Abstract:
In a non-destructive inspection device of the present invention which detects in a non-destructive manner a defect of a member to be inspected, based on a change in magnetic fluxes due to eddy currents that are generated by an inspection probe having a coil, a driving section which adjusts a position of the inspection probe, and measuring device for, based on a detection signal of the inspection probe, measuring a lift-off between the inspection probe and the member to be inspected are disposed. The driving section is controlled in accordance with a result of measurement of the measuring device, whereby a control of making the lift-off constant is performed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a non-destructive inspection device in which, when a defect of a member to be inspected (hereinafter, such a member is often referred to as inspection member) having a complex shape configured by combining planar and curved faces with one another is inspected in a non-destructive manner, the distance (lift-off) between an inspection probe and the inspection member is controlled to be always constant, thereby improving the inspection accuracy. 
     2. Description of the Related Art 
     As the advancement of downsizing and performance of a mechanical structure or a device, a stress which is applied as a load to a structure is increasing. Therefore, even when a defect is small from the viewpoint of a field of a material strength, stress concentration produces a large influence. As a result, it is requested to surely detect even a small defect (impact mark, crack, damage, grinding burn, or the like). 
     Conventionally, a device shown FIG. 5 is known as a defect inspection device which satisfies such a requirement. In the device, an AC current is supplied to a coil  110  to induce eddy currents in an inspection member  11  which is opposed to the coil. A defect is detected in a non-destructive manner on the basis of a phenomenon in which the amplitude and phase of the eddy currents are changed depending on the dimensions, kinds of the defect, the permeability, the conductivity, and the like of the inspection member  11 , i.e., a change of an impedance of the coil  110 . In FIG. 5, φ′ indicates magnetic fluxes generated by the eddy currents. FIG. 6 is a diagram of eddy currents in the case where the inspection member  11  has no defect. By contrast, in the case where the inspection member  11  has a defect, i.e., the case where a passage of a conductor is bent to change its shape as shown in FIG. 7, also eddy currents are changed in accordance with the change, so that the defect of the inspection member  11  is inspected in a non-destructive manner. 
     It is known that, when the coil  110  is wound into the absolute type as shown in FIG. 8, such a defect inspection device based on eddy currents is suitably used for detecting changes in all of the dimensions, kinds of the defect, the magnetic permeability, the conductivity, and the like. It is known also that, when two coils are wound in the differential type, a crack can be detected more sensitively. 
     An inspection member which is to be inspected by such a non-destructive inspection device often has a complex shape configured by combining planar and curved faces with one another. When an inspection member having such a complex shape is to be inspected, a moving mechanism having degrees of freedom in three or more axes is used, and inspection is performed while tracing the shape of the inspection member by interpolating the three axes. In a method of the conventional art, coordinates of an inspection probe for scanning the inspection member are preset, and inspection is performed in accordance with the preset coordinates. 
     Therefore, the distance (lift-off) between the inspection probe and the inspection member is affected by, for example, setting errors of the inspection probe and the inspection member, and the tolerance of the inspection member, and hence the lift-off is varied. When the lift-off is changed by scanning of the inspection probe, setting errors of the inspection probe and the inspection member, the tolerance of the inspection member, and the like, eddy currents in the inspection member due to magnetic fluxes generated by the coil in the inspection probe is changed, so that the impedance of the coil in the inspection probe is largely changed. The impedance change of the coil in the inspection probe is larger than that due to a defect to be detected. When the lift-off is varied, therefore, there arises a problem in that the detection accuracy of the inspection is not stable. 
     When inspection using an inspection probe is performed while setting based on design values is unchanged, a dimension difference of several tens of p.m is caused in the lift-off by the tolerance error of the workpiece, the error in chucking of the inspection member, the error in setting of the inspection member, and the like. An output signal of a non-destructive inspection device has relationships such as shown in FIG. 2 with the lift-off. When the lift-off is changed, therefore, also the level of the output signal is largely changed. 
