Download Control Engineering Laboratory Process control across network

Control Engineering Laboratory
Process control across network
Vincenzo Mattina and Leena Yliniemi
Report A No28, October 2005
University of Oulu
Control Engineering Laboratory
Report A No 28, October 2005
Process control across network
Vincenzo Mattina* and Leena Yliniemi**
• *University of Palermo, Department of Chemical Engineering
**University of Oulu, Control Engineering Laboratory
Abstract: This research analyzes and examines process control across wireless network.
The main focus is to examine the applicability of a wireless network to the control of a
laboratory-scaled process and to compare results with the results achieved by two different
wired networks (EthernetLAN and Internet). Especially the delay times and packet losses
will be compared using wired and wireless networks.
The theoretical part reviews the terminology and basic technologies of communication
networks, emphasizing the principles of various network types. The new wireless
technologies and their main characteristics have been presented, with particular interest on
new application fields of these technologies. The compensation methods of a network delay
have been also presented.
In the experimental part, the components and software used have been described and a
control application suitable for controlling a real-time process via network has been
presented. The programs LabVIEW and Commview have been used in developing the
control application and in monitoring the traffic of UDP packets on the network. UDP is a
protocol used to transport the measurement and control data. The experimental environment
consists of a laboratory-scaled flow process. Two different test series have been done. In the
first test series the controller and the process are in the same place, in a classroom of the
university and in the second test series the controller is in the main library of Oulu City,
while the process is in the same place as in the first test series.
The results show that the process control via wireless network is good and feels little the
effects of distance between the process and controller. The packet losses and delay times are
minimal and almost the same both for the wired and wireless network. These experiments
show that wireless technologies are applicable for process control taking into the delay time
and packet loss.
Keywords: wireless control, network control, remote control
ISBN 951-42-7875-5
ISSN 1238-9390
University of Oulu
Control Engineering Laboratory
Box 4300, FIN-90014 University of Oulu
Contents
1
2
3
4
5
6
7
8
Introduction
Networking
2.1
Network types for control systems
2.1.1 Ethernet with CSMA/CD mechanism
2.1.2 Token-passing bus (ControlNet)
2.1.3 CAN bus
Protocol architecture
3.1
Protocols
WLAN
4.1
Physical layer
Bluetooth
5.1
Physical link definition
5.2
Supported protocols
Networked-based control system
6.1
Structure of networked-based control systems
6.2
Stability of network control systems
6.3
Problems in network control systems
6.3.1 Time delays
6.4
Networked control methodologies
6.4.1 Robust control methodology
6.4.2 Fuzzy logic modulation methodology
6.4.3 Novel auto-tuning robust PID controller
for networked control system
Wireless network
7.1
Comparison between wireless and wired systems
7.2
Topology of wireless
7.3
Types of wireless networks
7.4
Wireless network control system for chemical plants
Comparison of experimental results between wireless and
wired networks
Conclusions
References
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1
INTRODUCTION
A traditional centralized control system is no longer suitable to meet new requirements
such as modularity, distributed control, integrated diagnostics, quick and easy
maintenance and low cost. Network systems with common bus architectures, called
networked control systems (NCSs) provide several advantages such as small volume of
wiring and distributed processing. Connecting the control system components such as
sensors, controllers and actuators via a network it is possible to reduce the complexity of
the systems.
The applications connected through the network can be remotely controlled from a longdistance source. Especially with respect to manufacturing systems, NCS architectures that
utilize processing power at each node allow for the modularization of functionality and
standard interfaces for interchangeability and interoperability. Many different network
types exist for control systems. The most common are the Ethernet with CSMA/CD
mechanism, token-passing bus (ControlNet), and controller area network (CAN) bus
(DeviceNet).
1
2
NETWORKING
Network is typically defined as a collection of terminals, computers, servers, allowing an
easy data flow and usage of common resources. There are several ways of assorting
networks comprehending size, type of connection, media, protocol or use. The network
size can be presented with three typical magnitudes as Figure 1 presents
•
•
•
Local Area Network (LAN)
Metropolitan Area Network (MAN)
Wide Area Network (WAN).
LAN is generally characterized by three basic features:
•
•
•
Diameter of not more than few kilometers
Total rate at least several Mbps
Complete ownership by a single organization.
WAN typically covers whole countries. It has slower data rates and the infrastructure is
usually owned by multiple organizations. WANs normally consist of several LANs,
which have been connected together with the third party networks.
