition unit 46 acquires the rotation number based on the rotation angle of the rotation shaft of the workpiece W. This eliminates the need to calculate extreme values ​​of the output signal from the eddy current sensor 20, thereby further reducing the computation load.

[0081] In the first embodiment, the inflection point B3 is a point where the direction of the vector of the locus of the output signal P output by the eddy current sensor 20, which is represented on the complex plane, changes in the imaginary axis direction of the complex plane. This makes it possible to easily and accurately detect the inflection point B3.

[0082] In the first embodiment, the grinding burn depth estimation unit 49 judges whether or not grinding burn has occurred at the rough grinding process end time T1 based on the first characteristic information, and evaluates the grinding burn state when it is judged that grinding burn has occurred. This makes it possible to reduce the calculation load because it is not necessary to evaluate the grinding burn state when no grinding burn has occurred at the rough grinding process end time T1.

[0083] Alternatively, in the case where the real axis value in the output signal P of the eddy current sensor 20 can be used as an index of the inflection point, the first feature information may be configured so that at least the real axis value in the output signal P of the eddy current sensor 20 is linked to the time information. In this case, the imaginary axis value in the output signal P of the eddy current sensor 20 is not required, so that the computation load can be reduced. In this case, the inflection point B3 is set as a point where the direction of the vector of the locus of the output signal P output by the eddy current sensor 20, which is represented on the complex plane, changes in the real axis direction of the complex plane, so that the inflection point B3 can be easily and accurately detected.

[0084] Alternatively, in the case where both the real axis value and the imaginary axis value of the output signal P of the eddy current sensor 20 can be used as indicators of the inflection point, the first feature information may be formed by linking the imaginary axis value and the real axis value of the output signal P of the eddy current sensor 20 with time information. In this case, the inflection point B3 can be a point where the direction of the vector of the locus of the output signal P output by the eddy current sensor 20, represented on the complex plane, changes in the real axis direction and the imaginary axis direction of the complex plane.

[0085] Moreover, the first embodiment includes a frequency setting unit 52 that sets the frequency of the excitation current applied to the workpiece W to be evaluated, and an inflection point determination criterion storage unit 43 that prestores an inflection point determination criterion for determining whether or not the inflection point B3 has been reached for each excitation current frequency. The inflection point time extraction unit 45 determines whether or not the inflection point B3 has been reached based on the inflection point determination criterion corresponding to the frequency of the excitation current applied to the workpiece W to be evaluated, and extracts the time when it is determined that the inflection point B3 has been reached as the inflection point time Ta. This makes it possible to appropriately extract the inflection point B3 according to the frequency of the excitation current.

[0086] In the first embodiment, the inflection point judgment criterion is created based on the correspondence relationship between the frequency of the excitation current, the first characteristic information acquired by applying an excitation current having the frequency to a workpiece for creating an inflection point judgment criterion during machining, and the grinding burn state of the workpiece for creating an inflection point judgment criterion. This makes it possible to change the inflection point judgment criterion according to the frequency of the excitation current, and to calculate the inflection point B3 more accurately.

[0087] In addition, in the above embodiment 1, the average data minimum value time Tb for defining the first range F1 and the second range F2 shown in Figure 4(a) was acquired by the average data minimum value time acquisition unit 48, but instead, in modified embodiment 1, the first range F1 and the second range F2 can be defined as follows.

[0088] First, the first characteristic information acquisition unit 41 acquires first characteristic information that links the imaginary axis value and the real axis value with time information that is the time when the workpiece W is ground. Then, the inflection point calculation unit 44 calculates, from the first characteristic information, a first inflection point that is a point where the direction of the vector of the trajectory of the output signal represented on the complex plane changes in the imaginary axis direction of the complex plane, and a second inflection point that is a point where the direction of the vector changes in the real axis direction of the complex plane.

