« DEVICE AND METHOD FOR TRANSCE IVING ALTERNATING OPTICAL SIGNALS »

Technical Field

The present invention relates to a device for transceiving alternating optical signals, comprising a first part and a second part electrically isolated from each other, a secondary emitter, comprised within the second part and facing the first part, adapted to transmit an alternating optical signal, a primary receiver, comprised within the first part and facing the second part, adapted to receive the alternating optical signal and generate a corresponding analog electrical signal, and a signal detector comprised within the first part and connected to the primary receiver, adapted to extract an AC component with substantially zero average from the above- mentioned analog electrical signal.

The invention also relates to a method for transceiving alternating optical signals between a first part and a second part electrically isolated from each other, comprising the steps of transmitting an alternating optical signal, receiving such alternating optical signal and generating a corresponding analog electrical signal, and extracting an AC component with substantially zero average from the analog electrical signal.

The present invention can be advantageously but not exclusively applied to the transmission of checking signals from a rotating part to a stationary part and relevant reception, in particular in a machine tool, which the description that follows will explicitly refer without loss of generality. Prior Art

Unwanted vibrations are generally present in a machine tool, caused by out-of-balance conditions of the rotating tool mounted on the machine, which different factors incidental to the same tool are responsible of. For instance, the out-of-balance conditions that cause the vibrations in a grinding machine are due to imperfections connected to shape and manufacture of the grinding wheel, such as non homogeneous materials and errors of concentricity between external abrading surface and internal centering hole, namely to the inaccurate assembling of the grinding wheel to the rotating flange so the center of gravity of the grinding wheel does not lie on the rotation axis, or to modifications due to wear and/or splinter during the grinding of the workpieces being machined. Such vibrations originate inaccuracies in the characteristics of the machined workpieces, for example ovality and lobing, and uprising of forces and tensions that may damage the machine tool itself.

In order to compensate said out-of-balance conditions, the grinding machines may include a balancing device with a balancing head connected to the grinding wheel, rotating with the latter with respect to supporting means of the machine tool, for example a wheel slide designated to reciprocally move the grinding wheel and the workpiece being machined, and comprising movable masses, the position thereof is adjustable and the corresponding displacement occurs by means of driving electric motors. In addition to the balancing device with the balancing head, a vibration sensor may be included within the grinding machine, for instance an acoustic sensor to detect acoustic emissions due to the contact between the grinding wheel and the workpiece being machined or between the grinding wheel and a dressing tool, and to generate checking electrical signals. These checking signals, together with the corresponding control signals for the wheel slide, that control the speed of the reciprocal displacement between the grinding wheel and the workpiece being machined, are sent to a processing and control unit associated to the stationary part through a system for bidirectionally contactless transceiving, including a transceiver device with suitable transceiver units or capsules. The system for transceiving also enables the contactless connection between the processing and control unit and the balancing device, said system being designated for sending and receiving, in addition or as an alternative to the already cited checking signals and control signals for the wheel slide, also diagnostic signals concerning the balancing head, as well as control signals for the driving electric motors, that control the movable masses displacements.

