Patent Publication Number: US-9853576-B2

Title: Control apparatus of vibration-type actuator and control method of vibration-type actuator

Description:
This application is a division of U.S. patent application Ser. No. 14/203,393, filed on Mar. 10, 2014 (allowed), which is a division of U.S. patent application Ser. No. 13/106,790, filed on May 12, 2011 (now U.S. Pat. No. 8,669,723, issued on Mar. 11, 2014). The contents of each of the foregoing applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a control apparatus of a vibration-type actuator and a control method of a vibration-type actuator. 
     Description of the Related Art 
     Actuators are conventionally proposed which subject a given mass point on a plate-like vibrator to elliptical motion (driving member) to drive a driving member. 
     As a basic configuration of a vibration-type actuator with a plate-like vibrator, such a configuration as illustrated in Japanese Patent Application Laid-Open No. 2004-320846 is known.  FIG. 8A  is a perspective view illustrating an example of the external basic configuration of the vibration-type actuator in Japanese Patent Application Laid-Open No. 2004-320846. 
     As illustrated in  FIG. 8A , a vibrator in the vibration-type actuator includes an elastic member  4  formed of a metal material shaped like a rectangular plate. The elastic member  4  includes a piezoelectric element (electromechanical energy transducer)  5  joined to a back surface thereof. A plurality of protrusions  6  are provided on a top surface of the elastic member  4  at respective predetermined positions. 
     According to this configuration, applying an AC voltage to the piezoelectric element  5  allows simultaneous generation of secondary bending vibration in a long side direction of the elastic member  4  and primary bending vibration in a short side direction of the elastic member  4 . This excites elliptical motion in the protrusions  6 . 
     Then, the driving member  7  is brought into contact with the top portions (contact portions) of the protrusions  6  under pressure and then linearly driven by elliptical motion of the protrusions  6 . That is, the protrusions  6  act as a drive unit for the vibrator. 
       FIG. 8B  is a schematic diagram illustrating an example of a polarization area of the piezoelectric element  5  in the vibration-type actuator illustrated in  FIG. 8A . 
     Furthermore,  FIGS. 9A and 9B  are perspective views illustrating a vibration mode of the elastic unit  4 .  FIG. 9C  is a diagram illustrating elliptical motion excited in the protrusions  6  of the elastic unit  4 . 
     The piezoelectric element  5  is subjected to a polarization process and includes two electrodes A 1  and A 2 , as illustrated in  FIG. 8B . 
     AC voltages V 1  and V 2  in phase with each other are applied to the two electrodes A 1  and A 2 , respectively, to excite the rectangular elastic unit  4  into primary bending movement with two nodes extending in a direction parallel to the long side direction. This corresponds to a first vibration mode illustrated in  FIG. 9A . 
     Furthermore, the AC voltages V 1  and V 2  out of phase with each other are applied to the two electrodes A 1  and A 2 , respectively, to excite the rectangular elastic unit  4  into secondary bending movement with three nodes extending in a direction parallel to the short side direction. This corresponds to a second vibration mode illustrated in  FIG. 9B . 
     Then, the first vibration mode and the second vibration mode are combined together to excite elliptical motion in the protrusions  6 . At this time, when brought into contact with the protrusions  6  under pressure, the driving member can be linearly driven. 
     Here, the first vibration mode illustrated in  FIG. 9A  allows activation of an amplitude (hereinafter referred to as a Z-axis amplitude) displaced in a direction perpendicular to the surface of the contact portion (hereinafter referred to as the contact surface brought into contact with the driving member under pressure in the protrusions  6 . 
     Furthermore, the second vibration mode illustrated in  FIG. 9B  allows an amplitude (hereinafter an X-axis amplitude) displaced in a direction parallel to the contact surface to be excited in the protrusions  6 . 
     Combination of the first vibration mode and the second vibration mode allows elliptical motion to be excited in a predetermined one of the protrusions  6  as illustrated in  FIG. 9C . The ratio in magnitude between the Z-axis amplitude and the X-axis amplitude is hereinafter referred to as ellipticity of elliptical motion. 
       FIG. 10A  is a graph illustrating the amplitudes in the first vibration mode and the second vibration mode observed when the difference in phase between the two-phase voltages V 1  and V 2  is changed between −180 degrees and 180 degrees. 
