Underwater piezoelectric sensor array receive sensitivity

[nauman]

Hi all

I have some confusion about piezoelectrical sensors combination. If i have three acoustic piezoelectrical sensors (with same receive sensitivity in dB ref V/1uPa) placed at specific distance, these sensors receive acoustic signal from a sound source placed at far field distance (Plane Wave) and from broadside. I receive output of these sensors through individual preamplifiers, add them through hardware like summer circuit adder or in software after digitization and in this way got an 'Array Gain' due to addition of phase aligned sinusoidal signals. This array gain ultimately adds in receive sensitivity of individual sensor since signal is coming from broadside and an overall inrease in receive sensitivity of combine 'array of 03 sensors' occurs. Till now, things are straight forward.

Now my question is that if i hardwired + polarity terminalof all three piezo sensors together and -polarity terminals together, and connect them to a single preamplifier, will i get the same increase in receive sensitivity due to 'Array Gain' as in above case?

Thanks

 

[Baluncore]

... will i get the same increase in receive sensitivity due to 'Array Gain' as in above case?

No, you will not. The array pattern may be the same, but the magnitude of the main-lobe energy will be less.

A transducer is impedance matched to its preamplifier, to maximise power transfer and prevent retransmission.

If you wire three transducers in parallel, you will not be matching their impedance, and you will be retransmitting some of the transducer signal energy.

 

[nauman]

A transducer is impedance matched to its preamplifier, to maximise power transfer and prevent retransmission.

But i am talking about transducers in reception mode only in which i think these act as a voltage sources not power source!

 

[Baluncore]

But i am talking about transducers in reception mode only in which i think these act as a voltage sources not power source!

I believe the transducer is a bidirectional or reciprocal device that can work as a transmitter or a receiver. It is connected to the electronics with a transmission line. Voltage implies current, which implies power, which is the rate of energy transfer.

If the transmission line has a characteristic impedance of; Zo,
and there is a voltage; v, present on the matched impedance line,
then a current of; i = v / Zo will be flowing.
The power will then be; W = v * i .

If the transducers include a preamplifier, then the problem becomes one of connecting four transmission lines with a four way coupler, without introducing an impedance mismatch. The efficient way to do that is with a three input summing amplifier, driving one output line.

 

[DaveE]

It should work if you wire them in series.
I wouldn't worry too much about impedance matching since your sound waves will (I think) be low frequency compared to your wiring lengths. Use a high input impedance amplifier.

 

[sophiecentaur]

Now my question is that if i hardwired + polarity terminal of all three piezo sensors together and -polarity terminals together, and connect them to a single preamplifier, will i get the same increase in receive sensitivity due to 'Array Gain' as in above case?

Arrays of piezo sensors can be designed to have a sensitivity pattern in just the same way as an RF array. It's a matter of the impedances of the sources and the impedance of the amplifier. An appropriate "summer circuit" will add the currents from all the sensors if the sensors have a high source impedance and if the summing amplifier has a low impedance. That way the currents will add because sensors won't affect each other's signals. This is a general consideration when using electronics. If the sensors have identical resonances then the pattern of the array will be as expected. You'd need to look at the spec of the sensors (which brand / type are they?). Buffering is often the best way for predictable results of summing.

Piezoelectric sensors tend to be frequency sensitive and will have a resonance and antiresonance (this link among many others). If signal levels are a possible problem then an amplifier on each sensor could match the impedance.

 

[Baluncore]

If you think of the transducers as voltage sources, then you would connect the transducers in series. Seen as current sources, you would connect transducers in parallel, but they are neither, and they are both. They are impedance matched energy transducers, being connected by transmission lines that, if not matched and correctly terminated, will change the phase-shift and gain across the band of operation.

Managing the energy budget is critical to maintaining the gain of the array. Short, miss-matched transmission lines will look like lumped impedances, that will distort the phase of the transducer signals, causing the broadside array to squint, looking in different directions, at different in-band frequencies.

