Microcontroller Based Dielectric Constant Measurement

Transcription

1 Sensors & Transducers Magazine, ol.4, Issue, March 004, pp.8-90 Sensors & Transducers ISSN by IFSA Microcontroller Based Dielectric onstant Measurement A. aendran, P. Neelamegam* PG and esearch Department o Applied Physics, Nehru Memorial ollege, Puthanampatti, Tiruchirappalli, Tamil Nadu-6 007, India *PG and esearch Department o Physics, AM Sri Pushpam ollege, Poondi, Thanavur, Tamil Nadu-6 50, India eceived: 5 February 004 /Accepted: 0 March 004 /Published: March 004 Abstract A dielectric constant measurement setup has been developed to measure dielectric constant o liquids using AT8955WD microcontroller. A modiied operational ampliier based A Schering Bridge network is used to compute capacitance and hence calculate dielectric constant o liquids. This instrument system permits recording o dielectric constant o liquids at various concentrations and temperature and sends data to a computer to enable the computer processing o such data. A dedicated AT8955WD (8-bit) based microcontroller and its associated peripherals are employed or the hardware. The details o its interace to measure dielectric constant o liquids, temperature and to control the temperature at desired range and evaluate results are explained in this paper. Keywords: A Shering Bridge, dielectric constant, temperature measurement, microcontroller. Introduction Dielectrics are basically insulator materials having a special property o storing and dissipating electrical energy when subected to electromagnetic ields. The dielectric constant o materials contains detailed inormation about physical and chemical composition and structure []. Studies o these materials particularly in ac ields provide an insight o the electrical nature o the molecular or atomic species, which constitute the dielectric materials. The decrease in unit cost and the increase o on-chip capabilities have enabled the use o single chip microcontroller in instrumentation and measurement technology. This development system enabled use o compensation, calibration and linearization techniques and application o Microcontrollers in system control and data acquisition and processing. This paper presents measurement o dielectric constant o liquids, which takes ull advantage o the microcontroller acilities. In the present work an operational ampliier based modiied A Shering bridge network have been used to measure dielectric constant o liquids. 8

2 . Experimental Sensors & Transducers Magazine, ol.4, Issue, March 004, pp.8-90 Figure shows the block diagram o dielectric constant measurement set up. The capacitance cell is connected to one arm o a modiied A Shering bridge network is kept in the block A. The cylindrical cell is immersed in a beaker containing liquids. The circuit is unbalanced depends upon the dielectric constant o a liquids. The output o the bridge network is rectiied using precision rectiier, which is kept in block B. Block consists o an AD590 is used as a sensor to measure temperature o the solution and instrumentation ampliier, which ampliies the signal rom temperature sensor. The block D is a multiplexer (I 405), which is used to select dielectric constant or temperature. The output o multiplexer is given to the analog to digital converter (I MP0), which is kept in the block E. The Microchip Technology Inc. MP 0 is a successive approximation - bit (A/D) converter with the device is done using a simple serial interace compatible with the SPI protocol. The device is capable o sample rates o up to 00ksps at a clock rate.6 MHz. The MP0 operates over a broad voltage range (.7-5.5). It is used to convert the analog dielectric constant and temperature into digital values. Block G is an AT8955WD microcontroller rom Atmel ompany, is a low power, high perormance MOS 8-bit microcontroller with 0KB o lash programmable and erasable memory and 56 bytes o AM. It has our parallel ports, three 6-bit timers/counters, eight interrupt sources, one programmable serial port, low power ideal and power down modes and it has the acility o three level program memory lock. Two o its Port and Port are being employed as input port and another Port 0 as an output port. The solid-state power controller with heater, which is kept in the block F, is used to heat the urnace. The rate o heating is controlled at any temperature by proper commands rom the microcontroller. Block H is a two-row 6 characters LD display rom Hitachi is interaced with microcontroller through Port to display the measured data and results. Block I consists I MAX is a dual S transmitter/receiver interace circuit that meets all EIA S speciication. It requires a single 5 supply. The output MAX is connected to OM port o computer, which is kept in the block J. A keyboard is connected kept in block K or getting inputs. A B H D D E G I J F K A Modiied operational ampliier based A Shering bridge BPrecision ectiier AD590 Temperature sensor & Signal conditioning circuit D Multiplexer E -bit Serial A/D converter (MP0) F Solid state controller with heater G AT8955WD Microcontroller H LD I MAX JP K Keyboard Fig. Block Diagram o AT8955WD Microcontroller Based Dielectric onstant o Liquids Measurement Setup 8

