Sensors & Transducers

Application Note - 
High Intensity Transducer Drive Circuits

1 Introduction

Many applications of piezo transducers require that the maximum acoustic pressure is developed from a given voltage supply, to maximise the power transfer. This implies that the drive face of the transducer vibrates with the maximum amplitude and operates under resonant conditions with high mechanical ‘Q’.

Therefore, the intention is to supply power at the resonant frequency of the transducer irrespective of any changes due to acoustic loading, caused mechanically or by temperature changes. The impedance seen by the power supply is thus minimised and the power absorbed maximised. The most direct route to design suitable drive circuits is to consider the equivalent electrical circuit for the transducer.

2 Equivalent Circuit

The equivalent circuit of a piezo transducer for operating frequencies around the resonant frequency, F_r, is usually considered as Fig 1.

If the transducer is acoustically loaded then R also represents an equivalent resistance in which acoustic power is dissipated in addition to the ‘mechanical’ resistance. The equivalent circuit values of fig 1 should ideally be inferred from direct measurements made on the transducer in the working environment, using an Impedance Analyser such as the Hewlett Packard HP4194A.

3 Transducer System Resonance

A typical resonance might appear as Fig 2.

Fig 2 Resonance Curve

The system is resonant at frequency, F_r, where the minimum impedance, Zmin, is practically resistive (real) and represents the sum of all dissipative loads. Generally, a good approximation for F_r is  

Due to the shunt effect of capacitance, C_b, an anti-resonance frequency, F_a, will also be found where the maximum impedance, Zmax occurs.

F_a approximates to

At frequencies away from these resonances, below F_r and above F_a, the system will appear capacitive to the driver and between F_r and F_a inductive.

As mentioned earlier, most acoustic transducers delivering high power (tens or even hundreds of watts) generally operate very close to F_r whenever possible, in order to minimise voltage drive levels. Should acoustic conditions change resulting in a variation of the resonant frequency, the power transfer will decrease as non-resonant operation ensues, unless the driver circuit compensates automatically.

4 Inductive Compensation

To reduce the current demand on the driver circuit due to possible large reactive currents in C_b, the accepted practise is to shunt C_b with inductance, L_s, to produce a second combination resonant at frequency F_r. The value of L_s is calculated from

The complete driver / transducer system thus appears as Fig 3.

Fig 3. The Compensated Equivalent Circuit

M - mechanical equivalent circuit
E - electrical compensating circuit
M and E - are resonant at Frequency Fr 
Rs - shunt resistance, inherent or added

5 Resonance of Compensated System

Typical impedance – frequency plots for the complete system are shown in fig 4.

Fig 4. Impedance Curves for Compensated System

Curves 1, 2 and 3 show effect of increasing R (mechanical acoustic loading)

Fig 4. clearly shows two anti-resonant frequencies, symmetrically placed around the centre resonance frequency, F_r, whose separation approximates to the useable bandwidth for the system.

As the acoustic loading is increased, effectively increasing R, the resonant impedance increases and the curve shape will become flat as curve 3. Under this load, the impedance remains practically constant within the bandwidth and allows for considerable latitude in the drive frequency. However, the impedance is higher at the centre frequency, F_r, than with lower loading in curves 1 and 2 and higher drive voltages would be necessary to deliver the same power.

In order to maximise power transfer at a given voltage, it is also necessary to ensure that the driver circuit output impedance is matched to the resonant impedance of the compensated transducer. To this end, shunt resistance, R_s, is sometimes added to the compensation circuit to optimise impedance matching.


6 Driver Considerations

The design of the driver is crucial to the successful operation of any resonant transducer system. The prime requirement is to supply electrical power at a well-controlled frequency thus minimising the voltages required to deliver a specified power.

Voltage can be supplied sinusoidally or via a square wave according to circuit design, and where voltage levels demand it, power may be supplied via an output transformer which can also provide a floating output if this is necessary.

Generally speaking, the driver should self-tune the frequency to match the transducer system. This is best achieved by arranging the equivalent circuit components to form the frequency determining element in the driver oscillator circuit.

Self-tuning drivers are essential when driving high intensity devices, such as welding converters and liquid pulverisers. These generally have very high ‘Q’ resonances, and operation at frequencies off F_r will result in a marked drop in delivered power under constant drive voltage conditions.

Transducers used in ultrasonic cleaning baths may be driven by circuits designed to sweep the frequency across F_r by a few percent, since generally there will be several transducers driven in parallel. Under this condition, the ‘centre’ frequency F_r is the group average and each individual transducer will be resonant at a frequency close to this. The driver is designed to modulate the frequency to cover the individual resonances, so that during the frequency sweep all will resonate with an average power delivered throughout the group.

Please note:-

The aforementioned application notes are provided as a guide for the considerations and requirements necessary when providing drive conditions to piezoelectric ceramics and transducers. Unfortunately we are unable to offer electronic design assistance or off-the-shelf driver units for applications. If further information or assistance is required relating to drive requirements we will be happy to put you in contact with suppliers and designers of driver electronics for various applications.

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