Drive circuit for an ultrasonic motor having noise cancellation metals and method for using same

A drive circuit for driving an ultrasonic motor which maintains frequencies of drive signals for the ultrasonic motor within a drive frequency band. Generation of audible sound is thereby prevented. Further, it is possible to drive an ultrasonic motor in which irregularity of a waveform of a detection signal does not occur in an audible sound generating band.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a drive circuit for driving an ultrasonic 
motor and a method of driving and controlling the ultrasonic motor. 
2. Related Art 
Conventionally, ultrasonic motors have been known in which ultrasonic 
vibration is utilized to generate a driving force. In a traveling-wave 
type ultrasonic motor, a piezoelectric body is adhered to an annular 
elastic body to form a stator, against which a rotor attached to a drive 
shaft is pressed so as to contact the stator. A drive circuit for the 
ultrasonic motor supplies the piezoelectric body with a sine-wave drive 
signal and a cosine-wave drive signal of predetermined frequencies. Due to 
mechanical vibration generated by these two drive signals, an ultrasonic 
vibration (traveling wave) whose loops and nodes travel in a 
circumferential direction along the elastic body is excited in the elastic 
body. Due to the traveling wave, the rotor which contacts the elastic body 
and the drive shaft are rotated. 
The amplitude of the mechanical vibration generated in the piezoelectric 
body becomes a maximum when the frequencies of the drive signals coincide 
with the resonant frequency. However, within a specific frequency band 
including the resonant frequency, abnormal vibrations of an audible 
frequency are generated in the elastic body, and the vibrations lower the 
rotational speed of the rotor and the efficiency of the ultrasonic motor. 
Accordingly, the ultrasonic motor is driven such that drive signals whose 
frequencies are sufficiently higher than a frequency band in which audible 
sound is generated (hereinafter referred to as "audible sound generating 
band") are initially supplied to the ultrasonic motor. The frequencies of 
the drive signals are then gradually lowered so as to enter a drive 
frequency band slightly higher than the audible sound generating band and 
are then maintained within this drive frequency band. 
The audible sound generating band and the drive frequency band of the 
ultrasonic motor change depending on the ambient temperature of the 
ultrasonic motor and the magnitude of the load acting on the ultrasonic 
motor. Accordingly, the proper frequencies of the drive signals must not 
be constant and must be changed in accordance with the ambient 
temperature, the load and the like. Therefore, a device has been proposed 
in which a piezoelectric element is adhered to the elastic body so as to 
control the frequencies of the drive signals based on an AC detection 
signal which is output from the piezoelectric element in accordance with 
the ultrasonic vibration of the elastic body. 
As an example of frequency control of drive signals, a technique is 
disclosed in Japanese Patent Application Laid-open No. 62-203575 in which 
a detection signal output from a piezoelectric element is subjected to 
half-wave rectification by a diode, followed by smoothing with a capacitor 
to obtain a smoothed signal. The frequency is controlled such that the 
level of the smoothed signal becomes a predetermined value lower than a 
predetermined level which is obtained at the above-mentioned resonant 
frequency. Another technique is disclosed in Japanese Patent Application 
Laid-open No. 3-159583 in which irregularity of the waveform of a 
detection signal output from a piezoelectric element is monitored, and the 
frequencies of drive signals are lowered when irregularity of the waveform 
does not occur. On the contrary, when irregularity of the waveform occurs, 
it is judged that the frequencies of the drive signals have entered the 
audible sound generating band, and the frequencies are raised. 
In the frequency control arrangement disclosed in Japanese Patent 
Application Laid-open No. 62-203575, the level of a signal obtained by 
smoothing the detection signal output from the piezoelectric element, 
namely, the average level of the detection signal, is compared with a 
predetermined level. Therefore, even when the frequencies of the drive 
signals enter the audible sound generating band and irregularity of the 
waveform occurs, the frequencies are lowered if the average level of the 
detection signal is lower than the above-mentioned predetermined level. 
Accordingly, this frequency control arrangement has the drawback that the 
frequencies of the drive signals enter the audible sound generating band 
so that audible sound is generated from the ultrasonic motor. 
