Experimental investigations of different force measuring systems

Transcription

1 Indian Journal Pure & Applied Physics Vol. 51, June 2013, pp Experimental investigations different force measuring systems Harish Kumar 1,2 * & Chitra Sharma 3 1 CSIR-National Physical Laboratory, New Delhi, India 2 University School Engineering & Technology, Guru Gobind Singh Indraprastha University, New Delhi, India 3 Department Mechanical & Automation Engineering, Indira Gandhi Institute Technology, Delhi, India * harishkec@gmail.com Received 25 July 2012; revised 10 December 2012; accepted 13 March 2013 transducers have been developed over the decades and equipped with different measuring devices like dial gauges, digital dial gauges, strain gauges etc for realization force. The findings a comparative study for different force measuring devices, generally, used with force transducers have been presented in the present paper. Two force transducers have been calibrated as per the standard calibration procedure and uncertainty force realization has been evaluated. The study suggests that the force transducers with strain gauges for realization force have the least uncertainty force realization and fers more realistic/reliable results along with the advantages digitization and measurement force in terms electrical signal. The electrical signals are free the losses that are inherited in measurement systems like dial gauges or digital dial gauges and fer results with improved resolution. Keywords: transducers, Dial gauge, Digital dial gauge, Strain gauge 1 Introduction transducers have been employed for realization force precisely in various engineering applications in the range from nano Newton to mega Newton. The force transducers may have varying degree accuracy depending upon the applications desired. The force transducer consists an elastic element equipped with a deflection measuring system for realization force, when force transducer is exposed to an external force. The force transducers may various shapes like ring, elliptical, square ring, extended octagonal ring etc. The shape the force transducer is very important and is decided on the basis design and manufacturing considerations. The force transducers may have suitable force measuring systems like dial gauge, digital dial gauge, strain gauge, Hall effect sensors etc to express the force applied in the form suitable form like deflection, strain, electrical signal etc. A number researchers have developed various types force transducers with different measuring devices. The dial gauges have been in use since the inception force transducers in 1927 and now digital dial gauges have been in use to eliminate observational errors for few past years, have mechanical systems for deflection measurement force transducer under action external forces. Since 1960, strain gauges have been used for realization force and to reproduce force in terms electrical signal with the help suitable data acquisition system 1-6. Though strain gauged force transducers have already proven their superiority over the dial gauged force transducers as reported by researchers earlier, but a detailed comparison study on the basis standard procedure is still awaited. Hence, the present paper investigates the metrological characterization study 20 kn and 50 kn ring shaped force transducers to evaluate uncertainty force realization using dial gauge, digital gauge and strain gauge. The force transducers have been calibrated according to the calibration procedure based on standard ISO and IS (reaffirmed 2003) for % the rated capacity. The uncertainty the force realization includes the relative deviation due to factors like repeatability error, zero error, resolution error etc. The results have been discussed and presented in the paper. 2 Experimental Details The force transducers have been equipped with dial gauge, dial gauge and strain gauges one by one. Suitable arrangements and fixtures required have been prepared and suitable fixed to the force transducers. The dial gauges have been manufactured by the reputed manufacturer through the globe. The resolution the dial gauge used is 0.1 divisions and it

