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Extensometric Hydrodynamic Transducer For Influence Sea Mine Fuses
Researchers Construct a New Sensor and Develop Signal-Processing Algorithms for Sea Mine Hydrodynamic Field Analysis

Dr. Henryk Chodkiewicz
Head of Underwater Weapon Systems Division
Jaroslaw Michalski
Dr. Michał Widlok
Senior Specialist
Research and Development Marine Technology Centre
Gdynia, Poland

Modern influence mine fuses are usually equipped with several measurement channels, including acoustic, magnetic, electric, seismic and, of course, hydrodynamic. Most of these channels can be simulated by modern sweeps quite accurately; however, the hydrodynamic channel cannot be simulated in any accurate way.

The lack of good simulators (sweeps) makes the hydrodynamic channel a natural choice for a main mine-activation method, especially in shallow water. However, the measurement conditions for a ship's hydrodynamic field are usually less than ideal. Waves, currents or high tides can severely distort sensor output, leading to errors in ship detection or, worse, to false activation of the mine. There are also other constraints that need to be taken into account when designing and building hydrodynamic sensors for sea mines, such as the power supply, working depths and required sensitivity.

The Research and Development Marine Technology Centre (OBR CTM) last year introduced a hydrodynamic sensor designed specifically for bottom and anchor sea mines, as well as new signal-processing algorithms to improve the probability of detection in difficult environmental conditions or during high tides. The sensor and the algorithms are currently being actively tested in the laboratory and at sea. After testing is complete, the sensor is set to be incorporated into the center's sea mines.

Cross-section of hydrodynamic sensor equipped with two extensometric transducers.

Ship Hydrodynamic Fields
Every moving ship generates a hydrodynamic field (pressure changes) that can be detected by a mine's sensor. This field depends on the ship's velocity, size, hull submersion and other factors. What will be detected by the sensor also depends on current sea conditions, and sometimes these conditions can have a major impact on the acquired data.

In ideal conditions, i.e., Sea State 0, a ship's hull and even its wake are clearly visible and also relatively easy to detect, even by simply looking at field waveforms. In such conditions, one can use almost any sensor with the required sensitivity, and very simple algorithms can be used.

Unfortunately, Sea State 0 occurs very rarely, so the sensor's transducer must be developed to work in different conditions. When OBR CTM performed sea trials last March in Sea State 3 (very irregular waves about one meter high, quite typical for the Baltic Sea near harbors), the ship passing over the mine was almost impossible to detect using only the data from the hydrodynamic sensor. In fact, using the acoustic 'peak' (recorded by another sensor) was the only way to do so. In such conditions, no simple detection algorithm can be used to accurately detect the ship with only hydrodynamic channel data. Irregular waves, and possibly also underwater currents, create signals that are comparable or even higher in amplitude than the signal generated by the ship.

Requirements for the Hydrodynamic Sensor
Prior to development, researchers factored in acquired data and the requirements of the Polish navy to determine the main characteristics of the sensor.

The most important qualities were determined to be the following: the sensor has to work with static depths (mine operating depths) up to 100 meters; it has to be powered from the mine's internal battery and consume very little power; it has to be turned on and off by the mine's main control processor and should be ready to work within two to three seconds; it must measure small pressure changes with a pass band from zero to a few hertz, with sensitivity that is constant over all working depths; and its sensitivity should be not less than one pascal, preferably better.

Sea mines usually have very long deployments and they have to be fully autonomic, so capacitance of the power supply is a major limiting factor for developers. The problem is normally solved by equipping mines with several 'standby' and 'combat' circuits that work together. Standby circuits are designed to have the minimum possible power consumption, and their only task is to detect an incoming ship (or signs of a ship) and turn on the combat circuits. The combat circuits then perform accurate detection of the ship and activate the mine. Combat circuits are turned off most of the time and do not consume power, but they still have to be designed with power efficiency in mind.

Almost all ship-detection algorithms work best when provided with accurate data closely following real pressure changes. This means that the sensor should not filter the signal (high-pass behavior is mostly problematic) and should respond irrespective of working depth. The sensor also must be able to pick up very low frequencies in order to detect slowly moving ships, but since a ship's motion will never generate very high frequencies, a pass band of up to a few hertz is normally more than enough for ship detection.

A major challenge of these requirements is that it is nearly impossible to achieve the required sensitivity at the requested working-depth range of up to 100 meters using standard measurement techniques, because the dynamic range would be too high for most low-power circuits.

Development of the Hydrodynamic Sensor
To solve the problem of achieving the required sensitivity with such large static depth changes, OBR CTM developed a special compensation circuit. The compensation circuit consists of resistors and a variable resistors network controlled from the sensor's main microcontroller.