Further, among various ultrasonic motors, there are some ultrasonic motors 
in which irregularity of the waveform of the detection signal does not 
occur even when the frequencies of the drive signals enter the audible 
sound generating band because of their shapes and sizes, pressing force of 
the rotor and the stator, and other factors. When the frequency control 
method disclosed in Japanese Patent Application Laid-open No. 3-159583 is 
used for driving the above-mentioned ultrasonic motor, the frequencies of 
the drive signals are lowered and enter the audible sound generating band, 
and are further lowered, passing the resonant frequency, because 
irregularity of the waveform of the detection signal does not occur even 
when the frequencies of the drive signals enters the audible sound 
generating band. Accordingly, this frequency control method is not 
suitable for ultrasonic motors in which irregularity of the waveform of 
the detection signal does not occur in the audible sound generating band. 
SUMMARY OF THE INVENTION 
The present invention has been accomplished by taking the above-mentioned 
facts into consideration, and the object of the present invention is to 
provide a drive circuit for an ultrasonic motor and a method of driving 
and controlling the ultrasonic motor which results in an ultrasonic motor 
in which audible sounds are not generated. 
Further, another object of the present invention is to provide a drive 
circuit for an ultrasonic motor and a method of driving and controlling 
the ultrasonic motor which is capable of driving even ultrasonic motors in 
which irregularity of the waveform of a detection signal does not occur in 
the audible sound generating band. 
To achieve the above-mentioned objects, a drive circuit for an ultrasonic 
motor according to the present invention comprises drive signal output 
means for outputting ultrasonic motor drive signals of predetermined 
frequencies, detection means for detecting vibration of a stator of the 
ultrasonic motor and for outputting a detection signal whose amplitude 
corresponds to the vibration, judging means for judging whether the level 
of the detection signal is less than or equal to a predetermined level 
which is set in advance, and frequency control means for bringing the 
frequencies of the drive signals into a drive frequency band by raising 
the frequencies of the drive signals only in a period in which the level 
of the detection signal exceeds the predetermined level in a case in which 
the level of the detection signal exceeds the predetermined level, and by 
lowering the frequencies of the drive signals only in a period in which 
the level of the detection signal is less than or equal to the 
predetermined level in a case in which the level of the detection signal 
is less than or equal to the predetermined level. 
The present invention also provides a method of driving and controlling an 
ultrasonic motor comprising the steps of outputting ultrasonic motor drive 
signals of predetermined frequencies, detecting vibration of a stator of 
the ultrasonic motor and outputting a detection signal whose amplitude 
corresponds to the vibration, judging whether a level of the detection 
signal is less than or equal to a predetermined level which is set in 
advance, and bringing the frequencies of the drive signals into a drive 
frequency band by raising the frequencies of the drive signals only in a 
period in which the level of the detection signal exceeds the 
predetermined level in a case in which the level of the detection signal 
exceeds the predetermined level, and by lowering the frequencies of the 
drive signals only in a period in which the level of the detection signal 
is less than or equal to the predetermined level in a case in which the 
level of the detection signal is less than or equal to the predetermined 
level. 
In the present invention, it is judged whether the level of a detection 
signal, which is output from the detection means and whose amplitude 
corresponds to vibration of the stator of the ultrasonic motor, is less 
than or equal to the predetermined level which has been set in advance. 
When the level of the detection signal exceeds the predetermined level, 
the frequencies of the drive signals are raised by the frequency control 
means only in a period in which the level of the detection signal exceeds 
the predetermined level. When the level of the detection signal is less 
than or equal to the predetermined level, the frequencies of the drive 
signals are lowered only in a period in which the level of the detection 
signal is less than or equal to the predetermined level. The 
above-mentioned predetermined level is set at a value such that an amount 
of change in the frequency control medium (for example, voltage) for 
raising the frequencies in a period in which the level of the detection 
signal exceeds the predetermined level and an amount of change in the 
frequency control medium (for example, voltage) for lowering the 
frequencies in a period in which the level of the detection signal is less 
than or equal to the predetermined level become equal when the ultrasonic 
motor is driven by the drive signals whose frequencies are within the 
drive frequency band. 
