Primary objective of this particular field of research is to apply the basic physical knowledge of mechanical vibration and wave theory for designing transducers for sound generation and reception.
The spectral domain of interest ranges from low-frequency transducers, utilized in seismics and in passive sonars, up to ultrasonic transducers that are used in acoustical imaging systems of medical diagnostics, imaging sonar tech- nology, and non-destructive material testing.
The application of ever-improving measurement data processing algorithms requires a higher information capacity of the measured signals and therefore an improved response characteristic of future transducers particularly with regard to their bandwidth. The design of such transducers requires complex simulation tools based on the Finite Elements Method (FEM).
For this reason the research activities within the scope of sensor technology is focused on adaptation and application of commercial available FEM-tools to design transducers with high quality and bandwidth and to optimize the arrangement of transducer groups regarding the mutual interactions of neighboring transducers and the acoustical coupling of the transducers to the propagation medium.
Design of hydroacoustic Sensor Arrays (Antennas)
A sensor group (antenna) is a geometrical arrangement of individual transducers. By applying a special processing to the single transducer signals (Beamforming) it is possible to only receive signals from acoustic sources whose sound waves are coming from directions that are lying within a predefined solid angular range (Spatial Filtering)
Application areas of beamforming are sensor groups in radar and sonar engineering as well as in seismic exploration and mobile communication. The spatial filtering effect depends on the assembling of the sensor group, i.e. the number and the geometrical arrangement of the transducers. The spatial filtering effect of beamforming depends on the design of the sensor group, i.e. the number and the geometrical arrangement of the transducers.
The objective of designing 1D, 2D and 3D antennas is the determination of a transducer arrangement that provides the claimed transmit/receive beampattern either by employing the smallest possible number of transducers or by using an optimized amplitude and phase shading of the individual transducers.
Since this class of optimization problems can be solved with the known, in particular with derivation-based methods, e.g. the gradient and Newton-Raphson method, only unsatisfactorily (convergent properties) or not at all (differen- tiability requirements), alternative optimization methods have to be developed.
Research objectives are therefore the development of new and the application of robust optimization algorithms for antenna design. As promising optimization strategies, the genetic algorithm resulting from the theory of evolution as well as the simulated annealing method derived from solid-state physics are considered.
Daten-Acquisition
The sensor signals of acoustical imaging systems can have a dynamic range of more than 160 dB. With available analog-to-digital converters (ADC), however, only a dynamic range of up to 120 dB can be achieved. In order to be able to sample and digitize the sensor signals undistorted, an optimum control of the ADC by means of a signal-dependent gain control is necessary.
For this reason hydroacoustic sensor systems use a so-called automatic gain control (AGC). In this case, the gain is adjusted as a function of the estimated instantaneous signal power. The control speed is essentially determined by the integration time for the power estimation. While large integration times lead to a sluggish and thus unadjusted control behavior, significant signal components might be filtered out by integration times that are too short.
An alternative to the AGC is the use of a time-variable gain (TVG). For a TVG, a fixed amplifier characteristic curve is used during each measurement cycle. In order to follow the changes in the sound propagation conditions adequately, an adaptation of the characteristic curve from measurement cycle to measurement cycle should also be possible. In this generalized case one also speaks of an Adaptive-Time-Variable-Gain (ATVG).
Based on the fundamentals for the digitization of analogue signals, the research focuses on specific topics for the optimization of the signal pre-processing chain consisting of preamplifiers, AGC and/or ATVG, mixers, anti-aliasing and pre-whitening filters as well as ADC.
In order to assess the performance of active and passive sensor array systems and to carry out a problem-oriented analysis and processing of the sensor group signals, a physical modeling of the generated and received wave fields is essential. The physical modeling is becoming increasingly important, especially through the use of ever more powerful signal processors and the associated realizability of ever more complex signal processing methods.
The research activities in the field of sound propagation modeling consist in a continuous observation of the theoretical and experimental research activities, in the assessment of the relevance of the new research results for the acoustic metrology as well as in an updating and supplementation of the own sound propagation models.
Acoustical imaging systems are e.g. used in oceanography for the visual representation of the sea floor and the objects on it. The quality of such acoustical imaging systems is strongly dependent on the backscattering behavior of the objects of interest compared to the backscattering behavior of the rough seabed.
The detection capability of an object can be determined a priori by means of its target strength. For simple geometric objects, formulas for the calculation of the target strength are available in tabular form. In order to assess the backscattering behavior of complex geometric bodies, simulation models are to be developed by using the Boundary Element Method (BEM) or Finite Element Method (FEM), which allow a determination of the target strength as a function of frequency, bandwidth, pulse shape and aspect angle.
