Vibration driven motor or actuator

A vibration driven motor or actuator has a piezoelectric element, arranged in a vibrating member, for generating a vibration in a first direction, and another piezoelectric element, arranged in the vibrating member, for generating a vibration in a second direction different from the first direction. In the motor or actuator, the vibrating member has a recess portion which shaved e.g., by a laser, or a portion added with a mass, so that the natural frequencies of a given vibration mode of the vibrations in the first and second directions coincide or substantially coincide with each other.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a normal mode adjustment position 
detection method and adjustment method for a bar-shaped or annular 
vibration driven motor or actuator. 
2. Related Background Art 
FIG. 7 is an exploded perspective view of a vibrating member of a 
bar-shaped vibration driven motor, and FIG. 8 is a longitudinal sectional 
view of the bar-shaped vibration driven motor. 
In a conventional vibrating member, a driving A-phase piezoelectric element 
al including a group of two piezoelectric element plates PZT1 and PZT2, a 
driving B-phase piezoelectric element a2 including a group of two 
piezoelectric element plates PZT3 and PZT4, and a sensor piezoelectric 
element s1 including a piezoelectric element plate are stacked, as shown 
in FIG. 7. Electrode plates A1 and A2 for supplying power to the 
piezoelectric elements, and a sensor signal output electrode plate S are 
sandwiched between respective adjacent piezoelectric elements. In 
addition, GND electrode plates G1, G2, and G3 are arranged for giving a 
GND potential. Metal blocks b1 and b2 formed of, e.g., brass or stainless 
steel, which causes relatively small vibration attenuation, are arranged 
to clamp these piezoelectric element plates and electrode plates. The 
metal blocks b1 and b2 are fastened by a fastening bolt c to obtain an 
integrated structure, thereby applying a compression stress to the 
piezoelectric element plates. In this vibrating member, since an 
insulating sheet d is inserted between the bolt c and the metal block b2, 
only one sensor piezoelectric element s1 need be used. 
The A- and B-phase piezoelectric elements a1 and a2 have an angular 
displacement of 90.degree. therebetween. These piezoelectric elements a1 
and a2 respectively excite bending vibrations in directions within two 
orthogonal planes including the axis of the vibrating member, and have a 
proper temporal phase difference therebetween. Thus, surface portions of 
the vibrating member are caused to form a circular or elliptic motion, 
thereby frictionally driving a moving member pressed against the upper 
portion of the vibrating member. 
FIG. 8 shows an example wherein such a vibrating member is used in a 
bar-shaped vibration driven motor. In this example, the fastening bolt c 
of the vibrating member has a small-diameter column portion c2 at its 
distal end portion. A fixing member g fixed to the distal end portion of 
the column portion c2 can fix the motor itself, and can also rotatably 
support, e.g., a rotor r. The rotor r contacts the front end face of the 
front metal block b1, and a pressure is given by pressing a coil spring h 
in a spring case i inserted in the rotor r through a bearing member e and 
a gear f. 
In order to obtain high efficiency, both a bar-shaped vibration driven 
motors and annular vibration driven motor are designed, so that the 
natural frequencies of normal modes of two phases excited in the vibrating 
member coincide with each other. 
However, in practice, each of these natural frequencies is shifted relative 
to the other due to variations in the material of the metal blocks 
constituting the vibrating member, pressure variations in the portions for 
clamping the PZT elements, and the like. Thus, when the two phases are 
driven at the same frequency, the amplitudes generated by the two phases 
have a difference therebetween, and a circular motion formed at the mass 
point of the vibrating member is distorted, resulting in a decrease in 
motor efficiency. 
SUMMARY OF THE INVENTION 
The present invention has been made to solve these conventional problems, 
and has as its object to provide an adjustment position detection method 
and an adjustment method for decreasing any difference between the normal 
frequencies of the two natural modes excited in a vibrating member as much 
as possible. 
It is another object of the present invention to provide a vibration driven 
motor or actuator, which has a structure for causing the natural 
frequencies of normal modes of two phases excited in a vibrating member to 
coincide with each other. 
Other objects of the present invention will become apparent from the 
following detailed description of the present invention. 
One aspect of the present invention is characterized in that a recess 
portion is formed in a predetermined portion of a vibrating member e.g., 
by a laser, or by adding thereto a member having a predetermined mass so 
as to cause the natural frequencies of normal modes of at least two phases 
excited in the vibrating member to coincide with each other. 
In order to achieve the above objects of the present invention, a normal 
mode adjustment position detection method for a vibration driven motor is 
characterized in that AC voltages are applied to two, i.e., A- and B-phase 
driving electro-mechanical energy conversion elements in a vibrating 
member of a vibration driven motor, and an adjustment position is detected 
on the basis of the magnitudes of the voltages and currents at a given 
frequency and phase differences therebetween. 