WLANs are the newest step in the networking technology. The purpose of WLAN
(Wireless Local Area Network) is to provide all features and benefits of traditional LAN
technologies such as Ethernet but wirelessly. WLAN works without limitations of cabling
using either infrared light (IR) or radio frequencies (RF) as a medium.
WAN(Wide Area Network)
MAN(Metropolitan Area Network)
LAN(Local Area Network)
Figure 1. Different networks with magnitudes.
2
2.1
Network types for control systems
A network is a collection of computing devices interconnected with cables, radio
transmissions or light (IR) which has the capacity to communicate via the connection
media. Thus, the devices on a network have the ability to connect and transmit data, if the
conditions of the media and transmission procedures are met. For control systems, the
common network types are Ethernet (CSMA/CD), token-passing bus
( ControlNet ) and CAN Bus ( Device Net ).
2.1.1
Ethernet with CSMA/CD mechanism /1, 2/
Ethernet uses the CSMA/CD mechanism for resolving contention on the communication
medium. When a node wants to transmit, it listens to the network. If the network is busy,
it waits until the network is idle, otherwise it transmits immediately. If two or more nodes
listen to the idle network and decide to transmit simultaneously, the messages of these
transmitting nodes collide and the messages are corrupted. While transmitting, a node
must also listen to detect a message collision. On detecting a collision between two or
more messages, a transmitting node stops transmitting and waits a random length of time
to retry its transmission. The data packet frame size is between 46 and 1500 bytes.
Ethernet uses a simple algorithm for operation of the network and has almost no delay at
low network loads. It is mainly used in data networks. Ethernet is a technology, which
does not support any message prioritization. At high network loads, message collisions
are the major problem because they greatly affect data throughput and time delay, which
may be unbounded.
2.1.2
Token-passing bus (ControlNet) /1/
The nodes in the token-passing bus network are arranged logically into a ring and each
node knows the address of its predecessor and its successor. During operation of the
network, the node with the token transmits data frames until either it runs out of data
frames to transmit or the time it has held the token reaches a limit. The node then
regenerates the token and transmits it to its logical successor on the network. Data frames
do not collide, as only one node can transmit at a time. The token- passing bus is
deterministic that provides excellent throughput and efficiency at high network loads.
Although the token bus is efficient and deterministic at high network loads, at low
channel traffic its performance cannot match that of contention protocols.
2.1.3
CAN bus (DeviceNet) /1/
CAN is a serial communication protocol developed mainly for applications in the
automotive industry and it is optimized for short messages and uses a CSMA/arbitration
on message priority medium access method. A node that wants to transmit a message
waits until the bus is free and then starts to send the identifier of its message bit by bit.
Conflict for access to the bus is resolved during transmission by an arbitration process at
the bit level of the arbitration field, which is the initial part of each frame. If two devices
want to send messages at the same time, they first continue to send message frames and
then listen to the network. If one of them receives a bit different from the one it sends out,
it loses the right to continue to send its message, and the other wins the arbitration. CAN
is a deterministic protocol optimized for short messages. Higher priority messages always
gain access to the medium during arbitration. The major disadvantage of CAN compared
with the other networks is the slow data rate and the throughput is limited. CAN is also
not suitable for transmission of messages of large data sizes.
3
3
PROTOCOL ARCHITECTURE /2, 3/
In the protocol architecture the modules are arranged in a vertical stack, where each
layer performs a related subset of functions required to communicate with another
system. Communication is achieved by having peer layers in two systems
communicate. The peer layers communicate by means of formatted blocks of data
that obey a set of rules or conventions known as protocol.
3.1
Protocols
The protocols on the network layer are the key components of a computer network.
The protocols are a set of standardized instructions, which the applications use to
communicate with each other over the network. Data communication protocols are
accurately standardized, documented and widely known. They are used to transport
data from an application to a remote application over the network. Two protocols are
special importance in the network control systems: TCP/IP and UDP/IP.
The communication tasks for TCP/IP (Transmission Control Protocol/Internet
Protocol) can be organized into five relatively independent layers:
•
•
•
•
•
application layer
transport layer
internet layer
network access layer
physical layer.
TCP provides a point-to-point channel for applications that require reliable
communication. It can be used to transport data with a wide variety of network
technologies and it functions on a Token Ring network as well as it does on an
Ethernet. TCP/IP can also be used to transmit data by using the serial
communication port and radio frequency links. It is the most supported protocol
worldwide.