     FIGS. 9 and 10 show relationships between the lift-off and the detection signal, and the state of a defect in the detection signal. When inspection is performed with placing an inspection probe  20  in position A (too close), B (normal), or C (too remote) with respect to an inspection member  21 , the level of the detection signal (output voltage) V is changed as indicated by V A , V B  or V C . Namely, as the lift-off is larger, the level of the output voltage V is lower, and, as the lift-off is smaller, the level of the output voltage V is higher. An inspection area AR A  of the output voltage V A  shows a signal corresponding to a defect, an area inspection AR B  of the output voltage V B  shows a signal corresponding to the defect, an inspection area AR C  of the output voltage V C  shows a signal corresponding to the defect, and ΔV A , ΔV B , and ΔV C  show the levels of the signals corresponding to same defect, respectively. 
     Conventionally, in an inspection member, the requested accuracy of the size of a defect is not very high. Even when the absolute value of the output signal is somewhat varied in degree, therefore, a defect can be detected because the level difference between outputs from a defect detection section are large. By contrast, with respect to a small defect, the output signal is low in level. When the lift-off is large and the level of the output signal is low, therefore, the output signal is buried in noises and cannot be detected. When V B &gt;V C  and ΔV B &gt;ΔV C  in FIG. 10, for example, it is impossible to judge which defect is larger. 
     On the other hand, when the distance between the inspection probe and the inspection member is too small, there is a fear that the probe may be in contact with the member. Usually, as the surface area where an inspection probe is opposed to an inspection member is larger, the detection accuracy is higher, and an output of a sufficient level cannot be obtained unless the probe and the member are somewhat closer. In the case where an inspection member having a complex shape configured by combining a planar face (straight line) and a curved face (curved line) with one another, a concave curved or spherical face, a cylindrical face, or the like is to be inspected, when an inspection probe is planar or larger in radius of curvature than the inspection member, therefore, a state may be caused in which the inspection probe is made excessively closer to the inspection member to be in contact therewith. 
     In such a non-destructive inspection device, usually, defect judging means makes a judgement in either of the following manners: (1) if the difference between a detection signal obtained from an inspection member with respect to a reference level is larger than a predetermined value, the inspection member is judged failure; and (2) if the difference between a detection signal with respect to a reference level is smaller than a constant value (for example, 20%), the inspection member is judged acceptable, and, if not, the inspection member is judged failure. However, such means has a problem as follows. In (1) above, when the probe position is excessively remote as in the case of position C shown in FIG.  9  and the signal level is low, the inspection member is judged acceptable in spite of failure. By contrast, in (2), the inspection member is judged failure not only when the probe position is excessively remote as in the case of position C with the output level is low, but also when the probe position is excessively close as in the case of position A with the output is high. 
     SUMMARY OF THE INVENTION 
     The invention has been conducted under the above-mentioned circumstances. It is an object of the invention to provide a non-destructive inspection device in which inspection can be stabilized by correcting a change of the lift-off due to setting errors of an inspection probe and an inspection member, or the tolerance of the inspection member, the time period required for the correction can be minimized so as to suppress an increase of the inspection time period, and an influence of the change of the lift-off can be minimized without adding a distance sensor or the like. 
     The invention relates to a non-destructive inspection device which detects in a non-destructive manner a defect of a member to be inspected, based on a change in magnetic fluxes due to eddy currents that are generated by an inspection probe having a coil. The object of the invention can be attained by comprising: a driving section which adjusts a position of the inspection probe; and measuring device for, based on a detection signal of the inspection probe, measuring a lift-off between the inspection probe and the member to be inspected, and controlling the driving section in accordance with a result of measurement of the measuring device, whereby a control of making the lift-off constant is enabled. 
