Functional fishfinder sidescan

[Rickard]

Hello everybody!

For some days I have tested a functional Dan Fountain style fishfinder sidescan sonar array and it is time to report. The array has four transducers. The transducers are unevenly spaced following an idea from René Pascal. Optimization of spacings are based on wave theory and implemented in Graphing Calculator for Windows. The heart of the system is a Humminbird Matrix 37. The transducers have four beams. 83 and 200 KHz, created with a dual frequency piezo, and two 455 KHz beams intended for "sidescanning" of fishes. The array is optimized for 83 and 200 KHz. The array is mounted on a metal pipe and the pipe is firmly attached to the boat side. The individual transducer cables are connected to a box on the boat. Thus the transducers are intact - no cutting of cables etc.

I thought success was most probable with 83 KHz but the result was terribly coarse and I have only used 200 KHz since. The vertical beam width is only about 20 degrees but that proved to be a smaller problem than expected. It's easy to adjust downtilt angle by twisting the array. Most scanning was done with an angle of only 15 degrees. In my shallow waters this proved ideal when having the array attached to the boat. The system seem usable down to about 30 meters and can scan up to about 150 meters. When trying to scan at larger depths and distances various stability problems appear. (The instrument is confused about depth causing varying scroll speed and ping rate).

As the instrument can't store or export data I have to take photos of frozen screens. Tests are done in Sweden, on lake Mälaren, close to the city of Uppsala. The wreck photo shows a small boat, depth 9 meters. Note the wavy pattern caused by the rolling boat but note how little this effects the picture. The photo of wave formed gravel bottom is typical for the lake (a surprise for me). Depth 20 meters. The sunken concrete ponton rests at 8 meters just outside the marina.

I think the system works just fine. I have already spent several concentrated hours on water and must say it's getting funnier the more I learn about it!


Rickard Nilsson

[Glennk]

As the instrument can't store or export data I have to take photos of frozen screens. Tests are done in Sweden, on lake Mälaren, close to the city of Uppsala. The wreck photo shows a small boat, depth 9 meters. Note the wavy pattern caused by the rolling boat but note how little this effects the picture. The photo of wave formed gravel bottom is typical for the lake (a surprise for me). Depth 20 meters. The sunken concrete ponton rests at 8 meters just outside the marina.

I think the system works just fine. I have already spent several concentrated hours on water and must say it's getting funnier the more I learn about it!


Rickard Nilsson

I have seen sidescans saved to a SD card on a Lowrance echosounder/chartplotter, and the saved data are much more clear and readable than the data presented on the Lowrance's screen. Maybe you should try to save some scans sometime, and see if the saved data are better viewed when played back on a PC?

I live in Norway, and I just came back from a hollyday trip to Älgsjön, and thats not to far away from Mälaren.

Cheers,
Glenn

[Rickard]

Hi Glenn!

Yes, it’s a pity my instrument can’t store data but I didn’t have sidescanning in mind when I bought the unit two years ago. I don’t think any Humminbird could save data at that time. Recent models have that option though.

Of course the photos presented here have much lower quality than the pictures presented on the units display. The display is excellent with a vertical resolution of 640 pixels. When documenting the display I have to take several photos of the frozen screen and use flash to reduce glare. The display makes some kind of fuzz as there seems to be a sweep interfering with exposure time causing shaded areas in the photos. I will work out a solution on this. Or buy another unit…

Perhaps the sonograms don’t impress on anyone but I think they are surprisingly good considering the theoretical preconditions. My simulations showed that an array made of circular piezos separated by more than a quarter of a wavelength (a few millimetres) will, at best, have a sidebeam suppression of only a few dB. In comparison, a professional array with rectangular piezos can have a suppression of 13 dB. Many times more than my array. Thus, I was prepared for a total failure However, curiosity, the possibility of miscalculations and the hope for an ability of the human eye to discriminate structures on a fuzzy display made me try after all. I certainly don’t regret it!

Perhaps I will have an opportunity to scan on lake Älgsjön some day?

