Abstract:
The invention relates to a piezoelectric drive device with a bimodal piezoelectric resonator, with at least a first control electrode for triggering the resonator in a first drive direction, with at least a second control electrode for triggering the resonator in a second drive direction, and with a trigger circuit for supplying control signals to the first and the second control electrode. The invention is characterized in that a regulating circuit is provided for regulating the control signals, the second control electrode is designed for supplying a feedback signal to the regulating circuit when the resonator is being triggered in the first drive direction by means of the first control electrode, and the first control electrode is designed for supplying a feedback signal to the regulating circuit when the resonator is being triggered in the second drive direction by means of the second control electrode.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a piezoelectric drive device with a bimodal piezoelectric resonator, with at least a first control electrode for triggering the resonator in a first drive direction, with at least a second control electrode for triggering the resonator in a second drive direction, and with a trigger circuit for supplying control signals to the first and the second control electrode. 
     Such a piezoelectric drive device is known from EP 633 616 A2. This known drive device comprises as its piezoelectric resonator a rectangular piezoelectric plate with four control electrodes, two control electrodes being provided for triggering the resonator in the first drive direction and the other two control electrodes for triggering the resonator in the second drive direction. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a drive device of the kind mentioned in the opening paragraph which has an improved efficiency. 
     According to the invention, this object is achieved in that 
     a regulating circuit is provided for regulating the control signals, 
     the second control electrode is designed for supplying a feedback signal to the regulating circuit when the resonator is being triggered in the first drive direction by means of the first control electrode, and 
     the first control electrode is designed for supplying a feedback signal to the regulating circuit when the resonator is being triggered in the second drive direction by means of the second control electrode. 
     More specifically, a piezoelectric drive device is provided comprising a bimodal piezoelectric resonator ( 1 ), and means for supplying control signals (S), wherein the resonator ( 1 ) comprises: 
     at least a first control electrode ( 2 ) for triggering the resonator ( 1 ) in a first drive direction, and 
     at least an other control electrode ( 4 ) for triggering the resonator in a second drive direction, and 
     said means for supplying control signals comprises a trigger circuit ( 6 ) for supplying control signals (S) to the first and the other control electrode, 
     characterized in that 
     a regulating circuit ( 7 ) is provided for regulating the control signals (S), 
     first means for supplying a feedback signal (k) from the other control electrode ( 4 ) to the regulating circuit ( 7 ) when the resonator is not being triggered by said other control electrode and the resonator ( 1 ) is not being triggered in the first drive direction by means of the first control electrode ( 2 ), and 
     second means for supplying the feedback signal (k) form the first control electrode ( 2 ) to the regulating circuit ( 7 ) when the resonator is not being triggered by the first control electrode and the resonator ( 1 ) is being triggered in the second drive direction by means of the other control electrode ( 4 ). 
     Such a drive device can be realized with a high efficiency. It is necessary for piezoelectric drive devices with high efficiency that the piezoelectric resonator should have a high performance. In the case of high resonator performances, the gradient of the performance over the frequency shows a narrow band. The optimum frequency, i.e. the frequency at which the resonator shows the best performance, is dependent on, for example, the amplitude of the control voltage supplied to the control electrodes, the mechanical prestress of the resonator, mechanical tolerances in the construction, and changes in the load. It is possible by means of the regulating circuit to adjust the control signal supplied to the control electrodes such that an optimum efficiency of the drive device is obtained at all times. 
     The relevant passive control electrode, i.e. that control electrode which is not being triggered in the instantaneous drive direction, is designed for obtaining the feedback signal necessary for the regulation at each moment. This renders possible an optimized utilization of the surface area of the piezoelectric resonator. No additional sensor electrodes are necessary for obtaining the feedback signal. Additional sensor electrodes would reduce the surface area available for the control electrodes and thus the active piezo volume, i.e. the volume lying under the control electrode surfaces. 
