Abstract:
When starting a vibration-type drive device, the frequency of a drive signal is lowered from a predetermined frequency at a first change rate. When the vibration state of a vibration member reaches a predetermined state, it is determined whether or not a relative movement speed has reached a reference speed. When the relative movement speed has not reached the reference speed, the frequency of the drive signal is lowered at a second change rate that is smaller than the first change rate. This avoids problems which could occur when moisture intervenes on a slide surface of the vibration-type drive device, such as the vibration-type drive device not starting, and a desired rpm not being achieved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a control apparatus and a control program for controlling drive of a vibration-type drive device at start-up.  
           [0003]    2. Related Background Art  
           [0004]    Recently, vibration-type motors have been used not only as actuators in camera lenses, but also as actuators to drive calendars in watches, to drive photosensitive drums in copiers, etc. Further, examples of the shape (constructions) of a vibration member in vibration-type motors include a ring shape, a rod shape, a disc shape, and many others.  
           [0005]    A general method of driving a vibration-type motor is shown in flowchart in FIG. 5 (see Japanese Patent Application Laid-Open No. 2001-25271, for example). Furthermore, FIG. 6 shows a pattern diagram of a frequency, a phase difference (the phase difference between an input voltage Va to the vibration-type motor (piezoelectric element) and an output voltage Vs of the sensor phase provided to the piezoelectric element), and the rotation number (speed) of a rotor (hereinafter referred to as the “rotor rpm”) when the vibration-type motor starts normally.  
           [0006]    In FIG. 6, a horizontal axis shows time (ms), and a vertical axis shows frequency (kHz), rotor rpm, and the above-mentioned phase difference θa−s.  
           [0007]    When the vibration-type motor starts, the frequency starts at the maximum frequency fmax within the frequency control range of the circuit (step S 501  in FIG. 5), and with a predetermined sweep down rate R 1  (Hz/sec) the frequency is swept down (step S 502  in FIG. 5). Then, when an encoder or the like detects (by pulse detection) that the vibration-type motor (the rotor) has started to rotate (step S 503  in FIG. 5), the phase difference θa−s is calculated (step S 504  in FIG. 5). Here, when the calculated phase difference θa−s exceeds a predetermined phase difference P 2  (step S 505  in FIG. 5), the frequency sweep is stopped (step S 506  in FIG. 5).  
           [0008]    Subsequently, with a frequency f 1  where the frequency sweep stopped serving as a reference, the motor is driven to a target point while controlling the frequency so that the phase difference θa−s falls between the above-mentioned phase difference P 2  and a predetermined phase difference P 1 .  
           [0009]    However, there is a fear that, when trying to start the vibration-type motor in a highly humid environment, it will not start on the first try, or it will only run at a very slow speed. This problem arises because, when the vibration-type motor is left in high humidity, trace amounts of moisture attach to minute gaps between the frictional surface between the vibration member and the rotor. This causes the vibration member and rotor friction coefficient to drop, reducing the torque generated when the motor starts. In particular, when using a SUS material, aluminum or other friction materials which have superior wear resistance and involves extremely little wear, the decrease in torque occurs more easily.  
           [0010]    When the motor is in a low-torque state such as described above, in the conventional control method, sometimes the vibration-type motor cannot be started normally. That is, as shown in FIG. 7, even when the frequency is swept down from a high frequency (fmax) upon motor start-up and the friction amplitude of the vibration member has increased to a sufficiently large value V 1  (frequency is f 1 ), the rotor rpm (N 1 ) is almost 0 due to the moisture and the like on the frictional surface.  
           [0011]    In this case, the drive pattern is such that as the frequency is further dropped from the frequency f 1 , it eventually sweeps down to the minimum frequency (fmin) where the drive ends.  
           [0012]    Here, since the vibration-type motor is driven by the frictional force between the vibration member and the rotor, the frictional heat on the frictional surface becomes extremely high, and when driven in a steady state the temperature on the friction surface reaches hundreds of degrees. However, when moisture intervenes on the frictional surface, within a short time period after the vibration-type drive device begins to start, the frictional heat generated on the slide surface is also very small and the conventional control method cannot eliminate the moisture.  
