Microcontroller Based LCR Meter Essay Sample

A microcontroller based strategy for LCR measuring is described. The unknown component ( an inductance or a capacitance or a resistance ) is measured using a non conventional Ac span. The component to be measured signifiers one arm ( side ) of the span and the 2nd ( series ) arm is made up of a simple resistance. A Multiplier type Digital to Analog Converter ( MDAC ) . controlled by a microcontroller. serves as the other two weaponries. The microcontroller. after obtaining quadrature status between the span end product and one of the designated span electromotive forces. acquires the current through and voltage across the series connected resistance. With these values and the value of digital input to the MDAC. the parametric quantities of L or C or R values are evaluated by the microcontroller and displayed in appropriate show Fieldss. The strategy was implemented utilizing an Inte18751 microcontroller. An overall truth of the order of +1. 0 % was achieved for the paradigm with a 12-bit MDAC holding an truth of 4-0. 2 % . Keywords: LCR measuring ; Microcontroller-application ; Quasi-balanced span ; PSD-application

1. Introduction

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Measurement of constituents ( induction L. Capacitance C or resistance R ) is indispensable in many Fieldss of electrical and electronics technology. Several strategies for LCR measuring have been developed for general every bit good as specific applications. These methods can be usually grouped into ( a ) span methods and ( B ) direct methods. In a category of direct methods o f measurings. a known current is passed through the unknown electric resistance Z ( where Z could be L. C or R ) and the resulting electromotive force across it is measured and used for calculation [ 1. 2 ] . Direct methods are besides reported. wherein the unknown electric resistance is connected in series with a standard opposition and the series combination is excited by a sinusoidal beginning [ 3. 4 ] . The assorted electromotive forces across the criterion opposition. electric resistance and the beginning are used for the calculation O f electric resistance. The earliest and the most accurate methods of measuring O f an unknown electric resistance are the span methods [ 5. 6 ] . Since in these ac Bridgess. balance is obtained by changing two parametric quantities. convergence towards balance would affect several stairss. Complex automatic reconciliation techniques have been developed for a specific application of these Bridgess [ 7. 8 ] . To get the better of the job of equilibrating in conventional Ac Bridgess. quasi equilibrating methods have been proposed.

These quasi balanced bridge-forms require accommodation of merely one variable component and hence convergence to the balance status. in general. is obtained with few stairss. However. in conventional quasi balanced span signifiers. two independent quasi balances are obtained and the parametric quantities of the unknown Z computed therefrom. In most of these types of Bridgess. equilibrating involves accomplishing a “minimum” on the detected end product. The detection of such a lower limit detected voltage/ current becomes hard in most instances and under certain conditions the sensing becomes impossible [ 9 ] . We now propose a new attack to a conventional quasi balanced span. With an added Phase Sensitive Detector ( PSD ) . the “quasi balance” status is indicated by obtaining a nothing at the end product of the interposed PSD. Hence detection of quasi balance can be easy implemented. Alternatively of obtaining a 2nd quasi balance. here certain circuit responses are measured and the value of Z is computed. Hence. unlike the conventional Ac Bridgess. the truth of the proposed method would be chiefly dictated by the truth with which the circuit responses are measured.

The unknown electric resistance Z ( L or C or R ) is connected in series with a opposition Rs and the combination forms one of the ratio weaponries of an Ac span. A resistive possible splitter. with one subdivision holding opposition ( 1 – m ) Rp and the other holding opposition mRp forms the other two weaponries of the span. The span is excited by a sinusoidal electromotive force Vs at the needed frequence a ; as shown in Fig. 1 ( a ) . The phasor diagram of the electromotive forces for fluctuation in “m” is represented in Fig. cubic decimeter ( B ) for an inductance. The possible splitter is adjusted by changing ‘m’ such that the span end product electromotive force venereal disease and VR. the electromotive force bead across Rs due to current i. are in quadrature ( i. e. a 90 ° stage difference between the signals ) . Then the value of the unknown inductance can be deduced as where Vd. VR and I are the several rms values ofvd. VR and I. The span is about the same for the measuring of a electrical capacity except that weaponries mRp and ( 1 – m ) Rp are interchanged. The balance status of the span for the present constellation is indicated by Vd and the electromotive force across the electrical capacity. Vz in quadrature.

For the measuring of an unknown opposition R x either of the above processs could be adopted. If Rx is measured with the inductance option. its value would be straight off indicated by Eq. ( 2 ) . On the other manus. if Rx is measured in the capacitance manner. the value 1/Rx is indicated by Eq. ( 4 ) .

From Eqs. ( 1 ) to ( 6 ) it can be seen that finding of the constituent parametric quantities. after quasi equilibrating the span as indicated above. involves merely the measuring of the rms values of relevant circuit responses and measuring appropriate equations. These stairss can be easy implemented with a microprocessor/microcontroller. Such a strategy using a microcontroller is described here.

