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1. e tm X ME Zi P communication range Figure 8 Location of a generic device P the relative location of a point on a 3D space at least four reference points are generally needed Sandwith and Predmore 2001 Chen et al 2003 Akcan et al 2006 In general a trilateration problem can be formulated as follows Given a set of nodes n With known coordinates x yj Zi and a set of measured distances M a system of non linear equations needs to be solved to calculate the unknown position of a generic point P u v w see Figure 8 ute ic Heo l ii x2 uy 02 vY 2 wy M5 3 Xn uy T Vn v T wy M If the trilateration problem is over defined four or more reference points it can be solved using a least mean squares approach Savvides et al 2001 Each unknown node generically P estimates its position by performing the iterative minimisation of an error function EF defined as ap e M Gi n l 4 where M measured distance between the i th node and the unknown device P G calculated distance between the estimated position of P u v w and the known position of the i th device C z x yj Zi n number of constellation devices Cj i l 7 within device P communication range Each Cricket mounted on the mobile probe to locate its own position uses the known locations of at least four constellation Crickets and the measured distance from them AII inf2. International Journal of Production Research 3879 oe P3 X3y Y3 Z3 Py Xa Yo Z2 V s Py X4 Yi Z4 By x4 ya z4 b a Figure 10 Location of constellation device B4 utilising distances from P1 P2 P3 P4 reference points device it is necessary to know distances from at least four reference points Chen et al 2003 Figure 10 a represents the procedure to determine distances from some reference points and a constellation Cricket The probe tip is placed next to the point P with the aim of calculating the distance from Cricket B4 point D The following distances are known e AD and BD from constellation Cricket B4 and devices A and B e AB and P gt B from devices A and B mounted on the mobile probe and from the device B and the probe tip P2 To calculate distance PD we can use the Carnot Theorem see Figure 10 b Applying this theorem to triangle ABD we obtain the following equation AD AB BD 2 AB BD cos a 11 From which AB BD AD cosa AB BD 12 Applying again Carnot theorem to triangle PBD P D P B BD 2 PB BD cos o 13 Substituting Equation 12 within Equation 13 we obtain AB BD AD PD P B BD 5B os 14 Equation 14 makes it possible to calculate the distance from the reference point P to the constellation device B4 point D The described procedure is re
3. Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research 3869 Table 1 Definition and description of MScMS basic requirements Requirement Description Portability Easy to move easy to assemble disassemble lightweight and small sized Fast installation and start up Before being ready to work system installation start up or calibration should be fast and not too complicated Low price Low costs of production installation and maintenance Metrological performances Appropriate metrological performances in terms of stability repeatability reproducibility and accuracy ISO 5725 1986 Working volume Area covered by the instrument should be wide enough to perform measurements of large size objects dimensions up to 30 60 metres Easy use System should be user friendly An intuitive software interface should guide the user through measurements Work indoor System should be able to work indoor inside warehouses workshops or laboratories Flexibility System should be able to perform different measurement typologies 1 e determination of point coordinates distances curves surfaces etc Table 2 Measuring systems comparison qualitative performance evaluation MEASURING erotica Wor SYSTEM Portabiy instalation Cour oO een THEODOLITE FAST H NOME E LASER TRACKER MEDIUM MEDIUM MEDIUM 3 other hand theodolites or GPS are smaller and lightweight bu
4. evaluate mutual distances MIT Computer Science and Artificial Intelligence Lab 2004 The system makes it possible to calculate the position in terms of spatial coordinates of the object points touched by the probe More precisely when a trigger mounted on the mobile probe is pulled current coordinates of the probe tip are sent to a PC via Bluetooth Acquired data is then available for different types of elaboration determination of distances curves or surfaces of measured objects Constellation devices Crickets operate as reference points or beacons for the mobile probe Spatial location of the constellation devices follows a semi automatic procedure described in Section 4 4 Constellation devices are distributed without constraint around the object to measure In the following subsections we describe the MScMS hardware focusing on e Wireless devices Crickets e Measuring method to evaluate mutual distances among Crickets e Mobile probe and procedure to locate touched points a 1e eu cH Figure 1 Practical application of MScMS University of Bath Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research 3871 3 1 Cricket devices Cricket devices are equipped with radiofrequency RF and ultrasound US transceivers Working frequencies are respectively 433 MHz on RF and 40kHz on US Cricket devices are developed by Massachusetts Institute of Technolo