           [0013]    As a common solution to the situation where moisture is present on the slide surface, the pressure applied to the contact surface between the vibration member and the rotor is increased so as to raise the surface pressure on the contact surface to thereby eliminate the moisture, or the frictional surface is roughed (creating concaves and convexes) to minimize the influence of the moisture.  
           [0014]    However, in the vibration-type motor, simply raising just the applied pressure increases the load on the frictional surface, and thus increases wear. Moreover, since a load also bears on the bearing that receives the reaction force when the pressure is applied, the durability of the bearing deteriorates and damage to the bearing increases. Furthermore, when the slide surface is simply roughened, this increases the wear of the frictional surface, which is detracts from the durability of the vibration-type motor.  
         SUMMARY OF THE INVENTION  
         [0015]    According to the present invention, there is provided a control apparatus for controlling drive of a vibration-type drive device in which a vibration member excited by receiving an input of a drive signal and a contact member brought into contact under pressure with the vibration member can move relative to each other, the control apparatus including: a vibration detection unit which detects a vibration state of the vibration member; a speed detection unit which detects a relative movement speed between the vibration member and the contact member; and a control unit which outputs the drive signal to the vibration member to control drive of the vibration-type drive device, characterized in that the control unit performs: a first frequency control step of lowering a frequency of the drive signal from a predetermined frequency at a first change rate, when the vibration-type drive device is started; a determining step of, when the vibration state detected by the vibration detection unit becomes a predetermined state in accordance with a change in the frequency in the first frequency control step, determining whether or not the relative movement speed detected by the speed detection unit has reached a predetermined reference speed; a second frequency control step of, when it is determined in the determining step that the relative movement speed has not reached the reference speed, lowering the frequency of the drive signal at a second change rate which is smaller than the first change rate; and a third frequency control step of, when the relative movement speed has reached the reference speed, changing the frequency of the drive signal such that the relative movement speed reaches a target speed.  
           [0016]    That is, at the time when the vibration-type drive device starts, if the frequency is changed at the first change rate but the relative movement speed still does not increase due to moisture, etc. on the frictional surface of the vibration-type drive device, the frequency is changed at the second change rate which is smaller than the first change rate, thereby securing sufficient time for the frictional heat generated on the frictional surface to eliminate the moisture.  
           [0017]    Furthermore, it is also possible for a program to cause a computer to execute the processing operations of the above-mentioned control circuit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a block diagram of a control apparatus according to a first embodiment of the present invention;  
         [0019]    [0019]FIG. 2 is a flowchart showing operations when starting a motor according to the first embodiment;  
         [0020]    [0020]FIG. 3 is a timing chart when starting the motor according to the first embodiment;  
         [0021]    [0021]FIG. 4 is a timing chart when starting a motor according to a second embodiment;  
         [0022]    [0022]FIG. 5 is a flowchart showing operations when starting a motor according to a conventional example;  
         [0023]    [0023]FIG. 6 is a timing chart (normal) when starting the motor according to the conventional example;  
         [0024]    [0024]FIG. 7 is a time chart (abnormal) when starting the motor according to the conventional example;  
         [0025]    [0025]FIG. 8A is an external view of a rod-shaped vibration-type motor;  
         [0026]    [0026]FIG. 8B is a diagram illustrating a vibration mode;  
         [0027]    [0027]FIG. 9 is a cross-sectional view of the above-mentioned vibration-type motor; and  
         [0028]    [0028]FIG. 10 is an external view of the above-mentioned vibration-type motor. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0029]    [0029]FIGS. 1 through 3 and  8 A through  10  are used to explain a control apparatus of a vibration-type motor, according to a first embodiment of the present invention.  
         [0030]    [0030]FIGS. 8A through 10 are explanatory diagrams of a rod-shaped vibration-type motor. FIG. 8A shows a construction of a rod-shaped vibration member, and FIG. 8B shows a vibration mode of the vibration member. FIGS. 9 and 10 are constructional diagrams of the vibration-type motor having the vibration member shown in FIGS. 8A and 8B. The vibration-type motor is used as an actuator for driving a lens provided to a camera, for example.  
         [0031]    In FIGS. 8A through 10, reference numeral  101  denotes a first elastic member, and reference numeral  102  denotes a second elastic member. Reference numeral  103  denotes a multi-layer piezoelectric element (or a multi-layer member with single-layers of piezoelectric elements) which serves as an electromechanical energy conversion element, and is sandwiched between the first elastic member  101  and the second elastic member  102 .  
