Piezoelectric motor

A piezoelectric motor comprises includes a stator (1) and a rotor (3). The stator (1) has a housing (7), a piezoelectric oscillator (6) generating radial mode vibrations and mounted on the housing (7), said oscillator including a piezoelectric cell (9) with electrodes (13) and at least two pushers (10). Each pusher (10) has one end secured to at least one flat surface of the piezoelectric cell (9) so that a gap (14) is provided between the piezoelectric cell (9) and the pusher (10). The other end of each pusher (10) rests against the rotor (3).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to piezoelectric devices and, more 
particularly, to piezoelectric motors. 
2. Description of the Prior Art 
Known in the art is a piezoelectric motor (GB,A, 2118374B), comprising a 
stator, a rotor frictionally interacting with the stator having a housing, 
a piezoelectric oscillator producing radial mode vibrations mounted on the 
housing and comprising a piezoelectric cell disposed coaxially with the 
rotor and made in the form of a disk with electrodes, the disk being 
polarized perpendicular to the electrodes, and at least two pushers, each 
pusher having one end secured on the cylindrical surface of the 
piezoelectric cell and another end resting against the rotor. 
In this piezoelectric motor any change of the rotational speed can be 
effected only by changing the diameter of the piezoelectric cell. This 
does not allow one to make a general-purpose piezoelectric cell suitable 
for any rotational speed of the rotor of the piezoelectric cell suitable 
for any rotational speed of the rotor of the piezoelectric motor. 
Therefore, the range of rotational speeds of the rotor for a given 
piezoelectric cell is limited. 
Furthermore, the robustness of the pusher increases with the increase in 
the thickness of the piezoelectric cell and this does not allow one to 
reduce the input power, for example, when running the piezoelectric motor 
under a low load, e.g. for driving the diaphragm of a photographic camera. 
SUMMARY OF THE INVENTION 
The basic object of the invention is to provide a piezoelectric motor 
having a design which permits the axis of symmetry of each pusher to be 
displaced so that it becomes parallel to the axis of symmetry of the 
piezoelectric cell by converting the radial acoustic vibrations in the 
piezoelectric cell into transverse vibrations and then into longitudinal 
acoustic vibrations in the pushers of the piezoelectric radial mode 
oscillator. This widens the range of rotational speeds of the rotor for a 
given piezoelectric cell and the range of input power of the motor. 
This object is attained by providing a piezoelectric motor comprising a 
stator, a rotor frictionally interacting with the stator, having a 
housing, a piezoelectric oscillator producing radial mode vibrations 
mounted on the housing and comprising a piezoelectric cell disposed 
coaxially to the rotor and made in the form of a disk with electrodes, the 
disk being polarized perpendicular to the electrodes, and at least two 
pushers, each of which has one end secured on the piezoelectric cell and 
has another end resting against the rotor. According to the invention, one 
end of each pusher is secured on at least one flat surface of the 
piezoelectric cell so that a gap is provided between the piezoelectric 
cell and the pusher. 
This invention makes it possible to produce a number of piezoelectric 
motors featuring a wide range of input power. 
The present invention also makes it possible to produce piezoelectric 
motors with general-purpose piezoelectric cells featuring a wide range of 
rotational speeds of the rotor and a wide range of input power. The 
invention allows one to reduce the cost of a piezoelectric motor so that 
it can be used as an actuating drive instead of electric magnets, and also 
to reduce the labour consumption in manufacture of piezoelectric motors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The piezoelectric motor comprises a stator 1 (FIG. 1) having terminals 2 
for connection to an a.c. voltage source (not shown). The stator 1 is a 
stationary part of the piezoelectric motor relative to the unit (not 
shown) in which it is mounted. The rotor 3 of the piezoelectric motor is 
mounted on the stator 1 so it can rotate in a bearing 4 and is fixed 
thereto to avoid axial motion by a detachable joint 5, e.g. a lock washer. 