With the help of so-called non-linear sound sources it is possible to build low-frequency, high-resolution imaging systems in compact form. A non-linear sound source is composed of a pump transducer for two high-frequency primary waves and an interaction region in which a low-frequency secondary wave is produced as a result of non-linear sound propagation phenomena. The extent of the interaction range (length and width), which depends on the directional characteristic and the attenuation coefficient of the primary waves, defines the characteristics of the secondary wave.
The research task in the field of non-linear acoustics is to develop a model that describes the characteristics of a non-linear sound source (formation and propagation of the secondary wave) so accurately that it can be used as an integral part of a design tool for non-linear sound sources.
The data processing of sensor group signals is playing a central rule in the area of
o passive localization, e.g. in passive sonar and interferometrical radio-astronomy, in which the properties of the
propagation medium are assumed to be known and the number, location and power of a signal source are to be
determined from the sensor group data,
o active localization, e.g. in radar and active sonar, in which a wave is generally emitted in a directed manner, in
order to subsequently analyze its on an object reflected wave,
o exploration, e.g. in geophysics and medical engineering, in which information about the propagation medium is
to be obtained from the sampled wave field that is generated by signal sources with known properties,
The scope of research at the IWSS in the field of data processing is summarized below.
Signalanalysis
o Detection Theory (CFAR, Transients)
o Parametrical and Non-Parametrical Spectral Analysiso Time-Frequency-Analysis (Wavelets, Wigner-Ville-Distribution)o Optimum-Filter-Theory (Wiener- und Kalman-Filter)o Automatic Detection und Track-Initiationo Multi-Sensor Track-Fusion
Sensor Group Signal Processing
The spatial filter effect (resolution and ambiguity) of a sensor group is determined by the number, position and directional characteristic of the sensors as well as by the signal frequencies of the waves. Since the resolving power of a sensor group can not be arbitrarily extended for constructive and physical reasons (finite coherence length), approaches which allow an increase of the radial and lateral resolution as new signal forms (signals with high bandwidths), the formation of synthetic sensor groups (synthetic aperture methods), and the application of more complex models (high-resolution methods) are subject of research.
- Signals with high Bandwidth
To improve the radial resolution, high-bandwidth waveforms and large time-bandwidth products are of particular interest. Approaches to this are
o bi- und polyphase coded signalso frecuency hopping coded signalso pseudo-random sequences
with pulse compression on the receiving side. The impulse compression is realized by a matched filter or a mis- matched filter with predefined properties
- Synthetic Aperture Methods
Another possibility to enhance the lateral resolution gives the synthetic aperture method. It can be distinguished between a synthetic aperture principle in stripmap or spotlight mode, dependent on the transmission or reception method used during the sensor movement. For the construction of a synthetic aperture, a precise compensation of the deviations of the sensor movement from the nominal path is necessary. Possible motion compensation methods are:
o Inertial System Supported Path Estimatorso Micro Navigation Techniqueso Auto Focus Methods
- High Resolution Methods
In the case of high-resolution methods, a distinction is made between parametric and nonparametric methods. The class of parametric methods includes methods in which a predetermined signal model is optimally adapted to the measured data by variation of the model parameters of interest in terms of the maximum likelihood principle or the least squares approach. The non-parametric methods, e.g. linear prediction methods (maximum entropy), methods of the Capon-Pisarenko type and projection methods (MUSIC, Minimum-Norm) use a priori known mathematical proper- ties of the measured data to determine the desired parameters.
o Image Generation (Interpolation- and Decimation Techniques, Geo Coding, On-Line 3D-Illustration
o Image Filtering and Normalization Methods, as well as Image Segmentation and Image Fusion Techniques
o Tomography and Stereoscopy (resolution and SNR improvement)
o Interferometry and Shape from Shading Techniques
Automatic Classification Methods
o Feature Extraction Techniques + Sonar Signals (aktive/passive)
+ Sonar Images (edge detection, texture analysis, echo- und shadow classification)
o Automatic Classification + Statistical Methods, Decision Theory
+ Neuronal Networks, Fuzzy Logic + Knowledge based Systems + Deep Learning
Broadband Communication in the Water Sound Channel
o Special channel-matched Signal Structures (e.g. Signal structures of dolphins, bionics)
o Robust Modulation Methods in Multipath Propagationo Adaptive Filters/Equalizers as well as Adaptive Beamforming o Simulation of Acoustic Data Transmission Scenarios
INSTITUTE OF WATER-ACOUSTICS, SONAR-ENGINEERING AND SIGNAL-THEORY (IWW)
Tel.: +49 421 5905 3512
Rene Ramson received the Dipl.-Ing. (B.Sc.) degree from Fachhochschule Zittau/Görlitz (University of Applied Sciences), Görlitz, Germany in 1982 and the Dipl.-Ing. (M.Sc.) degree from Dresden University of Technology, Dresden, Germany in 1990, in industrial electronics and information technology respectively.