         [0032]    Reference numeral  104  denotes a shaft, and reference numeral  105  denotes a nut. The shaft  104  and the nut  105  binds the first elastic members  101 , the second elastic member  102 , and the multi-layer piezoelectric element  103  so as to squeeze them with a given amount of force.  
         [0033]    Reference numeral  107  denotes a rotor (contact member). One surface (the bottom surface in FIG. 9) is formed with a contact portion  107   a . The contact portion  107   a  is arranged so as to make contact with a frictional piece  106  that is provided to an end surface of the vibration member, and has a structure with a small contact width and appropriate springiness. Further, the other surface of the rotor  107  (the top surface in FIG. 9) is formed with a convex portion (or concave portion) that engages with a concave portion (or convex portion) of a gear  108 .  
         [0034]    Reference numeral  108  denotes a gear that rotates together with the rotor  107  and transmits the output from the vibration-type motor. The gear  108  is held in position at a thrust direction across the shaft  104  by means of a flange  110  that mounts the vibration-type motor. Reference numeral  111  denotes a nut that fixes an end portion of the shaft  104  to the flange  110 . Reference numeral  112  denotes a pressure spring that applies pressure to the rotor  107  and is provided between the gear  108  and the rotor  107 .  
         [0035]    Reference numeral  109  denotes a bearing which is fixed to the gear  108  and regulates the position of the shaft  104 .  
         [0036]    The multi-layer piezoelectric element  103  has two grouped electrode groups (areas divided into + and −). When AC voltages with different phases are applied to each electrode group from an electrical source (not shown), the vibration member is excited with a curved vibration shown in FIG. 8B, and a vibration that is curved similarly to the curved vibration and occurs at an angle perpendicular to the surface of the paper.  
         [0037]    Here, by adjusting the phases of the applied voltages, it is possible to create a 90° chronological phase difference between the two curved vibrations. As a result, a curved vibration of the vibration member is generated around the axis of the vibration member.  
         [0038]    Accordingly, elliptic movement is formed on the upper surface of the first elastic member  101 , which makes contact with the rotor  107 . The rotor  107 , being pressed against the friction-resistant frictional piece  106 , creates frictional drive. Thus, the rotor  107 , the gear  108 , and the pressure spring  112  rotate as a single unit.  
         [0039]    [0039]FIG. 1 is used to explain a construction of the control apparatus of the vibration-type motor according to the present invention.  
         [0040]    In FIG. 1, reference numeral  1  denotes the control circuit. Reference numeral  2  denotes a voltage control oscillator (VCO), which outputs a frequency voltage corresponding to the output from the control circuit  1 . Reference numeral  3  denotes a distribution circuit, which divides the frequency voltage of the VCO  2  to output signals at a π/2 phase difference.  
         [0041]    Reference numerals  4  and  5  denote output circuits, which amplify the frequency voltage of the distribution circuit  3  to a voltage and electric current value that can drive the vibration-type motor  6 .  
         [0042]    The output signals from the output circuits  4  and  5  are inputted to the multi-layer piezoelectric element  103  provided to the vibration member of the vibration-type motor  6  and excite the above-mentioned curved vibration in the vibration member. This rotates the rotor  107  which is slidably or frictionally pressed against the vibration member. The rotation of the rotor  107  is transmitted through a force transmission mechanism (not shown) to a driven object  7  such as a photosensitive drum of a copier, a lens barrel of a camera, or the like, thus driving the driven object  7 .  
         [0043]    Reference numeral  8  denotes an encoder that detects the rotation of the rotor  107 . The encoder  8  is constituted by a slitted light-shielding plate that rotates together with the vibration-type motor, and a photo-interrupter that detects the rotation of the light-shielding plate. Reference numeral  9  denotes a position speed detection circuit that detects the rotational position and number of rotations of the vibration-type motor (the rotor  107 ), based on an output from the encoder  8 . Detection results from the position speed detection circuit  9  are sent to the control circuit  1 .  
         [0044]    Reference numeral  10  denotes a phase difference detection circuit, which detects the phase difference between the signals inputted from the output circuits  4  and  5  to the vibration-type motor  6  (the multi-layer piezoelectric element  103 ), and the output signal from the feedback sensor phase (“S” phase) provided to the multi-layer piezoelectric element  103 . These detection results are sent to the control circuit  1 .  