In a case in which the frequencies of the drive signals are sufficiently 
higher than the drive frequency band, the amplitude of vibration generated 
in the stator is small, and the amplitude of the detection signal is also 
small. Therefore, the length of each period in which the level of the 
detection signal exceeds the predetermined level is extremely short, or 
such a period does not exist. Accordingly, since the length of each period 
in which the level of the detection signal is less than or equal to the 
predetermined level is long, the frequencies of the drive signals are 
changed so as to become lower on the whole, i.e., so as to approach the 
drive frequency band. 
When the frequencies of the drive signals are lowered, the amplitude of 
vibration generated in the stator and the amplitude of the detection 
signal become larger so that the length of each period in which the level 
of the detection signal exceeds the predetermined level gradually becomes 
longer. When the frequencies of the drive signals reach the drive 
frequency band, as described above, an amount of change in the frequency 
control medium for raising the frequencies in a period in which the level 
of the detection signal exceeds the predetermined level and an amount of 
change in the frequency control medium for lowering the frequencies in a 
period in which the level of the detection signal is less than or equal to 
the predetermined level become equal. Accordingly, the frequencies of the 
drive signals are controlled so as to roughly maintain their present 
values. Therefore, the driving of the ultrasonic motor at frequencies 
within the drive frequency band is continued. 
Further, when the frequencies of the drive signals enter the audible sound 
generating band due to changes in ambient temperature, changes in load and 
the like, irregularity of the waveform occurs. Due to this irregularity, 
periods in which the amplitude of the detected signal far exceeds the 
predetermined level and periods in which the amplitude does not reach the 
predetermined level are produced. As a whole, compared to the case in 
which the ultrasonic motor is driven in the drive frequency band, the 
length of each period in which the signal exceeds the predetermined level 
due to the large amplitude becomes long. In this case, the frequencies of 
the drive signals are changed so as to increase as a whole. Accordingly, 
the frequencies are controlled to return to the drive frequency band when 
the frequencies of the drive signals enter the audible sound generating 
band. 
In the present invention, the frequencies of the drive signals are 
controlled to stay in the drive frequency band, as described above. 
Therefore, the ultrasonic motor can always be driven at an optimum 
frequency without causing any drawbacks such as the frequencies entering 
the audible sound generating band such that audible sound is generated 
from the ultrasonic motor. 
In a case of driving a ultrasonic motor in which irregularity of the 
waveform of a drive signal does not occur even when the frequencies of the 
drive signals enter the audible sound generating band, the amplitude of 
the detection signal increases when the frequencies of the drive signals 
enter the audible sound generating band even though irregularity of the 
waveform of the detection signal does not occur. As a result, the length 
of each period in which the level of the detected signal exceeds the 
predetermined level becomes longer, and the frequencies of the drive 
signals are changed to increase as a whole. Accordingly, the frequencies 
are changed so as to return to the drive frequency band when the 
frequencies of the drive signals enter the audible sound generating band 
even in a case of driving an ultrasonic motor in which irregularity of the 
waveform of a drive signal does not occur in the audible sound generating 
band. Accordingly, it becomes possible to drive ultrasonic motors in which 
irregularity of the waveform does not occur in the audible sound 
generating band. 
As described above, in the drive circuit according to the present 
invention, it is judged whether the level of a detection signal whose 
amplitude corresponds to vibration of the stator of the ultrasonic motor, 
is less than or equal to the predetermined level. The frequencies of the 
drive signals are raised only in a period in which the level of the 
detection signal exceeds the predetermined level, and are lowered only in 
a period in which the level of the detection signal is less than or equal 
to the predetermined level. Therefore, the present invention has an 
excellent effect in that it is possible to drive an ultrasonic motor 
without the generation of audible sound even when irregularity of the 
waveform does not occur in the ultrasonic motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will now be explained in detail with reference to the 
drawings. FIG. 2 shows a traveling-wave type ultrasonic motor 10 according 
to the present embodiment. The ultrasonic motor 10 has an annular elastic 
body 12 made of a copper alloy or the like, and a piezoelectric body 14 is 
adhered to the elastic body 12, thereby forming a stator. 
