1. technology and computing

Proceedings of TMCE 2014, May 19-23, 2014, Budapest, Hungary, Edited by I. Horváth, Z. Rusák
 Organizing Committee of TMCE 2014, ISBN 978-94-6186-177-1
DEVELOPMENT OF A FRAMEWORK FOR INFORMATION ACQUISITION
AND PROCESSING IN CYBER-PHYSICAL SYSTEMS
Yongzhe Li
Faculty of Industrial Design Engineering
Delft University of Technology and
State Key Laboratory of Advanced Welding and Joining
Harbin Institute of Technology
Netherlands/China
y.li-8@tudelft.nl
Yu Song
Imre Horváth
Eliab Z. Opiyo
Faculty of Industrial Design Engineering
Delft University of Technology
The Netherlands
{ y.song, i.horvath, e.z.opiyo }@tudelft.nl
Guangjun Zhang
Jun Xiong
State Key Laboratory of Advanced Welding and Joining
Harbin Institute of Technology
China
zhanggj@hit.edu.cn, changfeng0007@163.com
ABSTRACT
In the designing and modeling of CPSs, the
information acquisition and processing processes are
often application dependent and process oriented.
Those information management frameworks are
simple and effective for small scale systems. However,
many functions developed are not reusable or cannot
be directly re-used, when a large number of details
and relations need to be added. Aiming at designing
a flexible and scalable system with “plug-and-play”
components, a preliminary information acquisition
and processing framework for CPSs is proposed in
this paper based on the object oriented design (OOD)
method. The concept of informational hierarchy
within CPSs is identified first. Then it is further
elaborated as instantaneous information, dynamic
information and context information. Using these
three types of information, together with the physical
properties of a component in CPSs, the concept of
hybrid object is proposed as the basic component of
the proposed framework. By defining the inherent
and update operation of hybrid objects, the proposed
information acquisition and processing framework is
formed with hierarchical hybrid objects. To verify the
effectiveness and the efficiency of the proposed
framework, a case study on designing and modeling
a gas metal arc welding (GMAW) based rapid
manufacturing system is presented. Limitations of the
proposed framework and future research directions
are discussed as well.
KEYWORDS
Information acquisition and processing framework,
object oriented design, hybrid object, plug-and-play,
cyber-physical systems,
1. INTRODUCTION
Cyber-physical systems (CPSs) are physical and
engineered systems whose operations are monitored,
coordinated, controlled and integrated based on
computing and communication technologies [1]. By
merging computing and communication with
physical processes, CPSs have been manifested from
the nano-world to large-scale wide-area systems [2].
In the past several years, CPSs attracted enough
attentions in many application domains by the
potential of offering more effective and efficient
solutions [3].
CPSs bridge the gap between the cyber world and the
physical world [4] using the cyber technologies and
physical technologies (Figure 1) [5], furthermore, in
many cases, cyber technologies and physical
technologies in CPSs are integrated and bonded
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Figure 1 A conceptualization of cyber-physical
systems [6]
together to form synergic technologies [6] [7]. Using
the cyber, physical and synergic technologies, a CPS
is able to make virtual and physical interactions with
the social-techno-economic environment. The
interactions can be sensed, monitored and controlled
via communicating a specific type(s) of signal, data,
information or knowledge.
Development of generalized hardware, software,
knowledgeware, and hybrid platforms is one of the
major research challenges in the context of CPSs [3]
[8]. A hybrid CPS platform may include a hardware
architecture and a software framework, which enable
execution of specific functions and managing
different types of signals, data, information and/or
knowledge. In the past several years, remarkable
research efforts have been devoted to study different
aspects of platforms. For instance, Song and Dyke
developed an experimental platform for dynamic
model updating [9]. Based on their platform, a CPS
is able to capture nonlinear response of the system
and update itself according to different scenarios.
Wan et al. designed a general and low-priced test
platform for unmanned vehicles using CPSs
technologies [10]. An interesting feature of their
platform that the wireless sensor network (WSN) and
the unmanned vehicles are treated as separated
components in the CPSs, and each unmanned vehicle
is equipped with a standard kernel. This treatment
reduces the complexity and improves the scalability
of the system.
To facilitate the simulation of CPSs, Genge et al.
developed an experimentation environment that can
concurrently reproduce physical and cyber events in
the process of security analysis of CPSs [11].
Recently, based on several case studies, Al-
1256
Hammouri developed a co-simulation platform for
CPSs [12]. In comparison with other applicationoriented architectures, platforms not only simplify
and accelerate the modeling and designing processes
of CPSs, but also offer the advantages in many
aspects, such as robustness, flexibility, safety,
security and scalability [13]. Information acquisition
and processing is a key issue for the development
and building a CPS platform. The convergence
between the cyber and the physical worlds poses new
challenges for handling signals, data, information and
knowledge in platforms for CPSs [14]. The
challenges appear in different aspects, for instance, in
terms of data interpretation [15], data storage [14],
and heterogeneous information fusion [16].