UDP (User Datagram Protocol) is a protocol that sends independent packets of data,
called datagrams, from one computer to another with no guarantee about arrival.
UDP is not connection-based like TCP. The order of delivery is not important and is
not guaranteed, and each message is independent of any other. For many
applications, the guarantee of reliability is critical to the success of the transfer of
information from one end of the connection to the other. However, other forms of
communication do not require such strict standards.
The OSI reference model was developed by the International Organization for
Standardization (ISO) as a model for a computer protocol architecture and as a
framework for developing protocol standards. It consists of seven layers:
1. Physical layer: Provides electrical, functional, and procedural characteristics to
activate, maintain, and deactivate physical links that transparently send the bit
stream; only recognises individual bits, not characters or multicharacter frames.
2. Data link layer: Provides functional and procedural means to transfer data
between network entities and (possibly) correct transmission errors; provides for
4
activation, maintenance, and deactivation of data link connections, grouping of bits
into characters and message frames, character and frame synchronisation, error
control, media access control, and flow.
3. Network layer: Provides independence from data transfer technology and
relaying and routing considerations; masks peculiarities of data transfer medium
from higher layers and provides switching and routing functions to establish,
maintain, and terminate network layer connections and transfer data between users.
4. Transport layer: Provides transparent transfer of data between systems, relieving
upper layers from concern with providing reliable and cost effective data transfer;
provides end-to-end control and information interchange with quality of service
needed by the application program; first true end-to-end layer.
5. Session layer: Provides mechanisms for organising and structuring dialogues
between application processes; mechanisms allow for two-way simultaneous or twoway alternate operation, establishment of major and minor synchronization points,
and techniques for structuring data exchanges.
6. Presentation layer: Provides independence to application processes from
differences in data representation that is, in syntax; syntax selection and conversion
provided by allowing the user to select a "presentation context" with conversion
between alternative contexts.
7. Application layer: Concerned with the requirements of application. All
application processes use the service elements provided by the application layer. The
elements include library routines which perform interprocess communication,
provide common procedures for constructing application protocols and for accessing
the services provided by servers which reside on the network.
5
4
WLAN /4/
The wireless network is a system of data transmission that exploits the radio
frequency technology to the place of the connection with cables. It allows computer,
telephones, etc. to exactly be interconnected among them as through a net LAN or
traditional WAN. In this way the sharing of information and resources among distant
points geographically it becomes more flexible and fast guaranteeing the immediate
circulation of any data (audio/video, projects, documents, signals of trial, etc.).
In comparison to the transit of data in the internet the wireless system guarantees a
greater safety (the net is deprived) and a greater bandwidth to equivalent costs and
often even inferior. The wireless solution is to prefer in comparison to the solution
wired (copper or fibre optics) in how much it guarantees very competitive costs.
Beginning from 2002 the wireless network underline a notable development both for
the liberalization of the band 2,4 GHz and 5,4 GHz, that for the most greater
efficiency of the data transmission. Several standards are available for wireless
networking. They are referred to as the IEEE 802.11 standard set. Collision detection
is difficult to implement in wireless networks and a variant of CSMA/CD is used for
this purpose. This variant is called Carrier Sense Multiple Access with Collision
Avoidance(CSMA/CA).
4.1
Physical Layer
The physical layer specification includes two RF (radio frequencies) spread
spectrum technologies and one infrared technology ( IF ): DSSS, FHSS and IF.
DSSS (direct-sequence spread spectrum) operating in the 2.4 GHz ISM (Industrial
Scientific and Medical) band, at data rates of 1Mbps, 2 Mbps, 5.5 Mbps and 11
Mbps. That is the bandwidth reserved for general use by Industrial, Scientific and
Medical applications worldwide. It requires that each data bit is spread over a 22
MHz slice of the ISM band by mixing each data bit with a high rate pseudorandom
“chipping code”. After the mixing operation, the resulting data is carried by RF.
FHSS (frequency-hopping spread spectrum) operating in the 2.4 GHz ISM band, at
data rates of 1 Mbps and 2 Mbps. It uses a transmitter and receiver pre-defined
pseudorandom hopping pattern which must hop across 75.1 MHz frequency slots at
least once every 400ms. Each frequency slot must be utilized at least once every 30
seconds. The standard specifies that the entire data frame must be sent before it hops
to another frequency. The FHSS systems commonly work with 1 Mbps data rate.
