Implementation of a system for measuring velocity of ...

Proceedings of the IEEE SoutheastCon 2015, April 9 - 12, 2015 - Fort Lauderdale, Florida

    Implementation of a system for measuring velocity of   primary & secondary waves in rocks and soils.  

Prashant S Sengar      

Department of Electrical and Computer Engineering Engineering Center (EC) Florida International University Miami, Florida- 33174, USA email: [email protected]

Maryam S. Bhagini

D. N. Singh

Department of Electrical Engineering Indian Institute of Technology- Bombay, Powai   Mumbai 400076, India email: [email protected]

Department of Civil Engineering, Geotechnical Engineering division, Indian Institute of TechnologyBombay, Powai, Mumbai 400076, India email: [email protected]

 

  Abstract—Measurement of shear and compression wave   velocities in rock and soil samples play an important role in the detection of their types and characteristics. The instruments   for these measurements are too expensive and bulky. employed Thus, a low cost instrument is required to measure the primary (shear)  and secondary (compression) waves velocities in real time. This paper describes an implementation of an ultrasonic nondestructive measuring system using Piezoceramic   transducers. The operational frequency of the system is 0.2KHz 30KHz and 500KHz, 1MHz for measuring shear and compression wave velocities respectively. It is able to detect the   wave velocities in rock specimens and some compacted soils of different properties (dry & wet).

 

Keywords—Piezoceramic element, bender, compression wave, piezoceramic disc, extender, instrumentation amplifier, filter.  

I. INTRODUCTION   Piezoceramic elements are crystalline substances which generate   electrical charges when subjected to mechanical stress. Conversely, if placed in an electric field they experience a mechanical strain (Lings and Greening 2001). It is due to this   property, that they can be used as actuators and or sensors for various geotechnical engineering applications [1]. Piezoceramic  transducers are widely used for variety of applications such as ultrasonic cleaners, atomisers, dynamic force & pressure   measurement, SONAR, accelerometers, flowmeters, strain gauges, actuators and more. They can be effectively employed for the shear and compression wave velocity   measurements in rock and soil specimens. These polarized sensors when connected in different configurations can be used   or extender elements which can generate shear waves as bender (S) and compression waves (P) respectively. Bender elements   are commonly used for the measurement of shear wave velocity and the determination of small strain shear modulus (Gmax)  [2]. Whereas, the extender elements are less commonly used for the compression wave velocity measurement and determination of Young’s modulus (Emax) & Poisson’s ratio   (ν). Since the extender elements are not efficient for

 

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compression wave velocity measurement, piezoceramic discs of frequency 500KHz and 1MHz were used for the system development. Interestingly, both bender element and piezoceramic disc can act as a transmitter or receiver of waves at any given point of time [3]. Extensive investigations are made to account for the performance of the piezoceramic elements according to their configurations, such as the polarization of the elements and wiring configurations (series or parallel) [1]. The implemented system is divided into three functional blocks namely, Signal generator, amplifiers-filters and signal detection (DSO). Bender elements and piezoceramic discs are included in the amplifiers-filters block. These transducers are coupled to the surface of rock or soil specimens by using a coupling gel. The circuitry implemented for sensor system requires good Common mode rejection ratio (CMRR) and a precise range of band pass filter for the acquisition of the signal. Instrumentation amplifiers were employed to improve the CMRR of the received signal [7]. The operational amplifiers were chosen according to their slew rates in order to amplify the desired frequencies. II. BENDERS AND PIEZOCERAMIC DISCS Piezoceramic transducers (benders & peizoceramic discs) used to realize the present system were developed using a Lead Zirconate Titanate (LZT) based material, SP-5A US DOD Navy type material-II (Sparkler Ceramics LTD). These low power operated piezoceramic elements have high dielectric constant and piezoelectric sensitivity. The quality and strength of the transmitter signal & receiver signal depends upon the size of the piezoceramic elements [2]. These elements also show good response and minimum time lag between the excitation and wave generation. A bender element (S-wave) transducer is composed of two piezoceramic plates bonded together with a central metallic plate [8]. Depending upon the wiring configuration, the benders can be polarized in series or parallel type. In order to transmit and receive a S-wave, a piezoceramic element with the parallel wiring (same

Proceedings of the IEEE SoutheastCon 2015, April 9 - 12, 2015 - Fort Lauderdale, Florida

  polarization) acts as a transmitter and the other with series wiring  (opposite polarization) act as a receiver [3], depicted in Fig. 1. The dimensions of the bender elements used as transmitter   and receiver were 15mm x 12mm x 0.65mm.