Recent decades, a number of information processing
and management frameworks have been developed.
The ParcTab [17] system was the earliest attempt on
general context-aware framework [18]. Dey et al.
identified five categories of components (context
widgets, interpreters, aggregators, services execute,
discoverers) that implement distinct functions in
composing a context framework. A Context Toolkit
was built to instantiate this conceptual framework
and supports rapid prototyping of context-aware
applications [19]. Mehrotra et al. proposed an event
oriented model for CPS, which includes the physical
layer, semantic abstraction layer and the high-level
steam language layer [20]. Kim et al further
addressed the problem of distributed sensing,
optimization, and control in the networked CPS by
developing an application framework based on
partially ordered knowledge sharing for loosely
coupled systems. The framework consists of cyberhosts, cyber-engines, cyber-nodes, and cyberapplications [21].
Korpipaa [22] proposed an information managing
framework for distributed context-aware computing
in an event-based manner. With a hierarchical
structure, the framework includes four main
functional entities: context manager, resource servers,
context recognition services and applications.
Similar frameworks can be found in SOCAM [18],
RCM [23] [24], etc. Based on those frameworks,
examples of application oriented implementations
can be found in medical care [25], traffic control [26],
robotics [27], building automation [28] systems, etc.
Current information management frameworks are
typically process oriented, and address procedures
such as information acquisition, processing,
organization, and control [25]. Most of these
Yongzhe Li, Yu Song, Imre Horváth, Eliab Z. Opiyo, Guangjun Zhang, Jun Xiong
frameworks are simple and effective in the case of
small-scale system. However, a large number of
details and relations should be considered in the
implementation according to the requirements of the
targeted application [28] [29] [30]. This restricts the
safety, flexibility [31], security [32], and especially
scalability of CPSs. In addition, the process oriented
management frameworks are often application
dependent. Many of the developed functions are not
reusable, or cannot directly be re-used. For instance,
in [19], an active badge location system consisted of
a number of infrared sensors distributed throughout a
building. If another sensor nodes consisting of a
thermal sensor would be inserted into the existing
sensing system, the information manage system
should be redesigned regarding to the new scenario.
Thus, application oriented information management
frameworks cannot be used as a basis for a generic
platform for CPSs.
The objective of the proposed research is to
conceptualize and build a prototype framework for
handling and managing signals, data, information and
knowledge in CPSs. Differing from existing
information management frameworks, the framework
should address issues of generality, flexibility and
scalability for various CPSs. The proposed
information management framework applies the
principles of object oriented design (OOD). The
physical constituents and the related information
constituents of a CPS are encapsulated as a hybrid
object. Structurally, a hybrid object comprises a
knowledge base and multiple interfaces. The
knowledge base is able to acquire, process, and store
information that is collected both in the cyber world
and in the physical world. The interfaces of a hybrid
object enable its interactions with other objects. In
addition, several hybrid objects are able to join
together through the interfaces to form a cluster of
hybrid objects, for instance, a WSN. Relying on a
hybrid object, the proposed framework is able to
provide “plug-and-play” function, which accelerates
the modeling and designing of CPSs and facilitates
achieving generality, flexibility and scalability of the
framework.
In the rest of this paper, an explanation on the
applied informational hierarchy is first given. Aiming
at a better understanding of the information
acquisition and processing process in CPSs, the
interrelationships between signals, data, information
and knowledge are analyzed. In Section 3, the
informational hierarchy is further elaborated.
Information is categorized as instantaneous
information, dynamic information, and context
information. Based on these three types of
information, and using the hybrid object concept, an
information acquisition and processing framework is
proposed in Section 4. In Section 5, information
processing technologies are reviewed with the goal to
establish relations among instantaneous, dynamic and
context information in different types of hybrid
objects. In order to verify the effectiveness of the
proposed framework, Section 6 provides a case study.
It includes modeling and designing of gas metal arc
welding (GMAW) rapid manufacturing system.
Finally, the feasibility and effectiveness of the
proposed framework is presented in the conclusion.
2. INFORMATION ACQUISITION AND
PROCESSING FRAMEWORK IN CPSs
The information processed in a CPS depends on its
various constituents and interactions among them
[13]. The constituents of a CPS are often referred to
as the cyber world and the physical world, as shown
in Figure 2(a) [14]. The physical environment has a
local part and a remote part. Three types of
components constitute a physical world, namely: (i)
human, (ii) machines, and (iii) environment. Some of
machines have connections with the cyber world, for
instance, with mobile devices or portable computers.