         [0045]    Reference numerals  11  and  12  denote capacitors, which use coils  13  and  14  to increase the voltage of the signal inputted to the vibration-type motor (the multi-layer piezoelectric element  103 ).  
         [0046]    The control circuit  1  performs drive control on the vibration-type motor as descried below, based on the outputs from the position speed detection circuit  9  and the phase difference detection circuit  10 . Further, the control circuit  1  detects the vibration amplitude of the vibration member based on an output voltage (Vs) from the sensor phase.  
         [0047]    In this embodiment, description is given regarding a driving method performed after the vibration-type motor is started from a stopped state, up to the point where control begins at a predetermined speed. The drive controls, stop operations, and the like that are performed on the motor after that point are similar to the conventional techniques.  
         [0048]    For example, after the vibration-type motor is started, speed controls and phase controls are used to control the speed of the vibration-type motor closer to a desired rotation speed. Speed controls stop the motor drive.  
         [0049]    Here, the above-mentioned speed controls detect the rotation speed of the vibration-type motor at given periods, compare the detected rotation speed and the desired rotation speed, raise the drive frequency by a predetermined value when the actual rotation speed is faster than the desired rotation speed, and lower the drive frequency by a predetermined value when the actual rotation speed is slower than the desired rotation speed, thereby controlling the rotation speed of the vibration-type motor.  
         [0050]    Further, the above-mentioned phase controls detect the frequency voltage inputted to the piezoelectric element and the phase difference of the frequency voltage from the sensor piezoelectric element, and then control the frequency based on the phase difference information that was attained.  
         [0051]    [0051]FIG. 2 is a flowchart of controls at the time when the vibration-type motor is started. FIG. 3 is a diagram showing a time chart of a frequency, an “S” phase voltage Vs, and a rotor rotation number (hereinafter referred to as the “motor rpm”) N.  
         [0052]    In FIG. 2, at step S 101 , the frequency is set to the maximum frequency fmax within the frequency control range (frequency greater than the resonance frequency of the vibration member), and the vibration-type motor starts to be driven.  
         [0053]    Note that, the frequency at the start of the drive on the vibration-type motor does not have to be set to fmax, but may be set to another given frequency. For example, it is possible to store the frequency from the previous time when the motor was started, to set the frequency that can drive the vibration-type motor (i.e., rotate the rotor  107 ) without fail. Accordingly, the vibration-type motor can be started to a given number of rotations in a short time.  
         [0054]    At step S 102 , the frequency is swept down at a sweep rate R 1  (Hz/s), and the output signal and a vibration amplitude Vs from the encoder  8  are monitored.  
         [0055]    At step S 103 , it is determined whether or not the vibration amplitude Vs is equal to or greater than a predetermined vibration amplitude Vst. When Vs is greater than Vst, the processing advances to step S 104 .  
         [0056]    At step S 104 , the rotation number (hereinafter referred to as the “rpm”) N of the rotor  107  is detected based on the output from the encoder, to determine whether or not the rpm N is equal to or greater than a predetermined rpm Nt. Here, when N is equal to or greater than Nt, the processing advances to step S 107 . When N is less than Nt, the processing advances to step S 105 .  
         [0057]    [0057]FIG. 3 shows a case where, because the moisture and the like are intervening on the frictional surface in the vibration-type motor, the rpm N has not reached the predetermined rpm Nt at a time T 1  when the amplitude Vs reaches the predetermined amplitude Vst (Vs 1 ). N 1  indicates the rpm at this time.  
         [0058]    At step S 105 , the sweep rate is changed from R 1  to R 2 . Here, the sweep rate R 2  is smaller than the sweep rate R 1 . In FIG. 3, a frequency f 1  is a threshold where the sweep rate changes from R 1  to R 2 .  
         [0059]    At step  106 , the frequency is swept down at the sweep rate R 2 .  
         [0060]    At step S 107 , the sweep rate changes to R 3 . Here, the value of the sweep rate R 3  can be set as needed, but in order to quickly raise the rpm to the target rpm (N 3  of FIG. 3), the sweep rate R 3  should be greater than the sweep rate R 2 .  