For example the U.S. patent number US5688160, that refers to a known balancing device which a vibration sensor is associated to, shows a transceiving device that comprises a first transceiving part (stator) , which is tightly connected to a stationary part of the machine tool and may be connected to the processing and control unit, and a second transceiving part (rotor) , which is partially located in the spindle of the machine tool and rotates with the latter, and may be wired to the balancing head, the two transceiving parts facing to one another, in coaxial positions with respect to the spindle rotation axis which the grinding wheel is fixed to. In particular, the rotor transmits to the stator the signal coming from the vibration sensor and the possible diagnostic signals, containing for instance information about the condition and/or the proper operation of the balancing head or the whole rotating group. When a balancing device regularly operates, such as that one corresponding to the already cited U.S. patent number US5688160 and commercialized by Marposs S.p.A., the communication between rotor and stator is optically performed and generally starts in serial manner, that is digital signals are transmitted. When a more refined signal processing should be advantageously carried out, for example when the machine tool operates at high speed or in the presence of considerable background noise so it is useful to select a frequency band wherein the signal-to-noise ratio is optimum or amplify a weak checking or diagnostic signal, the communication between rotor and stator is preferably performed in exclusive analog manner, therefore any kind of serial communication between rotor and stator is interrupted. The analog optical signal that concerns such communication may be separated in two components: an AC component, bringing the information content coming from the acoustic vibration sensor, and a continuous component, only functional for the optimal transmission of the AC component and without information content, which is the reason why the continuous component is usually removed in the stator. In the optical analog communication any level of the signal received by the stator, within a determined range, is possible, zero included. Consequently, a transceiving device according to the known art does not allow recognizing whether a null signal is caused by the rotor properly operating in absence of signals detected by the acoustic vibration sensor or by a malfunctioning rotor. Disclosure of the Invention

Object of the present invention is to provide a device and the related method for transceiving alternating optical signals, such device and method being free from the previously described inconveniences and, concurrently, easily and cheaply implemented.

According to the present invention, this and other objects are reached by a device and a method for transceiving alternating optical signals according to claim 1 and, respectively, claim 12, that are an essential part of the present description.

These and other objects and advantages of the present invention will become apparent from the detailed description that follows, according to a preferred embodiment of the invention, given by way of non-limiting example. Brief Description of the Drawings

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

- figure 1 is a schematic view, with a longitudinal partial section, of a machine tool with a device according to a preferred embodiment of the present invention;

- figure 2 is an enlarged longitudinal section of a detail of figure 1;

- figure 3 is a functional block diagram of the detail of figure 2; and

- figure 4 is a functional block diagram of a detail of a machine tool with a device according to a different embodiment of the present invention.

Best Mode for Carrying Out the Invention

As a non-limiting example of implementation of the present invention, the diagram of figure 1 shows a machine tool M, in particular a grinder for machining a workpiece W, that includes a tool or grinding wheel 1 connected to a spindle 2 and rotating with the latter about a longitudinal axis A with respect to supporting means 3, for instance a wheel slide, under the control of a processing and control unit 4.

The machine tool M includes also a tool control system

9 comprising a balancing mechanism 6 and an acoustic vibration sensor 8, for instance piezoelectric, and a device for transceiving or transceiver 7 which is electrically connected by means of a cable 5 to the balancing mechanism 6 and to the piezoelectric sensor 8 in order to enable the contactless connection of these last two to the processing and control unit 4, as it will be better illustrated in the following. The balancing mechanism 6, the piezoelectric sensor 8, part of the transceiver 7 and the cable 5 are located within a longitudinal hole 20 of the spindle 2. The balancing mechanism 6 is designated for balancing the grinding wheel 1 according to a method per se known and uses, for example, movable masses, the position thereof is adjustable through driving electric motors controlled by means of signals sent by the processing and control unit 4 through the transceiver 7 (the masses and the motors are not shown in figure) . The piezoelectric sensor 8 is adapted to detect vibrations caused by the contact between the grinding wheel 1 and the workpiece W being machined, and send to the processing and control unit 4, through the transceiver 7, a corresponding checking signal that is processed in a known way to modify the speed of the reciprocal displacement between wheel slide and workpiece W during the grinding procedure .

The transceiver 7 substantially comprises two parts electrically isolated, for instance spaced apart, from each other, as it is schematically illustrated in figure 2 at a distance intentionally enlarged with respect to figure 1 for better understanding, a first part or stationary part 71 connected to the supporting means 3 through an arm 30, and a second part or rotating part 72 located within the hole 20 of the spindle 2 and adapted to rotate with the latter about a longitudinal axis A. The transceiver 7 realizes a contactless connection among the processing and control unit 4, connected to the stationary part 71, and components of the tool control system 9, the piezoelectric sensor 8 among these, connected to the rotating part 72.