Another aspect of the present invention is characterized in that a current 
value is measured while changing the magnitudes of AC voltages to be 
applied to two, i.e., A- and B-phase driving electro-mechanical energy 
conversion elements in a vibrating member of a vibration driven motor, and 
phase differences therebetween, and a position where the measured current 
value has a peak value is determined as a normal mode adjustment position. 
Still another aspect of the present invention is characterized in that the 
detected adjustment position is shaved e.g., by a laser, so that a phase 
having a higher normal frequency of a natural mode than the other phase is 
adjusted to lower the natural frequency, or vice versa.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram showing a circuit arrangement of a detection 
apparatus according an embodiment of the present invention, which 
apparatus can effectively practice a method of the present invention. 
The detection apparatus comprises an arithmetic microcomputer (.mu.-com) 1 
for controlling the entire apparatus, an oscillator 2, driving amplifiers 
3 and 4 (for A and B phases), a voltage detector 5 for an A-phase 
piezoelectric element a1, a voltage detector 6 for a B-phase piezoelectric 
element a2, a current detector 7 for the A-phase piezoelectric element a1, 
a current detector 8 for the B-phase piezoelectric element a2, a phase 
difference detector 9 for detecting a phase difference between an A-phase 
voltage and an A-phase current, a phase difference detector 10 for 
detecting a phase difference between a B-phase voltage and a B-phase 
current, and a phase difference detector 11 for detecting a phase 
difference between an A-phase current and a B-phase voltage. 
The arithmetic microcomputer 1 supplies a signal for sweeping the output 
frequency of the oscillator 2 within a predetermined frequency range to 
the oscillator 2. The output from the oscillator 2 is applied via the 
amplifiers 3 and 4 to the piezoelectric elements a1 and a2, which are the 
same as the A- and B-phase driving piezoelectric elements a1 and a2 shown 
in FIGS. 7 and 8. At this time, if the natural frequency of a given normal 
mode is present within the sweep frequency range, the admittance 
(current/voltage) is maximized at the natural frequency. The present 
invention detects the natural frequency by utilizing these 
characteristics. 
When the frequency sweep (scan) operation and the measurement of the 
voltages and currents are performed for both the A and B phases, the 
natural frequencies in two directions can be obtained. FIGS. 2A, 2B, and 
2C show the admittance characteristics obtained in this process. 
FIG. 2A is a graph showing the characteristics of a vibrating member (see 
FIG. 7), which has a high rigidity in, e.g., the direction for vibrating 
the piezoelectric element of A-phase (X-direction), and a low rigidity in 
the direction for vibrating the piezoelectric element of B-phase 
(Y-direction), different by 90.degree. from the X-direction. In FIG. 2A, a 
solid curve represents the characteristics between the frequency of a 
voltage applied to the A-phase piezoelectric element a1, and the 
admittance at each frequency. Also, a dotted curve represents the 
characteristics between the frequency of a voltage applied to the B-phase 
piezoelectric element a2, and the admittance at each frequency. 
FIG. 2B is a graph showing the admittance characteristics of a vibrating 
member different from that of FIG. 2A, i.e., a vibrating member which has 
a low rigidity in the X-direction and a high rigidity in the Y-direction. 
In FIG. 2B, a solid curve represents the admittance characteristics of the 
A phase upon application of an AC voltage to the A phase, and a dotted 
curve represents the admittance characteristics of the B phase upon 
application of an AC voltage to the B phase. 
FIG. 2C is a graph showing the characteristics of a vibrating member in 
which the directions of high and low rigidities of the member are 
different from those of the vibrating members shown in FIGS. 2A and 2B. In 
FIG. 2C, solid and dotted curves are the same as those in the cases shown 
in FIGS. 2A and 2B. 
A method of obtaining directions of high and low rigidities of the 
vibrating member on the basis of the admittances and phases will be 
described below. 
Assume that the angle of a shift from a vibration application direction APD 
(see FIG. 7) of the A-phase piezoelectric element a1 is represented by 
.theta. (see FIG. 7). 
A direction in which the rigidities of the components a1, a2, b1, b2, c, 
and d shown in FIG. 7 are originally high will be referred to as an X mode 
hereinafter, and a direction in which the rigidities of these components 
are low will be referred to as a Y mode hereinafter. 