5. intervals included between 150 and 350 milliseconds each device tries to synchronise itself with neighbours in order to exchange US signals Synchronisation information is transmitted through RF packets 2 When a Cricket receives a new distance from a neighbour or directly measured stores and sends it to its neighbours by an RF packet containing a new list of inter node distances Firmware coordinates the communication among Cricket devices making them able to cooperate and share information about inter node distances When the user pulls the mobile probe trigger all information is sent via Bluetooth to a PC for elaboration 4 4 Semi automatic location of the constellation Location of Cricket devices should be fast and automated as much as possible This operation if manually performed is tedious and conflicting with system adaptability to different working places As a consequence in order to minimise human moderation a method for a semi automatic localisation has been implemented It is important to remark that accuracy in the localisation of constellation nodes is fundamental for accuracy in the next mobile probe location Patwari et al 2005 Sottile and Spirito 2006 Two techniques for the location of constellation devices have been designed 4 4 First approach First technique consists of touching using the mobile probe different reference points within the measuring area It 1s good to select points
6. 0m To make their positioning easy we used different supports such as booms articulated arms and tripods see Figure 1 Mobile probe It is made by a metal structure containing the following elements a Two Cricket devices b A tip to touch the points of measured objects Tip V and Cricket devices A and B are aligned and spaced as indicated dA B 450mm and d A V 540 mm see Figure 12 c A bluetooth transceiver connected with one of the two Cricket devices by an RS232 serial port Personal computer Ad hoc application sofware runs on a standard PC In order to receive data sent by the probe the PC is equipped with a bluetooth transceiver Application software The purpose of this software is to drive the user through measurements and to make results display efficient Functions provided are similar to those typically implemented by CMM software packages MScMS likewise CMMs make it possible to determine the shape geometry of objects circumferences cylinders planes cones spheres etc on the basis of a set of measured surface points gathered from the mobile probe using classical optimisation algorithms Overmars 1997 In more detail the software is organised into three application modules to assist the user in the following operations e Initialisation This is a guided procedure to switch on wireless devices Crickets and bluetooth adapter and open the PC connection for data reception f
7. At the same time we present a taxonomy of the most common techniques and metrological equipment for dimensional measuring Major advantages and drawbacks will be highlighted The attention will be subsequently focused on MScMS The following aspects will be analysed in detail hardware and software configuration MScMS description and description of the first prototype trial runs and preliminary experimental results and critical aspects and possible improvements The final section of the document illustrates the possible improvements and research perspectives for MScMS enhancement 2 System requirements and comparison with other measuring techniques MScMS has been designed to perform dimensional measurements of medium large size objects dimensions up to 30 60 metres It should be easy to move and install low priced and able to work indoors inside warehouses workshops laboratories Table 1 identifies MScMS basic requirements Considering previous requirements we briefly analyse the most common measuring tools and techniques Table 2 shows the result of a qualitative comparison among five measuring instruments theodolite tacheometer CMM laser tracker photogrammetry system and GPS The last row of the table takes into account MScMS target performances Different considerations arise from Table 2 CMMs in spite of being very accurate measuring instruments are expensive bulky and not easily movable On the University of Bath
8. This article was downloaded by University of Bath Library On 27 May 2009 Access details Access Details subscription number 909050074 Publisher Taylor amp Francis Informa Ltd Registered in England and Wales Registered Number 1072954 Registered office Mortimer House 37 41 Mortimer Street London W1T 3JH UK International Journal of Production Research Publication details including instructions for authors and subscription information http www informaworld com smpp title content t7 13696255 Mobile Spatial coordinate Measuring System MScMS introduction to the system Fiorenzo Franceschini Maurizio Galetto Domenico Maisano Luca Mastrogiacomo Politecnico di Torino Dipartimento di Sistemi di Produzione ed Economia dell Azienda 10129 Torino Italy First Published January2009 To cite this Article Franceschini Fiorenzo Galetto Maurizio Maisano Domenico and Mastrogiacomo Luca 2009 Mobile Spatial coordinate Measuring System MScMS introduction to the system International Journal of Production Research 47 14 3867 3889 To link to this Article DOI 10 1080 00207540701881852 URL http dx doi org 10 1080 00207540701881852 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use http www informaworld com terms and conditions of access pdf This article may be used for research teaching and private study purposes Any substantial or Systematic reproduction re distribution re s
9. This information combined with the information on the five located Crickets 1s used to locate the whole constellation by means of an incremental algorithm Moore et al 2004 This algorithm starts with a set of five nodes with known coordinates Other nodes in the network determine their own coordinates using distances from them As an unknown node obtains an acceptable position estimate it may serve as a new reference point This process can be incrementally applied until all nodes in the network obtain their specific coordinates The procedure is driven by an ad hoc software routine Time required for self localisation is about 1 2 minutes Calculations are automatically performed by the central PC University of Bath Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research 3881 Figure 12 Mobile probe prototype Again accuracy in constellation location influences accuracy in future measurements The more Cricket positions are affected by uncertainty the less accurate measurements will be Taylor et al 2005 Franceschini et al 2007 5 MScMS prototype A first prototype of MScMS has been developed at the industrial metrology and quality laboratory of DISPEA Politecnico di Torino It is made by the following elements 1 2 3 4 Cricket constellation Twenty two Cricket devices have been freely distributed around a measuring area covering a volume of about 6
