Download PC Based RLC Meter

DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
PC-Based RLC Meter
Mare Srbinovska, Cvetan Gavrovski, Vladimir Dimcev,
Zivko Kokolanski
Department of Electrical Measurement and Materials,
Faculty of Electrical Engineering and Information Technologies,
Skopje, Macedonia
E-mail: [email protected]
Nis, October 2007
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DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
OUTLINE
I.
Introduction
II. Two measurement techniques:
a. Step response
b. Sinusoidal response
III. Relaxation oscillator
IV. Experimental results
V. Conclusions
Nis, October 2007
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DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Introduction


Popular method that converts resistance, capacitance or inductance into
corresponding voltage, time intervals or frequency. Two basic techniques
for R, L and C measurement are called “Step response” and “Frequency
domain” methods.
This paper describes how we developed a Virtual RLC meter, based on a
PC with a DAQ board (National Instruments PCI 6221) and an external
relaxation oscillator, using Lab VIEW as the development software
Nis, October 2007
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DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Step response
  RC
  L/R
Fig.1 RC and RL circuits
An unknown capacitor or inductor whose value is to be determined can be connected to a known resistor in
the appropriate circuit. The time constant of the circuit can be measured and the unknown component value
computed.
C  /R
L  R
Nis, October 2007
for the RC circuit
for the RL circuit
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DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Relaxation Oscillator
The oscillator circuit consists of a
comparator with a positive feedback loop,
which converts the time – varying signal
into a square wave with a varying time
period. This signal can be counted and
digitized into a value that is proportional to
the oscillator frequency and in turn
proportional to R, C or L measured.
R1
1   


T  2RC ln 

R1  R2
1   
R1  R2
f out 
Fig.2 Relaxation oscillator
For measurement of C, the
resistor is known or vice versa
Nis, October 2007
  1/ 2
1
kRC
f out 
R
kL
For measurement of L, the
resistor is known or vice versa
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DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Hardware and Software realization
The designed virtual RLC meter consists of the
following blocks: the relaxation oscillator, data
acquisition system, personal computer and
Labview software.
The designed virtual RLC meter has tree
operation modes for R, L and C measurement
respectively. The software of the RLC meter
was entirely developed using Lab VIEW.
Programming an application in Lab VIEW is
very different from programming in a text
based language such as C or Basic. Lab VIEW
uses graphical symbols (icons) to describe
programming actions.
Fig.3 Block diagram of Virtual RLC meter
Nis, October 2007
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Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Software realization – Frequency setup
The starting edge parameter is used to determine if the
counter will begin measuring on a rising or falling edge.
Nis, October 2007
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DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Software realization
The software of the RLC meter was
entirely developed using Lab VIEW.
For frequency measurement is used
one counter on a Counter Input
Channel. The edge, minimum value
and maximum value are all
configurable. The measurement edge
is used to specify the edge on which
the counter has started measuring.
Result for the
measured resistance
Fig.4 Front panel of the frequency measurement
Nis, October 2007
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DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Experimental results – Measurement of R
Rref 
1
1M
2
47k
3
33k
4
22k
5
10k
6
4,7k
7
2,2k
8
1k
9
680
10
470
Rcal 
1,23M
40k
35,3k
20,7k
10,4k
5,36k
2,45k
1,084
765
604,2
Nis, October 2007
Cref nF 
0,133
10
10
10
10
10
100
100
100
100
1 %  2 %
22,87
0,6
1
1,8
4
14,28
4,54
12,5
17,6
28,78
12
-14,8
2,7
-6
-2
10
9
8,4
12,5
15
The first error  1 is
calculated using the
operational amplifier TL,
while  2 is calculated by
using higher performance
comparator OP27. As it can
be seen errors can be lower
using OP27.
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DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Experimental results - Measurement of C
1
2
3
4
5
6
7
8
9
10
Cref F 
680p
1n
3,3n
6,8n
10n
22n
33n
100n
470n
1
Ccal F 
660p
1,067n
3,35n
7,1n
10,3n
22,7n
33,9n
112n
530n
1,3
Nis, October 2007
Rref k
105
105
105
10
10
10
10
1
1
0,1
1 %  2 %
10
6,7
1,5
4,4
3
3,2
2,7
12
12,7
30
-11,7
-5
-4
1,5
-2
-4,5
-6
8
-1
-8
The first error  1 is
calculated using the
operational amplifier TL,
while  2 is calculated by
using higher performance
comparator OP27. As it can
be seen errors can be lower
using OP27.
- 10 -
DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Conclusion
This approach offers a simple solution for an application that needs the low
cost measurement of resistors, capacitors and inductors. The realized virtual
system can be used to any applications where frequency-time parameters
have to be measured with highest resolution and programmable accuracy.
Using a relaxation oscillator with higher performance comparator would
significantly improve the performance and precision of this RLC meter.
Nis, October 2007
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DAAD
Deutscher Akademischer Austausch Dienst
German Academic Exchange Service
Projekt „ISSNB“
Thank you for your attention!
Nis, October 2007
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