Building the 3 MHz ultrasonic transducer

A first 3 MHz transducer prototype is ready for testing. A vintage Airmar ST200 speed transducer housing has been modified to receive the piezoelectric element.

WARNING: this particular transducer design is not intended to be installed on a real boat for safety reasons. The threads between the compression ring and the housing may be stripped following an underwater impact, causing massive ingress of water. This design is intended to be used only for development purposes on special floating devices.

A future improvement will be to modify the form of the yellow urethane matching layer so as to get rid of the recess. This first prototype will be used mainly to test the electronics outside of a real boat, so that the safety epoxy plug will not be cast for the moment, in order to keep full access to the transducer.

I was fortunate to get the help of a competent machine shop owner to achieve this. Here are some pictures.

Designing an ultrasonic transducer

Here is the conceptual design of the 3-MHz ultrasonic transducer. Apart from the piezoelectric element, the 2 important design parameters are the nature of the backing and matching layers.

Air is a good absorber of high-frequency ultrasounds, and has been chosen as backing layer. The matching layer will be a sheet of urethane (40 A Durometer), available from this source.

The first step has been to characterize the piezoelectric element. Two wires have been soldered to the piezo’s silver platings.

The following arrangement has been used to measure the resonance and antiresonance frequencies of the piezo disc. The frequency generator is first adjusted to give a maximum voltage across the resistor: this is the resonant frequency, where the piezo impedance is at its minimum. The frequency is then increased until a minimum voltage is measured: this is the antiresonance frequency, where the impedance is at its maximum.

The results of these measurements are as follows:

            Resonance frequency:           2,932,000 Hz
            Antiresonance frequency:     3,345,000 Hz

The resonance frequency is where the piezo element converts with the highest efficiency the electrical energy into mechanical energy. This will be the design frequency of the transducer. Note that these frequencies are for the unloaded piezo in free air. When the transducer will be completed, the resonance frequency will be measured again with the transducer in the water.

The powerful (and free) BioSono KLM software has been used to model the transducer performance. From the physical description of the transducer components, this software will calculate a whole range of outputs, including the resonance and antiresonance frequencies and impedances.

The model is in very good agreement with the measured frequencies, and calculates a resonance impedance of 0.37 ohm. This is in line with the piezo manufacturer figure of less than 1 ohm.

The design goal is to transmit a peak power of 12 W to the transducer. This means that during the pulses, a high-frequency AC current of 5.7 amperes will flow through the transducer. Usually, an impedance matching circuit would be placed inside the transducer housing so that the amplifier will see an impedance of 50 ohms with a reduced current ouput. The BioSono KLM software can be used to calculate the required impedance matching circuit.

However, for this project, the decision has been made to use a high-current Class-A amplifier with an output impedance matching the piezo’s impedance of 0.37 ohm.

There are 2 main advantages for this unusual design:

a)  The ouput current to the piezo will be a pure sinewave instead of the typical square wave used in continuously transmitting ultrasonic applications;

b) The amplifier will use a standard marine power supply voltage of 12 V.  With a higher impedance, a higher voltage power supply would be needed.

Class-A amplifiers are well known for their high-fidelity but poor efficiency (as low as 15%). In this application, this efficiency penalty is not significant, because the length of the pulses will be very short. Even with an instantaneous transmitting power of 12 W, the average consumed power will much less than 1 watt.