2 394 INDIAN J PURE & APPL PHYS, VOL 51, JUNE 2013 is equivalent to mm (0.2 m). The dial gauges are still used on a wide scale for realization force in developed/developing nations. The dial gauge converts the vertical movement the stylus into the rotation indicator. As the external force is applied, the deflection the force transducer is observed by stylus the dial gauge and indicator gives the readings inform divisions. Generally, the dial gauges are found to have repeatability error and maximum error ±1 m and ±2 m, respectively 7,8 (Fig. 1). The major drawback the dial gauges is its mechanical system for realization force in terms deflection the elastic element the force transducers. The contribution dial gauge s errors to the overall uncertainty force transducers is negligible and hence, not taken into account for uncertainty evaluation force transducers. The digital dial gauges has also been manufactured by world renowned manufacturer and the resolution is mm (or 1 m), which is the best possible to the best knowledge authors. The digital dial gauges may have repeatability error and maximum error up to ±2 m and ±5 m, respectively. The advantage digital dial gauges lies in their application and gives the deflection force transducer under action external forces directly in mm / inch despite divisions as in case dial gauges 8,9 (Fig. 2). The major drawback the digital dial gauges is their resolution and hence, their use is still limited. The strain gauges have been used for force measurement since 1960 s and made fine resistance wires and when they are strained, resistance change is a measure force applied. The strain gauges have been mounted on the force transducer as per Wheatstone bridge and a suitable high resolution precision digital indicator has been used with resolution mv/v for taking observations. This methodology gives advantages digitization and the results are free from mechanical systems as in case dial/digital dial gauges. The force transducers have been calibrated by 50 kn dead weight force machine which has already been discussed earlier elsewhere (Figs 3 and 4). The force transducer have been calibrated according to the calibration procedure based on standard ISO and IS (reaffirmed 2003) and the calibration as been done for % the capacity force transducers in equal steps with 10% incremental. The calibration procedure is as follows: Fig. 2 Typical digital dial gauge Fig. 1 Typical dial gauge Fig. 3 Metal foil strain gauge

3 KUMAR & SHARMA: DIFFERENT FORCE MEASURING SYSTEMS 395 Fig. 4 Wheatstone bridge circuit Fig. 6 (a) 20 kn and (b) 50 kn transducer with different measuring systems (ISO ) Fig. 5 (a) 20 kn and (b) 50 kn force transducer with different measuring systems (IS ) 2.1 ISO (a) Digital indicator is switched on for 30 min and no load output signal was noted. (b) The force transducer is preloaded thrice to its maximum capacity and kept at full load for 90 s. (c) Calibration force transducer has been done in compression mode. (d) Calibration is carried out by applying two series calibration forces in ascending order from 10 to 100% in steps 10% at initial position, considered 0. (e) Two series calibration forces have been applied at rotation positions 120 and 240. (f) transducer is subjected to full load once for 90 s each time before starting calibration to new rotated position. (g) Between loadings, readings corresponding to no load at force transducer, after waiting at least 30 s for return to zero are noted. Uncertainty measurement force transducer involves relative deviations due to zero error, repeatability error, reproducibility error and error resolution etc. (Eqs 1, 3 and 4) wc (tra) = w(rep) + w(rpr) + w(zer) + w (res) (1) 2.2 IS (reaffirmed 2003) The procedure for calibration force transducers is similar to ISO , but only one series is taken at 0 position and rest the procedure is the same. 1/2

4 396 INDIAN J PURE & APPL PHYS, VOL 51, JUNE 2013 Table 1 Uncertainty Estimations 20 kn force transducer with different measuring systems (IS ) Uncer. Uncer. Reap. Reso. Reap. Reso. Reap. Reso. Uncer Table 2 Uncertainty estimations 50 kn force transducer with different measuring systems (IS ) Uncer. Uncer. Reap. Reso. Reap. Reso. Reap. Reso Uncer. Uncertainty measurement force transducer involves relative deviations due to zero error, repeatability error and error etc resolution (Eqs 2-4) wc (tra) = w(rep) + w(zer) + w (res) (2) The relative combined standard uncertainty w c(tra) and the relative expanded uncertainty W (tra) for k=2 is calculated by the following Eqs 1 and 2. Other terms represent the relative variances relative deviations due to repeatability error (rep), zero error (zer) and resolution error (res). W (tra) = kw (3) c (tra) The relative uncertainty calibration W shall be determined by Eq. (3), considering the best measurement capability (bmc) the force standard machine. 1/ /2 (tra) (bmc) (4) W = W + W The metrological findings are shown in Figures 5 and 6 and presented in Table Results and Discussion The force transducers have been used with different force realizations systems as discussed in the paper for many years and every system has its own merits and demerits. Though mechanical system, dial gauges and digital dial gauges are very commonly used as force measuring systems and represent the force in terms deflection the elastic elements the force transducer. The strain gauges are resistance foil type gauges which are fixed to the surface the force transducer suitably and arranged in form Wheatstone bridge. When the force transducer is exposed to external force, the output the