Also, we have the following definitions: 
Axa=force factor when A-phase piezoelectric element a1 drives X mode of 
vibrating member 
Aya=force factor when A-phase piezoelectric element a1 drives Y mode of 
vibrating member 
Axb=force factor when B-phase piezoelectric element a2 drives X mode of 
vibrating member 
Ayb=force factor when B-phase piezoelectric element a2 drives Y mode of 
vibrating member 
Zmx=mechanical impedance of vibration in X mode of vibrating member 
An admittance Yaa of the A-phase piezoelectric element a1 upon driving of 
A-phase piezoelectric element a1 is given by: 
##EQU1## 
An admittance Ybb of the B-phase piezoelectric element a2 upon driving of 
the B-phase piezoelectric element a2 is given by: 
##EQU2## 
A relationship Yab between the current of the A-phase piezoelectric element 
a1 and the voltage of the B-phase piezoelectric element a2 upon driving of 
the B-phase piezoelectric element a2 is given by: 
##EQU3## 
At this time, it is assumed that the assembling errors of the piezoelectric 
elements a1 and a2 are negligible. 
Also, we have: 
##EQU4## 
Furthermore, we have: 
Yba=Yab 
where Yba is the relationship between the current of the B-phase 
piezoelectric element a2 and the voltage of the A-phase piezoelectric 
element a1 upon driving of the A-phase piezoelectric element a1, and is 
given by: 
##EQU5## 
When Zmx, Zmy, and A are eliminated from formulas (4) and (5), we have: 
##EQU6## 
These formulas are set in the arithmetic microcomputers 1, and the 
microcomputer 1 (FIG. 1) calculates the above-mentioned Yaa, Ybb, and Yab 
on the basis of the input data from the detectors 5 to 11, and obtains the 
above-mentioned .theta., i.e., one of the two directions of rigidities. 
Also, the microcomputer 1 obtains the direction of a high or low rigidity 
according to the admittance characteristics shown in FIGS. 2A to 2E. 
When a rigidity in a direction R1 (see FIG. 7) is high, the admittance 
characteristics are as shown in FIG. 2D; when a rigidity in a direction R2 
(see FIG. 7) is high, the admittance characteristics of the A- and B-phase 
piezoelectric elements a1 and a2 are respectively as shown in FIG. 2E. 
When .theta.=0, and the rigidity in the direction R1 is high, the 
characteristics shown in FIG. 2A are obtained; when the rigidity in the 
direction R2 is high, the characteristics shown in FIG. 2B are obtained. 
Thus, the admittance characteristics (FIGS. 2A to 2E) of the elements a1 
and a2 are obtained, and it is confirmed if the direction of the high 
rigidity of the vibrating element is the direction R1 or R2. 
Then, a recess portion is formed on a surface portion of the vibrating 
member, which portion is located on or substantially on the direction of 
the high rigidity, and suffers from a large distortion caused by a 
vibration so as to have a proper depth (this depth corresponds to a 
difference between frequencies f1 and f2 or between frequencies f11 and 
f12 in FIGS. 2A and 2B, i.e., a frequency difference (f1-f2) or 
(f11-f12)), thereby decreasing the rigidity of the vibrating member in the 
above-mentioned direction. 
Alternatively, a mass corresponding to the mass of the above-mentioned 
recess portion is decreased from a surface portion of the vibrating 
member, which portion is located along a direction different by 90.degree. 
from the above-mentioned direction, and suffers from a small distortion 
caused by a vibration. 
FIG. 3 is a block diagram showing a circuit arrangement of a detection 
apparatus according to another embodiment of the present invention. 
In the detection apparatus shown in FIG. 1, .theta. is calculated using the 
phase difference detectors 9, 10, and 11. However, in this embodiment, the 
outputs from amplifiers 3 and 4 can be varied by gain controllers 12, and 
13, thereby obtaining .theta., and determining the above-mentioned recess 
portion formation position or mass decreasing position. 
More specifically, a vibration in only the vibration application direction 
of the A-phase piezoelectric element a1, and a vibration in only the 
vibration application direction of the B-phase piezoelectric element a2 
can be excited in the vibrating member under the control of the gain 
controllers 12 and 13. The vibration application directions of the A- and 
B-phase piezoelectric elements a1 and a2 are formed perpendicularly to 
each other since the elements a1 and a2 are arranged perpendicularly to 
each other. If voltages having the same magnitude are applied to the A- 
and B-phase piezoelectric elements a1 and a2, the direction of the 
synthesized vibration corresponds to a direction shifted by -45.degree. or 
45.degree. from the vibration application direction of the A- or B-phase 
piezoelectric element a1 or a2. By utilizing this phenomenon, the 
above-mentioned .theta. is obtained. 
The frequency of an AC voltage to be applied to the A- or B-phase 
piezoelectric element a1 or a2 is sequentially changed to obtain the 
admittance characteristics shown in FIGS. 2A to 2C, thereby obtaining a 
frequency f1 or a frequency near the frequency f1, or a frequency f2 or a 
frequency near the frequency f2. Then, AC voltages having the frequency f1 
or f2 are applied to the A- and B-phase piezoelectric elements a1 and a2. 