Referring to figure 2, the stationary part 71 comprises a first casing 11 with a first reference and supporting element 12, and primary circuits 13 fixed to the first reference and supporting element 12, that contain optoelectronic devices with a primary receiver or photodiode 14 and a primary emitter or LED 15, both of them facing the rotating part 72. A first ferromagnetic cylindrical capsule 16, also fixed to the first reference and supporting element 12, defines a seat for primary electrical windings 17, electrically connected to the primary circuits 13 in turn. The rotating part 72 comprises a second casing 21, inserted into the hole 20 of the spindle 2, with a second reference and supporting element 22. Secondary circuits 23, that contain optoelectronic devices with a secondary receiver or photodiode 24 and a secondary emitter or LED 25, both of them facing the stationary part 71, are located and fixed within the second casing 21. A second ferromagnetic cylindrical capsule 26, connected to the second reference and supporting element 22, defines a further seat for secondary electrical windings 27, electrically connected to the secondary circuits 23.

For example, the secondary LED 25 is adapted to transmit an alternating optical signal, e.g. for checking, while the primary LED 15 is adapted to transmit an operation, e.g. control and/or request, alternating optical signal. The primary photodiode 14 and secondary photodiode 24 are adapted to receive, respectively, such optical signals, and generate corresponding and operation analog electrical signals.

The functional blocks of figure 3, that shows architectural elements already shown in figure 2, illustrate in a very schematic way some functions of the primary circuits 13 and secondary circuits 23, respectively of the stationary part 71 and the rotating part 72 of the transceiver 7.

In particular, the secondary circuits 23 include an interface 50 for the interaction of the rotating part 72 with the devices connected to the spindle 2, for instance with the driving electric motors and/or the piezoelectric sensor 8, none of them shown in figure 3, a second adapter 201 which connects the interface 50 to the secondary LED 25, a rectifier 202 of the supply alternating current transmitted through the electromagnetic inductive coupling between the secondary electrical windings 27 and primary electrical windings 17, for feeding all the circuits of the rotating part 72, and a second demodulator 204 that connects the secondary photodiode 24 to the interface 50. The second adapter 201 has the task of modulating in a suitable way, analogically or digitally, for instance the checking electrical signal sent by the piezoelectric sensor 8 via the interface 50, in order to optically transmit it to the secondary LED 25, and may comprise one or more signal amplifiers, an analogic modulator and an On/Off Keying modulator, these two being connected to a multiplexer adapted to control the analog or digital driving of the secondary LED 25. The one or more signal amplifiers, the modulators and the multiplexer are not shown in figure. The second demodulator 204 has the task of transforming the control and/or request analog electrical signal received by the secondary photodiode 24 in a corresponding suitably demodulated digital electrical signal, for instance to start the balancing procedure that is to say to control the driving electric motors via the interface 50.

The primary circuits 13 comprise a first adapter 101 that connects the primary LED 15 to the processing and control unit 4 and has a function alike that of the second adapter 201, being adapted to transform the control and/or request electrical signal coming from the processing and control unit 4 in a signal suitably modulated to be optically transmitted to the primary LED 15, an oscillator 102 connected to the processing and control unit 4 and to the primary electrical windings 17 to generate a supply alternating current adapted to be transmitted to the rotating part 72 through the already cited inductive coupling, and an enabling circuit 103 adapted to enable the functions of the oscillator 102 following an appropriate signal coming from the processing and control unit 4. In addition, the primary circuits 13 include three different blocks for processing the checking optical signal sent by the secondary LED 25 and received by the primary photodiode 14: a first demodulator 104 that, like the second demodulator 204, has the task of transforming the checking analog electrical signal output from the primary photodiode 14 in digital electrical signal, a signal detector 105, comprising for instance a band-pass filter and one or more amplifiers (not shown in figure) , and a bias detector 106, that can include a band-pass filter, one or more amplifiers and a dial gauge with hysteresis (not shown in figure) . The signal detector 105 and the bias detector 106 have the function of extracting from the checking analog signal, respectively, an AC component with substantially zero average, which is adapted to contain information useful for checking the machining of the workpiece W, and a continuous component or bias signal, which is adapted to include information about the presence and/or the proper operation of the secondary LED 25, so of the rotating part 72, and indicative of the validity of the AC component, namely of the fact that such AC component contains the above- mentioned useful information. The first demodulator 104, the signal detector 105 and the bias detector 106 are connected to the processing and control unit 4, which is able to receive and discriminate among the input analog and digital electrical signals to select that to process each time .

