JP2006262635A - Ultrasonic motor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce weight and cost, by changing the material of stator from a metal to ceramics and the material of a rotor to ceramics or to plastics. <P>SOLUTION: To improve the characteristics of an ultrasonic motor, the weight of the stator 2 is reduced to half, by changing the material of the stator to a metal or ceramics. The non-metal ultrasonic motor is obtained, by changing the material of the rotor 1, to ceramics or plastics to achieve further weight reduction, and by changing the material of stator 2 to ceramics, operation under high torque is further stabilized. Furthermore, for utilization of the characteristics of lightness in weight and high-torque function, a sensor introduced inside the motor to facilitate the control of the motor. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to an ultrasonic motor, and in particular, by improving the structure of the ultrasonic motor, an ultrasonic motor with stable operation and high efficiency is realized. Furthermore, by selecting an ultrasonic motor structural material, it is possible to reduce the weight and size, and realize an ultrasonic motor with high output and high controllability.

Conventionally, in order to obtain mechanical force, electromagnetic force, use of fluid (hydraulic pressure, air pressure), those using an internal combustion engine such as an engine, hydraulic wind power, and the like have been common methods. If it is limited to the area of a small motor, it is limited to electricity, fluid and spring force.
The ultrasonic motor extracts a part of the energy of the vibrating body as a mechanical force via a frictional force, and is a new method for extracting a mechanical output different from any of the above methods. Despite the fact that 20 years have passed since the invention of the ultrasonic motor, it is a motor with many features that are different from other motors, but these features are not fully utilized, so they are limited. It has become a range application. Even though it is not as wide as expected, the torque is large and the holding torque during non-operation is large, so that application of a precision motion mechanism to a servomotor is particularly useful at this time. The large torque is very advantageous when applied to small equipment. Generally, when a high torque is obtained with a small device, the output is inevitably gained by the rotation speed of the motor and the torque is obtained by a gear. At that time, vibration and rotation sound are generated. An ultrasonic motor does not have any problem of vibration or motor sound. As another application, in recent years, introduction to robots has become popular as an application utilizing the features of light weight and large torque.

Listed below are issues that must be addressed in ultrasonic motors.
The small size and high torque that are listed as one of the features of ultrasonic motors are the same at the research stage, and the features have been utilized at the practical stage so far. However, it has also been found that the application is limited by high torque alone. The following items need to be improved when considering future development. (1) Since the examination of the mechanism design of the ultrasonic motor is delayed in practical use, its features are not fully utilized. (2) Life is concerned because of the contact operation, and (3) Cost is high. The application range is limited due to the above three problems, and there is an application for driving a camera lens as an application example that is exceptionally used in large quantities. Compared to the significant development of conventional electromagnetic motors, ultrasonic motors are not used as much as the initial goals.
There are still problems with controllability that is one of the features of ultrasonic motors. Ultrasonic motors that have been put to practical use do not have a control sensor, so when applied as a servomotor, a new sensor must be added. When viewed from the application side, it is necessary to attach a sensor to detect the angle, and to develop software for exchanging between the ultrasonic motor and the sensor. This is not particularly limited to ultrasonic motors, but ultrasonic motors have a poor track record, so the burden on the application side is large, and extra work is required to use them beyond stepping motors, servo motors, etc. appear. This is the reason why the conventional servo motor using electromagnetic force cannot be caught up. There is a lack of efforts to provide the features of ultrasonic motors and to respond to the demands of the application side. In the end, it is only used in a very limited range of applications that require an ultrasonic motor.
However, among them, a lens driving motor is an application example that takes advantage of the advantages of shape and weight reduction. This is a fortunate combination because the development of the motor and system has progressed with a clear application intention from the beginning of the invention.

The next issue is cost. In recent decades, the number of small motors shipped has increased by orders of magnitude. Prices are also decreasing in inverse proportion. From the viewpoint of motor application, the cost is important, but there is a margin to absorb the motor cost at the device design stage, and the cost can be reduced even at the motor manufacturing side. It was. On the other hand, ultrasonic motors could not keep up with the flow, so the price could not be reduced. Even in the present situation, from the standpoint of system design, I would like to use it due to its characteristics, but there are many cases where I do not use it in terms of price.
In addition, in comparison with the electromagnetic force motor, the handicap inherent in the ultrasonic motor requires an oscillation circuit for obtaining vibration as drive energy. Since other motors do not require such a circuit, it is necessary to make an effort not to reflect this part in the price of the entire motor as much as possible. An oscillation circuit as low as possible is required. If a motor can be manufactured that has a moderate balance in terms of performance and price compared to conventional motors, more development is possible. One of the objects of the present invention is to show how a low-cost ultrasonic motor can be realized, which will be described in detail below.

