Acoustic sensor for monitoring machining processes in machining tools

An acoustic sensor for monitoring machining processes in machining tools, e.g. grinding machines, includes a stationary unit, or stator (21) and a rotating unit, or rotor (23), the latter being coupled, for instance, to the grinding wheel and rotating with it. The rotor includes a support (45;45′) and a piezoelectric transducer (33). The position of the transducer is defined and locked without using glues by means of mechanical elements (48, 53, 54; 63, 64; 74, 78) that may include elastic elements such as springs. The rotor also includes a printed circuit board (55), connected to the piezoelectric transducer, with a charge amplifier (37) for amplifying the output signal of the piezoelectric transducer. The rotor is electrically coupled to the stator by means of transformer type couplings (30, 31).

TECHNICAL FIELD

The present invention relates to an acoustic sensor including a stationary unit, a rotating unit with support and protection elements with a support, a vibration detecting transducer coupled to the support, positioning and clamping elements adapted to define and lock the position of the transducer, and power supply, processing and transmission circuits with at least one contactless coupling adapted for achieving the electric connection between the stationary unit and the rotating unit.

BACKGROUND ART

Acoustic sensors are known and used, for example, in apparatuses for monitoring the machining process of computer numerical control (“CNC”) machine tools, as lathes, grinding machines, milling machines, etc. such apparatuses are able to detect, by means of sensors, the magnitude of physical features connected to the process to be checked, for example the wear of the tool, and indicate to the machine numerical control, or directly to the operator, the need to perform maintenance and/or corrective procedures.

An apparatus of this kind is disclosed in International patent application No. PCT/EP02/07519, filed by the same applicants of the present patent application.

In machining process monitorings on grinding machines, the acoustic sensors are generally coupled to the flange for securing to the spindle the grinding wheel (as shown in simplified form inFIG. 2) or the dressing wheel and enable to detect vibrations generated by the occurrence of contact between the grinding wheel (or the dressing wheel) and the workpiece (or the grinding wheel), in the course of the machining.

More specifically, the acoustic sensors presently utilized in apparatuses of the type described in the formerly mentioned patent application include a rotor with a casing coupled to a movable part as, for example, the grinding wheel of a grinding machine. Furthermore, the rotor includes a piezoelectric transducer glued to the casing and electric circuits for conditioning the output signal of the piezoelectric transducer. The rotor is coupled, by means of a transformer type coupling, to a stator connected, for example, to the bed of the grinding machine, that includes further electric circuits for processing and transmitting the signal received from the electric circuits of the rotor.

An acoustic sensor of this type is disclosed in European patent application EP-A-0446849.

The structure and the dimensions of the components of an acoustic sensor as the one disclosed in the formerly mentioned European patent application, provide good standards of performance in response to acoustic signals with frequencies reaching up to a few hundreds of KHz.

In a grinding machine in which it is desired to reach high rotation speeds, the grinding wheel (and/or the dressing wheel) is coupled to an electrospindle. The use of high speed electrospindles generates an acoustic background noise that, in the frequency range utilized by the known acoustic sensors (50–250 KHz), is added to the acoustic signal generated by the occurrence of contact between the grinding wheel and the workpiece or the dressing wheel thus making the detecting of said contact extremely difficult. In the graph ofFIG. 1there are shown, in logarithmic scale and as a function of frequency F, the trends of the spectral densities DS1and DS2of the background noise generated, in a grinding machine, by a traditional type spindle and by a high speed electrospindle, respectively, carrying the grinding wheel or the dressing wheel.

Furthermore, the dashed line inFIG. 1indicates the trend of the spectral density DS3of the acoustic signal generated by contact occurring between grinding wheel and workpiece or dressing wheel (that is the signal to be detected), that does not depend on the type of operation of the grinding wheel and, within certain limits, on the rotation speed of the grinding wheel itself. The graph ofFIG. 1shows how, at high speed (DS2), the ratio between the signal generated by the occurrence of contact between grinding wheel and workpiece and background noise is critical in the range of frequencies 50–250 KHz. As a consequence, the checking of the machining process, performed by an acoustic sensor operating in said range of frequencies, is extremely problematic. The ratio between useful signal and background noise is acceptable at higher frequencies (in the range 500 KHz–1 MHz). Thus, in order to perform machining process checkings on grinding machines with a grinding wheel rotating at high speed, activated by an electrospindle, the acoustic sensors like the one disclosed in patent application No. EP-A-0446849 have poor performances in consideration of the limited range of frequencies within which there is guaranteed good response.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a broad band acoustic sensor that can transduce high-frequency acoustic signals in order to overcome the previously mentioned problems.

