Single Input Virtual Digital Multi-meter Design and Implementation

Kai-chao Yao Li-ling Chao National Chang-hua University of Education

E-mail:kcyao@cc.ncue.edu.tw

Abstract

In this research, a single input virtual digital multi-meter are designed and constructed. It consists of three major works: (1) Software design of front panel (2) Measurement circuit design (3) System integration. The related measurement theories are also described and introduced. This digital multi-meter is made by software part, Labviewhardware part, DAQ card and external measurement circuits. The measurement functions include (1)DC voltage (2) AC voltage (3) DC current (4) AC current (5) Resistance (6)Capacitance (7) Inductance (8) Diode test (9) Audio continuity. In the illustrations, demonstrate the design of front panels and also proceed the measurement test of the designed single input virtual digital multi-meter to show the capabilities. Key Words: virtual instrument, programmable, digital muti-meter, design, Labview

1. Introduction

Virtual instrument employ a computer screen as a front panel of a physical instrument. Buttons, knobs and signal windows can be operated in a similar way. The hardware behind the panel may vary widely, from a simple PC with a plugged in card, to a sophisticated VXI mainframe incorporating powerful cards[1]. Also, the instruments can be interconnected through a standard bus to create a very complex instrument [2]-[6].

Virtual instrumentation is today a common point that has dramatically changed the concept of traditional instrumentation, and it is therefore necessary to incorporate it as a main topic in the classroom. LabVIEW is a programming environment based on the concept of data flow programming. This programming paradigm has been wisely used for data acquisition and instrument control. These are three important components involved in test and measurement applications, namely data acquisition, data analysis and data visualization. Labview features an easy-to-use graphical programming environment, which covers these vital components [7]-[12]. Many exciting experiments can be design and demonstrated by integrating these powerful

virtual instrument technology products in a flexible laboratory environment with enormous possibilities of expansion and experimentation[13]-[16].

2. Systems

The experimental system is shown in Figure 1 and the required devices and tools are listed below: Computer: Pentine IV Signal Acquisition Device: (DAQ Card) PCI-6251 M

SeriesSoftware: Labview7.1 and NI ELVIS 3.0 Hardware: NI ELVIS systems shown in Figure 1.

Figure 1: Virtual instrument workstation In Figure 1, the parts of marked numbers are

explained below: 1. Desk Computer with Labview installed. 2. Data Acquisition Card. 3. 68 pin Shielded Cable. 4. NI ELVIS Benchtop Workstation.

3. Main Results

The construction of the single input virtual digital multi-meter can be divided into the following five important parts: 1.Front Panel Design: The designed digital multi-meter possesses the following functions: (1)Display Window - The Display Window has the following indicators: ( )% FS—Displays the percentage of the current range that is being used. ( )Activity LED—Indicates that the DMM is running. ( )Diode Status—Displays either GOOD or OPEN to indicate the status of the diode being measured. ( )Null Value—Displays the measurement that was active when the Null button is pressed.

Function Controls

0-7695-2882-1/07 $25.00 ©2007 IEEE

The function controls palettes includes: ( )DC voltage ( ) AC voltage ( ) DC current ( ) AC current ( ) Resistance ( )Capacitance ( )Inductance ( ) Diode test ( ) Audio continuity. (3) Null, Run, and Single Buttons ( )Null Button—Selects Relative mode. ( )Run Button—Starts or stops measurements.( ) Single Button—Acquires a single measurement.

Range Controls-determines the input range for the selected DMM measurements. 2. Data Acquisition devices – DAQ Card

In this research, M Series PCI-6251 is used as data acquisition interface. In Figure 2, showing the shape and pinout of PCI-6251. In the device, one side is connect to data acquisition card and the other side is connected to prototyping board of workstation as Figure 1.

Figure 2: M Series PCI-6251 pinout

A brief process to install the DAQ card: Step 1: Install application software - Labview 7.1Step 2: Install DAQ card driver first, before

assembling DAQ card into the desktop computer. The process can ensure WINDOWS to detect the DAQ card.

Step 3: Install the necessary devices, accessories and cables.

Step 4: Power on the computer. Step 5: Confirm that the device is recognized. Step 6: Run test Panel. 3.Connnecting Signals:

Table 1 shows how the NI ELVIS input channels map to the DAQ device input channels.