The stator 1 of a piezoelectric motor comprises a piezoelectric radial mode 
oscillator 6 acoustically insulated from the housing 7 of the stator 1 and 
secured thereon. The housing 7 is used for protection of the piezoelectric 
radial mode oscillator 6 against damage and for mounting of the 
piezoelectric motor in the unit by means of mounting holes 8. 
The piezoelectric radial mode oscillator 6 is an electromechanical device 
intended for conversion of electric energy into oscillatory motion of the 
particles of the solid body of the piezoelectric oscillator 6, 
substantially in the radial direction of a piezoelectric cell 9 made in 
the form of a body of revolution which serves as an active part of the 
oscillator 6. 
In contrast to the active part of the piezoelectric radial mode oscillator 
6, the passive part of the same does not convert one kind of energy into 
another. The passive part of the piezoelectric radial mode oscillator 6 
includes pushers 10 and an acoustically conductive body 11 connecting 
these pushers 10 to the piezoelectric cell 9. In the given embodiment of 
the piezoelectric radial mode oscillator 6, four pushers 10 are provided. 
To exclude any effort from the rotor 3 to the stator 1, the minimum 
quantity of pushers 10 must be two. When the number of pushers 10 is two 
or more (with uniform distribution of these pushers), the total force of 
each pusher 10 applied to rotor 3 is approximately zero. In this case, the 
more the number of pushers 10, the higher the torque of the rotor 3. It 
becomes necessary to step up the power supply voltage for the 
piezoelectric motor, and this limits the maximum number of pushers 10 to 
approximately thirty two. One end of each pusher 10 is fixed to at least 
one flat surface of the piezoelectric cell 9, while the other end rests 
against the rotor 3. 
Each pusher 10 is a flat spring made of an elastic sound conducting 
material, e.g. steel or SPR paper laminate. As a rule, each pusher 10 is 
rectangle-shaped. The pushers 10 may have another shape, for example, 
narrowing towards the free end or be provided with projections for fixing 
the piezoelectric radial mode oscillator 6 or for fixing the pushers 10 
themselves. In doing so, the parameters of the piezoelectric motor do not 
deteriorate. 
The pushers 10 are pressed to the rotor 3 due to the elastic properties of 
the pushers 10 themselves. For this purpose, the ends of the pushers 10 
are bent when installing the rotor 3 on the stator 1. However, the pushers 
10 can also be pressed to the rotor 3 by applying an axial force to the 
rotor 3, e.g. making use of the weight of the rotor 3 itself or by means 
of an electric magnet, a flat spring or a helical spring. (not shown in 
FIG. 1). 
The acoustically conducting body 11 connecting the pushers 10 to the 
piezoelectric cell 9 is solidified solder or a rigid mass of organic 
compound, for example, polymerized epoxy resin. 
To produce radial vibrations in the piezoelectric oscillator 6, the 
piezoelectric cell 9 is made in the form of a disk 12 with electrodes 13 
on the flat surfaces of the disk 12, to define major flat top and bottom 
surfaces 50 said electrodes being made in the form of a thin metallic 
coating obtained by evaporation of metal in vacuum, burning-in silver or 
chemical precipitation of metal in a solution. The voltage from the power 
supply source (not shown in FIG. 1) is applied to the electrodes 13 
through terminals 2 which are soldered to the electrodes 13 as shown in 
FIG. 1, or connected by the contact method. The part of the pushers 10, 
which is not coated by the binding conducting body 11, is installed on the 
flat surface of the piezoelectric cell 9 with a gap 14 between the major 
flat top surface 50 of the piezoelectric cell 9 and an associated pusher 
10. The gap 14 is adequate for the assembly of the piezoelectric motor so 
that the pusher 10 does not touch the electrode 13 of the piezoelectric 
cell 9. In other words, if during the assembly, for example, ten 
piezoelectric motors in one of them the pusher 10 touches the electrode 
13, the construction of this motor should be modified by making the gap 14 
twice as large. The disk 12 is made of a polycrystalline material 
featuring piezoelectric properties. The piezoelectric properties in the 
polycrystalline material are originated by providing remanent 
polarization. The process of producing remanent polarization is actually 
polarization of this material. The material for the piezoelectric cell 9 
of a piezoelectric motor is preferably based on a solid solution of 
titanate of lead zirconate. 