10. a Ena Se Formulas 10 univocally locate point V using spatial coordinates of Crickets A and B Distances d A B and d A V are a priori known as they depend on probe geometry The previous model is based on the assumption that US sensor A and B and probe tip V are punctiform geometric elements In practice the model 1s inevitably approximated University of Bath Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research 3877 because sensors A and B have non punctual dimensions see Figure 2 To minimise point P position uncertainty the following condition should be approached a B V d A V Zakrzewski 2003 4 3 Cricket firmware Firmware is essential to organise RF and US communication among Cricket devices Firmware is written in NesC language and works under the operating system TinyOS NesC is derived from C and it is currently utilised to program MICA Mote devices produced by Crossbow Technologies which Crickets are derived from NesC is object oriented and event based Programs are organised in independent modules They interrelate themselves by means of reciprocal queries replies MIT Computer Science and Artificial Intelligence Lab 2004 Moore et al 2004 Figure 9 shows a schematic flow chart of Cricket firmware Each Cricket device performs two types of operation 1 Time of flight measurement of US signals transmitted received from other devices At random time
11. aced on a plane XY perpendicular to the vertical Z axis see Figure 19 Since we have experimentally verified that the distribution of the point coordinates can be considered as normal both for repeatability and reproducibility data the variability range considering a 99 7 confidence level is given by 30 Reproducibility range is an index of the instrument actual accuracy whereas repeatability variation range 1s an index of the target instrument accuracy supposing to compensate the most important causes of systematic errors The most critical aspects of the whole measuring system are due to US sensors In particular 1 Dimensions of US transceivers 2 US signals are prone to different types of noises 3 Speed of sound dependence on environmental conditions 4 Working volume discontinuities 5 Use of amplitude threshold detection at receivers These aspects are individually discussed in the following subsections University of Bath Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research 3885 b e ultrasound points of departure arrival Figure 17 Points of departure arrival of US exchanged between two Crickets 6 1 Dimensions of US transceivers A source of uncertainty in US time of flight measurements is due to non punctiform US sensors The volume of each piezo electric crystal is about 1 cm As shown in Figure 17 it is difficult to determine the exact point of depar
12. als included between 150 and 350 milliseconds each device transmits an RF query packet to other devices within its communication range It checks if neighbouring Crickets are ready to receive a US signal Figure 3 a Priyantha et al 2000 2 Ready devices reply sending an RF acknowledgement authorising next signal transmission Figure 3 b Ultrasound receiver Integrated antenna for RF transceiving Ultrasound transmitter Perspective view Orthogonal projection Figure 2 Cricket device Crossbow Technology University of Bath Library At 13 30 27 May 2009 Downloaded By 3872 F Franceschini et al device 1 device 2 AMANN a Query RF b Reply RF and authorisation VV for signals transmission HF c Concurrent transmission of RF and US signals Figure 3 Communication scheme implemented by Cricket devices Priyantha et al 2000 device 1 device 2 query RF signal L RF c speed of electromagnetic radiation HUE nn PRR bd M eU d HF authorisation for m LE Ait adani anionin ene FB TRAD ISSIOE concurrent transmission ee gt of RF and US signals S RF i ee AUS s speed of sound a At TDoA T time lapse between im reception of RF 5s and US signals T i Figure 4 Time evolution of RF and US signals qualitative scheme 3 Querying Cricket is now authorised to concurrently send an RF and a US signal Figure 3 c 4 Receiving device measures the la
13. an T wh mu r d 1 t i t Li Figure 14 Display for the measurement of single points Circumference Function to determine a circumference on a horizontal plane points minimum Number of Points 8 Center x 1122mm y 40 mm Radium 219mm Ox t 13 mm ay i3 mm Probe Cricket 1 W NE a c M Probe Cricket n 2 a uw a b e Figure 15 Display for the measurement of a circle T a i Li a a acd dama moa mom mon oon Rao om om 3883 University of Bath Library At 13 30 27 May 2009 Downloaded By 3884 F Franceschini et al Measured single point a Hepeatability the mobile probe position is fixed b Reproducibility the mobile probe direction is during the measurements changed before every measurement Figure 16 Representation scheme of the practical tests carried out to evaluate MScMS performances Table 3 Results of the MScMS preliminary tests Repeatability Reproducibility Test Ox 05 0 O y one Mean standard deviation mm 4 8 5 1 J3 7 3 7 8 4 1 The statistical results of these preliminary tests are reported in Table 3 Take note that o value is basically lower than o and o both for repeatability and reproducibility tests This behaviour is due to the geometric configuration of the constellation devices in general network devices are mounted on the ceiling or at the top of the measuring area and for this reason they can be considered as approximately pl