FIG. 1 shows a basic configuration of a ring type ultrasonic motor. The structure of the ultrasonic motor is composed of a stator 2 that is a vibrating body of ultrasonic vibration and serves as a driving source, and a portion of the rotor 1 that is in contact with the stator and receives a rotational force from the stator via a frictional force.
In the stator, a resonance body for causing mechanical resonance of the vibration body and a piezoelectric body for vibration excitation shown in FIG. 3 are attached to the base (opposite the contact surface with the rotor). In order to obtain the rotational force of the rotor from the vibration energy, the contact surface with the rotor generates a circular motion by generating two sets of vibrations of 90 degrees different in phase at the same frequency in the stator, and the vibration of the circular motion is generated. At the apex, a rotational force is applied to the rotor portion in contact therewith via a frictional force.
FIG. 3 shows an electrode structure of a piezoelectric body (for generating circular motion) for generating two sets of vibrations having a phase difference of 90 degrees on a stator with one piezoelectric body. Two sets of vibration generating electrodes are arranged on the piezoelectric body at 90 degrees of phase difference (named A phase and B phase, the left side 31 is A phase and A1 +, A1-, A2 +, A2-, A3 + Furthermore, the right side 32 is provided with vibration excitation elements B1 +, B1-, B2 +, B2-, B3 +, B3- in the B phase). The resonance of the stator that becomes the source and the mode as a whole is obtained. There are two main parts that constitute the basic structure of an ultrasonic motor: a stator and a rotor. In addition, a pressurizing mechanism for obtaining a frictional force, a rotating shaft for extracting output, and a supporting part for the motor are added.
Although FIG. 1 shows the basic structure and principle of a ring type ultrasonic motor, FIG. 2 shows the structure of a disk type ultrasonic motor. The disk type ultrasonic motor uses a flexural vibration of a disk, and the vibration mode of the disk of FIG. 1 is different from that of the ultrasonic motor. Reference numeral 21 denotes a disk resonator which serves as a motor stator. Like the ring type stator, a vibration expansion portion is provided to obtain a driving force at the apex. 22 is a rotor, and 23 is a piezoelectric body bonded to the back surface. Reference numeral 24 denotes a lining material for obtaining a frictional force. FIG. 4 shows a piezoelectric body for a disk type ultrasonic motor. The piezoelectric body shown in FIG. 4 is used by attaching two sheets of which the phases are different from each other by 90 degrees on the stator. FIG. 5 shows a cross-sectional view of a disk-type ultrasonic motor.
In both the ring-type ultrasonic motor of FIG. 1 and the disk-type ultrasonic motor of FIG. 2, the distortion when the electrical input is simply applied to the resonator as the stator is very small and the displacement is small, and the mechanical output is taken out. I can't do that. In order to take out the mechanical output, mechanical resonance is generated in the stator, and the magnitude of the surface displacement is increased by the amount of resonance. The vibration amplitude of the resonator, that is, the magnitude of the circular motion is still small to take out as mechanical output even if it is enlarged by resonance. The greater the displacement amplitude is, the easier it is to extract the machine output. Several ideas have been made to increase the vibration amplitude. One of them is to increase the Q of the resonator.
In general, a resonator is evaluated by a value called Q (a ratio between an amount of kinetic energy stored as vibration energy and input electric energy). The stator as a resonator generates an amplitude Q times the displacement amount (very small amount) generated by bending of the stator due to distortion generated by the piezoelectric body when a constant direct current is applied. The stator Q is designed to be as large as possible. Q varies depending on the structure of the stator, the stator material to be used, the selection of the piezoelectric body, the assembly technique, and the like.
What has been described above holds true when the amplitude is small when the voltage is small (when the power to be extracted by the motor is small). On the other hand, since an ultrasonic motor is as small as possible and requires a high output, it is desirable to input as much electrical input as possible into a stator that is a vibrator having a fixed shape and extract as much mechanical output as possible. In that case, the condition of the small amplitude described above, that is, the condition that the mechanical vibration of Q times with respect to the electric input is generated is not satisfied.
In the case of large amplitude, distortion is generated in each of the piezoelectric material, adhesive, and stator material that constitutes the stator, but in the weakest material, Hook's law, which is the fundamental law of vibration, that is, force and strain Is not established. Not all of the increased electrical energy becomes mechanical energy that can be extracted to the outside, and most of the increased energy becomes thermal energy through internal loss, leading to a decrease in efficiency or motor breakdown.
On the other hand, even if the stator has the same Q value, the mechanical output can be easily taken out by devising the shape structure of the stator. The inventors have demonstrated that the efficiency of an ultrasonic motor can be increased to a practical range by adopting a so-called comb-tooth structure for a mechanical output for mechanical output proposed by the present inventors. Published Patent Publication Sho 61-191278, and IEEE ULTRASONICS SYMPOSIUM PROCEEDIBGS 1987, P747-P756
When the comb-tooth structure is not used, the electromechanical conversion efficiency is about several percent, but the conversion efficiency can be increased to about 60% under the introduction of the comb-tooth structure and the optimum input condition. This is because a sufficient mechanical amplitude can be extracted within the linear region (a range in which the relationship between the electric input and the mechanical output is proportional).

