Using a piezoelectric transducer as a sonar

[wrongbat]

Hi there!

I'm starting to work on my first real electronics project, which is to build my own ultrasonic range finder. I've figured out that I need to use piezoelectric transducers, and I think that I more or less understand the basic principles from reading here.

Sensing pulses:
The transducer has a piece of neutrally charged but polarized material which becomes polarized when it's mechanically distorted. The polarizations can be measured as a voltage, and so the transducer can be used to sense incoming sound waves, as the waves distort the material slightly.

Transmitting pulses:
When a voltage is applied over the terminals of the polarized material, the molecules will move to align themselves with the electrical field, and the material will mechanically distort. By reversing the direction of the electrical field the material will distort in the opposite direction, and you can thereby generate a sound wave pulse train.

Would you agree on that simple explanation?

So my question is: Is it possible to use a single piezoelectric element to both generate mechanical vibration (i.e., the ultrasonic wave) and sense waves that have been reflected from nearby surfaces?

Thank you for helping me understand this basic fact .

-Halfdan

 

[ericgibbs]

Yes it is possible to use the sensor as a transceiver.
E

 

[Colin]

The element takes so long to stop vibrating that you cannot detect the received signal.
In addition it is not sensitive to 40kHz.
You can use a single ultrasonic unit but the circuit works on a different principle. It works on interference of the receiver signal. The 40kHz device uses a crystal that produces a voltage when compressed.

 

[cowboybob]

https://link.springer.com/chapter/10.1007/978-0-387-76540-2_11

 

[audioguru]

You cannot use any old piezo transducer for ultrasonics because many resonate at audio frequencies and do nothing at ultrasonic frequencies.
Almost all ultrasonic range finders use a matched pair of ultrasonic transducers, a transmitter and a receiver.

 

[wrongbat]

Yes it is possible to use the sensor as a transceiver.
E

Thank you for your reply! Ok, so I can use something like this sensor to send out a sonar sound wave by applying a pulse train over the terminals?

 

[wrongbat]

The element takes so long to stop vibrating that you cannot detect the received signal.
In addition it is not sensitive to 40kHz.
You can use a single ultrasonic unit but the circuit works on a different principle. It works on interference of the receiver signal. The 40kHz device uses a crystal that produces a voltage when compressed.

Thank you too for your reply! The time to stop vibrating that you mention, would that be the decay time? Most of the units that I'm looking at have a decay time or around <1.3ms. So does this mean that it takes up to 1.3ms for the unit to be able to react to an incoming pulse after it's been used to transmit a pulse train?

 

[wrongbat]

You cannot use any old piezo transducer for ultrasonics because many resonate at audio frequencies and do nothing at ultrasonic frequencies.
Almost all ultrasonic range finders use a matched pair of ultrasonic transducers, a transmitter and a receiver.

Again, thank you for your reply. Ahh, so the voltage over the terminals of the piezoelectric transducer depends on the frequency

 

[audioguru]

Again, thank you for your reply. Ahh, so the voltage over the terminals of the piezoelectric transducer depends on the frequency

An ordinary audio piezo transducer resonates at about 4kHz where it is very sensitive as a transmitter or receiver. Because its resonant frequency has a high Q then it has a long decay time (it rings like a bell).
you need special transmitter and receiver 40kHz piezo transducers for a range finder. Some are in kits.

Here are frequency response curves for an ordinary audio piezo transducer and for a pair of 40kHz ultrasonic ones:

 

[cowboybob]

Thank you too for your reply! The time to stop vibrating that you mention, would that be the decay time? Most of the units that I'm looking at have a decay time or around <1.3ms. So does this mean that it takes up to 1.3ms for the unit to be able to react to an incoming pulse after it's been used to transmit a pulse train?

Yes.

Again, thank you for your reply. Ahh, so the voltage over the terminals of the piezoelectric transducer depends on the frequency

A piezo transducer element has a resonant frequency (https://www.dictionaryofengineering.com/definition/resonant-frequency.html), i.e., a level of vibration that is most vigorous at a specific frequency, whether induced by an electric current or an external mechanical vibration.

As to your question, if used as a receiver, the piezo element will produce its maximum voltage level output if exposed to its resonant frequency, with decreasing output as the frequency deviates (up or down) from that resonance value. Same goes for the element when used as a transmitter, except the output is a mechanical vibration that is the result of an electrical input to the piezo element.

 

[wrongbat]

An ordinary audio piezo transducer resonates at about 4kHz where it is very sensitive as a transmitter or receiver. Because its resonant frequency has a high Q then it has a long decay time (it rings like a bell).
you need special transmitter and receiver 40kHz piezo transducers for a range finder. Some are in kits.

Here are frequency response curves for an ordinary audio piezo transducer and for a pair of 40kHz ultrasonic ones:

Ahh, thank you! This has really helped clarify things for me. Now I understand why surrounding sounds not in the 40KHz range will not interfere with the operation of the circuit. Thank you!

 

[wrongbat]

Yes.

A piezo transducer element has a resonant frequency (https://www.dictionaryofengineering.com/definition/resonant-frequency.html), i.e., a level of vibration that is most vigorous at a specific frequency, whether induced by an electric current or an external mechanical vibration.

As to your question, if used as a receiver, the piezo element will produce its maximum voltage level output if exposed to its resonant frequency, with decreasing output as the frequency deviates (up or down) from that resonance value. Same goes for the element when used as a transmitter, except the output is a mechanical vibration that is the result of an electrical input to the piezo element.

Thank you for your explanation! Now I'm more confident that I'll be able to build a range finder using just a single transducer, as long as I accept the fact that it will not be able to sense objects that are very close (because of the decay time). Thank you!!

 

[wrongbat]

On a side node I think that my newbie mistake was starting out looking at very cheap components, but at the same time very badly documented ones which increased my confusion. Next time I will start with a standard components which is well documented and then switch to the cheap component later when I understand how the component works.

I guess the main trick for me was to realize that even though they are many many different piezoelectric transducers with a variety of specs, the underlying principle of operation is more or less the same. So the circuit for a normal component can more or less be assumed to work with more rare components of the same type. I’m probably generalizing too much, but on the other hand my previous understanding of “every components needs a completely different design” only helped to confuse me. So thank you all for your help!