Hydrodynamic field recorded by two different sensors in a sea mine (the largest peak is the ship passing the sensor). Data gathered in ideal conditions: shallow water at Sea State 0. Click to enlarge.

The microcontroller uses variable resistors to bring the sensor's output to zero at any given working depth. The speed of the compensation procedure is limited, mostly by the necessity of performing low-pass signal filtering to clean unwanted high-frequency noise from the amplifiers. To reduce compensation time to the absolute minimum, the design takes an idea from successive approximation register (SAR) analog-to-digital converters: The microcontroller tests all of the bits in binary compensation word, from most important to least important. Using this technique and 10-bit compensation word, the device achieves less than one-second compensation time, and by using the compensation results, depth can be measured to within a few centimeters.

It was decided that extensometric absolute pressure transducers were the most suitable for the hydrodynamic sensor. They have low power consumption, are highly linear and they make it possible to build a sensor that has no moving parts. Mechanical parts are a known source of unreliability, especially after they have been submerged in seawater for long periods. It was also decided to use two transducers instead of one to increase sensitivity, reduce noise and allow differential electronic circuits to process the transducers' output.

After its construction, the hydrodynamic sensor underwent various laboratory tests to verify its resolution, depth range, power consumption and other features. A special stand was built to test the compensation circuits and resolution at various static depths, as well as compensation range and depth-measurement accuracy. The noise floor has been tested using another stand, but the electronics and transducers' noise must be tested separately because normal pressure variations in a laboratory are within the sensor's detection range and can distort the results.

During testing, the sensor was subjected to various static pressures from 100 kilopascals (normal atmospheric pressure) to 1.1 megapascals (100 meters of water). Then, after the compensation procedure was finished, very small (100 to 5,000 pascals) dynamic pressure variations were administered with controlled rate of change, value and shape, and sensor output was recorded and reviewed. These measurements show that the sensor successfully fulfills the requirements, while its resolution and depth measurement accuracy are even better than required. Sensor output is directly proportional to input pressure without any unwanted filtration or disturbances, which simplifies the design of signal-processing algorithms for ship detection.

Signal-Processing Algorithms
A ship's hydrodynamic field can be severely distorted by sea conditions, especially by waves and underwater currents. To improve the probability of ship detection in various sea states, OBR CTM developed special signal-processing algorithms that make use of adaptive filtering and other advanced signal-processing methods.

Hydrodynamic field recorded by sea mine sensors in non-ideal conditions (green: acoustic, yellow: hydrodynamic). Acoustic field is shown as a reference to localize the ship passing the sensor. Click to enlarge.

Generally, all these methods try to 'learn' how the noise signal looks and then detect any disturbance that looks different—such disturbances might be a ship's field. Such filtration is also very effective in dealing with slow constant depth changes (high tide, for example), as well as with false signals of a constant frequency. In both cases, the filter removes the unwanted signals after a few seconds of learning.

Hydrodynamic fields generated by ships can be heavily distorted by waves or underwater currents, complicating the process of ship detection. To separate signals caused by waves and other disturbances, special signal-processing algorithms were created based on adaptive filters.

The sensor presented in this article features two extensometric transducers and no moving parts, and it can quickly compensate constant static pressure, irrespective of working depth (up to 100 meters) using an algorithm similar to SAR analog-to-digital converters.

Static pressure compensation allows the sensor to measure small hydrodynamic fields at these high static pressures without highly dynamic electronic circuits, which normally have too high power consumption.

The sensor can precisely reproduce hydrodynamic pressure changes irrespective of signal frequency, shape and static depth, which is required for proper operation of the signal-processing algorithms.

The sensor has undergone sea trials on Polish navy ships to fully test its parameters and to fine tune the signal processing algorithms, with further trials scheduled for March.

This work is being financed by the Polish Ministry of Science, Project Number 399/BO/A.

Dr. Henryk Chodkiewicz is the head of the underwater weapon systems division of the Research and Development Marine Technology Centre in Gdynia, Poland. He received his doctoral degree from the Military Technical University, Warsaw, in 1976. His main interests are ships' physical fields and underwater weapon systems.

Jaroslaw Michalski received his M.Sc. Eng. degree in digital signal processing from the Technical University of Gdansk, and he has worked at the Research and Development Marine Technology Centre in Gdynia since 2006. His research focuses on control systems for underwater sensors, and his hobbies include electronic music and military technique.

Dr. Michał Widlok has worked for the Research and Development Marine Technology Centre in Gdynia since 2006. He received his doctoral degree from the University of Science and Technology in Krakow, Poland. His research mainly focuses on sensors and control systems for sea mines and other underwater military equipment, and his hobby is restoring antique clocks.

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