The piezoelectric body 14 is made of a piezoelectric material which 
converts electric signals to mechanical vibrations. The piezoelectric body 
14 is divided into a plurality of portions arranged in the circumferential 
direction with a plurality of electrodes being formed thereon. A rotor 18 
attached to the drive shaft 16 is comprised of a rotor ring 20 made of an 
aluminum alloy or the like, and an annular slider 22 which is bonded to 
the rotor ring 20. The slider 22 is urged by a spring 24 against the 
above-mentioned elastic body 12 so as to contact the elastic body 12. To 
obtain a stable frictional force and frictional coefficient, the slider 22 
is made of an engineering plastic or the like. Accordingly, the rotor 18 
can be driven efficiently. 
Further, a piezoelectric element 32 (see FIG. 1) is adhered to the elastic 
body 12. As shown in FIG. 1, the piezoelectric element 32 is connected to 
terminals 30A and 30B of an ultrasonic motor drive circuit 30. The 
piezoelectric element 32 detects vibration of the elastic body 12, and 
outputs to the drive circuit 30 an AC signal whose amplitude and period 
change according to the vibration. 
One end of a resistor 38 is connected to the terminal 30A while the 
terminal 30B is connected to a ground terminal 36. Connected to the other 
end of the resistor 38 are one end of a resistor 40, the cathode of a 
diode 42, the anode of a diode 44 and the inverted input terminal of a 
comparator 46, which serves as a comparing means. The other end of the 
resistor 40 and the anode of the diode 42 are connected to the ground 
terminal 36 while the cathode of the diode 44 is connected to a power 
supply terminal 34. The power supply terminal 34 is connected to an 
unillustrated constant-voltage power supply so that a constant voltage 
(such as 5 V) is supplied from the power supply. 
Further, one end of a resistor 48 is connected to the power supply terminal 
34. Connected to the other end of the resistor 48 are one end of a 
resistor 50, whose other end is connected to the ground terminal 36, and 
the non-inverted input terminal of the comparator 46. Accordingly, a 
constant voltage obtained by the voltage division by the resistors 48 and 
50 is supplied to the input terminal of the comparator 46 as a reference 
voltage. The power supply terminal of the comparator 46 is connected to 
the power supply terminal 34 and the ground terminal of the comparator 46 
is connected to the ground terminal 36 so that power is supplied to the 
comparator 46 for operation. 
Diodes 42 and 44 are arranged for protecting the comparator 46 and are not 
limited to the arrangement shown in FIG. 1. For example, the diode 42 may 
be disposed between the terminal 30A and the resistor 38 such that its 
anode is connected to the terminal 30A and its cathode is connected to the 
resistor 38. Further, the diodes 42 and 44 may be disposed between the 
inverted input terminal and the non-inverted input terminal of the 
comparator 46 such that the orientations of the diodes 42, 44 are opposite 
to each other. 
Connected to the output terminal of the comparator 46 are one end of a 
resistor 52, one end of a resistor 54 and the anode of a diode 56. The 
other end of the resistor 52 is connected to the power supply terminal 34. 
Further, the other end of the resistor 54 is connected to the cathode of 
the diode 58. The anode of the diode 58 and the cathode of the diode 56 
are connected to a signal input terminal of a voltage controlled 
oscillator circuit 60. One end of a capacitor 62 is connected to a line 
connecting the anode of the diode 58 and the cathode of the diode 56 with 
the signal input terminal of the voltage controlled oscillator circuit 60, 
while the other end of the capacitor 62 is grounded. 
The power supply terminal of the voltage controlled oscillator circuit 60 
is connected to the power supply terminal 34 and the ground terminal 
thereof is connected to the ground terminal 36 so that power is supplied 
to the voltage controlled oscillator circuit 60 for operation. The voltage 
controlled oscillator circuit 60 outputs a signal whose frequency changes 
in accordance with the voltage level at the signal input terminal, namely, 
the voltage between both ends of the capacitor 62, such that the 
frequencies of the drive signals are lowered as the voltage level becomes 
higher. The output terminal of the voltage controlled oscillator circuit 
60 is branched into two lines, one of which is connected to the input 
terminal of an amplifier circuit 64 and the other of which is connected to 
the input terminal of an amplifier circuit 68 via a phase shifter 66. 
