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Advanced lock in amplifier for detection of phase transitions in liquid crystals


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Advanced lock in amplifier for detection of phase transitions in liquid crystals

  1. 1. INTERNATIONAL JOURNALEngineering and TechnologyRESEARCH IN International Journal of Advanced Research in OF ADVANCED (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME ENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)ISSN 0976 - 6499 (Online)Volume 4, Issue 2 March – April 2013, pp. 17-26 IJARET© IAEME: Impact Factor (2013): 5.8376 (Calculated by GISI) © ADVANCED LOCK-IN AMPLIFIER FOR DETECTION OF PHASE TRANSITIONS IN LIQUID CRYSTALS Bhagyajyothi, Immanuel J., P. Bhaskar*, L.S. Sudheer and Parvathi C. S. Department of Instrumentation Technology, Gulbarga University P.G. Centre, RAICHUR-584133, KA, INDIA ABSTRACT Lock-in amplifier (LIA) is the most important and essential instrument in signal recovery in the presence of large amount of noise. In this paper an indigenously designed microcontroller based advanced lock-in amplifier is proposed. The lock-in detection is done by quadrature sampling method. It is designed to work for the frequency range of 10Hz to 100kHz. The microcontroller based lock-in amplifier recovers signal of very small amplitude buried in large noise very efficiently. The designed lock-in amplifier is applied for a photoacoustic spectrometer (PAS) to detect the phase transitions in liquid crystal. In the present study N-(p-n-pentyloxybenzylidene)-p-n-hexylaniline (5O.6) liquid crystal compound is used as sample. The phase transitions of 5O.6 liquid crystal are detected by the system. The secondary transitions which are not observed in differential scanning calorimeter (DSC) are also observed in the proposed system. The amplitude and phase variations of the sample during the temperature scanning are measured and displayed on LCD module. The temperature scanning rate is kept at 0.3oC/min. The measured amplitude, phase and temperature information is sent to the PC, via serial port, for further data processing/analysis. Keywords: Lock-in amplifier, C8051F060 microcontroller, Phase, Amplitude, PAS. 1. INTRODUCTION In many scientific and industrial applications, a situation exists where it is necessary to measure the signal of much smaller amplitude than noise components present in the environment. In such cases, the lock-in amplifier is very essential. There are various types of lock-in amplifiers reported in the literature. Gabal et al [1] reported the application of analog lock-in amplifier to recover sensor signals buried in noise for embedded applications. Juh 17
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEMETzeng Lue [2] reported the junction impedance measurements of diodes by a simplified lock-in amplifier, which operated at frequencies from 20Hz to 100kHz, has a noise rejection ratioof 40dB. G. Busse et al [3] reported the lock-in vibrothermal inspection of polymercomposites.The magnitude and phase of the temperature modulation generated by modulatedstress were analyzed and investigations are made on various polymer and their compositeswere revealed. Adrian A. Dorrington et al [4] in their paper presented a small and simpledigital lock-in amplifier that uses a 20-bit current integrating analog to digital converterinterfaced to a microcontroller. Adrian successfully developed a simple, high sensitivity,small, and low-cost digital amplifier for the detection of low level optical signals, which hasa dynamic range of 103dB and is capable of recovering input signals in the pico-ampererange. The sample rate is set to twice the reference frequency placing the sampled lock-insignal at the Nyquist frequency allowing the lock-in procedure to be performed with onesimple algorithm. This algorithm consists of a spectral inversion technique integrated into ahighly optimized low-pass filter. He demonstrated a system with dynamic range of 103dBrecovering signals up to 85dB below the interference. Maximiliano O. Sonnaillon et al [5]proposed and validated experimentally a high frequency digital lock-in amplifier that usesnon-uniform sampling. They reported that, by using a random sampling strategy, it ispossible to process periodic signals of frequencies several times greater than Nyquistfrequency which is given by the sampling theorem. They implemented prototype of the LIAbased on 32-bit floating point DSP. The unknown system is excited by a reference signalgenerated by a direct digital synthesizer. The signal obtained at the unknown system output isamplified by a similar stage which presents high impedance at