         [0061]    As shown in FIG. 3, when the moisture is present on the frictional surface of the vibration-type motor, the sweep rate switches from R 1  to R 2 , and the frequency is changed according to R 2 , whereby the friction between the vibration member and the rotor can eliminate the moisture on the frictional surface. When the moisture is eliminated, the rpm N gradually rises and reaches the predetermined rpm Nt. After this point, the sweep rate can be switched to R 3  so that the rpm can reach the target rpm quickly.  
         [0062]    Performing the controls described above eliminates the problem in that the vibration-type motor drive stops before the rpm rises, as described with respect to the conventional technique.  
         [0063]    At step S 108 , the frequency sweeps down at the sweep rate R 3 . At step S 109 , it is determined whether or not the rpm N has reached the target rpm (N 3  of FIG. 3). Here, before the rpm N reaches the target rpm, the sweep down continues at the sweep rate R 3 . When the rpm N reaches the target rpm (T 3  of FIG. 3) the sweep down stops, and the drive control on the vibration-type motor begins.  
         [0064]    In this embodiment, the rotor rpm N was used as the reference to determine when to switch the sweep rate, but the present invention is not restricted to this configuration. Rather, any method can be used provided that it can judge whether or not the vibration-type motor is driving. For example, the number of pulses from the encoder can serve as the determination reference, and the sweep rate can be switched when  5  or more pulses are outputted at the time T 1  in FIG. 3.  
         [0065]    According to this embodiment, in a normal environment (i.e., an environment without high humidity), the rpm rises as the frequency changes, so the vibration-type motor can be started rapidly, without delays while starting. On the other hand, under poor conditions such as a high-humidity environment, by starting the vibration-type motor gradually in a high-amplitude state, the frictional heat generated on the slide surface by driving the vibration-type motor can eliminate the moisture present on the frictional surface. Accordingly, the vibration-type motor returns to the state where the rotor can rotate without fail and can be driven.  
       Second Embodiment  
       [0066]    [0066]FIG. 4 is used to explain a second embodiment of the present invention. Here, FIG. 4 is a diagram showing a time chart of the frequency, the “S” phase voltage Vs, and the motor rpm N. FIG. 4 is similar to FIG. 3, and explains a case where, because moisture is intervening on the slide surface of the vibration-type motor, even when the frequency is changed, the rpm still does not rise in response thereto.  
         [0067]    Note that, the circuit structure and the construction of the vibration-type motor in this embodiment are similar to those explained in the first embodiment (FIGS. 1 and 8A through  10 ).  
         [0068]    In the first embodiment, the sweep rate is changed to R 2  at step S 105  in FIG. 2. However, in this embodiment, instead of changing the sweep rate to R 2 , the frequency sweep is stopped (i.e., the frequency is kept constant).  
         [0069]    In FIG. 4, the frequency is swept down from the frequency fmax at the sweep rate R 1 , and when the amplitude Vs reaches the predetermined amplitude Vst (at the time T 1 ), a predetermined vibration amplitude at the frequency f 1  continues to be applied.  
         [0070]    Then, when the rpm N reaches the given rpm Nt (at a time T 2 ) the frequency sweep rate is set to R 3  and the sweep down starts again. The sweep down continues until the rpm reaches the target rpm N 3 . After the rpm reaches the target rpm N 3 , the drive control on vibration-type motor is performed. This embodiment also obtains effects similar to the effects explained in the first embodiment.  
         [0071]    In the above-mentioned operations, when the constant vibration at the frequency f 1  is applied for a given period of time and the vibration-type motor still does not start, the drive on the vibration-type motor may be stopped after a given period of time (e.g., 1 sec), and the processing may be repeated again from the start. By repeating this processing, the motor can be started quickly even when the state of the frictional surface in the vibration-type motor has deteriorated.  
         [0072]    Note that, in the above-mentioned first and second embodiments, the frequency was changed in a straight line as shown in FIGS. 3 and 4. However, the frequency may also be changed along a curved line.  
       Third Embodiment  
       [0073]    A third embodiment of the present invention uses the phase difference θa−s instead of the vibration amplitude Vs to detect the vibration amplitude of the vibration member. Note that the circuit structure and the construction of the vibration-type motor in this embodiment are similar to the constructions explained in the first embodiment.  
