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A Fully Automatic Multimeter Calibration System

ABSTRACTThis paper presents a ful ly

automatic multimeter calibration system using a programmable switch. The switch was specially designed to be programmable as well as portable and it also was designed to be low insertion loss and low thermal electromotive force (EMF). The switch can be programmed via a built-in micro-controller chipset and can be directly controlled by a personal computer via RS232 interface. Then the switch was inserted in the multimeter calibration system. Finally, the results of fully automatic calibration system were compared with that of the conventional manual system.

NIMT Vitawat Sittakul/Jutarat Tanarom/Narat Rujirat and Ajchara Charoensook

A Fully Automatic Multimeter Calibration System Using Programmable Switch

The obtained results confirm that the system can automatically operate without any signif icant negative effects and the calibration time is reduced from 35 minutes to 25 minutes.

Keywords: meter, automatic system, calibration

1. INTRODUCTIONNowadays many measurement

instruments have been used in all laboratories throughout the world. Unfortunately, their accuracies are mostly proportional to the 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 i m p r o v e m e a s u r e m e n t accu r acy , 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

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Vitawat Sittakul/Jutarat Tanarom/Narat Rujirat and Ajchara Charoensook

uncertainty components caused by statistical analysis (type A uncertainty) can be reduced [6-8]. This uncertainty source is based upon normal distribution and often results from random contribution such as human errors and repeated measurements. The human errors may come from several factors where human interface occurs, for example connecting loose cable or reading wrong data [2]. These considerations require human experience and correct instruction to avoid subjective judgment in cable handling and meter reading.

Unfortunately, not all calibration systems can run automatically. Based on author’s knowledge, one of the problems is that the calibration measurement instruments themselves may not have computer interface outputs to directly transmit measured data to Personal Computers (PC). This may due to cost reduction or it was not initially designed by manufactures but this issue currently can be solved using computer vision technology where the display screen is captured on a digital camera in image format and then performing image to data processing to obtain the measured data [1-5, 9-11]. Another problem is that there is necessity of cable re-arrangement even though some measurement instruments can be fully controlled via either Recommended Standard 232 (RS232) interface or General Purpose Interface Bus (GPIB) interface standardized by IEEE-488. This is because the system configuration might have to be changed during calibration processes. In these cases, a manual standard test method has to be applied and metrologists have to be standby entire calibration processes as a result labor cost is prohibitive. These processes are very time consuming and as mentioned earlier they increases human errors. Therefore, it is interesting if one could design a low-cost programmable switch which can perform as a cable router for cable re-arrangement. With the switch, a system

could be operated fully automatic. To date, similar devices known as multiple channel scanners may take this job but they have some limitations [12-15]. Most of them are no t des igned fo r par t icu la r jobs , unprogrammable, bulky and very expensive. For example, they may not endure high current, may not be portable, programmable and may contain high insertion loss and thermal EMF. These properties are very impor tant for ca l ibrat ing accurate measurement instruments such as accurate digital multimeters [16].

In this work, the signif icant contribution is the demonstration of automatic calibration system using implemented programmable switch. After programmed, the switch functions as a cable router which can re-configurate cables per manufacture calibration procedure and it can be controlled by a PC via RS232 interface. The RS232 interface was chosen to reduce necessity of expensive GPIB chipset usage and consequently the production cost was considerably reduced. The measurement instruments were controlled by a PC via GPIB interface. The programmable switch was

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specifically designed to be low-cost as well as low in un-preferred negative effects due to resistive, mutual coupling and potential effects. Therefore, the components inside the switch had to be carefully selected to minimize these effects [16]. Finally, the switch was introduced to the calibration system designed for multimeter calibration.

The commercial software (MET/CAL7.2) from FLUKE Company was used to automatically control the switch together with the measurement instruments via both GPIB and RS232 interfaces and the data were automatically recorded in the database. The measured data are the electrical quantities of multimeter which are electrical voltage, current and resistance. Then, the results were compared with that of a conventional manual calibration system. It has to be noted that the system can be operated by the other control software and thus other laboratories

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.

2. MULTIMETER AUTOMATIC CALIBRATION SYSTEM

The principle of automatic system can be seen in figure 1. The PC controller equipped with GPIB and RS232 built-in boards. The GPIB board is used to control the multimeter and the instrument standard (calibrator); a standard device to source constant and accurate values of DC/AC voltage, current and resistance.

These output values from the calibrator are generally used to compare with the reading values of the multimeter [Unit under Calibrator (UUC)] to ensure that they are still within the specification of the multimeter of the manufacturer. If the output of the multimeter is out of range, the instrument adjustment according to its manufacturer has to be performed. The RS232 interface is used to control the automatic switch which functions to re-route 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-preferred effects. The block diagram of the designed switch can be seen in figure 2.