the input in order to avoiddisturbing the measured system. Results show that application of random sampling strategyreduces significantly the speed requirements of the ADC and DSP. After elaborate literaturesurvey we came to know that most of the lock-in amplifiers are designed using DSP’s andsome are designed using microcontroller. In both the cases separate ADC and DAC areemployed and the signal is multiplied with reference and passed through the low-pass filtersto find the amplitude and phase of the signals. To design such advanced lock-in amplifiersdefinitely requires help of microcontroller for front panel control. This motivated the authorsto design and develop C8051F060 microcontroller based advanced lock-in amplifier, whereit contains all the features including on-chip ADC and DACs to handle the signal processingand front panel control.2. INSTRUMENTATION The lock-in amplifier contains single-chip mixed-signal processor C8051F060 fromCygnal Integrated Products Inc. This microcontroller has advanced features which are usefulto design an single instrument. Fig. 1 shows the block diagram of single chip lock-inamplifier. A pre-amplifier with high input impedance, high gain, and low noise is required toamplify very low amplitude signals buried in noise. A pre-amplifier is designed using lownoise op-amp (LM308). The pre-amplifier is designed with two stages to improve the gain-bandwidth product of the amplifier. The pre-amplifier has a gain of 10 at first stage and 100at second stage with an effective gain of 1000. The proposed lock-in amplifier is designedusing C8051F060TB microcontroller board from Cygnal Integrated Products, Inc., Austin,USA. The on-chip peripherals of microcontroller will facilitate to design a single chip lock-inamplifier. Except PCA module, rest of the features of microcontroller has been used in thedesign of lock-in amplifier. The microcontroller has the following features [6]. 18
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME • High-speed pipelined 8051-compatible CIP-51 microcontroller core (up to 25 MIPS) • Two 16-bit, 1 MSPS ADCs (ADC0 & ADC1) with a direct memory access controller • Two 12-bit, DACs (DAC0 & DAC1) with programmable update scheduling • 64KB of in-system programmable flash memory • 4352 (4096 + 256) bytes of on-chip RAM • External Data Memory Interface with 64KB direct address space • SPI, SM Bus/I2C, and two UART serial interfaces implemented in hardware • Five general purpose 16-bit Timers • Programmable Counter/Timer Array (PCA) with six capture/compare modules • On-chip Watchdog Timer, VDD Monitor, and Temperature Sensor In most of the digital lock-in amplifiers, the processing is done in digital domain usingsoftware and dedicated digital signal processor (DSP). The system still features a pre-amplifierand band-pass filter to remove any signal component higher than half of the sampling frequencyof ADC. The lock-in amplifier requires a reference signal to perform the phase sensitivedetection. The reference signal is generated internally or derived from sampling an externalsignal. In case of internally generated signal, the individual sample points of the reference signalcan be calculated to a high degree of accuracy, and therefore do not suffer from the typical errorsfound in analog lock-in amplifier. The reference signal is also phase-shifted by 90° by eitherlook-up table or simple mathematical operations. Here, the reference signal is derived internallyby look-up table with 256 sine codes, Timer3 module used for scheduled update, and DAC0module of the C8051F060. Since, it is essential for this routine to be never interrupted ordelayed; it is assigned a high priority level. A simple circular buffer counter moves through atable of values that are output to DAC0 for every 10µSec. This will produce a sine wave withmaximum amplitude of 2.4Vand frequency of 352Hz. The signal is acquired with on-chip ADC0 module with 16-bit resolution. The ADC0 canbe initiated from various sources such as AD0BUSY bit, Timer2 overflow, Timer3 overflow, andexternal trigger. In the present design, it is important that all the clocks for sampling, and signalgeneration need to be synchronized because of a possible change in phase relationship of thesignal with the change in timings. For this reason, the ADC0 conversion is also derived fromTimer3 overflow and it is set to produce the start-of-conversion signal for every 10µSec (at asampling frequency of 100 kHz) so that the signal generation and acquisition will be done at thesame time. This feature makes the system to lock the signal frequency to the reference frequency.The ADC0 acquires this signal every 10µsec and stores the sampled data directly on data RAMthrough DMA controller. 