      (a)

 

through receiver, placing a standard test specimen (concrete) between the transducers. A graph of input frequency at transmitter verses output voltage at receiver for both 500KHz and 1MHz piezoceramic transducer are plotted, as shown in the Fig. 3. It is clear from the graph that the resonant frequency of the piezoceramic disc transducers pair (transmitter & receiver) is different from the described frequency of single transducer (i.e. 563KHz for 500KHz elements and 1.124MHz for 1MHz elements). This frequency shift is due to the mismatch of transmitter & receiver disc frequencies. Thus, in order to retrieve better signal quality and strength from the receiver transducer, the setup must be operated at the resonant frequency.

          (b) Fig. 1.  (a) Bender element wiring configuration transmitter and receiver (adapted from Cristiana Maria da Fonseca Ferreira) (b) Dimension of the bender elements used for the system development.

 

Piezoceramic disc of resonant frequency 500KHz and 1MHz   (Diameter 54mm, thickness 4mm) were employed for compression wave generation and reception, Fig. 2. A longitudinal stress wave is generated by the piezoceramic disc   when excited through a pulse generator. The size of the piezoceramic discs and the excitation voltage influence the quality   and strength of the received wave [2].

(a)

        (b)

   

Fig. 3. (a)Frequency response of 500KHz piezoceramic transducer pair (transmitter & receiver with the test specimen concrete). (b) Frequency response of 1MHz piezoceramic transducer pair (transmitter & receiver with the test specimen concrete)

 

III. BENDER & PIEZOCERAMIC DISCS TEST SETUP

Fig. 2. Shape, mode, wave motion and direction of polarization of the Piezoceramic discs.

These piezoceramic discs (500KHz & 1MHz), transmitter and receiver are resonant at particular fundamental frequencies. It must  be noted that when the transmitter transducer is excited with the resonant frequency, the receiver generates maximum amount  of potential difference around its electrodes. The exact resonant frequency of the transmitter and receiver pair was found out by exciting the transmitter with a range of   frequencies at 5 Volts and detecting the output waves in DSO

 

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The test setup used to measure shear and compression wave velocity is shown in Fig. 4. The Function generator IC XR2206CP was used to develop the sine wave signal generator for bender elements (0.2KHz - 30KHz) and piezoceramic discs (500KHz & 1MHz). The operational voltage of the function generator is from 10V to 24V. Thus, it makes it suitable for the benders and piezoceramic discs test setup. The output sine

Proceedings of the IEEE SoutheastCon 2015, April 9 - 12, 2015 - Fort Lauderdale, Florida

  wave from the function generator has very low waveform distortion   and was fed to an buffer operational amplifier circuit. The piezoceramic element, bender and piezoceramic disc generate   shear waves (S) and compression waves (P) respectively. The piezoceramic transducers were directly interfaced with the buffer circuits for S-wave and P-wave   transmission. These transducers are coupled to the surface of rock or soil specimens by using a coupling gel or paraffin wax. S-waves   and P-waves are injected to the rock and soil samples through the transmitter transducer. The received wave (signal) is detected   through the receiver transducer which is filtered and amplified at the filters & amplifiers block. The signal is then fed to a dual channel digital signal oscilloscope (DSO) TDS 2001,  Tektronics for display of the wave. For storing the received data, TDS2MEM communication extension module (consists   of USB flash card & a RS232 port for computer interface) was used. The waves were displayed using LabVIEW software on the desktop computer which is an   integral part of the DSO, TDS2001.

        Fig. 4. The   block diagram of the piezoceramic transducer test setup (Block diagram adapted from A. patel, D.N. Singh).

Bender elements are placed on top and bottom surface of   the specimen by creating a thin slit at the center of each of the two planes. It is ensured that the bender elements are placed in   the same axis, parallel to each other. Piezoceramic disc have a large flat plane which can be easily coupled using gel with the top and  bottom flat surfaces of the specimen.