14. arch perspectives for future improvements are given Keywords mobile measuring system wireless sensor networks dimensional measurements coordinate measuring machine CMM 1 Introduction This paper introduces a new measuring system called Mobile Spatial coordinate Measuring System MScMS MScMS is designed to perform simple and rapid dimensional measurements of large size objects An essential requirement for the system is portability that is its aptitude to be easily transferred and installed Corresponding author Email fiorenzo franceschini polito it ISSN 0020 7543 print ISSN 1366 588X online 2009 Taylor amp Francis DOI 10 1080 00207540701881852 http www informaworld com University of Bath Library At 13 30 27 May 2009 Downloaded By 3868 F Franceschini et al Many types of metrological equipment utilising different kinds of technologies optical mechanical electromagnetic etc give physical representations of measured objects in a three dimensional Cartesian coordinate system Coordinate measuring machines CMM theodolites tacheometers photogrammetry equipments GPS global positioning systems laser trackers are typical instruments to do it Each of these systems is more or less adequate depending on measuring conditions a user s experience and skill and other factors like time cost size accuracy portability etc Classical CMMs that make performing of repeated and accurate measurements o
15. asic parts 1 a constellation of wireless devices distributed around the working area 2 a mobile probe to register coordinate points of the measured object and 3 a PC to store and elaborate data sent via Bluetooth by the mobile probe Furthermore MScMS 1s adaptable to different working environments and does not require long installation or start up times Before performing measurements constellation devices freely distributed around the measuring area automatically locate themselves in a few minutes System is supported by ad hoc software created in Matlab to drive user through measurements and online offline elaborations Today MScMS Achilles heel is represented by its low accuracy due to the use of ultrasound transceivers non punctiform dimension speed of sound dependence on temperature etc As research perspectives all factors affecting system accuracy will be analysed and improved in detail in order to reduce their effect References Akcan H Kriakov V Bronnimann H and Delis A 2006 GPS free Node Localization in Mobile Wireless Sensor Networks In Proceedings of MobiDE 06 25 June 2006 Chicago IL USA 35 42 ANSI IEEE Std 176 1987 1988 Standard on Piezoelectricity American National Standards Institute New York Balakrishnan H Baliga R Curtis D Goraczko M Miu A Priyantha N B Smith A Steele K Teller S and Wang K 2003 Lessons from Developing and Deploy
16. at the Cricket receivers and to periodically update s using an experimental relation s s T Bohn 1988 As a better alternative we implemented an optimisation procedure which makes it possible to estimate measurement by measurement the optimum s value using the following information e Times of flight among at least four constellation Crickets and the two mobile probe Crickets A and B e A standard of length for reference given by the a priori known distance between the mobile probe Crickets A and B By an automatic optimisation we calculate the s value which better satisfies the previous constraints with reference to a particular portion of the working volume In this way the s value can be recalculated for each single measurement 6 4 Working volume discontinuities A requirement of the measuring instruments is to measure uniformly and with no discontinuities all the points within the working volume Due to its technology MScMS is based on a network of distributed devices communicating through RF and US While RF sensors communication range is almost omni directional and up to 25 metres US sensors have a communication range limited by cone of visions with an opening angle of about 80 and a range of no more than 10 metres see Figure 19 Signal strength outside the cone drops to 1 of the maximum value Priyantha et al 2000 It is therefore important to provide a full coverage to the area served by network devi
17. can be covered varying the number of constellation nodes 4 MScMS software architecture This section describes software firmware features of MScMS to implement the following operations e Location of Crickets mounted on the mobile probe e Location of points touched by the probe University of Bath Library At 13 30 27 May 2009 Downloaded By 3874 F Franceschini et al Orthogonal view Cricket devices Bluetooth adapter to PC Probe tip touching measured object B BV Fixed distances G Trigger Figure 6 Schematic representation of the mobile probe Xy Y Z4 x Y Z5 X3 D 39 Z3 Distances from device A r V point touched by the probe Figure 7 Location of points touched by the probe e Communication and data sharing among Cricket devices e Semi automatic location of constellation devices Figure 7 represents the first two operations All operations are better described in the following subsections 4 1 Location of Crickets mounted on the mobile probe Spatial location of each Cricket probe is performed using a trilateration technique Trilateration uses the known locations of beacon reference points To uniquely determine University of Bath Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research 3875 C OO C ec Mg Xe Yor Z6 C Xa Yo Z4 d j Xs yg Zs x Y Z per 5 C X55 Y5 Zs C Xz Y Z e e
