The design and development of a force comparator standard machine to provide national traceability in force measurement to industry. Speaker/ Author: S. Dlamini Co-author: C. Gouws National Metrology Institute of South Africa (NMISA) Private Bag X34, Lynnwood Ridge, Pretoria, 0040, South Africa e-mail: sdlamini@nmisa.org Phone: +27 12 841 2075 Fax: +27 12 841 2131 Abstract Precise force measurements are important in testing of materials, weighing and balancing of heavy structures such as aircrafts, ships and engineering structures. For these applications, force transducers (artifact) are used to measure the static force generated in the system. Calibration of the transducer is done against force standard devices of higher accuracy traceable, through an unbroken chain of calibrations to the reference standards of force. For South Africa, the national measurement standards for force are a range of force transducers with different capacities ranging from 20 kn to 5000 kn. These are maintained at the National Metrology Institute of South Africa (NMISA). Dead weight machines are commonly used to realize or generate static force with significantly lower uncertainty. However, they are expensive to procure and maintain. Alternatively force comparator machines can be used to disseminate traceability in force measurements at slightly higher yet acceptable uncertainty. A force comparator standard machine at NMISA was designed and developed to provide national traceability in force. It has been developed to disseminate the unit of force from national level to the user industries in the range of 5 kn to 4500 kn. This type of force machine is easy to operate and maintain and makes the calibration of commercial force transducers economical viable without significant compromise in the uncertainty. The calibration and measurement capability (CMC) expressed as an uncertainty for the standard force transducers by comparison using this comparator machine was found to be ± 0.03% in the range of 5 kn to 200 kn for tension and compression, ± 0.04% in the range of 500 kn to 1000 kn for compression only and ± 0.11 % in the range 1000 kn to 4500 kn for compression only. The CMC is stated as the standard uncertainty of measurement multiplied by coverage factor of k = 2 at a confidence level of 95.45%. 1. Introduction There is wide range of force applications in industries all over the world. Force applications in industrial weighing, engine thrust measurements, civil aviation, automobiles, microelectronics, proof loading of bridge bearings, balancing of heavy structures, material and load testing. All these applications require precise force measurements. In each application, there will be an uncertainty requirement on the force measurement and the equipment used to make the measurement must be traceable with an unbroken chain of calibrations to a realization of the SI unit of force (the newton). The realization and
generation of accurate forces can be achieved by the use of dead weight force machine and precise force measurements can be achieved by using strain gauge based force transducers. Dead weight machines are commonly used to realize or generate static force with significantly lower uncertainty. However, they are expensive to procure and maintain. Alternatively force comparator machines can be used to disseminate traceability in force measurements at slightly higher yet acceptable uncertainty. Comparator machines are easy to operate and maintain. As a result, calibration of commercial force transducers is economical without significant compromise in the uncertainty. The National Metrology Institute of South Africa (NMISA) through its mandate, "The maintenance and dissemination of the National Measurement Standards (NMS)" improves the competitiveness of the South African industry in support of the Department of Trade and Industry (dti) strategic goals [1]. As such effort has been made to design and fabricate a force comparator standard machine at the NMISA s Force lab to realize the unit of force in the range 5 kn to 4500 kn to fulfill the demand for force calibration in South African Industries and neighboring countries. The force comparator machine is based on direct comparison of the output of the force proving or measuring devices against that of a reference force transducer of lower uncertainty. Calibration of the reference force transducers of NMISA is done against force standard devices of higher accuracy traceable, through an unbroken chain of calibrations to the reference standards of force from international NMIs. Therefore NMISA maintains the National Measurement Units and Measurements Standards of force at an internationally recognized level. 2. Design and function of the Force Comparator Machine The force comparator machine at NMISA mainly encompasses a stainless steel frame (Fig. 1), a hydraulic electric pump and stepper motor coupled to the Budenberg pressure balance (Fig. 2). The comparator machine is based on a reference force transducer having a low uncertainty value which is incorporated into the machine and its output is used to monitor the force generated by the machine. Reference force transducer standards with capabilities ranging from 5 kn to 4500 kn are Hottinger Baldwin Messtechnik (HBM), type C4 (5 kn to 1000kN) and C16A (2000 kn to 5000 kn) force transducers of class 05. All the force transducer standards, with the exception of the 5000 kn transducer, are sent to an international accredited National Metrology Institute (NMI) which uses a dead weight testing machines to calibrated the force standards. The 5000 kn force transducer is calibrated by means of hydraulic amplification method. HBM digital readouts of model DMP40S2 are used together with in-house developed software to capture results and interface with the readout units. All readout units are used in calibrated state with traceability via BN100A ratio divider. The force comparator machine can perform calibration of force transducers by comparison in compression and tension. The machine setups in compression and in tension are shown in Figure 3 and Figure 4 respectively. In compression calibration measurements (Fig. 3), the reference force transducer is attached to the upper coupling using Allen head type screws. The device to be calibrated is coaxially aligned with the reference force transducer and the ram to within 1 mm, using a steel rule.