This and other objects are achieved by an acoustic sensor according to claim1.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 2is an extremely simplified and partial plan view of a machine tool, more specifically a computer numerical control (“CNC”) grinding machine1including a bed2, a tool-carrier3, more specifically a wheel-carrier, coupled to a slide4, that can displace relative to bed2along an axis Y and is, on its turn, coupled to a slide5that can displace relative to bed2along an axis X, in such a way that the tool-carrier3can perform movements in plane XY relative to bed2. A spindle, for instance an electrospindle7, is carried by the tool-carrier3and a grinding wheel9is coupled, by means of a flange11, to the electrospindle7. The electrospindle7causes the grinding wheel9to rotate about a longitudinal axis M.

The grinding machine1is utilized for the machining of a mechanical piece13, with rotational symmetry, for example a shaft, supported, referenced and made to rotate about a longitudinal axis P, parallel to the longitudinal axis M, by means of a support and reference system of the known type that consists, for example, of a live center15and a dead center17.

The grinding machine includes an apparatus for checking the machining process by means of an acoustic sensor (or AE sensor)19, illustrated in more detail inFIG. 4, including a stationary unit21(or stator) and a movable, rotating unit23(or rotor). The stator21is coupled to the bed2, in a known and herein not shown way, while rotor23is coupled to flange11for the coupling of the grinding wheel9to the electrospindle7.

FIG. 3is an elementary circuit diagram of the acoustic sensor19.

The acoustic sensor19includes power-supply, processing and transmission circuits with a sinusoidal voltage generator27in stator21, power-supply circuits with a rectifier29in rotor23, and a first contactless coupling, more specifically a transformer type inductive coupling30, for the electric connection between generator27and rectifier29. A vibration detecting transducer, more specifically a piezoelectric transducer33, is located in rotor23, the latter also houses a first amplifier, or charge amplifier37that receives the signal output by the piezoelectric transducer33, and is power supplied by rectifier29. The charge amplifier37is connected—by means of a second contactless coupling, more specifically a transformer type inductive coupling31—to a second amplifier, or line amplifier39, in stationary unit21, the latter amplifier being in turn connected to a processing and display unit40, external to sensor19, for the processing and the displaying of the signal.

The acoustic sensor19operates in the following way. When a vibration is generated, for example, by the occurrence of contact between grinding wheel9and piece13, the piezoelectric transducer33emits an electric signal that is sent to the amplifier37, amplified by the latter and sent, by means of the transformer type coupling31, to the line amplifier39that amplifies and transmits it, on an adapted line, to the processing and display unit40. Amplifier37is power supplied by the sinusoidal voltage generator27through the rectifier29that is connected, by means of the transformer type coupling30, to the voltage generator27.

FIG. 4is an axonometric, cross-sectional view of rotor23of the acoustic sensor19according to a first embodiment of the invention. Rotor23includes support and protection elements with a support45, for example made from steel, defining a seat47, and positioning and clamping elements with an annular centering element48, for example made from plastic material like PVC. The positioning and clamping elements include also mechanical clamping devices with a thrust device having an elongate element54and a ball53, made from conductive material, welded in a seat43defined by the elongate element54. The elongate element54is in turn coupled, by means of an adjustable threaded coupling, to a bushing56, integral to the support and protection elements of the rotor23.

The piezoelectric transducer33has a cylindrical shape with two plane faces, and is arranged in a hole defined by the annular element48, that determines the transversal position of the transducer33within seat47.

The ball53defines an abutment surface that operates on the piezoelectric transducer33by urging and locking it against a surface49of the support45, with a force that can be set thanks to the threaded coupling between the elongate element54and the bushing56. Moreover, the support45, the elongate element54and the ball53have the function of electrically connecting the electrodes50,52of the transducer (that coincide with the plane faces of the piezoelectric transducer33) and a printed circuit board55that carries the bushing56and includes the rectifier29and the charge amplifier37. More specifically, the lower electrode52is connected to the printed circuit board55by means of the surface49of the support45and three screws57set 120° apart, only one shown inFIG. 4, that also serve to secure the printed circuit board55to the support45. The upper electrode50is electrically connected to the printed circuit board55by means of the ball53and the elongate element54that is in contact, by means of the bushing56, with a conductive portion on printed circuit board55. Moreover, the printed circuit board55includes the secondary winding of the transformer type coupling30and the primary winding of the transformer type coupling31. The support45includes a threaded spigot51for securing rotor23to flange11, in a known way. The support and protection elements include also a first, substantially cylindrical-shaped, casing46, sealed at an end and made, for example, from plastic material and a second, substantially cylindrical-shaped, casing44made, for example, from steel. The manufacture of the first casing46from plastic material enables the electromagnetic coupling between the windings of rotor23and stator21.

An important characteristic that an acoustic sensor19according to the invention provides is that of detecting and transducing high frequency acoustic signals. This is achieved thanks to a piezoelectric transducer33with small dimensions and, consequently, high resonance frequency. The direct connection of the piezoelectric transducer33to the charge amplifier37, in other words the amplification of the output signal of the piezoelectric transducer33, enables to minimize the influence of parasitic electric parameters that could lower the sensitivity and the transducing frequency range. Moreover, the direct amplification of the transducer signal diminishes the influence of the electric noise generated by the subsequent processing circuits. More specifically, the transmission to the stator21of a signal having amplified amplitude and power improves the immunity of the whole acoustic sensor19from external electromagnetic interferences.