18. ces by proper alignment of the US transmitters towards the measuring area To increase the working volume coverage it is necessary to increase the number of network devices In general the best solution 1s mounting the network devices on the ceiling or at the top of the measuring area as shown in Figure 19 University of Bath Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research 3887 cel ing cone of vision of network device Ds Figure 19 Representation scheme of the US sensors cone of vision Since for locating the mobile probe by trilateration the mobile probe should communicate with at least four network devices on the basis of practical tests we determined that the coverage of an indoor working volume about four metres high can be achieved using about one network device per square metre considering a plant layout 6 5 Use of amplitude threshold detection at receivers To evaluate time difference of arrival TDoA receivers can detect signals with amplitude equal or greater than a threshold value Since US transceivers operate at 40 kHz frequency the time period of a complete wave cycle is 1 40000 s 25 us US waves are saw tooth shaped with a linear rise see Figure 20 Considering fresh US signals at the transmitter their amplitude may decrease depending on two basic factors 1 Attenuation signal amplitude decreases depending on the distance cover
19. ed 2 Transmitter orientation since US transmitters are not omni directional signal amplitude changes depending on their orientation In particular the maximum signal strength is related to the direction perpendicular to the transducer surface at the centre of the cone of vision while signal amplitude drops to 1 of the maximum value at 40 away from it Priyantha et al 2000 University of Bath Library At 13 30 27 May 2009 Downloaded By 3888 F Franceschini et al Signal amplitude f l w Full amplitude signal Error in TDoA ra Signal with decreased amplitude P r l I I 4 I 4 I l I I I I I I Amplitude threshold set at the receiver xL e Time difference of arrival TDoA Figure 20 Representation scheme of the error produced by the use of amplitude threshold detection The consequence of the use of amplitude threshold detection is the occurrence of errors in TDoA evaluation see Figure 20 However considering that amplitude threshold value is usually about half of the full amplitude signal the range of these errors is included within half of the US time period Since the speed of sound is about 340 m s it is typically included between 0 and 4 5 mm 7 Conclusions The MScMS measuring system can be considered as complementary to CMMs It 1s portable not too expensive and suitable for measuring large size objects not easy on conventional CMMs MScMS is made up of three b
20. elling loan or sub licensing systematic supply or distribution in any form to anyone is expressly forbidden The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date The accuracy of any instructions formulae and drug doses should be independently verified with primary sources The publisher shall not be liable for any loss actions claims proceedings demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material University of Bath Library At 13 30 27 May 2009 Downloaded By Taylor amp Francis Taylor amp Francis Group International Journal of Production Research Vol 47 No 14 15 July 2009 3867 3889 Mobile Spatial coordinate Measuring System MScMS introduction to the system Fiorenzo Franceschini Maurizio Galetto Domenico Maisano and Luca Mastrogiacomo Politecnico di Torino Dipartimento di Sistemi di Produzione ed Economia dell Azienda Corso Duca degli Abruzzi 24 10129 Torino Italy Received 29 January 2007 final version received 7 December 2007 In many industrial fields for example automotive and aerospace dimensional measurements of large size objects should be easily and rapidly taken Nowadays the problem can be handled using many metrological systems based on different technologies optical mechanica
21. gy and manufactured by Crossbow Technology Each device uses an Atmega 128L microcontroller operating at 7 4 MHz with 8 Kb of RAM 128 Kb of FLASH ROM program memory and 4Kb of EEPROM mostly as read only memory Alimentation is provided by two AA batteries of 1 5 V Balakrishnan et al 2003 Cricket devices are quite small see Figure 2 easy to move and cheap each unit would cost about 10 20 if mass produced Due to these characteristics they are optimal for ad hoc wireless sensor network applications Priyantha et al 2000 Crickets are equipped with US transceivers quartz crystals which transform electric energy into acoustic and vice versa piezo electric effect They generate receive 40kHz ultrasound waves Transducers excited by electric impulses vibrate at the resonance frequency producing acoustic ultrasound impulses ANSI IEEE Std 176 1987 1988 3 2 Evaluation of distances between Cricket devices Cricket devices continuously communicate with each other in order to evaluate mutual distances Device communication range is typically 8 10 metres in the absence of interposed obstacles The technique implemented by each pair of Crickets to estimate mutual distance is known as time difference of arrival TDoOA It is based on the comparison between the propagation time of two signals with different speed RF and US in this case Savvides et al 2001 TDoA technique is described as follows 1 At random time interv
22. h point we have calculated the standard deviations 0 0 0 related to the registered Cartesian coordinates x y Z 2 Reproducibility test this test 1s similar to the previous one with the only difference that the mobile probe orientation 1s changed before each measurement with the aim of approaching each single point from a different direction see Figure 16 b University of Bath Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research muacrimmm gi mA E z nu F d LI u L1 mm c a rj d w r mma mg yirt Eom 1 i Ll z i E z a E r t L i 3 E 1 LLL S ELE LLL ELT E T a z D i j s J i a L a a i Li A i 454a eui ee eee deed et Pr ep Ere i E E a r La p P i H i Ld uu E 1 Li 1 SEDE m s ir Tho i r 1 1 i L t 1 1 h k b i i n oe E oo iba bma k ihg fum 4 E y t i a i n i t i i i b E ae a t t i 1 Li b k I F T 1500 La m EEG REGGG 4 EW EG zm VERG EE EHREREGA 5 i 1 k i L 1 1 t i d r 1 Ld de 100 an m naw a o T TX um o eater n i i 1 i d r 1 T i Ld i i d Li 1 Qu a3 Htcahdzca upra Peaaeia ba coma ua mag hus BEMAGRZ a x i LI d r 1 f T E k i E F i L H u i u i i 1 i a kh 1 i tl efter fre rtm brr mad E r r 1 7 E F t 1 t u b k 1 1 L k F i z i i Li i a i
