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MBES/sonar question


(Pierson) #1

I work on a AUV and we have been thinking about implementing a sonar on the vehicle. Ideally we would like to use a multi beam forward looking imaging sonar, but seeing as those are a little pricy, we are looking into side scan or a sector scan unit. While looking at these units I ran into many “Multi Beam Echo Sounders” or MBES, and I have had difficulty determining what the difference between a MBES and a Side Scan Sonar or a standard altimeter is. I was wondering if any of the more experienced guys on this forum could explain.


(Kevin) #2

Take a look at these links:

http://www.furuno.com/special/en/sonar/index.html

http://oceanservice.noaa.gov/facts/sonar.html


(Pierson) #3

Thank you that was very helpful, so side scan sonars measure the intensity of the reflected signals and are useful in identifying objects to the left and right of the vehicle on the sea floor, where as MBES generate ranges to the sea floor. At least thats what i got out of the NOAA pages.

My followup question is, if effectively a MBES measures the distance each beam travels, could you use a MBES in a forward looking orientation, and if not why not?


(Harold Scadden) #4

Frankly any SONAR is capable of providing you distance to an object. It all depends on the software package that comes with the unit. I have used military SONAR but I don’t have no experience with commerical units but they all work the same.

The distance is just a function of the velocity of sound in the water, turn around time to the object and your distance resolution will be based upon the frequency that you are transmitting with. Higher the frequency, the better the resolution; however, the trade off is the amount of power you have to put out because of attenuation etc.

 


(Kevin) #5

Here is another way to think about it. If you have a single transducer that generates a narrow beam, you can point it in any direction and measure the distance to objects that generate a return signal or echo (e.g., the bottom, fish, wrecks, ships, etc.). If it points down, you have a “depth finder” if you you are on the surface, or it can measure height-over-bottom for a submerged AUV. If you point it forward, you might use if for collision avoidance with ships, rocks, etc. You can even point it upward to measure depth below the surface as is done for AUVs operating under the ice. The range that you can measure is a function of both the frequency and the power. The lower the frequency and the higher the power, the greater the range. It is also possible to combine multiple transducers in 1D, 2D or 3D arrays, which allows you to create focused and steerable beams of different shapes (see https://en.wikipedia.org/wiki/Synthetic_aperture_sonar). At higher frequencies, such systems allow for high-resolution acoustic imaging, not all that different from medical ultrasound instruments. Sidescan sonars typically use fairly short, linear (i.e., 1D) arrays of transducers to synthesize their fan-shaped beams and are most often used to measure the bathymetry (i.e., contours) of the bottom on either side of the vehicle’s track, but can also detect other objects (e.g., wrecks) on the bottom. Once again, their range and resolution are a function of frequency and power.


(Pierson) #6

Right, fundamentally all sonars work by measuring a combination of reflected sound intensity and time of flight. My concern is that there is something in the software that would cause a MBES not work in a forward looking capacity.

For instance picotech makes both a Forward looking sonar and a MBES, the specifications appear the same, the only difference i noticed was in output data format, and transducer shape, it appears that they both measure time of flight and magnitude of 64 beams spaced every .7 degrees. Imagenex also does a similar thing in their DT line.

We are not too concerned with max range, most of these have maximum ranges well over a hundred feet and our vehicle only operates at depths of less than 15ft and in pools so less than 300ft. we are more interested in object recognition and avoidance than bottom profiling and bathymetry, also our DVL takes care of altitude.

On the topic of power and frequency, I am a little concerned about interference between the Sonar and DVL. I know we need to stay away from the DVL’s frequency, but should we be worried about resonances?


(Harold Scadden) #7

SInce you are talking about being in really shallow water … I would be more worried about just scattering and reflection from the bottom. Since you are talking about operating in pools … the reverberation could drive the equipment crazy too.

Stupid question time. If you are operating in basically very restricted waters such as a pool … and you stated that you are looking for object recogition and avoidance, why not cook up your own system and use much higher frequencies to get a good profile for avoidance etc.? At least at high frequencies you can range gate your system to basically create a geo fence around the unit. You ignore everything outside of your range gate and allow enough time between pulses to allow reverb etc. to die down … which should be pretty quick.

 

 


(Pierson) #8

Reverberation is something that we are concerned about, we know that some of our competitors successfully use tritech microns, and blueview P series units. I have some concerns stepping away to the lesser known brands.

We briefly considered attempting to make our own. Unfortunately we are an undergraduate team, so we don’t have people who have strong enough DSP skills to do it. we have been struggling to even get a passive sonar system for acoustic pinger locating to work. like you said in another post, getting a transducer to ping and receive a signal is easy, its getting that signal into a computer in a use full manner, and processing the data. Building a high frequency sonar I feel would be a multi year project. High frequency analog electronics are no trivial matter. But as it is clear from above I am no expert perhaps I am wrong, but I feel like getting 60+ channels of 15MHz+ sampling ADCs running would be miserable.


(Harold Scadden) #9

Pierson the passive system should be the easiest thing in the World to do in comparison to what you are trying to do for active.

To help my old fart brain out … just what is the objective? I assume from the way you are talking that you are running an course with objects that you have to avoid and others that you have to deal with? What are the basic parameters of life that you can deal with? Sometimes it is a lot better to just sit back, have a cold one or two and look at things in the most simple terms as possible and make whatever the device has to do to just do it.

All of these systems that I see appear to have linear arrays and they are doing a lot of beamforming etc. Have you ever considered going to a spherical array and form your beams with it? Lets look at the problem of finding the pinger in the water. If you have a spherical array the very shape can help you determine a direction because of the phase relationship of the “acoustic” wave hitting the array. By just dealing with the phase angle difference you can tell if the wave front is coming from the left or the right … or dead on. Now add layers for declination and elevation. Now you can sort out up and down. For that matter a cross shape array can allow you to zero in on a noise source.

