# Ultrasonic generator introduction

__ Ultrasonic generator__ is used to generate ultrasonic transducer to provide ultrasonic energy devices. Ultrasonic generator on its incentives in two ways: one is he excited. The other is self-excited. If you press the final amplifier tube type device used points, and divided into four types: tube-type ultrasonic generator; thyristor inverter type ultrasonic generator; transistor-type ultrasonic generator and ultrasonic generator power modules. Tube type and thyristor inverter has now basically eliminated, the current widespread use of the transistor type generator.

He excited ultrasonic generator mainly consists of two parts, the first stage is an oscillator, amplifier after the class is. Generally coupled through the output transformer, added to the ultrasonic energy transducer. The ultrasonic generator self-excited oscillation is to, amplifier, output transformer and transducer set as a whole, form a closed loop circuit to meet the amplitude, phase feedback conditions, the formation of a power amplifier of the oscillator. And resonance in the transducer mechanical resonance frequency.

Based on the characteristics of ultrasonic generator, the main discussion, analysis, design, resonant ultrasonic generator, power amplifier and matching and other related issues.

I. On the resonance problem <frequency automatic tracking>

The so-called resonance problem is to ask the generator's output signal frequency can change in the work of the transducer resonant frequency tracking, automatic tracking frequency that is known. The commonly used frequency automatic tracking roughly the following methods:

1. Acoustic tracking

In acoustic coupling mode, collected from the transducer resonant frequency of the telecommunications number, and then fed back to the pre-amplifier, so that the formation of self-excited oscillator.

Circuit is a closed-loop system, the circuit in the power of the moment to produce a shock pulse, the pulse after the preamplifier, power amplifier to motivate the transducer, the transducer at its natural frequency of vibration. Resulting in the feedback sound the same frequency on the receiver available telecommunications numbers. After a phase-shift circuit, frequency selection, pre-amplifier and power amplifier to go excitation transducer, if the oscillator phase and amplitude conditions, the system will self-oscillation, and oscillation frequency tracking of the transducer resonance frequency.

2. Power tracking

The so-called "power track" also known as self-excited oscillator feedback. Roughly the following form

(1) impedance bridge in the form of dynamic feedback system

Impedance bridge in the form of dynamic feedback system consisting of the frequency of automatic tracking circuit works as follows; it is the principle of using the bridge balance compensation transducer with electrical arm of the active component of reactive power, by means of a variable differential transducer to extract and arm oscillation current is proportional to the feedback voltage, the closed-loop system in the mechanical resonance frequency of the transducer on the self-oscillating. The method of transducer electrical parameters of compensation may be independent of frequency, and thus good tracking in a wide frequency band.

Differential variable automatic frequency tracking device bridge circuit shown in Figure 1.29. Map Tf is the differential variable device, the positive feedback voltage Uf, leads from the secondary winding W3; primary winding W1, W2 and impedance Z1, Z2 constitute four-arm bridge, Z1 is the transducer impedance (which by the arm impedance Zm and electrical impedance Ze arm formed in parallel), Z2 is the impedance of the compensation components. Z3 is used to compensate the bridge reactance. Let Zm "Ze is Z1 = Ze. Obviously, if meet conditions W1I1 = W2I2 (W1W2 of primary turns), bridge-balance, Tf secondary winding voltage feedback Uf = 0. This equilibrium can be table Z2/Ze = W2/W1

X2/Xe = R2/Re = W2/W1 = n

That is, where Re / Xe Ze and Z2, respectively the real and imaginary parts. N represents the differential coefficient of the variable control of the primary winding turns ratio of two parts, which is equal to the compensation components electrical transducer impedance and arm ratio.

Under normal circumstances, the current Im flowing through the Zm make the bridge out of balance, Tf sense to give birth to the secondary winding is proportional to the transducer arm oscillating current Im feedback voltage Uf

Uf = Im (W1/W3) Rim

Where Rim is the oscillator input resistance.

When the system of self-excited frequency fo = fr (transducer of mechanical resonance frequency), the current Im. Reach the maximum. System feedback strongest, Uf feedback voltage maximum, to meet the range of conditions, and Uf and Uin-phase, but also to meet the phase condition, the system self-excited in the transducer mechanical resonance frequency. If the transducer's mechanical resonance frequency reduced due to some factors, the fo> fr. , Zm was inductive load, Im the phase angle is negative, resulting in Uf lag Uin (the original input voltage), the system oscillation frequency is reduced so as to achieve the purpose of tracking.