IF operates at wavelengths between 850 and 950 nanometers and achieves data rates
of 1 Mbps and 2 Mbps. Of all sections, this solution is quite rare.
6
5
Bluetooth /5/
The Bluetooth technology eliminates the need for wires, cables, and the
corresponding connectors between mobile phones, modems, computers, and so on.
The main aim of Bluetooth is to be widely available, inexpensive, convenient, easy
to use, small and low power. One of the most characteristics of the Bluetooth
specification is that it should allow devices from a lot of different manufacturers to
work with each another. Bluetooth allows up to eight devices to connect together in a
group called a piconet. It uses packet-based transmission. The information stream is
fragmented into packets. All packets have the same format, starting with an access
code, followed by a packet header, and ending with the user payload. Bluetooth
devices operate at 2.4 GHz, in the globally available, license-free, ISM band. Since
this radio band is free to be used by any radio transmitter as long as it satisfies the
regulations, the intensity and the nature of interference can not be predicted.
Therefore, the interference immunity is very important issue for Bluetooth.
Interference immunity can be obtained by interference suppression or avoidance.
Suppression can be obtained by coding or direct-sequence spreading. Instead,
interference avoidance is more attractive since the desired signal is transmitted at
points in frequency and time where interference is low or absent. Avoidance in
frequency is more practical. Since the 2.4 GHz band provides about 80 MHz of
bandwidth and most radio systems are band-limited, with high probability a part of
the radio spectrum can be found where there is no dominant interference. Filtering in
the frequency domain provides the suppression of the interferers at other parts of the
radio band. To implement the multiple access scheme for the Bluetooth, FH-CDMA
(Frequency Hopping – Code Division Multiple Access) technique has been chosen.
5.1
Physical link definition /6/
There are two basic types of physical links:
•
•
Synchronous Connection Oriented ( SCO )
Asynchronous Connection-Less ( ACL )
A SCO link is a point-to point link and it provides a symmetric link between the
master and the slave as shown in Fig. 2, with regular periodic exchange of data in the
form of reserved slots. The master is the unit that controls FH channel, whereas the
slaves are all devices involved in the session have to adjust
their spreading sequences and clocks to the master´s. Thus, the SCO link provides a
circuit-switched connection where data are regularly exchanged and as such it is
intended for use with time-bounded information. A master can support up to three
SCO links to the same or to different slaves.
An ACL link is a point-to-multipoint link between the master and all the slaves on
the piconet as shown in Fig. 2. It can use all of remaining slots on the channel not
used for SCO links. The ACL link provides a packet-switched connection where data
are exchanged sporadically. The traffic over the ACL link is completely scheduled
by the master.
7
Figure 2. Point-to-point and point-to-multipoint piconets /5/.
5.2
Supported protocols /7/
Bluetooth is designed in such a way to allow many different protocols to be run on
top of it. Some of these protocols are:
•
The Wireless Access Protocol (WAP) – WAP provides a protocol stack
similar to the IP stack, but it is tailored for the needs of mobile devices. It
supports the limited display size and resolution typically found on mobile
devices by providing special formats for Web pages which suit their
capabilities. It also provides for the low bandwidth of mobile devices by
defining a method for WAP content to be compressed before it is
transmitted across a wireless link. WAP can use Bluetooth as a bearer
layer in the same way as it can use GSM, CDMA and other wireless
services. The WAP stack is joined to the Bluetooth stack using User
Datagram Protocol (UDP), Internet protocol (IP) and Point to Point
Protocol (PPP).
•
Object Exchange Protocol (OBEX) – OBEX is a protocol designed to
allow a variety of devices to exchange data simply and spontaneously.
Bluetooth has adopted this protocol from the Infrared Data Association
(IrDA) specifications. OBEX has a client/server architecture and allows a
client to push data to a server or pull data from the server.
•
The Telephony Control Protocol – Bluetooth´s Telephony Control
protocol Specification (TCS) defines how telephone calls should be sent
across a Bluetooth link. It gives guidelines for the signaling needed to set
up both point to point and point to multipoint calls. By use of TCS, calls
from an external network can be directed to other Bluetooth devices. TCS
is driven by a telephony application which provides the user interface,
and provides the source of voice or data transferred across the connection
set up by TCS.
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6
Networked-based control system
In many control applications as Figure 3 shows, networks are used as medium to
transfer data, such as control signals, alarm signals and sensor measurements for
different purposes. The two major types of control systems that utilize networks are
complex control systems and remote control systems. The first type is a large-scale
system containing several subsystems. A subsystem by itself can be thought of as a
control system with three system components as sensors, actuators and controllers.
