m. helmy a. raouf, rasha s.m. ali and m.s. gadelrab

Construction and Remote Calibration of an Automated Resistance Measuring System

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MAPAN - Journal of Metrology Society of India, Vol. 26, No. 2, 2011; pp. 125-131ORIGINAL ARTICLE

1. Introduction

In spite of the great efforts and time spent bymetrologists for manual calibrations; there is apossibility of several problems due to human errors.Generally, the accuracy of these measurements is oftenless than that of the automated measurements.Traditional resistance calibrations are limited to becontrolled inside the laboratory. For these reasons,several efforts have been made to develop new systemsfor the remote calibrations which can be controlledthrough the Internet or local Ethernet networks [1-2].One of the important demands of the remotemeasurements is the safety in hazardous fields suchas high voltage systems. The method of remotelysensing the voltage in a substation would be cheaperbecause insulation is not required [3]. So remotesensing of voltage using optical assessment of coronawas investigated.

In this paper, an automated calibration system isestablished to perform automatic calibrations of theresistances at NIS for the first time. Design andimplementation of the system are described. Its remoteoperation is obtained via internet using the "remotepanels" from National Instrument (NI) LabVIEW.Results of the straightforward resistancemeasurements are obtained to validate our automatedremote system.

2. Architecture of the Resistance Automated Remote Calibration System

As shown in Fig.1, this system consists of theResistors Automatic Changer (RAC), an instrumentfor resistance measurement, and a personal computer(PC1) to control the system inside the lab. It can beconnected remotely to any other personal computer(PC2) located at distant place through an internetconnection.

© Metrology Society of India, All rights reserved 2011.

Construction and Remote Calibration of an AutomatedResistance Measuring System

M. HELMY A. RAOUF*, RASHA S.M. ALI and M.S. GADELRAB

Department of Electrical Quantities Metrology, National Institute of Standards (NIS), Egypt*e-mail: [email protected]

[Received: 26.11.2010 ; Revised: 16.02.2011 ; Accepted: 21.02.2011]

AbstractThe calibration of resistances was being performed manually at National Institute of Standards (NIS),Egypt till now. In this paper, a fully automated system for the remote calibrations of resistances isdescribed. This sytem is mainly used for routine calibrations for low precision calibrations. The calibrationof many resistors can be performed in this system automatically through the Resistors AutomaticChanger which is controlled by a LabVIEW program developed and described in the present researchwork. Not only these calibrations of resistances are performed automatically but are also controlledremotely via an internet connection. Some simple remote and automated resistance measurements arecarried out just to confirm our new measuring system.

M. Helmy A. Raouf, Rasha S.M. Ali and M.S. Gadelrab

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Each one of the four resistors R1, R2, R3, and R4 isconnected to the corresponding electronic relay of theRAC as shown in Fig.1. Therefore, if relay1 is closed,by the micro-controller command, then R1 is onlyconnected to the RAC output and its resistance valuecan be measured by the indicated measuring device.Also if relay2 is closed then R2 is only connected tothe RAC output and its resistance value can bemeasured,……. and so on. Then the measurementresults are sent to the PC1 through GPIB card andIEEE-488 cable; using a specially designed LabVIEWprogram. Hence, these results can be analyzed andsaved as described in section 5. In addition, PC1 isconnected to the RAC by the computer serial port as

illustrated in Fig. 1. This system is accessed andcontrolled directly from PC1 as well as remotely fromany PC2 by using remote panels of NI LabVIEWprogram.

3. Fabrication of the Resistors Automatic Changer

It mainly consists of AT89C2051 micro-controllerand four electronic relays of the type PRME 15005A.This device can be controlled from outside or insidethe computer according to the selected position of thecontrol switch (CS) shown in Fig. 1. For the samenumber of readings, the same accuracy level isobtained using either manual or automatic operation

Internet Remote

PC 2

Rel ay3

Relay4 l 2

R1

R2

R3

R4

Rel ay1

Relay2

PC 1 RS232

AT89C2051

S1 S2 S3 S4

Serial Port

Measuring Device

GPIB Cable

RAC C S

Fig.1. Remote automated calibration system of resistance at NIS


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of this RAC. By using a prepared assembly programthat is stored in the micro-controller ROM, then anyresistor can be selected and its resistance value canbe measured at the RAC output terminals. Thisselection takes place manually by pressing one of thecorresponding 4- push-button switches named S1 toS4 without using the computer as illustrated in Fig.1.The manual operation is just as an additional optionfor this device, but the main and the principal aim ofthis research is the automatic operation which isaccomplished by the designed LabVIEW programshown in Fig. 2.