     The object of the invention can be attained more effectively by winding the coil into the absolute type, and outputting a detection signal of the inspection probe as an absolute value, or by configuring the coil as two coils which are wound in the differential type. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing an embodiment of the invention; 
     FIG. 2 is a view showing an example of characteristics of an inspection probe used in the invention; 
     FIG. 3 is a flowchart showing an example of the operation of the invention; 
     FIG. 4 is a diagram showing a manner of inspecting a component of a disk of a continuously variable transmission; 
     FIG. 5 is a diagram showing the principle of a non-destructive inspection device used in the invention; 
     FIG. 6 is a diagram showing a distribution of eddy currents in the case where there is no defect; 
     FIG. 7 is a diagram showing a distribution of eddy currents in the case where there is a defect; 
     FIG. 8 is a view showing the manner of winding a coil in the case where an output of an absolute value signal is to be obtained; 
     FIG. 9 is a view showing relationships between an inspection probe and an inspection member, and 
     FIG. 10 is a waveform chart showing examples of a defect detection signal and relationships of detection signals corresponding to the lift-off between an inspection probe and an inspection member. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the non-destructive inspection device of the invention, an absolute value signal of an output of the inspection probe which is used for detecting a defect such as an impact mark, a crack, or the like is employed, an absolute value signal component of a detection signal from the inspection probe is used as means for measuring the distance (lift-off) between the inspection probe and the inspection member, and the position of the inspection probe is feedback-processed on the basis of a result of the measurement of the lift-off to adjust the position. According to this configuration, the inspection can be performed while always maintaining the lift-off constant, and hence it is possible to realize highly accurate non-destructive inspection. Since the position of the inspection probe can be adjusted without adding a position sensor, the size of the device is not increased. 
     Hereinafter, an embodiment of the invention will be described with reference to the accompanying drawings. 
     FIG. 1 shows an embodiment of the invention. An inspection coil  1 A of an inspection probe  1  which is moved in the X- and Y-axis directions and rotated about the θ-axis is set so as to be opposed to the surface of an inspection member  2  with being separated therefrom by a distance D (for example, 100 to 1,000 μm, preferably, 100 to 500 μm). An orbit B indicated by the solid line is a target orbit which is to be traced by the coil in order to inspect the surface of the inspection member  2 . Orbits which are indicated by broken lines and deviated from the target orbit B by a distance E or F are indicated by A and C, respectively. The distance E or F is a setting error, a process error, or the like, and usually exists in the range of about ±100 μm. A detection signal DS of the inspection probe  1  is supplied to a signal processing circuit  3  to be sequentially subjected to processes such as signal amplification, A/D conversion, calculation of deviation amounts of the coordinates, and calculation of the amount of interpolation using an interpolation coordinate table. An interpolation signal PDS of the detection signal is supplied to a controller  4  configured by a CPU, etc. Motors  5  to  7  are connected to the controller  4  via a driving circuit  4 A. The motor  5  drives the X-axis, the motor  6  drives the Y-axis, and the motor  7  drives the θ-axis. A feedback control of the position of the inspection probe  1  is performed by driving the X- and Y-axes by means of the motors  5  and  6 . In the embodiment, the position of the inspection probe  1  is adjusted in the X- and Y-axes. Alternatively, the position may be adjusted in three axes in which the Z-axis is added to the X- and Y-axes. 
     Conventionally, calculation is performed on the basis of the shape and dimensions of the inspection member  2 , and the shape, dimensions, and locus of the inspection probe  1  to determine the center of setting of the inspection probe  1  to position  0  (origin) of FIG. 1, and measurement is then performed. By contrast, in the invention, before measurement, the distance (lift-off) between the inspection probe  1  and the inspection member  2  is measured in plural positions along the locus to be measured, and it is checked whether or not the center position  0  is within an allowable range of the lift-off (locus B) to be measured. If the lift-off is deviated from the locus B, a feedback control is performed until the lift-off enters a given range with respect to the locus B. Then, inspection is performed while setting the inspection probe  1  to be moved along the locus B. 
     The relationship between the detection signal DS (or the signal PDS), and the lift-off D between the tip end of the inspection probe  1  and the inspection member  2  is indicated by an inverse-proportion curve such as shown in FIG.  2 . Therefore, the distance D can be detected from the level of the detection signal DS (or the signal PDS). In FIG. 2, the detection signal DS of the locus A in which the lift-off D is close to the inspection member  2  has a larger value DS A , the detection signal DS of the locus B has a value DS B , and the detection signal DS of the locus C which is remote from the inspection member  2  has a smaller value DS C . As the lift-off D is larger, the detection signal DS is smaller in level. 
