Apparatus for detecting surface microdefects in a metal workpiece.

This invention relates to an apparatus for detecting surface microdefects in a metal workpiece.

Particularly, but not exclusively, the invention relates to an apparatus suitable for checking vehicle components such as the camshaft, crankshaft, live axles, injection pump pistons etc., in order to identify microdefects such as surface microfissures. Components of the aforesaid type are generally subjected to grinding, during which tensile stresses can be created due to local overheating of the material. In this respect, sudden cooling of the material can give rise to microfissures which are known to act as a triggering point for cracks which can lead to subsequent fracture of the material in which they have been formed.

The checking systems used up to the present time during production utilise either magnetoscopic techniques or penetrating liquids, these being systems in which the final check is solely visual and therefore depends on the subjective judgement of the operator. In order to obtain an objective evaluation, systems using optoelectronic sensors have also been proposed. However, these systems can be used only at the experimental level in that the operating conditions prevailing in the workshop (imperfect cleaning of the workpieces under examination, difficulty of inspecting workpieces of complicated geometry etc.) render them unpractical. Furthermore, the relative apparatus is extremely costly.

It is also known to use differential induced current sensors formed conveniently from two bars of ferrite (or suitable ferromagnetic material) disposed parallel to each other side-by-­side at a distance apart of a few millimetres. Copper windings are provided on the lower part of the two bars. An open magnetic circuit is therefore created which is able to detect the presence of cracks by measuring the difference in the impedance of the two said winding. However, differential induced current sensors of the aforesaid type are insufficiently sensitive. This is because such sensors offer poor spatial resolution by virtue of the fact that the magnetic field established between the two ferrite elements is not concentrated (or focused) in the measuring region. Moreover, the resolution power of the sensor depends on the distance between the ferrite bars which, because of the thickness of the conductors used for the windings, cannot be disposed closer together than a certain limit.

The object of the present invention is to provide an apparatus for detecting surface microdefects in a metal workpiece which obviates the aforesaid drawbacks of apparatuses of known type. Said object is attained according to the present invention by an apparatus for detecting surface microdefects in a metal workpiece, characterised by comprising at least one induced current sensor formed from at least two cores of ferromagnetic material, on each of which at least one conducting winding is provided; said cores having adjacent terminal portions which converge to define an air gap of the order of a few tens of µm. The present invention will be more apparent from the description of some preferred embodiments thereof given hereinafter by way of non-limiting example with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a diagrammatic elevational view of a first embodiment of an apparatus constructed in accordance with the present invention;</li><li>Figure 2 is a section on the line II-II of Figure 1 to an enlarged scale, with some parts removed for clarity;</li><li>Figure 3 is an elevation view of a second embodiment of the apparatus constructed in accordance with the present invention;</li><li>Figure 4 is an elevational view of a third embodiment of the apparatus according to the invention;</li><li>Figure 5 is a section on the line V-V of Figure 4 to an enlarged scale;</li><li>Figure 6 is a diagrammatic elevational view of a fourth embodiment of the apparatus according to the present invention;</li><li>Figure 7 is a diagrammatic elevational view of a fifth embodiment of an apparatus according to the present invention; and</li><li>Figure 8 is a section on the line VIII-VIII of Figure 7.</li></ul>

In Figure 1, the reference numeral 1 indicates overall an apparatus for detecting surface microdefects in a metal workpiece 2. The apparatus 1 comprises essentially a sensor 3 and a processing unit 4. More specifically, the sensor 3 consists of two cores of ferromagnetic material (conveniently ferrite) 5 and 6 respectively, on each of which a respective conducting winding 7, 8 is provided. The cores 5, 6 are essentially of bar structure, their respective lower ends 9, 10 being not covered by the windings 7, 8 and conveniently converging to define an air gap a of the order of a few tens of µm (see also Figure 2). It can be seen in particular that the end portions 9, 10 are of semicircular cross-section, and face each other along a diameter. The cores 5, 6 with their relative windings 7, 8 are housed in a support casing 11 conveniently of plastics material, such that only the front surfaces of the end portions 9, 10 of the cores 5, 6 are exposed to the outside. These front surfaces can be placed in a position facing a surface 12 of the said workpiece 2. Only two pairs of electrical terminals, ie of the windings 7, 8 respectively, emerge from the casing 11 and are connected to respective inputs of the processing unit 4. This latter is of essentially known type and is specifically able to measure the difference in the impedances of the windings 7, 8 respectively. The apparatus illustrated in Figure 3 and indicated overall by 21 comprises a sensor 23 and a processing unit 24. The sensor 23 is substantially analogous to the sensor 3 of Figure 1 and consequently the common parts are indicated by the same reference numerals. The sensor 23 comprises a further winding 25 which is disposed externally to the cores 5, 6 and to the relative windings 7, 8. In this specific case, the winding 25 acts as an excitation winding, and its terminals are connected to respective power outputs of the processing unit 24. In contrast, the windings 7, 8 have their respective terminals connected to the corresponding inputs of the processing unit 24, which is structured in such a manner as to measure the amplitude and phase difference in the voltages induced in the windings 7, 8 by the winding 25.

Figure 4 shows an apparatus 31 also provided with a sensor 33 and a processing unit 34. In particular, the sensor 33 comprises two pairs 35, 36 of ferromagnetic cores on which respective windings 37, 38 are wound. Specifically, the pairs 35 and 36 are disposed such that the respective cores are diametrically opposite. The relative windings 37, 38 are connected in series. Four terminals constituting the end terminals of the pairs of windings 37, 38 therefore lead to the processing unit 34. Again in this case, the pairs of cores 35, 36 have their respective end portions 39, 40 conveniently converging so as to present opposing facing surfaces separated by an air gap b (see Figure 5), the width of which is of the order of a few tens of µm. As can be seen in Figure 5, the end portions 39, 40 of the cores 35, 36 are delimited externally by a cylindrical surface, and each of them is shaped in cross-section as a circular sector with a 90° angle at its centre.