     When the difference in phase between the two-phase AC voltages V 1  and V 2  applied to the respective two electrodes A 1  and A 2  of the polarized piezoelectric element  5  is changed between −180 degrees and 180 degrees, the amplitudes in the first vibration mode and the second vibration mode (P 2 ) are as illustrated by P 1  and P 2  in  FIG. 10A , respectively. 
     In  FIG. 10A , the axis of abscissas indicates the phase difference. The axis of ordinate indicates the amplitudes in the first amplitude mode and in the second amplitude mode. 
     Combination of the first vibration mode and the second vibration mode allows elliptical motion to be excited in the protrusions  6 . Changing the phase difference between the AC voltages V 1  and V 2  to be applied allows adjustment of ellipticity of elliptical motion excited in the predetermined protrusion  6 . 
       FIG. 10A  illustrates, in the lower part thereof, elliptical shapes corresponding to the phase differences on the axis of abscissas. The direction of driving by the vibration-type actuator, which provides linear driving, can be switched by switching between the positive sign and negative sign of the phase difference between the AC voltages V 1  and V 2 . 
     Moreover, the direction and speed of driving can be consecutively changed by consecutively changing the phase difference starting with any value, with the sign appropriately changed between the positive one and the negative one (for example, consecutively changing the phase difference between 90 degrees and −90 degrees, with the sign appropriately changed between the positive one and the negative one). 
     Concerning the driving speed, the following phenomenon (which is called a cliff drop phenomenon) occurs as illustrated in  FIG. 10B . The driving speed peaks at the resonant frequency and decreases slowly on a higher frequency side of the resonant frequency, while decreasing rapidly on a lower frequency side of the resonant frequency. 
     Furthermore, as is generally known, the speed can be increased by setting the frequency of the AC voltage applied to the piezoelectric element closer to the resonant frequency. The speed can be reduced by setting the frequency of the applied AC voltage further away from the resonant frequency. 
     As such a vibration-type actuator, an apparatus can be provided which exerts a driving force increased using a plurality of vibrators. 
     However, when the vibration-type actuator is configured to drive the driving member, using a plurality of vibrators, the following problem may occur. 
     When a common frequency is applied to each of the plurality of vibrators in order to simplify the circuit configuration of a control apparatus of the vibration-type actuator, the vibration-type actuator operates unstably if the resonant frequency varies among the vibrators. Thus, the vibration-type actuator needs to drive the object without using frequency regions corresponding to the unstable operation. 
     In view of the above-described problem, an object of the present invention is to provide a control apparatus of a vibration-type actuator and a control method of a vibration-type actuator in which the vibration-type actuator configured to drive the driving member using a plurality of vibrators can drive the object stably even with a variation in resonant frequency among the vibrators. 
     SUMMARY OF THE INVENTION 
     The present invention provides a control apparatus and method for a vibration-type actuator described as follow. 
     A control apparatus for a vibration-type actuator, wherein the vibration-type actuator includes a plurality vibrators each having a contact portion contacting an object to be driven, such that, responsive to a common alternating current signal to the plurality vibrators, the plurality vibrators move, through the contact portions, the object relative to the vibrators, to generate an elliptical motion of the object, wherein the control apparatus comprises a frequency determining unit for setting a frequency of the alternating current signal, and the frequency determining unit sets the frequency of the alternating current signal for changing an ellipticity of the elliptical motion, within an overlapping frequency range in which frequency ranges of ellipticity changing each set for each of the plurality of vibrators are overlapped, and the frequency ranges of changing the ellipticity are set for the vibrators as a frequency ranges between an upper limit and a lower limit, such that the lower limit is a maximum resonant frequency in all case of changing the ellipticity, and the upper limit is larger than the lower limit and is a maximum frequency for the relative movement of the driving member. 
     A control method for a vibration-type actuator, wherein the vibration-type actuator includes a plurality vibrators each having a contact portion contacting an object to be driven, such that, responsive to a common alternating current signal to the plurality vibrators, the plurality vibrators move, through the contact portions, the object relative to the vibrators, to generate an elliptical motion of the object, wherein the control method comprises a step for setting a frequency of the alternating current signal, and, in the step frequency determining, the frequency of the alternating current signal is set for changing an ellipticity of the elliptical motion, within an overlapping frequency range in which frequency ranges of ellipticity changing each set for each of the plurality of vibrators are overlapped, and the frequency ranges of changing the ellipticity are set for the vibrators as a frequency ranges between an upper limit and a lower limit, such that the lower limit is a maximum resonant frequency in all case of changing the ellipticity, and the upper limit is larger than the lower limit and is a maximum frequency for the relative movement of the driving member. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating the external configuration of a vibration-type actuator according to a first exemplary embodiment of the present invention. 