3 . ircuit Description Sensors & Transducers Magazine, ol.4, Issue, March 004, pp.8-90 A general A Shering bridge network is modiied as shown in igure, where two very high gain operational ampliiers U and U are connected with the bridge network [] with the non-inverting terminal connected to the circuit ground. This enables the bridge output nodal points B and D to be almost at the same potentials with respect to the ground and hence the eect o stray capacitance that will exist between them and also between them and ground, may be assumed to be minimized. Since B and D are at virtual ground, so or the sinusoidal supply voltage i m sint, the currents through the bridge impedances,, and 4 are respectively given by I I I 4 I 4, () 4 where is the output voltage o the operational ampliier U. I 0 is the output voltage o the operational ampliier U then the current through its eedback resistance is given by I 0 Applying Kirchho s current law and I I 0 () I I I 0 () 4 The equation () and () give i 0 i (4) The equations () and () give i 0 0 (5) 4 The equations (4) and (5) give (6) ( 4 ) i 0 At balance condition o the bridge, 0 0, which is identical with the conventional bridge network. For the modiied A Shering bridge circuit 8

4 Sensors & Transducers Magazine, ol.4, Issue, March 004, pp Hence, rom the equation (6), the bridge output voltage is given as [ ] i x x 0 I the bridge is balanced at the minimum value o the process variable or which the capacitance o a transducer is 0, then 0 0 or X 0, also, 0-0 and 0-0. is the change in capacitance or a given change o the process variable above this minimum value. The capacitance cell is connected instead o. The capacitance o a liquid is determined by the given equation 0 i 0 i π π π, (7) where 0 is the bridge output voltage; i is the input excitation voltage; is the eedback resistance; is the A excitation requency; is the capacitance o a solution. A dielectric constant o liquid: air medium r ε, (8) where medium is the capacitance when the cell illed with a dielectric liquid; air is the capacitance in air. Figure shows the circuit diagram or the modiied A Shering Bridge with a precision rectiier. The precision rectiier (U & U4) is implemented with the operational ampliier. The temperature transducer is also presented in Figure, includes an integrated circuit temperature sensor AD590, which delivers a current proportional to the temperature and a signal conditioning circuit, which converts the current to a voltage. The temperature sensor is almost insensitive to line voltage drops due

5 Sensors & Transducers Magazine, ol.4, Issue, March 004, pp.8-90 to its high output impedance and its high intererence reection results rom the output being current rather than a voltage. The output current rom AD590 lows through a KΩ resistance, thereby developing a voltage o m/ ºK. The output o the.5 dc reerence voltage obtained rom a voltage regulator is divided down by resistors to provide an oset o 7 m required to have º as the reerence temperature. This oset is subtracted rom the voltage across KΩ resistor and ampliied by a second ampliier (U5&U6). Fig. Modiied A Shering Bridge and Temperature Measurement ircuit The circuit diagram o AD interace with microcontroller is shown in Figure. It consists o AT8955WD microcontroller (U9), AD MP0 (U8) and MAX and LD. P0.0, P0. and P0. are connected to S, D OUT and LK signals o MP 0 bit serial A/D converter. LD is connected to Port o microcontroller. P. and P.4 bits o Port are used to select either dielectric constant or temperature using the multiplexer 405(U7). Figure 4 shows solid-state power controller, which is used to control the power to the heater. The power controller is establishing a variable duty cycle switch with zero cross over switching. The circuit is built around I 555 timers, triac driver (MO 04), triac (BT6). U0 wired as a variable duty cycle oscillator with a constant time period o around 0. second. Duty cycle can be varied rom 0 to 00% using the resistance to 7, which are selected by the multiplexer (U). The duty cycle can be changed by proper words rom the microcontroller to pin 9,0 and o 405. U wired as comparator with hysteresis (i.e.) Schmitt trigger. The transormer T with rectiying diodes D and D delivers unidirectional. The low A voltage is given to pin and 6 o U. The output o U8 rom pin is connected to pin o triac driver as shown in the igure. MO 04, which incorporates a zero voltage crossing bilateral triac driver providing low power dc control o power triac, is used to drive 85

6 Sensors & Transducers Magazine, ol.4, Issue, March 004, pp.8-90 the triac. By outputting 0 to 7 to port o microcontroller the duty cycle o oscillator o an U0 can be changed and hence the power to the heater can be varied. Fig. ircuit Diagram o Microcontroller and-bit serial A/D onverter Fig. 4 ircuit Diagram or Solid Sate Power ontroller 86

