Design and development of a new large-scale metrology system: MScMS (Mobile Spatial coordinate Measuring System)

2 Chapter 1 
1.1 The Mobile Spatial coordinate Measuring System (MScMS) 
This thesis introduces a new measuring system called Mobile Spatial coordinate Measur-
ing System (MScMS), developed at the Industrial Metrology and Quality Laboratory of 
DISPEA – Politecnico di Torino. MScMS has been designed to perform simple and rapid 
indoor dimensional measurements of large-size objects (large scale metrology). An essen-
tial requirement for the system is portability Ŧ that is the aptitude to be easily transferred 
and installed. 
MScMS is made up of three basic parts: (1) a “constellation” of  wireless devices 
(Crickets), (2) a mobile probe, and (3) a PC to store and elaborate data [MIT C.S.A.I.L., 
2004]. Crickets and mobile probe exploit ultrasound (US) transceivers in order to evalu-
ate mutual distances. The constellation devices act as reference points, essential for the 
location of the probe. 
Each US device has a communication range limited by a cone of transmission within 
an opening angle of about 170° and a maximum distance of no more than 8 m. The mo-
bile probe location in the working volume is obtained by a trilateration. Consequently, the 
probe can be located only if it communicates with at least 4 constellation devices at once 
[Akcan et al., 2006]. 
The system makes it possible to calculate the position – in terms of spatial coordinates 
– of the object points “touched” by the probe. Acquired data are then available for differ-
ent types of elaboration to determine the geometric features of the measured objects (dis-
tances, curves or surfaces). 
One of the most critical aspects in the system set-up is the constellation devices posi-
tioning. Constellation devices operate as reference points, or beacons, for the mobile 
probe. In principle, Crickets can be positioned without restriction all around the measured 
object. However, the number and position of constellation devices are strongly related to 
the dimensions and shape both of the measuring volume and the measured object. It is 
important to assure a full coverage of the space served by constellation devices by a 
proper alignment of US transmitters. The spatial location of the constellation devices fol-
lows a semi-automatic procedure. The accuracy in the location of constellation devices is 
fundamental for the accuracy in the next mobile probe location [Patwary et al., 2005].  
Introduction  3
1.2 The new paradigm of the distributed measuring systems 
For the purpose of discussion, the large-scale dimensional measurement systems can be 
classified into centralized and distributed systems. In the case of centralized instruments, 
measurements may independently arise by a single stand-alone unit which is a centralized 
complete system (i.e. a CMM, a laser-scanner or a Laser Tracker), while distributed in-
struments are made of two or more distributed units (i.e. MScMS or other innovative sys-
tems like the indoor-GPS, described in Chap. 6 [Metris, 2007]).  
Distributed measurement systems introduce a new paradigm in the field of large-scale 
metrology. Due to their nature, they are portable and can be easily transferred around the 
area where the measurand is. Compared to centralized systems, distributed systems may 
cover larger measuring areas, with no need for repositioning the instrumentation devices 
around the measured object [Kang et Tesar, 2004]. 
MScMS can be classified as a modular distributed measuring system for large volume 
objects. Even if at present time MScMS is still a prototype and needs to be further devel-
oped, the system enables factory-wide location of multiple objects, applicable in manu-
facturing and assembly. Mainly, it can be used by aerospace manufacturers, but can also 
be adopted by automotive and industrial manufacturers both for positioning and tracking 
applications. Since MScMS main components are a number of wireless devices distrib-
uted around the measuring area, this not rigidly connected frame makes the system easy 
to handle and to move, and gives the possibility of placing its components freely around 
the workpiece, adapting to the environment and not requiring particular facilities. As a 
consequence, MScMS is suitable for particular types of measurement, which can not be 
carried out by traditional frame instruments, like conventional CMMs, because they are 
bulky and cannot be comfortably moved.  
The introduction of distributed measuring systems will probably have important effects 
on simplifying the current measuring practices within large scale industrial metrology 
[Maisano et al., 2007]. This tendency is confirmed by other recent distributed measuring 
systems based on laser and optical technology: the indoor-GPS (iGPS), the Portable-
CMM and the Hi-Ball [ARC Second, 2004; Metris, 2007; Metronor, 2007; Welch et al., 
2001]. All these systems – even they use different technologies – are composed of a 
number of sensors, arranged around the measuring area, which can be viewed by a sensor 
probe measuring the object surface. 
4 Chapter 1 
1.3 Literature review 
Dramatic advances in integrated circuits and radio technologies have made the use of dis-
tributed wireless sensor networks (WSNs) possible for many applications [Neil, 2005]. 
Recently, the attention towards the utilization of systems based on distributed sensor de-
vices in manufacturing is increasing. Since sensor devices do not need cables and may be 
easily deployed or moved, they can be practically utilized for a variety of industrial appli-
cations – factory logistics and warehousing, environmental control and monitoring, sup-
port for assembly processes, industrial dimensional measuring and real-time surveillance 
are only some possible applications. While outdoor localization applications are wide-
spread today (e.g. Global Positioning System – GPS), indoor applications can also benefit 
from location determination knowledge [Gotsman and Koren, 2004]. To make such ap-
plications feasible, the device costs should be low and the network should be organized 
without significant human involvement. 
