Development and characterization of thin film nichrome strain gauge sensor for load applications

International Journal of Advanced Research in Engineering (IJARET)
International Journal of Advanced Research in Engineering and Technology
and Technology (IJARET), ISSN 0976 – 6480(Print), IJARET
ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 1, Number 1, July - Aug (2010), © IAEME
ISSN 0976 – 6499(Online) Volume 1,
Number 1, July - Aug (2010), pp. 59-67 © IAEME
© IAEME, http://www.iaeme.com/ijaret.html

DEVELOPMENT AND CHARACTERIZATION OF THIN
FILM NICHROME STRAIN GAUGE SENSOR FOR LOAD
APPLICATIONS
Latha H K E
Assistant Professor
Department of Instrumentation & Electronics
Siddaganga Institute of Technology, Tumkur-572103
E-mail: lathahke@yahoo.com

Stephen R John
Professor
Department of Instrumentation & Electronics
Siddaganga Institute of Technology, Tumkur, Karnataka.

ABSTRACT
This paper describes the design & development of a sputter deposited Nickel-
Chromium (sensing film) strain gauge sensor for load applications. Beryllium copper
(Be-Cu) strip is used as a spring element (cantilever beam). The various steps followed to
prepare thin film strain gauges on the spring element are described. M-bond 610 adhesive
(Measurements Group) has been employed as the insulating layer. The strain gauges were
deposited using DC magnetron Sputtering technique on either side of the Be-Cu strip.
The developed strain gauges were connected in wheat stone bridge configuration to
measure load. The NiCr thin film strain gauges developed can be used in micro machined
load sensors.
Key words:- Cantilever, Load, Thin film Strain Gauge.
1. INTRODUCTION
Thin-film load transducers on rigid substrates have been well received in the field
of sensing technology. For the measurement of force, vibration and load by electronic
means, we usually make use of a system that has a spring element and a collection of four

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International Journal of Advanced Research in Engineering and Technology (IJARET)

strain gauges as the basic unit [1]. Although metal foil type strain gauges were commonly
employed in load sensor, in recent years thin film strain gauges have become more
popular because of their advantages [2]. Different types of materials that are used for thin
film strain gauges are metals, alloys, cermet and semiconductors. The high resistivity of
alloys along with their low temperature coefficient of resistance (TCR) and good thermal
stability make them prime candidates for strain gauge applications. Since Nickel-
Chromium material has satisfy above mentioned properties. Hence this material was
chosen as sensing material.
In this paper, we report the preparation and study of load transducer with
Nichrome (NiCr) thin film strain gauges as sensors.
2. PREPARATION OF THIN FILM LOAD SENSORS
The strain gauge development involves the design of strain gauge, preparation of
the cantilever beam (substrate), deposition of thin film strain gauges and electrical wiring.
The suitable pattern required for the thin film strain gauge was designed using AutoCAD.
A photo plot of designed gauge pattern was obtained and with the help of it, Be-Cu
mechanical masks of thickness 150µm were made.
A cantilever beam is a high strain, low force structural member & offers
convenient means for implementing a full bridge circuit by mounting opposed pairs of
gauges on each of the two surfaces. If the beam is reasonably thin, the arrangement will
result in good temperature compensation because the temperature differences between
gauges can be kept low. Hence cantilever beam of size 150mm x15mm x0.5mm was
fabricated.
In the present work Beryllium-copper (Be-Cu) material has been chosen as
substrate, because of the following reasons. Be-Cu is a highly ductile material, which can
be stamped and formed into very complex shapes with the closest tolerances. It can be
strengthened by precipitate hardening. The surface condition of the substrate and the type
of the substrate material used influence the performance of thin films [3]. The surface
roughness of the substrate affects the adhesive property of thin films [4]. Hence the
surface of the substrate was prepared using standard polishing and cleaning procedures.

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In order to electrically isolate the thin film strain gauges from the metallic surface,
a thin layer of a 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 0C. The polymer layer was applied uniformly on the required
region and heat treatment of the substrate was done at 150oC for 2hrs.
After application of the polymer layer and its curing, the cantilever beam was
placed in a sputtering system for deposition of Nickel-Chromium films.
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 because of the 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 is maintained between the anode or substrate (Be-Cu) and the cathode or target
(NiCr). The chamber of the sputtering 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 argon gas of purity 99.999%. The deposition parameters were
optimized to achieve the required properties for the film.
In order to attach electrical leads bonding pad was sputter deposited. Also, a
terminal pad was bonded to the cantilever for convenience of external wire connections.
These were done on either side of the cantilever.
3. TRANSDUCER PARAMETRIC ANALYSIS
A mechanical setup was designed and developed to 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 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.

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Figure1 Photograph of the cantilever set up with strain gauges.
The resistance of a strain gauge is not in itself a performance characteristic. To
satisfy the basic functioning of strain gauge, the free end of the cantilever was deflected
by means of a digital micrometer and resistance measurements were made for each load.
The fractional change in resistance values of each strain gauge with respect to deflection
of the cantilever applied load were found to be linear for all the four thin films strain
gauges and one set of such characteristic for compression & tensile mode is shown in
Figures 2 & 3 respectively. The strain gauges are arranged in a wheat stone bridge
configuration as shown in Figure 4.