64KB of data-RAM is available on C8051F060TB board. Hence, about100 cycles (284 data samples for each cycle of 350 Hz signal) at the sampling rate of 100 kHzwill be stored. If the sampling is performed with 16-bit resolution at 100 kHz rate, then an anti-aliasing filter needs to be set at 50 kHz to attenuate any signals above 50 kHz. As the on-chipADC0 conversion time is 1µSec, it can be extended up to 1MHz. After collecting these samples,the results of any subsequent data points are ignored until the current data has been processed.The data collected will be processed by using quadrature sampling to find amplitude and phase ofthe signal and are displayed on LCD. In this method the reference and phase shifted referencevalues are multiplied directly to generate the intermediate X’ and Y’ signals. These X’ and Y’ of100 waves are averaged to eliminate random noise of the signal and to get X and Y. These Xand Y values are used to calculate amplitude and phase by using the relations Amplitude = X 2 + Y 2 , Phase = tan −1 (Y X ) .Since, on-chip ADC of microcontroller is unipolar, themicrocontroller board is provided with a circuit to convert input bipolar wave to unipolar wave. A2-line, 16-characters LCD module is interfaced to microcontroller. The amplitude and phase aredisplayed on the LCD. A temperature controller has been employed to study the phase transitions 19
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEMEas a function of temperature. The scanning temperature is also displayed on LCD module. Thetemperature, amplitude, and phase values are transmitted to the PC through serial port. The dataare stored and further used to plot the graph. The photograph of indigenously designed lock-inamplifier system is shown in Fig. 2. C8051F060TB Microcontroller Signal Address Pre- Band Pass DMA Data ADC0 Amplifier Filter Controller Quadrature XRAM (64KB) Sampling and On-chip/Off-chip Interrupt Averaging Timer3 Reference X Y Reference sine Calculate DAC0 wave codes Serial Amplitude & PC (Look-up Table) Port Phase LCD Module Fig. 1. Microcontroller based lock-in amplifier Fig. 2. Photograph of lock-in amplifier instrument3. SOFTWARE DETAILS An embedded ‘C’ program has been developed for lock-in detection. The flowchart of theprogram is shown in Fig. 3. The code is developed using Silicon Laboratories IDE and Keil full-version embedded ‘C’ cross compiler. The program first initializes the on-chip peripherals suchas, ADC0, DAC0, DMA, UART0, Oscillator, and LCD module interfaced externally. Afterinitialization the program generates sine wave with help of on-chip DAC0 and Timer-3 modules.The sine codes are placed in the look-up table. These sine codes are scheduled updated to DAC0using Timer-3. The Timer-3 is programmed to generate an interrupt every 10µSec. Wheninterrupt occurs, the program reads the sine code from table by calculating the step through phaseaccumulator algorithm and sends to DAC0. By varying the step size, the frequency of sine wave,thus generated, can be varied. Next, the program reads signal through ADC0 and calculatesamplitude and phase of the signal by averaging 100 waves, to eliminate random noise, anddisplays on LCD. Finally, it enters the serial communication subroutine to send the measuredamplitude and phase to PC through UART0. The above procedure is repeated continuously. 20
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME4. APPLICATION The proposed microcontroller based lock-in amplifier is applied for photoacousticspectrometer to measure amplitude and phase of the PA signal to study the phase transitions. Nibu AGeorge et al [7] presented analog lock-in amplifier to detect the PA signal amplitude variationsduring the temperature scanning. They proposed laser induced photoacoustic technique for thedetection of phase transitions in liquid crystals. The liquid crystals such as 7OCB and 8OCB arestudied and phase transitions of the same are presented. They reported the detection of first order andsecond order phase transitions in these liquid crystals using PAS. The detected phase transitions arecompared with the standard DSC results of the same liquid crystals. Start Initialize on-chip peripherals viz., ADC0, DAC0, DMA0, Timer3, UART0, & Oscillator and LCD module Generate reference from microcontroller for chopping laser light Acquire generated acoustic wave from microphone and store on to XRAM through DMA Controller Perform quadrature sampling on data points i.e., read first point (X´) and 72nd data point (Y´), out of 284 data points in a wave Calculate X and Y for 100 waves X = X+X´, Y = Y+Y´ Average X = ( X 100) , Y = (Y 100 ) (To eliminate random noise) −1 Amplitude = X 2 + Y 2 , Phase = tan (Y X ) Display amplitude & phase on LCD Get temperature from PAS and send amplitude, phase and temperature values to PC through serial port. Fig. 3. Flowchart of the lock-in algorithm 21