The phase shifter 66 shifts the phase of the input signal by 90 degrees and 
outputs the shifted signal. Accordingly, the amplifier circuits 64 and 68 
are supplied with signals whose frequencies and amplitudes are the same 
but whose phases differ by 90 degrees from each other. The output terminal 
of the amplifier circuit 64 is connected to one end of the piezoelectric 
body 14A through a terminal 30C while the output terminal of the amplifier 
circuit 68 is connected to one end of the piezoelectric body 14B through a 
terminal 30D. These piezoelectric bodies 14A and 14B form the 
piezoelectric body 14 of the ultrasonic motor 10. The other end of each of 
the piezoelectric bodies 14A and 14B is grounded through a terminal 30E. 
Operation of the present embodiment will now be explained. When the 
ultrasonic motor 10 is driven, the voltage controlled oscillator circuit 
60 of the drive circuit 30 outputs a signal whose frequency (indicated as 
"FREQUENCY AT BEGINNING OF DRIVE" in FIG. 3) is sufficiently higher than 
the drive frequency band (see FIG. 3). The signal output from the voltage 
controlled oscillator circuit 60 is divided into two, and the phase of one 
divided signal is shifted by 90 degrees by the phase shifter 66. These 
signals are amplified by the amplifier circuits 64 and 68, respectively, 
so that a sine-wave drive signal and a cosine-wave drive signal are 
generated and supplied to the piezoelectric bodies 14A and 14B of the 
ultrasonic motor 10. 
The drive signals are converted to mechanical vibration by the 
piezoelectric bodies 14A and 14B so that a traveling wave is excited in 
the elastic body 12 of the ultrasonic motor 10, whereby the drive shaft 16 
and the rotor 18 are rotated. Further, the vibrations of the elastic body 
12 are converted into an electric signal by the piezoelectric element 32, 
and the electric signal is supplied to the drive circuit 30. At this time, 
since the frequencies of the drive signals are sufficiently high, the 
amplitude of the vibrations of the elastic body 12 is small and the 
amplitude of the detection signal from the piezoelectric element 32 is 
also small, as shown in FIG. 5A. 
The detection signal is subjected to half-wave rectification by the diode 
42 to obtain a rectified signal as shown in FIG. 5B, and the signal is 
input to the inverted input terminal of the comparator 46. The negative 
components of the signal shown in FIG. 5B are produced due to the forward 
direction voltage drop of the diode 42. The level of the signal input to 
the inverted input terminal of the comparator 46 is compared with the 
voltage level of the reference voltage (the level illustrated by the 
broken line in FIG. 5B), which is obtained by the voltage division by the 
resistors 48 and 50 and which is supplied to the non-inverted input 
terminal of the comparator 46. 
As shown in FIG. 5B, since the amplitude of the detection signal is small, 
the level of the signal input to the inverted input terminal does not 
exceed the voltage level of the reference voltage, so the comparator 46 
always outputs a signal of high level (see FIG. 5C). While the signal 
output from the comparator 46 is at a high level, current flows to the 
capacitor 62 via the diode 56 so that the capacitor 62 is gradually 
charged. Accordingly, the voltage V between both ends of the capacitor 62 
gradually increases as shown in FIG. 5D so that the frequencies of the 
drive signals output from the voltage controlled oscillator circuit 60 are 
gradually lowered toward the drive frequency band. 
The amount of charge to the capacitor 62 per unit time depends on the 
electrical resistance of the resistor 52. In a case in which the 
electrical resistance of the resistor 52 is low, the speed of charging 
becomes faster (the inclination of the line shown in FIG. 5D becomes 
larger). On the other hand, in a case in which the electrical resistance 
of the resistor 52 is large, the speed of charging becomes slower (the 
inclination of the line shown in FIG. 5D becomes smaller). 
