Ultrasonic Transducers

[Charles Creations]

So I am working on an ultrasonic range finder, and I came across some transducers on Mouser made by Kobitone Audio Company. First of all, I could not find any website for such a company, so I was wondering if anyone has used any of their products before or knows of them.

Secondly, many of the ultrasonic transducers are labeled as specifically being transmitters or receivers. For instance, the 255-400SR16-ROX and 255-400PT160-ROX which can be both found on Mouser. The R16 denotes receiver, and the T16 denotes transmitter. In my application, I would like to use the same transducer for sending and receiving. I have only found a couple that can do this like the Murata MA40H1S-R. However, this is a very new part and is proving difficult to track down. Any other ultrasonic transducers I should look at that are easily found?

One questions is what is physically different within a ultrasonic transducer that is optimized for sending or receiving? From my understanding, the transducer is essentially a crystal sandwiched between two plate electrodes that you then input or output from. What is the result of driving a receiver to emit a signal? Will it eventually destroy it?

Your help is greatly appreciated.

[Niklas]

We are using a piezo as both transmitter and receiver to measure properties in liquid. The frequency is typically around 1 MHz instead of the lower frequencies that are usually used for distance measurement in air. There is also a large difference in signal levels where the excitation is measured in Volts and the echos in millivolts. The mechanical structure is also important as that can store oscillating energy long enough to drown you echo.

[coppice]

We are using a piezo as both transmitter and receiver to measure properties in liquid. The frequency is typically around 1 MHz instead of the lower frequencies that are usually used for distance measurement in air. There is also a large difference in signal levels where the excitation is measured in Volts and the echos in millivolts. The mechanical structure is also important as that can store oscillating energy long enough to drown you echo.

What is the distance over which your 1MHz signal travels? In ultrasonic water flow measurement, with a path of maybe 200mm, the receive transducer typically see about half the signal amplitude that was transmitted. In gas measurement you get a pretty weak output, especially if your sensors lack mechanical impedance matching between the piezo element and the gas, but most liquids are pretty low loss at 1MHz. At the sort of frequencies used for ultrasonic microscopes (1GHz or more) water is phenomenally lossy, but at 1MHz it carries acoustic energy very well.

[awallin]

If you want to build your own transducer you can get the bare piezo-element from e.g. here:
http://www.ferroperm-piezo.com/
they may not have hobby-friendly prices, but you might find the same thing from china or ebay?

You would need to solder a GND-wire to one side and the signal wire to the other side of the piezo. There is a critical temperature which must not be exceeded or otherwise the piezoelectric effect is lost from the material.

To make a transducer the back side is usually damped with some 'heavy' lossy material e.g. epoxy mixed with iron or lead powder.
The front side would need to be isolated if you want to use it in water. Two-part silicone between PZT and water would probably work. The ideal impedance matching calls for an acoustic impedance at the geometric mean of PZT and water IIRC.

This thing will resonate at a frequency that is related to the thickness of the PZT transducer. Thinner transducers give higher resonance frequencies and vice versa. If you want to send a short pulse you need to have good damping, i.e. wide bandwidth.

The same transducer will work for both Tx and Rx, or pulse-echo with just one transducer. The pulsers that drive these things usually have an analog switch that switches out the sensitive Rx amplifier while a "main bang" Tx circuit drives a negative-swing spike (100V or so) to the transducer. Once the tx pulse is gone the rx amplifier is switched in again.

[Charles Creations]

Right now, I have stripped the transducers off of a Parallax Ping sensor and am driving them with a LC resonant circuit and a N-Channel MOSFET at roughly 40Khz (I have can control it with a pot between 38KHz and 42KHz to find the center frequency). I was thinking of using Zeners to clamp the voltage so the amplifier stages will not be damaged. I have measured by LC circuit at 80Vpp with a 12V supply.

I have attached the LC drive voltage. The purple waveform is a subtraction between the high and low side of the transducer. Also, I attached a rough sketch of my circuit.

At the moment, I am using a separate RX transducer and getting good results, but to reduce part count and size eventually I would like to implement the circuit I have attached so I can send and receive with the same traducer.

I like this idea of switching out the amplifier stages. How do you think my Zener clamping technique will fair for the op amp? Is it enough protection?

From my testing in air, I get about 1/100th of the signal to return, but this quickly falls of with distance as well. My current amplifier is setup for about 10,000 times gain before I feed it through a comparator and into an arduino. I am getting good data with this setup.

[Conrad Hoffman]

Maybe a couple different avenues to try- search for transceivers. Also look up the Polaroid ultrasonic rangefinder. I think they used a single unit in their design.

[Niklas]

We are using a piezo as both transmitter and receiver to measure properties in liquid. The frequency is typically around 1 MHz instead of the lower frequencies that are usually used for distance measurement in air. There is also a large difference in signal levels where the excitation is measured in Volts and the echos in millivolts. The mechanical structure is also important as that can store oscillating energy long enough to drown you echo.

What is the distance over which your 1MHz signal travels? In ultrasonic water flow measurement, with a path of maybe 200mm, the receive transducer typically see about half the signal amplitude that was transmitted. In gas measurement you get a pretty weak output, especially if your sensors lack mechanical impedance matching between the piezo element and the gas, but most liquids are pretty low loss at 1MHz. At the sort of frequencies used for ultrasonic microscopes (1GHz or more) water is phenomenally lossy, but at 1MHz it carries acoustic energy very well.

The distance to the fixed reflector is approx. 6 cm. Due to the environmental conditions, the piezo is enclosed in a thin metal cup made of stainless steel. The piezo itself is not only sensitive to one frequency. The excitation pulses are sent in short bursts and the echo contains both the 1 MHz and the 1/(2*burst period).
The application is automotive where a widespread temperature range, possible misfillings and bubbles are among the major issues to cope with.

[Marco]

One questions is what is physically different within a ultrasonic transducer that is optimized for sending or receiving? From my understanding, the transducer is essentially a crystal sandwiched between two plate electrodes that you then input or output from. What is the result of driving a receiver to emit a signal? Will it eventually destroy it?

Transmitters/transceivers might have thicker electrodes, because they take more current. So if you want to experiment with a device beyond it's purpose I'd experiment with the transmitters. That said, have you tried searching for ultrasonic transceivers? I see quite a few when I search for that. With the first page having readily available cheap options and higher end looking more compact options.

PS. the zeners respond in nanoseconds, so yeah the opamp should be fine. If the transducer keeps resonating too long you could try shorting it for a bit at the end of the pulse train with a second FET.