1. International Journal of Advanced Research in Engineering (IJARET)
International Journal of Advanced Research in Engineering and Technology
and Technology (IJARET), ISSN 0976 – 6480(Print),
ISSN 0976 – 6499(Online) Volume 1,
IJARET
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
Number 1, July - Aug (2010), pp. 68-76 © IAEME
© IAEME, http://www.iaeme.com/ijaret.html
VIRTUAL INSTRUMENTATION FOR MEASUREMENT
OF STRAIN USING THIN FILM STRAIN GAUGE
SENSORS
Kalpana H.M
Department of Instrumentation
Siddaganga Institute of Technology, Tumkur-572103
E-mail: hm_kalpanaravi@yahoo.co.in
John R Stephen
Department of Instrumentation
Siddaganga Institute of Technology, Tumkur– 572103
ABSTRACT:
This paper describes the development of thin film strain gauge sensor for strain
measurement and indication using LabVIEW. The strain gauges designed were deposited
on either side of the cantilever of Beryllium copper (Be-Cu) using DC magnetron
sputtering technique. LabVIEW version7.1, signal conditioning connecter block
SCC2345 with half bridge type II strain gauge input module SG03 and Data Acquisition
(DAQ) board 6221 have been utilized for the acquisition and indication of strain. Strain
was also calculated & compared with the indicated value. The error found to within
0.5%. The developed strain gauges are expected to be used in aerospace and biomedical
applications for the measurement of micro strain.
Keywords: strain, Thin film strain gauge, LabVIEW.
INTRODUCTION
Measurement of strain is important in automotive and biomedical applications,
aerospace industries, and also in seismic testing of bridges and other structures [1]. Strain
depends on the amount of deformation of a body due to an applied force. More
specifically, strain is defined as the fractional change in length. While there are several
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2. International Journal of Advanced Research in Engineering and Technology (IJARET)
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
methods of measuring strain, the most common is with a strain gauge, a device whose
electrical resistance varies in proportion to the amount of strain in the device.
Strain gauge is a sensing device or sensor used to measure the linear deformation
occurring in the material during loading [2]. Based on the principles of operation, strain
gauges are classified as electrical, mechanical, optical, acoustical and pneumatic [3].
Among these the electrical strain gauges have become widely accepted and at present
dominate this field. The different types of electrical strain gauges are metallic foil,
semiconductor and thin film. Foil type strain gauge sensor is the most commonly used
one, but it has limitations such as thermal degradation, relatively low output signals,
requirement of careful installation procedures and degradation in performance due to
moisture effect [4]. These limitations are overcome by using thin film sensors. Hence thin
film strain gauges have widespread use in industry. Different kinds of materials that are
used for thin film strain gauges are metals, alloys, cermets and semiconductors. The high
resistivity of alloys along with their low temperature coefficient of resistance (TCR) and
good temporal stability make them prime candidates for strain gauge application.
Nichrome (NiCr 80/20 wt.%) is the most widely used alloy because of its high resistivity,
low TCR, commercial availability, low temperature dependence of gauge factor and
similar properties[5].
The aim of this work is to design and develop Nickel-Chromium (NiCr) thin film
strain gauge sensors and interface with associated instrumentation for strain measurement
and indication using Virtual Instrumentation
A virtual instrumentation system is the use of customizable software and modular
measurement hardware to create user defined measurement systems.
National Instruments (NI) LabVIEW data acquisition hardware and software
modules have become one of the most widely used tools to capture, view, process and
control. Hence, NI hardware has been utilized in our work also.
2. DEVELOPMENT OF THIN FILM STRAIN GAUGES
The strain gauge development involves the design of strain gauge, preparation of
cantilever beam (substrate), deposition of thin film strain gauges on it and wiring. The
suitable pattern required for the thin film strain gauge was designed using AutoCAD. A
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3. International Journal of Advanced Research in Engineering and Technology (IJARET)
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
photo plot of designed gauge pattern was obtained and, with the help of it, mechanical
masks of Be-Cu material were made.
The surface condition of the substrate and the type of the substrate material used
influence the performance of thin films [6 ]. The surface roughness of the substrate
affects the adhesive property of thin films [7,8]. Hence prior to the application of the
polymer layer, the surface of the substrate was prepared using standard polishing and
cleaning procedures. In the present work Beryllium-copper (Be-Cu) material (98%
copper and 2% Beryllium) has been chosen as substrate, because Be-Cu is a highly
ductile material, which can be stamped and formed into very complex shapes with the
closest tolerances. Be-Cu can be strengthened by precipitate hardening. Heat treated Be-
Cu features excellent dimensional stability, fatigue resistance and corrosion resistance
and mechanical strength.
