«DEVICE FOR CHECKING DIMENSIONS OF A MECHANICAL PIECE»

Technical field

The present invention relates to a device for checking dimensions of a mechanical piece comprising at least one contactless inductive transducer.

Background art

There are known contactless inductive transducers comprising a core, made for example of ferrite, with windings. These transducers can be mounted on a support frame, so as to face the surface of the mechanical piece, to check features of the material of which the mechanical piece is made and/or surface finish defects.

For example, the patent No. US 4306455 describes a device for checking, besides the diameter, the surface roughness of the mechanical piece. The device includes a nosepiece which is inserted in a hole of the mechanical piece to be checked, a couple of transducers connected by means of arms, each of which is movable around a fulcrum, two feelers to check the diameter of the piece and a couple of contactless inductive transducers for checking the surface roughness of such piece. The inductive transducers for checking the surface roughness comprise a core, for example made of ferrite, with windings and are fixed to the nosepiece of the device or, according to an alternative solution, to the arms, which carry also the couple of transducers for checking the diameter of the hole. In both cases, the cores are arranged in such a way as to face the surface to be checked.

The core made of ferrite - a material which can't be machined by machine tools - reduces significantly the possible uses. The shapes and dimensions of the transducers available on the market don't enable to properly check surfaces which have particular shapes or a radius of curvature below a certain value. In these cases, for example, the transducer doesn't cooperate correctly with the surface to be checked because, as schematically shown in figures la and lb, the distance between core and surface, more specifically the distance between a surface 21 of the core facing the piece and the surface of the piece to be checked, can vary in a substantial way in the checked area, in particular when the transverse dimensions of the core are relatively large with respect to the radius of curvature of the checked surface, e.g. as in the examples shown in figures la and lb. Moreover, according to the patent No. US 4306455, the contactless inductive transducers facing the surface to be checked are not used to check dimensions of the hole. All the alternative embodiments described in the US patent are provided with additional components and transducers to check diameter dimensions of the piece. The presence of such additional components and transducers makes the mechanical structure rather complex.

Disclosure of the invention

Object of the present invention is to obtain a device for checking dimensions of a mechanical piece comprising at least one contactless inductive transducer which overcomes the application constraints due to shapes and dimensions of the cores available on the market. This and other objects are achieved by a device for checking dimensions of a mechanical piece according to the attached claims.

Brief description of the drawings

A checking device according to the present invention is hereinafter described with reference to the attached sheets of drawings given by way of non-limiting examples, wherein:

- figures la and lb are schematic representations of transducers according to the prior art;

- figures 2a and 2b are a front view and a perspective view, respectively, of two inductive transducers for different embodiments of a device according to the present invention,

- figures 3a, 3b and 3c are perspective front views of inductive transducers for different embodiments of a device according to the present invention;

- figure 4a is a perspective front view of an inductive transducer according to an alternative manufacturing technique, for a device according to the present invention ;

- figure 4b is a sectional view of the transducer of figure 4a taken along the line IV-IV of figure 4a;

- figure 5 is a very schematic, sectional representation, with some details shown in front view, of a device according to an embodiment of the present invention featuring a plurality of contactless inductive transducers connected to a common support;

- figure 6 is a schematic, cross-sectional representation, with a detail shown in front view, of a device according to the present invention for checking dimensions of a hole of a mechanical piece; and

- figure 7 is partial view of a device for checking internal diameters of a mechanical piece according to the present invention.

Best mode of carrying out the invention

A device according to the present invention, for checking dimensions of a mechanical piece 1, for example radial dimensions of a hole defining a cylindrical surface 2, is schematically shown in figure 6. The device includes a support frame 3 and a transducer, fixed to the support frame 3 and in particular a contactless inductive transducer comprising a core 4 which faces the surface 2 of the hole. An external unit with power supply and processing circuits, which is schematically shown and marked with the reference number 6, is connected to the transducer. The transducer generates signals indicative of the distance between the surface 2 of the piece 1 and the transducer, more specifically a sensing surface 5 of the core 4. The core 4 of the contactless inductive transducer is made of a particular material called MUMETAL ^® , that is a metallic alloy composed of different percentages of nickel, iron, copper and molybdenum and notable for its high magnetic permeability. The core 4 includes at least one portion about which the windings are arranged.

