DEVELOPMENT AND CHARACTERIZATION OF THIN FILM NICHROME STRAIN GAUGE SENSOR FOR LOAD APPLICATIONS

Transcription

1 International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN (Print), ISSN (Online) Volume 1, Number 1, July - Aug (2010), pp IAEME, IJARET International Journal of Advanced Research in Engineering and Technology (IJARET) IAEME DEVELOPMENT AND CHARACTERIZATION OF THIN FILM NICHROME STRAIN GAUGE SENSOR FOR LOAD ABSTRACT APPLICATIONS Latha H K E Assistant Professor Department of Instrumentation & Electronics Siddaganga Institute of Technology, Tumkur lathahke@yahoo.com Stephen R John Professor Department of Instrumentation & Electronics Siddaganga Institute of Technology, Tumkur, Karnataka. 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 59

2 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. 60

3 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 C. The polymer layer was applied uniformly on the required region and heat treatment of the substrate was done at 150 o C 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 %. 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. 61

4 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 Strain Gauge under Compression R/R Deflection in mm Figure 2 Deflection of cantilever versus R/R for the strain gauge under Compression 62

5 strain gauge under Tension R/R 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

6 [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) 2 Ebd 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, GF Where R R ε = (2) 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. 64

7 Bridge Output voltage in mv Bridge Excitation Voltage=3V Bridge Excitation Voltage=4V Bridge Excitation Voltage=5V Bridge Excitation Voltage=7V 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 Bridge excitation voltage=5v Bridge output in mv Mass in milligrams 5. CONCLUSIONS Figure 6 Variation of the mass versus bridge output Voltage 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

8 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 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 Maissel L I, Glang R, Hand book of Thin Film Technology, International Business Macines Corporation Components Division, East Fishkill Facility Hopewell Junction N Y 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, E O Doebelin (1985), Measurement Systems-Applications and Design 3 rd edn (London: McGraw Hill)pp 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 Dally J W and Riley W F, (1978), Experimental stress analysis, 2 nd 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 John Stephen R, Rajanna K, Vivek Dhar, Kalyan Kumar K G, and Nagabushanam S, Comparative performance Study of Strain Gauge Sensors for Ion Thrust Measurement, conference proceedings Vol. 3-1 pp

9 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 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 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 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. 67