Fig.1: Block Diagram of Automatic Calibration System

Fig.2: Block Diagram of Automatic Switch

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Figure 2 shows the main parts of automatic switch. The switch was designed to consist of 8 input ports and 7 output ports. The numbers of ports of the switch were defined from the minimum port requirement of calibration system. These input and output ports could be modified in future to fit with any different measurement system being used. The switch can be divided into two main par ts ; re lay c i rcu i t and the microcontroller circuit board. They will be detailed in the following sections. 3.1 Relay Circuit Design

The relay circuit here was designed as shown in figure 3. Here, eleven relays with the same part number of FTR-H1CA012V from Fujitsu component limited company were used. They all have maximum insertion losses of 100mΩ (measured at 1A and 6VDC) and endure the maximum current of 14A. The normal operation is at 12V with the operation current of 10A. It can be seen that the connection of circuit within the automatic switch can be re-configurated by turning operation of each relay ON and OFF.

For example, if the relay 1 is on, the input 3 will be connected to the output 1. If the relays 5 and 11 are on, the input 4 will be directly connected to the input 7 and thus the input short circuit between inputs 4 and 7 is achieved. It has to be noted here that the number of relays has to be minimized to reduce the insertion loss as much as possible. The material of the relay contact is made of gold plate silver alloy which is the same as that of the cables. This is to reduce the thermal EMF effects which are caused by the use of different materials. Theoretically, the thermal EMF is zero and independent to temperature if the same materials are used. 3.2 Microcontroller Design

The microcontroller circuit of the switch was designed using an 8 bit flash microcontroller (P89LV51RB2) with 80C51 CPU core. This allows specific commands to be programmed on the switch for different operations. The switch operation commands can be seen in the Table 1.

Fig.3: Block Diagram of Relay Circuit

No. Commands Status Input to Output

1. 1W1E LED 1 ON CH 1 to CH 1CH 2 to CH 2

2. 1W2E LED 2 ON CH 1 to CH 1CH 2 to CH 2CH 3 to CH 3

3. 1W3E LED 3 ON CH 1 to CH 1CH 2 to CH 2CH 3 to CH 1CH 4 to CH 2

4. 1W4E LED 4 ON CH 1 to CH 5CH 2 to CH 2

5. 1W5E LED 5 ON CH 5 to CH 5CH 2 to CH 2

6. 1W6E LED 6 ON CH 6 to CH 5CH 7 to CH 2

7. 1W7E LED 7 ON CH 6 to CH 7CH 7 to CH 2

8. 1W8E LED 8 ON No connected

Table 1: Commands for Switch Control

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Each command represents a specific set of cable arrangement. For instance, if the command <1W7E> is sent to the switch, the input channels 6 and 7 will be automatically connected to output channels 7 and 2 respectively. These commands can be re-programmed in the microcontrol ler regarding what the measurement system performed. In the microcontroller board, Light Emitting Diodes (LEDs) were included so that the operation status of the switch could be monitored.

The power supply was designed as can be seen in figure 4 and it was implemented to be full- wave bridge rectifier

with smoothing capacitors to supply voltages of 5 and 12 V to the microcontroller board and the relay circuit respectively. The dual transformers were used to down convert the home voltage from 220V. The full-wave bridge rectifier has been used here since it provides good output power stability which is necessary for calibration. 3.3 Switch Assembly and Testing

All components in the block diagram in figure 2 were brought together into a switch module as shown in figure 5. It can be seen that all components were assembled in a plastic block orderly to 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 control led condit ions (Temperature 23+/- 2 degree and Humidity 50+/- 15%). Figure 6 shows the configuration of automatic calibration system using the programmable switch.

The conf igura t ion was se t according to the block diagram in figure 1

Fig.4: Block Diagram of Power Supply

Fig.5: Automatic Programmable Switch

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Fig.6: Automatic Calibration System

and Unit under Calibration (UUC) service manual where the instrument standard in this test is the calibrator (Fluke 5720/5725) and the UUC is the multimeter (Fluke 45). The calibration procedures was written and programmed to the commercial calibration software (MET/CAL 7.2). The calibrator sourced AC/DC Voltage, AC/DC Current and resistance and the reading data from the multimeter were recorded and saved to the PC. Each measurement data point was averaged from five raw data points. To check the effects of the use of automatic switch in the system, the comparison test between the manual and automatic systems were performed. The results can be seen in the Table 2. The results reveal that both measured reading values are close to the applied inputs and they are within the mulitmeter specification [17]. Moreover, the system with the switch can operate without any negative effects. Both measurements are very similar and the variation of the measured data comes from the standard deviation of statistic. It can be seen that the resistance reading values for 0Ω is exactly zero since the cable and relays insertion losses were zeroed by the meter. The calibration time were reduced from 35 minutes to 25 minutes since the time of manual cable arrangement was removed.