[0089] Further, the inflection point time extraction unit 45 extracts a first inflection point time, which is the time when the first inflection point is exhibited, and a second inflection point time, which is the time when the second inflection point is exhibited. Then, the rotation count acquisition unit 46 acquires the rotation count based on the maximum value data of the output signal of the eddy current sensor 20 in a first section F1 from the precision grinding step S12 to the earlier of the first inflection point time and the second inflection point time, and acquires the rotation count based on the minimum value data of the output signal of the eddy current sensor 20 in a second section F2 from the earlier of the first inflection point time and the second inflection point time to the later of the first inflection point time. Note that other configurations in the modified embodiment 1 are the same as those in the above-mentioned embodiment 1, and therefore description thereof will be omitted.

[0090] According to the first modified embodiment, it is not necessary to calculate the average data minimum value time Tb for defining the first range F1 and the second range F2. Note that the first modified embodiment also provides the same advantageous effects as the first embodiment.

[0091] In the second grinding burn depth estimation process S2 in the present embodiment 1 shown in FIG. 9, in step S280, the rotation angle of the workpiece W from the start time T1 of the fine grinding process is acquired by the rotation angle acquisition unit 47, and in step S290, the rotation number of the workpiece W is calculated by the rotation number acquisition unit 46 based on the rotation angle, thereby calculating the rotation number of the workpiece W without extracting the extreme value data of the output signal of the eddy current sensor 20. However, instead of this, the following modified form 2 may be used. In modified form 2, in the output signal of the eddy current sensor 20, extreme value data for each rotation of the workpiece W from the start time of fine grinding is extracted from the rotation angle acquired by the rotation angle acquisition unit 47, and the real axis value and the imaginary axis value at the extreme value are represented on the complex plane. The extreme value is represented by a black circle and a white circle in FIG. 4(b). Then, the extreme value data on the complex plane is counted up to the inflection point time Ta of the output signal P of the eddy current sensor 20, and the counting result is used as the rotation number from the start time T1 of the fine grinding process. The other configurations are the same as in the first embodiment.

[0092] According to the second modified embodiment, the extreme value data is represented on a complex plane, so that the inflection point B3 can be analyzed more accurately and quickly. Note that the second modified embodiment also provides the same advantageous effects as the first embodiment.

[0093] In addition, in the first embodiment, the inflection point calculation unit 44 calculates the inflection point B3 based on the inflection point judgment criterion. Alternatively, however, extreme value data of the output signal of the eddy current sensor 20 may be displayed on a complex plane, and an operator may determine the inflection point by visual inspection or the like based on the displayed content, and information on the inflection point on the complex plane that has been judged to be an inflection point may be input from an inflection point information input unit (not shown) and the inflection point information may be used as the calculation result of the inflection point calculation unit 44.

[0094] As described above, according to the first embodiment and the first and second modifications, it is possible to provide the grinding burn depth estimation device 1 that can estimate the grinding burn depth of the workpiece W with high accuracy.

[0095] The present invention is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the gist of the present invention. [Explanation of symbols]

[0096] 1. Grinding burn depth estimation device 2 Cylindrical grinding machine 3 Processing section 20 Eddy current sensor 40 Signal Acquisition Unit 41 First characteristic information acquisition unit 42 Time storage section 43 Inflection point judgment criteria storage unit 44 Inflection point calculation section 45 Inflection point time extraction part 46 Rotation count acquisition unit 47 Rotation angle acquisition unit 48 Average data minimum value time acquisition section 49 Grinding burn depth estimation section 50 Grinding condition adjustment section 51 Margin calculation section 52 Frequency setting section 53 Grinding burn depth display