23. ification tests for coordinate measuring machines CMM MIT Computer Science and Artificial Intelligence Lab 2004 Cricket v2 User Manual Available from http cricket csail mit edu v2man html Accessed 22 January 2007 Moore D Leonard J Rus D and Teller S S 2004 Robust distributed network localization with noisy range measurements In Proceedings of SenSys 2004 November Baltimore MD 50 61 Overmars M H 1997 Designing the Computational Geometry Algorithms Library CGAL Applied computational geometry towards geometric engineering Berlin Springer Verlag 53 Patwari N Ash J Kyperountas S Hero A Moses R and Correal N 2005 Locating the nodes cooperative localization in wireless sensor networks IEEE Signal processing Magazine 22 4 54 69 Pozzi F 2002 Comparison of 3D Measurement Techniques in Cultural Heritage Application User Point of View Jn Proceedings of the lst international symposium on 3D data processing visualization and transmission IEEE computer society 19 21 June Padova Italy 762 765 Priyantha N B Chakraborty A and Balakrishnan H 2000 The Cricket Location Support System In Proceedings of the 6th ACM MOBICOM August Boston MA 32 43 Rooks B 2004 A vision of the future at TEAM Sensor Review 24 2 137 143 Sandwith S and Predmore R 2001 Real time 5 Micron Uncertainty with Laser Tracking Interferometer Systems using Weighted Trilateration I
24. ing the Cricket Indoor Location System Preprint University of Bath Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research 3889 Bohn D A 1988 Environmental effects on the speed of sound Journal of the Audio Engineering Society 36 4 223 231 Bosch J A 1995 Coordinate Measuring Machines and Systems New York Marcel Dekker Chen M Cheng F and Gudavalli R 2003 Precision and Accuracy in an Indoor Localization System Technical Report CS294 1 2 University of California Berkeley USA Franceschini F Galetto M and Settineri L 2002 On Line diagnostic tools for cmm performance The International Journal of Advanced Manufacturing Technology 19 2 125 130 Franceschini F Galetto M Maisano D and Mastrogiacomo L 2007 A review of localization algorithms for distributed wireless sensor networks in manufacturing Forthcoming International Journal of Computer Integrated Manufacturing Gustafsson F and Gunnarsson F 2003 Positioning using Time Difference of Arrival measurements Jn Proceedings of the IEEE international conference on acoustics speech and signal processing ICASSP 2003 6 10 April Hong Kong Vol 6 553 556 ISO 5725 1986 Precision of test methods determination of repeatability and reproducibility for a standard test method by inter laboratory tests ISO 10360 Part 2 2001 Geometrical Product Specifications GPS acceptance and rever
25. l electromagnetic etc Each of these systems is more or less adequate depending on measuring conditions a user s experience and skill or other factors like time cost dimensions accurateness portability etc In general for measuring medium large size objects portable systems can be preferred to fixed systems Transferring the measuring system to the measured object place is often more practical than vice versa The purpose of this paper is to introduce a new system called Mobile Spatial coordinate Measuring System MScMS The system has been designed to perform dimensional measurements of medium large size objects MScMS is made up of three basic parts a constellation of wireless devices liberally distributed around the working area a mobile probe to register the coordinate points of the measured object and a PC to store data sent by the mobile probe via Bluetooth and to process them by means of ad hoc application software MScMS is easily adaptable to different measuring environments and does not require complex procedures for installation start up or calibration This document presents the system hardware software firmware architecture and functionalities and describes the peculiarities and metrological performances of MScMS first prototype which has been developed at the industrial metrology and quality laboratory of DISPEA Politecnico di Torino Finally the most critical aspects of MScMS are illustrated and the rese
26. n Proceedings of 2001 boeing large scale metrology seminar St Louis MO Savvides A Han C and Strivastava M B 2001 Dynamic fine grained localization in ad hoc networks of sensors In Proceedings of ACM IEEE 7th annual international conference on mobile computing and networking MobiCom 01 16 21 July Rome Italy 166 179 Sottile F and Spirito M 2006 Enhanced Quadrilateral based Localization for Wireless Ad hoc Networks Jn Proceedings of IFIP fifth annual mediterranean ad hoc networking workshop Med Hoc Net 2006 14 17 June Lipari Italy 224 231 Taylor C Rahimi A Bachrach J and Shrobe H 2005 Simultaneous Localization Calibration and Tracking in an ad hoc Sensor Network Tech Rep Computer Science and Artificial Intelligence Laboratory of MIT Available from https dspace mit edu handle 1721 1 30541 Accessed 22 January 2007 Welch G Bishop G Vicci L Brumback S and Keller K 2001 High performance wide area optical tracking the Hi Ball tracking system Presence Teleoperators And Virtual Environments 10 1 1 21 Zakrzewski J 2003 Error and uncertainty reduction challenge for a measuring systems designer Measurement Science Review 3 31 34