They can be treated as physical machines also acting
as cyber terminals. In the cyber world, databases and
algorithms are the main components that are
organized by the controller and operate on network
infrastructure. Compared to the local physical
environment, the cyber world is much broader due to
its effective communication range. This offers CPSs
the advantages of linking different local physical
environments together via communications in the
cyber world.
To characterize the status of components in CPSs and
the interactions among them, the data-informationknowledge-wisdom (DIKW) hierarchy has been
partly adopted to form the basis of the information
structure [33]. Here, the phase informational
hierarchy is used to describe the relations among a
phenomenon, the produced physical signals, the
descriptive data, the conveyed information, and the
implied knowledge within CPSs.
In CPSs, the interactions among components lead to
the changes of status of components. The status
changes of the components always correspond to
different types physical and/or cyber phenomena, e.g.,
to the network traffic [34]. The various phenomena
THE DEVELOPMENT OF A FRAMEWORK FOR INFORMATION ACQUSITION
1257
[36]. In CPSs, knowledge can be
obtained either by transferring it
from some original owners, or by
extracting
it
from
collected
information [37]. It can be
represented as relations among
different components in CPSs, the
intelligence in the algorithms, or the
intelligence in the controllers.
Figure 2 CPSs and its informational hierarchy
are captured by using sensors, and transferred as
electro- magnetic signals [35]. The signals are then
converted to digital forms, and represented as
digitized data after scaling.
In the proposed DIKW informational hierarchy, data
are defined as symbols that represent properties of
objects, events and their environment. Data are
products of observation and serve as the source of
inferring information which has meaning for both
humans and system components. Information is not
only contained in descriptions, but it is also provided
by answers to questions that begin with such words
as who, what, when and how many [36]. In CPSs,
information interpreted from data may contain
features, relations, etc. For instance, the data
expressing the value of the temperature of a given
environment is 295. In the context of interpreting the
temperature of the environment, 295 can be inferred
as 295 degrees Kelvin. However, depending on the
setup of the sensor, it can also be 22 degrees
Centigrade. These are equal characterization of the
information of the feature named temperature.
Data can be stored in the databases and semantically
processed as information by algorithms via the
controller software components of CPSs. However,
the algorithms for interpreting data can be
constructed based on knowledge. This knowledge is
actually know-how on the objectives, the context and
the way of achieving the objectives, and makes it
possible to transform information into instructions
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Based on collected information and
the possessed knowledge, CPSs can
make the desired interventions into
the physical world in an intelligent
manner. That is, instructions
captured in the knowledge bases are
transformed into signals, which are
needed to operate the actuators that
actually intervene with the physical
world, either in a human controlled
(manned) or autonomous (unmanned)
way. Figure 2(b) shows a rough
architecture of such a CPS and its
relations with the informational hierarchy. In the left
side of the figure, CPS components are categorized
according to their functions as sensors, actuators
controllers,
database and
algorithms. The
informational hierarchy is presented in the right side
of the figure. The concrete relations between the
various components of CPSs and the informational
hierarchy are indicated by solid lines. The possible
relations are represented by dashed lines.
The modeling and designing of the components and
the CPSs as a whole are based on the DIKW
informational hierarchy. Our literature survey
indicates that designing of the cyber world and
physical world components of CPSs are often done
independently, when conceptualization of the cyberphysical system has been completed [38]. That is, the
interactions within and between the cyber world and
physical world components are defined as cyber
information flows and the physical information flows,
respectively [13]. Due to the various geographical
locations, the physical information flows are usually
further decomposed into many local physical
information flows, as shown in Figure 3. These
function orientated designs are simple and effective
when the complexity of the CPS is low. However, for
large CPSs, limitations have been identified in the
following aspects:
Firstly, the phenomenon addressed by the DIKW
informational hierarchy typically pertains to the
Yongzhe Li, Yu Song, Imre Horváth, Eliab Z. Opiyo, Guangjun Zhang, Jun Xiong
knowledge
management
system of the CPS, and for
the implementation of “plugand-play” functions in terms
of
the
local
physical
environments.
3. A DEEPER
CONTEMPLATION OF
INFORMATIONAL
HIERACHY IN CPSs
Designing an information
acquisition and processing
framework that is capable to
address all challenges needs a
better
understanding
of
information hierarchy in
CPSs. The above-introduced
DIKW information hierarchy
framework
provides
an
opportunity for a clear
description of the relations
Figure 3 A typical information acquisition and processing framework of
among phenomena, signals,
existing CPSs
data,
information
and
physical world (though it may pertain to the cyber
knowledge within CPSs. It can serve as the basis of
world too). The analogue signal, which is converted
abstraction-based modeling and functional, structural
into digital data, creates a bridge (informational link)
and behavioral designing of CPSs. However, further
between the physical world and the cyber world. The
specialization of this informational hierarchy seems
data undergo semantic interpretations in contexts
to be necessary according to the properties of CPSs.