     In the one embodiment of the invention, wherein the other control electrode is a third electrode ( 4 ), the piezoelectric resonator ( 1 ) comprises a first pair of control electrodes ( 2 , 3 ) for triggering the resonator in the first drive direction, and a second pair of control electrodes ( 4 , 5 ) for triggering the resonator in the second drive direction, the second pair of control electrodes ( 4 , 5 ) is designed for supplying a feedback signal (k) to the regulating circuit ( 7 ) when the resonator is not being triggered by the second pair of electrodes and the resonator ( 1 ) is being triggered in the first drive direction by the first pair of control electrodes ( 2 , 3 ), and the first pair of control electrodes ( 2 , 3 ) is designed for supplying a feedback signal (k) to the regulating circuit ( 7 ) when the resonator is not being triggered by the first pair of electrodes and the resonator ( 1 ) is being triggered in the second drive direction by the second pair of control electrodes ( 4 , 5 ). In this embodiment, a first and a second control electrode pair are provided for triggering the relevant drive directions. The active piezo volume, i.e. the volume lying below the control electrodes, should be as great as possible so as to obtain a high energy density and a maximum output power. The arrangement of the control electrodes in pairs renders possible a good space utilization on the surface of the piezoelectric resonator. 
     The arrangement of the control electrodes in pairs is particularly favorable in an embodiment of the invention wherein the piezoelectric resonator ( 1 ) is substantially rectangular in shape, and one control electrode is provided in each quadrant of the substantially rectangular piezoelectric resonator ( 1 ). The substantially rectangular piezoelectric resonators are polarized in the thickness direction, and the oscillation modes are preferably stimulated by the D-31 piezo effect. 
     The amplitude evaluation provided in another embodiment of the invention wherein the regulating circuit ( 7 ) is designed for evaluating the amplitude of the feedback signal (k), takes place preferably by means of an analog-digital converter. The control circuit compares either the amplitude of the feedback signal with a programmable reference value or the amplitude of the feedback signal with the amplitude of the control signal, and derives the regulating signal therefrom. 
     In another embodiment of the invention, the regulating circuit ( 7 ) is designed for evaluating the phase difference between the control signal (s) and the feedback signal (k) by means of a phase control (PLL) circuit. This embodiment can be realized in a particularly simple and inexpensive manner, because no analog-digital converter is required. 
     The regulating circuit ( 7 ) is designed for regulating the frequency of the control signal (s). Such frequency regulation of the control signal renders it possible to operate the piezoelectric resonator continuously with the highest possible performance. 
     The regulating circuit ( 7 ) is also designed for regulating the amplitude of the control signal (s). This has the advantage that the output power of the motor can be regulated thereby. 
     It is particularly advantageous to combine the regulation of the frequency and the regulation of the amplitude. In the advantageous embodiment of the invention as defined in claim 8, this takes place in a first step through regulation of the frequency of the control signal. This safeguards an optimum performance of the piezoelectric resonator. Advantageously, the amplitude of the control signal can subsequently be regulated in a second step, for example for achieving a desired output power or a desired torque. 
     The drive device may preferably be used for driving the shaving head of a shaver or for driving the write/read unit of an electronic device for reading data stored on disc-type data carriers, in particular CDs and DVDs, and/or writing data on such disc-type data carriers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A few embodiments of the invention diagrammatically depicted in FIGS. 1 to  6  will now be explained in more detail with reference to these Figures, in which: 
     FIG. 1 is a perspective view of a piezoelectric resonator with four control electrodes, wherein each pair of diagonally opposed control electrodes can be coupled on the one hand to a regulating circuit and on the other hand to a control circuit, 
     FIG. 2 is a lateral elevation of the piezoelectric resonator of FIG. 2, 
     FIG. 3 shows a CD drive unit with a piezoelectric drive device as shown in FIG. 1, 
     FIG. 4 shows a further embodiment of a CD drive unit with a piezoelectric drive device as shown in FIG. 1, 
     FIG. 5 shows a third embodiment of a CD drive unit with a piezoelectric drive device as shown in FIG. 1, and 
     FIG. 6 is a sketch showing the construction principle of a beard trimmer of an electric shaver which is driven by a piezoelectric drive device as shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a substantially rectangular piezoelectric resonator  1  in perspective view, comprising a first control electrode  2 , a second control electrode  3 , a third control electrode  4 , and a fourth control electrode  5 . The first control electrode  2  and the second control electrode  3  are arranged diagonally opposite one another and form a first control electrode pair. The third control electrode  4  and the fourth control electrode  5  are also arranged diagonally opposite one another and form a second control electrode pair. A trigger circuit  6  is provided for triggering the piezoelectric resonator  1 . The trigger circuit  6  supplies a control signal S to the piezoelectric resonator  1 . A regulating circuit  7  is provided for regulating the control signals S of the trigger circuit  6 . A first switch  8  is connected between the trigger circuit  6  and the piezoelectric resonator  1 . The trigger circuit  6  has an output  6   b , which is coupled to an input  8   a  of the first switch  8 . The control signal S is supplied to the input  8   a  of the first switch  8 . The first switch  8  has a first switch position  8   b  and a second switch position  8   c  at its output side. The first switch position  8   b  is coupled to the third control electrode  4  and the fourth control electrode  5 . The second switch position  8   c  is coupled to the first control electrode  2  and the second control electrode  3 . A second switch  9  is arranged between the piezoelectric resonator  1  and the regulating circuit  7 . The second switch has a first switch position  9   a  and a second switch position  9   b  at its input side. The second switch  9  has an output  9   c  which is coupled to an input  7   a  of the regulating circuit  7 . The first switch position  9   a  of the second switch is coupled to the first control electrode  2  and the second control electrode  3 . The second switch position  9   b  of the second switch is coupled to the third control electrode  4  and the fourth control electrode  5 . The piezoelectric resonator  1  has a planar ground electrode  10  over its entire lower surface. The trigger circuit  6  has a ground output  6   c  which is coupled to the ground electrode  10  of the piezoelectric resonator  1 . The regulating circuit  7  has an output  7   b  which is coupled to an input  6   a  of the trigger circuit  6 . 