In order to electrically isolate the thin film strain gauges from the metallic surface,
a thin layer of an epoxy adhesive (M-Bond 610) was applied on either side of the
substrate. This polymer provides the highest level of performance and is suitable for
temperatures up to +230° C. The polymer layer was applied uniformly on the required
region and heat treatment of the substrate was done immediately in a temperature-
controlled oven at 150°C for about 2hrs.
Using the mechanical masks, thin film strain gauges were deposited on either side
of the cantilever beam using the DC Magnetron sputtering technique. This technique has
been chosen because of its high ionization efficiency and good adhesion of the deposited
films & better molecular bonds that will ensure faithful transfer of strain experienced by
the strain gauges.
The sputtering system used consists of an arrangement in which a plasma
discharge was maintained between the anode or substrate (Be-Cu) and the cathode or
target (Nickel-Chromium). The chamber of the system was initially evacuated to a
pressure of 10-6 torr using a combination of rotary and diffusion pump and back filled to
sputtering pressure with the inert gas argon [9,10]. The deposition parameters were
optimized to achieve the required properties for the film. Thin film strain gauges of
Nichrome were sputter deposited on either side of the cantilever.
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4. International Journal of Advanced Research in Engineering and Technology (IJARET)
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
3. TRANSDUCER PARAMETRIC ANALYSIS
A mechanical setup was designed and developed for study the characteristics of
the strain gauge developed. The setup consists of a rectangular base plate made of brass
material with a cylindrical rod fixed vertically at the left wherein a fixture has been
provided for holding the strain gauges in the form of a cantilever. The free end of the
cantilever can be deflected by means of a digital micrometer fixed to a holder. Fig.1
shows the photograph of the cantilever set up with strain gauges.
Figure 1 Shows the photograph of the cantilever set up with strain gauges.
The fractional change in resistance with respect to deflection of the cantilever
were stored in the data files.Using National Instruments LabVIEW version8.0,VI block
diagram was created with spread sheet read icon and express xy plotter as shown in fig2.
Data read by spread sheet reader and plotted in the xy plotter is found to be linear for
both compression & tensile mode and is as shown in figs 3 & 4
Figure 2 VI Block diagram to read & plot compression & tensile mode resistance
values
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5. International Journal of Advanced Research in Engineering and Technology (IJARET)
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
Figure 3 Deflection of Cantilever versus Figure 4 Deflection of cantilever versus strain gauge
resistance under Strain gauge resistance under
Compression tension.
The gauge factor of the strain gauges developed were determined to be ~ 2.7
using four point bending method.
4. HARDWARE DESCRIPTION
The block diagram of the strain measurement system developed is shown in Fig.5
and the function of each of the blocks is explained below.
Sensor Signal DAQ
Unit conditioning
unit
unit
PC with
LabVIEW
Figure 5 Block diagram of strain measurement system
4.1 Sensor Unit.
The sensor unit consists of a cantilever with thin film strain gauges on either side.
The cantilever was fixed to the mechanical setup.
4.2 Signal conditioning unit.
This unit consists of NI signal conditioning connector block SCC2345 and strain
gauge input module SG03 [11].The SCC2345 has 20 SCC sockets, labeled J1 through
J20. Sockets J1 through J8 accommodate SCC modules for conditioning signals on the
analog input channels of the DAQ card.
SCC-SG03 is a dual-channel strain gauge module for conditioning half bridge
strain gauges. This module has two strain gage input channels, offset nulling circuitry for
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6. International Journal of Advanced Research in Engineering and Technology (IJARET)
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
each channel and a 2.5V excitation source. Each input channel includes an
instrumentation amplifier with differential inputs with a fixed gain of 100. The output of
each amplifier is filtered and buffered to prevent settling time delays.
4.3 DAQ unit:
DAQ card PCI6221 is required for the interfacing purpose. Hardware
functionality includes 16 analog inputs, 2 analog outputs, 24 digital I/O, counter/timers,
triggering and synchronization circuitry.
4.4 Personal Computer with Labview
Personal computer with labVIEW version7.1 has been used for acquisition and
indication of strain, in our present work.
5 DATA ACQUISITION AND PRESENTATION USING LABVIEW
The cantilever was fixed to the mechanical setup fabricated and strain gauges
were connected to the strain gauge input module SG03. The bridge completion resistors
present in SG03 completes Wheatstone bridge configuration. SG03 module was inserted
into one of the analog input sockets of the SC2345 connector, which in turn was
connected to the PC through DAQ card 6221.The experimental set up is as shown in fig.6
Figure 6 Photograph of the experimental set up
The data acquisition board was configured for strain measurement using the
measurement & Automation Explorer (MAX) of LabVIEW. Configuration was done by
entering the gauge factor, gauge resistance, minimum & maximum values of strain to be
indicated & setting the excitation voltage as internal. The strain gauge input module
SG03 was configured as half bridge type II which measures only bending strain [12].
The block diagram VI & front panel VI were constructed using appropriate icons
& function pallets of NI DAQmx on LabVIEW platform. These are shown in Figure 7 &
Figure 8 respectively.