         FIG. 2  is a graph illustrating the relationship between the drive frequency and driving speed of a vibrator in the vibration-type actuator. 
         FIG. 3  is a block diagram illustrating the configuration of a control apparatus of the vibration-type actuator according to the first exemplary embodiment of the present invention. 
         FIG. 4  is a block diagram illustrating an ellipticity determining unit and a frequency determining unit in the vibration-type actuator in detail according to the first exemplary embodiment of the present invention. 
         FIG. 5  is a graph illustrating the relationship between the drive frequency and driving speed of the vibrator in the vibration-type actuator according to the first exemplary embodiment of the present invention. 
         FIG. 6  is a flowchart for a vibration-type actuator according to a second exemplary embodiment of the present invention. 
         FIG. 7  is a diagram illustrating the configuration of a vibration-type actuator according to a third exemplary embodiment of the present invention. 
         FIG. 8A  is a perspective view illustrating an example of an external basic configuration of a vibration-type actuator in a conventional example, and  FIG. 8B  is a schematic diagram illustrating an example of a polarization area of a piezoelectric element in the vibration-type actuator illustrated in  FIG. 8A . 
         FIGS. 9A and 9B  are perspective views illustrating a vibration mode of an elastic member of the vibration-type actuator in the conventional example, and  FIG. 9C  is a diagram illustrating elliptical motion excited in protrusions of the elastic member. 
         FIG. 10A  is a graph illustrating amplitudes in a first vibration mode and a second vibration mode, and  FIG. 10B  is a graph illustrating the relationship between the frequency and speed of the vibrator. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. 
     First Exemplary Embodiment 
     The configuration of a control apparatus of a vibration-type actuator according to a first exemplary embodiment of the present invention will be described with reference to  FIG. 1 . 
     The vibration-type actuator according to the present exemplary embodiment includes a plurality of vibrators each at least with an electromechanical energy transducer and an elastic member. 
     An alternating current of a drive frequency common to the electromagnetic energy transducers in the respective plurality of vibrators is applied to the electromagnetic energy transducers to drive a driving member which is in contact with the plurality of vibrators via contact portions thereof. 
       FIG. 1  illustrates a vibration-type actuator including two vibrators configured to relatively drive one driving member. The vibration-type actuator is configured to cause relative movement of a linearly extending driving member in a longitudinal direction thereof. The two vibrators can generate a double thrust. 
     Vibrators  8   a  and  8   b  illustrated in  FIG. 1  move integrally owing to a holder (not shown in the drawings), relative to a driving member  3 . 
     As illustrated in  FIG. 1 , the vibrator  8   a  of the vibration-type actuator includes an elastic member  4   a  formed of a metal material shaped like a rectangular plate. A piezoelectric element (electromechanical energy transducer)  5   a  is joined to a back surface of the elastic member  4   a.    
     Two protrusions  6   a  are provided on a top surface of the elastic member  4   a  at respective predetermined positions. 
     The vibrator illustrated in  FIG. 1  subjects the protrusions to elliptical motion so as to drive the driving member which is in frictional contact with upper portions (contact portions) of the protrusions. Similarly, in the vibrator  8   b , a piezoelectric element  5   b  is joined to an elastic member  4   b . Two protrusions  6   b  are provided at respective predetermined positions on the elastic member  4   b.    
     Each of the piezoelectric elements  5   a  and  5   b  includes two groups of electrodes A 1  and A 2  illustrated in  FIG. 8B . Each of the piezoelectric elements  5   a  and  5   b  is subjected to a polarization process in the direction of the sheet of  FIG. 8B . 
     In the vibrator configured as described above, the ellipticity of elliptical motion excited in the protrusions  6   a  and  6   b  illustrated in  FIG. 1  can be changed by changing the phase difference between two-phase AC voltages (two-phase alternating current) to be applied. 
     The vibrator illustrated in  FIG. 1  can be increased in speed by making the drive frequency of the AC voltage applied to the piezoelectric element of the vibrator closer to a resonant frequency as illustrated in  FIG. 10B . 
     Furthermore, the speed of the vibrator can be reduced by making the frequency of the AC voltage applied to the piezoelectric element of the vibrator further away from the resonant frequency. 
     Additionally, the vibrator is characterized in that the driving speed peaks at the resonant frequency and gradually decreases on a higher frequency side of the resonant frequency, while decreasing rapidly on a lower frequency side of the resonant frequency. 
