Introduction Of Ultrasonic Impedance Analyzer

Introduction Of Ultrasonic Impedance Analyzer

Date:Nov 27, 2019

Traditional ultrasonic impedance analyzer all need a computer running software to realize scanning analysis function, and HS520A provided by Altrasonic series ultrasonic impedance analyzer not only have the functions of the computer scanning analysis, also provide display in the instrument directly piezoelectric device scan function, customers no longer need a computer for each instrument configuration. This method not only ensures the test efficiency, but also reduces the test cost. This is HS520A series of products in the field of piezoelectric testing to provide customers with another super value solution.

At the same time, HS520A has good measurement accuracy, ultra-wide frequency range and excellent stability, which can meet the measurement requirements of most ultrasonic devices and materials.

Ultrasonic impedance analyzer is mainly used for the measurement of impedance characteristics of all kinds of ultrasonic devices, including: piezoelectric ceramics, transducers, ultrasonic cleaning machines, ultrasonic ranging, ultrasonic motors, ultrasonic flowmeters, ultrasonic flaw detectors and other ultrasonic equipment.

Measurement parameter

For a piezoelectric device, its impedance characteristics vary with frequency. A complete description of a piezoelectric device requires an extremely complex circuit network, and a simpler network is selected in the frequency band we are interested in. (including inductors, resistors, capacitors), a more complete description of the characteristics of the piezoelectric device. It has been proved that the network is constructed by using the inductors, resistors and capacitors included in the following network, and the required network characteristics can be better reproduced.

For a general piezoelectric device, there is no other resonance in the frequency domain away from a certain resonant frequency. In the frequency domain near the resonant frequency, the device can be simulated with a plurality of inductors, resistors, and capacitors, and the corresponding equivalent circuit is as shown below. Shown as follows:

Figure 1: General piezoelectric device equivalent circuit diagram

Figure 2: Admittance characteristics of piezoelectric devices

In Fig. 1, (a) is a symbol indicating a piezoelectric device, and (b) is an equivalent circuit of the piezoelectric device. Where C0 is a static capacitor, R1, C1, and L1 are resistance, capacitance, and inductance in dynamic impedance, respectively, and R0 is the insulation resistance of the material. In the above equivalent circuit, since the circuit is expressed in parallel, it is convenient to use admittance analysis, so that the admittance of the whole circuit is Y, and the parallel branch (consisting of R0, C0, called static admittance) Admittance is Y0, series branch

The road (composed of R1, L1, and C1, called dynamic admittance) is admitted to Y1.

Y= Y0 + Y1 Y0 = 1/R0+1/(j2πfC0), Y1 = 1/{R1+j2πf L1+1/( j2πfC1)}

The calculation can be used to obtain the variation of the total admittance Y and the dynamic admittance Y1 with the frequency f (admittance-frequency characteristic). Y and Y1 are vectors, which should be decomposed into real parts (conductance G) and imaginary parts (susceptance B) in graphical form.

Figure 2 shows two different representations of admittance characteristics. The upper part is the characteristic diagram of the conductance/suspension with frequency, the yellow line represents the B(S)--f characteristic diagram, and the red line is the G(S)--f characteristic diagram. The lower half is an admittance vector plane, the abscissa is the conductance G (the real part of the admittance), and the ordinate is the susceptance B (the imaginary part of the admittance), which shows how it varies with frequency.

The admittance variation characteristics of the device.

When the signal frequency changes in the range around the resonance frequency (series resonance), the trajectory of the vector Y1 is a circle whose center is (1/2R1, 0) and the radius is 1/2R1.

When the trajectory of the vector Y1 around the resonant frequency is rotated by one round, the vector Y0 varies generally with frequency and can be regarded as a constant. Therefore, the trajectory circle of Y1 is translated along the longitudinal axis on the admittance plane. You can get the trajectory circle of the admittance Y as a function of frequency, the so-called admittance circle.

Using the admittance chart, the equivalent circuit of the piezoelectric device and other important parameters can be obtained.