To give a concrete idea of the potential of the systems based on WSNs in manufactur-
ing, here are briefly introduced some of the most interesting research issues with the cor-
responding bibliographic references.  
Support for final assembly. Ultrasonic sensors are mounted on power tools – for example 
screwdrivers – to detect their real position and activate them if they are in the right posi-
tion, during final assembly [Pepperl+Fuchs, 2005]. 
Industrial control and monitoring. Sensor devices can be deployed to perform industrial 
control and monitoring (for instance control of the air conditions of pollution, tempera-
ture, and pressure in different areas of the factory) or for emergency responses in case of 
incidents [Doss and Chandra, 2005; Pan et al., 2006; Koumpis et al., 2005; Oh et al., 
2006].
Factory logistics and warehousing. In an industrial warehouse mobile forklifts generally 
move along corridors in order to reach the shelves where goods are stored. Forklifts and 
shelves can be equipped with ultrasound transceivers that communicate with each other, 
with the purpose of evaluating mutual distances [Intel Corporation, 2005]. This type of 
wireless sensor network can be utilized to calculate the position of the forklifts for: 
x Indoor Navigation. Mobile forklifts, equipped with wireless transceiver, are automati-
cally guided towards their destination [Wang and Xi, 2006]; 
Introduction  5
x Traffic Monitoring. The physical traffic can be monitored in order to identify the most 
congested areas or to improve goods distribution [Capkun et al., 2001]. 
Large-scale dimensional measuring. Besides the MScMS, two innovative measuring sys-
tems for large scale dimensional measurements are the 3rd Tech Hi-Ball and Metris iGPS 
[Welch et al., 2001; Rooks, 2004; Metris, 2007]. These systems Ŧ all based on optical 
technologies and recently industrialised Ŧ are lightweight and very accurate, but they are 
relatively high priced and generally require a relatively large time for installation and 
start-up. Recently, the iGPS performance was studied and tested during a three months 
research activity carried out at the University of Bath (UK), attending the project LVMA 
(Large Volume Metrology Assembly Ŧ http://www.bath.ac.uk). A detailed description of 
this system and a comparison with MScMS is presented in Chap. 6. 
1.4 Organization of the dissertation 
The remainder of this dissertation contains a detailed description of the principle func-
tioning and the implementation of MScMS. Then, the system performance is evaluated 
and compared with two other existing systems for large-scale dimensional measurements: 
the CMMs and the iGPS. More specifically, the thesis is structured like this: 
x Chap. 2 presents the MScMS design features and modus operandi. In particular, the at-
tention is focused on the system principle functioning and the hardware/software archi-
tecture.
x Chap. 3 describes the first MScMS prototype, presenting a preliminary experimental 
evaluation of its metrological performance. Also, this  chapter identifies the system 
critical aspects and possible improvements. 
x Chap. 4 concentrates the attention on the main features of the US transceivers equip-
ping the system. They are deeply analysed by means of a structured experimental plan. 
x Chap. 5 provides a structured comparison between MScMS and the classical CMMs.  
x Chap. 6, discusses the iGPS technological features and principle, and provides a com-
parison with the MScMS. 
x Chap. 7 presents a short general analysis of the development of WSNs. This can be in-
teresting, considering that MScMS and other innovative measuring systems are based 
on distributed WSNs. 
6 Chapter 1 
x Finally, Chap. 8 summarizes the thesis contributions and mentions possible future di-
rections for improving the MScMS performance.
2. Principle functioning and MScMS architecture 
2.1 Introduction 
The purpose of this chapter is to describe the MScMS hardware/software/firmware archi-
tecture and functionalities.
Before introducing MScMS, in Section  2.2 we provide a structured description of re-
quirements and functionalities that a generic system for large-scale dimensional meas-
urements should meet. At the same time, we present a taxonomy of the most common 
techniques and metrological equipments for dimensional measuring. Major advantages 
and drawbacks are highlighted. The attention is subsequently focused on the MScMS de-
sign, analysing in detail the following aspects: hardware and software configuration, dis-
cussion of the location algorithms implemented by MScMS, description of the semi-
automatic procedure for the spatial location of the MScMS constellation devices. 
2.2 System requirements and comparison with other measuring 
techniques
MScMS has been designed to perform dimensional measurements of medium-large size 
objects – with dimensions up to 30÷60 meters. It should be easy to move and install, low-
priced and able to work indoor (inside warehouses, workshops, laboratories).  
Tab. 2.1 identifies the MScMS basic requirements.  
Considering them, we briefly analyse the most common measuring tools and tech-
niques. Tab. 2.2 shows the result of a qualitative comparison among five measuring in-
struments: theodolite/tacheometer, CMM, Laser Tracker, photogrammetry system, and 
GPS. The last row of the table takes account of MScMS target performances. 