When the frequencies of the drive signals are gradually lowered as 
explained above, the amplitude of vibration of the elastic body 12 becomes 
larger so that the amplitude of the detection signal output from the 
piezoelectric element 32 also becomes larger, as shown in FIG. 6A. 
Accordingly, a period is periodically produced in which the level of the 
signal, which has been rectified by the diode 42 and input to the inverted 
input terminal of the comparator 46, exceeds the level of the reference 
voltage, as shown in FIG. 6B. Accordingly, the signal output from the 
comparator 46 is at a low level during periods in which the level of the 
rectified signal exceeds the level of the reference voltage, as shown in 
FIG. 6C. 
In each period in which the signal output from the comparator 46 is at a 
low level, the electric charge accumulated in the capacitor 62 is 
discharged via the diode 58 and the resistor 54 so that the voltage 
between both ends of the capacitor 62 is lowered (see FIG. 6D). Due to the 
discharge, the voltage V between both ends of the capacitor 62 is 
maintained at a constant level Vo, as illustrated by the imaginary line in 
FIG. 6D. The amount of discharge from the capacitor 62 per unit time 
depends on the electrical resistance of the resistor 54. In a case in 
which the electrical resistance of the resistor 54 is low, the speed of 
the discharge becomes faster (the inclination of the line shown in FIG. 6D 
during periods in which voltage decreases becomes larger). On the other 
hand, in a case in which the electrical resistance of the resistor 54 is 
large, the speed of the discharge becomes slower (the inclination of the 
line shown in FIG. 6D during periods in which voltage decreases becomes 
smaller). 
In the present embodiment, the level of the voltage supplied to the 
non-inverted input terminal of the comparator 46 is determined in advance 
such that, when the voltage V is at a constant level, the frequency of the 
signal output from the voltage controlled oscillator circuit 60 coincides 
with the optimum drive frequency in the drive frequency band (see FIG. 3) 
at which both the rotational speed and the efficiency of the ultrasonic 
motor 10 become maxima. Accordingly, the frequencies of the drive signals 
are gradually lowered from the frequency band sufficiently higher than the 
drive frequency band, as explained above, and the frequencies are 
maintained after the frequencies have reached the optimum drive frequency. 
On the other hand, in a case in which the temperature increases or in a 
case in which a load acts on the ultrasonic motor 10, the impedance 
characteristic of the ultrasonic motor 10 changes as shown in FIG. 4 so 
that the frequencies corresponding to the optimum drive frequency, the 
drive frequency band and the audible sound generating band (see FIG. 3) 
also change. When the frequencies of the drive signals become higher than 
the optimum drive frequency, the amplitude of the detection signal becomes 
smaller, as was explained with reference to FIG. 5, so that the 
frequencies of the drive signals are controlled by the drive circuit 30 so 
as to be lowered. 
In a case in which the frequencies of the drive signals become lower than 
the optimum drive frequency and enter the audible sound generating band, a 
large irregularity is produced in the amplitude and in the period of the 
detection signal, as shown in FIG. 7A. As a result, with regard to the 
signal input to the inverted input terminal of the comparator 46, periods 
in which the amplitude far exceeds the level of the reference voltage and 
periods in which the amplitude does not exceed the level are mixed, as 
shown in FIG. 7B. As a whole, the length of each period in which the 
signal exceeds the level of the reference voltage becomes longer compared 
to the case in which the ultrasonic motor 10 is driven at the optimum 
drive frequency (see FIG. 6B) because of the above-mentioned large 
amplitude. 
Accordingly, as illustrated in FIG. 7C, the length of each period in which 
the signal output from the comparator 46 is at a low level becomes longer 
so that the amount of discharge from the capacitor 62 becomes larger than 
the amount of charge thereto. As a result, the voltage V between both ends 
of the capacitor 62 is lowered as a whole, as illustrated by the imaginary 
line in FIG. 7D so that the frequencies of the drive signals are raised 
and return to the drive frequency band. As described above, the 
frequencies are controlled by the drive circuit 30 such that the 
frequencies return the drive frequency range when the frequencies of the 
drive signals deviate from the drive frequency band due to the effects of 
temperature and load. Accordingly, the ultrasonic motor 10 is controlled 
such that the drive frequencies pursue the optimum drive frequency without 
causing drawbacks such as continuous generation of audible sound from the 
ultrasonic motor 10. 