Regards,
Rickard

[Glennk]

How are the transducers interfaced with the Humminbird unit? Maybe you could try to do a scan with my Lowranve connected some time?
I probably will go back to Älgsjön later this summer, and maybe we could meet some where and try it out?
Still, I think the pictures you had where suprisingly good, to be from an "fishfinder" unit. This proves that there really are ways to get a "home buildt" sidescan to work as intended.

Glenn

[Rickard]

The transducers are connected with their original connectors to a box. The box also has a male connector for the cable to the instrument. The box is my own creation (it took more effort than building the array!) and allows for use of original Humminbird seven pin cable connectors (only five pins are in use). All that is needed for interfacing a Lowrance is a cable with Humminbird female features in one end and something that connects to a Lowrance in the other. Pins in the Humminbird connector are 1 mm thick. I built the connectors in the box using parts from a computer printer cable. I have several left so the problem can have a provisional solution with single cables (I guess two are enought) from pin to pin. If we are lucky there might appear an opportunity to test this this summer.

(Today I found a small wreck, about 14 metres long at a depth of 12 metres. It is well hidden in a slope. The highest point above the surrending bottom is only about 50 cm but my array did it!)

Rickard

[Glennk]

Are the transducers connected in series or in paralell? Or do you need some sort of interface betwen the 4 transducers and the Humminbird to "fool" the echosounder to think there are only one transducer connected?

And have you interfaced the Humminbird with an GPS?

Glenn

[Rickard]

The transducers are connected in paralell inside the connector box. So, the Humminbird sees only one transducer. For temperature, only one of the transducers is used. The paralell connection will decrease impedance and it could have caused the instrument not recognizing the signal. Dan Fountain used a combination of paralell/serial to keep impedance at the level of a single transducer but I followed his advice to ignore the issue of impedance as the impedance is very high. Also, serial connection might introduce phase shifts between the transducers.

It's possible to connect a GPS but I haven't done this since the cable was very expensive (they have an irritating belief in high prices). For keeping track of my scanning I use a Garmin GPSmap 176c chartplotter. When I detect something I press the "mark position" button. Thereafter I can sweep around the spot until I have a good view of what's down there. The plotter affords tremendeous support showing track, current position, marked spot and a distance scale. This information in combination with depth/distance data from the Humminbird makes it possible to determine position of things down there. When I'm sure about the position I use to try with a "zoomed" run by manually setting a narrow depth interval on the Humminbird and pass the spot at a precise distance to get a larger picture of the thing down there. Much work involved though, but fun.

Rickard

[Rickard]

Glenn,
Refering to your post elsewhere, good luck with the connectors and the welding! I understand you must be very impatient right now. I think there must be a way to connect my array to your equipment, or vize versa. The ports have probably the same pin dimensions so a simple pin to pin arrangement using standard connector parts will work.
As for pictures... yes I have too many! Most of them junk but I can present some pictures from scannings of the 14 meter wreck I found the other day. They are not selected for promoting the system. Then I would have kept them as secrets. I think they tell something about what it's really like when I try to detect difficult objects down there. The wreck doesn't look like a wreck but somehow it's obious that it's a wreck anyway (or a rock?). Depth is 18 metres. The "thing" rests in a slope causing difficulties when scanning.

Rickard

[Rickard]

I'm adding some more pictures. First shows a profile from a pass over the 14 m wreck using one transducer at 200 KHz. Direction was paralell with the shore so the slope at the site is not visibel. Next photo shows a very small wreck of a sailing boat that people say went down with sails up. The wreck is 5.5 metres long. Depth 20 metres. Below the wreck there is a cable! Third photo shows part of the "concrete ponton area" outside the marina. One can see parts of the blocks and in the lower part a mysterious structure wich I think is the bearing structure of a submerged bridge. To the left and under that mystery there are some rows of old poles. Conditions at the site are harsh for scanning with muddy water, boats going in and out causing waves in all directions and bubbles in the water. A faire wind was pushing from open "sea" making my boat jumping around. I suspect there is a fleet of several pontons at the bottom.