Differential variable automatic frequency tracking device bridge circuit advantages in its electrical resistance of the transducer elements of the compensation is independent of frequency, in order to ensure the feedback voltage within a wide frequency band with only the mechanical oscillation current, reliable tracking, offset smaller .

(2) partial pressure of the load the way feedback system.

This system is shown in Figure 1.30, the figure form a closed loop of the circuit circuitous road. Circuit in the power of the moment produces an electrical pulse, the transducer amplifier added to both ends, so excited vibration transducer. The vibration frequency of the transducer itself, the natural frequency of the transducer at both ends of the oscillation signal sent by the partial pressure on the adjustable phase shifter, and then sent to the amplifier. When the adjustable phase shifter is adjusted to meet the self-excitation phase conditions, the system self-excited in the transducer's natural frequency. Transducer resonant frequency of small changes, the circuit system can keep track of the work is always at its best.

(3) phase-locked type frequency automatic tracking

Phase-locked type frequency automatic tracking system and the first two self-excited systems, the circuit is much more complex, but it can get a better frequency automatic tracking performance. Therefore, the generator of ultrasonic plastic welding machine to get more and more widely used.

The use of phase-locked frequency technology for automatic tracking of the key is how to get the load circuit, the phase difference between voltage and current. Piezoelectric transducer in the vicinity of the resonant frequency of the equivalent circuit shown in Figure 1.31. Diagram inside the dashed border equivalent circuit for the transducer. C0 is the static capacitance transducer. Rm is the mechanical impedance reflected to the electrical side of the resistance (if you ignore mechanical loss is reflected in the electrical side of the radiation resistance), Lm is the dynamic equivalent quality reflected in the electrical side of the value; Cm for the equivalent force along the reflected power side of the dynamic capacitance value; L. Work for the transducer resonant state in the parallel matching inductor.

The circuitous path of the resonant frequency

ω2 = 1/LoCo = 1/LmCm

In the resonant state, the voltage across the transducer U = iRm,

The voltage and current are in phase. When the transducer when the detuning, u and i no longer in phase, and frequency of its reactance characteristic curve shown in Figure 1.32. Figure ωs for the series resonant frequency. ωp for the parallel resonant frequency.

When ωs <ω <ωp the circuit was emotional, u ahead of i, the phase difference is positive. When ω = ωs, the circuit was pure resistance characteristics, i-phase current and voltage U. Therefore, the voltage across the transducer and the flow through the transducer current, positive and negative phase difference between them, size, representing the excitation signal frequency and the vibration system natural frequency relationship. Therefore, if the voltage and current signals out of phase, as an incentive to change the resonant frequency of vibration system control signal, to achieve frequency tracking purposes. This is the phase-locked type frequency tracking of the basic principles.

Phase-locked type frequency automatic tracking system practical circuit diagram shown in Fig. The figure by the phase comparator, voltage comparator, a low-pass filter, VCO, incentive amplifier, power amplifier, current sensing and voltage sampling and so on. This is a closed-loop system. However, due to voltage-controlled oscillator itself can work alone. Therefore, the open-loop system can also work. Open-loop similar to his when excited ultrasonic generator. This is not the same as self-excited oscillator. The system uses a transducer on the final stage of the phase difference between voltage and current, the phase comparison to obtain the phase error signal, and then low-pass filtered, to control the VCO output signal frequency. So that it remains with the mechanical resonant frequency of vibration of the system consistent.

Although the frequency and phase relationship between the dependent transformation, but the feedback system consisting of two, from the control results, is not the same. Frequency of the feedback results, will lead to the input and output signal frequency difference between the smallest possible, and the feedback phase of the phase difference between the results of the two signals as small as possible. Posed based on the frequency and phase differential - integral relationship between the phase feedback system eventually led to two signal frequency difference is zero.

Phase-locked type frequency locked loop automatic tracking system has application specific integrated circuit, such as CD4046 and so on. Not be detailed here, please refer to the principle of phase-locked loop integrated circuit data sheets and books.

Finally, we summarize. Phase-locked type frequency automatic tracking system has the following characteristics;

(1) The phase-locked loop is an excellent band-pass filter, therefore, no error with the system to other non-resonant frequency

On;

(2) automatic tracking system frequency control signal and sampling the voltage waveform of the waves good or bad, does not matter much;

(3) The output power is relatively stable. Changes will not load a significant change,

(4) The control system operates in the small-signal state, they are able to work long hours.

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