Next, an explanation will be given of a case in which in the ultrasonic 
motor 10, irregularity of the waveform of detection signal does not occur 
even when the frequencies of the drive signals enter the audible sound 
generating band. In this case, the amplitude of the signal output from the 
piezoelectric element 32 becomes larger, as shown in FIG. 8A, when the 
frequencies of the drive signals becomes lower than the drive frequency 
band and enter the audible sound generation band. However, the period and 
amplitude are constant and no irregularity occurs in the waveform. In such 
a case, similar to the previously-described case, the length of each 
period in which the level of the signal input to the inverted input 
terminal of the comparator 46 exceeds the level of the reference voltage 
becomes longer compared to the case in which the ultrasonic motor 10 is 
driven at the optimum drive frequency (see FIG. 6B) although periods in 
which the signal exceeds the level of the reference voltage cyclically 
emerge, as shown in FIG. 8B. 
Accordingly, the length of each period in which the signal output from the 
comparator 46 is at a low level becomes longer, as shown in FIG. 8C, so 
that the voltage V between both ends of the capacitor 62 is lowered as a 
whole, as illustrated by the imaginary line in FIG. 8D. As a result, the 
frequencies of the drive signals are raised so as to return to the drive 
frequency band. As described above, the drive circuit 30 according to the 
present embodiment is capable of driving ultrasonic motors, in which 
irregularity of the wave form of the detection signal does not occur in 
the audible sound generating band, without causing drawbacks such as 
generation of audible sound. 
The above-mentioned operation of the drive circuit for driving the 
ultrasonic motor 10 is illustrated in brief by the flowchart in FIG. 9. 
Namely, in step 100, the driving of the ultrasonic motor 10 is started by 
using drive signals of a frequencies sufficiently higher than the drive 
frequency band. In step 102, the level of the detection signal (or a 
signal after rectification) is compared with the level of the reference 
voltage by the comparator 46. When the level of the signal obtained by 
rectifying the detection signal exceeds the level of the reference 
voltage, the frequencies of the drive signals are raised in step 104 
during the period in which the level of the signal exceeds the level of 
the reference voltage. When the level of the signal obtained by rectifying 
the detection signal does not exceed the level of the reference voltage, 
the frequencies of the drive signals are lowered in step 106 during the 
period in which the level of the signal is below or equal to the level of 
the reference voltage. 
By repeating the processes in steps 102, 104 and 106, the ultrasonic motor 
is driven without causing drawbacks such as continuous generation of 
audible sound whether the ultrasonic motor is an ultrasonic motor in which 
irregularity of the waveform of the detection signal occurs in the audible 
sound generating band or is an ultrasonic motor in which irregularity of 
the waveform of the detection signal does not occur in the audible sound 
generating band. When the frequencies of the drive signals coincide with 
the optimum drive frequency, the ratio of the length of each period in 
which the detection signal exceeds the level of the reference voltage to 
the length of each period in which the detection signal does not exceed 
the level of the reference voltage becomes constant. Accordingly, the 
amount of change in the frequency in step 104 becomes equal to the amount 
of change in the frequency in step 106 so that the frequencies of the 
drive signals are maintained so as to coincide with the optimum drive 
frequency. 
In the above-mentioned embodiment, the frequencies of the drive signals are 
controlled by charging and discharging the capacitor 62 in accordance with 
the level of the signal output from the comparator 46. The present 
invention, however, is not limited to this arrangement. For example, the 
level of the detection signal may be monitored by a microprocessor or the 
like so as to calculate the amount of change in the frequencies of the 
drive signals based on the length of each period in which the detection 
signal exceeds the level of the reference voltage and the length of each 
period in which the detection signal does not exceed the level of the 
reference voltage within a give period of time.