WO2015189177A1

WO2015189177A1 - System and method for checking position and/or dimensions of an edge of a workpiece

Info

Publication number: WO2015189177A1
Application number: PCT/EP2015/062784
Authority: WO
Inventors: Roberto Toso
Original assignee: Marposs Societa' Per Azioni
Priority date: 2014-06-13
Filing date: 2015-06-09
Publication date: 2015-12-17

Abstract

A system for checking position and/or dimensions of an edge (5, R) of a workpiece (1, 1R) that defines a longitudinal axis, comprises two matching elements (6, 7) which have respective tapered matching surfaces (8, 9) adapted to assume a certain arrangement with respect to the edge to be checked, and a transducer system (11) that provides electrical signals (M, M6, M7) depending on said certain arrangement and on the position of the edge. The matching elements have different geometrical features, for instance the tapered matching surfaces define slope angles (α, β) with respect to the longitudinal axis which are different from each other. A method that uses this checking system involves bringing the matching elements in a checking condition wherein the respective matching surface assumes the certain arrangement with respect to the edge to be checked, detecting the electrical signals provided by the transducer system, and processing such signals together with reference signals depending on the arrangement of a reference edge. System and method are advantageously applied to the checking of the axial position of the internal edge of a valve seat (2).

Description

DESCRIPTION

« SYSTEM AND METHOD FOR CHECKING POSITION AND/OR DIMENSIONS OF AN EDGE OF A WORKPIECE »

Technical Field

The present invention relates to a system and a method for checking position and/or dimensions of an edge of a workpiece, with respect to a reference surface.

In particular, the present invention may be advantageously, but not exclusively, applied in checking the axial position of circular edges of an object generated from the intersection of two surfaces having rotational symmetry. The object may be, for example, a component of an injection system with a valve seat comprising the edge to be checked, or a valve seat in the cylinder head, or a valve intended to be housed in such seat, in an internal combustion engine. Reference to checking of an internal edge of a valve seat will be explicitly made in the following specification without loss of generality.

Prior Art

In a typical form, a valve seat, present in a cylinder head of an engine, comprises a tapered sealing surface at one end of a cylindrical guide aperture. The tapered sealing surface, intended to cooperate with the valve head, typically comprises two or more adjacent conical surfaces, each of which slopes down of a certain angle with respect to the central axis of the seat itself. The contact between the valve head, that defines a conical surface in turn, and the circular edge originated from two adjacent conical surfaces realizes the seal.

Checking very carefully the position of parts most directly involved in the operation of the system can be requested, in particular checking the position of the operating surfaces, and, among them, of the circular edge. Possible modifications with respect to the nominal positions, indeed, may be responsible of a not proper cooperation between the valve and the relevant seat and/or a cooperation in a position unlike the desired one, as well as of a cylinder compression ratio unlike the nominal one, with resulting decrease of the engine performance and increase of the levels of exit discharge, besides having a risk that the valve may collide with the cylinder itself.

The known to date techniques for checking operating surfaces which may include the circular edge use different technologies and are mainly of two types. A first known checking technique is of contact type, such as that shown in U.S. patent application published with number US2010119104A1, wherein, for example, a touch probe is used, that scans the object in a sufficient number of points to reconstruct a digital image on the basis of which the features of the object are checked. This type of checking, however, generally implies the run of a high number of data which, in most cases, requires expensive tools and/or long processing times. Moreover, it can not be used for checking valve seats having small dimensions. A second known checking technique is of non-contact type, such as that shown in U.S. patent number US7643151B2, wherein, for example, optical measurement devices are used, that take advantage of the interferometric technique to digitally reconstruct the image of the object on which the checking is run. However, this second known technique is, among other things, extremely sensitive to vibrations and dirt, and generally not suitable for checking in workshop environment .