before they can be used in the cyber world. This
For instance, existing knowledge can be adopted to
explains why different parts of the entire
understand a phenomenon in the CPSs, but it can also
informational hierarchy belong to different
be built during the operation of CPSs. It is hard to
information flows. On the other hand, this lends itself
identify the best origin of specific bodies of
to an extra complexity in the information
knowledge for a CPS. On the other hand, the issue of
management of CPSs. For instance, if a physical
obtaining knowledge from various sources causes
component is removed from a system, then often
difficulties in the management of knowledge. In the
extra operations are needed to identify and dump the
proposed framework, time and context are used as
unused data that belonged to that component.
two extra dimensions for managing information. In
Secondly, the sensor, controller, actuator, database
the DIKW informational hierarchy, information is
and algorithm components belonging to the CPS are
categorized and treated as: (i) instantaneous
also separated. This implies that often specific
information, (ii) dynamic information, and (iii)
functions are needed to interconnect them. For
context information. They are graphically shown in
instance, this may be needed when one type of data
Figure 4.
have relations to many components in the system. In
Instantaneous information
these cases, complexity of the relation management
system may increase exponentially by the number of
Instantaneous information is a derivative of the
components.
signals collected by a sensor regarding a particular
aspect of a physical-phenomenon or a cyberThirdly, as part of the increase of the complexity of
phenomenon at a given moment [39]. As shown in
the CPS, larger number of local physical
Figure 4, for instance, signals collected by a sensor at
environments will be introduced and integrated into
time a can be scaled as data. With the context
the system. This also poses challenges for the
information of the system, the environment, and the
THE DEVELOPMENT OF A FRAMEWORK FOR INFORMATION ACQUSITION
1259
Figure 4 The instantaneous information, the dynamic information, and the context information of a
component
component (yellow blocks at the bottom in Figure 4),
the information contents of data are explored and
interpreted, or even further abstracted and aggregated
as knowledge (circles in the vertical plane at time a).
Instantaneous information also includes all basic
information that can be derived and extracted from
signals collected by an arrangement of independent
sensors [40]. Typical examples of instantaneous
multi-source information include temperature,
displacement, force, network traffic, etc. Some
sensors can also collect more than one type of signals
for instantaneous information. For example,
Poghossian et al. utilized an ion-sensitive field-effect
transistor (ISFET) in a hybrid sensor module for
detecting four physical signal values: (i) flow
velocity, (ii) flow direction, (iii) diffusion coefficient
of ions, and (iv) liquid level [41]. Another sensor
developed by Yang et al is also able to detect four
physical parameters, including: (i) plant diameter, (ii)
head diameter, (iii) plant weight, and (iv) head
weight from aerial photographs and field reflectance
spectra [42].
Dynamic information
In the real-world scenario of CPS applications,
complex
operation
domains
and
dynamic
environments are to be counted for [43~45].
Dynamic information represents the changes of a set
of instantaneous information in a given time span.
For instance, as shown in Figure 4, dynamic
information contains a set of pieces of instantaneous
information and their reflective or objective changes
at times a, b, c, d (see blue dashed line in Figure 4).
Capturing dynamic information is important in the
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case of many CPS applications, especially for the
intelligent control algorithms of CPSs. For instance,
Wu et al. analyzed model-based control of serial and
parallel robotic system and proposed a method to
identify dynamic parameters [46]. Our literature
research indicates that dynamic information also
plays an important role in system modeling of largescale CPSs applications with respect to their
dynamical uncertainties [47] [48]. Dubey and
Crowder proposed a dynamic tactile CPS,
implementation of which is capable to detect slip, as
well as to provide normal force effect [49].
Context information
Context information is related to: (i) the external
characterization of individual components, (ii) their
relationships with other components, and (iii) their
operating environments. There have been many
definitions of context information published in the
literature. In our research, we relied on the definition
given by Debes et al. concerning their CPS
application [50]. Regarding a particular component
of a CPS, the context information describes four
main aspects of existence:
 Identity
As context information, an explicit and unique
identifier is given to components;
 Location
Context information includes position data and
orientation data, as well as information about
regional relations to other components. In our
research, the abovementioned relations comprise
spatial relations and cyber relations as well. These
Yongzhe Li, Yu Song, Imre Horváth, Eliab Z. Opiyo, Guangjun Zhang, Jun Xiong
relations are continuously updated
instantaneous and dynamic information;
by
the
 Status
Context information also contains properties,
which can be perceived by a user. Status
information has a close link with instantaneous
information and dynamic information. For instance,
as status information, the temperature of a system
at a given moment can be updated by instantaneous
information and the average temperature of a
system in a given time span can be updated by
dynamic information;
 Time
As context information, time registers the moment
when a signal (information) is recorded or provided.