The electrodes 13 are required for polarization of the material. A 
polarizing voltage is applied to these electrodes. In this case the 
direction of polarization of the piezoelectric cell 9 coincides with the 
vector of the polarizing field intensity, i.e. perpendicular to the 
surface of the electrodes 13. In order to produce radial vibrations in the 
piezoelectric radial mode oscillator 6 at the working frequency of the 
piezoelectric motor, it is expendient that the maximum diameter D.sub.1 of 
the piezoelectric cell 9 or the difference of the diameters D.sub.1 -D, 
where D is the diameter of the hole of the disk 12, is equal to the half 
length of the wave of the radial acoustic vibrations. This is attained 
using the following formula: 
##EQU1## 
wherein N is the frequency constant of the material of the piezoelectric 
cell 9 depending on the velocity of sound in this material; 
f is the resonance frequency of radial vibrations. 
The rotor 3 of the piezoelectric motor consists of three parts. The first 
part is a frictional part 15 consisting of a ring with a cylindrical or 
conical surface. This part frictionally interacts with the pushers 10. The 
frictional part 15 must be made of a wear-resistant material with minimal 
roughness of the surface for reducing the wear of the pushers 10. 
Piezoelectric motors rated for a long service life are provided with a 
frictional part made of alumina-based powdered ceramic material or 
thermosetting plastics such as Getinaks (paperbased laminate). The 
frictional part 15 is rigidly secured on the second part of the rotor 3 of 
the piezoelectric motor, i.e. a cup 16. Mounted in the cup 16 is the third 
part of the rotor 3 of the piezoelectric motor, i.e. a shaft 17, which is 
capable of rotating in the bearing 4, axial motion being prevented by a 
detachable joint 5 squeezing the shaft 17. In piezoelectric motors whose 
life is not a decisive factor, all three parts of the rotor 3 or a 
combination of any two parts are made of the same material, e.g. of steel. 
The piezoelectric radial mode oscillator 6 is secured with respect to the 
housing 7 by means of slots 18 in the housing 7, the ends of the pushers 
10 resting in these slots. The use of other methods of fixing the 
piezoelectric radial mode oscillator 6, for example by means of an elastic 
adhesive does not affect the essence of the invention provided that the 
piezoelectric radial mode oscillator 6 is acoustically insulated from the 
housing 7. In the piezoelectric motor under consideration the acoustic 
insulation of the piezoelectric radial mode oscillator 6 from the housing 
7 is effected by means of a gasket 19, e.g. made of rubber. In order to 
increase the piezoelectric motor longevity, the pushers 10 are damped by 
means of dampers 20, which are made of rubber. The material of the dampers 
20 in a liquid state (prior to polymerization) is applied in the form of 
drops on the pushers 10 so that this material flows over the surface of 
the electrode 13 encompassing the pushers 10; then this material 
polymerizes. 
FIG. 2 shows the piezoelectric motor, taken along a section line II--II in 
FIG. 1. The angle .alpha. (FIG. 2) is formed by a tangent drawn from the 
surface of the pusher 10 through the point of contact with the frictional 
part 15 and by a tangent drawn to the surface of the frictional part 15 
through the point of contact with the pusher 10. 
The angle .alpha. must be less than 90.degree. and be in the range from 
45.degree. to 85.degree.. If the angle .alpha. is less than 45.degree., 
the efficiency of the piezoelectric motor is reduced. If the angle .alpha. 
exceeds 85.degree., the torque of the rotor 3 drops down and this also 
reduces the efficiency. The rotational speed .omega. and the torque M of 
the rotor 3 depend on the angle .alpha. in the following way: 
##EQU2## 
The angle .alpha., which is formed by the tangent drawn to the surface of 
the pusher 10 at the point of its fixing and the line of the plane 
extending through the fixed end of the pusher 10 and the axis of rotation 
of the rotor 3 has a small effect on the rotational frequency .omega. and 
the torque M of the rotor 3. 
