1. Field of the Invention
The invention relates to an eddy current sensor used for eddy current measurement that is performed in order to inspect the surface texture, cracks, and the like, of a steel product by utilizing eddy current, and also to an inspection method using the eddy current sensor.
2. Description of the Related Art
In an existing art, for the purpose of nondestructive inspection on the surface texture, cracks, and the like, of a quench-hardened layer, or the like, in a steel product, eddy current measurement has been conducted. The eddy current measurement is a measurement utilizing eddy current. The eddy current measurement is conducted using an excitation coil and a detection coil. The excitation coil is used to apply an alternating-current excitation signal to the steel product. The detection coil is used to detect a detection signal, generated by eddy current, from the steel product to which the alternating-current excitation signal is applied by the excitation coil. That is, in the eddy current measurement, a target steel product is magnetized by the excitation coil, and an induction magnetic field induced by the thus generated eddy current is detected by the detection coil. Thus, for eddy current measurement, the configuration that includes the excitation coil and the detection coil is used as a sensor (hereinafter, referred to as “eddy current sensor”) included in a measurement system.
A technique related to eddy current measurement is, for example, described in Japanese Patent Application Publication No. 2008-134106 (JP-A-2008-134106). JP-A-2008-134106 describes a technique for inspecting a quenching pattern (quench-hardened layer) formed in a surface portion of a quenched component, such as an automobile component, by means of eddy current measurement. Specifically, in the technique described in JP-A-2008-134106, to determine whether the quenching pattern of a work piece (target steel product) is acceptable, a tolerance zone is set as a criterion in advance. Measured points of acceptable products are present in the tolerance zone within a plane in which a measured point determined by a value based on the phase difference of a detection signal from an alternating-current excitation signal and a value of the magnitude of the detection signal. Then, on the basis of whether a measured point regarding an inspection region of a work piece measured using the eddy current sensor falls within the tolerance zone, it is determined whether the quenching pattern of the work piece is acceptable.
Of course, with the technique described in JP-A-2008-134106, 100 percent inspection may be performed by means of in-line nondestructive inspection in terms of the quenching quality (quench depth, quenched hardness) of a component quenched by induction quenching, or the like. The quenching quality has been assured only through cutting inspection, or the like, by sampling. However, the eddy current measurement described in JP-A-2008-134106 has the following problem in terms of the structure of the eddy current sensor.
That is, as shown in FIG. 21A, in JP-A-2008-134106, a through-type coil 110 is used as an eddy current sensor. The through-type coil 110 is formed so that an excitation coil and a detection coil are accommodated in a case 111 made of synthetic resin, or the like, in a state where the excitation coil and the detection coil are arranged coaxially. The through-type coil 110 has a through hole 111a defined by the case 111. The through hole 111a is provided at a portion (inner peripheral portion) corresponding to a coil hollow portion of the excitation coil and detection coil that are accommodated in the case 111. That is, in the through-type coil 110, the excitation coil and the detection coil are provided so that the position of the common central axis substantially coincides with the position of the central axis of the through hole 111a. 
The through-type coil 110 is used so that a shaft portion 112 of a work piece, which is a shaft-like (columnar) component, is inserted through the through hole 111a of the case 111. That is, with the through-type coil 110, in a state where the shaft portion 112 of the work piece is inserted through the through hole 111a, eddy current measurement for inspecting the quenching pattern is conducted on the shaft portion 112.
With the thus configured through-type coil 110, a sufficiently strong magnetic field tends to be obtained in eddy current measurement. In addition, with the through-type coil 110, because of the distribution of magnetic flux densities, there is a small influence of a variation in distance between the measurement portion of a work piece and the through-type coil 110 (distance in the radial direction (for example, the direction indicated by the arrow X1) of the coil) on a measured value, so it is possible to obtain a stable measured value.
However, in the eddy current measurement conducted using the through-type coil 110, a measurement target will be only a shaft portion of a shaft-like component, such as the shaft portion 112 of the work piece. That is, in the eddy current measurement conducted using the through-type coil 110, only the shaft portion of a shaft-like component, which may be inserted through the through hole 111a, can be a measurement target. Thus, the through-type coil 110, which serves as an eddy current sensor, cannot accept a portion, which cannot be inserted through the through hole 111a, as a measurement target. The portion that cannot be inserted through the through hole 111a, for example, includes a journal portion and pin portion of a crankshaft, and a curved portion formed between a shaft portion and proximal portion of an outer race that constitutes a bearing.
On the other hand, the eddy current sensor used in the existing art includes a probe-type coil 120 as shown in FIG. 21B. The probe-type coil 120 is formed so that a coil, such as a solenoid coil, is provided inside along a shaft-like (columnar) case 121, and has a detecting portion 122 at one end thereof. With the probe-type coil 120, the detecting portion 122 provided at one end of the columnar probe-type coil 120 is brought close to a measurement portion to conduct eddy current measurement. Thus, as shown in FIG. 21B, with the probe-type coil 120, a journal portion 123a or pin portion 123b of a crankshaft 123, which cannot be measured by the through-type coil 110 shown in FIG. 21A, may be set as a measurement target.
However, in the case of the probe-type coil 120, the strength of a magnetic field is lower than that of the through-type coil 110, so it is difficult to obtain a magnetic field having a strength sufficient for eddy current measurement. In addition, with the probe-type coil 120, because of the distribution of magnetic flux densities, there is a large influence of a variation in distance between the measurement portion of a work piece (crankshaft 123) and the probe-type coil 120 (distance in a direction (see the arrow X2) in which the detecting portion 122 is brought close to the work piece) on a measured value. Therefore, with the probe-type coil 120, a measured value is extremely sensitive to a variation in distance between the measurement portion and the probe-type coil 120, so it is difficult to obtain a reproducible and stable measured value. For these reasons, the probe-type coil 120 is currently not practical for inspecting the quenching quality.
Note that, in order to increase the strength of the magnetic field generated by the probe-type coil 120, it is conceivable that the number of turns of the coil is increased or an electric current that flows through the coil is increased. However, an increase in the strength of the magnetic field through these methods is limited because of a resistance in the coil.
As described above, in the foregoing description, practical eddy current measurement that enables 100 percent inspection by means of in-line nondestructive inspection is a measurement only on a shaft portion of a shaft-like component using the through-type coil. That is, currently, it is desired to implement an eddy current sensor that allows eddy current measurement on a component having a relatively complex shape, such as a journal portion or pin portion of a crankshaft (other than a shaft portion of a shaft-like component).