The parallax error is kept within a 1mm alignment band. This alignment band ensures that the force applied is in alignment with the device s principal load axis. Figure 1: The main stainless steel frame of the comparator machine. Figure 2: A hydraulic electric pump coupled with a stepper motor.
Figure 3: Typical setup of calibration of force transducers in compression Figure 4: Typical setup of calibration of force transducers in Tension In tension calibration measurements (Fig. 4), the crossbeam is placed on the appropriate piston fittings. The reference force transducer is placed in the center of the crossbeam and the appropriate loading button is placed between the tension to compression rig and the reference force transducer. The tension to compression top half is placed over the reference transducer and bottom part fasten to the applicable screws. The device to be calibrated is fitted with suitable tension bars and attached to the tension rig. A hydraulic pump is used for movement of the ram up or down in compression measurement and in tension measurements, the piston is rammed to its maximum position and the hydraulic pipe coupled to the rig. For sensitive and fine application of applying loads, the stepper motor coupled to the Budenberg pressure balance is used together with in-house developed software. 3. Calibration and measurement capability (CMC) and development of the force comparator machine 3.1 Calibration and measurement capability (CMC) of the force comparator machine The force transducer standards are used along with the digital indicator (DMP40S2, HBM, Germany) to evaluate the uncertainty associated with the force measured by the comparator machine. These transducers have been calibrated in accordance with the requirement of ISO 376 [2] at Physikalisch-Technische Bundesanstalt (PTB) which is an international accredited NMI to obtain their force reference values. The relative measurement uncertainty of the force
scale in the measuring range used of the force standard machines at PTB is 0.002% (k = 2) throughout the range up to 1000 kn and 0.01% (k = 2) in the range up to 5000 kn. The relative combined standard uncertainty of the reference force values of the transducer standards calibrated as per ISO 376 considers the uncertainty contribution from various quantities such as repeatability, reproducibility, resolution of the readout, creep, drift in zero and uncertainty of instrument due to temperature changes during the measurement. Uncertainty in the force comparator machine is practically determined using these reference values and contributing factors from the comparator machine. The CMC for the machine is derived from this uncertainty. The document Guide to the Expression of Uncertainty in Measurement [3] is used to determine the uncertainty of calibration, where the expanded Uncertainty is expressed in percentage of applied load. Fig 5, 6 and 7 shows the CMC of the force comparator machine at each force load from 10 % to 100 % of maximum force and is expressed as a percentage. CMC of the machine across the whole force range is overestimated and the values are shown in table 1. Table 1: The CMC stated as the expanded uncertainty of measurement. Force range (kn) 5-200 200-1000 1000-4500 CMC (%) ±0.03 ±0.04 ±0.11 The CMC is stated as the expanded measurement uncertainty which is the standard uncertainty of measurement multiplied by coverage factor of k = 2 at a confidence level of 95.45%. Figure 5: CMC of the Comparator force machine in the range of 5kN to 200 kn.
Figure 6: CMC of the Comparator force machine in the range of 300 kn to 1000 kn. Figure 7: CMC of the Comparator force machine in the range of 1000 kn to 4500 kn. 3.2 The development of the Comparator force machine As it is currently not physically possible to control the hydraulic system to maintain a steady load, the force values are captured on the fly as the reference transducer passes through the applied force measurement point with increasing or decreasing force. Values are captured with the help of in-house developed software and a camera. The stepper motor coupled to the Budenberg pressure balance is used together with in-house developed software for sensitive and fine application of force.
Therefore for practical purposes, the CMC is over estimated across the force ranges in the view of the influencing components due to machine interaction. The traceability chain in force measurement is thus achieved efficiently with a lowest possible uncertainty as depicted in Fig 8. 4. Conclusion The CMC of the force comparator machine is found to be ±0.03% in the range of 5 kn to 200 kn, ±0.04% in the range of 200 kn to 1000 kn and ±0.11% in the range of 1000 kn to 4500 kn. Due to machine interactions particularly the inability to maintain the load steady for sufficient time, the CMC of the force comparator machine has been over estimated. A stepper motor with Budenberg pressure balance is incorporated into the system for fine applications of force. A camera together with in-house developed software is utilised to capture the readings on the readout units as the reference transducer passes through the applied force measurement point. Therefore a reliable and sustainable dissemination of national traceability in the field of force is established using this comparator force machine to transfer the force unit from NMISA to industry with a minimum loss of accuracy. References 1. http://www.dti.gov.za/about_dti.jsp [accessed on 24 Jul. 14] 2. ISO 376: 2004 (as amended), Metallic materials Calibrations of force-proving instruments used for the verification of uniaxial testing machines.
3. BIPM IEC ISO IUPAP OIML Guide to the Expression of Uncertainty in Measurement International Organisation for Standardisation, Geneva, 1993