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Guide to the Measurement of Force The Institute of Measurement and Control 87 G

Guide to the Measurement of Force The Institute of Measurement and Control 87 Gower Street London WC1E 6AF ISBN 0 904457 28 1 Originally published 1998 Reviewed and re-issued 2013 Guide to the Measurement of Force COMMITTEE RESPONSIBLE FOR THIS GUIDE This Guide to the Measurement of Force has been prepared by the National Physical Laboratory and the Institute of Measurement and Control, supported by the National Measurement System Policy Unit of the Department of Trade and Industry. An independent panel of force specialists, as listed below, developed the structure and content of the Guide. Andy Hunt, Coordinator Andrew Hunt Consultants Ltd Anthony Cross Nobel Systems Ltd Mike Crossman UKAS John Davis Davis Decade Ltd Ural Erdem Negretti Automation Ltd Allan Herbert DERA Pyestock Ray Jenkins National Physical Laboratory Andy Knott National Physical Laboratory Steve Maclean Veccer Leonard March Kistler Instruments Ltd Dave Tame Sheffield Testing Laboratories Gary Thompson Eagle Star Insurance Co Ltd David Viney Nobel Systems Ltd John Webb Avery Berkel Ltd Gavin Wort DERA Pyestock The panel would like to thank Colin Carter, Assistant Secretary of the Institute of Measurement and Control for his support in operating the panel meetings and Ken Chatters and Wilf Fry for their valuable comments on the document. This Guide is subject to review at any time by the responsible technical group of the Institute. It was reviewed and re-issued in PDF format in 2013 - members of the Weighing and Force Measurement Panel at the time of this review were: Andy Knott, Chairman National Physical Laboratory Ural Erdem, Deputy Chairman Consultant Thomas Allgeier Flintec UK Ltd Mike Baker Sherborne Sensors Ltd Paul Dixon National Measurement Office Peter Harrison UKAS Mark Hopkins Procon Engineering John Pugh Glasgow Caledonian University David Smith Avery Weightronix Ltd Ian Turner United Kingdom Weighing Federation Clarry Whittingham Tata Steel The Institute welcomes all comments on this Document and requests that these be addressed to the Institute. Users of this Institute of Measurement and Control Guide shall be responsible for its correct application. This guide refers to other publications that provide information or guidance. The editions listed are believed to be current at the time of publication, but reference should be made to the latest editions. No part of this publication may be reproduced in any form without the prior permission in writing of the Institute of Measurement and Control. Published by the Institute of Measurement and Control.  Crown copyright 1998. Reproduced by permission of the Controller of HMSO. ISBN 0 904457 28 1. Guide to the Measurement of Force ORIGINAL FOREWORD This Guide has been written to meet the need for a basic advisory document for users of force instrumentation. As in other fields of measurement, a consistent and harmonised approach is increasingly important, as is a common understanding of the terms used to define and describe force measurement. This Guide brings together information which exists already in the public domain but is in many cases difficult to obtain, poorly expressed, or widely misunderstood. The Guide is intended to be practical; readily applicable; widely acceptable; accessible; and to contain objective criteria against which good practice can be judged. The advice given here is carefully selected to represent conventional good practice in force measurement, to be consistent with recognised standard specifications relevant to force, and to be free from commercial bias. While this document provides a general introduction to force measurement it is not an in-depth scientific treatment of the subject. The further information sources section is provided as a starting point for those wishing to develop a more detailed understanding. It is in the interest of many groups and individuals that information about good measurement practices should reach all those who can contribute and benefit. Accordingly, this document has been written in collaboration between the Institute of Measurement and Control, the National Physical Laboratory and an independent panel of experts involved in the supply, calibration and use of force measurement systems, and in consultation with a wide circle of users in the UK and further afield. The creation of the document was made possible by support from the National Measurement System Policy Unit of the Department of Trade and Industry, and by the voluntary effort of many of the individuals involved. All readers of this Guide owe a debt of gratitude to those who have contributed to its preparation. C.R. Howard President The Institute of Measurement and Control CONTENTS 1 SCOPE ........................................................................................................................................................................................ 1 2 INTRODUCTION ..................................................................................................................................................................... 1 3 CONCEPTS IN FORCE MEASUREMENT ........................................................................................................................... 2 3.1 The SI system of units .................................................................................................................................................................... 2 3.2 Force and other physical quantities......................................................................................................................................... 2 3.3 Measurement terminology........................................................................................................................................................... 3 3.4 Force measurement systems ...................................................................................................................................................... 4 3.5 Characteristics of force measurement systems .................................................................................................................. 4 4 INTRODUCTION TO METHODS OF FORCE MEASUREMENT ................................................................................... 8 4.1 Overview .............................................................................................................................................................................................. 8 4.2 Strain gauge load cells .................................................................................................................................................................... 9 4.3 Piezoelectric crystal force transducers ............................................................................................................................... 13 4.4 Measuring force through pressure ........................................................................................................................................ 15 4.5 Other types of force measurement system ........................................................................................................................ 15 5 APPLICATIONS ..................................................................................................................................................................... 18 5.1 Range of applications .................................................................................................................................................................. 18 5.2 During development of a product .......................................................................................................................................... 18 5.3 During the manufacturing process ........................................................................................................................................ 19 5.4 Systems that measure or control force for safe operation ......................................................................................... 20 5.5 Process weighing ........................................................................................................................................................................... 21 5.6 Requirements on uncertainty .................................................................................................................................................. 21 6 CHOICE, DESIGN AND OPERATION OF SYSTEM ........................................................................................................ 22 6.1 Choice of force measurement system .................................................................................................................................. 22 6.2 Design considerations ................................................................................................................................................................. 24 6.3 Operating the force measurement system ......................................................................................................................... 25 6.4 General DOs and DON’Ts ........................................................................................................................................................... 26 6.5 Working with the supplier ........................................................................................................................................................ 27 7 CALIBRATION ....................................................................................................................................................................... 