In case of many CPSs, it is assumed that the basic
context information can be defined based on the
knowledge adopted from third party (as indicated by
the yellow block in Figure 4). Part of the context
information, for instance, Identity, does not change
with respect to time. However, Location and Status
may change based on the derived instantaneous
information and dynamic information (as indicated
by the yellow vertical planes in Figure 4). As time
elapses, part of the context information will be
updated. The richness of the context information may
be increased (as indicated by the heights of the
vertical planes in Figure 4). In our research, richness
of context information is represented for the amount
of context information, neglecting the method of
measurement.
Using context information in CPSs enables the
context-awareness of individual components, thus
improves the “smartness” of the system as a whole.
For instance, Lee et al. proposed a medical
application system that is not only able to detect the
physiological parameters, but also offers the context
information related to the patient [51]. Using this
system, care-givers were brought into the control
loop around the patient and they can offer a better
service. They can analyze information in context and
use delivery devices to initiate treatment. Though it
seems to be promising, we have to encounter further
issues. For example, in the case of an automotive
application of a CPS, information should be provided
not only regarding the vehicle itself, but also
concerning the engine temperature or the fatigue of
the components [52]. However, the deployed CPSs
have limited ability to provide high-level
contextualized functions, such as safety assessment,
optimal route planning, or location-based services.
Relations within a component
As mentioned above, instantaneous information,
dynamic information, and context information are
closely related. Both instantaneous information and
dynamic
information
necessitates
context
information in order to achieve a proper (meaningful)
interpretation of data (as indicated by the solid
arrows in Figure 5). Dynamic information has been
defined as a set of instantaneous information related
to a particular phenomenon in a given time span. The
context may also be changing if we face a longer
time span. The changes will cause dynamic context
information (which is influenced by the
instantaneous information, and even more by the
dynamic information), as indicated by the dashed
arrows in Figure 5. We note that varying context
information can also have an influence on the
elicitation and interpretation of instantaneous and
dynamic information (as shown by the yellow
vertical planes in Figure 4).
4. INTRODUCING THE PROPOSED
INFORMATION ACQUISITION AND
PROCESSING FRAMEWORK
Below we provide information on the conceptualized
element and architecture of the proposed framework.
The objective of our research was to support the
development of heterogeneous and distributed CPSs,
which work according to complex application
scenarios, by a multi-resource platform. The
Figure 5 Relations among three types of
information
THE DEVELOPMENT OF A FRAMEWORK FOR INFORMATION ACQUSITION
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informational framework is supposed to serve it with
a purposeful information processing workflow by
which multiple elements, components and subsystems are co-working in synergy. Thus, the
framework should be able to take care of information
acquisition and processing, scalability and flexibility.
Technically, the most important is the “plug-and-play”
ability, that is, the ability by which a system can
automatically configure and operationalize new
components [53]. Towards this end, the concept of
hybrid object has been developed.
Hybrid objects
Hybrid object is a conceptual entity that compounds
both the cyber part and physical part in CPSs when
an object is used for modelling purpose. The concept
of hybrid object has been proposed based on the
understanding of the natural informational hierarchy
that underpins the information processing of the
different physical components and cyber components
of CPSs. It has been used as the basic constituent in
our research. Hybrid object is inspired by the concept
of object in object oriented design (OOD) [54]. In
OOD, an object contains encapsulated data and
procedures grouped together to represent an entity of
the system. In our research, the notion of ‘object’
from the cyber world has been extended to the
physical world and these two worlds are bridged
together by software techniques. Thus, from an
overall view, hybrid object has logical, functional,
structural and operational characters from both
internal view and external view
Internal view of a hybrid object
A typical hybrid object contains five major parts:
hardware entities, software entities, cyberware
entities, a database and an interface, as shown in
Figure 6. Hardware entities (HE) are the collection of
physical elements of the object that exist in the real
world. Cyberware entities (CE) are the virtual entities
that exist in the cyber world which reflect the
information of physical world as a knowledge
repository in CPSs. Software entities (SE) bridge the
cyberware part and hardware part of the hybrid
object. In addition, SE can decide on the desired
interactions and receive information from both the
hardware part and cyberware part, and handle the
information by a number of managing functions.
Figure 6 A hybrid object
In a hybrid object, the database can be seen as a
collection of data that can be stored, collected,
organized, shared, searched and utilized in both
software entities and cyberware entities [55] [56].
Interfaces are used to represent the relationships
between hardware, software and cyberware entities
of this hybrid object to other objects by sensing,
actuating and communication methods.
Types of hybrid objects in the framework
Two types of hybrid objects, namely: (i) the hybrid
system object, and (ii) the hybrid component object,
are used in the framework, as shown in Figure 7.
