Impedance Analyzer How to Guide Production

Evaluating the performance of a vibrator or vibration system requires analysis from both the parameters and the admittance curve diagrams:

  1. Parameters: An impedance analyzer can assess the performance of various devices and equipment such as piezoelectric ceramic plates, piezoelectric transducers, and entire vibration systems (transducers plus amplitude-modulating rods, molds). The most important parameters for analyzing ultrasonic devices and equipment with an impedance analyzer are as follows:
  • Fs: Mechanical resonant frequency, which is the operating frequency of the vibration system and should be as close as possible to the desired value in design, and must match the operating point of the power source. For cleaning machines, the consistency of the vibrator’s resonant frequency is better, the higher it is. For plastic welding machines or ultrasonic processing, if the amplitude-modulating rod or mold is designed unreasonably, the resonant frequency of the vibrator will deviate from the working point.
  • R1: Dynamic resistance, the resistance of the series branch of the piezoelectric vibrator, the smaller the better under the same support conditions. For cleaning or welding vibrators, it is generally between 5Ω and 20Ω. If it is too large, there will be problems with the operation of the vibrator or vibration system, such as circuit mismatch or low conversion efficiency, short vibrator life.
  • 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 efficiency of the vibrator; but Qm must be matched with the power source, and if the Qm value is too high, the power source cannot be matched. For cleaning vibrators, the higher the Qm value, the better. Generally, the Qm of cleaning vibrators should be between 500 and 1000. If it is too low, the vibrator efficiency is low, and if it is too high, the power source cannot be matched. For ultrasonic welding or processing, the Qm value of the vibrator itself is generally around 500 to 1000, and the whole machine system is between 1500 and 3000. If it is too low, the vibration efficiency is low, but it should not be too high either, because the higher the Qm, the narrower the working bandwidth, and the power source is difficult to match, that is to say: the power source is difficult to operate at the resonant frequency point, and the equipment cannot work.
  • CT: Free capacitance, the capacitance value of the piezoelectric device at a frequency of 1 kHz, which is consistent with the value measured by a digital capacitance meter. This value minus the dynamic capacitance C1 can get the real static capacitance C0, C0 = CT – C1. When using it, the C0 should be balanced with the inductance. In the circuit design of cleaning machines or ultrasonic processing machines, correctly balancing C0 can improve the power factor of the power source. There are two methods for balancing with inductance, parallel tuning and series tuning.
  • Fp: Anti-resonant frequency, the resonant frequency of the parallel branch of the piezoelectric vibrator, at this frequency, the impedance Zmax of the piezoelectric vibrator is maximized. If the anti-resonant impedance Zmax is very low, then there is a problem with the vibrator.
  1. Graphics

The impedance analyzer provides five types of coordinate characteristic diagrams, among which the logarithmic characteristic diagram is of great significance for the inspection of piezoelectric devices. The vibration performance of the piezoelectric vibrator or vibration system can be judged directly through the logarithmic coordinate diagram, which is intuitive and practical.

Under normal circumstances, the admittance circle and the conductance curve are as follows, the admittance circle is a single circle, and the logarithmic coordinate diagram only has a pair of minimum and maximum values:

In abnormal conditions, the admittance circle and conductance curve are as shown in the figure below; multiple parasitic small circles appear on the admittance circle diagram, and there are multiple pairs of minimum and maximum values on the logarithmic coordinate diagram:

In the following situations, the admittance circle and conductance curve of the piezoelectric ceramic or transducer will be abnormal:

  1. The transducer experiences crystal cracking during assembly.
  2. There are inherent issues with the piezoelectric ceramic, such as internal delamination.
  3. Problems arise in the design or assembly of the ultrasonic amplitude-modulating rod or mold.
  4. Stress on the rod and surrounding parts due to poor concentricity of the transducer, causing collisions.

Generally speaking, the admittance curve diagram and parameters are interrelated. If the vibrator’s admittance curve diagram is normal, then R1 is relatively low and Qm is relatively high. Conversely, if the vibrator’s admittance curve diagram is abnormal, R1 is generally higher and Qm is smaller.

For transducers, there are often many resonant points, such as the first vibration, second vibration, third vibration, etc. Generally, the further apart the first vibration (usually the thickness mode used by the user) and the second vibration, the better. This is because the second vibration is of another mode (such as bending, twisting, etc.). When operating in the first vibration mode, the second vibration can also occur, thereby affecting the life of the transducer. The farther apart they are, the less the impact.