Table 1. Analog Input Signal Mapping NI ELVIS input DAQ card input

AH0+ AI 0ACH0– AI 8ACH1+ AI 1ACH1– AI 9ACH2+ AI 2ACH2– AI 10ACH3+ AI 3ACH3– AI 11ACH4+ AI 4ACH4– AI 12ACH5+ AI 5ACH5– AI 13AISEN AI SENS

AIGND AI GND

These six channels can be used for external measurement circuits design and other signal acquisition. For example, the Labview control switch of external measurement circuits in this research use one of these six channels to control the selection of measurement circuits. These channels make the virtual digital multi-meter possess more flexibility and expansibility. 4. External measurement Circuits (1) The measurement of DC voltage, AC voltage, Resistance, Capacitance, Inductance, Diode test and Audio continuity.

The differential channel of the DAQ device (AI 7 and AI 15) is used to read the voltage signal and applies a gain of 0.5 to the voltages that are applied to VOLTAGE HI and VOLTAGE LO of the measurement side.

Figure 3 shows a basic block diagram of the NI ELVIS voltmeter.

Figure 3. A basic block diagram of the NI ELVIS voltmeter.

A manual adjustment is made for common-mode rejection. The adjusted common-mode rejection is typically above 80 dB. In Figure 4. a common mode rejection adjustment circuit that can be used in voltage measurement circuit of Figure 3 is illustrated.

Figure 4. a common mode rejection adjustment circuit

(2) The measurement of DC current and AC current When the differential channel of the DAQ

device (AI 7 and AI 15) is also used to read the current signals. The current signal is referenced to the NI ELVIS GROUND. The current is measured across the CURRENT HI and CURRENT LO terminals of the measurement side. The current flowing across the shunt is converted to voltage by a difference amplifier.

Figure 5 shows a basic block diagram of the NI ELVIS current meter.

Figure 5. A basic block diagram of the NI ELVIS current meter.

Integrate Figure 3, Figure 4, Figure 5 and a Labview controlled circuit selection switch, a single input virtual digital multi-meter can be design and constructed. Figure 6 show the block diagram of this virtual digital multi-meter.

Figure 6. The block diagram of this virtual digital multi-meter

In Figure 6, the Labview controlled circuit selection switch applies 4066IC , 4069IC and designed software of front panel to achieve a single input measurement feature of this virtual digital multi-meter. The designed circuitry of this Laview controlled switch is show as Figure 7.

Figure 7: Laview controlled I/O circuit selection switch

A complete external measurement circuit design illustrated in Figure 8.

Figure 8. External measurement circuit design

5. The specifications of Virtual Digital Multi-meterTable 2. The specifications of Virtual Digital

Multi-meter Capability Specifications

Resistance Accuracy: 1 % Range: 5 to 3M

DC Voltage Accuracy: 0.3% Range: ±20 V Input impedance: 1M

AC Voltage Accuracy: 0. 3% Range: ±14 Vrms

Current DC accuracy: 0.25% +3mA AC accuracy: 0.25% +3mA Proper null correction at the common mode voltage can reduce +3mA error to 200 μA noise Range: ±250mA Shunt resistance: 0.5 Maximum Common mode voltage: ±20 VCommon mode rejection: 70dB min

Capacitance Accuracy: 1% Range: 50pF – 500μF Test voltage: 1Vp-p

Continuity Resistance threshold: 15 max

Inductance Accuracy: 1% Range: 100 μH to 100mH Test frequency: 950Hz Test frequency voltage: 1V p-p

DiodeMeasurement

Voltage threshold: 1.1 V max

4. Measurement Test and Designed Panels

In Figure 9, the a single input virtual digital multi-meter is used to measure the 1N4001 diode for barrier potential of forward bias. The measured results show 0.71(V) which is similar to the value, 0.7 (V), expected.

In Figure 10, the designed single input virtual

digital multi-meter is used to measure a resister which has R= k6.4 . The display of front panel shows the measured number that is nearly close to the tag value.