     Furthermore, the movement speed of the driving member can be increased by increasing an X-axis amplitude of elliptical motion excited in the protrusions  6   a  and  6   b.    
     In addition, the movement speed of the driving member can be stably reduced by decreasing the X-axis amplitude, with a Z-axis amplitude maintained at a predetermined value. 
     Additionally, the direction of relative movement of the driving member can be switched by switching the phase difference between the two-phase AC voltages. 
     The present exemplary embodiment enables adjustment of ellipticity of elliptical motion excited in the predetermined protrusions  6   a  and  6   b  when the phase difference between the AC voltages applied to electrodes V 1  and V 2  of the piezoelectric element  5  illustrated in  FIG. 8B  described above is changed between −180 degrees and 180 degrees. 
       FIG. 10A  illustrates, in the lower part thereof, elliptical shapes corresponding to the phase difference on the axis of abscissas. 
     The driving direction and speed can be consecutively changed by consecutively changing the phase difference between 90 degrees and −90 degrees with the sign appropriately changed between a positive one and a negative one. 
     The above-described configuration excites vibration in each of the vibrators  8   a  and  8   b  in  FIG. 1  to enable driving of the driving member (slider)  3 . 
     Furthermore, the vibrator according to the present invention is not limited to the above-described exemplary embodiment. The vibrator may subject the protrusions to elliptical motion based on a combination of vibration in the vibration mode in which the Z-axis amplitude is excited in the protrusions and vibration in the vibration mode in which the X-axis amplitude is excited in the protrusions. Specifically, besides the above-described embodiment, an embodiment is possible in which the protrusions are subjected to elliptical motion based on a combination of longitudinal stretching vibration of the vibrator and bending vibration thereof. 
     Now, the driving characteristics of one vibrator illustrated in  FIG. 1  will be described. 
       FIG. 2  is a graph illustrating the relationship between the phase difference and drive frequency and driving speed of the AC voltages applied to the electrode A 1  and the electrode A 2  (see  FIG. 8B ) when one vibrator drives one driving member. For example, as illustrated in  FIG. 2 , as the phase difference starts at 90 degrees and approaches 0 degree, the amplitude in the second amplitude mode decreases to reduce the driving speed. Furthermore, as the phase difference starts at 90 degrees and approaches 0 degree, the resonant frequency increases. 
     Here, the driving characteristics obtained with the phase difference changed will be described. 
     For example, if the drive frequency is set to the value of the resonant frequency corresponding to a phase difference of 60 degrees, and the phase difference is shifted from 90 degrees toward a smaller phase difference side, then the cliff drop phenomenon is avoided while the phase difference is between 90 degrees and 60 degrees, because the drive frequency is higher than the resonant frequency. However, when the phase difference is shifted toward values smaller than 60 degrees, the cliff drop phenomenon occurs abruptly. That is, when the drive frequency is set to the value of the resonant frequency obtained when the phase difference is 60 degrees, and the phase difference is changed, the operation is unstable when the phase difference is smaller than 60 degrees. 
     However, if the drive frequency is set to the value of the resonant frequency corresponding to a phase difference of 10 degrees, and the phase difference is shifted from 90 degrees toward a smaller phase difference side, then the cliff drop phenomenon is avoided while the phase difference is between 90 degrees and 10 degrees, because the drive frequency is higher than the resonant frequency corresponding to the phase difference of 10 degrees. 
     That is, if the vibrator is driven with the drive frequency fixed and with the phase difference changed, the likelihood of the cliff drop phenomenon can be reduced by driving the vibrator with the phase difference set to correspond to a frequency higher than the resonant frequency corresponding to a smaller phase difference. 
     A resonant frequency  24  corresponding to a phase difference of 0 degree as described above is highest, and is thus defined as a lower limit value for control with the phase difference changed. 
     Now, the driving characteristics obtained with the frequency changed will be described. 
     For example, when the drive frequency is swept up toward frequencies higher than the resonant frequency with the phase difference set to 90 degrees, the driving speed decreases gradually. Then, the driving speed lowers abruptly and the driving member stops operation at a portion  23  of the drive frequency range. Thus, a frequency higher than the lower limit value and at which the driving member stops relative movement is defined as an upper limit value for phase difference control. Furthermore, the upper limit value may be, instead of the portion  23  at which the driving member stops operation during the above-described sweep-up process, a frequency corresponding to a portion at which the speed rises rapidly when the drive frequency is swept down from a value sufficiently higher than the resonant frequency. That is, according to the present invention, the upper limit value may be set equal to a frequency higher than the lower limit value and at which the driving member stops relative movement. 