Figure 2a shows a contactless inductive transducer 27 with a filiform core made of MUMETAL, that is a MUMETAL wire about which the windings are arranged, while a different contactless inductive transducer, shown in figure 2b, has a core including a base portion 23, opposite to the sensing surface, and two orthogonal portions 24, 25 with respect to the base portion, in particular a central portion 24 and a perimetrical portion 25. The windings are arranged about the central portion 24 of the core and are enclosed by the perimetrical portion 25 of the core.

The core portion including the windings can be sealed with non-hygroscopic material, for example araldite, to ensure that the core is sealed.

The main advantage of using this metallic alloy instead of traditional materials, such as ferrite, for the core is the possibility to have high flexibility in getting shapes and dimensions. The high workability of the MUMETAL enables indeed to obtain cores whose dimensions and shapes are suitable for the specific type of checking which has to be performed, so increasing the number of checking operations that can be carried out with high reliability and precision .

The geometrical shapes and dimensions of the cores made of ferrite and available on the market are limited and the possibilities of using such cores are limited, too, as shown in figures la and lb. On the contrary, MUMETAL can be machined by machine-tools and ensures a high flexibility, allowing to obtain particular shapes not bound to predetermined dimensions.

It is possible, for example, to obtain cores, even of small dimensions, which have surfaces that are properly shaped depending on the kind of surface to be checked. This is particularly advantageous when the surface to be checked is curved or, in general, not plane.

As schematically shown in figures 3a and 3b, it is possible for example to obtain cores in which the surface that faces the surface to be checked is cylindrical, more specifically convex to check internal diameters of the mechanical piece (such as the one of figure 6) or concave to check external diameters . By machining the core in such a way that its surface facing the piece, or sensing surface, is shaped - that is it has a trend substantially corresponding to the trend of the surface to be checked - the gap between the whole surface of the core and the surface of the piece is remarkably reduced with respect to a solution provided with a known transducer which has the same transverse dimensions and a core made of ferrite whose sensing surface facing the piece is plane (figures la and lb) .

This implies a remarkable reduction in the leakage of magnetic flux between core and surface of the piece to be checked, and enables a reliable checking in a larger measuring range. In other words, it enables to detect variations in the distance between core and surface to be checked, corresponding to possible variations in the dimensions of the piece, in a relatively wide range of values. Using contactless transducers like the ones described hereinbefore, it is thus possible to get usable information about dimensional variations, even of a certain extent, of the mechanical piece even when the mechanical pieces have curved or shaped surfaces.

Moreover, as it is possible, in principle, to increase the transverse dimensions of the core keeping the distance between the surface of the core and the surface of the piece to be checked substantially constant, the contactless transducers according to the present invention may even have large measuring ranges. This is not possible using a traditional core made of ferrite in which, as already explained with reference to the figures la and lb, the distance with respect to the surface to be checked increases as the transverse dimensions of the core increase .

As the machining process causes mechanical stresses which may alter the magnetic properties of MUMETAL, making them unstable, at the end of the machining the cores are preferably subjected to annealing process which stabilizes the magnetic properties again.

In the checking device of figure 6, the sensing surface 5 of the core 4 facing the piece 1 to be checked is shaped, that is it has a convex shape, more specifically it is substantially a portion of a cylindrical surface, which corresponds to the outline of the surface 2 of the hole. As previously stated, this enables to considerably reduce the gap between the surface 5 of the core 4 and the surface 2 to be checked ensuring that the leakage of magnetic flux is negligible .

As stated above, the core surface facing the mechanical piece to be checked, or sensing surface, can have different shapes from what has been described. It can be for example concave if external dimensions of the mechanical piece must be checked.

If necessary, for example to reduce overall dimensions, it is possible to shape even the core surface opposite to the sensing surface, that is the surface which is used to fix the transducer to the respective support. For instance, if the outline of the support area to which the transducer is fixed is not plane - it includes for example a cavity which houses a core portion - the core can be machined in such a way that the outline of the surface that is fixed to the support substantially matches the outline of the support.