  6. 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME The Fig. 4 shows block diagram of application of LIA to PAS [8]. It consists of 10mW IRlaser (830nm) source, PA cell, microphone, pre-amplifier, band-pass filter, and microcontroller(lock–in amplifier). In PAS, the sample is irradiated by modulated laser beam; as a result, theabsorption of light energy by the sample generates excited internal energy levels. All or part ofthe absorbed light energy is then transferred into heat through non-radiative relaxation process inthe sample. Since, radiation incident on the sample is intensity modulated, the internal heating ofthe sample is also modulated at the same frequency. The air at the sample surface undergoescompression and rarefactions by this internal heating of the sample, which in turn producesacoustic signal of same frequency as that of the modulating signal. The acoustic signal generatedfrom the PA cell is converted into electrical signal by a microphone. Since, the PA effect is basedon the absorption of light energy by a sample resulting in the production of electric signal of avery low amplitude, it is amplified by a high input impedance, high gain, and low noise amplifierdesigned using LM308 operational amplifier. The signal to noise ratio is further improved bypassing through a band-pass filter. The band-pass filter is designed using op-amps LM308 forQ=10, G=10, and fc=350Hz. Finally, the filter output is given to on-chip ADC0 of C8051F060microcontroller. The data from the ADC0 are stored on to the XRAM. The indigenouslydesigned lock-in amplifier performs lock in detection from stored values to recover PA signaland calculates amplitude and phase. The Phase transition of the samples is studied by varying thetemperature of the sample. The C8051F350 microcontroller based temperature controller hasbeen employed to vary the temperature. The amplitude and phase of PA signal are measuredduring the temperature scanning. The scanning rate is of 0.30C/Min. In this application, the lock-in amplifier is used to study the phase transition of 5O.6compound. The structure of 5O.6 compound is shown in Fig 6. The 5O.6 N-(p-n- LCD Module C8051F060-TB Laser MICROCONTROLLER Source Comparator & DAC0 Driver Buffer Lock-in Computer Personal UART0 Amplifier Algorithm Microphone DMA ADC0 PA Cell Controller Pre-Amplifier Band Pass Filter UART1 Sample Temperature UART0 Controller (C8051F350-TB) LCD Module Fig. 4. Application of lock-in amplifier for photoacoustic spectrometerpentyloxybenzylidene)-p-n-hexylaniline compound is unique in nO.m series as it exhibitsmaximum phase variantions viz., nematic, smectic-A, smectic-C, smectic-B, smectic-F, smectic-G (NACBFG). The variations include both first and second order transitions. Both amplitude andphase variations studied through the PAS technique, developed by the authors, facilitate toidentify all the phase transitions. On comparison with DSC thermogram, it is interesting to note 22
  7. 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEMEthat the PAS studies could resolve the second order smectic-A to smectic-C and smectic-G tosmectic-F transitions. While these transition could not be observed from DSC thermograms[9].The photograph of complete system is shown in Fig. 6. The detailed flow chart of application ofLIA to photoacoustic spectrometer is shown in Fig. 7. H11C5O CH = N C6H13 Fig. 5. Structure of 5O.6 Compound Temperature Controller (C8051F350) PA Cell Fig. 6. Photograph of complete lock-in amplifier application to photoacoustic spectrometer5. EXPERIMENTAL RESULTS From the experimental results it is found that the C8051F060 microcontroller based lock-in amplifier system recovers the very small amplitude PA signal under external disturbances. Thesystem measures PA signal amplitude variations during the heating of the liquid crystal 5O.6starting from crystalline phase to isotropic phase. Fig. 8 shows the amplitude variations duringheating of 5O.6 with a temperature scanning rate of 0.3oC/min. Fig. 9 shows the magnified viewof fig 8, which shows the remarkable phase transitions of 5O.6. The results, provide a evidenceof the sensitivity and significance of PAS technique in detecting the weak first order and secondorder phase transition over other techniques viz. the DSC and TM. From the graph it is observedthat there is a remarkable phase transition at 34oC and 37.16oC. These transitions were notobserved in DSC [9]. The results of the present PAS, TM and DSC studies on 5O.6 compoundare consolidated in Table 1. 23