     The range of frequencies between the upper limit value and the lower limit value described above is set to be an ellipticity changing frequency range. Changing the ellipticity within the ellipticity changing frequency range allows avoidance, during driving, of the cliff drop phenomenon and the portion at which the driving member stops operation as a result of a rapid decrease in driving speed. 
     The configuration of the control apparatus of the vibration-type actuator according to the present exemplary embodiment will be described with reference to a block diagram in  FIG. 3 . 
     The control apparatus includes a position instruction generating unit  17  configured to generate a target value for the driving member and to which an operation quantity determining unit  22  is connected via a comparing unit  18  on the output side thereof. 
     The comparing unit  18  compares the target value output by the position instruction generating unit  17  with the current position of the driving member which is output by a position detecting unit  16 . 
     The operation quantity determining unit  22  calculates the quantity e of operation of the vibration-type actuator based on the comparison result from the comparing unit  18 . 
     The operation quantity determining unit  22  includes a PI controller or a PID controller. The position detecting unit  16  detects the position of the driving member  3 , and includes, for example, a linear scale or an encoder. 
     The vibrator  8   a  is configured as illustrated in  FIG. 1  described above. The vibrator  8   a  includes the elastic member  4   a  formed of a metal material shaped like a rectangular plate, the piezoelectric element  5   a  joined to the back surface of the elastic member  4   a , and the plurality of protrusions  6   a  provided on the front surface of the elastic member  4   a.    
     Similarly, the vibrator  8   b  is configured as illustrated in  FIG. 1  described above. The vibrator  8   b  includes the elastic member  4   b  formed of a metal material shaped like a rectangular plate, the piezoelectric element  5   b  joined to the back surface of the elastic member  4   b , and the plurality of protrusions  6   b  provided on the front surface of the elastic member  4   b  as a driving portion. 
     The driving member  3  is illustrated in  FIG. 1  and connected to the output side of the vibrators  8   a  and  8   b.    
     An ellipticity determining unit  19  and a frequency determining unit  20  are connected to the output side of the operation quantity determining unit  22 ; the ellipticity determining unit  19  sets a ratio for an ellipse in the above-described elliptical motion, and the frequency determining unit  20  sets the frequency of an alternating current. 
     The ellipticity determining unit  19  is configured to be able to set the ratio of the X-axis amplitude and Z-axis amplitude in the elliptical motion caused in the protrusions of the vibrators  8   a  and  8   b  based on an output from the operation quantity determining unit  22 . The ellipticity determining unit  19  is further configured to be able to set a phase difference allowing the ratio to be achieved. 
     The drive frequency determining unit  20  is configured to be able to set the driving frequencies of AC voltages to be applied to the vibrators  8   a  and  8   b  based on the output from the operation quantity determining unit  22 . 
     Moreover, output sides of the ellipticity determining unit  19  and drive frequency determining unit  20  are connected to a drive signal generating unit  21 . 
     The drive signal generating unit  21  generates a two-phase alternating current with the frequencies determined by the frequency determining unit  20  and the phase difference determined by the ellipticity determining unit  19 . 
     A booster  25  is connected to an output side of the drive signal generating unit  21 . The booster  25  boosts the two-phase alternating current generated by the drive signal generating unit  21 , and applies the boosted two-phase alternating current to the piezoelectric elements of the vibrators  8   a  and  8   b.    
     The booster  25  may be formed of a power amplifier, a switching element, a DC/DC circuit, or a transformer circuit. 
     The functions of the ellipticity determining unit  19  and drive frequency determining unit  20  according to the present embodiment will be described with reference to  FIG. 4 . 
     In each graph in  FIG. 4 , the axis of abscissas indicates the operation quantity e. The axis of ordinate indicates one of a phase difference θ and a drive frequency fr. 
     The value of the operation quantity e output by the operation quantity determining unit  22  is input to the ellipticity determining unit  19 . 
     The ellipticity determining unit  19  uses this input value to determine the ellipticity based on settings expressed in the graphs. 
     The operation quantity e output by the operation quantity determining unit  22  is also input to the drive frequency determining unit  20 . The drive frequency determining unit  20  then determines the drive frequency fr based on the settings expressed in the graphs. 