(1) Fs: The mechanical resonance frequency, that is, the operating frequency of the vibration system, should be as close to the expected value as possible in the design. For a cleaning machine, the higher the resonant frequency consistency of the vibrator, the better. For plastic welders or ultrasonic machining, if the horn or mold design is unreasonable, the resonant frequency of the vibrator will deviate from the operating point.

(2) Gmax: Conductance in series resonance, the conductance value when the vibration system is operating, which is the reciprocal of the dynamic resistance R1. The bigger the better under the same supporting conditions, Gmax=1/R1. Generally, for cleaning or welding vibrators, it is between about 50 mS and 500 mS. If it is too small, in general, the vibrator or vibration system may have problems, such as circuit mismatch or low conversion efficiency, and short life of the vibrator.

(3) C0: Capacitance of the static branch in the equivalent circuit of the piezoelectric device, C0=CT-C1 (where: CT is the free capacitance at 1 kHz, and C1 is the capacitance of the dynamic branch in the equivalent circuit of the piezoelectric device ). In use, balance C0 with inductance. In the circuit design of a washing machine or ultrasonic processing machine, properly balancing C0 can increase the power factor of the power supply. There are two methods for using the inductor balance, parallel tuning and series tuning.

(4) Qm: mechanical quality factor, determined by the conductance curve method, Qm=Fs/(F2-F1), the higher the Qm, the better, because the higher the Qm, the higher the vibrator efficiency; but the Qm must match the power supply, Qm When the value is too high, the power supply cannot match.

For cleaning the vibrator, the higher the Qm value, the better. Generally speaking, the Qm of the cleaning vibrator should reach 500 or more. If it is too low, the vibrator efficiency is low.

For ultrasonic machining, the Qm value of the vibrator itself is generally around 500. After adding the horn, it generally reaches about 1000, plus the mold, generally reaching 1500-3000. If it is too low, the vibration efficiency is low, but it should not be too high, because the higher the Qm, the narrower the working bandwidth, the hard power supply is difficult to match, the power supply is difficult to work at the resonance frequency point, and the device cannot work.

(5) F2, F1: vibrator half power point frequency. For the entire vibration system (including the horn and the mold) for ultrasonic machining, F2-F1 is greater than 10 Hz, otherwise the frequency band is too narrow, the power supply is difficult to operate at the resonance frequency point, and the device cannot work.

F2-F1 is directly related to the Qm value, Qm=Fs/(F2-F1).

(6) Fp: anti-resonant frequency (mainly the resonance generated by C0 and L1), the resonant frequency of the parallel branch of the piezoelectric vibrator. At this frequency, the impedance of the piezoelectric vibrator is the largest and the admittance is the smallest.

(7) Zmax: anti-resonant impedance. Under normal conditions, the anti-resonant impedance of a transducer is above several tens of kilohms. If the anti-resonance impedance is relatively low, the life of the vibrator is often short.

(8) CT: Free capacitance, the capacitance value of the piezoelectric device at 1 kHz. This value is consistent with the value measured by the digital capacitance meter. This value minus the dynamic capacitor C1 can get the true static capacitance C0, C0 needs to be balanced by an external inductor, C1 participates in energy conversion when the system is working, no need to balance.

(9) Dynamic resistance R1: This is the resistance of the series connection of the piezoelectric vibrators in the figure. The formula is: R1=1/D, where D is the diameter of the admittance circle.

(10) Dynamic inductance L1: It is the inductance of the series branch of the piezoelectric vibrator in the figure.

The calculation formula is: L1=R1/2π(F2-F1) ,where R1 is the dynamic resistance and F1 and F2 are the half power points.

(11) Dynamic capacitance C1: This is the capacitance of the series branch of the piezoelectric vibrator in the figure.

The calculation formula is:C1=1/4π2Fs2L1 , where Fs is the resonant frequency and L1 is the dynamic inductance.

(12) Static capacitance C0: The calculation formula is C0=CT-C1, where CT is the free capacitance and C1 is the dynamic capacitance.

(13) Keff: effective electromechanical coupling coefficient. Generally speaking, the higher the Keff, the higher the conversion efficiency.

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