  8. 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME Start Initialize on-chip peripherals viz., ADC0, DAC0, DMA0, Timer3, UART0, & Oscillator and LCD module Set the desired temperature(set point) and rate of increase of temperature in temperature control system for the sample under investigation Recover the photoacoustic signal though Microcontroller based Lock-in amplifier. Temperature of the PA cell is varied by using C8051F0350 Microcontroller based temperature control system Calculate Amplitude & phase and display on LCD module Get current temperature of PA cell and display on LCD module Send Amplitude, Phase and temperature to computer through serial port. No Is stipulated temperature Yes attained? Yes End Fig. 7. Flowchart of the photoacoustic spectrometer application 24
  9. 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME 0.90 0.85 0.80 Amplitude(mV)-----> 0.75 0.70 0.65 0.60 0.55 0.50 0.45 Crys G F B+C A N Iso 0.40 20 30 40 50 60 70 80 90 100 Temperature(Degree Centi)-----> Fig 8. Amplitude variations of PA signal of 5O.6 during temperature scanning 0.75 Amplitude(mV)-----> 0.74 0.73 0.72 0.71 0.70 0.69 32 34 36 38 40 42 44 46 48 50 Temperature(Degree Centi)-----> Fig 9. Magnified view of Amplitude variations of PA signal of 5O.6 emperature scanning Table1. Comparative study of phase transitions in TM, DSC, & PASCompound Phase Variant Instruments Phase Transition Temperatures 0C I N A C B F G TM[9] 72.9 61.7 53.3 51.8 44.4 40.6 35.0 DSC[9] 72.0 60.0 - 49.0 - - - 5O.6 NACBFG PAS 74.0 59.5 53.5 44.49 42.55 40.36 35.83 (Present Study) 25
  10. 10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME6. CONCLUSIONS The present work focuses on the design and development of C8051F060microcontroller based Lock-in amplifier. Also the designed lock-in amplifier is used inphotoacoustic spectrometer to recover the PA signal and to study the phase transition of thesolid samples. This lock-in amplifier is very effective in recovering the signal of very smallamplitude. On-chip peripherals of microcontroller facilitate the generation and acquisitionsimultaneously. The designed lock-in amplifier is applied to study 5O.6 liquid crystalsample. Phase transitions are effectively detected with indigenously designed single chiplock-in amplifier. The system is compact and reliable for studying the phase transitions ofvarious liquid crystal samples. Also the system is facilitated with serial interface to PC tostore the data for processing/analysis. The results obtained by the designed instrument arecompared with that of other methods like TM and DSC. They found to be in good agreement.ACKNOWLEDGEMENTS Authors are thankful to the University Grants Commission, New Delhi, Indiaproviding financial assistance to carry out this project successfully. Also the authors arethankful to Prof. V.G.K.M. Pisipati, Director, Centre for Liquid Crystal Research andEducation, Nagarjuna University, Guntur for providing the liquid crystal samples to carry outthe present work.REFERENCES[1] M Gabal., N. Medrano, B. Calvo, P. A. Martinez, S. Celma, M.R. Valero,A completelow voltage analog lock-in amplifier to recover sensor signals buried in noise for embeddedapplications, Proc. Eurosensors XXIV, Sept. 5-8, 2010, Linz, Austria.[2] Juh Tzeng Lue., Junction Impedance Measurement of Diodes by simplified lock inamplifier, IEEE, Trans. On Instrumentation and Measurement, vol.IM.26, No. 4. Dec, 1977[3] G. Busse, M. Bauer, W. Rippel and D. Wu, Lockin vibrothermla inspection ofpolymer compositesQIRT 92- Eurotherm Series 27 EETI ed., Paris 1992.[4] Adrian A. Dorrington and Rainer Kunnemeyer, A simple microcontroller baseddigital lock-in amplifier for the detection of low level optical signals, Proc. of IEEE, Inter.Workshop on Electronic Design, Test and Applications, 2002.[5] Maximiliano O.Sonnaillon., Raul Urteaga and Fabian., Jose Bonetto and MartinOrdonez., Implementation of A High frequency digital lock in amplifier, CCECE/ CCGEL,Saskatoon, May, 2005.[6] C8051F350 Data Manual, Cygnal Integrated Products, Inc, Austin, USA.[7] Nibu A. George et al, Laser induced photoacoustic technique for the detection ofphase transitions in liquid crystals, Nondestructive Testing and Evaluation, vol. 17, pp. 315-324, 2001.[8] P. Bhaskar, Immanuel J., and Bhagyajyoti, Design and development ofmicrocontroller based photoacoustic spectrometer, Sensors & Transducers, vol. 141, issue 6,pp-26-34, 2012.[9] P. Bhaskar, Design and development of computer based instrumentation system forphotoacoustic studies, doctoral diss., Sri. Krishnadevaraya University, Anatpur, August 2000. 26