1. For the best signal levels and cleanest pattern, you can use a three-input inverting amplifier that terminates and isolates all input lines, by virtue of the "virtual earth" summing node. There is also an opportunity there, to amplify the combined signals by changing the amplifier feedback impedance.
2. The next best thing is a "hybrid" of resistive pads, that would combine the transmission lines with less interaction of the transducer impedance, but with significantly higher energy losses.
3. The worst you can do, is to connect the transducers directly together, as an internal echo chamber, making a local energy scatterer, without consideration of energy loss or impedance mismatch.

 

[DaveE]

I wonder why you care about a combination. Yes, one amp can work. But, as others have said, 3 are better. They aren't that fancy, just op-amps really. You'll have minimal extra cost/size and more flexibility to optimize things.

 

[nauman]

I wonder why you care about a combination. Yes, one amp can work. But, as others have said, 3 are better. They aren't that fancy, just op-amps really. You'll have minimal extra cost/size and more flexibility to optimize things.

Actually, i came across some underwater echo sounder transducers in which multiple piezo electrical transducers were hardwired in the way described above and only single connection was available for both transmission and reception.
In case of Transmission, it is obvious that each of these piezo transducers will draw current separately and will tranmit power acoustically as per its Transmit Voltage Response (TVR), contributing to overall acoustic power of combine array.
However, in case of Reception, voltage signals at the outputs of these piezo transducers will 'NOT' add up and Receive Sensitivity of combined array will be same as that of single piezo transducer. This is what i conclude after seeing the discussion above.

 

[DaveE]

I guess you could try summing the currents from the parallel pzts into a low input impedance amp. But pzts are high impedance devices, I don't think you'll get much current signal to work with.

 

[tech99]

I would endorse the reply #5 from DaveE. Piezo devices have very high resistance but some capacitance. There is no disadvantage in wiring them is series and working into a high impedance amplifier. But be careful about cable capacitance, as this will form a potential divider with the piezo capacitance and reduce the signal. For this reason, you may need the amplifier to be close to the transducers. It might be the reason for having three separate head amplifiers.
Considering now the noise performance of either direct series connection or using pre amplifiers. If you use three amplifiers with outputs in parallel, then you will have three times the current and three times the noise power. As the current is now x3, that is 9 times the signal power, but the noise power will be x3, (as it is incoherent). So the S/N will improve 3 times for both cases.

 

[sophiecentaur]

It should work if you wire them in series.

That would work in some circumstances but induced volts (or currents) from interfering sources could upset that idea. The normal form of connecting signals is an 'unbalanced' line with signal and earth wires. That would not be possible in your suggestion.

But we really don't know enough about the detail of this setup. What frequency is involved and what would be the spacings between the elements.

Interactions between nearby elements can affect the resulting radiation pattern but that can be dealt with with so called active elements that use buffering amplifiers.

 

[sophiecentaur]

these sensors receive acoustic signal from a sound source placed at far field distance (Plane Wave) and from broadside.

I went back to this, from the original post and I would like to know how directivity can be achieved - or if it is relevant here. What frequency would you be operating at? It may be relevant. Equal lengths of feeder from the elements will produce in-phase result for broadside conditions - OK. But what spacing are you planning? Do you want any particular directivity pattern? (Rejecting off-boresight signals) There is a list to how close the sensors can be placed but they could be mounted pretty close together if their mutual effect (mutual impedance) is small.

It may be worth while thinking in terms of first principles before choosing any design. You use the term "array" so you are assuming what that term involves. Frequency and geometry can be important. The surroundings / mounting arrangement can be relevant because reflections from elsewhere can add to or subtract from the wanted signal.

 

[DaveE]

i came across some underwater echo sounder transducers in which multiple piezo electrical transducers were hardwired in the way described above and only single connection was available for both transmission and reception.

Other engineer's designs aren't always the best. They may have been ignorant, or more likely they had constraints that aren't applicable in your case. It makes sense to investigate a bit, but at some point you need to choose for yourself.

I've done pieces of designs that looked kind of stupid. But if you asked me to explain it it would make more sense. Like "It is stupid, but I had no other choice", or "What about this requirement/feature that you haven't been told about yet".