FIGS. 3a, 3b, 3d, 3e, 3g show the piezoelectric cell carrying the pushers 
10 with different distances between the fixed end of the cell and the 
center of the disk 12. The shift of the middle line of the pusher 10 
relative to the middle line of the disk 12 by a value h exceeding the 
thickness of the disk 12 makes it possible to move the pusher 10 in the 
radial direction. In this case the pushers 10 can be fixed either on the 
edges of the disk 12, as shown in FIGS. 3a, 3b, 3c, 3d, or in the central 
portion of the disk 12, as shown in FIGS. 3b, 3e. Such fixing of the 
pushers 10 can be provided both for the piezoelectric radial mode 
oscillator 6 (FIGS. 3a, 3b, 3c) designed for low-speed piezoelectric 
motors with a rotor 3 encompassing pushers 10 and for the piezoelectric 
radial mode oscillator 6 (FIGS. 3d, 3e, 3g) designed for high-speed 
piezoelectric motors having a rotor 3 disposed between the pushers 10. By 
changing the point of fixing the pushers 10, as well as by changing the 
length of the pushers 10, one can change the diameter of the rotor 3 thus 
widening the range of rotational speeds of the rotor 3 for one type and 
size of the piezoelectric cell 9. If the pushers 10 considerably extend 
beyond the range of the piezoelectric cell, their damping is effected by 
means of a damper 20 made in the form of a rubber tube put on the end of 
the pusher 10, for example, as shown in FIG. 3 or 3g. 
FIG. 4 shows a piezoelectric motor, in which the pushers 10 are fixed on 
both flat surfaces of the piezoelectric cell 9. This is equivalent to 
connection of two piezoelectric radial mode oscillators 6 which are 
conditionally separated by a broken line. The fixing of the pushers 10 on 
both flat faces of the piezoelectric cell 9 is used for increasing the 
volume of the piezoelectric radial mode oscillator 6 so that the power on 
the shaft of the piezoelectric motor is increased. 
An embodiment of a low-speed piezoelectric motor used as an electric magnet 
is shown in FIG. 5. In this case the piezoelectric radial mode oscillator 
6 is fixed by means of slots 18 and a shaped gasket 19 with projections 21 
disposed between the pushers 10. The detachable part 22 of the housing 7, 
which also serves as a bearing, presses the projection 21 to the 
piezoelectric cell 9 by means of a nut 5 so that the angular position of 
the piezoelectric radial mode oscillator 6 is fixed. The cup 16 and the 
shaft 17 are made, like the components of the housing 7, of plastic 
material by injection moulding. This makes it possible to provide teeth 23 
of a gear wheel, an eccentric projection 24 for converting rotary motion 
into reciprocatingg motion and a side projection 25 on the cup 16. The 
side projection 25 interacts with a contact pair 26 forming therewith a 
position indicator to indicate the position of the rotor 3. 
An embodiment of a high-speed piezoelectric motor having a rotor 3 disposed 
between the pushers 10 is shown in FIG. 6. In this motor the housing 7 has 
a textolite plate 27 metallized at both sides and a cover 28 made of 
metal. The cover 28 is electrically connected to one of the electrodes 13 
of the piezoelectric cell 9 through a gasket 19 made of an elastic 
current-conducting material and pushers 10. The other electrode is pressed 
by the contact method to the metallized surface of the plate 27. The 
terminals 2 from the metallized surfaces 27 are used for applying an 
electric voltage to the piezoelectric motor, in which case the metallized 
surface is electrically connected to the cover 28 by means of soldering. 