     As illustrated in the graphs in  FIG. 4 , when the operation quantity e has a small absolute value, the phase difference determined by the ellipticity determining unit changes. When the operation quantity e has a large absolute value, the frequency determined by the frequency determining unit  20  changes. 
     Thus, areas with the frequency and the phase difference constant therein are provided so as to prevent the phase difference from changing when the frequency changes, while preventing the frequency from changing when the phase difference changes. The frequency fixed when the phase difference changes are hereinafter referred to as an ellipticity controlling frequency (fe). 
     Now, the ellipticity controlling frequency (fe) set when a plurality of vibrators with different resonant frequencies drive the driving member using a common drive frequency will be described; this aspect is most important in the present invention. 
       FIG. 5  illustrates the relationship between the drive frequency and the phase difference and the driving speed observed when one of the two vibrators with different resonant frequencies drives one driving member. 
       FIG. 5  is a graph illustrating the driving speed observed when the phase difference between the two-phase voltages V 1  and V 2  applied to the piezoelectric element illustrated in  FIG. 8B  is changed. 
     The drive frequency vs. driving speed observed when the phase difference for the vibrator  8   a  is changed between 90 degrees and 10 degrees is as illustrated at 90 degrees (a) to 10 degrees (a) in  FIG. 5 . 
     The resonant frequency corresponding to a phase difference of 0 degree for the vibrator  8   a  is a resonant frequency  24   a  corresponding to a phase difference of 0 degree illustrated in  FIG. 5 . Similarly, the drive frequency vs. driving speed observed when the phase difference for the vibrator  8   b  is changed between 90 degrees and 10 degrees is as illustrated at 90 degrees (b) to 10 degrees (b) in  FIG. 5 . The resonant frequency corresponding to a phase difference of 0 degree for the vibrator  8   b  is a resonant frequency  24   b  corresponding to a phase difference of 0 degree illustrated in  FIG. 5 . Furthermore, when the vibrator  8   a  places the driving member in relative movement, the maximum frequency at which the driving member stops relative movement is illustrated at  23   a . When the vibrator  8   b  places the driving member in relative movement, the maximum frequency at which the driving member stops relative movement is illustrated at  23   b.    
     In the present embodiment, the ellipticity changing frequency range (a), the range of driving frequencies set by the frequency determining unit based on the characteristics of the respective plurality of vibrators, is determined as follows. 
     That is, as described above, the lower limit value of the ellipticity changing frequency range is defined as the resonant frequency corresponding to the phase difference of 0 degree (the lower limit value is defined as the maximum resonant frequency obtained when the ellipticity determining unit changes the ellipticity). 
     Furthermore, the upper limit value is defined as the maximum frequency at which the driving member can be driven when the driving member is driven at a frequency higher than the lower limit value. 
     The ellipticity changing frequency range (a) lies between the thus determined lower limit value and upper limit value. 
     Similarly, the ellipticity changing frequency range (b) is determined based on the characteristics of the vibrator  8   b . In the following description, the vibrators  8   a  and  8   b  drive one driving member. 
     For example, when the vibrators  8   a  and  8   b  drive one driving member, the cliff drop phenomenon does not occur even if the phase difference for the vibrator  8   b  is changed at the resonant frequency of the vibrator  8   a  corresponding to a phase difference of 30 degrees. 
     However, the cliff drop phenomenon occurs when the phase difference for the vibrator  8   a  is changed from 90 degrees toward a smaller phase difference side. 
     Then, the driving speed of the vibrator  8   a  decreases rapidly, thus suppressing the driving speed of the vibrator  8   b.    
     Furthermore, the driving speed becomes unstable. To avoid this state during driving is the object of the present invention as described above. 
     That is, when the driving member is driven at a frequency lower than the ellipticity changing frequency range (c), the cliff drop phenomenon may occur in any of the plurality of vibrators which has a higher resonant frequency. 
     On the contrary, when the driving member is driven at a frequency higher than the ellipticity changing frequency range (c), the driving speed of any of the plurality of vibrators which has a lower resonant frequency may decrease and pass beyond the portion of the frequency range at which the vibrator stops operation. 
     In contrast, according to the present embodiment, the ellipticity determining unit is configured to be able to change (control) the ellipticity within the overlapping range of driving frequencies between the ellipticity changing frequency ranges set by the frequency determining unit based on the characteristics of the respective plurality of vibrators. 