Published Patent Publication 61-191278 IEEE ULTRASONICS SYMPOSIUM PROCEEDIBGS 1987, P747−P756

Although it has been shown that the structure of the vibrator is a comb-teeth structure for solving the problems for practical use, many problems still remain. In the present invention, an improved ultrasonic motor is realized by improving the stator material.
The mechanical Q of the vibrator is described above, but the Q value varies greatly depending on the material. Conventionally, materials used for the stator are materials with high mechanical Q and materials that can be easily machined, and iron-based alloys, particularly stainless alloys, phosphor bronze, and the like are used. The point that must be added here is that the Q value is different for the above-mentioned specific material and even a specific shape. The reason is caused by the non-uniformity inside the stator material. If the material properties are partially different even in a constant-shaped vibrating body, there are density non-uniformity, hardness non-uniformity, internal strain non-uniformity, and the like. As a result, Q is lowered. This is because if the non-uniformity is viewed in detail, the speed at which vibration is transmitted in the micro area is different, so that it is not possible to perform a consistent movement as a whole. When a material having internal non-uniformity is used, the vibration of the entire stator becomes non-uniform, and the entire stator cannot be uniformly vibrated.
In an actual motor embodiment, since the stator generates non-uniform vibration, contact with the rotor is not efficient because the entire surface cannot be evenly contacted (the piezoelectric body shown in FIG. 1). In the motor, it is determined whether or not the seven contact surfaces all perform vibrations with equal driving force). Furthermore, non-uniform contact results in the generation of audible sound, which is a low-order vibration mode, through vibration nonlinearity. Of course, the mechanical Q value is also small. Differences in mechanical dimensions can be easily checked at the time of stator production, but there is no knowledge to find out about the uniformity of the materials used, so one of the challenges of stator design is that it must rely on the experience of the designer. is there.

  As a result of studying the stator material, the present inventor has found that the ceramic material is most suitable judging from several viewpoints. The conditions required for the stator include that a complicated stator structure can be easily produced, that machined dimensional accuracy can be obtained, that the Q value is sufficiently large, that the cost is reasonable for motor parts, and that the motor output is The conventional metal can be used under the conditions that the mechanical strength at the time of resonance is guaranteed, does not change with time, can withstand environmental tests (minus 30 ° -90 °), and does not become a harmful substance. As a result of comparison with the stator, it was found that the ceramic material is most suitable in many respects.

In addition, the fact that the specific gravity of ceramic is less than half that of metal also contributes to further enhancing the characteristics of lightweight motors.
A comparison is made between engineering ceramic materials used in ultrasonic motors and conventional metal materials. When the ceramic and the metal are compared with respect to the Q value, when the shape is strictly secured, the ceramic has less elastic loss than the metal (Q is large). However, a ceramic with a fixed shape with strict dimensional accuracy is disadvantageous compared to a metal in that it is difficult to machine, especially when it is in the shape of a stator, leading to an increase in cost. In application to an ultrasonic motor, there is no significant difference between the two in the case of a small amplitude. In other words, ceramic is slightly better, but in that case, it varies greatly depending on the fabrication conditions. What must be considered now is a comparison in the case of a large amplitude. In relation to stress and strain, ceramic does not have a clear yield point, and strain increases with increasing stress, eventually leading to failure. In the case of metal, the relationship between stress and strain is proportional, but it disappears as the input increases. Even if it passes the so-called yield point that deviates from the proportional relationship, it does not lead to destruction immediately. Although it depends on which strain point is operated (how far the machine output is taken out), generally, a metal stator tends to generate unstable operation when operated at a point exceeding the proportional relationship. In that respect, the ceramic stator has a wider linear region, so that stable operation can be easily obtained.
When the stator that becomes the drive source of the ultrasonic motor is reviewed again, the electromechanical transducer that becomes the drive source of vibration is a piezoelectric body. A piezoelectric body is attached to the stator and a composite of the piezoelectric body and the stator metal is used as a resonator. The resonator is desired to vibrate under as uniform conditions as possible. Optimization must be measured in a form that includes a piezoelectric body and a stator. One of the reasons for making the stator ceramic is that the piezoelectric body is ceramic, so the composite of the piezoelectric body and the stator can provide the most natural vibration, combined with the large line formation of the ceramic stator section. Stable operation can be obtained rather than using metal. One of the reasons is that the thermal expansion coefficients are the same and the mechanical properties are similar, and that it is easy in that it does not give excessive residual strain when the two are bonded.
One advantage of using ceramic for the stator of an ultrasonic motor is that the motor is lighter. If the weight of metal and ceramic is compared, it is clear that the ceramic is light. It is clear that when the weight of the ultrasonic motor is reduced to half or less, the characteristics are more easily utilized. Furthermore, in the present invention, the rotor is also made lighter by using ceramic or plastic to reduce the weight of the entire motor. With regard to the structure of the rotor, there is no problem simply by converting from metal, which is easy to make conventionally.
A third advantage is cost reduction. Until now, stators were manufactured by machining metal, which required high machining accuracy, leading to increased costs.
The factor that requires machining accuracy is to obtain a stable resonance that guarantees Q. In particular, the groove processing of the stator causes an increase in cost.

