Ultrasonic actuator

The ultrasonic actuator, as described below, is not applicable to very high frequency (> 1MHz) emitter applications. The application in VHF is mainly for medical ultrasonic imaging and nondestructive testing, and the thickness mode d33 is used. What this section discusses is the low frequency (20~100kHz) application, the piezoelectric film works in the length change mode d31.

Advantages of piezoelectric films used in low frequency ultrasound

It's the flexibility of this material. It can be easily

A circular curl oscillator (see Figure 33). Beampattern

The shape depends on the number and diameter of the half - garden unit. work

The frequency is determined by the diameter of the half - garden unit. Should refer to

The difference between figure 33 (a) and figure 33 (b) is the difference between

The number and diameter of the effective unit are different. Want to show

The coverage of a wide beam should reduce the effective unit

Number? A cylindrical vibrator, you can get 360 degrees

The beam pattern.

For the application of ultrasound, for long distance measurement,

A sharp beam with a minimum sidelobe is required. But for all

As for the application of collision detection in the rear of the car

A wide beam of more than 180 degrees is needed. Figure 33

The design shape of the ultrasonic oscillator with two beams of wide and narrow beams is shown.

The application of piezoelectric membrane through air as an ultrasonic actuator includes vehicle reversing, safety ranging, personal safety system, air velocity (Doppler) detection, and object internal communication. Similar structural forms can also be used for underwater and liquid detection, including flow, liquid level sensors, and communication.

Thermoelectric Foundation

Piezoelectric polymeric materials, such as PVDF and their VF2/VF3 copolymers, are also thermoelectric polymers. Thermoelectric sensing materials are usually dielectric materials with dipole moments that vary with temperature. As the material absorbs heat energy, they also expand or shrink, thereby inducing two piezoelectric signals. When the piezoelectric film is heated, the membrane dipole excitation under thermal random motion. This makes the average polarization in the film decreases, resulting in charge on the membrane surface, the output current and the temperature change rate (the T) is proportional to the temperature increase (or decrease) the number of charge was generated, you can use a hot charge coefficient to represent P.

The charge and voltage produced by a piezoelectric film with an area of A, a dielectric constant of epsilon and a thickness of T, are calculated by the press.

The TA Q = P

V = the T/ epsilon p t

Example five:

A film thickness of 9 m (T), Permittivity (epsilon) is 106 x 10 '12C/Vm and pyroelectric coefficient (P) is 30 x 10' 6C/ (M2 ~ K) piezoelectric film pyroelectric detector due to infrared radiation, and the temperature (the T) increased by 1 degrees K (f), following the calculation of output voltage:

The thermoelectric coefficient of the piezoelectric thin film is about one order of magnitude higher than that of lead zirconate titanate (PZT) and BaTiO3 (BaTiO3). Table 5 compares the thermoelectric properties of these materials.

Table 5: Thermoelectric comparison table

The advantages of a piezoelectric film include:

- no absorption of moisture (< 0.02% H2O water absorbency)

Low thermal conductivity

Low permittivity

Chemical inertness

Large size detector

The thermoelectric response of the piezoelectric thin film is also the noise source of the piezoelectric sensor in the low frequency application. For low frequency stress induction, there are several convenient methods "common mode suppression" of thermoelectric response. For example:

Two of the same shape and size of the electrode used in a piezoelectric film device; an electrode with the same d31, and the other electrode is d31 and perpendicular to the direction of signal generating thermal response signals of the two electrode is the same, but the vertical electrode and the d31 electrode area to the 10 times. After the signal is subtracted, the pure piezoelectric response is obtained.

Two piezoelectric devices with the same size are all stacked structures. One film is d31 oriented and parallel to the stress plane. The other membrane is d31 oriented, but perpendicular to the stress surface. As mentioned above, the signal is subtracted, which separates the piezoelectric response from the thermal response.

There are other common mode suppression techniques that can be explained by the application engineers of the MSI company.

When applied to higher frequency, the temperature change rate of piezoelectric thin film device is lower than that of the stress process to be measured. The frequency filter can remove the unnecessary heat signal conveniently.