Claims

1. A grinding burn depth estimation device that estimates the grinding burn depth of a processed portion of a workpiece that is machined into a final target shape by grinding a surface of the workpiece using a cylindrical grinding machine, comprising: an eddy current sensor disposed opposite to a portion to be machined of the workpiece, which uses an excitation current to induce an eddy current inside the workpiece and outputs an AC output signal corresponding to a magnetic field generated by the eddy current; a first characteristic information acquiring unit that acquires first characteristic information obtained by linking at least one of an imaginary axis value and a real axis value when an output signal output by the eddy current sensor is represented on a complex plane in a Cartesian coordinate system with time information indicating a time when the workpiece is ground; an inflection point calculation unit that calculates an inflection point in a locus of the output signal represented on the complex plane from the first feature information; an inflection point time extraction unit that extracts a time at which the inflection point occurs as an inflection point time; a rotation count acquisition unit that acquires the number of rotations of the workpiece from a rotation count acquisition start time to the inflection point time; a grinding burn depth estimation unit that estimates a grinding burn depth of the workpiece based on the number of rotations acquired by the number of rotations acquisition unit and a cutting depth per rotation of the workpiece; A grinding burn depth estimation device comprising:

2. The grinding process of the workpiece is configured to perform a fine grinding process after a rough grinding process, The start time of acquiring the number of rotations is the end time of the rough grinding process, 2. The grinding burn depth estimation device according to claim 1, wherein the grinding burn depth estimation unit estimates the grinding burn depth of the workpiece based on the number of rotations acquired by the number of rotations acquisition unit and a cutting amount per rotation of the workpiece in a precision grinding process.

3. 3. The grinding burn depth estimation device according to claim 1, wherein the rotation count acquisition unit acquires the rotation count based on extreme value data of an output signal of the eddy current sensor.

4. An average data minimum value time acquisition unit that acquires an average data minimum value time, which is a time when average data of the output signal of the eddy current sensor becomes a minimum value, 4. The grinding burn depth estimation device according to claim 3, wherein the rotation count acquisition unit acquires the rotation count based on the maximum value data of the output signal of the eddy current sensor in a first interval from the rotation count acquisition start time to the average data minimum value time, and acquires the rotation count based on the minimum value data of the output signal of the eddy current sensor in a second interval from the average data minimum value time to the inflection point time.

5. The first characteristic information acquisition unit acquires the first characteristic information by linking the imaginary axis value and the real axis value with time information that is a time when the workpiece is ground, the inflection point calculation unit calculates, from the first feature information, a first inflection point which is a point at which a direction of a vector of the trajectory of the output signal represented on the complex plane changes in an imaginary axis direction of the complex plane, and a second inflection point which is a point at which a direction of the vector changes in a real axis direction of the complex plane; the inflection point time extraction unit extracts a first inflection point time which is a time at which the first inflection point is exhibited and a second inflection point time which is a time at which the second inflection point is exhibited; 4. The grinding burn depth estimation device according to claim 3, wherein the rotation count acquisition unit acquires the rotation count based on the maximum value data of the output signal of the eddy current sensor in a first section from the rotation count acquisition start time to the earlier of the first inflection point time and the second inflection point time, and acquires the rotation count based on the minimum value data of the output signal of the eddy current sensor in a second section from the earlier of the first inflection point time and the second inflection point time to the later of the first inflection point time and the second inflection point time.

6. A rotation angle acquisition unit that acquires a rotation angle of the rotation axis of the workpiece, The grinding burn depth estimating device according to claim 1 , wherein the rotation count acquiring unit acquires the rotation count based on a rotation angle of a rotation shaft of the workpiece.

7. 7. The grinding burn depth estimation device according to claim 6, wherein the rotation count acquisition unit extracts extreme value data of the output signal of the eddy current sensor for each rotation of the workpiece from the rotation count acquisition start time to the inflection point time based on the rotation angle acquired by the rotation angle acquisition unit, represents the extreme value data on the complex plane, and acquires the rotation count by counting the extreme value data on the complex plane.

8. 3. The grinding burn depth estimation device according to claim 1, wherein the inflection point is a point where a direction of a vector of a locus of the output signal output by the eddy current sensor, which is represented on the complex plane, changes in an imaginary axis direction and a real axis direction of the complex plane.

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