A Fully Automatic Multimeter Calibration SystemDocument Transcript
NIMTSittakul/Jutarat Tanarom/Narat Rujirat and Ajchara Charoensook Vitawat A Fully Automatic Multimeter Calibration System Using Programmable Switch ABSTRACT The obtained results confirm that the system This paper presents a fully can automatically operate without any automatic multimeter calibration system significant negative effects and the using a programmable switch. The switch calibration time is reduced from 35 minutes was specially designed to be programmable to 25 minutes. as well as portable and it also was designed16 to be low insertion loss and low thermal Keywords: meter, automatic system, electromotive force (EMF). The switch can calibration be programmed via a built-in micro-controller chipset and can be directly controlled by 1. INTRODUCTION May-June 2012/Vol.14, No.67 a personal computer via RS232 interface. Nowadays many measurement Then the switch was inserted in the multimeter instruments have been used in all laboratories calibration system. Finally, the results of fully throughout the world. Unfortunately, their automatic calibration system were compared accuracies are mostly proportional to the with that of the conventional manual system. time period. As time passes, they may function incorrectly and generate some errors. The mistaken results from such instruments can cause serious problems in economic system and life safety since they will be used for validating product standards in the importing and exporting industries. In order to ensure that they work perfectly, the calibration process is required. In the past, the calibration has to be performed manually and this process usually takes long time. Presently, fully automatic calibration systems have been used worldwide and they play an important role in the calibration of measurement instruments [1-5]. They can improve measurement accuracy, repeatability and minimize routine jobs. Also they are ease of use, provide faster and convenient operation. Moreover, total uncertainties of such systems are basically improved due to the fact that one of
uncertainty components caused by statisticalanalysis (type A uncertainty) can be reduced[6-8]. This uncertainty source is based uponnormal distribution and often results fromrandom contribution such as human errorsand repeated measurements. The humanerrors may come from several factors wherehuman interface occurs, for exampleconnecting loose cable or reading wrongdata . These considerations requirehuman experience and correct instructionto avoid subjective judgment in cablehandling and meter reading. Unfortunately, not all calibrationsystems can run automatically. Based onauthor’s knowledge, one of the problems isthat the calibration measurement instrumentsthemselves may not have computer interface 17 May-June 2012/Vol.14, No.67outputs to directly transmit measured datato Personal Computers (PC). This may due could be operated fully automatic. To date,to cost reduction or it was not initially similar devices known as multiple channeldesigned by manufactures but this issue scanners may take this job but they havecurrently can be solved using computer some limitations [12-15]. Most of them arevision technology where the display screen not designed for particular jobs,is captured on a digital camera in image unprogrammable, bulky and very expensive.format and then performing image to data For example, they may not endure highprocessing to obtain the measured data current, may not be portable, programmable[1-5, 9-11]. Another problem is that there is and may contain high insertion loss andnecessity of cable re-arrangement even thermal EMF. These properties are verythough some measurement instruments can important for calibrating accuratebe fully controlled via either Recommended measurement instruments such as accurateStandard 232 (RS232) interface or General digital multimeters .Purpose Interface Bus (GPIB) interface In this work, the significantstandardized by IEEE-488. This is because contribution is the demonstration of automaticthe system configuration might have to be calibration system using implementedchanged during calibration processes. In programmable switch. After programmed,these cases, a manual standard test method the switch functions as a cable router whichhas to be applied and metrologists have to can re-configurate cables per manufacturebe standby entire calibration processes as calibration procedure and it can be controlleda result labor cost is prohibitive. These by a PC via RS232 interface. The RS232processes are very time consuming and as interface was chosen to reduce necessity ofmentioned earlier they increases human expensive GPIB chipset usage anderrors. Therefore, it is interesting if one could consequently the production cost wasdesign a low-cost programmable switch considerably reduced. The measurementwhich can perform as a cable router for cable instruments were controlled by a PC via GPIBre-arrangement. With the switch, a system interface. The programmable switch was