         [0074]    Here, the phase difference θa−s itself does not directly express the vibration amplitude of the vibration member, but can be used to learn which location is currently vibrating with respect to the resonance frequency. Therefore, if the value of the phase difference θa−s is designated, substantially the same vibration amplitude can be regenerated on the an identical vibration-type motor. Thus, the vibration amplitude of the vibration member can be indirectly monitored based on the phase difference θa−s.  
         [0075]    Specifically, a phase difference θa−st that corresponds to the vibration status at the time when the above-mentioned predetermined vibration amplitude Vst is excited in the vibration member is pre-stored in the memory, and instead of performing step S 103  in FIG. 2, the phase difference θa−s detected based on the output from the phase difference detection circuit  10 , and the phase difference θa−st, are compared.  
         [0076]    That is, when the phase difference θa−s reaches the phase difference θa−st (which corresponds to when the amplitude Vs reaches the predetermined amplitude Vs), it is judged whether or not the rpm of the rotor has reached the predetermined rpm Nt.  
         [0077]    Then, before the rpm reaches the predetermined rpm Nt, the sweep rate switches from R 1  to R 2  as explained in the first and second embodiments, and when the rpm reaches the predetermined rpm Nt, the sweep rate switches to R 3  to raise the rpm N to the target rpm.  
         [0078]    Here, when the vibration amplitude Vs of the vibration member is extremely small, the signal at the phase difference θa−s is not precise, and erroneous signals may occur. Therefore, in the case where the phase difference θa−s serves as the guideline for the determination, it is necessary to use a signal obtained after Vs becomes reasonably large.  
         [0079]    In this embodiment, when the phase difference between Va and Vs is to be measured, their signals are passed through a comparator and converted into rectangle wave signals. For Vs, the comparator is offset by a fixed level, so that the signal will not be outputted from the comparator if Vs is below a certain level.  
         [0080]    That is, when Vs is small and at a level where the phase difference θa−s may be an erroneous signal, the comparator signal from Vs is not outputted. Thus, the phase difference θa−s is not measured and a default value is shown. Accordingly, the phase difference θa−s at the level that will produce erroneous signals is automatically eliminated, and the measurement is taken once the phase difference θa−s produces an accurate signal. Accordingly, extremely reliable control can be performed.  
       Fourth Embodiment  
       [0081]    In a fourth embodiment of the present invention, Vs is not used as in the above-mentioned first and second embodiments. Instead, a mechanical arm electric current element in the electric current that is applied to the vibration-type motor serves as a guideline to detect the vibration amplitude of the vibration member, and the drive control on the vibration-type motor is performed.  
         [0082]    The mechanical arm electric current increases in proportion to the distortion of the piezoelectric elements, and thus is substantially proportionate to the vibration amplitude of the vibration member. Because of this, if the relationship between the mechanical arm electric current and the vibration amplitude is obtained in advance, based on the mechanical arm electric current, it is possible to perform controls similar to the above-mentioned controls (FIG. 2) that were explained in the above-mentioned embodiments.  
         [0083]    In this embodiment, an absolute value of the mechanical arm electric current is detected, and this value serves as the guideline of the vibration amplitude instead of Va that was used in the above-mentioned first embodiment and second embodiment.  
         [0084]    Specifically, a mechanical arm electric current Imt that corresponds to the vibration status at the time when the above-mentioned predetermined vibration amplitude Vst is excited in the vibration member, is pre-stored in the memory, and instead of performing step S 103  in FIG. 2, a detected mechanical arm electric current Im and a mechanical arm electric current It are compared.  
         [0085]    That is, when the mechanical arm electric current Im reaches the mechanical arm electric current Imt (which corresponds to when the amplitude Vs reaches the predetermined amplitude Vst), it is judged whether or not the rotor rpm has reached the predetermined rpm Nt.  
         [0086]    Then, before the rpm reaches the predetermined rpm Nt, the sweep rate switches from R 1  to R 2  as explained in the first and second embodiments, and when the rpm reaches the predetermined rpm Nt, the sweep rate switches to R 3  and raises the rpm N to the target rpm.  
         [0087]    Note that the above-mentioned embodiment was explained with respect to the case using the rod-shaped vibration-type motor shown in FIGS. 8A through 10, but the present invention is not restricted to this configuration. Any type (e.g., ring-shaped type, disc-shaped type) can be applied if it is a vibration-type motor that performs frictional drive.