Implicit Digital RMS Meter Design - IEEE Xplore

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A digital voltmeter is designed here to measure the rms value of a fluctuating voltage utilizing sampling and digital techniques. The technique used is based.
IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-33, NO. 4, DECEMBER 1984

Implicit Digital

RMS

257

Meter Design

SALEEM M. R. TAHA AND MAJID A. H. ABDUL-KARIM

Abstract-This paper introduces the design of an implicit-type digital rms-value measurement device. It employs TTL and LSI components throughout the design. It is based on the difference of squares principle. The instrument is simulated and analyzed through the aid of a digital computer program. The software analysis developed reflects the hardware design of the implicit digital rms meter. System accuracy is about (±0.1) percent.

I. INTRODUCTION A LTERNATING voltages can be measured by the conventional methods of finding the peak, mean, and rms values. The rms value is the most widely accepted representation of an ac signal level. A digital voltmeter is designed here to measure the rms value of a fluctuating voltage utilizing sampling and digital techniques. The technique used is based on the difference of squares principle. The instrument which measures the rms voltage is shown in block diagram form in Fig. 1. Gilbert [1] uses the same technique, but in analog form. This paper gives a digital counterpart that measures the rms value implicitly.

1A,

N

Ei~Vj2- N V2 =

(2)

-NV

1=1

I

: o N

N E

=i2

1/2

(3)

A zero logic detector is incorporated in front of the accumulator I output to capture the measurement process. The output will be taken from the accumulator II output digital terminals. This may be converted to its decimal equivalent for readout purposes. The accuracy of measurement is dependent on the word length and number of samples taken per cycle. This design is supported by an analysis executed by a digital computer program. The computer-aided design (CAD) program simulates the instrument and calculates its error and accuracy [2]. This leads to the appropriate choice of the digital hardware. In this case, N = 1024 samples/cycle and n = 7 bits (i.e., 7 bits ADC resolution), yielding an error of around 0.056 percent.

II. INSTRUMENT PRINCIPLE

Fig. 1 shows the block diagram of the proposed digital system of rms-value measurement. The input voltage Vi is converted to its digital equivalent via the appropriate analog-todigital converter (ADC) circuit. Then, the sum of the input/ output and the difference between them for each input sample are calculated and applied to a digital multiplier. The product will be in the form of the difference between the squares of input and output, i.e., the equivalent of (V12 - Vj). The products of all input samples are accumulated by the digital register (accumulator I). Accumulator II is used to accumulate the output of accumulator I at the end of each input cycle to its contents. A state will be reached when the accumulator II output V0 will force accumulator I output to zero. This condition happens during the recording of the rms value of Vi, i.e.,

III. CONCLUSIONS

The inplicit digital rms meter described here (Fig. 2) will measure the rms value of any complex varying signal by computing numerical values of simultaneous samples of the instantaneous input voltage. Usually, in such systems, the time taken to measure the rms value differs from one input wave shape to another. For instance, if the input is a sine wave of 50-Hz frequency, the rms value is measureable within 0.2 s [3]. The rms value is obtained as five decimal digits. The input frequency range is (30-70) Hz. It is capable of expansion by some simple additional circuitry. The accuracy of measurement is in the (±0.1 percent) class. The described system may be utilized as a multipurpose digital processing unit to measure power, energy, volt-amperes, and volt-amperes reactive. Although there is a commercially available single chip (BurrN Brown Model 4340) that converts the rms to dc, it has a poorer -V2) = .(1 E(V2 accuracy and has more limited applications compared with the i=1 described digital implicit system. The implicit digital rms-value measurement techniques calSince VO has been regarded as constant during one input culate the rms value implicitly and the system does not need cycle, the above equation becomes to use a square rooter, as is the case with existing explicit techniques. Hence, it offers less circuit complexity and lower cost. Manuscript received April 6, 1983; revised March 30, 1984. A microprocessor-based implicit system that provides the same The authors are with the Department of Electrical Engineering, Uniwould be very useful. capability versity of Baghdad, Baghdad, Iraq.

0018-9456/84/0900-0257$01.00

© 1984 IEEE

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-33, NO. 4, DECEMBER 1984

258

VI - Vo

2 2 Vj - VO

Fig. 1. Block diagram of the implicit digital rms meter.

Fig. 2. Complete circuit diagram of implicit digital rms meter.

REFERENCES [11 B. Gilbert, "Novel technique for R.M.S.-D.C. conversion based on the difference of squares," Electron. Lett., vol. 11, no. 8, pp. 181182, Apr. 17, 1975.

[2] S. M. R. Taha and M. A. H. Abdul-Karim, "CAD of implicit digital R. M. S. meter," IASTED 84, submitted for publication. [3] S. M. R. Taha, "Implicit digital R. M. S. voltage measurement," M.Sc. thesis, University of Baghdad, Baghdad, Iraq, Nov. 1981.