  SIGNAL GENERATOR, FILTERS & AMPLIFIRES IV. The electronics involved for the test setup of measuring shear   and compression wave velocity in rocks and soils consists of a function generator, filters, instrumentation amplifiers, oscilloscopes and data storage/ processing device. Function   generator equipments are bulky and fail to provide high strength signals. Thus, a signal (sine wave) generator was   developed for the testing purposes. A sine wave generator circuit can provide high strength and quality waves required by   the piezoceramic transducers.   A. Signal generator The Function generator IC XR-2206CP was used to develop the sine   wave signal generator for bender elements (0.2KHz 30KHz) and piezoceramic discs (500KHz & 1MHz), shown in Fig. 5.  The operational voltage of the function generator is from 10V to 24V. The output sine wave from the function generator (XR-2206CP) has very low waveform distortion and   to an non-inverting buffer operational amplifier was fed LM358 circuit. The sine waves of frequency 0.1Hz to 2MHz with more   than 20Vpeak to peak can be generated.  

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Fig. 5. Sine wave generator and Op amp buffer.

B. Filters and Amplifiers for Benders. An input sine wave of 20Vp-p from the signal generator having a frequency between 0.5 KHz to 20 KHz is given to the transmitter which injects this signal to the soil or rock sample. The signal received from the receiver transducer is then fed to filters and amplifiers in order to eliminate noise and to increase the amplitude of the received attenuated waves. The R C filters and amplifiers implemented consist a combination of three operational amplifiers ICs (INA121, OP07 and TL082). INA121 is an instrumentation amplifier that provides high input impedance and amplification to the signal from the transducer. For eliminating DC errors which may arise during amplification of the signal, the high pass filter was used. The low pass and high pass cut-off frequencies were fixed as 150 KHz and 1 KHz, respectively. The output waves received from the sample after excitation of the bender setup is passed to an active High pass filter having a cut-off frequency of 1 KHz, allowing only high frequencies over 1KHz to pass through it. The filtered wave is amplified using an operational amplifier OP07 which has typical slew rate of 0.3V/µs and very low input offset voltage (75μV max for OP07E). The low offsets and high open-loop gain makes OP07 useful for high-gain instrumentation applications. The output of high pass filter is given as an input to the low pass filter having a cut off frequency of 150 KHz. The active low pass filter is also implemented using R C components and IC OP07 which allows frequencies only up to 150 KHz to pass. Both the operational amplifiers (IC OP07) were operated in non- inverting mode as shown in Fig. 6.

Fig. 6. Filters and Amplifiers circuit diagram for Bender transducers.

Proceedings of the IEEE SoutheastCon 2015, April 9 - 12, 2015 - Fort Lauderdale, Florida

  The output from the filter stage is amplified using IC TL082CP which  is a high speed, dual JFET input operational amplifiers. It requires low supply current yet maintains a large gain bandwidth   product and fast slew rate of about 13V/µs. In order to get correct and clear waveforms, various gain levels were tried for the output signal, by placing a potentiometer as a   feedback resistor to the amplifier. Further the output variable signal from the amplifier is displayed and stored using DSO and in  desktop computer (LabVIEW) for signal analysis. C. Filters and Amplifiers for Piezoceramic discs.   A piezoceramic disc of 500 KHz or 1 MHz when excited with its resonant frequency, it injects maximum amount of   longitudinal stress wave through the rock and soil samples [6]. The test setup utilized for the rock sample is shown in Fig. 7.

             

(a)

analysis. LabVIEW software which is provided with the DSO (Tektronix, USA) was utilized for the storage and analysis of waves.

Fig. 8. Filters and Amplifiers circuit diagram for 500KHz Piezoceramic disc transducers

Band pass filter and amplifier circuit for 1MHz piezoceramic discs was implemented in a same manner as shown in Fig. 9. The centre frequency of the band pass filter was kept as 1.124 MHz with a bandwidth of 50 MHz [5]. The resistor values were determined with the same equations (refer equation (1)) used before for 500 KHz piezoceramic discs. Operational amplifier ICs AD797 and LM318 were utilized for filter and amplification purposes due to their high slew rate, very low noise, low distortion and wide gain bandwidth. IC AD797 has a slew rate of 20 V/μs and a 110 MHz gain bandwidth, which makes it highly suitable for our low frequency ultrasonic application. The output gain can be varied by using a potentiometer as variable feedback resistor to the amplifier IC LM318 which has a high slew rate of 60 V/μs.

(b)

Fig. 7. (a) 500KHz Piezoceramic disc transducers with granite sample test setup. (b)  1MHz Piezoceramic disc transducers with granite sample test setup.