0.001
Strain Gauge under Compression
0.000

-0.001

-0.002

-0.003
∆R/R

-0.004

-0.005

-0.006

-0.007

0 1 2 3 4 5
Deflection in mm

Figure 2 Deflection of cantilever versus R/R for the strain gauge under Compression

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400

strain gauge under Tension
350

300

250

200

-6
∆R/R 10
150

100

50

0

-50
0 1 2 3 4 5 6
Deflection in mm

Figure 3 Deflection of cantilever versus R/R for the strain gauge under tension

Figure 4 Bridge circuit with strain gauges

4. GAUGE FACTOR DETERMINATION
Gauge factors of the strain gauges were determined from the change in electrical
resistance in response to a longitudinal strain using cantilever technique. Cantilever
characteristically a high strain, low force structural member, is widely used for load
measurement. With the cross section symmetrical about the bending axis, there are
always two surfaces subjected to equal strains of opposite sign. This offers a convenient
method for implementing a full bridge circuit, by connecting the pair of strain gauges on
either side of cantilever in the opposite arms of the wheat stone bridge. Thus the two
gauges which undergo tensile form one pair of opposite arms of the bridge and two
gauges which undergo compressive strain form the other pair of opposite arms of the
bridge. This produces the maximum differential output voltage for a given deflection of
the beam. In addition, it also offers the advantage of adequate temperature compensation

63


[5]. Cantilever beam bending was used to measure the gauge factor of the developed
strain gauge. In this technique [6] a bending moment is applied to the beam by fixing one
end and loading the other end with weights. The applied load can be determined by
measuring the strain undergone by the load cell at the fixed end.
The strain can be calculated using the relation
6WL
ε= (1)
Ebd 2
Where E is the young’s modulus for the Be-Cu beam,
b is the beam width, d is the beam thickness, L length of the beam and W the applied
load.
The gauge factor not only depends on the type of strain gauge material used but
also on the thickness of gauge, temperature and surface imperfections. Gauge factor was
calculated using the relation,

∆R
GF = R (2)
ε

Where ∆R is the fractional change in resistance of the strain gauge. The measured gauge
R

factor was found to be ~ 2.7 for thin film NiCr material.

Also, the free end of the cantilever was deflected using digital micrometer in steps
and the corresponding bridge output voltages were recorded. The variation of the
deflection versus bridge output voltage for different excitation voltages are shown in fig.5
and are found to be linear over the entire range. The bridge excitation was limited to 5V
because higher excitation voltage results in excessive heating of the thin film strain
gauges.

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0.65
Bridge Excitation Voltage=3V
0.60 Bridge Excitation Voltage=4V
Bridge Excitation Voltage=5V
0.55 Bridge Excitation Voltage=7V

Bridge Output voltage in mV
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0 1 2 3 4 5 6
Deflection in mm

Figure 5 Deflection of Cantilever versus Bridge output voltage
Further, small masses were added in steps to the free end of the cantilever and the
corresponding bridge output voltages were recorded. Variation of the mass versus bridge
output voltage for an excitation of 5V is as shown in fig 6 and is found to be linear.

4.72

4.70 Bridge excitation voltage=5V

4.68

4.66
Bridge output in mV

4.64

4.62

4.60

4.58

4.56

4.54

4.52

0 100 200 300 400 500 600
Mass in milligrams

Figure 6 Variation of the mass versus bridge output Voltage
5. CONCLUSIONS

Nichrome thin film strain gauge sensor was developed by employing DC
sputtering technique for load measurements. The characterization of the strain gauge
sensors indicated good linearity and sensitivity. The load measured was in the range of 5
to 550 mgs. The thin film developed can be used in micro machined load sensors.

65


ACKNOWLEDGEMENT
The authors thank Siddaganaga Institute of Technology, Tumkur, Karnataka,
India, for supporting this research work.
REFERENCES
1. Benjamin Varghese P Satish John and Madhusoodanan, K N (2009), “Weighing
system with ordinary load cell and a multimode fiber”, Jl. Of Instrum. Soc. Of India,
Vol.39 No.1, pp 65-66.
2. Nayak M M, Gunasekaran N, Muthunayagam A E, Rajanna K and Mohan S, (1993 ),
“Diaphragm-type sputtered platinum thin film strain gauge pressure transducer”,
Meas. Sci. Technol. 4, pp 1319-1322.
3. Maissel L I, Glang R, “ Hand book of Thin Film Technology”, International Business
Macines Corporation Components Division, East Fishkill Facility Hopewell Junction
NY
4. Koski K, Holsa J, Ernoult J, Rouzaud A, (1996), “ The connection Between Sputter
Cleaning and Adhesion of thin solid film”, surface and coatings Technology, 195-199.
5. E O Doebelin (1985), Measurement Systems-Applications and Design 3rd edn
(London: McGraw –Hill)pp 428-429.
6. Window A L and Holister G S (1982), Strain gauge technology, Applied Science
publishers, London,
7. William M Murray, what are strain gauges – what can they do?, (1962), ISA Journal,
Vol9, No.1, P.30.
8. Dally J W and Riley W F, (1978), Experimental stress analysis, 2nd edition, McGraw-
Hill, Kogakusha..
9. 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.
10.John Stephen R, Rajanna K, Vivek Dhar, Kalyan Kumar K G, and Nagabushanam S,
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Measurement”, conference proceedings Vol. 3-1 pp121-126.

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Latha H K E obtained B E degree in Instrumentation Technology from
Bangalore University, India in year 1995 & ME in power Electronics from
the same university in the year 2000. Presently working as Assistant
Professor in the Dept. of Instrumentation, Siddaganga Institute of
Technology, Tumkur, India. Currently she is pursuing PhD in the field of
thin film pressure sensors under Visvesvaraya Technological University,
Belgaum.

R John Stephen was born in Tiruchirapalli, Tamilnadu, India, in 1957.
He received B.E. degree in Electronics/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 IISc, Bangalore in
the year2005. At present he is Professor & Head, Department of
Instrumentation, SIT, Tumkur. He has publications in both National and
International Journals.

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