💊 🙍🏼 🤳🏾 Underwater Ultrasonic Rangefinder Module. Part three ◀️ 👄 ⏪

Without buildup.

The most significant changes in the project, compared to previous versions of the sonar ( one and two ), are the simplification of the analog part and the evolution of the assembly process of modules towards greater manufacturability. Various combinations of adhesives and compounds have been tested to seal the board and emitter inside the case.

Well and the fact that now it is a commercial product, brought to the production stage.

Production

Notably tormented with a very inconvenient Temporary Gain Control, which did not solve the problem of a narrow dynamic range, it was decided to use an integrated logarithmic amplifier. God (if you exist), thank you and the guys at Analog Devices for this miracle! A third of all sonar functionality, without exaggeration, is he. Feedback? Clipping? Self-excitation? No, have not heard. Our answer is AD8310!

The previous option is also working, but with minuses. Firstly, two cascades of active very narrow-band filters are difficult to configure. Secondly, the gain control circuit - it needs to be controlled. Thirdly, the cascade of amplitude detection is non-linear closer to the boundaries of its dynamic range. For the last two points, the characteristics, in part, depend on the temperature and the spread of the parameters of the key components. So we came to the logarithmic amplifier. The new analog path has a significant advantage - it is a measuring device with a logarithmic scale. This allows you to capture the entire available dynamic range (95 dB) with a conventional 12-bit ADC with high fidelity, and gain control can be done in post-processing on the software side.

As for changes in manufacturing technology. The main thing is to solve the problems of adhesion.
The issue of adhesion of materials to each other is very important, since it is this property that mainly provides protection against leaks. The module’s interior is filled with a compound and the weakest point is the junction of the cable sheath with the casting compound, which has the main responsibility for withstanding pressure and sealing the circuit. This compound must have high strength and good adhesion to the components. The fact is that there is air inside the cable, which is compressed under pressure, causing the outer shell to break away from the surrounding compound. There are several solutions to this problem. The main action in our case was the use of a cable in the polyurethane sheath Helukabel DataPUR-C. It has the best adhesion to the selected compound from the many cables we tested. That part of the cablewhich goes into the case and is filled with a compound, in additionplasma treated for even greater grip on the compound. Cutaway

module:

Some of the buttherts brought the story of RoHS . In short, RoHS is the EU rules governing the content of hazardous substances in products (Lead, mercury, cadmium and other byak). And since we were originally focused on exporting, priority in the selection of components was given to those that had RoHS certificates / declarations of conformity. Therefore, by the way, Russian components were not considered. The only exception was lead piezoceramics, used as an electro-acoustic transducer in sonar. These materials are exempted from RoHS regulation, like any ceramics with bound lead (2011/65 / EN Exemption 7 © -I).

Since export was mentioned ...

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The sonar case is made of stainless steel, which also acts as a screen for the internal circuit. It should not have direct galvanic contact with circuit elements, therefore it is connected to the ground via a ceramic capacitor welded to the body using spot welding.

All shipped modules are pressure tested. Despite the declared working depth of 100 meters, the test pressure is 35 Atmospheres (equivalent to almost 350 meters). As they say, it is better to overdo it ...

The installation of pressure testing to the limit is simple. The source of pressure is a pressure testing system for heating systems with a manometer. The pressure chamber is a segment of a high-quality polypropylene pipe, on the one side of which there is a fitting connecting the pressure testing hose, and on the other there is a cover with a hole and a seal for the cable of the tested module.

One of the key parameters is the radiation pattern. The following construction was used as a measuring bench: The

sonar is fixed in the aquarium permanently. The mobile receiving antenna is located at the same level with the sonar emitter and has the ability to rotate within 180 ° in the plane perpendicular to the emitter plane. Thus, we are able to measure the amplitude of the received acoustic signal depending on the direction of the receiver relative to the sonar emitter. One of the sonar pins was tuned to the output of the clock for the oscilloscope, so that you could clearly distinguish the direct sonar signal from the reflected ones. The measurements showed an angle of 12 ° to half the maximum amplitude of the received acoustic signal.

Graph based on the data received:

Use experience

It is naive to believe that by sending samples of sonars to testers for free, we can get high-quality feedback, we lost time and did not get significant test results. There is a feeling that testers would have more motivation to deal with the device, if they paid at least 50% of the cost.

Still, quality feedback is when you yourself become a user of your product.

Here we smoothly move on to one of the possible uses of our sonar.

We are still far from underwater robots, but we assembled the autopilot boat for bathymetry on our own.

PixHawk (software - ArduPilot) was chosen as the autopilot controller.

The boat moves along a preformed route. Sonar data on the distance to the bottom are recorded autopilot on a memory card along with data from the GPS receiver. Combining these data, it is possible to make a map of the bottom of the reservoir.

In general terms, the connection diagram is as follows:

And this is the Gretta-2 scientific research vessel with the equipment installed:

A strong wind blew on the test day, and our pelvis was decently sausage, as a result of which the sonar periodically gasped, which affected the readings. The image below shows these abnormal peaks in the measured depth plot. So, if you want to repeat this design, you should pay some attention to this nuance.

Tests on a better day made it possible to obtain more accurate data, thanks to which a map of the bottom of a small part of the local reservoir was built:

By the way, the module implements two types of protocol: binary, and text NMEA. The latter is supported by the ArduPilot platform, so there are no problems with the connection of the sonar and the autopilot controller.

Information on how to pre-configure our sonar to work in conjunction with ArduPilot can be found here.

To conduct experiments with a sonar, it is not necessary to have sophisticated external equipment, or possess advanced programming skills. It’s enough to have an Android smartphone preloaded with our GUI, any USB-> UART converter and OTG cable. You can change such parameters as the frequency of radiation, the number of pulses in the packet, the period of generation of the probe pulses, and much more. The result of changing these parameters is immediately visible on the screen.

If the smartphone has a GPS module, then you can record sonar data and data from the GPS of the smartphone itself.

Changing the emitter from the disk to an oblong parallelepiped, you can get a kind of HBO - side - scan sonar from a narrow-beam sonar . At the minimum, of course.

Although its small power and ultra-compact dimensions will not allow it to shine for tens of meters, it’s enough to get acquainted with the principles of operation of HBO, play with the settings and instantly see the result of changing these settings.

HBO on the minimum:

I would also like to dwell on the GUI.

Along with the fact that he knows how to change the parameters of the sonar, write logs with coordinates, send data to a remote server (a feature in development), this is also an open source project written in Java in Android Studio. For those who are engaged in mobile development and communication of the application with external devices, perhaps the solutions that are implemented in our application will be useful.

I would also add that the sonar, in addition to the UART interface, has several additional discrete inputs / outputs that can be used, for example, to synchronize an array of sonars and build a similarity of a locator with a synthetic aperture. But this is a completely different story ...

PS I almost forgot ...

MEMS IMU.
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Underwater Ultrasonic Rangefinder Module. Part three

Production

Use experience

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