A method for transceiving alternating optical signals according to the present invention is extensively described in the following, such method being useful for instance for the operation of the piezoelectric sensor 8 of the grinding machine M.

During an initial grinding phase, the grinding wheel 1 of the grinding machine M rotates with a not high speed, the signals are weakly affected by noise and the communication between rotating part 72 and stationary part 71 occurs in serial manner. The processing and control unit 4 sends a signal to the enabling circuit 103 to enable the oscillator 102, which generates a supply alternating current that is transmitted to the rotating part 72 through the inductive coupling between the secondary electrical windings 27 and the primary electrical windings 17, and further control and/or request electrical signal to the first adapter 101, which transforms it in the signal suitably modulated to be optically transmitted by the primary LED 15 to the secondary photodiode 24. In the rotating part 72, the secondary electrical windings 27 transmit to the rectifier 202 the inducted alternating current for feeding the secondary circuits 23, and the second demodulator 204 transforms the control and/or request analog electrical signal received by the secondary photodiode 24 in the corresponding demodulated digital electrical signal to control the driving electric motors via the interface 50. In addition, the second adapter 201 digitally modulates the checking electrical signal sent by the piezoelectric sensor 8 via the interface 50 so it is optically transmitted by the secondary LED 25 to the primary photodiode 14. The first demodulator 104 transforms the analog electrical signal output from the primary photodiode 14 in the checking digital electrical signal, and sends the latter to the processing and control unit 4. The signal detector 105 and the bias detector 106 are also operating and, from the same checking analog electrical signal, extract an AC component and a bias signal respectively. The processing and control unit 4 receives and discriminates among the input analog and digital electrical signals to select that to process. In particular, according to the fact that in this phase the communication between rotating part 72 and stationary part 71 occurs in serial manner, the processing and control unit 4 ignores the AC component and the bias signal, selects only the digital electrical signal output from the first demodulator 104 and properly processes it in a known way in order to carry out the checking and modify the reciprocal displacement speed between wheel slide and workpiece W. In this phase, the checking of the presence and the proper operation of the secondary LED 25, that is of the rotating part 72, is committed to the serial communication protocol.