  On the other hand, in the case of ceramic, since the basic structure of the stator is manufactured once by a ceramic “mold”, it is mass-productive and the cost can be reduced. Ensuring accuracy is related to the inability to control the shrinkage rate when firing the entire ceramic. Ceramics shrink by 15% inside and outside upon firing. The initial shape is determined in anticipation of this contraction. The problem is whether the shrinkage occurs uniformly. At present, it has been confirmed that the balance of shrinkage in the entire stator is less than 0.5%. Even at this value, sufficiently stable stator vibration can be obtained without additional work for securing the shape.

According to the present invention, the use of ceramic for the stator of the ultrasonic motor can realize an ultrasonic motor having stable operation and high torque and high efficiency. Furthermore, the rotor part is made of a non-metallic material, and a light weight ultrasonic motor in which the entire motor is made of a non-metal can be realized. In particular, by providing an angle sensor inside the above-described lightweight ultrasonic motor, it has become possible to wait for the characteristics as a servo motor.

The ceramic material used for the stator was selected from so-called engineering ceramics because it must be mechanically strong with low vibration loss. Engineering ceramics include alumina (Al2O3),
Mullite (3Al2O3 ・ 2SiO2), silicon carbide (SiC), silicon nitride (Si3N4), aluminum nitride (AlN),
Examples include zirconia (ZrO3) and titania.

Of these ceramics, the three most commonly used alumina, zirconia, which is the strongest ceramic, and titania, which is similar in composition to piezoelectric ceramics, are used in the stator of an ultrasonic motor. The ultrasonic motor was produced by making it.
Here, it can be said that the major feature of comparing the material characteristics when used for a metal and ceramic stator is that the ceramic is lighter and stronger than the metal. When ceramic is used, it must be designed and fabricated with sufficient knowledge that it is an application as a resonator. Table 1 summarizes the physical constants of the metal material and ceramic material used in the stator.

Table 1 Stator material comparison

  When comparing two stator materials, it is not possible to obtain information on which is superior or not. The reason why ceramic materials have not been used so far is that the machining accuracy requirement for the stator is too strict, and when strict accuracy is required, it cannot be obtained at a low cost.

