High-power ultrasonic transducer

At present, the commonly used methods of frequency automatic tracking are as follows: collect the electrical signal of resonant frequency from the transducer by acoustic coupling, and then feed it back to the front amplifier to form a self-excited oscillator. The principle of acoustic tracking transducer is that the circuit is a closed-loop system. At the moment of power on, the circuit generates an impact pulse, which is excited by the preamplifier and power amplifier to vibrate the transducer according to its natural frequency. Thus, electrical signals of the same frequency can be obtained on the feedback acoustic receiver. After phase shifting, frequency selection, preamplifier and power amplifier of the circuit to excite the transducer, if the phase and amplitude conditions of the oscillator are met, the system will Self-oscillation, and the oscillation frequency will track the resonance frequency of the transducer.
Practical circuit box for phase-locked frequency automatic tracking system. It consists of a phase comparator, a voltage comparator, a low-pass filter, a voltage controlled oscillator, an excitation amplifier, a power amplifier, current sampling, and voltage sampling. This is a closed-loop system. But due to the fact that the voltage controlled oscillator itself can work independently. So, the open loop of this system can also work. When open loop, it is similar to a separate excitation transducer. This is different from a self-excited oscillator. This system utilizes the phase difference between the voltage and current on the final stage transducer. After phase comparison, a phase error signal is obtained, which is then filtered through low-pass filtering to control the frequency of the output signal of the voltage controlled oscillator. Keep it consistent with the mechanical resonance frequency of the vibration system. Although there is a dependency transformation relationship between frequency and phase, the feedback system composed of the two is different from the control results. The frequency feedback result will result in minimizing the frequency difference between the input and output signals, while the phase feedback result will minimize the phase difference between the two signals. According to the differential integral relationship between frequency and phase, the phase feedback system ultimately results in a zero frequency difference between the two signals.
Feedback system for load sharing method. The entire circuit forms a closed loop circuit. At the moment of power on, the circuit generates an electrical pulse that is applied to both ends of the transducer through a power amplifier, causing the transducer to vibrate. The vibration frequency is the inherent frequency of the transducer itself, and the oscillation signals at both ends of the transducer are divided and sent to the adjustable phase shifter before being sent to the power amplifier. When the adjustable phase shifter is adjusted to meet the self-excited condition, the system self-excited at the natural frequency of the transducer. The small changes in the resonant frequency of the transducer can be tracked in a timely manner by the circuit system to ensure that the work is always in a state. Compared with the first two types of self-excited systems, the phase-locked frequency automatic tracking system has a much more complex circuit, but can achieve better frequency automatic tracking performance. Therefore, it has been increasingly widely used in the generators of ultrasonic plastic welding machines. The key to using phase-locked technology for automatic frequency tracking is how to obtain the phase difference between voltage and current in the load circuit. Therefore, the phase difference between the voltage at both ends of the transducer and the current flowing through the transducer represents the relationship between the frequency of the excitation signal and the natural frequency of the vibration system. So, if the phase difference signals of voltage and current are taken out as control signals to excite the resonance frequency change of the vibration system, the purpose of frequency tracking is achieved. This is the basic principle of phase-locked frequency tracking.
Electrical tracking, also known as feedback self-excited oscillator. There are generally several forms: a dynamic feedback system in the form of an impedance bridge. The principle of a frequency automatic tracking circuit composed of a dynamic feedback system in the form of an impedance bridge is as follows; It utilizes the principle of bridge balance to compensate for the reactive and active components of the transducer's electrical arm, and uses a differential variable to extract a feedback voltage proportional to the oscillation current of the transducer's mechanical arm, enabling the closed-loop system to self oscillate at the transducer's mechanical resonance frequency. This method may compensate for the electrical parameters of the transducer independently of frequency, thus achieving good tracking over a wide frequency range.