1. Introduction

Low amplitude ultrasonic propagation in a medium is characterized by the changes in velocity of propagation (how fast it travels) and attenuation (how fast it decays). In this case, the medium retains its intrinsic state and the wave gets modified, hence, it is a non-destructive technique. Most of the instrumentation design is focused around the measurements of these two parameters. There are several methods for the measurement of ultrasonic velocity of propagation such as: continuous wave, standing wave (resonance method), optical, reverberation, impedance, pulse etc. Amongst these methods, the pulse method is the most popular due to its high accuracy, reproducibility of results and simplicity of operation. The pulse method includes pulse echo overlap, pulse superposition, sing around and pulse echo techniques.

Among the pulse techniques, sing-around technique is widely used in the measurement of ultrasonic velocity with better accuracy in relative measurements. The principle of this technique was first proposed by Cedrone and Curran ^[1]; while its high accuracy was first attempted by Forgacs ^[2]. The technique was patented by H. Asada ^[3]. Since then, several workers [4-26]^[4] have improved this technique in terms of its performance, accuracy and stability. Length of the optical fiber cable has been determined by Nakamura et al ^[22], using this technique. Ghodki et al ^{[23, 27, 28, 29]} have designed virtual sing around technique with improved accuracy and Singh et al ^{[30, 31, 32, 33]} have controlled a virtual sing around technique wirelessly using e-mail and mobile phone and over internet to be operated remotely.

An electrical pulse is sent to the transmitting transducer by a triggered pulse generator. The pulse after passing through the specimen is received by the receiving transducer, amplified and used to generate a trigger signal that initiates a new pulse for the transmitter. This loop runs continuously and a counter measures the frequency of occurrence of triggered signals. The conventional sing-around technique is shown in Figure 1.

Figure 1. Block diagram of Sing-around Technique

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2. Experimental

In present work, an embedded sing-around technique is implemented using a matched pair of 2 MHz piezoelectric transducers. These transducers are mounted to the two end of the sample holder, designed in our laboratory. An 8-bit microcontroller PIC 18F4550 is used to provide intelligence to the sing-around system. The entire system is designed in such a manner that number of electronic components used in the design of pulse generator, amplifier and the detector circuits, drastically gets reduced. In a conventional and PC based instrumentation designs, a rf burst is generated using electronic components, which consumes more power. Moreover, the system is not portable. In such a system, GUI control panel is designed in VB or Lab VIEW to control the operations. These problems are overcome in the present embedded sing-around system. The system becomes simple in design, compact, reliable, precise and portable. There is no need to have the control panel as the system is stand-alone. The required parameters can be displayed as a rolling display and the use of PC can be discarded, once the micro-controller is programmed.

Figure 2 shows the block diagram of the embedded sing around system, designed in the present work, using PIC micro-controller 18F4550. An rf burst of 2 MHz is generated by the microcontroller itself, which is amplified using buffer 7407 and pull-up resistor and fed to the transmitting transducer, Tx. The received signal, after receiving transducer, Rx, is amplified by an operational amplifier, LM 7171. This signal is detected and fed to the µC 18F4550. The program to generate the necessary control signals for the start/stop of the counter, trigger pulse and synchronization of data acquisition, analysis and display, is embedded into the micro-controller, via USB port of the PC. The time of flight is measured using the internal timer of the micro-controller having frequency of 12 MHz.

The sample under investigation is placed in the liquid cell and Julabo ME-32 circulating thermostat maintains the constant temperature to ± 0.1°C. The system is programmed to measure the time of flight for 1000 readings and displays the average time of flight on the LCD display. The system is calibrated as per the procedure mentioned in the earlier work carried out in our laboratory ^[23]. This stand-alone embedded system, designed, adds an intelligence ^{[33, 34]} to the present sing-around system.

Figure 2. Block diagram of Embedded Sing-around System

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Figure 3. (a) Photograph of the embedded sing-around system (b) Liquid cell used in the system and (c) Transmitted pulse and received echoes

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Table 1.

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3. Results and Discussion

The system developed in the laboratory has been tested for the ultrasonic velocity measurements in different standard liquids. It is observed that the experimentally measured values match those reported in the literature ^[35]. Ultrasonic velocity measurements in ethanol show large deviation from the literature values, due to the grades of the chemicals used. Table 1 shows the results of our measurements carried out in standard liquids at different temperatures.

Acknowledgement

The Authors are thankful to VI Labs, Department of Electrical and Electronics Engineering, Indian Institute of Technology, Bombay, India, for providing PIC µC 18F4550 boards (AURUM v1.2) for the experimental work. One of the authors (SJS) would like to acknowledge UGC, New Delhi for the financial support to carry out the present research work.

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