or industry companies could perform the same test. However, in this work MET/CAL 7.2 software was used for the test due to its availability, friendly interface and ease of use. This paper is organized as follows. Section 2 gives a basic concept of the designed automatic calibration system. Section 3 explains the designed components of the programmable automatic switch. Section 4 demonstrates the fully automatic calibration system using the programmable switch and the measurement results. Section 5 provides the summary. Fig.1: Block Diagram of Automatic Calibration System 2. MULTIMETER AUTOMATIC specifically designed to be low-cost as well CALIBRATION SYSTEM as low in un-preferred negative effects due The principle of automatic system to resistive, mutual coupling and potential can be seen in figure 1. The PC controller effects. Therefore, the components inside equipped with GPIB and RS232 built-in the switch had to be carefully selected to boards. The GPIB board is used to control18 minimize these effects . Finally, the the multimeter and the instrument standard switch was introduced to the calibration (calibrator); a standard device to source system designed for multimeter calibration. constant and accurate values of DC/AC The commercial software (MET/ voltage, current and resistance. May-June 2012/Vol.14, No.67 CAL7.2) from FLUKE Company was used to These output values from the automatically control the switch together with calibrator are generally used to compare the measurement instruments via both GPIB with the reading values of the multimeter and RS232 interfaces and the data were [Unit under Calibrator (UUC)] to ensure that automatically recorded in the database. The they are still within the specification of the measured data are the electrical quantities multimeter of the manufacturer. If the output of multimeter which are electrical voltage, of the multimeter is out of range, the current and resistance. Then, the results instrument adjustment according to its were compared with that of a conventional manufacturer has to be performed. The manual calibration system. It has to be noted RS232 interface is used to control the that the system can be operated by the other automatic switch which functions to re-route control software and thus other laboratories cable configuration wired between the instrument standards, multimeter and the automatic switch. After the calibration procedure is end, the reading results are sent back to the PC controller to compare with the specification issued by the manufacturer. 3. PROGRAMMABLE AUTOMATIC SWITCH DESIGN As mentioned earlier, the switch has to be carefully designed to reduce un- Fig.2: Block Diagram of Automatic Switch preferred effects. The block diagram of the designed switch can be seen in figure 2.
Figure 2 shows the main parts ofautomatic switch. The switch was designedto consist of 8 input ports and 7 output ports.The numbers of ports of the switch weredefined from the minimum port requirementof calibration system. These input and outputports could be modified in future to fit withany different measurement system beingused. The switch can be divided into twomain parts; relay circuit and themicrocontroller circuit board. They will bedetailed in the following sections. 3.1 Relay Circuit Design The relay circuit here was designedas shown in figure 3. Here, eleven relays withthe same part number of FTR-H1CA012Vfrom Fujitsu component limited companywere used. They all have maximum insertion Fig.3: Block Diagram of Relay Circuitlosses of 100mΩ (measured at 1A and6VDC) and endure the maximum current of14A. The normal operation is at 12V with the 19 May-June 2012/Vol.14, No.67operation current of 10A. It can be seen thatthe connection of circuit within the automatic Table 1: Commands for Switch Controlswitch can be re-configurated by turningoperation of each relay ON and OFF. No. Commands Status Input to Output For example, if the relay 1 is on, the 1. 