In addition, a point-scanning of the operating surfaces profile of the object is run in both known techniques. The discrete data thus obtained may be, for example, interpolated to reconstruct the image of the object. A position depending on the point-scanning and on the interpolation formula will be associated to the edge. Except for the fortuitous and unlikely event that the profile of the operating surfaces is scanned exactly at the junction edge, such position is not the real one and does not take into account for example imperfections in manufacturing, wear or deposition of material. Disclosure of the Invention

Object of the present invention is to provide a system and a method for quantitatively and accurately defining the position of the edge of an object with respect to a reference surface, such system and such method being free from the previously described inconveniences and, concurrently, easily and cheaply implemented.

According to the present invention, this and other objects are achieved by checking system and method according to the attached claims, which are an integral part of this description.

A system according to the present invention comprises: a support and locating frame, checking elements connected to the support and locating frame and adapted to cooperate with a workpiece along a longitudinal direction, a transducer system connected to the checking elements, and a processing unit connected to the transducer system. The checking elements include two matching elements which have different geometrical features, for instance respective tapered matching surfaces defining slope angles with respect to the longitudinal direction which are different from each other. The matching surfaces are adapted to assume a certain arrangement with respect to the edge to be checked that depends on the geometrical features of the matching elements, for instance on the slope angles of the matching surfaces; the transducer system includes at least a transducer element, which provides electrical signals depending on the arrangement assumed by the matching surfaces and on the position of the edge; the processing unit receives and processes the electrical signals in order to determine the position of the edge with respect to the reference surface. The matching elements are, for example, cone-shaped, or pyramid-shaped with a polygonal base, or exhibit the shape of spherical caps.

A system according to the present invention, for example for checking the axial position of a circular edge, may consider that the two matching elements are feelers adapted to touch the edge to be checked, connected to the support and locating frame by means of at least one shaft, the latter being connected to the transducer element that provides electrical signals depending on the longitudinal position of the shaft, hence of the matching element that it brings .

Advantageously, the shaft is designed for automatically centering each of the two matching elements with the workpiece, in particular with the circular edge, with respect to the longitudinal direction.

The matching elements of a system according to the present invention may have such structural features that they are adapted to touch simultaneously or in a substantially simultaneous way the edge to be checked.

Alternatively, a system according to the present invention may be of the fluid type and comprise a source of pressure fluid (i.e. gas), and a mechanism for positioning the matching elements, which defines predetermined positions of said matching elements along the longitudinal direction and consequent arrangements of the tapered matching surfaces with respect to the edge to be checked that vary as the position of the edge varies. In this case the electrical signals provided by the transducer element (i.e. a pneumo-electrical converter) are indicative of variations of features (i.e. flux or pressure) of said pressure fluid passing through a flow zone delimited by the matching surface of each matching element and the edge to be checked.

Preferably, the support and locating frame includes two checking stations, each of which comprises a transducer element and one of the two matching elements. As an alternative, the support and locating frame may include a Coordinate Measuring Machine (CMM) , which alternatively mounts the matching elements on a movable arm and performs sequential checkings of the workpiece.

In a method according to the present invention, in order to perform checkings by means of a system with the features so far mentioned, the matching elements are brought in a checking condition wherein the respective tapered matching surface assumes a certain arrangement with respect to the edge to be checked, for example it leans on such edge. Signals provided by the transducer system, which depend on the certain arrangement assumed by the matching surfaces and on the position of the edge to be checked, for example signals relative to the longitudinal position of the matching elements in the checking condition, are detected and then processed together with reference signals depending on the position of a reference edge, in order to determine the gap of the edge with respect to the reference edge .

Objects and advantages of the present invention will be clear from the detailed description that follows, concerning a preferred embodiment of the invention, given only by way of non-restrictive example, with reference to the attached drawings.