Fig. 2. Block ofthemicrocontroller LCR diagram based metre. given in Fig. 2. The unknown component in seriescombination with a opposition Rs is connected to Vs as explained earlier. The resistive possible splitter [ mRp and ( I – m ) Rp ] is implemented with a Multiplying type Digital to Analog Converter ( MDAC ) . whose mention is connected to v s through a buffer. Instrumentation amplitierAl and A2 convert Vz and venereal disease to individual ended ( land referenced ) measures. The current I is converted to a relative electromotive force by an opamp current-to-voltage convertor. The signals i. vR. venereal disease and Vz are connected to an rms-to-dc convertor through an 8-to-1 parallel multiplexer. The electromotive force venereal disease is besides given as input to a synchronal type Phase Sensitive Detector ( PSD ) [ 10 ] . The mention input of the PSD is selected as either VR or Vz. by the DPDT switch S1. depending on either L or C is to be measured. The other pole of S 1 is employed to bespeak the type of electric resistance being measured to the microcontroller through the choice of a logical input as “0” for C and “1” for L.

A 2nd switch S2. of SPST type. indicates to the microcontroller whether RL or Q for the induction ( Gc or tan 6 for the electrical capacity ) is to be computed and displayed. The District of Columbia end product of the PSD and the end product from the rrns-to-dc convertor are multiplexed by a 2-to- 1 parallel multiplexer and the multiplexer end product is given to the input of an parallel to digital convertor ( ADC ) . The ADC is interfaced to the microcontroller with needed interface logic. The microcontroller controls the MDAC. 8-to-1 and 2-to-1 multiplexers. the ADC and a set of seven section shows through necessary interfaces. For the measuring of excitement frequence w. the span electromotive force Vs. converted to a square moving ridge by a sine to square convertor. is given to one of the counters of the microcontroller. The quadrature status to be met for quasi-balancing is achieved with the aid of the PSD. For a synchronal type PSD end product is a dc electromotive force VpSD. given by: Vps D = Kps D Vp cos ? where KpsD is the P S D invariable. Vp the peak electromotive force of the input sinusoid and ~ the stage between the input and the mention signals of the PSD. If V ~ D is zero with a finite Vp so it turns out nicely that the input and mention sinusoids of the P S D are at quadrature. the needed status for quasi balance in our instance. The rnicrocontroller is programmed to quasi equilibrate the span by puting a suited binary value for “m” on the M D A C so as to obtain a zero end product on the PSD. After quasi balance is achieved. the rms values of relevant electromotive forces and current are acquired. appropriate equations are evaluated and the consequences displayed by the microcontroller. The possible systematic mistakes of the proposed strategy are discussed in the following subdivision.

By utilizing rms-to-dc convertors and ADC of high truth. the mistake due to this beginning can be minimised. The truth of frequence measuring depends upon the ratio of the span excitement frequence to the clock frequence of the microcontroller and can be improved by utilizing higher clock frequences. The declaration of the M D A C is the confining factor for the scene of “m” and its influence in the overall mistake can be minimised by taking a more accurate and high declaration MDAC. The systematic mistakes due to the footings in Eq. ( 7 ) merely add up to supply the worst instance mistake AL/L. The value of m should be big for understating the constituent Am/m. which could be realised by appropriately taking an appropriate R s. It may be mentioned here that in the instance of measuring of a pure resistance. the mistake due to frequency measuring does non calculate. Hence the overall mistake in the instance of measuring of pure resistances will be smaller than that compared to measuring of other parametric quantities. To look into the feasibleness of the proposed strategy. a paradigm was built utilizing an Intel 8751 microcontroller [ 11 ] and tested. The inside informations of the paradigm and trial consequences are discussed following.

5. Experimental consequences and decisions
The paradigm was built with normally available ICs like AD 7521 as MDAC. AD 637 as rms-to-dc convertor. MAX 134 as ADC. The of import IC Numberss are marked in Fig. 2. The plan for the 8751 is burnt into the E P R O M of 8751. The complete flow chart of the package is shown in Fig. 3 and is self explanatory. The paradigm was employed for measuring on laboratory standard inductances and capacitances ( available in decennary box signifier ) at frequences of 100 Hz and I kHz. The consequences were compared with measurings made with a Hewlett Packard HP 4274A multi frequence LCR metre holding a basic truth of ±0. 1 % .

Accuracies of the order of ±0. 5 % for inductances in the scope 20 mH to 200 mH and + 1 % for capacitances in the scope 1 nF to 10 nF were obtained with the paradigm. The major beginning of mistake in the paradigm is due to the limited declaration and one-dimensionality of the M D A C used ( for AD 7521. the declaration is +0. 025 % and truth. +0. 2 % severally ) for puting “m” . This would take to a finite mistake in the scene of nothing at the end product of the PSD. As mentioned earlier the truth of the rmsto-dc convertor and the ADC besides contribute to the overall inaccuracy of the system but are undistinguished compared to that of the MDAC. Hence using an M D A C holding better declaration coupled with good truth. one can increase the truth gettable. To reason. a new microcontroller based method for the measuring of L. C and R has been devised and by experimentation verified.


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