     In FIG. 1, the orbit A is closer than the target orbit B which is to be traced, and the orbit C is remoter than the target orbit B. From the relationships shown in FIG. 2, deviation amounts E and F from the target orbit B are obtained on the basis of the level of the detection signal DS. Interpolation data are obtained from the interpolation coordinate table on the basis of the deviation amounts E and F. A feedback control is performed on the motors  5  and  6  based on the interpolation data, thereby correcting the position of the inspection probe  1  to the target orbit B (lift-off D) which is to be traced. This measurement of the lift-off serves also as acquisition of signals which are to be used in the defect inspection. Therefore, the position of the inspection probe  1  can be corrected without adding a distance sensor. 
     FIG. 3 shows an example of the inspection operation in the invention. First, the inspection probe  1  is set to an inspection start position (step S 1 ), and it is judged whether the inspection member  2  is linear or curved (step S 2 ). If it is judged that the inspection member is linear, the lift-off D which is the distance between the inspection probe  1  and the inspection member  2  is measured by the method that has been described with reference to FIG. 2 (step S 3 ). It is then judged whether or not the lift-off D is deviated toward E from the target orbit B which is to be traced, to be closer to the inspection member  2  (step S 4 ). The measurement and judgements are performed by the controller  4 . If the lift-off is closer to the inspection member  2 , the motors  5  and  6  are driven by the controller  4  on the basis of the interpolation amount which is calculated by the signal processing circuit  3 , thereby correcting the position (step S 5 ). Thereafter, the defect inspection is performed while moving the inspection probe  1  (step S 20 ). 
     If it is judged in step S 4  that the lift-off D is not closer than the target orbit B which is to be traced, it is then judged whether or not the lift-off is deviated toward F from the target orbit B to be remoter from the inspection member  2  (step S 6 ). If the lift-off is remoter from the inspection member, the motors  5  and  6  are driven by the controller  4  on the basis of the interpolation amount which is calculated by the signal processing circuit  3 , thereby correcting the position (step S 7 ). Thereafter, the defect inspection is performed while moving the inspection probe  1  (step S 20 ). 
     By contrast, if it is judged in step S 2  that the inspection member  2  is curved, the lift-off D which is the distance between the inspection probe  1  and the inspection member  2  is measured in two or more positions (step S 10 ). From a result of the measurement, a deviation between coordinates for interpolation or the curvature, and the actual curved face is calculated (step S 11 ), and it is then judged whether there is a deviation or not (step S 12 ). If there is no deviation, the inspection is performed without performing correction (step S 20 ). If there is a deviation, the motors  5  and  6  are driven in the same manner as described above to perform correction (step S 13 ), and the defect inspection is then performed while moving the inspection probe  1 . 
     The non-destructive inspection device of the invention implements the defect inspection while repeating the above-mentioned operations. Alternatively, the lift-off may be adjusted before the defect inspection, and the defect inspection may be then performed. 
     When correction of the radius is relatively small with respect to the design value, the center position O of the XY coordinates can be determined by performing the measurement in at least two positions. Correction of the radius is enabled by performing the measurement in three or more positions. 
     In the case where the shape of a trapezoid-like inspection member (a disk of a continuously variable transmission)  8  such as shown in FIG. 4 is to be inspected, inspection is continuously performed in the sequence of an end face (locus # 1 ) → a spherical face (locus # 2 ) → an upper end face (locus # 3 ) → a spherical face (locus # 4 ) → an end face (locus # 5 ). Also in this case, the lift-off which is the distance from the inspection face is previously measured, identification is conducted so that movement of the inspection probe is continuously constant, and the inspection probe is moved along the locus, whereby inspection can be accurately performed. 
     FIG. 4 shows an example in which the invention is applied to a disk of a continuously variable transmission. The invention is effective also in a defect inspection of the surface of a complex shape in which strength must be ensured, such as a hub unit of an automobile, a raceway surface of a constant-velocity joint, a contact surface of a universal joint, a spherical roller bearing, a bearing with a shaft, a grooving surface of a ball screw, or a guiding surface of a linear guide. 
     As described above, according to the non-destructive inspection device of the invention, since the detection signal from the inspection probe is used also as means for detecting the lift-off between the inspection probe and the inspection member, the lift-off can be controlled to be constant without newly disposing a distance (lift-off) sensor, whereby the accuracy of inspection can be improved and inspection can be stably performed. Since a special additional device is not required, the performance of the device can be improved while maintaining the present production cost. 
     While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.