29 7.1 Overview ........................................................................................................................................................................................... 29 7.2 Standards and traceability ........................................................................................................................................................ 29 7.3 Multi-axis and dynamic calibration....................................................................................................................................... 30 7.4 Methodology .................................................................................................................................................................................... 30 7.5 Calibration procedures ............................................................................................................................................................... 31 8 QUALITY ASSURANCE OF FORCE MEASUREMENTS ................................................................................................ 36 8.1 Measurement accreditation ...................................................................................................................................................... 36 8.2 Competence in force measurement ...................................................................................................................................... 36 9 EXAMPLE APPLICATIONS ................................................................................................................................................ 37 9.1 Example: Measurement of web tension ............................................................................................................................. 37 9.2 Example: Force measurement on a mechanically-driven power press ............................................................... 39 9.3 Example: Spring force test on a personal computer mouse ...................................................................................... 41 9.4 Example: Force measurement in bio-mechanics ........................................................................................................... 41 10 FURTHER INFORMATION SOURCES ............................................................................................................................. 42 10.1 Bibliography .................................................................................................................................................................................... 42 10.2 Useful addresses ............................................................................................................................................................................ 44 11 INDEX ...................................................................................................................................................................................... 45 FIGURES Figure 1. Typical output characteristics of a force measurement system .................................................................................... 5 Figure 2. Creep curve of a typical force transducer ................................................................................................................................ 6 Figure 3. Basic diagram of an elastic transducer element ................................................................................................................... 8 Figure 4. Typical elastic elements and their usual rated capacities ............................................................................................. 10 Figure 5. Typical metal foil strain gauges ................................................................................................................................................ 10 Figure 6. Basic arrangement of four strain gauges in a load cell ................................................................................................... 12 Figure 7. A range of industrial load cells .................................................................................................................................................. 13 Figure 8. Industrial piezoelectric load washers .................................................................................................................................... 13 Figure 9. Mounting of a load washer .......................................................................................................................................................... 14 Figure 10. Multi-component crystal force sensor ................................................................................................................................ 14 Figure 11. An example of a hydraulic load cell. ..................................................................................................................................... 15 Figure 12. A shear pin force transducer ................................................................................................................................................... 19 Figure 13. Vessel supported by four load cells ...................................................................................................................................... 21 Figure 14. Typical instrumentation combinations .............................................................................................................................. 23 Figure 15. An example calibration certificate ........................................................................................................................................ 34 Figure 16. Forces on the roller ...................................................................................................................................................................... 37 Figure 17. Vector diagram of forces ........................................................................................................................................................... 37 Figure 18. Alignable force transducers ..................................................................................................................................................... 38 Figure 19. Force transducers in solution 2.............................................................................................................................................. 39 Figure 20. Sensor locations on a C-frame press .................................................................................................................................... 40 Figure 21. Blank and draw tool .................................................................................................................................................................... 40 Figure 22. Load curve examples ................................................................................................................................................................... 41 Figure 23. Response curve from mouse ‘click’ testing ....................................................................................................................... 41 Figure 24. A transparent multi-component force plate ..................................................................................................................... 42 Figure 25. Example of human gait analysis ............................................................................................................................................. 42 TABLES Table 1. SI notation for large and small numbers ................................................................................................................................... 2 Table 2. Approximate conversion factors to some non-SI force units ........................................................................................... 3 Table 3. Guide to force transducer types and characteristics ............................................................................................................ 9 Table 4. Instrumentation associated with common transducer types ....................................................................................... 24 Table 5. Checklist of potential requirements for a force measurement system ..................................................................... 28 Table 6. Types of force standard machine ............................................................................................................................................... 30 Guide to the Measurement of Force 1 Guide to the Measurement of Force 1 SCOPE This guide aims to help anyone wishing to measure force in any industrial or laboratory environment. It is essentially a guide for the user of the measurement and should help him or her to understand the range of measurement techniques available, how to characterise the requirements of his or her particular problem, and how to communicate those needs to manufacturers and suppliers of force measurement equipment. It also aims to give an understanding of the needs for, and methods of, calibrating, operating and maintaining force measurement systems. The techniques described cover the majority of problems encountered in force measurement, but where the guide cannot go into sufficient detail, the reader is referred to further published documents. The range of forces covered is from approximately 10-3 newtons (1 mN) to 109 newtons (1 GN), and those forces may be static or dynamic, and single or multi-axis. The guide is to the measurement of force, including the use uploads/s1/ force-guide.pdf