These two types of hybrid objects are generated
based on a basic template of the hybrid object, and
each of them has its unique attributes. Generally, the
hybrid component object represents a particular real
component when it is “plugged in” the system.
Hybrid system object is the basic functional unit to
handle the hybrid component objects through
interfaces within a local world (LW). LW means the
functional range (both sensing range and working
range) of the system object when a task is handled. It
has to be mentioned that a component object can also
be seen as a system object when its child components
need to be handed. And a system object can also be
described as a component object when the system as
a whole is managed by another system object of
upper level. This kind of property could increase the
flexibility of the system when modelling at different
levels.
Figure 7 Hybrid system object and hybrid
component object
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Yongzhe Li, Yu Song, Imre Horváth, Eliab Z. Opiyo, Guangjun Zhang, Jun Xiong
Architecture
two
types
objects
for
of
A
simple
architecture of a
CPS, which is
based on the two
types of hybrid
objects,
is
presented in Figure
9. There are two
system objects at
the
superlative
Figure 8 Information exchange in the system object and component object
level, named hybrid
system object A and B, included in this CPS. In case
Furthermore, the hybrid system object describes the
of the hybrid system object B, there is only one
basic context information, as shown in Figure. 8. It is
component object, named B1, which inherits from
formed when a local physical environment is plugged
the object B, as indicated by the arrow. In case of the
or designated in the CPSs. Actually, the context
system object A, two component objects, named A1
information is the only information that it has and the
and A2, are plugged in. As shown in Figure 9, hybrid
context information is formed when a system is
object A1 is used to simulate a WSN, which has three
established. Differently of the system object, the
nodes. This hybrid object can also be seen as another
component objects have their own instantaneous
system object and has three child component objects,
information and dynamic information.
named A1-1, A1-2 and A1-3, which correspond to
Two important operations, called inheritance
three respective nodes of the WSN. The black arrow
operation and update operation, have been defined to
in Figure 9 indicates the inherence operations and the
establish relations between the information
blue dashed arrows represent the update operations.
associated with the abovementioned two objects, as
External Interactions among hybrid objects
shown in Figure 8. The component object inherits
from the system object. The update function reflects
In the proposed framework, hybrid objects are
the information collected from the component object
connected to each other through their interfaces.
onto the system object. That is, changes in the
Figure 10 illustrates the interactions of hybrid system
inherited context information will also be reflected
object A with other objects in a CPS through the
by these objects. This way of operation guarantees
respective interfaces. In Figure 10, the hybrid system
that the system object always has the latest (“fresh”)
object A has a cyber-sensor, two physical sensors
context information.
(sensor 1 and 2), a cyber-actuator and two physical
actuators (actuator 1 and 2). For a typical cyber
interaction, for
instance,
the
hybrid object A
tries to acquire
information
from
hybrid
object G, as the
arrow indicates
in Figure 10,
object
A
acquires
information via
its cyber sensor,
and object G
delivers
the
Figure 9 A simple architecture of a CPS based on hybrid objects
THE DEVELOPMENT OF A FRAMEWORK FOR INFORMATION ACQUSITION
1263
For instance, Ye et al. developed a type of highly
sensitive, surface acoustic wave-based blood pressure
sensor relying on finite element simulation results,
[60]. Using Kalman filter, Yim et al. was able to
track the local position of the user based on the
strength and directions of WiFi signals [61].
Figure 10 Interactions among hybrid objects in a
CPS
information through its cyber actuator.
Object A is able to sense information within its
sensing range 1 by its physical sensor 1, i.e., it is able
to acquire a type of physical information regarding
objects B, C and D. The information of objects B, D
and E can be sensed within the sensing range 2 using
physical sensor 2. Here, an interesting phenomena is
that objects B and D are located in the overlapped
sensing ranges 1 and 2. That is, information
regarding objects B and D is acquired twice from
different perspectives corresponding to sensor 1 and
2, respectively. These may help object A to get a
better image of objects B and D, by combining the
information by information fusion. Details of
information fusion will be discussed in the next
Section. Concerning the physical actuators of object
A, two working ranges are defined with regards to
physical actuator 1 and 2, respectively. Each physical
actuator is able to perform certain actions on the
objects that are located in its working range.
5. INFORMATION PROCESSING WITHIN
THE FRAMEWORK
In a hybrid component object, instantaneous
information is acquired by processing raw data
collected by sensor(s). In the past decades, many
sensors had been developed, together with their
information processing algorithms, and adopted in
CPSs [57-59]. The known information processing
algorithms can perform a wide spectrum of tasks,
ranging from simple threshold detection to the
execution of rather complex mathematical algorithms.
1264
Dynamic information is constructed in the
framework based on instantaneous information.