When the switch CS shown in Fig. 1 is convertedto another position, then the control from inside thecomputer is enabled through the computer serial port.Also, inside the LabVIEW program either manual orautomatic operation can be selected (Fig. 2). Thissnapshot shows that an automatic operation isselected and the time between each value and the nextone is adjusted to be three minutes. Furthermore, therequired sequence for obtaining these resistancesvalues is selected before running the program. Whenthe program runs, the stored code of the requiredresistance, R2, is transmitted by the serial port cable tothe corresponding electronic relay through the micro-

controller as shown in Fig. 1. Then this resistancevalue can be measured at the RAC output terminals,and so on for the rest three resistors. Accordingly anyfour resistors can be connected to this RAC, then anyof the four resistance values can be obtained with anydesired sequence and delaying time. This system isflexible enough so that the resistance can be measuredby any measurement device such as resistance bridge,DMM etc.

4. Remote Control

In order to make resistance calibration systemremotedly accessible, we opted for an Internet-enabledsolution. For this purpose, a feature called "remotepanels" from NI LabVIEW is used [4]. It can beconsidered as kind of client-server application.Enabling this feature is transparent for end-users whointend to use our system remotely at any place.Furthermore, we can embed the front panel into a webpage and operate it from a web browser. If we properlyconfigure the LabVIEW server on (PC1) and create apage web for the virtual instrument (VI), we only needto execute it from the client machine (PC2) via a webbrowser with LabVIEW run-time engine plug-in. Dueto space limitations, the configuration details are out-

Resistors Automatic Changer

Fig.2. Front panel of the designed LabVIEW program to control the RAC by the PC


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of-scope of this paper. However, the options are,generally, self-explained and straight forward toconfigure and in many cases default values can beused.

To realize this remote control, some steps shouldbe applied for both server and client sides. Hence aset of configuration steps on the server machine iscarried out (PC1 in our system) as shown in Fig. 1. Inthe following, we will explain briefly the main stepswhereas more details can be found in [5]:

i) Enable the LabVIEW Web Server on the servermachine, which is a networked computer that isrunning the LabVIEW Web Server.

# Tools>>Options>>Web Server: Configuration

ii) Specify a list of client IP addresses that are allowedto access the Web Server.

# Tools>>Options>>Web Server: Browser Access

# Using an asterisk (*) grants access to any IPaddress, which is not good for security concerns.

iii) Specify a list of VIs that can be accessed remotely.

# Tools>> Options>>Visible VIs

iv) Publish a web page to be accessible through theweb server for each VI you intend to make itremotely accessible.

# Tools>>Web Publishing

On the client-side (PC2), the steps usesummarized as follows:

i) Launch a browser (e.g., Internet explorer) withLabVIEW run-time engine installed on it.

ii) Write the IP address of the LabVIEW server in theaddress field followed by the name of VI web pagethat you have obtained from step 4 above. Forexample: http://192.168.1. 11/MyVi.html

iii) Begin using and/or controlling the device remotely.

As shown in Figs. 2 and 4, the front panels of theprograms of our application are kept as simple aspossible because the size and complexity of the frontpanel impacts control latency when executedremotely. Therefore we can control them remotely fromthe client machine PC2.

Fig. 3. Fully Automated system for 2-termianl resistance calibrations


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5. Fully Automated System for Resistance Calibrations

The values of the resistors needed to be calibratedare 100 kΩ , 1 MΩ , 10 MΩ and 100 MΩ . They arecalibrated directly by using digital referencemultimeter Model Fluke 8508A. The 8½ digitresolution and filter off, were some of the measurementconditions used. Internal circuits and their operationalprinciples are given in [6], illustrating in details allfunctions that can be measured and their possibleranges. Figure 3 shows the 4 resistors connected tothe RAC, which is in turn controlled automatically bythe PC1 through LabVIEW program shown in Fig. 2,via the illustrated serial port cable. The two outputterminals of the RAC are connected to the DMM bythe red and black terminals of the low thermal cableshown in Fig. 3. Then the resistances values aremeasured by DMM with the pre adjusted required

sequence and separating time. These results aretransmitted to the PC1 via a GPIB cable as depicted inFig.1, and controlled by Lab-VIEW program.

The results of the calibrated resistances arerecorded automatically, and the expandeduncertainty values are also estimated and computedfollowing the guidelines [7-8]. Basically this RAC is ascanner with 4 channel. Its performance was verifiedusing suitable validation method according to itsfunction [9]. The brief results of the performed two-terminal resistance measurements are listed in Table1 to illustrate the effect of the RAC circuit. Our RACeffect is included in the uncertainty budget shown inTable 2. The contribution of each uncertainty parameteris mentioned in Table 3, and the final expandeduncertainty values for all of the performedmeasurements are also evaluated.