FIG. 7 shows piezoelectric motor having a piezoelectric cell 9 in the form 
of a disk 12 having no hole. In this motor the cover 28 is connected 
through projections 29 to a plate 27 of the housing 7. The slots 18 made 
in the cover 28 are used for fixing the piezoelectric radial mode 
oscillator 6 in the radial direction, while in the axial direction the 
piezoelectric radial mode oscillator 6 is fixed by means of a gasket 19 
made in the form of a rubber ring. To prevent any displacement of the 
rotor 3 along its axis, the shaft 17 of the rotor 3 is provided with an 
annular groove, which accommodates the annular projection 30 of the cover 
28. 
The claimed piezoelectric motor operates as follows. The supply voltage 
from a power source having a frequency equal or close to one resonance 
frequency of the longitudinal radial vibrations of the piezoelectric 
radial mode oscillator 6 (FIG. 1) is applied to the electrodes 13 of the 
piezoelectric cell 9 exciting therein longitudinal radial vibrations. 
These vibrations are transferred to the pushers 10 through the 
acoustically conductive body 11. The pushers 10 are moved along the 
surface of the piezoelectric cell 9, therefore, the longitudinal radial 
vibrations are first converted into transverse acoustic vibrations at the 
point of fixing the pushers 10 to the piezoelectric cell 9. The transverse 
vibrations are reflected from the end of the pushers 10 and are again 
converted into longitudinal radial vibrations spreading in the pushers 10. 
These longitudinal radial vibrations are shifted with respect to the 
longitudinal radial vibrations excited in the piezoelectric cell 9 by a 
value h (FIG. 3a). In this case the converted portion of acoustic energy 
in the form of energy of transverse vibrations, but the larger portion of 
the energy is energy of longitudinal vibrations. By selecting the 
frequency of the a.c. voltage supply source close to the resonance 
frequency of the longitudinal vibrations, one can eliminate the effect of 
the transverse vibrations on the operation of the piezoelectric motor. 
Owing to the fact that the angle .alpha. is lower than 90.degree., a force 
originates in the zone of contact of the pushers 10 with the rotor 3 
tangentially to the rotor 3 causing its rotation in the bearing 4. This 
force is transmitted to the shaft 17 through the cup 16. If the 
piezoelectric motor is not fixed, the stator 1 and rotor 3 move in the 
opposite directions. In order to prevent rotation of the stator 1, the 
latter is secured to the unit through the holes 8. The forces, which can 
arise in the rotor 3, are balanced by the members fixing the shaft 17 in 
the direction of the axis of the rotor 3, in particular, by the detachable 
joint 5. The torque of the rotor 3 arising due to the tangential forces 
tends to turn the piezoelectric radial mode oscillator 6 in the housing 7. 
However this is prevented by the slots 18. In the zone of contact of the 
pushers 10 with the rotor 3, in addition to the tangential force, there 
arises an alternating force tending to excite subharmonic flexural 
vibrations in the pushers 10, i.e. vibrations with a frequency below the 
frequency of the power supply voltage of the piezoelectric motor. The 
damper 20 effectively attenuates the flexural vibrations, since the 
pliability of the pushers 10 to flexure is comparable to the elasticity of 
the damper 20. As a result, having no effect on the longitudinal 
vibrations in the pushers 10, the dampers 20 exclude self-excitation of 
subharmonic vibrations that would reduce the life of the piezoelectric 
motor and amplify audio noise. When the pusher 10 contacts the electrode 
13, a chopper contact of the pushers 10 with the piezoelectric cell 9 
takes place. The longitudinal vibrations in the pusher 10 become unstable 
and parasitic subharmonic vibrations appear, which drastically reduce the 
efficiency. For eliminating this phenomenon, the pushers 10 are mounted 
relative to the surface of the piezoelectric cell 9 with a gap 14. The 
value of the gap 14 is selected so that under the effect of impact and 
vibrational loads and temperature effects there is no mechanical contact 
between the pushers 10 and the surface of the piezoelectric cell 9. The 
gap 14 is usually taken in the range of 0.2 to 0.3 mm and is increased 
with an increase in the size of the piezoelectric radial mode oscillator 
6. If the gap 14 is lower than 0.2 mm a mechanical contact occurs between 
the pushers 10 and the surface of the piezoelectric cell 9 under the 
effect of external vibrational loads on the piezoelectric motor. This 
results in higher acoustic noise. When the gap 14 exceeds 0.3 mm, the 
piezoelectric motor efficiency drops. 