Dynamic information provides information about the
changes of instantaneous information. This allows a
CPS to monitor, observe, record and respond to
phenomena according to complicated scenarios.
Based on the generated dynamic information and
using the advanced control algorithms, system
controllers are able to achieve a better performance
[62]. For instance, Vivas and Poignet applied
predictive control algorithms in a parallel robot
operation scenario. Using predictive control strategy,
together with a proportional-integral-derivative (PID)
controller as a low-level controller, the system
offered advantages compared with the two other
types of controller [63].
In real-life situations, context information represents
a broad range of information, including the
information about the location and the status of the
components. Using user context information can add
a lot to an aware operation of systems. For instance,
Göker and Myrhaug used mobile applications to
evaluate the effectiveness of context information
among tourists [64]. Kim et al. developed a webbased application to adapt to the user context.
According to their experiences, the system which
used context information offered more convenience
for the user, helped user save time, and reached a
higher satisfaction level, when it was evaluated in
comparison with other current systems [65].
Sensors are hardly used alone in CPSs. Due to
functional requirements and technical limitations, in
many applications, typically multiple sensors are
used in order to provide those pieces of information
that the system exactly needs. CPSs offer the
advantage of using multi-sensor data elicitation
across the system based on information fusion
technology. Information fusion is a technology that
enables combining data from sensors or sensor nodes
of WSNs [66]. There are many potentials and
advantages of using information fusion, such as
availability and accuracy, comparing to the outcome
of a single sensor [67-69]. Data collected from
sensors may be imperfect, correlated, inconsistent
and disparate [70] - information fusion algorithms
Yongzhe Li, Yu Song, Imre Horváth, Eliab Z. Opiyo, Guangjun Zhang, Jun Xiong
information fusion. The information fusion
algorithm, which is operated by the controller,
can improve the context information possessed
by object A by incorporating information
acquired from objects B and C related to the
phenomenon.
6. AN APPLICATION CASE STUDY
The main objective of this section is to provide
evidence on the practical relevance and utility
of the proposed informational framework.
Toward this goal we analyze an application
case. We have applied the proposed
framework in abstraction-based modeling and
designing a Gas Metal Arc Welding (GMAW)
system as a part of a cyber-physical system.
As reflected by the literatures, GMAW is
currently often used for rapid manufacturing
Figure 11 Information fusion in the proposed framework
(RM) of fully-functional 3D metal objects
directly from CAD models [77]. GMAW uses
can help solving these problems. They are capable to
the
welding
process for deposition of melted metal in
integrate data not only from the same type of sources,
a
target
object.
The scalability of current rapid
but also from different type of sources. Furthermore,
manufacturing
system
is not strong enough that an
they are also capable to enrich and improve the
existing system can only produce a category of parts
quality of the static, dynamic and context information
with similar physical parameters. Aiming at realizing
in CPSs [71]. Recently, sensor information fusion
the “plug-and-play” function for the welding system,
has received a large attention in a many application
the principles and technologies of CPSs are adopted
fields, including complex data processing [72],
in the development of the GMAW system. The
remote sensing [73], machine intelligence [74],
manifestation of the experimental GMAW system as
medical instruments [75] and machine vision [76].
a CPS is largely influenced by its application domain
In the proposed framework, information fusion is
and procedure [78].
also exploited. Figure 11 describes the applied
The major constituents of a GMAW system include:
procedure of improving the quality of context
(i) the welding power supply, (ii) a welding gun held
information by information fusion. The descriptive
by a motion platform, and (iii) a work piece. For a
data or the signals produced by a physical or cyber
better control of the GMAW system, two CCD
phenomenon are captured by the hybrid
object A through its interface. Within
the hybrid object A, dynamic and
context information is generated based
on information processing techniques
such as the ones indicated by the blue
arrows in Figure 11. We may also
assume that the phenomenon is also
captured by hybrid objects B and C
from different perspectives. To improve
the quality of context information
included in object A, information
related to the same phenomenon is
taken over from objects B and C
through the shared interfaces, as shown
by the green arrows in Figure 11. This
aggregated information is the subject of
Figure 12 Current experimental setup of the GMAW system
THE DEVELOPMENT OF A FRAMEWORK FOR INFORMATION ACQUSITION
1265
(charge-coupled devices) cameras are used to
monitor the welding bead in the welding process (see
Figure 12).
The architecture of the experimental GMAW system
is presented in Figure 13. This architecture resembles
the generic CPS system presented in Figure 2. In the
experimental system: (i) the height of the welding
bead, (ii) the width of the welding bead, and (iii) the
height of the welding gun are monitored by the
sensors. The signals produced by or on the physical
phenomena are captured, translated, interpreted and
saved in the database via the controller.
