Ultrasonic transducers play a very important role in ultrasonic welding machines. A high-quality ultrasonic transducer can ensure strong and stable output power. Only when you understand the parameters of the ultrasonic transducer can you accurately match your ultrasonic generator. So, how to judge the performance of ultrasonic transducers?

1. Parameters

1) Mechanical resonance frequency (Fs)

This refers to the operating frequency of the vibration system, which should be as close to the expected value as possible in the design, and must match the operating point of the power supply.

2) Dynamic resistance (R1)

The resistance of the series branch of the piezoelectric vibrator should be as small as possible under the same supporting conditions! For cleaning or welding vibrators, it is generally between 5Ω-20Ω. If it is too large, there will be problems with the vibrator or vibration system, such as circuit mismatch or low conversion efficiency, short life of the vibrator, etc.

3) Mechanical quality factor (Qm)

Determined by the conductance curve method, Qm=Fs /(F2-F1), the higher the Qm, the better, because the higher the Qm, the higher the efficiency of the oscillator. But Qm must match the power supply. When the Qm 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 be between 500-1000. If it is too low, the efficiency of the vibrator will be low, and if it is too high, the power supply cannot be matched. For ultrasonic welding or processing, the Qm value of the vibrator itself is generally around 50-1000, and the whole system is 1500-3000. If it is too low, the vibration efficiency will be low, but it should not be too high, because the higher the Qm, the greater the working bandwidth. Narrow, the power supply is difficult to match, that is, the power supply is difficult to work at the resonance frequency, and the device cannot work normally.

4) Free capacitance (CT)

The capacitance value of the piezoelectric device at a frequency of 1kHz is consistent with the value measured by the digital capacitance meter. This value subtracts the dynamic capacitance C1 to get the real electrostatic capacitance C0, C0=CT-C1. When using inductance to balance C0, in the circuit design of the ultrasonic processing machine, the correct balance of C0 can improve the power factor of the power supply, there are two ways to use inductance balance, parallel tuning and series tuning.

5) Anti-resonance frequency (Fp)

The resonance frequency of the parallel branch of the piezoelectric vibrator. At this frequency, the impedance Zmax of the piezoelectric vibrator is the largest. If the anti-resonance impedance Zmax is very low, the vibrator has a problem.

2. Graphics

Generally speaking, the vibration performance of the vibration system or the piezoelectric transducer can be judged directly through the logarithmic coordinate diagram, which is simple and intuitive.

1) Under normal circumstances, the admittance circle is a single circle, and the logarithmic graph has only a pair of minimum and maximum values.

2) Under abnormal conditions, multiple small parasitic circles appear on the admittance circle graph, and the logarithmic coordinate graph has multiple pairs of minimum and maximum values.

So under what circumstances will the admittance circle and conductance curve of piezoelectric ceramics or transducers appear abnormal? There are mainly the following situations:

1) Piezoelectric ceramics have problems, such as internal delamination.

2) Chip cracks occurred during the assembly of the transducer.

3) Problems with the design or assembly of the ultrasonic horn and mold.

4) The parts around the stress rod collide due to the poor concentricity of the transducer.

Generally speaking, the admittance curve and parameters are related to each other. If the admittance curve of the vibrator is normal, R1 is lower and Qm is higher. On the contrary, if the admittance curve of the vibrator is abnormal, generally R1 is larger and Qm is smaller. As an important part of ultrasonic equipment, the quality of ultrasonic transducer is directly related to the service life of the equipment.