     Specifically, the ellipticity controlling frequency (fe) illustrated in  FIG. 4  is set equal to the ellipticity changing frequency range (c), which is the overlapping portion between the ellipticity changing frequency ranges of the two vibrators. 
     Thus, the driving speed can be prevented from being unstable during driving when one driving member is placed in relative driving and when a common frequency is applied to a plurality of vibrators. 
     Second Exemplary Embodiment 
     A second exemplary embodiment will be described which corresponds to an example of a configuration for setting the upper limit of the ellipticity changing frequency, the highest frequency of a frequency range used for driving by a vibration-type actuator, and the lower limit of the ellipticity changing frequency, the lowest frequency of the frequency range. 
     The present embodiment provides a control method of a vibration-type actuator in which a plurality of vibrators drives one driving member. In this method, a characteristics detector detects the characteristics of each of the plurality of vibrators. 
     Then, the following are set based on the results of detection of the characteristics of each vibrator: the upper limit value of the ellipticity changing frequency, the highest frequency of the frequency range used by the plurality of vibrators to drive one driving member, and the lower limit value of the ellipticity changing frequency, the lowest frequency of the frequency range. 
     A control apparatus of the vibration-type actuator in which a plurality of vibrators drive one driving member has a configuration similar to that in the first exemplary embodiment. Thus, description of the configuration is omitted. 
     Now, detection of the output characteristics of each vibrator according to the present embodiment will be specifically described with reference to a flowchart in  FIG. 6 . 
     First, in STEP  1 , the vibrator  8   a  of the vibration-type actuator, in which the vibrators  8   a  and  8   b  illustrated in  FIG. 1  drive one driving member  3 , is set in a characteristics detector (not shown in the drawings). 
     The characteristics detector (not shown in the drawings) is an apparatus including a sensor configured to detect the driving characteristics of an vibrator in an actuator including the one vibrator and one driving member as illustrated in  FIG. 8A . 
     Then, in STEP  2 , the frequency to be applied to the vibrator  8   a  is set to a sufficiently large value. 
     Then, in STEP  3 , the phase difference for the vibrator  8   a  is set to 90 degrees. 
     When the phase difference is changed, the driving speed of the vibration-type actuator is maximized at the set phase difference. 
     Then, in STEP  4 , the frequency and phase difference set in STEP  2  and STEP  3  are applied to the vibrator  8   a  to start driving. 
     Then, in STEP  5 , the speed is detected. The speed detected in this case is that of the relative driving between the vibrator  8   a  and the driving member. 
     Then, in STEP  6 , comparison is carried out to determine whether the speed detected in STEP  5  is equal to or larger than 0. If the detected speed is higher than 0, the process proceeds to STEP  8 . 
     Furthermore, if the speed detected in STEP  5  is zero, the detector determines that the driving member and the vibrator fail to make relative movement. The process proceeds to STEP  7 . 
     Then, in STEP  7 , the frequency is reduced. The process then proceeds to STEP  5 . 
     Here, if the driving member and the vibrator fail to make relative movement, the operation between STEP  5  and STEP  7  is repeated. 
     Then, in STEP  8 , the frequency being applied to the vibrator  8   a  is stored in a memory (not shown in the drawings) as the upper limit value of the ellipticity changing frequency. 
     Then, in STEP  9 , the phase difference applied to the vibrator is set to 1 degree. 
     This is to detect the resonant frequency at as small a phase difference as possible because the resonant frequency of the vibrator increases when the phase difference is shifted from 90 degrees toward a smaller phase difference side. 
     Here, the phase difference is set to a sufficiently small value, 1 degree. 
     In subsequent STEP  10 , the detector detects whether or not the cliff drop phenomenon is occurring, in which the driving speed decreases rapidly. If the cliff drop phenomenon is not occurring, the process proceeds to STEP  11 . If the cliff drop phenomenon is occurring, the process proceeds to STEP  12 . 
     In STEP  11 , the frequency is reduced. If the cliff drop phenomenon is not detected, the operation in STEP  10  and STEP  11  is repeated. 
     In STEP  12 , the frequency being applied to the vibrator  8   a  is stored in the memory (not shown in the drawings) as the lower limit value of the ellipticity changing frequency. 
     Then, in STEP  13 , the vibrator set in the characteristics detector is changed from the vibrator (a) to the vibrator (b). Thereafter, the process proceeds to STEP  2  to repeat the above-described operation. The operation allows the output characteristics of the vibrator  8   b  to be detected after detection of the output characteristics of the vibrator  8   a.    