As previously stated, the use of this metallic alloy grants the possibility to obtain cores whose shapes are different from the traditional ones and depend on mounting needs or structure of the assembly. It is possible, for example, to obtain parallelepiped shaped cores, in particular cores having square or rectangular cross-sections as schematically shown in figure 3c, which are easier to machine than a cylindrical core. It is even possible to obtain spherical cores. It is also possible to obtain cores with indented portions adapted to enclose, at least partially, the surface of the piece to be checked. For example, if it is necessary to check a grooved outline of a mechanical piece, more specifically shape and/or dimensions of a protruding portion of said surface, the core can be machined in such a way that cuts are provided in the perimetrical portions in order to enclose the protruding portion of the piece to be checked and the height of the central portion is smaller in order that it doesn't interfere with such portion of the piece to be checked. This enables to reduce the gap between the core of the transducer and the surface of the piece to be checked, and thus to reduce the leakage of magnetic flux .

To facilitate the correct positioning of the core with respect to the support or support frame to which is fixed, the core can be machined so as to include additional portions integral with the core and adapted to house fastening elements. The core can be provided, for instance, with two lateral slots integral with the core itself and adapted to be fixed by means of screws to suitable seats in the support frame. This ensures not only a proper positioning of a core, but also the possibility to substitute the core with another one, in case for example of malfunction of the transducer or mounting needs.

A further, important advantage of using MUMETAL is the possibility to fix the magnetic core to the corresponding support by laser welding. Contrary to the cores made of ferrite which are fixed by gluing, the welding is considerably stabler and enables to avoid problems due to wrong positioning of the transducer caused for example by a gluing which is wrongly made or is less resistant, or by unwanted and undetected displacements of the parts owing to thermal expansions of the glue. The fixing technique by welding can be used for any core independently from its shape or dimension.

The operation of the transducer is known and is hereinafter described in general terms.

The core is fed by an alternating current generating a magnetic field which interacts with the surface of the mechanical piece facing the core, and induces a current in the surface to be checked. This current generates its own magnetic field, which opposes the magnetic field of the core and causes a variation in the impedance of the system depending on the cooperation between transducer and surface of the piece, in particular on the mutual distance therebetween. The impedance variation can be observed by detecting the voltage variation across the electrical circuits to which the system is coupled.

Obviously, a transducer of the kind described above can be used for checking mechanical pieces made of conductive materials, that is materials which allow the movement of the magnetic flux of the transducer through them, or provided with at least one portion made of conductive material .

The transducer made of MUMETAL can be obtained using an alternative technique consisting in welding a plurality of laminae 28, 29 made of MUMETAL which are suitably shaped to obtain a core with desired shape and dimensions. Such laminae 28, 29 can be obtained for example from a MUMETAL sheet by laser cutting.

To obtain, for example, a cylindrical core (schematically shown in figures 4a and 4b) it is possible to weld to a cylindrical base lamina 28, with greater thickness, several annular laminae 29 which form one of the portions orthogonal to the base portion 23 of the core, more specifically the perimetrical portion 25. The number of laminae depends on the dimensions that the perimetrical portion 25 of the core must have. Finally, a cylindrical element, for example a pin 30 forming the central portion 24 of the core, is welded in the centre of the cylindrical base lamina 28. By applying the lamination technique used in the transformers and welding the MUMETAL laminae 28, 29 in such a way that there is a minimal gap between one lamina and the other, it is possible to reduce eddy currents .

Another technique to reduce eddy currents consists in replacing the central pin of the core with a plurality of thin pins made of MUMETAL jointed together, or with a MUMETAL sheet wound as a spiral.

As previously stated, the described transducer ensures high flexibility in use.

As described above with reference to figure 6, one or more contactless inductive transducers can be used in a device for measuring and/or checking the dimensions of a mechanical piece according to the invention. Such device comprises a support frame to which the transducer is connected in such a way as to face the surface of the mechanical piece to be checked. The transducer ( -s ) is/are coupled to power supply and processing circuits housed in the support frame or arranged outside the support frame.