Range Applied Input UUC Reading Manual Automatic switch

DC Voltage

0 mV 0 mV -0.003 mV -0.003 mV

/90 mV 90 mV 89.990 mV 89.991 mV

/900 mV 900 mV 899.94 mV 899.94 mV

/3 V 3 V 2.9997 V 2.9997 V

/30 V 30 V 29.995 V 29.995 V

/300 V 300 V 299.95 V 299.95 V

/1000 V 1000 V 999.88 V 999.88 V

AC Voltage

/30 mV 15 mV @ 1 kHz 14.994 mV 14.993 mV

15 mV @ 100 kHz 13.985 mV 13.984 mV

/300 mV 300 mV @ 1 kHz 299.78 mV 299.77 mV

300 mV @ 100 kHz 295.06 mV 295.10 mV

/3 V 3 V @ 1 kHz 2.9977 V 2.9977 V

/30 V 30 V @ 1 kHz 29.976 V 29.976 V

/300 V 300 V @ 1 kHz 299.84 V 299.84 V

/1000 V 750 V @ 1 kHz 749.82 V 749.81 V

Resistance

0Ω 0 Ω 0.00 Ω 0.00 Ω

/300 Ω 190 Ω 190.05 Ω 190.04 Ω

/3 kΩ 1.9 kΩ 1.9001 kΩ 1.9000 kΩ

/30 kΩ 19 kΩ 18.997 kΩ 18.996 kΩ

/300 kΩ 190 kΩ 189.97 kΩ 189.97 kΩ

/3 MΩ 1.9 MΩ 1.8999 MΩ 1.8998 MΩ

/30 MΩ 19 MΩ 19.0 MΩ 19.0 MΩDC 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 1 A 0.9990 A 0.9991 A

/10 A 9 A 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

Table 2: Measurement Results

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5. SUMMARYA fully automatic calibration system

using a programmable switch has been successfully demonstrated. The switch can be included in the calibration without negative effects for calibrating the multimeter (Fluke 45) using the source Calibrator (Fluke 5720/5725). The calibration time is reduced from 35 minute to 25 minute. This is because the cable re-configuration time can be eliminated since it was automatically performed.

6. ACKNOWLEDGEMENTThe work was co-funded by the

National Institute of Metrology (Thailand) and Thai government. The authors would like to acknowledge and appreciate their help. In addition, the authors would like to thank for the best support of the members of the electrical working instrument laboratory; Mr. Adithep Jang-on and Mr. Kongsak Tongboon.

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for Evaluating and Expressing the uncertainty of NIST Measurement Results, NIST Technical note, pp.1297, 1994.[7] NIS-81, The Treatment of Uncertainty in EMC Measurements, National Physical Laboratory, 1994[8] International Organization for Standardization / International Electrotechnical Commission, ISO/IEC Guide 17025 General requirements for the competence of testing and calibration laboratories, 2005.[9] A. T. P. So, W. L. Chan and K. C. Li, “A Computer Vision Based Fire Detection, Lighting and A i r -cond i t ion ing Cont ro l Sys tem” , Proceedings of CIBSE National Conference, Computers in Construction Industry, pp. 345-355, 1993.[10] W. L. Chan, L. S. L. Pang, C. F. Ma, “Computer V is ion App l ica t ions in PowerSubstations”, Proceedings of the 2004 IEEE International Conference on Electric Utility Deregulation, pp. 383-388, 2004[11] L. S. L. Pang, “The First Successful Application of Computer Vision Technology in Automating Multi-meter Calibration”, CLP Power TSD Technical Bulletin, Vol. 6, 2004.[12] Associated research company, “High voltage/high current modular scanning metrixes – model SC6540 datasheet”, 2011.[13] Hioki company, 3950 High Voltage Scanner datasheet, 2011[14] Guildline instrument company, Low thermal quad scanner model 6664B datasheet, 2011.[15] Transmille company, “10-Channel Low thermal scanner model 8500 datasheet”, 2011.[16] National instrument company, “How to reduce error when switching low-voltage signals”, National instrument white paper”, 2011[17] Fluke company, Fluke 45 datasheet, 2011.