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7. International Journal of Advanced Research in Engineering and Technology (IJARET)
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
Figure 7 Block Diagram VI of strain Figure 8 Front panel VI of strain measurement
measurement system system
Known weights were added in steps to the free end of the cantilever and the
corresponding amplified & filtered bridge output voltage of SG03 was acquired by
analog input (AI) channel by continuous sampling and was converted to strain, using
convert to strain inbuilt functions present in block diagram VI and was stored in the
system for indication in the front panel VI. The true value of strain was also calculated
using physical dimensions of cantilever. Variations of mass versus indicated & calculated
strain are as shown in Figure 9.
300
Bridge Excitation Voltage =2.5V
250 Indicated Strain
Calculated strain
200
Micro strain
150
100
50
0
0 100 200 300 400 500
Mass in milli grams
Figure 9 Variation of mass versus indicated & calculated strain
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8. International Journal of Advanced Research in Engineering and Technology (IJARET)
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
6. CONCLUSIONS
Using Labview strain was indicated as micro strain. The LabVIEW measurement
platform has dramatically reduced test times as results were automatically collected. This
type of Strain measurement system can be used in closed loop for aerospace and
biomedical applications.
ACKNOWLEDGEMENTS
The authors thank Siddaganaga Institute of Technology, Tumkur, Karnataka,
India, for supporting this research work.
REFERENCES
[1] Willshire, J., Niewczas,Dziuda, Fusiek, and McDonald,R., February 2004,
“Dynamic strain measurement using an Extrinsic Fabry-perot interferometric sensor
and an Arrayed Waveguide Grating Device” IEEE Transactions on instrumentation
and measurement, VOL.53.No,1.
[2] Murray, M., January 1962,what are strain gauges – what can they do?, ISA Journal,
Vol9, No.1, , P.30
[3] Dally.J.W andRiley,W.F., .Experimental stress analysis, 2nd edition, McGraw-Hill,
Kogakusha, 1978.
[4] Rajanna,K, Mohan,S and. Gopal,E.S.R 1989.’Thin film strain gauges – An
overview”, Indian J. pure and Appl. Physics, Vol. 27, pp 453.
[5] Kazi.H, Wild.P.M, Moore.T.N, Sayer.M.,2003. ‘The electromechanical behavior of
nichrome(80/20 wt.%) film. Thin Solid Films 337-343, 433
[6] Maissel, L. I, Glang, R, “ Hand book of Thin Film Technology”, International
Business Macines Corporation Components Division, East Fishkill Facility Hopewell
Junction N Y
[7] Koski, K., Holsa, J., Ernoult 1996, “ the connection Between Sputter Cleaning and
Adhesion of thin solid film”, surface and coatings Technology, 80) 195-199.
[8] Liu, C S,. Chen, L.J.,1996 “Effects of Substrate Cleaning and Film Thickness the
Epitaxial Growth of UltraHigh Vacuum Deposited Cu Thin Films on (001)Si”,
Applied Surface Science, 92,) 84-88
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9. International Journal of Advanced Research in Engineering and Technology (IJARET)
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
[9] Stephen R J, Rajanna, K, Vivek Dhar, K G Kalyan Kumar, and Nagabushanam, S
2004., “Thin Film Strain Gauge Sensors for Ion Thrust Measurement”, IEEE sensors
Journal, Vol. 4, No.3, June.
[10] Stephen, R,J., Rajanna, K, Vivek Dhar, K G Kalyan Kumar, and Nagabushanam, S
“Comparative performance Study of Strain Gauge Sensors for Ion Thrust
Measurement”, conference proceedings Vol. 3-1 pp121-126
[11] National Instruments SCC-SG Series Strain gage Modules User Guide
[12] National Instruments Data Sheet
H.M.Kalpana working as assistant professor in the dept.
Department of Instrumentation, Siddaganga Institute of Technology,
Technology, Tumkur. She has completed her Graduation in Instrumentation
& Electronics from Bangalore university and MTech in Industrial Electronics
from Mysore university. She is currently pursuing her PhD in thin film
sensors under Visvesvaraya Technological University, Belgaum.
R John Stephen was born in Tiruchirapalli, Tamilnadu, India, in 1957. He
received B.E. degree in Electronics and Instrumentation in the year 1982
from Annamalai university. After staying in industries for a period of about
two years he joined as Lecturer, Siddaganga Institute of Technology (SIT),
Tumkur, Karnataka, in the year 1984. He obtained M.Tech degree in
Instrumentation technology from Indian Institute of Science (IISc),
Bangalore, India, in the year 1988 & PhD degree from the IISc, Bangalore in
the year 2006. At present he is an professor & HOD in the department of
Instrumentation, SIT, Tumkur. He has publications both in National as well
as in International Journals.
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