It is thus possible to use several transducers of this kind concurrently. In this case the transducers are fixed to the support frame so as to be placed at proper distances from each other or separated by insulating elements to prevent possible interferences between the magnetic fields of the single transducers. The signals of the transducers are combined to get information about shape and/or dimensions of the surface to be checked.

To check complex outlines of mechanical pieces, it is possible to use, as schematically shown in figure 5, a common support 26 which is suitably shaped so as to substantially match the outline of the piece to be checked, to which the transducers 4 are fixed.

Figure 7 shows a further, possible embodiment of a checking device according to the present invention, in particular a gauge for checking internal dimensions of a mechanical piece comprising a support frame 10, or handle, with a centering nosepiece 11 fixed to the handle 10 by means of screws 12. The surface of the nosepiece 11 fixed to the handle 10 has a central area 17 which is indented with respect to the perimetrical areas, where the screws 12 are inserted. In this way the nosepiece 11 rests only on the perimetrical areas of the handle 10 so as to ensure a correct positioning. The end of the nosepiece 11 is provided with two diametrically opposite seats 13, 13' in which two transducers 14, 14', more specifically two MUMETAL cores with windings, are fixed. The dimensions of the seats are larger than the dimensions of the cores so that the electrical wires connected to each transducers can be inserted in the nosepiece 14. The nosepiece 11 is provided with a longitudinal through hole 15 which houses the electrical wires. The through hole 15 communicates at one end with the seats 13, 13' and at the other end with the indented area 17 of the lower surface of the nosepiece 11, which houses the electrical wires in excess. The electrical wires are connected to a connector 16, placed inside the handle 10, through a passage 18 covered with insulating material, for example rubber, arranged in the part of the handle 10 facing the indented area 17 of the nosepiece 11.

The gauge is coupled - in a known way - by wire or wirelessly to an external processing unit including power supply and processing circuits 19. The processing unit processes and combines the signals received by the transducers 14, 14' to get measuring information about the dimensions of the piece. The described gauge comprises two transducers, but it can be provided with a higher number of transducers, for example three arranged in such a way as to form angles of 120° with respect to each other, or four arranged in such a way as to form angles of 90° with respect to each other. In this case the nosepiece has a number of seats - suitably arranged - corresponding to the number of transducers.

The device shown in figure 7 can be manually operated by an operator. However, analogous devices, with suitable changes, can be used in automatic measuring assemblies.

To prevent the magnetic flux generated by the core from leaking in the support frame of the gauge and being transmitted partly, or not being transmitted at all, to the surface of the piece to be checked, the support frame can be made of nonmagnetic material, for example stainless steel .

To increase the sturdiness of the device, the nosepiece can be made of magnetic material and the core of the transducer can be inserted in a protection casing made of nonmagnetic material and fixed thereto, for example by laser welding. The protection casing insulates the transducer from the support frame and prevents the magnetic flux from leaking inside the frame so affecting or preventing the measurement .

For gauges adapted to check large diameters in which the nosepiece has large dimensions and is thus very expensive, it is possible to fix the transducer using an alternative method, instead of welding, which enables to remove the nosepiece and use it again, in case for example of fastening or mounting errors. According to such alternative method, the end of the nosepiece is provided with an opening which communicates with the seats housing the transducers. Thanks to the opening, it is possible to fix the core by means of a removable fastening element of a known type.

To calibrate the gauge it is possible, for example, to use calipers, or masters, whose portions adapted to cooperate with the transducers include inserts made of the same material as the one of the piece to be checked or measured. The electronics compensate thermal drifts due to the difference between the material of the gauge and the material of the mechanical piece.

In an alternative embodiment of the device according to the invention, the traditional windings of the transducer are replaced with a properly processed annular fiberglass disc fixed to the core. As the arrangement of the fiberglass windings can't be altered, this solution provides advantages in terms of linearity of the transducer and repeatability of performance.

According to a different embodiment, the windings can be provided in an annular silicon oxide disc fixed to the core. This solution provides remarkable advantages in terms of core miniaturization.