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Research on Piezoelectric Ceramic Polarization Device for Ultrasonic Motor

Views:4     Author:Site Editor     Publish Time: 2019-10-08      Origin:Site

Ultrasonic motor is a new type of motor driven by piezoelectric ceramic excitation ultrasonic wave. It has the advantages of low speed and large torque, low inertia, fast response and no interference from magnetic field. Due to the above advantages of ultrasonic motor, ultrasonic motor technology has been rapidly developed in the 20th century, and has been successfully applied in the fields of camera, medical, aerospace, robot, automobile, precision potentiometer, micro-machinery, etc., and has become a hot research topic today. The driving principle of ultrasonic motor with different structures is basically the same, that is, the micro-deformation generated by electric vibration is converted into macroscopic mechanical motion by resonance amplification and friction coupling by using the inverse piezoelectric effect of piezoelectric ceramic. Piezoelectric ceramics are components of ultrasonic motors that convert electrical energy into vibrational energy. The piezoelectric properties are related to the mechanical properties of ultrasonic motors. Polarization process of piezo disc sensor is the key process for obtaining piezoelectricity of piezoelectric ceramic materials. In order to improve the performance of piezoelectric ceramics, a new piezoelectric ceramic pole for traveling wave ultrasonic motor is developed according to the requirements of thermal polarization process. The device makes the piezoelectric ceramics have the same characteristics after polarization, which plays a key role in improving the performance of the ultrasonic motor.

1 Proposed polarization
The traveling wave type ultrasonic motor relies on the traveling wave in the elastic body to obtain the rotational moment by the friction between the rotor. According to the basic working principle of the traveling wave type ultrasonic motor, the conventional method adopted for the piezoelectric vibrator is to first partition according to the vibration mode, how many partitions, and how many times of polarization. The study found that this polarization of piezoelectric vibrators has major drawbacks. Due to the multiple polarizations, the polarization effects of the partitions after polarization are different, so the amplitudes of the bending waves excited by the two sets of piezoelectric vibrators cannot be balanced. At this time, the friction coefficient between the rotor and the stator is lower, thereby affecting the output efficiency of the motor. In order to solve the aboving problems and improve the polarization uniformity, this paper proposes a simultaneous isotropic polarization mode. Firstly, the piezoelectric ceramic is first partitioned, and then the positive polarity of the output voltage of the high voltage generating circuit is connected to the positive polarization partition of the piezoelectric ceramic through the electric field distribution device; the negative polarity of the output voltage is connected to the reverse of the piezoelectric ceramic. After the polarization device is energized, the piezoelectric ceramics are simultaneously polarized in the same direction.

2 overall structure
In addition to the commonly used thermal polarization, the piezo ceramic crystal quartz rod polarization process also has a high temperature and low voltage polarization, a resonant polarization process, and the like. Different polarization processes have different design requirements for polarizing devices. Under the premise of meeting the polarization requirements, in order to make the designed polarization device simple and reliable, the operation is simple, they chooses the thermal polarization process, that is, the piezoelectric ceramic to be polarized is placed in the silicone oil and heated to 100 ° C ~ 150 ° C, and then the current is applied across the piezoelectric ceramic so that the domains are aligned as far as possible in the direction of the external electric field and taken out after a period of time. The polarization mechanism is simply under the action of a strong DC electric field, the originally chaotically oriented polarization regions (electric domains) are arranged in an order along the direction of the electric field. After the electric field is removed, the domains in the ceramic body still retain a certain direction along the electric field. Thus, the isotropic polycrystal (there is no offset of the electric center of gravity and no piezoelectricity) becomes an anisotropic polycrystal, so that the piezo ceramic body has piezoelectric properties. According to the process requirements of thermal polarization, the polarization device needs to provide a constant polarization temperature, a suitable polarization electric field and a desired electric field distribution. According to the design requirements, the piezoelectric ceramic polarization device should mainly consist of the following three parts:
1 heating circuit. Mainly by heater, temperature controller.It consists of a polarizing groove.
2 polarization high voltage generating circuit.
3 electric field distribution device.

3 temperature controller
The system is based on the microcontroller as the core to manage and coordinate the entire controller. It uses temperature sensor as temperature measuring component, V/F circuit as A/D conversion, strong anti-interference ability. In the temperature controller, functions such as setting and running of temperature and control parameters can be performed by a keyboard and a display circuit. The controller program adopts the algorithm. The control principle is to first find the deviation value of the measuring polarization temperature and the set temperature, and then process the deviation value to obtain a control signal to adjust the duty ratio of the output signal, thereby adjusting the ratio of heating time in a certain period is used to control the temperature of the polarization bath. The working process of the system is as follows: the temperature signal is converted into a voltage signal by an integrated temperature sensor, and the voltage signal is amplified and filtered to perform voltage-frequency conversion, and a pulse train compatible with the logic circuit is obtained, and the pulse frequency and the temperature are accurately proportional. After the pulse is isolated by optocoupler, it is input into the single-chip microcomputer. After the program is processed, the current temperature value can be obtained. The temperature is compared with the set value of the temperature to obtain the temperature deviation. The control quantity is calculated by the algorithm and controlled. The on-off time of the voltage applied to the heater to achieve the purpose of controlling the temperature.

4 polarization high voltage generating circuit
The function of the polarization high voltage piezoelectric disc transducer generating circuit is to provide suitable electric field conditions for polarization, which is a key part of the piezoelectric ceramic polarization device. In the polarization high-voltage generating circuit, the central tapping voltage regulating transformer and the step-up transformer are used to boost the commercial power, output an AC high voltage, and then rectify through a bridge rectifier circuit to obtain a DC high voltage required for polarization. During polarization operation, the voltage regulating transformer is slowly adjusted to increase the output DC voltage slowly, providing a suitable DC electric field for polarization. In the polarization experiment, it is found that when the output voltage is adjusted to a certain value, the output voltage will slowly fall as the polarization process progresses. In order to keep the polarization process stable, we need it by adjusting the voltage regulating transformer, the input voltage is continuously increased to stabilize the output voltage.

5 Conclusion
The research shows that the piezoelectric constant d33 of each zone of piezoelectric ceramics polarized simultaneously in the same direction is more consistent and uniform, which is beneficial to improve the output efficiency of ultrasonic motor. In order to improve the piezoelectricity of piezoelectric ceramics and improve the performance of ultrasonic motor, the author has carried out basic research on the piezoelectric device's polarization device and achieved certain results, but the further improvement of piezoelectric ceramic performance will also depend on the following aspects of work:

1 Further study of piezoelectric ceramic materials, improving their preparation process and material properties.
2 Study new polarization methods and try to maximize the performance of the materials.
3 Improve the polarization high voltage generating circuit to improve the power quality and supply range.
4 Through experiments, the optimal polarization conditions for different piezoelectric ceramics were sought.

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