In a phase, for example, of fine grinding, due to and depending on factors incidental to the machining type and the grinding wheel 1 used by the grinding machine M for finishing the workpiece W being machined, the signal-to- noise ratio decreases and the frequency band wherein the useful signal is, actually wherein the optimum signal-to- noise ratio is, tightens. As a consequence, the processing of the signals transmitted from the rotating part 72 to the stationary part 71 must be refined, since the selection of the frequency band wherein the signal-to-noise ratio is optimum, for instance, is essential. Similarly to what already described, the feeding of the secondary circuits 23 still occurs through the inductive coupling between the secondary electrical windings 27 and primary electrical windings 17. On the contrary, the communication from the rotating part 72 to the stationary part 71 must occur no longer in serial manner, but in exclusive analog manner. With this aim, the processing and control unit 4 sends to the rotating part 72 the request signal, according to the already-described procedure, requiring a shift from serial to analog communication, and the rotating part 72 sends to the processing and control unit 4 an acknowledge digital signal, acknowledging the reception of the request and validating the shift to analog communication. At this point, the second adapter 201 analogically modulates the checking electrical signal sent by the piezoelectric sensor 8 via the interface 50, so the secondary LED 25 optically transmits it to the primary photodiode 14. From the checking analog electrical signal output by the primary photodiode 14, the signal detector 105 and the bias detector 106 respectively extract an AC component, containing information useful to check the machining of the workpiece W, and a bias signal, containing information about the presence and/or the proper operation of the secondary LED 25, and indicative of the fact that the AC component contains the above-mentioned useful information, and send them to the processing and control unit 4. The first demodulator 104 is also operating and, from the same checking analog electrical signal, reconstructs the digital electrical signal. The processing and control unit 4 receives and discriminates among the input digital and analog electrical signals to select that to process. In particular, according to the fact that in this phase the communication between rotating part 72 and stationary part 71 occurs in analog manner, it ignores the digital electrical signal received from the first demodulator 104, selects the AC component extracted by the signal detector 105 and the bias signal output from the bias detector 106. Then, the processing and control unit 4 verifies, depending on the bias signal, whether the secondary LED 25 is present and properly operating, that is to say whether the AC component contains useful information, for instance it considers the secondary LED 25 is present and properly operating, or the AC component contains useful information, when the bias signal exceeds a predetermined value. If it is so, the processing and control unit 4 suitably processes in a known way the AC component in order to carry out the checking and modify the reciprocal displacement speed between wheel slide and workpiece W.

When the phase of fine grinding is completed, the processing and control unit 4 may return to carry out the checking depending only on the digital electrical signals received from the first demodulator 104. With this purpose, for example, the processing and control unit 4 may send a suitable signal to the enabling circuit 103 to disable the oscillator 102, which, by turning off, interrupts the inductive coupling between the secondary electrical windings 27 and primary electrical windings 17 and, consequently, prevents any communication between rotating part 72 and stationary part 71. If it is required to start a further phase of initial or fine grinding, the processing and control unit 4 sends a new signal to the enabling circuit 103 to enable the oscillator 102, the inductive coupling between the secondary electrical windings 27 and primary electrical windings 17 is reactivated, the secondary circuits 23 are fed again and the optical communication from the rotating part 72 to the stationary part 71 starts again in serial or analog manner, depending on the grinding type and according to what has been already described .

The bias signal output from the bias detector 106, in addition to check the presence and the proper operation of the rotating part 72, can be advantageously used for further checking. As it is known, the intensity of the alternating optical signal that propagates from the secondary LED 25 to the primary photodiode 14 suffers an attenuation that is a monotone function of the distance between the two, namely the distance between the stationary part 71 and the rotating part 72. The bias signal is a function of said attenuation and, in particular, decreases as the distance between the stationary part 71 and the rotating part 72 increases. As a consequence, the processing and control unit 4 may process the bias signal in order to obtain information about said distance, then advantageously control the latter depending on the bias signal. In addition, the AC component of the checking analog electrical signal features a dynamics the amplitude thereof is a function of the bias signal of the same analog electrical signal. The less the dynamics of the AC component is, the higher the gain of the one or more amplifiers of the signal detector 105, adapted to extract such AC component from the checking analog electrical signal, must be. According to what said so far, this gain may be automatically controlled depending only on the bias signal, with the clear advantage of adapting the sensitivity of the signal detector 105 to the features of the checking analog electrical signal, that is, as a last step, of adapting the sensitivity of the signal detector 105 to the intensity of the alternating optical signal received by the primary photodiode 14.

