Method and apparatus for detecting defect in elongated metal strip

Abstract: An apparatus for detecting crank on an elongated metal strip employs a plurality of surface condition detecting means operating in synchronism with each other so as to detect the surface conditions of the orientations of the steel belt, which orientations are longitudinally the same position but transversely at different position. The apparatus further includes means for deriving a difference of outputs plurality of the surface condition detecting means. A discriminator means is also provided in the apparatus for discriminating the surface condition of the strip on the basis of the difference derived by the difference deriving means.

Description: The present invention relates generally to a method and apparatus for detecing defect or crack in an elongated metal strip, such as a steel belt. More specifically, the invention relates to a technology for discriminating defect or crack from intersection of two strips which are joined by way of welding. In an apparatus employing an endless steel belt, such as a belt conveyor, crack can be formed during operation of the apparatus. Once cracking occurs on the steel belt, breakage of the belt may subsequently occur to cause damage for associated components. Therefore, it is preferred to detect occurrence of crank in order to know a timing of maintenace of the steel belt. Conventionally, it is typical to monitor the condition of the steel belt by means of a non-contact type proximity switch. In the conventional technology, the proximity switch generates high frequency magnetic field to induce eddy current on the steel. Magnitude of induced eddy current is variable depending upon the surface condition of the steel. Namely, when crack is formed on the surface of the steel, magnitude of the induced eddy current fluctuates from that induced at the normal surface condition. With such conventional crank detecting apparatus, a difficulty is encountered when the endless steel belt has a welded joining portion. Namely, the welded section has higher electric resistance than that in the general section of the steel belt. Therefore, magnitude of induced eddy current at the welded portion becomes different from that of the normal surface. This brings judgement that the crack is caused on the steel belt. US-A-3 141 952 discloses an electronic seam follower wherein an electromagnetic core having two pole pieces with general vertical axes. The magnetic core is energized by a source of alternating voltage for generating an alternating magnetic flux between the ends of the pole pieces. The ends of the pole pieces are positioned adjacent the surface of a metallic workpiece in order to direct the magnetic flux through a portion of the workpiece. A single pickup coil electrically insulated from the core and positioned adjacent the workpiece substantially between the pole pieces in a plane passing through the major axes of both poles is provided for sensing flux intensity variations occuring in the flux filed pattern judgement the workpiece due to discontinuities in the workpiece. Therefore, it is an object of the present invention to provide a method and apparatus for detecting crack on the metal strip, which has capability of discriminating the welded portion from crack. In order to accomplish aforementioned and other objects, an apparatus for detecting crank on an elongated metal strip, according to the present invention, employs a plurality of surface condition detecting means operating in synchronism with each other so as to detect the surface conditions of the orientations of the steel belt, which orientations are longitudinally the same position but transversely at different position. The apparatus further includes means for deriving a difference of outputs plurality of the surface condition detecting means. A discriminator means is also provided in the apparatus for discriminating the surface condition of the strip on the basis of the difference derived by the difference deriving means. According to one aspect of the invention, a defect detecting apparatus comprises: a first sensor placed opposite to an elongated metal strip being transferred along its longitudinal axis for monitoring surface condition at a first position of said elongated metal strip to produce a first sensor signal of variable signal level according to the variation of the surface condition at said first position; a second sensor placed opposite to said elongated metal strip for monitoring surface condition at a second position of said elongated metal strip, which second position is so located for monitoring a transversely offset position with respect to said first position of the elongated metal strip to produce a second sensor signal of variable signal level according to variation of the surface condition at said second position; means for receiving said first and second sensor signals and deriving an error signal depending on a difference of said first and second sensor signals; and means for comparing said error signal with a predetermined threshold value so that judgement that defect occurs in said elongated metal strip, is made when said error signal level is greater than said threshold value. The elongated metal strip may be formed into an endless belt by adjoining both ends, and the adjoining portion of the endless belt extending substantially in transverse with respect to the axis of the path. Preferably, the first and second sensors may comprise proximity switch producing the first and second sensor signals variable of signal levels depending upon variation of resistance at the first and second transverse positions. According to another aspect of the invention, a method for detecting a defect in an elongated strip being transferred through a predetermined path, comprises the steps of: providing a first sensor within said path of an elongated metal strip for monitoring surface condition at a first position of said elongated metal strip to produce a first sensor signal of variable siganl level according to the variation of the surface condition at said first position; providing a second sensor within a path of said elongated metal strip for monitoring surface condition at a second position of said elongated metal strip, which second position is so positioned for monitoring a transversely offset position with respect to said first position of the elongated metal strip to produce a second sensor signal of variable signal level according to variation of the surface condition at said second position; deriving an error signal depending on a difference of said first and second sensor signals; and comparing said error signal with a predetermined threshold value so that judgement that defect occurs in said elongated metal strip, is made when said error signal level is greater than said threshold value. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Fig. 1 is a perspective view of a belt drive apparatus employing an endless belt for which the preferred embodiment of a defect detecting apparatus according to the present invention; Figs. 2 to 4 are chart showing variation of signal level of sensor employed in the preferred embodiment of the defect detecting apparatus of Fig. 1; and Fig. 5 is a section showing another embodiment of a defect detecting apparatus according to the invention. Referring now to the drawings, particularly to Fig. 1 , a belt drive apparatus, for which the preferred embodiment of the defect detecting apparatus according to the present invention, is applicable, comprises an endless metal belt 13 extending over a pair of rotary drums 11 and 12 . The metal belt 13 may comprises a endless steel belt formed with a flexible steel strip with constant width by joining both ends together by way of welding. The welded joining portion 14 of the metal belt 13 extends generally in transverse with respect to the belt transferring axis. In order to drive the metal belt, one of the rotary drums 11 and 12 is connected to an electric motor or other driving power source (not shown) which provides rotational torque for the drum. The metal belt 13 is thus driven by the rotational torque applied from the driving power source via one of the rotary drums 11 and 12 . A pair of surface sensors 15a and 15b are provided in the vicinity of the belt path through which the metal belt is driven. The sensors 15a and 15b are arranged in transverse alignment with respect to the belt transferring axis and in spaced apart relationship with a predetermined distance to each other. Each of the sensors 15a and 15b comprises a proximity sensor which is per se well known construction. As is well known, the proximity switch generates high frequency magnetic field for inducing eddy current on the metal belt surface and detects magnitude of current induced on the metal belt surface to produce a sensor signal representative of the detected induced current magnitude. Since the magnitude of the eddy current to be induced on the metal belt surface is variable depending upon the resistance, the signal values of the detector signals output from the sensors 15a and 15b varies according to variation of the resistance in the metal belt 13 . The sensors 15a and 15b are connected to an error signal generator 16 . The error signal generator 16 receives the sensor signals from both of the sensors 15a and 15b to derive a difference of the signal values. The error signal generator 16 thus generates an error signal indicative of the derived difference. The error signal generator 16 outputs the error signal to a discriminator circuit 17 . The discriminator circuit 17 checks the error signal level against a predetermined threshold level to make judgement that the crank occurs or not depending upon the error signal level. Here, assuming only the sensor 15a is used and the crack formed substantially in axial direction passes across the sensor, the signal level of the sensor signal produced by the sensor 15a varies as illustrated in Fig. 2 . The similar waveform of Fig. 2 may appear when the welded joining portion of the metal belt 13 passes across the sensor 15a . Therefore, it makes discrimination of presence or absence of the crack difficult. In the shown embodiment, the welded joining portion 14 can be detected simultaneously by the sensors 15a and 15b so that the essentially identical waveform appears on the sensor signal of the sensor 15b . Therefore, by obtaining the difference of the sensor signals of the sensors 15a and 15b , the signal level of the error signal at the welded joining portion is maintained substantially small, as shown in Fig. 3 . On the other hand, when the local crack is created on the surface of the metal belt, the sensor signal level of the sensor 15a or 15b across which the crank containing portion of the belt travels, drops substantially. On the other hand, since the other sensor 15a or 15b does not detect the crack, the sensor signal level is maintained unchanged. In such case, the error signal level becomes substantial as shown in Fig. 4 . Therefore, by utilizing a plurality of sensor and utilizing the difference of the sensor signals, the influence of the welded joining portion of the metal belt can be successfully avoided. Fig. 5 shows another embodiment of the defect detecting apparatus according to the present invention. In the shown embodiment, three sensors 15a , 15b and 15c are employed in spaced apart relationship with a predetermined distance and aligned transversely with respect to the axis of belt travel. In such case, two error signals are derived on the basis of the sensor signals of the sensors 15a and 15b and 15b and 15c . Based on the error signals thus derived, discrimination whether the defect, e.g. crack, is present or absent can be performed effectively and in more precise manner. Therefore, according to the present invention, occurrence of crack can be successfully detected irrespective of the presence of the welded joining portion.