From 1990-1999 he was with Vero Electronics, Bremen, Germany as a designing engineer for power supplies. In 1999 he joint the Faculty of Electrical Engineering and Computer Science at the Hochschule Bremen - City University of Applied Sciences, where he has been responsible for the power electronics and power grid laboratories.
Since September 2009 he is affiliated to the Engineering Acoustics Laboratory as well as to the Institute of Water-Acoustics, Sonar-Engineering and Signal-Theory (IWSS) at the Hochschule Bremen, where he is now designing instrumentation and measurement electronics, especially power amplifier and signal conditioning components for activ sonar systems.
o Power and Measurement Electronics
o Audio and Video Signal Conditioning
o Power Amplifier for Piezo-Ceramic Sound Transducers
o Microphone Array Processing
INSTITUTE OF WATER-ACOUSTICS, SONAR-ENGINEERING AND SIGNAL-THEORY (IWW)
E-mail: ziliang.qiao@ieee.org
Tel.: +49 421 5905 3470
Ziliang Qiao received his bachelor's degree in Electronics and Information Engineering in July 2010 and master's degree in signal and information processing in April 2013, both from Northwestern Polytechnical University, China. His bachelor graduation project is to develop a simulation system for real-time underwater target detection and parameter estimation. His master thesis focuses on robust DOA estimation algorithm in impulsive noise environment.
Since September 2013 he is conducting research as an external PhD candidate of SPG, TU Darmstadt in the Institute of Water-Acoustics, Sonar-Engineering and Signal-Theory (IWSS) at the Hochschule Bremen - City University of Applied Sciences. His research is supported by a Chinese Scholarship Council (CSC) scholarship under the State Scholarship Fund of China. He is working towards his PhD degree on the application of compressive sensing for active and passive Sonar applications, and he is especially interested in high resolution synthetic aperture sonar imaging.
o Underwater Acoustics and Sonar Signal Processing
o Synthetic Aperture Sonar (SAS)
o Motion Compensation and Interferometric SAS
o Compressive Sensing and its Application for SAS
INSTITUTE OF WATER-ACOUSTICS, SONAR-ENGINEERING AND SIGNAL-THEORY (IWW)
Bergische Universität Wuppertal / ATLAS ELEKTRONIK GmbH
School of Electrical Engineering and Computer Science
E-mail: christoph.zimmer@atlas-elektronik.com
Tel.: +49 421 457 1098
Christoph Zimmer received the M.Sc. degree in Electronics Engineering from the Hochschule Bremen - City University of Applied Sciences, Germany, in November 2015.
Within a joint research project between the Institute of Water-Acoustics, Sonar-Engineering and Signal-Theory (IWSS) and ATLAS ELEKTRONIK GmbH, Bremen, he prepared his Master Thesis entitled “Development of a Simulation Environment for the Design of a Sonar Signal Processing Chain”. His Bachelor Thesis entitled “Mean Shift Algorithmus zur Bildsegmentierung” was also part of a research collaboration between the IWSS and ATLAS ELEKTRONIK GmbH.
Since March 2016 Christoph Zimmer is working as a research associate at the University of Wuppertal in collaboration with ATLAS ELEKTRONIK GmbH and the IWSS at the Hochschule Bremen. He is working towards his PhD degree in the field of Numerical Methods for Designing and Optimizing active and passive sonar antannae. Furthermore, he is lecturer at the school of Electrical Engineering and Computer Sciences where he is teaching Mathematics for Engineers as well as Underwater Acoustics and Sonar Signal Processing.
o Underwater Acoustics
o Sonar Signal Processing and Beamforming
o Sonar Transducer and Array Design
o Numerical Methods / Optimization
INSTITUTE OF WATER-ACOUSTICS, SONAR-ENGINEERING AND SIGNAL-THEORY (IWW)
INSTITUTE OF WATER-ACOUSTICS, SONAR-ENGINEERING AND SIGNAL-THEORY (IWW)
Hochschule Bremen - City University of Applied Sciences
INSTITUTE OF WATER-ACOUSTICS, SONAR-ENGINEERING AND SIGNAL-THEORY (IWSS)
Neustadtswall 30
28199 Bremen
Germany