1W1E LED 1 ON CH 1 to CH 1input 3 will be connected to the output 1. If CH 2 to CH 2the relays 5 and 11 are on, the input 4 willbe directly connected to the input 7 and thus 2. 1W2E LED 2 ON CH 1 to CH 1the input short circuit between inputs 4 and CH 2 to CH 27 is achieved. It has to be noted here that CH 3 to CH 3the number of relays has to be minimized toreduce the insertion loss as much as 3. 1W3E LED 3 ON CH 1 to CH 1possible. The material of the relay contact is CH 2 to CH 2made of gold plate silver alloy which is the CH 3 to CH 1same as that of the cables. This is to reduce CH 4 to CH 2the thermal EMF effects which are caused 4. 1W4E LED 4 ON CH 1 to CH 5by the use of different materials. Theoretically, CH 2 to CH 2the thermal EMF is zero and independent totemperature if the same materials are used. 5. 1W5E LED 5 ON CH 5 to CH 53.2 Microcontroller Design CH 2 to CH 2 The microcontroller circuit of theswitch was designed using an 8 bit flash 6. 1W6E LED 6 ON CH 6 to CH 5microcontroller (P89LV51RB2) with 80C51 CH 7 to CH 2CPU core. This allows specific commands 7. 1W7E LED 7 ON CH 6 to CH 7to be programmed on the switch for different CH 7 to CH 2operations. The switch operation commandscan be seen in the Table 1. 8. 1W8E LED 8 ON No connected
Fig.4: Block Diagram of Power Supply Each command represents a with smoothing capacitors to supply voltages specific set of cable arrangement. For of 5 and 12 V to the microcontroller board20 instance, if the command <1W7E> is sent and the relay circuit respectively. The dual to the switch, the input channels 6 and 7 will transformers were used to down convert the be automatically connected to output home voltage from 220V. The full-wave channels 7 and 2 respectively. These bridge rectifier has been used here since it May-June 2012/Vol.14, No.67 commands can be re-programmed in the provides good output power stability which microcontroller regarding what the is necessary for calibration. measurement system performed. In the 3.3 Switch Assembly and Testing microcontroller board, Light Emitting Diodes All components in the block (LEDs) were included so that the operation diagram in figure 2 were brought together status of the switch could be monitored. into a switch module as shown in figure 5. It The power supply was designed can be seen that all components were as can be seen in figure 4 and it was assembled in a plastic block orderly to implemented to be full- wave bridge rectifier reduce the coupling effects from metallic materials. 4. EXPERIMENT AND EXPERIMENTAL RESULTS In this section, the programmable switch was employed in the multimeter calibration system in the laboratory of National Institute of Metrology (Thailand) [NIMT] under controlled conditions (Temperature 23+/- 2 degree and Humidity 50+/- 15%). Figure 6 shows the configuration of automatic calibration system using the programmable switch. The configuration was set Fig.5: Automatic Programmable Switch according to the block diagram in figure 1
and Unit under Calibration (UUC) service Table 2: Measurement Resultsmanual where the instrument standard in this Range Applied Input UUC Reading Manual Automatic switchtest is the calibrator (Fluke 5720/5725) and DC Voltagethe UUC is the multimeter (Fluke 45). Thecalibration procedures was written and 0 mV 0 mV -0.003 mV -0.003 mVprogrammed to the commercial calibration /90 mV 90 mV 89.990 mV 89.991 mVsoftware (MET/CAL 7.2). The calibrator /900 mV 900 mV 899.94 mV 899.94 mVsourced AC/DC Voltage, AC/DC Current and /3 V 3V 2.9997 V 2.9997 Vresistance and the reading data from the /30 V 30 V 29.995 V 29.995 Vmultimeter were recorded and saved to the /300 V 300 V 299.95 V 299.95 VPC. Each measurement data point was /1000 V 1000 V 999.88 V 999.88 Vaveraged from five raw data points. To AC Voltagecheck the effects of the use of automaticswitch in the system, the comparison test /30 mV 15 mV @ 1 kHz 14.994 mV 14.993 mVbetween the manual and automatic systems 15 mV @ 100 kHz 13.985 mV 13.984 mVwere performed. The results can be seen in /300 mV 300 mV @ 1 kHz 299.78 mV 299.77 mVthe Table 2. The results reveal that both 300 mV @ 100 kHz 295.06 mV 295.10 mVmeasured reading values are close to the /3 V 3 V @ 1 kHz 2.9977 V 2.9977 Vapplied inputs and they are within the /30 V 30 V @ 1 kHz 29.976 V 29.976 Vmulitmeter specification . Moreover, thesystem with the switch can operate without /300 V 300 