Variations can be made to the device and method described hitherto for transceiving optical signals, without departing from the object of the present invention. In a different embodiment of the invention, the secondary circuits 23 include a further bias detector, similar to the bias detector 106, connected to the secondary photodiode 24, and a gain adaptive unit connected to the second adapter 201 and to the further bias detector. According to the well-known radiation diagrams of the light emitter diodes, the alternating optical signal emitted by the secondary LED 25 is transmitted in a large part to the primary photodiode 14, and in a small part to the secondary photodiode 24. The secondary photodiode 24 receives such small part of the alternating optical signal emitted by the secondary LED 25 and generates a corresponding auxiliary analog electrical signal. The further bias detector has the function of extracting an auxiliary continuous component or auxiliary bias signal from said auxiliary analog electrical signal, the auxiliary bias signal containing information about the intensity of the alternating optical signal transmitted by the secondary LED 25, to be sent to the gain adaptive unit. A method that uses such different embodiment considers that the gain of the one or more amplifiers within the second adapter 201 may be advantageously controlled in automatic and adaptive way depending only on the further bias signal. In other words, according to this method, the gain adaptive unit controls the intensity of the alternating optical signal transmitted by the secondary LED 25 on the basis of the auxiliary continuous component, independently of a possible variation of the functional features, specifically of the operation point, of the secondary LED 25, due for instance to the thermal drift of the latter.

In a device according to the present invention, the balancing mechanism 6 may advantageously comprise additional sensors of known kind, like Hall effect magnetic sensors that emit signals indicative of a predetermined configuration of the movable masses or thermal sensors for checking the temperature of the tool 1, conveniently located within the spindle 2 and electrically connected to the transceiver 7 via dedicated cables, said additional sensors having the function of sending to the processing and control unit 4 corresponding checking signals to process in a known way for controlling the workpiece W grinding procedure.

For example, the second adapter 201 may further digitally modulates a communication signal sent by the interface 50 and including service information of the relative protocol, such as identification codes of the grinding wheel or another rotating tool 1 of the machine tool M, and/or a diagnostic signal, containing information about the condition of the secondary circuits 23 and other elements of the rotating part 72, for instance to detect possible short circuits, and/or a so-called acknowledge signal, for acknowledging the correct reception by the secondary photodiode 24 of the control and/or request signal, so that they can be transmitted to the processing and control unit 4 and correctly understood.

Alternatively, as it is shown in figure 4 , the processing and control unit 4 may comprise a logic unit 40, for instance comprised within the first casing 11 and connected to the stationary part71, in particular to the first demodulator 104, the signal detector 105 and the bias detector 106, and adapted to implement at least some of the functions carried out by the processing and control unit 4, that is, when the transmission occurs in serial manner, select and process only the digital electrical signals output from the first demodulator 104 and, as an alternative, when the transmission occurs in exclusive analog manner, select the AC component extracted by the signal detector 105 and the bias signal output from the bias detector 106, verifies, depending on the bias signal, whether the secondary LED 25 is present and properly operating, that is to say whether the AC component contains useful information, and suitably process the AC component if it is so. When the phase of fine grinding is completed, for example, and it is desired to return to carry out the checking depending only on the digital electrical signal received by the first demodulator 104, as an alternative to what described, that is disabling followed by enabling of the oscillator 102 respectively to interrupt and reactivate the inductive coupling between the secondary electrical windings 27 and primary electrical windings 17, the processing and control unit 4 may send to the rotating part 72 the request signal, requiring a shift from analog to serial communication, which the rotating part 72 may reply with an acknowledge analog signal, acknowledging the reception of the request and validating the shift to serial communication .

A device for transceiving according to the present invention may be employed in a tool control system that comprises the only acoustic sensor 8, or a similar vibration sensor, or that does not include the balancing device 6 anyway, with appropriate circuit changes and simplifications.

The device and method for transceiving alternating optical signals described above, and the relative variations, shows many advantages as they are easily and cheaply implemented and enable a reliable transmission in analog way too, being employed in particular for transceiving checking signals from a rotating part 72 to a stationary part 71. It is indeed clear to a man skilled in the art that the device and the method for transceiving alternating optical signals according to the present invention enable to verify in simple and effective way whether the reception of an alternating optical signal with zero average, for instance transmitted from a rotating part 72 to a stationary part 71, is or is not caused by a malfunctioning of the same rotating part 72.