27. n objects which are even complexly shaped possible are widespread On the other hand CMMs are generally bulky and not always suitable for measuring large size objects for example longerons of railway vehicles airplane wings fuselages etc because the working volume is limited ISO 10360 Part 2 2001 In general for measuring medium large size objects portable systems can be preferred to fixed systems Transferring the measuring system to the measured object place is often more practical than vice versa Bosch 1995 Systems such as theodolites tacheometers photogrammetry equipment laser trackers or GPS rather than CMMs can be easily installed and moved Pozzi 2002 However they can have some other drawbacks as mentioned in the next section MScMS has been designed to be portable easy to install and start up low priced and adequate for measuring medium large size objects Innovative measuring systems which have been recently industrialised only partially fulfil previous desiderata In particular we mention 3rd Tech Hi Ball Leica T Probe and Metris Laser Radar Welch et al 2001 Rooks 2004 These systems all based on optical technologies are lightweight and very accurate but they are relatively high priced and generally require a large time for installation and start up Before introducing MScMS in the next section we provide a structured description of requirements and functionalities that a generic system should meet
28. ormation needed for the location is sent to a PC for centralised computing University of Bath Library At 13 30 27 May 2009 Downloaded By 3876 F Franceschini et al 4 2 Location of points touched by the probe tip The probe tip V lies on the same line of devices A and B see Figure 6 This line can be univocally determined knowing coordinates of points A xA va Za and B xg yp Zp and the distance d A V The parametric equation of this line 1s X XA xg xa f y yatQOs ya t 5 Z ZA t Zp ZA The distance d A V can be expressed as MA V y xa xy va yyy ZA zv 6 Coordinates of point V x y z are univocally determined solving a system of four equations in four unknown values xy yy Zy and tj Xy XA XB Xa fv yv YA QB YA fv Zy ZA ZB ZA lv 7 d A V J xa xv a wy a zvY Replacing terms x y and z in the fourth equation XA xa Xa xA S MA V ya QA Qs YA NT 8 za za Za za WT then i d A V A V da TAA B 9 xa xp va yp ZA ZB The denominator of Equation 9 1s the distance d A B between the two Cricket devices installed on the mobile probe In conclusion coordinates of the point V can be calculated as xy xa Gm a d A Y ya OB IA So 10 d A ay z
29. peated for all reference points 1 e P P4 in Figure 10 Once all required distances have been taken a trilateration technique can be applied in University of Bath Library At 13 30 27 May 2009 Downloaded By 3880 F Franceschini et al DB3 B4 Distances utilised in these mi automatic location of B1 B5 constellation Crickets the constellation A B probe Crickets Figure 11 Constellation location using the mobile probe as an ear order to localise each constellation Cricket The acquisition procedure is driven by an ad hoc software routine Calculations are automatically performed by the central PC 4 4 2 Second approach The second approach is an extension of the first The previous localisation approach is not adequate for constellations with a large number of Crickets since each device needs to know distances from at least four reference points For that reason we have implemented a semi automatic localisation technique which also uses the information on the mutual distances among constellation Crickets This technique is based on two steps 1 As described for the first approach the mobile probe is used to touch four reference points in order to locate five constellation Crickets 2 Subsequently the mobile probe is used as an ear to receive the mutual distances of all the constellation Crickets including the five which have been located Signals gathered are sent to the PC see Figure 11
30. pse time between reception of RF and US signals see Figure 4 The distance between two devices 1s calculated by the following formula eee 2Q 5 0 0 where c is the speed of electromagnetic radiations s the speed of sound and At is TDoA Gustafsson and Gunnarsson 2003 Since there is a large difference d 1 University of Bath Library At 13 30 27 May 2009 Downloaded By International Journal of Production Research 3873 Distances received by device B8 Distances discardedby B8 Distances stored by B8 and sent to its neighbours B8 communication range Figure 5 Distance information handled by a single device B8 The shadow highlights the B8 communication range between c about 300 000 km s and s about 340 m s in air with temperature T 20 C and relative humidity RH 50 then Xs At 2 3 3 Cricket communication Cricket devices build a wireless network of cooperating sensor nodes To preserve network scalability that is to make sure that the amount of information stored by each node is independent from network dimension in terms of nodes each node memorises the distances from its direct neighbours contained in the communication range see Figure 5 3 4 The mobile probe The mobile probe is equipped with two Cricket devices aligned with the tip see Figure 6 The system has been designed to be deployed over small or wide areas depending on the dimension of measured objects The measuring area
31. rom the mobile probe University of Bath Library At 13 30 27 May 2009 Downloaded By 3882 F Franceschini et al Figure 13 MScMS software menu e Self localisation of the constellation This procedure is described in Section 4 4 Figure 13 a e Measurements Execution of different kinds of measurements single points measurements distance measurements curves and surfaces evaluation see Figures 13 b 14 and 15 Measurements are taken by the probe tip When the probe trigger is pulled the application software calculates Cartesian coordinates of the touched point If measure ment is correctly taken an acoustic signal is emitted Measured results are displayed using numeric and graphical representations Figure 13 shows some screenshots of the software main menu and sub menus Figures 14 and 15 show some displays of the MScMS software 6 MScMS actual performances critical aspects and possible improvements A preliminary prototype of MScMS has been set up and tested with the purpose of verifying system feasibility and to evaluate the performances The prototype actual performance has been estimated carrying out two practical tests 1 Repeatability test a single point within the working volume is measured repeating the measurement about 50 times leaving the mobile probe in a fixed position see Figure 16 a The test is repeated measuring at least 20 different points in different areas of the working volume For eac