5 KUMAR & SHARMA: DIFFERENT FORCE MEASURING SYSTEMS 397 Table 3 Uncertainty estimations 20 kn force transducer with different measuring systems (ISO ) Uncer. Uncer. Reap. Repr. Res. Reap. Repr. Res. Reap. Repr. Res. Uncer Table 4 Uncertainty estimations 50 kn force transducer with different measuring systems (ISO ) Uncer. Uncer. Reap. Repr. Res. Reap. Repr. Res. Reap. Repr. Res. Uncer unbalanced Wheatstone bridge is in the form an electrical signal which may be suitably expressed in form mv/v, units force (N, kn, kgf etc.) or divisions. The force transducers capacities 20 kn and 50 kn have been studied with different force measuring systems and metrologically studied according to the calibration procedure based on standard IS (reaffirmed 2003). The uncertainty measurement force transducers has been evaluated for different force measuring systems by taking relative deviations due to repeatability error, zero error, resolution error etc. The uncertainty measurement is found to be maximum in case force transducers (FT) with digital dial gauge and this may be attributed due to relative resolution error mainly. The relative resolution error is much for digital dial gauges as it was the best possible digital dial gauge to the best authors knowledge. Hence, if resolution the dial gauge may be improved, then uncertainty measurement such force transducers may be considerably improved as in case dial gauged force transducers and strain gauged force transducers as evident from Figs 1-4 and Tables 1-4. Also, it may be given attention that once the force transducers are strain gauged and digitized, they may have interpolation and their intermediate force step values may be known from interpolation equation, which is not possible in case dial gauged or digital dial gauged force transducers. Also, the strain gauged force transducers are free from errors like friction in case dial or digital gauges. Besides it, they also have improved resolution in comparison other measuring devices listed. 4 Conclusions The force transducers capacities 20 kn and 50 kn have been studied according to the calibration

6 398 INDIAN J PURE & APPL PHYS, VOL 51, JUNE 2013 procedure based on the standard ISO and IS (reaffirmed 2003) and they have been studied with different force measuring systems like dial gauge, digital dial gauge and strain gauges. The dial or digital dial gauges are essentially a mechanical system and suffers losses due to mechanical components and mechanisms. The force transducers when used with digital dial gauges or dial gauges, are found to have poor metrological properties in terms uncertainty realization in comparison to strain gauged force transducers. Hence, the force transducers with strain gauges prove to be a better measuring system for force realization and have optimal metrological performance. The strain gauged force transducers are found to have improved resolution and the intermediate force step values may be computed from the interpolation equation once known, which is not possible to determine with dial gauges / digital dial gauges. Besides it, the strain gauged force transducers have better sensitivity. Symbols w rep Relative error to relative repeatability error w rpr Relative error to relative reproducibility error w res Relative error to relative resolution error w zer Relative error to relative zero error w c (tra) Combined uncertainty device under calibration W (tra) Expanded uncertainty device under calibration W bmc Best measurement capability the standard / calibration machine W Overall uncertainty measurement References 1 Rehman M A & Rehman S, Journal the Institution Engineers (India) Mechanical Engineering Division, 88 (2007) 3. 2 Josue N L, World Transaction on Engineering and Technology Education, 5 (2004) Bray A, Exp Mech, 21 (1981) 1. 4 Diddens D, Reynaerts D & Hendrik Van Brussel, Sensor Actuator A-Phys, 46 (1995) O Dogherty M J, J Agr Eng Res, 63 (1996) 9. 6 Karaby S, Mater Des, 28 (2007) Dial indicators/dial test indicators-quick guide to precision measuring instruments ( Kumar H, Kumar A & Yadav P, Meas Sci Rev, 11 (2011) Kumar H, Sharma C & Kumar A, J Sci Ind Res India, 70 (2011) Kumar H & Sharma C, T I Meas Control, 34 (2012) Metallic materials Calibration force proving instruments used for verification uniaxial testing machines, ISO Method for calibration force proving instruments used for verification uniaxial testing machines IS (re-affirmed 2003). 16 Kumar H, Kumar A & Gupta S, Indian J Pure & Appl Phys, 50 (2012) Kumar H & Kumar A, Meas Sci Rev, 11 (2011) Kumar H & Sharma C, Indian J Pure & Appl Phys, 49 (2011) Kumar H & Kumar A, NCSLI Measure J Meas Sci, 6 (2011) 64.