Currently, we are focusing on the introduction of
more robots and sensors to enlarge the working
envelope and improve the accuracy and efficiency of
the GMAW system in the further phases of the
research and development. Traditionally, the model
of a mechanical device-robot cooperation system for
RM is quite complicate to create and there are many
related challenges and many technical problems may
occur in the process of designing the new system. In
the current conceptualization phase of the double
agents GMAW system, the increased complexity of
control and information processing have been
identified as major technical issues. The main
reasons are as follows.
First, the multi-agent configuration makes the
management of the data and knowledge of the system
non-trivial. It implies a new abstraction in modeling.
In addition, additional parameters are needed for the
correct process and artifact representation. For
instance, the geometric shape of the work piece is
changing in the welding process. This change
necessitates the introduction of a global parameter.
Similarly, the other changes in the phenomena
require new parameters. Consequently, an extra layer
of parameters, named global parameters, has been
considered.
Secondly, with the increasing scale of the system, the
number of relations among components grows
exponentially. This poses challenges for function and
information management of CPSs. For example,
adding a temperature sensor may lead to the demand
that the function of “get temperature” need to be
added to every component of the system.
Thirdly, it is very hard to foresee and plan any
adaptation to the dynamic scenario of RM in
conceptualization of the CPS-augmented GMAW
system. Flexibility is one of the key features of an
RM system. Therefore, it is assumed that, in the case
of prototyping a small and simple work piece, the
system will be able to identify the optimal working
scenario and adapt to it, for instance, by using only
one motion agent. When a larger and more complex
work piece is fabricated, a robot will be switched on
and added to the system automatically. However, the
above mentioned switching on and off a robot actions
bring in the needs for an open-system type of
operation, but also supposes a series of additional
operations regarding every component in the system.
Using
the
proposed
information acquisition and
processing framework, the
system has been redesigned
as shown Figure 14. In the
new system design, there are
six hybrid objects: (i) a
welding system object, (ii)
the work piece, (iii) two
power supplies, (iv) a
motion platform, (v) a robot,
and (vi) sensors.
Figure 13 The design of the experimental system
1266
By the blue color used in the
graphical representation of
the interface, we wanted to
indicate that that object is
virtually connected. By the
blue-green gradient color in
the interfaces, we wanted to
indicate that the objects are
connected both virtually and
Yongzhe Li, Yu Song, Imre Horváth, Eliab Z. Opiyo, Guangjun Zhang, Jun Xiong
Figure 14 Abstract modeling of the two-agent GMAW system using the proposed framework
physically. The key information regarding each
hybrid object is listed adjacent to the database.
Significantly, the database of sensors records the
information of the work piece and the work piece is a
hybrid object which only contains a hardware entity.
Furthermore, sensors can be seen as another system
object and each sensor in it can be considered as a
hybrid component object. The design presented in
Figure 14 is more scalable and flexible, compared
with the previous design. For instance, a robot can be
easily switched off by removing the hybrid object
(indicated in yellow in Figure 14).
7. CONCLUSION
In this paper, a novel framework is proposed for
information acquisition and processing in CPSs. The
main findings of the presented research can be
summarized as follows:
1. It has proved to be advantageous to categorize and
process the kinds of information used in CPSs as
instantaneous, dynamic, and context information.
They represent interrelated pieces or bodies of
information;
2. Hybrid objects, which combines the physical and
cyber aspects of a component or subsystems of
CPS, can be used as the basis of abstraction-based
modeling and designing CPSs;
3. By using the incorporated information processing
and information fusion algorithms, the proposed
framework is suitable for heterogeneous and
distributed CPSs;
4. Using the proposed framework, a GMAW-based
RM system has been redesigned. The application
case was able to demonstrate the scalability and
the flexibility of the proposed framework.
Besides the above positive findings, some limitations
of the framework were also identified:
1. The proposed framework maybe too sophisticated
and/or complicated for a small scale CPS;
2. In the current implementation, the system level
and environmental level context information is
rather application dependent. More case studies
are needed to synthesize some sort of general
description schemes of the context information.
Our follow up research focuses on further
consolidation and empirical testing of the proposed
informational framework for a more complicated
implementation. This will be based on a WSN
subsystem and context sensitive reasoning. In
addition, our research will also consider the
development of an effective information fusion
algorithm, which may lead to a better performance
even in the case of CPSs with high complexity and
non-linear behavior.
ACKNOWLEDGMENTS
THE DEVELOPMENT OF A FRAMEWORK FOR INFORMATION ACQUSITION
1267
The presented research work is conducted at the
Faculty of Industrial Design Engineering of the Delft
University of Technology. The implementation of the
experimental GMAW system has been done in China
State Key Laboratory of Advanced Welding and
Joining. This part of the project has been supported
by the National Natural Science Foundation of China
under Grant No. 51175119. The work is also
sponsored by Chinese Scholarship Council (CSC).
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