A sine wave of frequency 563 KHz and 1.124 MHz having 10Vp-p  voltage is given to the piezoceramic discs (transmitter) of 500 KHz and 1 MHz respectively. The output signal from   sample is fed to an active band pass filter having the rock centre frequencies equal to the respective resonant frequencies of the piezoceramic discs that is 563 KHz and 1.124 MHz [5].   The filtered signal is amplified by using operational amplifiers ICs TL082, LM318. Two band pass filters and amplifiers were   implemented for both the piezoceramic disc of 500 KHz and 1 MHz. Filter amplifier circuit implemented for 500 KHz piezoceramic  discs is as shown in Fig. 8. The centre frequency of band pass filter was kept as 563 KHz and a bandwidth of 50 KHz. IC   TL082 having a slew rate of 13V/µS was used for the amplification of the filtered signal. A potentiometer of 10KΩ was placed at the feedback path of the amplifier in order to   vary gain of the received waves within ±5V range. The equations used to determine the resistor values of the filter are   as follows: R1 = Q  / (G*C*2*Pi*F), R2 = Q / ((2*Q^2)-G)*C*2*Pi*F), R3 = (2*Q) / (C*2*Pi*F),   Q = Center frequency/ Bandwidth

Fig. 9. Filters and Amplifiers circuit diagram for 1MHz Piezoceramic disc transducers.

V. SIGNAL GENERATOR, FILTERS & AMPLIFIERS BOX The signal generator, filter and amplifier circuitry for benders and piezoceramic discs (500KHz & 1MHz) was implemented in a single sided printed circuit board which was a general purpose board of 10×8cm. A 15×10 cm Acrylic box was fitted with four potentiometers, each for adjusting gain of the output waves and six bores were made for receiver, transmitter, function generator, DSO inputs & outputs as shown in Fig. 10.

... equation (1)

Center frequency = √(Lower freq. * Upper freq.)   where Q is the Quality factor. The filtered and amplified output waves can be seen on DSO which   can also be transferred to the computer for further

 

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(a)

Proceedings of the IEEE SoutheastCon 2015, April 9 - 12, 2015 - Fort Lauderdale, Florida

            (b)

7.3cms, shear wave velocity was calculated. The shear wave velocity obtained was 2471.2 m/s for the granite rock which approximately matched the standard known values [4]. Table no. I, Fig. 14 refers to the calculated and known shear wave velocity in granite rock sample. (B) Piezoceramic discs Sine wave of frequency 563 KHz and 1.124 MHz having 10Vp-p voltage is given to the piezoceramic discs (transmitter) of 500 KHz and 1 MHz respectively. The graph below, Fig. 12 shows the transmitter transducer input voltage versus output voltage at receiver transducer for both 500 KHz and 1 MHz piezoceramic discs.

 

Fig. 10. (a) Internal view of the developed system. (b) Fully fuctional Signal generator, Filters and Amplifiers Box .

 

VI. RESULTS The transmitter transducer was excited with the continuous sine wave produced by the signal generator. Also a single   sinusoidal pulse wave generator using a timer circuit was employed for the rock and soil testing. The developed system   was tested with four types of granite rocks and two types of compacted soils.   (A) Bender elements Bender element circuit was tested with a granite rock and a soil sample.  The wave velocity in the granite and soil sample obtained were in accordance with the standard values [1]. It must be  noted that the circuit developed for the bender element is for low power application, thus the rock & soil samples were reduced dimensionally. An input sine wave of 20Vp-p from the signal  generator having a frequency between 0.5 KHz to 20 KHz is given to the transmitter. The wave travels through the sample  with some distortion and acquires a time lag before being detected at the receiver. This 'Time Lag' is observed through  the DSO by comparing the transmitted and the received wave. In order to find the wave velocity in the sample, the distance between the transducers is divided by the transit time of  the wave (time lag). Fig. 11 shows the wave acquired on LabVIEW when tested with the granite rock sample.

 

(a)

              Fig. 11. Signal displayed in the LabVIEW front panel. Time Lag observed was 29.5 microseconds.  

Considering the time lag observed through DSO or LabVIEW   and the distance between the transducers which was software

 

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(b) Fig. 12. (a) Input voltage at transmitter verses output voltage at receiver for 500 KHz transducers, (b) Input voltage at transmitter verses output voltage at receiver for 1 MHz transducers.

Granite rock samples were tested for the verification of the piezoceramic disc transducer system. Fig. 13 depicts the waves obtained in DSO for a test granite sample. The distance between the transducers was approximately 6.8 cms for 500 KHz piezoceramic discs and 1.8cm for 1 MHz piezoceramic discs. The time lag between the transmitted (yellow) and received (blue) wave is 15.5 μs for 500 KHz and 3.88 μs for 1 MHz transducers. The compression wave velocity (P-waves) calculated from piezoceramic disc transducer test setup matched closely with the known granite P-wave velocity values. Below is the result table obtained through the developed system, refer Fig. 14.