At this time, the amplitudes of the AC voltage to be applied to the A- and 
B-phase piezoelectric elements a1 and a2 are sequentially changed to 
satisfy .vertline.Va.vertline.+.vertline.Vb.vertline.=constant (where 
.vertline.Va.vertline. is the absolute value of the voltage to be applied 
to a1; .vertline.Vb.vertline. is the absolute value of the voltage to be 
applied to a2 ), and a current Ia flowing through the piezoelectric 
element a1 at that time is detected by a detector 7. In FIG. 3, the same 
reference numerals denote elements having the same functions as those 
shown in FIG. 1. 
Then, the maximum value of the current Ia (a current Ib flowing through the 
piezoelectric element a2 may be used; ideally, Ia+Ib is preferable) is 
obtained. 
A change in current Ia is plotted on the coordinate system shown in FIG. 4. 
In FIG. 4, an angle .theta. is plotted along the abscissa. The angle 
.theta. is defined as follows. That is, the magnitude of the voltage to be 
applied to the A-phase piezoelectric element a1 is plotted along the 
Y-axis, and the magnitude of the voltage to be applied to the B-phase 
piezoelectric element a2 is plotted along the X-axis. The angle .theta. is 
defined between a direction of a synthesized vector of these two voltages 
and the Y-axis. The magnitude of the current Ia is plotted along the 
ordinate. 
The angle .theta. coincides with a synthesized vibration application 
direction generated in the vibrating member components a1, a2, b1, b2, c, 
and d (see FIG. 7) upon application of vibrations by the piezoelectric 
elements a1 and a2. 
The angle .theta. corresponding to the maximum value or a value near the 
maximum value of the current Ia is obtained from FIG. 4 (of course, the 
angle .theta. need not be obtained by actually drawing the graph of FIG. 4 
on paper, but may be obtained by utilizing a computer). 
Since the angle .theta. represents the same content as that of the angle 
.theta. described in the first embodiment (i.e., the angle .theta. 
represents an angle formed between the direction of a high or low rigidity 
and the direction APD shown in FIG. 7), a recess portion or a hollow 
portion may be formed in a surface portion of the vibrating member, which 
portion is located along a direction coinciding or substantially 
coinciding with the direction R1 or R2 (see FIG. 7) based on the obtained 
.theta.. Alternatively, when the direction of the low rigidity is 
obtained, a mass may be added to the corresponding portion. 
In this manner, the rigidities in the two directions can be caused to 
coincide with each other. 
The direction of the high rigidity is determined by measuring the 
admittance characteristics of the piezoelectric elements shown in FIGS. 2A 
to 2C like in the first embodiment. 
Note that the rigidities in the two directions need not be caused to 
perfectly coincide with each other. In other words, the natural frequency 
of a given mode of the vibrating member upon excitation of the 
piezoelectric element a1 need not perfectly coincide with the natural 
frequency of the mode of the vibrating member upon excitation of the 
element a2. That is, no practical problem is posed if the two natural 
frequencies have a difference of about 200 Hz therebetween. FIG. 5 is a 
sectional view of the main part of an embodiment for forming the 
above-mentioned recess portion on the surface of the vibrating member 
using a laser. 
In FIG. 5, a laser 15 radiates a laser beam onto a constricted portion b11 
of the metal block b1. The vibrating member held by a vibrating member 
holder 16 is rotated, so that a surface portion of the vibrating member, 
which portion coincides or substantially coincides with the direction 
obtained according to the first or second embodiment, is irradiated with 
the laser beam. The correction amount of the natural frequency is adjusted 
by controlling the radiation time or a scan width of the laser beam. 
In the above embodiment, a laser is used. Alternatively, the surface 
portion may be shaved using a file, a drill, a grindstone, or the like. 
The present invention is not limited to a bar-shaped vibration driven 
motor, but may be applied to an annular vibration driven motor. 
FIG. 6 shows a driving apparatus using a bar-shaped vibration driven motor 
worked by the method of the present invention. The apparatus includes a 
slit plate 56, a photocoupler 57, and a shaft 54 for coupling the motor 
and the apparatus 55 (in this case, a photographing lens driving unit for 
a camera). The output from the bar-shaped vibration driven motor is 
transmitted to the apparatus 55 through the shaft 54. 
As described above, according to the present invention, two natural 
frequencies can be adjusted to have desired values, and motor performance 
can be improved. Since the normal mode of the vibrating member can be 
effectively adjusted in an assembled state, a motor or actuator can be 
manufactured at low cost.