7 Sensors & Transducers Magazine, ol.4, Issue, March 004, pp Measurement and Details o Sotware A ixed sinusoidal excitation voltage o is applied to the bridge. The dielectric constant were measured at KHz using a water circulating arrangement in the temperature is maintained at desired temperatures by giving data to microcontroller through the keyboard. The dielectric constant is also measured or various concentrations. Sotware is developed in assembly language and language to initialize LD display, serial port, selection o temperature or dielectric constant (capacitance) by multiplexer, to select 0 A/D converter, to receive bit data serially rom the data output pin, measure the dielectric constant o the sample rom equation (9), temperature o the sample, data computation or acquired data, storage o data or dielectric constant and temperature, and to send data to P. To transmit data to P, sotware is written to select UAT mode, to select timer or non gated auto reload, to set 4800 baud rate, to start timer to send data to transmit buer, so that a P can read data through OM port. For the development o hardware and sotware 8055-microcontroller kit, which has six parallel ports is used. Ater development the codes are stored in the program memory (lash EPOM) o the AT8955WD microcontroller and program is executed. The lowchart or perorming the above tasks is shown in Figure 5. Sotware is also developed to initialize OM port in the P, to receive data rom the microcontroller, to store data in a ile and to send commands to microcontroller. 5. esults and Discussion The perormance o the microcontroller-based instrument is measuring dielectric constant o liquids, is investigated by comparing its response with result by other methods [-4]. The instrument is calibrated prior to the actual sample measurement with A grade pure organic solvents like benzene, carbon tetra chloride, acetic acid etc., whose dielectric constants are precisely known rom the literature [5]. Table shows dielectric constant or various liquids. The results o dielectric constant measurement using this instrument are compared with the Digital capacitance meter (asavi Electronics, India) to check the accuracy o the instrument and it is ound that both results agree and hence this instrument can be used to measure dielectric constant. The Figure 6 shows the dielectric constant o diethanolamine, 4 dioxan, cyclohexylamine benzene and triethylenetertramine benzene or various concentrations at 0º. The range o the instrument is used to measure the dielectric constant up to 0.The error in measurement is ound to be less then %. The variation o dielectric constant o the solutions or various temperatures is given in the Figure 7. The measurement system was tested with dierent samples to check the reproducibility. One common eature o the system is that the microcontroller can handle the process o dielectric constant, temperature measurement and control unctions in addition to data acquisition, data storage, data manipulation, displaying and decision making operations. Moreover, the system is easily operated and does not require any programming expertise. In this instrument the manual supervision involved is little. The system is highly reliable, low cost, and portable. 87

8 Sensors & Transducers Magazine, ol.4, Issue, March 004, pp.8-90 STAT INTIALIE POTS, SEIAL POT AND LD ALL AD MEASUE OLTAGE NO DISPLAY SELET TEMPEATUE Is key ALL AD YES MEASUE OLTAGE POESS TEMPEATUE AND ONTOL THE TEMPEATUE BY POPE OMMANDS DISPLAY SELET DIELETI ONSTANT POESS APAITANE IN MEDIUM (π ) π (π ) i POESS DIELETI ONSTANT medium ε r air DISPLAY TEMPEATUE AND DIELETI ONSTANT Is key? NO YES DISPLAY MEASUE APAITANE IN AI SEND DATA TO P THOUGH OM POT ONTINUE? ALL AD MEASUE OLTAGE POESS APAITANE IN AI YES END NO (π ) π (π ) i STAT DISPLAY MEASUE APAITANE IN MEDIUM SELET AD, SEND LK SIGNAL, EEIE DATA SEIALLY FOM THE OUTPUT PIN NO Is key YES ETUN Fig.5 Flow hart or Perorming Dielectric onstant Measurement System 88

9 Sensors & Transducers Magazine, ol.4, Issue, March 004, pp.8-90 Table. Dielectric constants or various liquids Dielectric constant (ε r ) Samples Present eerence work Benzyl chloride hloro benzene l Benzene.6.84 Toluene.4.80 Acetic Acid Fig. 6 urves or Dielectric onstant o Liquids at arious oncentrations 89

10 Sensors & Transducers Magazine, ol.4, Issue, March 004, pp.8-90 Fig. 7 The Dielectric onstant o Liquids at arious Temperatures eerences [] Hoppe Walter, Bio Physics, Springer-erlog, NewYork, 98. [] S.. Bera, S. hattopadhyay and J.K.oy, A Modiied approach to the Measurement o capacitance o a capacitive transducer by an A Bridge network, IE (I) ournal-el, 00, ol.8, pp [] N. helliah,. Sabesan, Dielectric studies o hydrogen bonded complexes o Mono and dihydric alcohols with amines, Indian Journal o Pure & Applied Physics, 994 ol., pp [4] T. Thenappan, M. Subramanian, Studies on luid structure and molecular interactions o amines, Indian Journal o Pure & Applied Physics, 00,ol.9, pp [5] John. A. Dean, Handbook o Organic hemistry, McGraw-Hill Book ompany, New Delhi opyright, International Frequency Sensor Association (IFSA). All rights reserved. ( 90