Fig. 4. Front panel of the LabVIEW program for resistance calibrations

Table 1 Values of the measurands with and without the RAC using the DMM shown in Fig. 3

Nominal Measured value Direct measured Relative differencevalue using the value without RAC (D) (C-D)/C

RAC (C)

100 kΩ 101.076052 k Ω 101.006961 k Ω 7*10-4

1 M Ω 1.00952857 M Ω 1.00829572 Ω 1*10-3

10 M Ω 10.5422899 M Ω 10.5354714 M Ω 6*10-4

100 M Ω 99.991257 M Ω 99.980274 M Ω 1*10-4


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Table 3Uncertainty values for each resistor

Nominal value 100 k Ω 1 M Ω 10 M Ω 100 M Ω

Repeatability 0.33 Ω 2.17 Ω 29.08 Ω 80.63 ΩDMM Calibration 0.145 Ω 2.85 Ω 60 Ω 2.2 k ΩRAC Effect 40 Ω 710 Ω 3.94 k Ω 6.34 k ΩExpanded Uncertainty (± %), k=2 0.081 0.141 0.075 0.013

Table 2 Uncertainty sources of the two-terminal resistance measurements using the RAC circuit

Source of uncertainty Type of uncertainty Divider

Repeatability (n=28) A 1DMM Calibration Certificate B 2RAC Effect B √3

Fig. 5. Excel sheet of the 1.0 M resistance values


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Although the uncertainty is dominated by RAC,we will try to improve the performance of RAC toreduce its uncertainty enabling its use for highprecision calibration. Details of the measurements ofTable 1 are recorded automatically for each resistorand saved on PC1 in a separate excel sheet to avoidthe possibility of human errors while encoding orwriting data. Figure 5, shows a sample excel sheet ofthe 1.0 MΩ resistor as an example. This sheet caneasily be obtained and accessed remotely from theclient machine PC2. The Type A uncertainty valueindicated in this sheet is one of the components of theexpanded uncertainty recorded in Table 3, which isassociated to the 1.0 MΩ resistance value.

7. Conclusion

A fully automated resistance calibration systemis designed and developed with the ability to beremotely-controlled. The system is successfully usedfor resistance measurements which are carried outautomatically and remotely in the NIS for the first time.Its remote operation is easily accomplished throughsome simple steps which are applied to both serverand client machine sides. The establishedmeasurement system reduces the intensive labour ofmanual operation and improves the efficiency. It ismore suitable for routine calibrations where low levelof uncertainty is not required.

References

[1] C. Cassiago, L. CaIIegarol and G. La Paglia,Internet Calibration for Electrical Metrology:First Application at IEN, IEEE Instrumentation

and Measurement Technology Conference(IMTC), Como, Italy, (2004) pp. 102-105.

[2] Instrument-grade Data Acquisition withEthernet Brings Precision Measurement andControl to Networks, Keithley Instruments, Inc.,Printed in the U.S.A, Copyright, (2001).

[3] P. J. Moore, D. L. Hickery and M.G.G. UrbanejaRemote Sensing of Voltage Using OpticalAssessment of Corona, IEEE Power EngineeringSociety Summer Meeting, 2 (2000) 1164 - 1159.

[4] National Instruments "Lab-VIEW Website:http://www.ni.com/Lab-VIEW/optin/ ".

[5] National Instruments Tutorial, "Remote Panelsin Lab-VIEW - Distributed ApplicationDevelopment", September , (2006). URL:http://zone.ni.com/devzone/cda/tut/p/id/4791.

[6] Fluke Corporation 2002-2006, "Users Manual",Digital Reference Multimeter, Type 8508A,Manual Part Number 1673798, Rev., 3 (2002)3/06.

[7] The Expression of Uncertainty and Confidencein Measurement, United KingdomAccreditation Service (UKAS), M 3003 Edition2 (2007).

[8] M.H.A. Raouf, Manual/ AutomatedCapacitance Box Using Micro-ControllerTechnique, MAPAN- Journal of MetrologySociety of India, 26 (2011) 105-113.

[9] S.K. Jaiswal, Complete Characterization of aLow Thermal Scanner for Automatic VoltageMeasurement, MAPAN- Journal of MetrologySociety of India, 23 (2008) 31-38.

m. helmy a. raouf, rasha s.m. ali and m.s. gadelrab

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