The glueless coupling of the transducer33to the support45, achieved by means of a rigid mechanical coupling that, in the embodiment ofFIG. 4, includes the ball53and the adjustable coupling between the elongate element54and the bushing56, guarantees a low acoustic impedance between the piezoelectric transducer33and the support45and ensures a good response of the acoustic sensor to high frequencies. Furthermore, this mechanical coupling is advantageous over the known solutions that foresee the use of glues also because it gives the possibility of removing and substituting the transducer by performing simple operations, for example in the case of transducer failure. The use of the line amplifier39, i.e. an amplifier with output impedance equal to the cable impedance, enables the analogic transmission of the signal to the processing and display unit40even when there are long connecting cables, without there being problems of attenuating the signal and limiting its frequency range. This is particularly advantageous when it is required that the connecting cables between acoustic sensor19and processing unit40be some tens of meters long, owing to the layout dimensions and the build of the grinding machines.

The amplifier37is power supplied by a voltage transmitted by stator21to rotor23by means of a transformer type coupling (30). In order to avoid the irradiation of electromagnetic fields, generated by this type of transmission of the signal, the frequency of the power supply sent to the primary winding of the transformer30is stabilized by a quartz oscillator27. Furthermore, the emission of harmonics is furtherly reduced by a capacitor C that, connected in parallel to the primary of the transformer30, achieves a circuit resonant at the same frequency as the quartz oscillator27.

Variants with respect to what is herein described are feasible. In the event the space available on the grinding machine1does not permit the mounting of the stator21as shown inFIG. 2, it can be mounted on bed2of the grinding machine1at the end of the electrospindle7opposite the one carrying the grinding wheel9. In this case, the rotor23is still coupled to the flange11for coupling the grinding wheel9to the electrospindle7but the windings of the transformer type couplings30and31integral to rotor23are coupled to the electrospindle7at the end opposite the one carrying the grinding wheel9and the stator21is arranged on slide4in a position that enables the electromagnetic coupling with the windings of rotor23. The connection among these windings, rectifier29and amplifier37can be made by means of cables, through a hole traversing the electrospindle7, or by means of a wireless connection, for example by means of optical signals, also transmitted through a hole in the electrospindle7.

Acoustic sensors with other types of rigid mechanical coupling between the piezoelectric transducer33and the support45also fall within the scope of the present invention.

For example, the thrust device can include, instead of ball53, a per se known elastic element between the elongate element54and the electrode50, like a “cup” spring. According to another possible embodiment, the elongate element54with the ball53are replaced with an elastic element like a spring that operates on the electrode50—in a direct way or by means of an element for protecting the surface of the electrode itself—for urging and locking the transducer33against the surface49of the support45. In this case the spring may achieve both the previously described rigid mechanical coupling and the electric connection between the electrode50and a suitable conductive portion on the printed circuit board55.FIG. 5shows an embodiment of the invention that includes these features, wherein the spring, identified by reference number64, is arranged between the support and protection elements and the transducer33. More specifically, the spring64is coupled on one side to a centering element66, coupled to the printed circuit board55, and on the other side to a conductive plate63abutting against the transducer33. The conductive plate63has an annular plane surface in contact with the electrode50, and this prevents the surface of the electrode50from deteriorating owing to the action of the spring64.

Another possible embodiment is shown inFIG. 6, where the mechanical clamping devices still include an elastic element, in this case an annular leaf spring74. A modified annular centering element78housed in seat47′ of support45′, defines a through hole—with two concentric portions of different diameters and an annular abutment surface72—housing the piezoelectric transducer33and a conductive protection plate73similar to plate63of the embodiment ofFIG. 5. An abutment ring70is partially housed in an annular seat71defined by an internal cylindrical surface of support45′, and the annular leaf spring74is arranged between the abutment ring70and the centering element78as shown inFIG. 6. More specifically, the outer and inner rims of the annular leaf spring74cooperate with the abutment ring70and the centering element78, respectively, pushing the latter—and, through the annular abutment surface72, the piezoelectric transducer33—against the surface49′ of the support45′.

In this case the leaf spring74achieves the rigid mechanical coupling between the transducer33and the support45, but not any electric connection. An axially arranged bushing75is coupled to the printed circuit board55through a cap76(e.g. by means of welding) and houses and guides a movable rod77, the latter having a protruding end in contact with protection plate73. The bushing75also houses a spring (not shown in the figure) outwardly thrusting the rod77with a light force sufficient to keep the contact between rod77and plate73in order to perform and guarantee the electric connection between the electrode50and a suitable conductive portion of the printed circuit board55.

The embodiment ofFIG. 6allows to safely lock in position the transducer33with a high force without applying an excessive thrust on possibly delicate parts of the support and protection elements, e.g. including the printed circuit board55.