Proceedings of the IEEE SoutheastCon 2015, April 9 - 12, 2015 - Fort Lauderdale, Florida

  VII. CONCLUSION A system consisting of signal generator, filters and amplifiers was successfully implemented to measure primary (compression) and secondary (shear) wave velocities in rock or soil samples using piezoceramic transducers. The system developed is for low power, laboratory applications. It is handy, portable and can be utilized for in-situ rock & soil testing. High quality sine waves are produced by signal generator which drives the piezoceramic transducers for signal transmission. The received signal is filtered and amplified to detect the desired waves which travelled through the sample for the determination of sample type and its characteristics (small strain shear modulus (Gmax), Young’s modulus (Emax) & Poisson’s ratio (ν)), refer equation (2).

            (a)  

 

..... equation (2) where, Cp is longitudinal pulse velocity. Detection of quality waves depend upon the size of the piezoceramic transducers. Waves detected through the developed system were stored and analyzed using LabVIEW software.

   

REFERENCES

  [1]

    Yellow- Input wave 1.124MHz (b) Blue- Output wave 1.125MHz

[2]

Fig. 13. (a) Input (top) and output (bottom) waves displyed on the DSO for 500 KHz and  (b) 1 MHz piezoceramic transducer test setup.

TABLE   I. Transducer

 

Benders   (Swave) Piezo-  ceramic   discs (Pwave)

Transducer distance (cm)

Time lag (μs)

Calculated value of Granite (m/s)

Known value of (Granite) (m/s)

0.1 30KHz

7.3

29.5

2471.2

2500 3300

500KHz

6.8

15.5

4427.3

1MHz

1.8

3.88

4483.2

Frequency

42006000 42006000

[3]

[4] [5]

[6]

[7]

  Fig. 14. Table for calculated and known shear and compression wave velocities in granite rock sample.  

Thus, it is clear from the above table that the shear wave (S) and compression wave (P) velocities calculated by using the   system matches the known granite values. The wave developed velocity values obtained through the test is beneficial in determination of Young’s modulus (Emax) & Poisson’s ratio   (ν). This system can also detect the S & P wave velocities in concrete and various stiff rocks.

       

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[8]

[9]

A.Patel, D. N. Singh and K. K. Singh, " Performance Analysis of PiezoCeramic Elements in Soils," in Geotechnical and Geological Engineering, vol. 28, Issue 5, pages 681–694, Springer Netherlands, DOI 10.1007/s10706-010-9328-2. Leong EC, Cahyadi J, Rahardjo H (2009) Measuring shear and compression wave velocities of soil using bender–extender elements. Can Geotech J 46(7):792–812. Ferreira, Cristiana M. F; Fonseca, António V.; Fahey, Martin. 2008. "Seismic wave measurements in true triaxial tests on residual soil from granite", Trabalho apresentado em IS Atlanta'08, In Proceedings of IS Atlanta'08, Atlanta. Bartake PP, Patel A, Singh DN (2008) Instrumentation for bender element testing of soils. Int J Geotech Eng 2(4):395–405 Gomez-Garcia, R.; Alonso, J.I.; Briso-Rodriguez, C., "On the design of high-linear and low-noise two-branch channelized active bandpass filters," Circuits and Systems II: Analog and Digital Signal Processing, IEEE Transactions on , vol.50, no.10, pp.695,704, Oct. 2003 Li, X.S. 1997. Discussion: Pulse transmission system for measuring wave propagation in soils. Journal of Geotechnical and Geoenvironmental Engineering, 123(9): 883-884. Worapishet, A.; Demosthenous, A.; Liu, X., "A CMOS Instrumentation Amplifier With 90-dB CMRR at 2-MHz Using Capacitive Neutralization: Analysis, Design Considerations, and Implementation,"Circuits and Systems I: Regular Papers, IEEE Transactions on , vol.58, no.4, pp.699,710, April 2011. Maslan, M.N.; Mailah, M.; Mat Darus, I.Z., "Identification and Control of a Piezoelectric Bender Actuator," Intelligent Systems, Modelling and Simulation (ISMS), 2012 Third International Conference on , vol., no., pp.461,466, 8-10 Feb. 2012. Book: Design with operational amplifiers and analog integrated circuits, Author, Franco, S. ISBN 9780072320848