ROV SONAR

[rossrov]

Far out - enjoyed the webpage. Some info on the ebay page too. I've been looking up SONAR stuff using different keywords in Google. Found alot of info but not come across that site before. Thanks.

That array in the fish on the website is oriented the "right" way for what I am doing/want to do. Will produce a "beam" that is narrow in azimuth but wider in elevation. If you spun the fish around so the head pointed north, east, south, west, and show the echoes as an azimuth-time plot on a polar display it would produce an image similar to the one below.

[a_shorething]

I WANT THAT.

[kenl]

There was a guy called Per Pelin that wrote a program called Dr Depth, he has since sold out to Hummingbird. But he was making similar scans to the one show using a rotating Lowrance side scan transducer, the purpose was for Ice fishing in his case.

Unfortunately since he sold out to Hummingbird his DIY instructions have been taken down. I think he called it a fishing Radar?

[rossrov]

a_shorething: Yeah, I like that image, especially the shadows. The scanner or array needs to be on a stable-ish platform. Looking at some fishing forums I read comments noting calm conditions. With an ROV though, down below the influence of waves, I think the only thing to be concerned about is maintaining a steady heading. If the beam is narrow and the ROV yaws, some echoes will be missed. A wider beam will be more forgiving, though of course less detail. Initially I only wanted to identify navigational reference points such as pilings, and maybe get a bounce off a mooring chain or mooring block and maybe some stuff on the bottom. No way though will I be hoping for anything of such detail as that picture.
Ken: I saw what I think was the Hummingbird at a boat show about a month ago. Transom-mount sort of thing that lowers into the water a couple of feet. I recon it must use a mechanical scanner because the transducer housing looked too small to contain an electronically steered array. Was not in the water, and no simulator running on any display. I think the people at the stand were more interested in selling boat shoes and stubby holders (beer can holders)...
Some links:
http://www.upl.cs.wisc.edu/~bmartin/sonar/
http://www.hnsa.org/doc/sonar/chap4.htm

[SSN626B]

Your experiments are very interesting and I look forward to seeing your results.
I too am hoping to build a sonar system in time, when I have completed my ROV. To help with this I bought Mark Thompson's PDF book called 'Home built Sonar'. Whilst he uses a specific type of display, I hope to use the front end of his design and adapt it to feed an Arduino and LCD.
Mark designed a sonar array and I think an array narrows the sonar beam width, but I don't know why that is. The results he achieved enabled him to locate wrecks that had been lost, so his system is quite successful.
I sourced the sonar transducer, they cost about $9.50 each from a Chinese company. The transducers are used in ultra sonic cleaners, probably why they are low cost. Mark designed his array for 6 transducers.
The design uses low frequency transducers. Mark said that higher frequencies mean better resolution but more difficulty in the design.

Hi Oldsirhippy,
Where did you find the ultrasonic transducers for $9.50 each? Sunnytec, the source recommend in the Manual, is no longer in business and the recommended p/n is showing up as $75 each from a couple of on-line companies.
I ordered the Sonar Manual from Mark and after reviewing the design, I do not believe that it is worth pursuing for hobby ROV work unless you plan on descending to depths of ~600 feet. Even after building the transmitter/receiver, unless you want to use the thermal paper printout, there is still the need to design a suitable display that will convert the return signal into a visible depth display. In my way of thinking, building a virtually discrete component transmitter/receiver that is designed for depths down to ~1000 ft is way overkill in cost and usability.
My suggestion, which I think that I will pursue, is to buy a used fish finder and convert the display output to a signal that can be transmitted and recorded for viewing. I plan to build a small autonomous twin pontoon survey craft that will map small bodies of water with the modified fish finder sonar and then use my SRV-II to view interesting areas detected by the survey craft.
Regards,
TCIII/SSN626B
Regards,
TCIII/SSN626B

[Oldsirhippy]

@SSN626B

The Sunnytec information is below and the specification for the transducer is attached. They also sell transducers with higher frequencies. I believe that these would provide higher resolution. Mark uses low frequency transducers to keep the processing away from the towed fish, which he says simplifies the design.