Brief Description of the Drawings

The present invention is now described with reference to the attached sheets of drawings, given by way of non- limiting examples, wherein:

- figure 1 schematically represents a checking system according to a possible embodiment of the present invention, with a partially sectioned workpiece to be checked;

figure 2 schematically illustrates the operating principle of the checking system shown in figure 1 according to the present invention; - figures 3a and 3b schematically show two different conditions of a method according to the present invention for checking radial and axial positions of the edge of an object with respect to a reference position;

- figure 4 schematically illustrates two different conditions of said checking method according to a preferred embodiment of the present invention;

figure 5 schematically illustrates two different conditions of said checking method according to a different embodiment of the present invention; and

figure 6 schematically illustrates two different conditions of said checking method according to a further different embodiment of the present invention. Best Mode for Carrying Out the Invention

Figure 1 shows the main components of a system for checking the position of an edge 5 of a workpiece 1, in particular for checking an injection system for internal combustion engine, with a reference surface, for example a rest surface 20, and a valve seat 2 that defines a longitudinal axis. The system includes, for example, a support and locating frame 10 apt to locate the rest surface 20 of the workpiece 1. The frame 10 comprises two checking stations A and B essentially identical, each of which includes a checking element connected to the support and locating frame 10 and adapted to cooperate with the workpiece 1 along a direction parallel to the longitudinal axis, namely a longitudinal direction, and a transducer element, i.e. an inductive transducer, schematically represented in figure 1 and referred to with reference 11. The transducer elements 11 of the two stations A and B are part of a transducer system connected to the checking elements .

Each of the checking elements includes, for example, a shaft 12 connected to the inductive transducer 11 and movable along the longitudinal direction with respect to the support and locating frame 10, and a matching element, in particular a feeler 6 (7) adapted to touch the edge 5 to be checked, substantially having rotational symmetry, connected to a free end of the shaft 12. The feelers 6 and 7 have geometrical features different from each other. For example, the feelers 6 and 7 are cone-shaped, having respective tapered matching surfaces, in particular sloped matching surfaces 8 and 9, and differ from each other for a distinct inclination of the respective matching surfaces 8 and 9. Each of said tapered matching surfaces 8 and 9 is adapted to assume a certain arrangement with respect to the edge 5 to be checked that depends on the geometrical features of the respective feeler 6 and 7, more specifically on the inclination of the same tapered matching surface 8 and 9. A processing unit 13, that comprises display devices, is connected to the inductive transducers 11 and receives from them signals M depending on the certain arrangement assumed by the tapered matching surfaces 8 and 9 and on the position of the edge 5, in particular on the longitudinal position of the respective shaft 12, that is on the longitudinal position of the feelers 6 and 7.

To better illustrate the operation of the system according to the invention, figure 2 schematically shows the feelers 6 and 7 superimposed while checking the same workpiece 1 in the respective checking station A and B (as well as figures 3a, 3b and 4 that will be taken into account herein below) . The shaft 12 defines a measurement axis Z that, during the checking of the workpiece 1, is substantially overlapped to the longitudinal axis of the seat 2 thanks to proper reference systems, belonging to the checking stations and not shown in figure, for referring the workpiece 1. Typically, the shaft 12 is properly sized and has structural features of small flexibility that enable limited transverse displacements of the relative feeler 6 or 7, in order to ensure the centering of said feelers with respect to the workpiece 1, enabling the overlapping of the measurement axis Z of the shaft 12 to the longitudinal axis of the seat 2.

With respect to the measurement axis Z, then with respect to said longitudinal axis, the matching surface 8 of the feeler 6 defines a slope angle a, whereas the matching surface 9 of the feeler 7 defines a slope angle β. The slope angles a and β are different from each other.

The seat 2 comprises two surfaces substantially cone- shaped, in particular a central surface 3 and an internal surface 4. Generally, with respect to the longitudinal axis defined by the seat 2, the central surface 3 has a wide slope angle, i.e. greater than 45°, whereas the internal surface 4 has a smaller slope angle, i.e. less than 45°. The intersection between the central surface 3 and the internal surface 4 forms the edge 5 whose axial position has to be checked. The angle a of the matching surface 8 that identifies the feeler 6 is a little bit smaller than the slope angle of the central surface 3 of the seat 2, whereas the angle β of the matching surface 9 that identifies the feeler 7 has a value smaller than both of them but a little bit greater than the slope angle of the internal surface 4.