     The piezoelectric resonator  1  comprises a drive tappet  11  at its front side. The piezoelectric resonator  1  may be used for driving drive elements (not shown in FIG. 1) by means of this drive tappet  11 , for example for driving the shaving head of an electric shaver. The piezoelectric resonator  1  can be excited into oscillation in an oscillation plane (x/y plane), wherein the oscillation results from a superimposition of two mutually perpendicular vibrations in the x- and y-directions. The resonance frequencies of the two mutually perpendicular vibrations may be adjusted through a suitable triggering and through a suitable choice of the geometry of the piezoelectric resonator  1  such that the two vibrations are excited with a sufficient amplitude and in a desired phase relation, and the drive tappet  11  of the piezoelectric resonator  1  moves along a curve lying in the x/y plane, in particular an ellipse-shaped curve e. Such an oscillation along the ellipse-shaped curve e may be generated both in a first, clock-wise drive direction  12  and in a second, anti-clock-wise drive direction  13  through a suitable triggering of the piezoelectric resonator  1 . The first control electrode  2  and the second control electrode  3  are provided for triggering the piezoelectric resonator in the first drive direction  12 . The third control electrode  4  and the fourth control electrode  5  are provided for triggering the piezoelectric resonator  1  in the second drive direction  13 . 
     For triggering the piezoelectric resonator  1  in the second drive direction  13 , the first switch  8  will be in the switch position  8   b  shown in FIG. 1, and the second switch  9  will be in the first switch position  9   a  shown in FIG.  1 . For this second drive direction  13 , the control signal S is supplied by the trigger circuit  6  via the switch  8  in its first switch position  8   b  in parallel both to the third control electrode  4  and to the fourth control electrode  5 . This control signal S excites the piezoelectric resonator  1  such that it moves in the second drive direction  13 . The first control electrode  2  and the second control electrode  3  are connected to the input  7   a  of the regulating circuit  7  via the second switch  9  which is in its first switch position  9   a . The first control electrode  2  and the second control electrode  3  thus supply a feedback signal K to the regulating circuit  7 . The feedback signal K supplies the regulating circuit  7  with a status description of the oscillation of the piezoelectric resonator  1 . The regulating circuit  7  evaluates the feedback signal K and supplies a regulating signal R to the trigger circuit  6 . The control signal S supplied by the trigger circuit  6  to the piezoelectric resonator  1  is modified by means of this regulating signal R. 
     The regulating circuit  7  is designed either for evaluating the amplitude of the feedback signal K or for evaluating the phase difference between the control signal S and the feedback signal K. For evaluating the amplitude of the feedback signal K, it is necessary for the regulating circuit  7  to comprise an analog-digital converter. If the phase difference between the control signal S and the feedback signal K is to be evaluated, the regulating circuit  7  will comprise a phase detector. Such a phase detector can be realized in a particularly simple and inexpensive manner. 
     The regulating signal R supplied by the regulating circuit  7  may be used for regulating the frequency of the control signal S, for regulating the amplitude of the control signal S, or for regulating the frequency and amplitude of the control signal S. 