A transducer at $75 is rather excessive - the transducers are used in ultrasonic cleaners and at that price would most likely be uncompetitive for the ultrasonic manufacturers.
From the specification attached the construction of the transducer looks simple, but perhaps I am oversimplifying it

The following is the information I received:

####The unit price is USD9.50/pcs FOB shantou

#### SHANTOU SHIANLIN ELEC.CO.,LTD

#### Tel:0086-754-88463744
#### Fax:0086-754-88269960
#### Web site: http://www.sunnytec.com.cn

I am not sure what you meant about descending to 600 feet depth. Mark's side scan sonar is towed a few feet below the surface behind his boat to reveal wrecks which he then dives on.
The display Mark uses is not that easy to source (I found one on ebay but it was pricey). Also I think it requires a lot of electronic knowledge to adjust correctly.
What I would like to do (eventually, project number 952!!!) is to adapt Mark's transducer and front end design, digitize the signal, process it and then display the data through a LCD monitor. This would not be built in the ROV, but used as Mark has done.

[SSN626B]

@Oldsirhippy,
Thanks for the info on the ultrasonic transducer, much appreciated.
My statement concerning the 600 ft depth was in reference to the range of the sonar. This home brew sonar is very powerful in relation to the standard fish finder and in my opinion somewhat overkill for hobby ROV usage. If your ROV will operate in depths of less than 100 ft, then this sonar, considering its build complexity, is overkill. I have built a lot of point to point hand wired circuits in my time and this one is somewhat complex. There is a fair amount of cut and try tuning involved and you need a fair amount of test equipment to tune the receiver and characterize the transducers. The transformers are not easy to wind even though they look simple. Just my two cents.
Regards,
TCIII/SSN626B

[rossrov]

Been looking around the net to see how to design an array to narrow the azimuth beamwidth of my mechanically rotated Sonar, and found these posts which will interest you side-scanners

http://www.geotech1.com/forums/showthre ... r-sidescan
http://www.geotech1.com/forums/showthre ... idescan-II

I was wondering to what amount, if any, phase errors would be introduced by making an array simply by putting transducers in parallel, maybe using a transformer so the fish-finder sees them electrically as just a single transducer. The guy on the forum describes just putting them in parallel, did not see any mention of transformer.

Rather than use the transom-mount transducers, maybe the 200KHz Chinese "toy" handheld fishfinder transducers would work if you could source them. Have not looked into that yet.

Have not bought Mark's DIY book, but I agree that there are going to be easier and cheaper ways to get a result

[Oldsirhippy]

Wow...
The images produced are remarkable. They are far superior to those obtained by Mark from his book. But they are operating at 200KHz rather than 28KHz which would significantly increase the resolution.

It looks like the images are from shallow waters, is the power feed to their transducers limiting their range?

Are the parallel transducers coupled directly into the Sonar display system, or is there some pre-processing done before the box displays the results?

From their experiments it looks like spacing of the transducers is critical.

[rossrov]

Other things being equal, higher frequencies have less range.

Going by Rickard's posts, the array is connected straight to the fish-finder as per a regular fish-finder setup.

One advantage Mark's 28KHz system has is that being a longer wavelength, spacings are further apart so you do not have the problem of having to cram the transducers together. Also tolerances can be sloppier than at 200KHz. Alignment of the transducer faces is important considering how short the wavelength is. The bare cleaner transducers also allow much more control over alignment. Rickard suggests using bare pucks (transducer elements), not a complete housing. In commercially made FF transducers, I imagine there would be some variation between potting and therefore location of the puck within the housing, even though transducers appear identical from the outside.

The arrays I've seen on the 'net appear to use mostly rectangular transducer elements. Rickard mentions rectangular vs circular somewhere in the threads

If I was going to make an array for the rotating ROV-mounted scan, I would start off doing a bit of research on the spacings, then set up a test rig in a largish body of water, bounce the beam (beams when you consider there will be sidelobes) off a target and adjust the transducer spacings on the fly to get the best compromise between narrow beam and smallest sidelobes. Would use exactly the same approach for a side scan.