In its own checking station, each of the feelers 6 and 7, being able to make forward/backward movements along the measurement axis Z to perform longitudinal displacements S, is urged into contact with the edge 5 and assumes positions that depend on the axial position and diametral dimensions of the edge 5 and on the respective slope angle a and β. In each of the checking stations A and B where the workpiece 1 is sequentially checked, the shaft 12 transmits to the inductive transducer 11 the longitudinal displacements S of the respective feeler 6 and 7. The inductive transducer 11 sends electrical signals M to the processing unit 13, such electrical signals M depending on the certain arrangement of the matching surfaces 8 and 9 and on the axial position of the shaft 12, hence of the feeler 6 or 7, as well as of the position of the edge 5. The processing unit 13 processes the signals M coming from both checking stations A and B in order to detect the gap between the position of the edge 5 and a reference position of a reference edge R of a seat 2R belonging to a calibration master 1R, and shows it in a suitable display.

A method for checking the axial position of the edge of an object according to the present invention is described in the following with reference to figures 3a and 3b. For example, checking the radial and longitudinal gap of the edge 5, for instance circular, of the valve seat 2 with respect to the reference edge R, circular too, of the calibration master 1R may be used for checking the axial position P of said circular edge 5 with respect to the rest surface 20, the centering of the feelers 6 and 7 with respect to the workpiece to be checked, that is the substantial overlapping of the measurement axis Z to the longitudinal axis of the valve seat 2, being ensured. The following embodiment shows such application of the method according to the present invention.

The method may comprise a preliminary calibration condition and at least one subsequent checking condition. During the calibration condition, each of the feelers 6 and 7 within the respective checking station A and B, not shown in figure, is brought in a calibration position defined by the contact between the respective matching surface 8 and 9 and the circular reference edge R featuring a known reference axial position Pr with respect to the rest surface 20. In this calibration condition, the inductive transducer 11 provides reference signals M6r (and M7r) depending on the arrangement of each matching surface 8 and 9 with respect to the reference edge R, and on a reference longitudinal position S6r (and S7r) of the relative feeler 6 (and 7) , as well as of the reference position of the reference edge R with respect to the rest surface 20. The processing unit 13 detects and stores the reference signals M6r (and M7r) , and associates them to the reference axial position Pr. During the checking condition, each of the feelers 6 and 7 within the respective checking station A and B is brought in a control position defined by the contact of the respective matching surface 8 and 9 with the circular edge 5 to be checked. The inductive transducer 11 provides and transmits to the processing unit 13 signals M6 (and M7) depending on the longitudinal position S6 (and S7) of the relative feeler 6 (and 7) , as well as on the position of the edge 5 with respect to the rest surface 20. The processing unit 13, processing the signals M6 and M7 together with the reference signals M6r and M7r, compares the longitudinal positions S6 and S7 with the reference longitudinal positions S6r and S7r. From this comparison, and from the known geometrical features of the two feelers 6 and 7 used, in this example the slope angles a and β of the respective matching surfaces 8 and 9, the processing unit 13 evaluates the gap between the reference axial position Pr and the axial position P to be checked. Knowing the reference axial position Pr and such gap, the axial position P may be determined in an easy and accurate way.

In order to better illustrate the method for checking the axial position of the edge of an object according to the present invention, the schematic of figure 4 is referred to, wherein the gap namely the distance between the circular reference edge R and the circular edge 5 to be checked is intentionally and excessively oversized for the sake of clarity. Said gap may be decomposed into a longitudinal component ΔΖ and a radial component ΔΧ.

During the calibration condition, for example, the feeler 6 (and 7) , moving forward along the measurement axis Z within its own checking station A (and B) not shown in figure, is brought in a reference longitudinal position S6r (and S7r) , defined by the contact of the matching surface 8 (and 9) with the reference edge R, that the processing unit 13 associates to the reference axial position Pr . Also for the sake of clearness, in the top of the schematic of figure 4 the feelers 6 and 7 show plane upper surfaces that are aligned with each other when the respective matching surfaces 8 and 9 cooperate with the circular reference edge R. In the checking condition, the feeler 6 (and 7) , moving forward along the measurement axis Z, is urged into contact with the circular edge 5 to be checked, in a longitudinal position S6 (and S7) .