5. Conclusions

In this research, a single input virtual digital multi-meter are designed and constructed. The process consists of three major works: (1) Front panel software design (2) Measurement circuit design (3) System integration of hardware and software. The measurement display and function selection of front panel is programmable. In the external measurement circuit , a quit different design comparing with traditional one is combining Labiew I/O control features, switch IC and inverter IC to achieve automatic selection switch between voltage and current measurement circuits. This kind of circuit design method possesses more flexibility and expansibility.

6. References

[1] A.Manuel, F.Sanchez. J.pral, D.Biel y J.Olive. Inslrumentaciu virtual. Adquisicio, pmcessamenr i analisi de senyals. Edicions UPC, 1997. [2] Fengying Cui,Gang Tong, Chunling Fan, Yibin Zhao,“Sonar Power Amplifier Testing System Based on Virtual Instrument”, Proceedings of the 6th World Congress on Intelligent Controland Automation, Vol 2, pp.5314 ~ pp. 5317, China, June 21~23, 2006. [3] Ventura J., Drake R., McGrory, J.,“NI ELVIS has entered the lab”, SoutheastCon, 2005. Proceedings. IEEE, pp. 670 ~ pp. 679, Apr 8 ~ 10, 2005. [4] K. Ashoka Reddy, J. Rezuana Bai, Boby George, N. Madhu Mohan, etc.,“Virtual Instrument for the Measurement of Haemo-dynamic Parameters Using Photoplethysmograph”, IMTC 2006 -

Instrumentation and Measurement Technology Conference, pp.1167 ~ pp.1171, Italy, Apr 24 ~ 27, 2006. [5] Xiaofei Ji, Jiangtao Cao, Yibo Li,“Design of Speech Lock System Based on RBF Neural Network and Virtual Instrument Technology”, Proceedings of the 2006 IEEE International Conference on Mechatronics and Automation, pp.1318 ~ pp.1323, China, June 25 ~ 28, 2006. [6] Hernandez Cid J.M., Velazquez Moran J.F.,“Adjustable Speed Drives (ASD) Test Bench for Harmonic Distortion Evaluation”, Systems and Informatics, pp. 1 ~ 4, Sept, 2006. [7] Ferrero and V. Piuri, “A Simulation Tool for Virtual Laboratory Experiments in a WWW environment,” IEEE Trans. On Instrumentation and Measurement, Vol. 48, No. 3, pp. 741-746, June 1999. [8] M. Santori, “An Instrument that Isn't Really (Laboratory Virtual Instrument Engineering Workbench),” IEEE Spectrum, Volume: 27 8, pp. 36 –39, Aug. 1990. [9] H. J. W. Spoelder, “Virtual Instrumentation and Virtual Environments,” IEEE Instrumentation & Measurement Magazine, Vol. 2, No. 3, Sept. 1999, pp. 14 –19. [10] “LabVIEW User Manual,” National Instrument, 2007. [11] Foster, K.R., “New Math for LabVIEW,” IEEE Spectrum, Vol. 346, June 1997, pp. 66-66.[12] Jon Wilson, A Practical Approach to Vibration Detection and Measurement, Sensor Magazine, Feb, March & April, 1999. [13] Yong He, Guang-Lin Tian, Yi-Dan Bao,“Application of network-based virtual instrument technology on remote vehicle inspection”, Proceedings of the Fourth International Conference on Machine Learning and Cybernetics, Vol 3, pp. 1428 ~ pp. 1436, China, Aug 18 ~ 21, 2005. [14] Hsiung Cheng Lin,“Remote real-time power system harmonics detection and monitoring via the Internet”, Department of Automation Engineering, Vol 3, pp. 2867 ~ pp. 2872 ,Taiwan, Oct 10 ~ 12, 2005. [15] Al-Shoaili, S.H., Eren H.,“Intranet-Based Real-Time Laboratory A Case Study in Electrical Engineering”, IMTC 2003- Instrumentation and Measurement, Technology Conference, Vol 2, pp. 1210 ~ pp. 1215, Canada, MAY 17 ~ 19, 2005. [16] Rapuano S., Zoino F.,“A Learning Management System Including Laboratory Experiments on Measurement Instrumentation”, IEEE Transactions on Instrumentation and Measurement, Vol. 55, NO. 5, pp.1757 ~ pp.1766, Oct, 2006.

Acknowledges This study was funded by a grant provided by

the National Science Council, Republic of China, under Grant No. NSC 95-2516-S-018-007-MY2