The processing unit 34 is analogous to the unit 4 of Figure 1 and also performs the function of measuring the difference in the impedances of the windings 37, 38.

The apparatus 41 illustrated in Figure 6 also comprises a sensor 43 and a processing unit 44. The sensor 43 is essentially equal to the sensor 33 of the apparatus 31, and consequently common parts are indicated by the same reference numerals. The sensor 43 is provided with a further winding 45, the terminals of which are connected to respective outputs of the processing unit 44 in order to induce respective voltages in the windings 37, 38 carried by the respective cores 35, 36. The opposing terminals of the windings 37, 38 lead to respective inputs of the processing unit 44, which measures the respective induced voltages and calculates their amplitude and phase difference. In Figures 7 and 8, the reference numeral 51 indicates overall a further embodiment of an apparatus constructed in accordance with the present invention and also comprising a sensor 53 and a processing unit 54. More specifically, the sensor 53 comprises a pair of cores 55, 56, on each of which there is externally provided a respective winding 57, 58. In particular, the core 55 is of tubular structure and internally houses the conveniently cylindrical core 56. Again in this case, the cores 55, 56 have end portions 59, 60 which conveniently converge to define (see also Figure 8) an air gap having a width c of the order of a few tens of µm.

The terminals of each winding 57, 58 lead to respective inputs of the processing unit 54, which calculates the difference in the impedances of the windings 57, 58.

The operation of the apparatuses described heretofore is as follows. Firstly, the various processing units feed the respective windings with a signal having an optimum frequency of the order of one or a few Mhz. Each sensor is then made to traverse the surface 12 of the workpiece 2 so as to scan the entire relevant surface of said workpiece 2.

The apparatuses 1, 31 and 51 operate in the same manner. Specifically, they calculate the difference in the impedances of the respective windings as heretofore described. More specifically, as the two windings 37 of the apparatus 31 are connected in series, as are the two windings 38, it is apparent that the impedance difference calculated by the processing unit 34 represents the difference between the sum of the impedances of the windings 37 and the sum of the impedances of the windings 38. From the aforegoing, the apparatuses 21 and 41 can also be considered analogous from the operational viewpoint. In this respect, their relative processing units 24, 44 generate an excitation voltage through the respective windings 25, 45, and calculate the amplitude and phase difference of the induced voltages measured across the terminals 7, 8 or 37, 38 respectively. In this latter case, analogously to that described with reference to Figure 4, this voltage induced by the pairs of windings 37, 38 obviously consists of the sum of the voltages across the ends of each winding, as the windings indicated by the same reference numeral are connected in series.

If microdefects such as surface microfissures are absent, the difference in the impedance or induced voltage is zero, whereas it assumes a significant value if a microfissure lies in correspondence with the air gap. The air gap has a very small width (of the order of tens of µm), and it has been observed that this allows high resolution and sensitivity to be obtained, thus enabling microfissures to be identified which would not be identified by conventional methods.

It has been noted that the microfissure is more easily identified if it runs in the same direction as the air gap. This could constitute a drawback with regard to the apparatus 1, but this could be overcome by mounting the casing 11 of the sensor 3 on a head which rotates about its axis so as to obtain constant sensitivity whatever the orientation of the microdefect. In any event it is obvious that the aforesaid drawback is not encountered in the embodiments shown in Figures 4, 6 and 7, in which the sensitivity zones of the respective sensor device are not limited to a single direction.

The advantages of the apparatuses according to the present invention are apparent from an examination of their characte­ristics. In this respect, these apparatuses are able to identify microdefects such as microfissures which would be practically unidentifiable by the aforesaid systems of known type, and in addition they have the advantage of being able to be used in workshops in that their operation does not require scrupulous cleaning of the workpieces under examination, neither is their application limited substantially by the geometry of the workpiece to be examined.

Finally, it is apparent that modifications can be made to the apparatuses heretofore described but without leaving the present invention. For example, in order to prevent the air gap in correspondence with the end portions of the said cores becoming excessively occupied by undesirable material which could falsify the measurements, a non-ferromagnetic separating element of conducting or non-conducting type could be used to entirely occupy the space lying between said adjacent ends. This element could for example be a common copper foil.

In a more complete embodiment, each sensor of the aforesaid apparatuses could be supported by a pneumatically-sustained movement system. This would eliminate direct contact between the sensor and the workpiece to be inspected, with consequent longer life of the sensor, which would not deteriorate by friction. Correspondingly, the speed of inspection of the workpiece would also be increased, allowing checking to be carried out within a time in line with the production rate.

Conveniently, the casing of each sensor could be enclosed within a conducting housing, to thus obtain the advantage of limiting dispersed flux only to the region involved in the measurement, while at the same time substantially reducing any edge effect.

It will also be noted that it is not necessary for the end portions of the cores of the sensors shown in Figures 1, 2, 3, 4 and 6 to be delimited externally by cylindrical surface portions, in that they could be of any convenient polygonal cross-section (rectangular, square etc.).

According to a further embodiment, an apparatus could be conceived provided with a plurality of side-by-side sensors, for example disposed at the nodes of a lattice, so as to be able to be placed directly in contact with the surface of the metal workpiece under examination. In this case, the apparatus would have to be provided with suitable processing means able to electronically scan the signals fed by the sensors.