     For triggering the piezoelectric resonator  1  in the first drive direction  12 , the first switch  8  will be in the second switch position  8   c  and the second switch  9  will be in the second switch position  9   b . The trigger circuit  6  supplies the control signal S to the first control electrode  2  and to the second control electrode  3  for triggering the piezoelectric resonator in the first drive direction  12 . During operation in the drive direction  12 , the third control electrode  4  and the fourth control electrode  5  are designed for supplying the feedback signal K to the regulating circuit  7 . The latter again supplies a regulating signal R to the trigger circuit  6 , so that a modification of the control signal S is possible also in the first drive direction  12 . 
     The regulation of the control signal S and the resulting regulation of the oscillation of the piezoelectric resonator  1  render it possible to operate the piezoelectric resonator  1  at the optimum frequency at all times, i.e. at that frequency for which the efficiency is an optimum. This is particularly advantageous for piezoelectric resonators  1  with a high vibration performance, because the gradient of the performance over the frequency lies within a very narrow band. Final trimming of the frequency of the control signal S can thus continuously safeguard an optimum operation of the trigger device with the highest possible resonator performance. 
     In the first drive direction  12 , the passive control electrodes  4  and  5 , i.e. those not used for triggering the piezoelectric resonator  1 , are used for obtaining the feedback signal K. In the second drive direction  13 , the control electrodes  2  and  3  passive in this drive direction are used for obtaining the feedback signal K. This alternate utilization of the control electrodes on the one hand for active triggering of the resonator  1  and on the other hand for passively obtaining the feedback signal K renders possible a particularly efficient utilization and partition of the surface of the piezoelectric resonator  1 . No additional sensor electrodes for obtaining the feedback signal are necessary in such an arrangement. The active piezo volume, i.e. the volume present under the control electrodes which are active at any time, is not reduced by additional sensor electrodes. The drive device is particularly efficient also for this reason. 
     FIG. 2 shows the piezoelectric resonator of FIG. 1 in side elevation. The piezoelectric resonator  1  comprises a piezoelectric base body  15  with a first surface  16  and a second surface  17 . The second surface  17  is fully covered by the ground electrode  10 . The first surface  16  supports the first control electrode  2 , the second control electrode  3 , the third control electrode  4 , and the fourth control electrode  5 . The polarization direction of the piezoelectric base body  15  is the same below the control electrodes  2 ,  3 ,  4 , and  5 . The polarization direction is indicated with two arrows  18  in FIG.  2 . The oscillation modes of the piezoelectric resonator  1  are stimulated by the D-31 piezo effect. 
     FIG. 3 shows a CD drive unit with a piezoelectric drive device as shown in FIG.  1 . In this drive unit, an arm  22  provided with a read/write head  23 ,  24  is moved in radial direction over a CD  21 , which is driven by a motor  20 . The arm  22  is arranged on a shaft  25  which is rotatable about an axis of rotation  26  and which is driven by a piezoelectric drive device according to FIG.  1 . The piezoelectric resonator  1  is for this purpose accommodated in a housing  27  which is pretensioned against the shaft  25 . A force is transmitted to the shaft  25  through the drive tappet  11 , which shaft is rotatable in both directions depending on the triggering of the piezoelectric resonator  1 , so that the arm  22  moves in the desired direction over the CD  21 . 
     FIG. 4 shows a further embodiment of a CD drive unit with a piezoelectric drive device  35 . The read/write head is concealed behind a lens  30  here. The entire read/write unit is mounted on a carriage  31  and is moved linearly in radial direction over the CD  21  along two guides  32 ,  33  by means of the piezoelectric drive device  35 . The piezoelectric drive element  35  may here be fixedly incorporated in the housing of the CD drive unit and hit against the carriage  31  with its drive tappet  11 , thus moving this carriage. 
     A further embodiment is shown in FIG. 5, where the piezoelectric drive unit  35  is mounted on the carriage  31 . The drive tappet  11  hits against the drive unit housing  34 . The result is that the piezoelectric drive device  35  and the carriage  31  move jointly. 
     FIG. 6 is a sketch showing the construction principle of a beard trimmer of an electric shaver. The beard trimmer, which is provided in particular for cutting longer hairs, comprises a stationary comb  40  and a moving comb  41 . The moving comb  41  can be driven into oscillation in the direction of the double arrow  43  by means of the piezoelectric drive device  35 . The moving comb  41  preferably comprises a T-shaped extension  44  with which the drive tappet  11  makes contact. 
     The triggering and regulating mechanisms for the piezoelectric resonator  1  are not shown in FIGS. 2 to  6  to keep the drawings clear and simple.