     In the method carried out between STEP  1  and STEP  14 , the output characteristics of the vibrator  8   a  are detected and then the output characteristics of the vibrator  8   b  are detected. This allows the output characteristics of each of the vibrators to be detected as illustrated in  FIG. 5 . 
     Then, in STEP  15 , an ellipticity changing frequency range is determined which lies between the upper limit value and lower limit value detected in STEP  1  to STEP  14  when the vibrator  8   a  is set in the characteristics detector. Furthermore, an ellipticity changing frequency range is determined which lies between the upper limit value and lower limit value detected when the vibrator  8   b  is set in the characteristics detector. Then, the overlapping range (common range) between the two ellipticity changing frequency ranges is calculated. 
     This enables calculation of the overlapping ellipticity changing frequency range (c) between the ellipticity changing frequency ranges (a) and (b) of the respective vibrators illustrated in the first exemplary embodiment. 
     Then, in STEP  16 , the vibrators  8   a  and  8   b  can be incorporated into the vibration-type actuator in which two vibrators relatively drive one driving member. The vibration-type actuator can then carry out controllable driving as in the case of the first exemplary embodiment based on the value calculated in STEP  15 . 
     In the present embodiment, when a plurality of vibrators relatively drive one driving member, and a common frequency is applied to the plurality of vibrators, each of the vibrators is mounted in the output characteristics detector, which thus detects the ellipticity changing frequency range. 
     Thus, the ellipticity changing frequency range (c) can be set based on the overlapping portion between the detected ellipticity changing frequency ranges. 
     Hence, when a plurality of vibrators relatively drive one driving member, pre-detection of the ellipticity changing frequency range of each of the vibrators allows easy setting of the ellipticity changing frequency range required when the plurality of vibrators relatively drive the one driving member. 
     Third Exemplary Embodiment 
     With reference to  FIG. 7 , a third exemplary embodiment will be described which corresponds to an example of a configuration in which three vibrators rotationally drive a ring-shaped driving member. 
     In the first and second exemplary embodiments, the vibration-type actuator in which two vibrators relatively drive one driving member has been described by way of example. 
     In the present exemplary embodiment, as illustrated in  FIG. 7 , three vibrators can rotationally drive a ring-shaped driving member. 
     The ring-shaped driving member can perform no operations other than the one of being rotated by a guide (not shown in the drawings). In this configuration, when the vibrators drive the driving member within the overlapping portion between the ellipticity changing frequency ranges of the respective vibrators, the driving can be achieved with the driving speed prevented from being unstable. 
     In the present embodiment, vibrators  8   c ,  8   d , and  8   e  relatively drive one driving member  2 , and a common frequency is applied to the vibrators  8   c ,  8   d , and  8   e , as illustrated in  FIG. 7 . When the vibrators drive the driving member within the overlapping ellipticity changing frequency range between the ellipticity changing frequency ranges of the respective vibrators  8   c ,  8   d , and  8   e , the driving is achieved with the driving speed prevented from being unstable. 
     Furthermore, the upper limit value of the ellipticity changing frequency range corresponding to the highest frequency and the lower limit value of the ellipticity changing frequency range corresponding to the lowest frequency can be set in a manner similar to that in the second exemplary embodiment. 
     Specifically, the characteristics of the respective vibrators are detected by different characteristics detectors. 
     Then, based on results for the characteristics of each of the vibrators, the upper limit value of the ellipticity changing frequency range corresponding to the highest frequency and the lower limit value of the ellipticity changing frequency range are set for the frequency range used for driving by the vibration-type actuator in which the vibrators  8   c ,  8   d , and  8   e  drive the one driving member  2 . 
     Thus, when a plurality of vibrators relatively drive one driving member, pre-detection of the ellipticity changing frequency range of each of the vibrators allows easy setting of the ellipticity changing frequency range required when the plurality of vibrators relatively drive the one driving member. 
     As described above, the configuration according to each embodiment of the present invention allows stable driving to be achieved when a plurality of vibrators drives one driving member and when a common frequency is applied to the plurality of vibrators. 
     That is, even with a variation in resonant frequency between the vibrators, stable driving can be achieved in the overlapping portion between the ellipticity changing frequency ranges of the respective vibrators. 
     Furthermore, stable driving can be achieved by adjusting the ellipticity changing frequency range based on the output characteristics of each vibrator. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2010-130156, filed Jun. 7, 2010, which is hereby incorporated by reference herein in its entirety.