The present inventor studied using the following three types of engineering ceramics.
(1) Alumina 96% or more Apparent density 3.7g / cm3
(2) Alumina 99.5% Apparent density 3.9g / cm3
(3) Zirconia Apparent density 5.6g / cm3
(4) Titania Apparent density 4.5g / cm3
(1) (2) (3) The reason for selecting the material (4) has been studied focusing on alumina, zirconia, and titania, which are the most common materials for industrial machine ceramics. ) Was selected from the viewpoint of cost priority among ceramics, which has a relatively low firing temperature. (3) is higher in density and mechanical strength than (1). The zirconia ceramic of (3) has the highest mechanical strength and is considered to be most suitable for the stator of an ultrasonic motor.
The outline of stator production is described below.
Raw material preparation Crushing and mixing to produce small-sized mud and small molds Making rough shape with rubber press Cutting external shape (rough production of external shape at this stage)
Sintering surface flattening in an atmosphere according to the firing material Lapping process piezoelectric bonding of the contact surface with the rotor The stator is manufactured by the above process. It should be noted that in the case of firing ceramics, the original shape must be determined after knowing that the dimensions shrink by about 15% when firing at high temperatures. Furthermore, it should be noted that the final geometry varies from 1% to 0.5%. This leads to variations in driving frequency.
The absolute value of the dimensional accuracy required for the stator of the ultrasonic motor is not required.
It is important that there is no variation in the dimensions of the entire stator. The part that contributes most in terms of accuracy is the thickness of the vibrating part. Currently designed 2mm inside and outside (considering shrinkage), 0.5% of 2mm is 10μ, and this value is comparable to the general machining accuracy of metal machining accuracy if only numerical comparison is made. It is a satisfactory value without any additional work.
Surface wrapping is required as an operation unique to ceramic stators. The reason for this is that it is difficult to ensure the flatness of the entire stator, so warpage during firing is recognized, to ensure uniform contact of the contact surface with the rotor during motor assembly, and the surface of the baked ceramic is common. Therefore, it is necessary to polish the ceramic surface in order to stabilize the frictional force of the surface and reduce the wear of the friction material provided in the rotor.
There is adhesion of the piezoelectric body at the end of the stator fabrication. Since both the stator material and the piezoelectric body are ceramic, their workability is easier when compared to a metal stator.
Although this does not appear in the table as normal production know-how, residual strain occurs due to the effect of pressure and temperature at the time of bonding (usually raising the temperature to cure the adhesive after bonding), but materials with similar properties to each other There is little influence between them. Furthermore, it is convenient for application of the adhesive that the stator surface is rough with an appropriate roughness.
Since the conventional stator used metal, one electrode could be taken from the stator. However, in the case of a ceramic stator, since it is non-conductive, it is necessary to change the electrode structure so that electricity can be applied from the piezoelectric body. .
One reason for using a ceramic stator is to reduce the weight of the motor. When the stator is lighter, the rotor must be lighter. As a material for reducing the weight of the rotor, almost all materials can be used as they are if they are lightweight and rigid. The specific material is ceramic. Engineering plastic, aluminum alloy, magnesium alloy etc. are raised. If the rotor is made of plastic, the main part of the entire motor can be made of non-metal. In the embodiment, it is cut out by machining using engineering plastic. There is no particular problem in terms of characteristics.
Conventional motors were thought to be a lump of iron and copper, but all of them changed, and the ripple effect was great.
In order to increase the advantage of using an ultrasonic motor as a servo motor, it is particularly desirable to incorporate an angle sensor. The structure of the ultrasonic motor is very simple with only a pressurizing mechanism such as a spring for obtaining a frictional force in addition to the stator and the rotor.
In the present invention, an ultrasonic motor having an angle detection unit on a stator or a rotor is realized. As the angle detector used, an encoder was attached to the rotor, and the angle was detected by a combination of a photodiode and a photodetector provided on the stator.
By providing the angle sensor, there is almost no increase in the overall weight or the like. Furthermore, the installation of the photodiode and the photo detector provided on the stator did not affect the vibration of the stator.

Ring type ultrasonic motor Disc type ultrasonic motor Ring type ultrasonic motor piezoelectric ceramic Disc type ultrasonic motor piezoelectric ceramic Cross section of disk type ultrasonic motor

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Ring type ultrasonic motor rotor 2 ... Ring type ultrasonic motor stator 3 ... Ring type ultrasonic motor fixed frame 21 ... Disc type ultrasonic motor stator 22 ... Disc type ultrasonic motor rotor 23 ... Disc type ultrasonic motor piezoelectric ceramic 24 ... Lining material 25 ... Bearing 26 ... Pressure spring 27 ... Nut 28 ... Motor fixing base 29 ... Rotating shaft 31 ... Stator excitation piezoelectric electrode 32 ... Stator excitation piezoelectric electrode 90 degrees out of phase

Claims (7)

Ultrasonic motor using ceramic as stator for vibrating body for driving ultrasonic motor Non-metallic lightweight ultrasonic motor consisting of a ceramic stator and a rotor using a plastic or ceramic material as the main drive source Lightweight ultrasonic motor consisting of a rotor made of ceramic stator and plastic or ceramic or aluminum (including alloy) or magnesium (including alloy) as the main driving source Ultrasonic motor using any material such as alumina, zirconia, titania, silicon nitride, aluminum nitride, silicon carbide, mullite as the stator material of ultrasonic motor An ultrasonic motor using ceramics for a stator, which is a vibrator for driving an ultrasonic motor, wherein a groove for expanding vibration amplitude is provided on the stator. 5. An ultrasonic motor according to claim 1, wherein a sensor for detecting a rotation angle is mounted on a stator or a rotor to control an angle and a speed. 4. The ultrasonic motor according to claim 1, wherein a stator shape is produced by using a mold particularly when the stator is produced.