AS6 and AS! are the differences of the longitudinal positions S6 and S7 of the matching elements 6 and 7 in said checking condition with respect to the reference longitudinal positions S6r and S7r, respectively AS6=S6-S6r and AS7=S7-S7r, and are due to both the longitudinal component ΔΖ and the radial component ΔΧ of the gap between the circular edges R and 5. The longitudinal component ΔΖ gives a contribution to the differences of the longitudinal positions AS6 and AS! that is the same for both the feelers 6 and 7, whereas the radial component ΔΧ gives a contribution to said differences that is dissimilar, in particular dependent on the slope angle a and β of the matching surfaces 8 and 9.

In more detail, the longitudinal positions of the feelers 6 and 7 undergo variations, for example, equal to

AS6 = ΔΖ + ΔΧ/tanga, and

AS7 = ΔΖ + ΔΧ/tangP

respectively .

The expression of the longitudinal component ΔΖ may be obtained in an easy and accurate way by deriving from each of the formulae the expression of the radial component ΔΧ and matching the second members of the derived expressions. According to the resulting expression:

ΔΖ = AS6 - AS!^■ (tangP/tangg) .

1- tangp/tanga

From the longitudinal component ΔΖ calculated this way, that is the longitudinal gap of the circular edge 5 to be checked with respect to the reference edge R, and from the reference axial position Pr, the axial position P may be determined, by applying for example the formula

P = Pr + ΔΖ. Advantageously, a checking system according to the present invention may be utilized for checking, for example, both the axial position P, as illustrated hitherto, and the diametral dimension D of the circular edge 5 as a function of the radial component ΔΧ and a known reference diametral dimension Dr of the circular reference edge R. During the calibration condition, the inductive transducer 11 provides the reference signals M6r (and M7r) depending on the reference longitudinal position S6r (and S7r) of the relative feeler 6 (and 7) , as already described. The processing unit 13 detects and stores the reference signals M6r (and M7r) , and associates them to said reference diametral dimension Dr. During the checking condition, substantially analogous to the checking condition already described, the processing unit 13 evaluates the gap between the reference diametral dimension Dr and the diametral dimension D to be checked. Knowing said gap and the reference diametral dimension Dr, the diametral dimension D may also be determined in a just as much easy and accurate way.

In particular, from the formulae of the variations of the longitudinal positions of the feelers 6 and 7, for example, analogous to those already seen and equal to

AS6 = ΔΖ + ΔΧ/tanga, and

AS7 = ΔΖ + ΔΧ/tangP

respectively, the expression of the longitudinal component ΔΖ may be derived. Subtracting the first derived expression from the second one, the expression of the radial component ΔΧ may be obtained in an easy and accurate way, i.e.

ΔΧ = tangg-tangP · [AS! - AS6] .

(tanga-tangP)

From the radial component ΔΧ calculated this way, that is the radial gap of the circular edge 5 to be checked with respect to the reference edge R, and from the reference diametral dimension Dr, the diametral dimension D may be determined, by applying for example the formula

D = Dr + 2 · ΔΧ . A checking system according to the present invention may also be utilized, for example, for checking the position of an edge originated from the intersection of two surfaces that complies with at least one of the following conditions: the central surface 3 is flat and perpendicular to the longitudinal axis of the seat 2, or the internal surface 4 is cylindrical and parallel to the same axis.

A checking system according to the present invention may present various structural modifications as compared with what is schematically described above.

For example, the support and locating frame 10 may include a lock mechanism for locking the workpiece 1. Such lock mechanism may have structural features that enable restricted transversal displacements of the workpiece 1, in order to ensure the overlapping of the measurement axis Z of the shaft 12 to the longitudinal axis of the seat 2 of said workpiece 1 and the centering of the measurement, in replacement of or in addition to the mentioned features of small flexibility of the shaft 12.

In a system according to the present invention, the feelers 6 and 7 may have shape other than conical, for example pyramidal with a polygonal base. In this case, a checking method like that so far illustrated assumes that the contact between the feelers 6 and 7 and the circular edge 5 (and R) occurs at the converging edges of the pyramidal surface, but it doesn't show significant differences with respect to what previously described.

