Improvements on Fishfinder Sidescan Transducers

[René Pascal]

This may be of interest for all of you who want to build or run a Fishfinder Sidescan Sonar.

Perhaps one year ago I saw some pages dealing with this technique, especially the page by Dan Fountain and this discussion forum. Since my original field of interest is optics and astronomy I have some experience on wave optics, and sonar transducers are wave-optical components. I realized that it could be an improvement on the row of fishfinder transducer, if you don`t mount them uniformly spaced as up to now all home-builders do. This helps to suppress unwanted sidelobes, that are the main problem in this setup with individual, more or less widely spaced transducers. To find an optimal configuration I ran some diffraction-simulations, and I was really astonished what a large improvement can be made. You can find the results on my homepage:



http://www.beugungsbild.de/sidescan/sidescan.html

[GBurton]

Re: Improvements on Fishfinder Sidescan Transducers

Hi,
I build my own sidescan transducers (see the results in postings below), and they are indeed used in a modified 'fish finder' type of echo sounder unit. Your plot is good but assumes that the entire active surfaces of the individual elements are moving uniformly. The bars may or may not exhibit other modes of oscillation which could degrade the beam pattern. For several reasons, the reality of it is that you will see much higher sidelobes with that arrangement than you think. I cut my own ceramic with diamond tooling on a modified grinding machine. If you wish to build your own transducers, expect to pay a premium for cut ceramic,(and 50 KHZ 'ducers will take a LOT of ceramic) which is why I don't have others cut it for me. I have produced 200KHZ transducers for modest operating depth that have worked excellent for me, and I'm in the process of trying to bring some economical sidescans to market. Hope to have one of them in semi 'kit' form for under $2500.00 WITH gps.
Regards

[Charles, Oak Island,NC]

Re: Improvements on Fishfinder Sidescan Transducers

How about without GPS. I already run two other items on the boat with them ??

[GBurton]

Re: Improvements on Fishfinder Sidescan Transducers

Hi,
The easy way is to just use the GPS that's made for it. Very likely there's a way to bring your existing GPS data in, but I've got to investigate that further. It would involve wiring jacks and stuff, making interfacing cables. It would not take much off the price. That base version semi-kit sidescan would involve you making your own towfish out of PVC or whatever suitable material, then you would mount the transducer to that. A lot of people here would like to make a sidescan, so just having to make a towfish assembly should be a piece of cake.
Regards

[Charles, Oak Island,NC]

Re: Improvements on Fishfinder Sidescan Transducers

I did Dan's SSS it works ok for what it is am ready for the next step, so lets get it in gear and good luck

[Jim Kennard]

Re: Improvements on Fishfinder Sidescan Transducers

Charles, Do you have any images that you can post to show us the capability of the Dan SSS fish finder transducer array?



Click Here

[Jim Kennard]

Re: Improvements on Fishfinder Sidescan Transducers

Rene, Excellent work, the link to your site provides some of the best analysis work that I have seen presented on this forum in a very long while. Congratulations, Jim



Click Here

[Charles, Oak Island,NC]

Re: Improvements on Fishfinder Sidescan Transducers

Sorry but I used a Liquid crystal depth finder last year. I will be using the X16 paper graph on it starting the 1st of March. (possible sooner depending on the weather and holidays)

I used it in depths of 30-40 feet sea water and was able to see the structure of a vessel we were searching for. It indicated bottom structure of about 5 feet. When dove it was in the 4 to 6 ft range in hight.

[Jim Kennard]

Re: Improvements on Fishfinder Sidescan Transducers

Congratulations on your success in building the transducer and obtaining a sonar image of the shipwreck.

What was the range of detection that you were able to achieve with your Dan transducer arrangement?

At what range were you first able to recognize the structure of the shipwreck?

How large was this wreck?

[Charles, Oak Island,NC]

Re: Improvements on Fishfinder Sidescan Transducers

Jim I'll have to guess because we made a sweep on either side of it . We were in about 35 to 40 ft of water with 2 to 3 ft swells. I would say we picked it up at about 60 to 70 ft from the fish which was only about 3 ft under water. Remembering we were working in rather shallow water. With a lot of the wreck under sand I would guess we were seeing about 15 ft of it in length.

[René Pascal]

Re: Improvements on Fishfinder Sidescan Transducers

Thank you for the compliment on my pages. Within the next time I will simulate transducer rows with 6 elements and try to find the best configuration. I will publish the results on my page. I hope that I will finish a sytem on the basis of a fishfinder next year.

[René Pascal]

Re: Improvements on Fishfinder Sidescan Transducers




Yes, that's right, if the transducers are not in phase or have different modes of oscillation the pattern may look very much different. For that reason I simulated the transducers only with a dynamic of 25dB, not with 50 dB, as many diagrams are shown (an exception is the section "can we trust the results"). What lies below -25dB is dominated by such side effects. Luckily "in phase" in this case means that the perhaps10 to 50 oscillations within a single pulse are in phase, so that differences in the resonance frequency should be lower than about 3 percent estimated.

I have tried to make a sketch what happens if the transducers are only partially in phase, its simply a drawing, not a simulation.
A is the pattern of a single transducer of the row. B would be the pattern of this row, if all transducers are firing, but are completely incoherent (not in phase, they all have different frequencies). The pattern is the same as for the single transducer (when viewed from far away from the transducer), with the only difference that you can pass more energy through it. C is the pattern for all crystals in phase, as desired for a real system, and as assumed by all of my simulations. D is for a transducer when the elements are only partially in phase, it is an intermediate state of the coherent case C and the completely incoherent case B. This transducer would have a very bad signal-to-noise ratio, because the broad lobe would introduce a high diffuse background. But real sonar systems are rather case C than D, as can be seen in measured beampattern.

[Pavel]

Re: Improvements on Fishfinder Sidescan Transducers

Hi, Rene
Your work is impressing,
Can you simulate pattern for my current built
X-ducers, which is two:
row of 8 5X5X50 mm spaced 0.8 mm 500 KHZ
row of 12 14X14X50 mm spaced 2 mm 150 KHZ
Would be nice to see it before job is finished.

I will wait with epoxy untill you answer.
Have you think about using matlab for simulation?
BRGDS,
Pavel