Rickard

[Glennk]

I'm adding some more pictures. First shows a profile from a pass over the 14 m wreck using one transducer at 200 KHz. Direction was paralell with the shore so the slope at the site is not visibel. Next photo shows a very small wreck of a sailing boat that people say went down with sails up. The wreck is 5.5 metres long. Depth 20 metres. Below the wreck there is a cable! Third photo shows part of the "concrete ponton area" outside the marina. One can see parts of the blocks and in the lower part a mysterious structure wich I think is the bearing structure of a submerged bridge. To the left and under that mystery there are some rows of old poles. Conditions at the site are harsh for scanning with muddy water, boats going in and out causing waves in all directions and bubbles in the water. A faire wind was pushing from open "sea" making my boat jumping around. I suspect there is a fleet of several pontons at the bottom.

Rickard

Clearly there are something down there! No doubt that your sidescan is working well. I think many of the "problems" caused by movements in the vessel will be eliminated if you made a towfish for the sidescan.
You could easily make one out of PVC tubing and a few flat PVC boards.
Se pictures of my M1 sidescan in other thread. This system is made by tubing and should be just fine for installing an array of transducers, just like yours.

Glenn

[Rickard]

I will try making i towfish later when I have finnished developing the array itself. Therer are still some tuning to be done. For exemple, last nights simulations indicated a slight improvement of sidelobe suppression if I extend the length of the array about 20 mm. But then I have to buy a longer tube to begin with (4 dollars). There are some critical tests to be done like spacing the transducers evenly, trying with 3, 2 and only one transducer in the array to see how these conditions differ from the optimized array. Perhaps the results will be the same independent of how the array is configured?

I will arrange for the option to mount the coming towfish to the boat as my experience shows that this could be very convenient in shallow water. The sidescanning transducer for the Humminbird 900 series is mounted to the hull and the results seem very good. I know what it's like to control towed lures when trolling - you really don't know what happens down there. When the array is attached to the boat you don't have to bother about speed and radius of curves when turning. And the result could still be accaptable - provided relatively calm weather.

Rickard

PS Nice transportation box! DS

[Rickard]

What's this??

I "stumbled" over this structure. Depth 30 m. Clay bottom. Long range. 5 knots, medium scroll speed. Ping rate at its lowest, maybe little more than two per second. Left window shows the 83 KHz echo and the right 200 KHz.

Found this when trying out a new method for taking photos. Photos could still be better but I'm satisfied with the performance of the fishfinder array.

Rickard

[Glennk]

Looks like you found you a new wreck?
A bit difficult to tell, but it might be a wreck lying 90degre on its side, showing the bow.
If you have the positions, you can scan the same area from different sides, maybe getting more perspective of the object.

Glenn

[Glennk]

There is another thing about fishfinder sidescan that I have som wondering about.
When I used the Sportscan 330kHz when my 200kHz echosounder was turned on, the image on the Sportscan never showed up as clear as it did when the echosounder was turned off......
My thoughts was that maybe your sidescan would perform better if you used only one of the frequencies? I assume that all the "noise" in the water from the transducers could interference with each other.

Glenn

[Rickard]

I want to go back and investigate yesterdays wreck but it's much too windy!:mad:

It's not possible to turn off the frequencies individually. They are emitted by the same dual frequency piezo. I guess the Humminbird people have designed their system to eliminate interferences in the system itself. Somewhere in the manual there is some talk about a specific function for filtering out sound from echosounders in neighboring boats also. I learned from a manufacturer of echosounders that most noise comes from the vessel itself and various electrical installations. As for example another echosounder, the engine, etc. I can see changes in the display when running the engine at different RPM:s. Filtering out irrelevant sound frequencies was a minor problem someone said.

I think the most important source for noise in my system, apart from the sources inherent in the array design, is surface clutter. The array is just two feet under water, the main beam directed almost paralell with the surface (yes, it happens that the array is up in the air!). Thus, the sound has to travel a long distance before escaping under the clutter zone. Under wavy conditions and when many other vessels have stirred up the water there is a lot of this clutter. It also happens that waves interfer.

Rickard