Alternatively, the feelers 6 and 7 may exhibit the shape of spherical caps having the tapered matching surfaces 8 and 9 as spherical surfaces, respectively, as it is shown in figure 5. The spherical caps have radii different from each other, for instance the feeler 6 has radius rad6 and feeler 7 has radius rad7, and height that is less or at most equal to the corresponding radius. In this further example, since the feelers 6 and 7 are adapted to touch the circular edge 5, the matching surfaces 8 and 9 define, at the point of contact with the circular edge 5, tangent planes featuring slope angles a and β respectively. The slope angles a and β defined this way depend on the radius rad6 and rad7 of the respective spherical caps and on the position of the circular edge 5 and, contrary to what previously described, are not known a priori. The formulae of the variations AS6 and AS7 of the longitudinal positions of the feelers 6 and 7 as a function of the longitudinal component ΔΖ and the radial component ΔΧ, nonetheless, may be obtained by using simple trigonometry calculus. In more detail, the longitudinal positions of the feelers 6 and 7 undergo variations, for example, equal to

AS6 = ΔΖ + Vrad6^ - (rr-ΔΧ)^ - rad6^ - rr", and

AS7 = ΔΖ + rad7^ - (rr-ΔΧ)^ - rad7 - rr"

respectively, where rr refers to a known reference radius of the circular reference edge R. The expression of the longitudinal component ΔΖ may be obtained in an easy and accurate way by deriving from each of the formulae the expression of the radial component ΔΧ and matching the second members of the derived expressions. According to the resulting expression:

ΔΖ = AS6² - AS7² + 2 (AS6- Vrad6^-rr^ - AS6 · Vrad7^-rr^)

2 (AS6 - AS7 + Vrad6^-rr^ - Vrad7^-rr^)

The checking method then proceeds analogous to the method previously illustrated, without substantial differences.

Referring to a possible alternative embodiment of a system according to the present invention, the feelers 6 and 7 may have such structural features that they may simultaneously, or in a substantially simultaneous way, cooperate with the edge to be checked. For example, one of the feelers 6 (or 7) may be provided with an internal recess, for housing the other feeler 7 (or 6) , and transit holes on the matching surface 8 (or 9) for enabling portions of the matching surface 9 (or 8) of the other feeler 7 (or 6) , conveniently shaped, to protrude and be radially aligned with portions of the matching surface 8 (or 9) of the first feeler 6 (or 7) . In a checking method carried out by means of this alternative system, the contact between the feeler 6 and the edge 5 (and R) and the contact between the feeler 7 and the same edge 5 (and R) may occur in an essentially simultaneous way. This alternative solution enables the time for checking to be reduced .

Other possible checking systems according to the present invention comprise feelers with different shape, i.e. with internal matching surfaces for checking external diameters, and may be used for checking the edge of a valve intended to be housed in a relative seat with the task of closing the intake and discharge ducts that are headed to the combustion chamber in the cylinder head of an internal combustion engine. Other possible checking systems according to the present invention may exhibit different structural modifications for further types of checking of position, even with respect to reference surfaces other than the rest surface 20, or dimensions of edges, closed or open, with profiles unlike that illustrated in the figures.

