Generation of PWM Signal for Active Sonar Applications

IJSTE - International Journal of Science Technology & Engineering | Volume 2 | Issue 08 | February 2016 ISSN (online): 2349-784X

Generation of PWM Signal for Active Sonar Applications Aryalakshmy M.S P.G. Scholar Department of Electronics & Communication Engineering Toc H Institute of Science and Technology, Arakkunnam Kochi

Prof. M. Mathurakani Associate Professor Department of Electronics & Communication Engineering Toc H Institute of Science and Technology, Arakkunnam Kochi

Abstract The quality of the transmission signal should be very good for active sonar applications like detection, tracking, communication, fish location, acoustic oceanography etc. One of the best ways to achieve it is by using a PWM signal. This paper presents various schemes for generating PWM signals for active sonar applications. There are two types of PWM signals and they can be generated using microcontroller, hardware and software schemes. The analysis of types and generation of PWM is also described. The analysis is done using Xilinx and Matlab.

Keywords: PWM, SONAR, Unipolar, Bipolar, Microcontroller ________________________________________________________________________________________________________

I. INTRODUCTION Pulse width modulation is a modulation technique used to encode information for transmission. The main advantage of PWM is that power loss of switching device is very low. PWM are classified into two Unipolar and Bipolar PWM. Among these two classes Unipolar PWM is more efficient. Bipolar PWM signals contain more harmonics components than Unipolar. Unipolar PWM makes filtering easier and have large bandwidth compared to Bipolar PWM. Both light and electromagnetic waves, commonly used under normal atmospheric conditions, undergo strong attenuation underwater and as such have very limited range. As a result, the ability of sound to propagate great distances under water has been extensively researched and the resulting sonar technology has been developed as a means to navigate and identify obstructions under water. “SONAR” is defined by Winder .It is an acronym for Sound Navigation and Ranging. It is the method or equipment for determining by underwater sound the presence, location or nature of objects in the sea. There are two classes of sonar, namely passive and active sonar.

Passive or listening sonar consists of a receiver system that monitors all incoming sound. The second class of sonar, active sonar, consists of both a transmitter and receiver .The transmitter produces an acoustic pulse and the receiver system listens for the resulting reflections or echoes. For active sonar applications require high resolution PWM signal to obtain good quality transmission signal at all power levels. Unipolar PWM is the best option for active sonar applications.

II. PWM In Sonar applications PWM signal is used as a switching technique. Efficiency using PWM technique is almost 90%.When switch is off, there is practically no current and when switch is on, power is being transferred to the load. Two types of pulse width modulation are;  Unipolar  Bipolar A. Bipolar PWM PWM scheme, when switching occurs, the output voltages switch between +v to –v are called Bipolar PWM. For generating Bipolar PWM signal, a modulating signal is compared with carrier signal. Here modulating signal is a sine wave and carrier is a triangle wave. When sine wave amplitude is greater than triangle wave, it is ON period and triangle wave amplitude is greater than sine wave, it is OFF period. In Fig: 1(a) shows bipolar PWM. Spectrum of Bipolar PWM is shown in Fig: 1(b). The spectrum shows that harmonics occur at all the odd and even components. From that it can be conclude that the amount of harmonics is very high. Therefore Bipolar PWM cannot be used for SONAR application.

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Generation of PWM Signal for Active Sonar Applications (IJSTE/ Volume 2 / Issue 08 / 035)

Fig. 1(a): Bipolar PWM

Fig.1 (b): Spectrum of Bipolar PWM

B. Unipolar PWM PWM scheme, when switching occur, the output voltages switches between either between zero and +V during positive half cycle or between zero and 窶天 during negative half cycle of the fundamental frequency thus this scheme is called Unipolar PWM. For Unipolar PWM generation requires two sinusoidal modulating waves which are of same magnitude and frequency but 180 degree out of phase. The two modulating wave are compared with a common triangular carrier wave and produce two PWM signal, subtract this two PWM signal produce Unipolar PWM. Fig: 2(a) shows unipolar PWM.

Fig. 2(a): Unipolar PWM

The use of Spectrum of unipolar PWM is shown in Fig: 2(b). The spectrum shows that harmonics occurs at only even components. Unipolar PWM switching scheme is better in terms of efficiency and lower harmonic distortion as compared to Bipolar PWM. Therefore unipolar PWM is used in SONAR applications.

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Generation of PWM Signal for Active Sonar Applications (IJSTE/ Volume 2 / Issue 08 / 035)

Fig. 2(b): Spectrum of Unipolar PWM.

III. GENERATION OF PWM SIGNAL PWM signal can be generated mainly in three ways.  Software scheme.  Microcontroller.  Hardware scheme.

A. Software Scheme In Software scheme, sine wave and triangle wave is generated and these waves are compared to produce PWM. These waveforms are generated in Matlab and this is implemented in DSP processors. Fig 2(a) show the generation of PWM signal in Matlab. The reconstructed signal using this scheme is shown in Fig: 4.

Fig. 4: Reconstructed signal in software scheme

In this scheme, high frequency sonar applications require more computation time and memory requirements.

B. Microcontroller Now a day’s all the microcontrollers are coming with PWM modules. These modules are hardware peripherals supporting high resolution PWM. The major bottleneck in using these modules in SONAR systems is the limited number of pulse width modulation modules available on a single chip. The maximum number of PWM modules available in a typical chip is 16; we need two modules to implement unipolar switching.

C. Hardware Scheme In hardware scheme, triangle wave is generated using counter and it is compared with sine wave to produce PWM. PWM is generated in Xilinx design suite using VHDL coding and it is implemented in FPGA. The obtained PWM signal is taken from Xilinx to software environment to verify it .The reconstructed signal using this scheme is shown in Fig: 5.

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Generation of PWM Signal for Active Sonar Applications (IJSTE/ Volume 2 / Issue 08 / 035)

Fig. 5: Reconstructed signal in Hardware scheme.

Table - 1 Comparison of different PWM scheme

PWM Scheme Software scheme Microcontroller Hardware scheme

Software complexity Very high Very high Low

Hardware resources required High Very High Medium

IV. CONCLUSION The paper explains about unipolar PWM, bipolar PWM and analysis of various schemes for generating PWM. From the analysis it can be concluded that single microcontroller, is not enough for sonar applications. More number of microcontrollers is needed but it will increase the complexity. In software scheme memory requirement and computation time is high. In hardware scheme, complexity is less.

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Tingting Lin, Under water acoustics ,Analysis, design and performance of SONAR”, Published 2019,John wiley Ltd. Jean-Paul marage, Yuon monn,“SONAR and Underwater Acoustics ,” Published 2010,United States by ISTE Ltd. Anuja Namboodiri,Harshal, “Unipolar and Bipolar PWM Inverter”, International Journal of Innovative Research in Science and Technology ,volume 1,issue 7.December 2014.

Alejandro R. Oliva,, “A Multi-Loop Voltage-Feedback Filterless Class-D SwitchingAudio Amplifier using Unipolar Pulse-WidthModulation”,IEEE,Volume 50,2004

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