Think back to the earliest days of SONAR systems. You had some poor smuck on a receiver stack with a single hydrophone. The guy would rotate the hydrophone 360 degrees and listen to get a feel of what direction was stronger than another to say “Captain! The destroyer is 47 degrees to Port” etc. Think of the problem in very simple terms. I had a guy in a Hackerspace, that I hung out at, trying to use a ton of formulas to get a fix on a simple system that I was using to make our glass window a touch pad for people to come up and select stuff by tapping on the window in different places to drive a computer display. He was going nuts and I was sitting back snickering. I did it with four accelerometers and just dealt with the timing differences between them when the window was tapped to plot a cross fix. To eliminate false taps, because of street traffic, I had schmitt trigger circuits tied in with the output so I could generate a clean square wave pulse to run counters with … to get my timing differences.

Just a though, at least for the pinger locator.

 


(Kevin) #10

Pierson - I assume that you are talking about RoboSub? You might want to check out this Forum thread:

https://www.bluerobotics.com/forums/topic/low-cost-sonar/


(Pierson) #11

@Harold: We want to use a sonar to supplement our vision, use the sonar to confirm objects and determine the distance we are away from the objects we have to manipulate. For our passive sonar array your are correct by having 3 tightly spaced hydrophones we plan on analyzing the phase on arrival, to determine the X and Z heading to the pinger. The difficulty we are having is taking the analog signal, amplifying, passing into a synchronized ADC sampling fast enough, from there passing the signal into a FPGA, microcontroller or DSP block, Programing the board to receive the signals, and process them. It isn’t the transducers or the math analysis that is the hard part, it’s getting the analog signal from the hydrophones into a computer in a meaningful way.

@Kevin: Your absolutely right we compete in the RoboSub competition this is going to be my 6th year in the competition. I have read through that thread, I think that Garmin’s Panoptix has a lot of potential and I have been regularly checking up on Paul’s website to see if he publishes any follow ups on that, My major concern with that unit was I haven’t seen it used with anything other than Garmin’s monitors, and being an AUV we have to have a computer interpret the data. Luckily though our team has access to fairly substantial funds, so we can purchase one of the more costly of the shelf units. The difficulty I have been encountering is that it is difficult to determine the difference between similar sonar units, and if a unit will be suitable for our purposes. right, like it is easy to tell the difference between chirp units like the Micron and a multi beam FLS like the M900 or Gemini, but the more subtle differences like the difference between the Gemini 720 and the Gemini NBI what is the advantage of the narrow beam. There are many of these subtle differences which for a new comer to sonar is difficult to navigate.


(Harold Scadden) #12

@Pierson: Can I recommend something simple … how about stop the DSP stuff etc. If you know the frequency that the pinger operates at then look for that. Take your transducer and don’t worry about FFT’s and the rest of the crap to see if there is a signal there … run the blasted thing through a filter network so the only thing you are looking at is a narrow band containing the pinger. Just as I did with the accelerometers, when the signal strength representing an SPL high enough to be a valid pinger signal comes in, then use a trigger circuit to give you square waves etc. so you can just use high speed counters or something to get the phase difference from.

Please remember, SIMPLE STUPID! No fancy stuff just a GO / NO GO. The pinger is either there or it isn’t there. You can figure out your filter width by adding any doppler effect that you believe will be in there so you don’t have to worry about up/down shift of the frequency.

 


(Kevin) #13

@Pierson: You mention using the sonar for “object recognition and avoidance” and “being an AUV we have to have a computer interpret the data”. I believe you will find that even the high end commercial AUVs do not “interpret” their sidescan data, e.g., in real time on the vehicle. Instead, the data is recorded and post processed after recovery with humans looking at images on monitors. The AUVs searching for the Malaysian Airlines plane could fly right over it but not know it was there until later. Navies are working on real-time recognition of mines on the sea floor using imaging sonar, which is a more constrained problem and similar to what I assume you are contemplating, but likely well beyond your capabilities. As for differentiating among the various commercial imaging sonars, I would suggest that you let the sales reps for each system help you with the comparisons and appropriateness for your specific application. If you indeed have the budget, they should be more than happy to assist you.


(Ed) #14

This is one of my favorite topics. The problem of object recognition and avoidance has long been solved in air-based robots using cheap ultrasonic sensors, here is an example;

https://www.parallax.com/product/28015

The range of these devices in air is usually 3m – 10m.

The Youtube video clip in the middle of the webpage gives some examples on how the ultrasonic sensor can be incorporated into a small microprocessor robot.

The software is written for the robot’s microprocessor controller – it can be Arduino, Propeller or any microprocessor. It is not difficult and there are many pre-coded modules, tutorials and examples available.

The problem is getting a simple low cost underwater ultrasonic transducer. Fortunately, the definitive paper by Brigid Benson shows you the way. There are many circuits on the web for transmit and receive boards for ultrasonic sensors.

 

brigid-benson-phd-paper-on-underwater-piezo (5.35 MB)


(Harold Scadden) #15

@Ed - hate to break the news but your ultrasonic device and how life is in a water borne environment is a whole different story. If it was as easy as those toys that Parallax puts out … the whole Chinese Navy would have SONAR on their submarines by now along with everyone else on the planet.

Pinging some thing with ultrasonics in air is extremely easy in comparison to how a ton of different variables in water effect a transmission and how you look at the return signals. Just a small list of things that can bend your sound wave, attenuate it, absorb it, etc. are things like temperature, depth, salinity, marine life (not talking about whale size here either … microscopic) and again this list continues.