32. t not very flexible in terms of different types of measurements offered Furthermore GPS systems are less accurate and cannot operate indoors Interferometrical laser trackers and digital photogrammetry equipment are extremely accurate but complex and expensive at the same time Sandwith and Predmore 2001 Points to be measured need to be identified by the use of reflective markers or projected light spots Theodolites tacheometers are typically used in topography but are not suitable to measure complex shaped objects In conclusion none of the examined measuring systems fulfil all previous require ments MScMS is a system based on the wireless sensor network technology able to make a trade off among these requirements University of Bath Library At 13 30 27 May 2009 Downloaded By 3870 F Franceschini et al 3 MScMS hardware equipment MScMS is made up of three basic parts 1 A constellation of wireless devices distributed around the measuring area 2 A mobile probe to register the coordinates of the object touched points 3 A PC to store data sent via Bluetooth by the mobile probe and ad hoc application software The mobile probe is equipped with two wireless devices identical to those making up the constellation These devices known as Crickets are developed by Massachusetts Institute of Technology and Crossbow Technology They utilise two ultrasound US transceivers in order to communicate and
33. that are easily reachable and easy to be manually located in a reference coordinate system For example points lying on objects with a simple and known geometry like parallelepiped vertexes Spatial coordinates X Yi Zi of the distributed constellation devices are the unknown parameters of the problem Location of each device 1s performed using a trilateration To identify a new t al Ini F Francesch 3878 OJEAULII JOYA 3y JO 11eq9 MO onewoyos y 6 oJn3rq uo eorunwwo Jopso pnppou qns sojnpour uui uoneano so npour ou 107 sjuoAo JL 7 gui pueu ouueuo Sf gui pueu ouueuo Ay TY BIA sinoqusrou qw BIA sanoquarou SpIBMO SUIDIEAIO SUNE pu JJ SPIVMO SUIPIEMIOJ vjep pue Sunepdn BIA UOISSTUISUUI o jsonboi vjep pue Sunepdn PP Dep IA UOISSI 1I SN JO uorssrusuen sr PP nep JO Uonestoyyne Jo ysonbad 4p u10012n g BIA Dd 91 uorssrusuezJ gyep pounseour TD suryoyspuvy onseour oouejsrp MOU uoneJoqe o geep UOISSIUISUPI Sf 1noqe uonesriogine Jo qp BIA 1d1o0o1 74242 UOISSIUISUEI Sf Jo isonbel Mou 10J no ouin 74242 uondoooi peusis Sf 242 soouejsrp Mou 1dio201 vyep AY u242 UOISSIUISUUI S 10 poayD 08 ou Jo 1dio2o1 7u242 peusis Sf Jo uorssrmurse 6007 ACW LZ OF ET aW Aaxeaqr1 Uuaegd jo JTSI ATUN g pepeo uwodq 13 30 27 May 2009 University of Bath Library At Downloaded By
34. the basis of the localised position We have experimentally verified that the minimum value of the EF 1s generally of the order of one tenth of mm When one or more measured distances are wrong due to systematic effects the EF minimum value explodes becoming three or University of Bath Library At 13 30 27 May 2009 Downloaded By 3886 F Franceschini et al obstacle 9000904 eset eate T 1 V D unuumuwvrryb d 4A bpbb dXa Fbpbb4Idd hhhaazA2 numumumaA mum ee HEHEEHHA heed hunnnn 44 0000 AA EEEEHEEA EHEEEENY EEEEEEHEY PP ad US recel er 9000904 x p d 090904 9290909 _ tete ox a oo Figure 18 US diffraction four order of magnitude greater In practical terms during the location of devices A and B if the EF minimum is included below a threshold value say 70 mm then the position is considered to be reasonable Otherwise it 1s rejected 6 3 Speed of sound dependence on environmental conditions Speed of sound s value makes it possible to turn US time of flight into a distance formula 2 It 1s well known that the speed of sound changes with air conditions temperature and humidity which can exhibit both temporal and spatial variations within large working volumes As a consequence 5 needs to be updated often depending on the time and the position A partial solution to this problem is to use the temperature T information evaluated by embedded thermometers
35. ture arrival of a US signal exchanged between a pair of Crickets These points are placed on the US sensors surfaces and may vary depending on their relative position see Figure 17 Regarding the future Cricket devices will be modified in order to minimise this problem for example reducing the size of US sensors 6 2 US signal are prone to different types of noises During measurement the user should not obstruct US signal propagation Two possible drawbacks may occur 1 Transmitted US signal does not reach the receiver because it 1s completely shielded by an obstacle 2 Transmitted US signal diffracts and goes round interposed obstacle reaching the receiver In this case the path covered by US is longer than the real distance between transmitter and receiver see Figure 18 Case 2 is more complicated to handle than case 1 In general it is not easy to notice possible path deflections Probe can be prone to other types of noise like external sources of US For example US produced by metal objects jingling However wrong distance measurements like the ones described can be indirectly detected and rejected To that purpose an effective diagnostic tool is the error function EF see Equation 4 Franceschini et al 2002 This function evaluated for both the mobile probe devices A and B during localisation is an index of the bias between measured distances evaluated by means of US transceivers and calculated distances on
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