V @ 1 kHz 299.84 V 299.84 V 21 /1000 V 750 V @ 1 kHz 749.82 V 749.81 V May-June 2012/Vol.14, No.67any negative effects. Both measurementsare very similar and the variation of the Resistancemeasured data comes from the standard 0Ω 0Ω 0.00 Ω 0.00 Ωdeviation of statistic. It can be seen that the /300 Ω 190 Ω 190.05 Ω 190.04 Ωresistance reading values for 0Ω is exactly /3 kΩ 1.9 kΩ 1.9001 kΩ 1.9000 kΩzero since the cable and relays insertion /30 kΩ 19 kΩ 18.997 kΩ 18.996 kΩlosses were zeroed by the meter. The /300 kΩ 190 kΩ 189.97 kΩ 189.97 kΩcalibration time were reduced from 35 /3 MΩ 1.9 MΩ 1.8999 MΩ 1.8998 MΩminutes to 25 minutes since the time of /30 MΩ 19 MΩ 19.0 MΩ 19.0 MΩmanual cable arrangement was removed. DC Current 0 mA 0 mA -0.0003 mA -0.0003 mA /3 mA 3 mA 2.9996 mA 2.9995 mA /30 mA 30 mA 29.997 mA 29.997 mA /100 mA 100 mA 99.982 mA 99.981 mA /1A 1A 0.9990 A 0.9991 A /10 A 9A 8.9995 A 8.9997 A AC Current /3 mA 3 mA @ 1 kHz 2.9982 mA 2.9981 mA /30 mA 30 mA @ 1 kHz 29.986 mA 29.985 mA /100 mA 100 mA @ 1 kHz 99.953 mA 99.951 mA /1A 1 A @ 1 kHz 1.0004 A 1.0004 A /10 A 9 A @ 1 kHz 9.0021 A 9.0023 A Calibration Time 35 minutes 25 minutes Fig.6: Automatic Calibration System
5. SUMMARY for Evaluating and Expressing the uncertainty A fully automatic calibration system of NIST Measurement Results, NIST using a programmable switch has been Technical note, pp.1297, 1994. successfully demonstrated. The switch can  NIS-81, The Treatment of Uncertainty in be included in the calibration without EMC Measurements, National Physical negative effects for calibrating the multimeter Laboratory, 1994 (Fluke 45) using the source Calibrator (Fluke  International Organization for Standardization / 5720/5725). The calibration time is reduced International Electrotechnical Commission, from 35 minute to 25 minute. This is because ISO/IEC Guide 17025 General requirements the cable re-configuration time can be for the competence of testing and calibration eliminated since it was automatically laboratories, 2005. performed.  A. T. P. So, W. L. Chan and K. C. Li, “A Computer Vision Based Fire Detection, Lighting and 6. ACKNOWLEDGEMENT Air-conditioning Control System”, The work was co-funded by the Proceedings of CIBSE National Conference, National Institute of Metrology (Thailand) and Computers in Construction Industry, pp. Thai government. The authors would like to 345-355, 1993. acknowledge and appreciate their help. In  W. L. Chan, L. S. L. Pang, C. F. Ma, addition, the authors would like to thank for “Computer Vision Applications in the best support of the members of the PowerSubstations”, Proceedings of the 200422 electrical working instrument laboratory; Mr. IEEE International Conference on Electric Adithep Jang-on and Mr. Kongsak Tongboon. Utility Deregulation, pp. 383-388, 2004  L. S. L. Pang, “The First Successful 7. REFERENCES Application of Computer Vision Technology May-June 2012/Vol.14, No.67  M. J. Flynn, S. Sarkani, and T.A. Mazzuchi, in Automating Multi-meter Calibration”, CLP “Aggression Analysis of Automatic Power TSD Technical Bulletin, Vol. 6, 2004. Measurement Systems”, IEEE Transactions  Associated research company, “High on instrument and Measurement, Vol. 58, voltage/high current modular scanning pp.3373 – 3379, 2009. metrixes – model SC6540 datasheet”, 2011.  E. M. Warner and J. L. West, “Programmable  Hioki company, 3950 High Voltage Automatic Multimeter Calibration System”, Scanner datasheet, 2011 IEEE Transactions on instrument and  Guildline instrument company, Low thermal Measurement”, Vol.IM-27, pp. 156-159, 1978. quad scanner model 6664B datasheet, 2011.  C. L. Chen and S. C. Wang, “A PC-Based  Transmille company, “10-Channel Low Adaptive Software for Automatic Calibration thermal scanner model 8500 datasheet”, 2011. of Power Transducers”, IEEE Transactions  National instrument company, “How to on instrument and Measurement, Vol. 46, reduce error when switching low-voltage pp. 1145-1149, 1997. signals”, National instrument white paper”, 2011  F. C. Alegria and A. C. Serra, “Automatic  Fluke company, Fluke 45 datasheet, Calibration of Analog and Digital Measuring 2011. Instruments Using Computer Vision”, IEEE Transactions on instrument and Measurement, Vol. 49 pp. 94-99, 2000.  S. L. Pang and W. L.Chan, “Computer Vision Application in Automatic Meter Calibration”, Industry Applications Conference Fourtieth IAS Annual Meeting, Vol. 3, pp.1731 – 1735, 2005.  B. N. Taylor and C. E. Kuyatt, Guidelines