Claims: A defect detecting apparatus comprising: a first sensor (15a) placed opposite to an elongated metal strip (13) being transferred along its longitudinal axis for monitoring surface condition at a first position of said elongated metal strip (13) to produce a first sensor signal of variable signal level according to the variation of the surface condition at said first position; a second sensor (15b) placed opposite to said elongated metal strip (13) for monitoring surface condition at a second position of said elongated metal strip (13), which second position is so located for monitoring a transversely offset position with respect to said first position of the elongated metal strip (13) to produce a second sensor signal of variable signal level according to variation of the surface condition at said second position; means (16) for receiving said first and second sensor signals and deriving an error signal depending on a difference of said first and second sensor signals; and means (17) for comparing said error signal with a predetermined threshold value so that judgement that defect occurs in said elongated metal strip (13), is made when said error signal level is greater than said threshold value. A defect detecting apparatus as set forth in claim 1, wherein said elongated metal strip (13) is formed into an endless belt by adjoining both ends, and said adjoining portion of said endless belt extending substantially in transverse with respect to the axis of said metal strip (13). A defect detecting apparatus as set forth in claim 2, wherein said first and second sensors (15a 15b) comprise proximity switch producing said first and second sensor signals of variable signal levels depending upon variation of resistance at said first and second positions. A method for detecting a defect in an elongated strip being transferred through a predetermined path, comprising the steps of: providing a first sensor (15a) within said path of an elongated metal strip (13) for monitoring surface condition at a first position of said elongated metal strip (13) to produce a first sensor signal of variable signal level according to the variation of the surface condition at said first position; providing a second sensor (15b) within a path of said elongated metal strip (13) for monitoring surface condition at a second position of said elongated metal strip (13), which second position is so positioned for monitoring a transversely offset position with respect to said first position of the elongated metal strip (13) to produce a second sensor signal of variable signal level according to variation of the surface condition at said second position; deriving an error signal depending on a difference of said first and second sensor signals; and comparing said error signal with a predetermined threshold value so that judgement that defect occurs in said elongated metal strip, is made when said error signal level is greater than said threshold value. A method as set forth in claim 4, wherein said elongated metal strip (13) is formed into an endless belt by adjoining both ends, and said adjoining portion of said endless belt extending substantially in transverse with respect to the axis of said path. A method as set forth in claim 2, wherein said first and second sensors (15a, 15b) comprise proximity switch producing said first and second sensor signals of variable signal levels depending upon variation of resistance at said first and second positions.

Inventors: NAWA MICHIO, NAWA, MICHIO
Applicants: MEIDENSHA ELECTRIC MFG CO LTD, KABUSHIKI KAISHA MEIDENSHA
Application year: 1990
Main classification: G01N27
IPC codes: G01N27