In a different embodiment of the invention, shown in figure 6, a fluid, for example pneumatic, checking system enables the checking to be executed without directly contacting the workpiece 1. The checking elements include a suitable locating mechanism 15, and the feelers 6 and 7 may be substituted for example with two matching elements 16 and 17 that do not touch the edge 5 to be checked but are intended to be located by means of the locating mechanism 15 to a predetermined position along the longitudinal direction in order to assume a certain arrangement with respect to the edge 5 to be checked, in other words at a distance from the edge 5, so that the respective tapered matching surfaces are facing such edge 5 and delimit a flow zone with it. The matching elements 16 and 17 keep the same geometrical features of the feelers 6 and 7, in particular they differ from each other for the distinct slope angles a and β with respect to the measurement axis Z, defined by the tapered matching surface 8 and 9 they respectively have. Said checking system further comprises a source of a pressure fluid, i.e. a gas source, and the transducer system includes for example a pneumo-electrical converter that, for both matching elements 16 and 17, detects in said flow zone variations of features of the pressure fluid, i.e. variations of pressure or flux (the fluid being represented with dotted curved lines in figure) , and transforms such variations in electrical signals M depending on the arrangement of each tapered matching surface 8 and 9 with respect to the edge 5, more specifically the distance between the former and the latter. As a consequence, the electrical signals M are indicative of the variations of features of the pressure fluid passing through the flow zone delimited by the tapered matching surface 8 (and 9) of each matching element 16 (and 17) and the edge 5 to be checked. The pneumo- electrical converter then sends said electrical signals M to the processing unit 13. The gas source and the pneumo- electrical converter are not shown in figure 6.

A checking method according to this different embodiment may comprise, similarly to what previously described, a preliminary calibration condition and a subsequent checking condition. In each condition, the matching elements 16 and 17 are sequentially brought in a calibration position, or in a control position, which in both cases is defined by said predetermined position along the longitudinal direction, set for example by the contact with an abutment plane or element that realizes the so- called locating mechanism 15. In such predetermined position, in calibration condition the tapered matching surfaces 8 and 9 assume a known arrangement with respect to the circular reference edge R, more specifically a known distance from the latter, in checking condition the tapered matching surfaces 8 and 9 assume an unknown arrangement with respect to the circular edge 5 to be checked, more specifically an unknown distance from the circular edge 5. In each condition and for each matching element 16 (17) , the pneumo-electrical converter performs, in a way known per se, the checking on the basis of the features of the fluid passing through the cooperation zones between the matching surface 8 (9) and, respectively, the reference edge R and the edge 5 to be checked, and transmits the results to the processing unit 13. The processing unit 13 receives and processes such results and the known geometrical features of the two matching elements 16 and 17 used, in particular the slope angles a and β, to determine the gap between the reference axial position Pr and the axial position P to be checked.

As an alternative to the two checking stations A and B, the system may include Coordinate Measuring Machine, or CMM, for sequentially checking a workpiece whose exact position is known instant by instant. Such CMM may comprise in a manner known per se a storage wherein the feelers 6 and 7 are arranged, a suitable automatic change mechanism and a device for locking them. In this embodiment, the feelers 6 and 7 are alternatively mounted on a movable arm of the CMM to be brought in a preliminary calibration condition and in at least one subsequent checking condition. The checking method is one of those previously described .

In a method according to the present invention, the reference longitudinal positions S6r and S7r may be obtained, as described hitherto, at each checking operation, or may be the result of calibration operations performed just once at the beginning of the checking of a sequence of workpieces and/or periodically performed after a certain amount of checked workpieces, or may be a data known a priori.

Advantages resulting from the application of the present invention are clear.

First of all, a system and a method according to the invention enable the position and, advantageously, the dimensions of an edge to be directly checked, avoiding indirect checkings of the adjacent surfaces and consequent interpolations .

This is also the reason why the number of data to run for the checking is significantly lower with respect to that required by the technique known to date.

Consequently, and due to the application of an analytical formula which is function of the known geometric features of the matching elements used, times for processing are greatly reduced.

The matching elements can be chosen for checking dimensions also of very small pieces.

The system object of the present invention has simple, robust and compact components, and is little sensitive to disturbances (vibrations, dirt) that are present in workshop environment.

Claims

1. System for checking position and/or dimensions of an edge (5) of a workpiece (1) that defines a longitudinal axis, comprising:

- a support and locating frame (10) ,

- checking elements connected to said support and locating frame (10) , adapted to cooperate with said workpiece (1) along a direction parallel to the longitudinal axis,

- a transducer system connected to said checking elements, and

- a processing unit (13) connected to said transducer system,

characterized in that:

- said checking elements include two matching elements (6, 7; 16, 17) having geometrical features different from each other and respective tapered matching surfaces (8, 9) , each of the tapered matching surfaces (8, 9) being adapted to assume a certain arrangement with respect to the edge (5) to be checked that depends on the geometrical features of the respective matching element (6, 7; 16, 17),

- said transducer system includes at least a transducer element (11) , which provides electrical signals (M; M6,

M7) depending on said certain arrangement and on the position of said edge (5) ,

- said processing unit (13) being adapted to receive said electrical signals (M; M6, M7) and process them in order to determine an axial position of said edge (5) with respect to a reference surface.