The mechanism of the excitation of longitudinal vibrations in the pushers 
10 is not changed if the place of fixing the pushers 10 is displaced along 
the radius from one end of the piezoelectric cell 9 to the other end. In 
this case, a turn of the pusher 10 through 180.degree. relative to the 
fixing point changes the phase of the longitudinal vibrations. However, 
this has no effect, for example, on the efficiency of the piezoelectric 
motor. The operation of the piezoelectric motor does not change if the 
pushers 10 are located at both sides of the piezoelectric cell 9 (FIG. 4). 
The location of the pushers 10 on the two flat surfaces of the 
piezoelectric cell 9 makes it possible to double the volume of the 
piezoelectric cell 9, thus increasing the power of the piezoelectric 
motor, and to widen the range of the input power. 
The piezoelectric motor which has an external rotor 3 (FIG. 5) encompassing 
the pushers 10 develops high torques, which are converted into 
longitudinal forces by means of an eccentric projection 24. Such motors 
are momentum piezoelectric motors. They are equipped with limit switches 
in the form of a side projection 25 interacting with a contact pair 26. 
These motors are used for replacement of electric magnets. When provided 
with teeth 23, these piezoelectric motors can also be used as low-speed 
rotary drives. 
High-speed piezoelectric motors have found applications in tape transport 
mechanisms. In these motors the current conducting wires are led through a 
cover 28 (FIG. 6) and the gasket 19, and the power supply can also be 
provided due to direct contact of the electrode 13 with the metal coating 
of the plate 27. 
A piezoelectric motor with a piezoelectric cell 9 (FIG. 7) in the form of a 
disk 12 having no hole may find application in toys. Its design simplicity 
is obtained due to the shape of the cover having projections 29, which 
provide quick assembly of the stator 1 and the rotor 3, as well as due to 
the annular groove and the annular projection 30 fixing the rotor 3 in the 
axial direction. 
Thus, the present invention makes it possible to widen the range of 
rotational speeds of the rotor 3 for a given piezoelectric cell 9 and to 
widen the range of input powers of the piezoelectric motor. 
Since the piezoelectric motor is provided with an external rotor 3 (FIG. 5) 
and an enveloping pusher 10, substantial rotational moments can be 
achieved and converted into longitudinal thrust by the eccentric 
projection 24. Such motors are torque piezoelectric motors. They are 
equipped with end switches made as the lateral projection 25 interacting 
with the contact pair 26. These motors are used to replace electric 
magnets. When provided with teeth 23, these piezoelectric motors can be 
operated as low-speed rotational drives. 
Tape transport devices use high-speed piezoelectric motors. In these 
motors, current passes through the cover 28 (FIG. 6) and the gasket 19 
and, also, by direct contact of the electrode 13 with the metal coating of 
the plate 27. 
The piezoelectric motor equipped with the piezoelectric element 9 (FIG. 7) 
made as a disk 12 without the opening may find application in childrens 
toys. The construction is very simple featuring a cover 28 with 
projections 29. The stator 1 and rotor 3 can be quickly assembled due to 
the annular groove and annular projection 30 securing the rotor 3 axially. 
Thus, this invention can help expand the range of speed of the rotor 3 for 
a particular piezoelectric element 9 and provide a wider range of power 
consumption of the motor. 
The invention can be used as a nongeared electric motor performing 
continuous or discontinuous (step-by-step) rotary motion. These motors are 
used in actuators of automatic systems instead of electric magnets, in 
drives of tape transport mechanisms of tape recorders and in drives of 
movie equipment.