Different considerations rise from Tab. 2.2. CMMs Ŧ in spite of being very accurate 
measuring instruments Ŧ are expensive, bulky and not easily movable. On the other hand, 
theodolites or GPS are smaller and lightweighter but not very flexible, in terms of differ-
ent types of measurements offered. Even more, GPS systems are less accurate, and cannot 
operate indoor. Interferometrical Laser Trackers and digital photogrammetry equipments 
8 Chapter 2 
are extremely accurate, but complex and expensive at the same time [Sandwith and Pred-
more, 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.  
Tab. 2.1 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 easy to perform.  
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 Vol-
ume
The area covered by the instrument, should be wide enough to perform 
measurements of large size objects (dimensions up to 30÷60 meters). 
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 (i.e. de-
termination of point coordinates, distances, curves, surfaces etc..). 
Tab. 2.2. Measuring systems comparison: qualitative performance evaluation 
Portability
Installation 
and Start-Up
Cost
Metrological 
Performances
Working 
Volume
Easy Use
Work 
Indoor 
Flexibility
THEODOLITE HIGH FAST LOW LOW LARGE MEDIUM YES LOW
CMM LOW SLOW HIGH HIGH SMALL HIGH YES HIGH
LASER TRACKER MEDIUM MEDIUM MEDIUM HIGH LARGE LOW YES MEDIUM
PHOTOGRAMMETRY MEDIUM SLOW MEDIUM MEDIUM MEDIUM LOW YES MEDIUM
GPS HIGH FAST MEDIUM LOW LARGE HIGH NO LOW
MScMS (Purpose) HIGH MEDIUM LOW MEDIUM LARGE HIGH YES HIGH
-
Key .
/
MEASURING 
SYSTEM
REQUIREMENTS
In conclusion, none of the previous measuring systems fulfil all previous requirements. 
MScMS is a system, based on the WSN technology, able to make a trade-off among these 
requirements.
2.3 MScMS hardware equipment 
MScMS is made up of three basic parts [Franceschini et al., 2008-II]: 
Principle functioning and MScMS architecture  9 
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 an 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 In-
stitute of Technology and Crossbow Technology. They utilize two US transceivers in or-
der to communicate and evaluate mutual distances [MIT C.S.A.I.L., 2004; Crossbow 
Technology, 2008]  
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, the current coordinates of the probe tip are calculated and sent 
to a PC via Bluetooth. Acquired data are 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. The spatial location of the constellation devices follows a semi-automatic proce-
dure, described in Subsection 2.4.4. Constellation devices are distributed without con-
straint around the object to measure. In the following subsections, we describe the 
MScMS hardware, focusing on: 
x the wireless (Crickets) devices (Subsection 2.3.1); 
x the measuring method to evaluate mutual distances among Crickets (Subsection 2.3.2); 
x the mobile probe (Subsection 2.3.3). 
2.3.1 Cricket devices 
Cricket devices are equipped with radiofrequency (RF) and ultrasound (US) transceivers. 
Working frequencies are respectively 433 MHz (on RF) and 40 kHz (on US). Cricket de-
vices are developed by Massachusetts Institute of Technology and manufactured by 
Crossbow Technology. Each device uses an Atmega 128L microcontroller operating at 
7.4 Mhz, with 8 kBytes of RAM, 128 kBytes of FLASH ROM (program memory), and 4 
kBytes of EEPROM (as mostly read-only memory). Alimentation is provided by two 
“AA” batteries of 1.5 V [Balakrishnan et al., 2003]. 
10 Chapter 2 
Cricket devices are quite small (see Fig. 2.1) easy to be moved, and cheap (each unit 
would cost about 10÷20 €, if mass-produced). Due to these characteristics, they are 
optimal for ad hoc WSN applications [Priyantha et al., 2000]. 
a
b
c
(b)
(c)
Integrated antenna for 
RF transceiving 
perspective view orthogonal projection 
| 9 cm
| 4 cm
(a)
Ultrasound Receiver  
photo 
|1.2 cm
Ultrasound Transmitter 
Fig. 2.1. Cricket Device (Crossbow Technology) 
The US transceivers equipping Crickets are quartz crystals, which transform electric 
energy in acoustic, and vice-versa (piezo-electric effect). They generate/receive 40 kHz 
ultrasound waves. Transmitters, excited by electric impulses, vibrate at the resonance fre-
quency producing acoustic ultrasound impulses [ANSI/IEEE Std. 176-1987, 1988]. On 
the other hand, receivers transform the vibration produced by ultrasonic waves in electric 
impulses. A detailed characterization of these transducers is presented in Chap. 4. 
2.3.2 Evaluation of distances between Cricket devices 
Crickets devices continuously communicate each other in order to evaluate mutual dis-
tances. Devices communication range is typically 6-8 meters, in absence of interposed 
obstacles.
The technique, implemented by each pair of Crickets to estimate mutual distance, is 
known as Time Difference of Arrival (TDoA). 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: 
a) At random time intervals, included between 150 and 350 milliseconds, each device 
transmits a RF query-packet to other devices within its communication range, checking