2. System according to claim 1, wherein said geometrical features of the two matching elements (6, 7) include slope angles (α, β) of the tapered surfaces (8, 9) with respect to said longitudinal axis which are different from each other .

3. System according to claim 1, wherein said matching elements (6, 7) exhibit the shape of spherical caps having said tapered surfaces (8, 9) as spherical surfaces, and said geometrical features include radii (rad6, rad7) of the spherical caps which are different from each other.

4. System according to any one of claims 1 to 3, wherein said two matching elements (6, 7) are feelers adapted to touch said edge (5) to be checked, said at least a transducer element (11) being adapted to provide electrical signals (M; M6, M7) depending on a longitudinal position of the feelers (6, 7) .

5. System according to any one of claims 1 to 3, comprising a source of a pressure fluid, wherein said checking elements include a locating mechanism (15) , the matching elements (16, 17) being intended to be located by means of the locating mechanism (15) to predetermined positions along the direction parallel to the longitudinal axis, and the electrical signals (M) provided by said at least a transducer element being indicative of variations of features of said pressure fluid passing through a flow zone delimited by the tapered matching surface (8, 9) of each matching element (16, 17) and the edge (5) to be checked.

6. Method for checking position and/or dimensions of an edge (5) of a workpiece (1) that defines a longitudinal axis, by means of a system with two matching elements (6, 7; 16, 17) having geometrical features different from each other and tapered matching surfaces (8, 9) , each of the tapered matching surfaces (8, 9) being adapted to assume a certain arrangement with respect to the edge (5) to be checked that depends on the geometrical features of the respective matching elements (6, 7; 16, 17), and a transducer system adapted to provide electrical signals (M; M6, M7) depending on said certain arrangement and on the position of said edge (5) , the method including the steps of:

- bringing each of said matching elements (6, 7; 16, 17) in a checking condition, wherein the respective matching surface (8, 9) assumes said certain arrangement with respect to the edge (5) to be checked,

- detecting the electrical signals (M, M6, M7) provided by the transducer system, and

- processing said signals (M, M6, M7) and reference signals (M6r, M7r) depending on a reference arrangement of each matching surface (8, 9) with respect to a reference edge (R) and on an axial position of the latter with respect to a reference surface, in order to determine a gap between the edge (5) to be checked and the reference edge (R) , and an axial position of said edge (5) with respect to the reference surface.

7. Method according to claim 6, wherein said geometrical features of the two matching elements (6, 7) include slope angles (α, β) of the tapered surfaces (8, 9) with respect to said longitudinal direction parallel to the longitudinal axis which are different from each other, and the gap between the edge (5) to be checked and the reference edge (R) may be decomposed into a longitudinal component (ΔΖ) and a radial component (ΔΧ) , the longitudinal component (ΔΖ) being obtained by means of the following formula

ΔΖ = AS6 - AS7 · (tangP/tangg) ,

1- tangp/tanga

where AS7 and AS6 are differences of longitudinal positions (S6, S7) of the matching elements (6, 7) in said checking condition with respect to reference longitudinal positions (S6r, S7r) , the latter being defined at the reference arrangement of each matching surface (8, 9) with respect to the reference edge (R) .

8. Method according to claim 7, wherein the radial component (ΔΧ) is obtained by means of the following formula ΔΧ = tangg-tangP [AS7 - AS6] .

(tanga-tangP)

9. Method according to claim 8, wherein the edge (5) to be checked is circular, and the radial component (ΔΧ) is used for checking diametral dimension (D) of said circular edge (5) .

10. Method according to any one of claims 6 to 9, wherein the reference surface is a rest surface (20) of the workpiece (1) .