In many cases, sensors and actuators are attached to a plant at different locations
from the controller. The complex system applications such as chemical processes,
automobiles, aircraft, and manufacturing plants have successfully used networks for
system control.
Figure 3. Applications of network-based control systems.
Remote control systems, instead, include remote data acquisition systems and remote
monitoring systems. To implement a remote control system, communication between
local and remote sites must be established. Many remote control systems have their
own specific communication connections. These connections are usually
conservative and are limited to only one single platform.
A more efficient way is to connect remote systems and controllers through a sharedmedium network. Using this scheme, several remote control systems can operate
with more utilization of the medium.
There are two general ways to design a control system via a network. The first way
is to have several subsystems, then form a hierarchical structure, in which each of
the subsystems contains sensors (S), actuators (A), and controllers (C) by itself, as
depicted in Figure 4.
9
A1
S1
A2
S2
C1
C2
Network
CM
Figure 4. Hierarchical structure of a control system with subystems.
These system components are attached to the same control plant. In this case, a
subsystem controller receives a set point from the main controller CM as shown in Figure
5.
Set point
A1
S1
CM
Network
C1
Measurement
Figure 5. Hierarchical structure of a control system.
The measurement data or status signal is then transmitted back via a network to the
central controller.
The structure shown in Figure 6 is denoted as the direct structure because sensors and
actuators of a control loop are connected directly to a network directly.
10
A1
Control
Network
C1
Measurement
S1
Figure 6. Direct structure of a control system.
According to Figure 6 a set of sensors and actuators are attached to a plant, while a
controller is separated from the plant via a network connection. The controller has to
perform the following tasks:
•
•
•
Read measurements via the network
Compute control signals
Send the control signals to the actuators through the network.
Both the hierarchical and direct structures have their own advantages. The
hierarchical structure is more modular. The control loop is simpler to reconfigure.
The direct structure has better interaction between system components because the
data are transmitted to components directly.
A controller in the direct structure can observe and process every measurement,
whereas a (central) controller in the hierarchical structure may have to wait until the
set point is satisfied before transferring the complete measurements, status signals, or
alarm signals. A control system in the direct structure is known as a network-based
control system.
A general problem of network-based control is the problem of network delays. The
major system components of a network-based control system are: controller, sensor,
and actuator. A shared-network acts as a medium to connect all these components
together. This network can also be shared with other control loops and network
resources. The message arrivals at a controller or an actuator are discrete events. As
such, they cannot be easily handled by a continuous-time model. Therefore, a
discrete-time model is more preferred.
6.1
Structure of network-based control systems
There are a number of possible arrangements for network control loops. A sensor,
controller, and actuator can either be a time-driven device or an event-driven device.
In a time-driven device, input reception or output transmission is controlled by a
sampling time, which could be represented by a clock signal. On the other hand, an
event-driven device starts to process immediately at the arrival of a device input.
11
6.2
Stability of network control systems
The stability of a network itself is defined by the number of messages in the queue of
each node. If this number is larger than a certain constant or tends to infinity as time
increases, the network is said to be unstable. On the other hand, the stability of a
networked control system should be defined by the performance of both the network
and the control system. That is, when the networks are unstable and network-induced
time delays degrade the control system sufficiently, the networked control system
can become unstable. It is possible to have a system with an unstable network but a
stable control system, and vice-versa. If sensors sample the data faster than the
network can transmit the data, then the network will be saturated and data will be
queued at the buffer. Although high sampling rates improve performance in
traditional control systems, they also induce high traffic loads on the network
medium. High traffic loads increase the message time delays and can degrade control
performance.
6.3
Problems in network control systems /8/
When designing a control loop over a NCS, an important point to consider is that the
behaviour of the distributed architecture largely depends on the characteristics and
performance parameters of the network, such as medium access protocol or available
communication bandwidth. Network providing real-time guarantees refers to the fact
that the exchanged data over the communication media must be sent and received
within a bounded time interval, otherwise a timing fault is said to occur. Therefore,
for control systems, candidate control networks must meet two main criteria:
bounded time delay and guaranteed transmission; that is, a message should be
transmitted successfully within a bounded time delay. Unsuccessfully transmitted or
large and unpredicted time-delayed messages may deteriorate the control system
performance, even causing a critical failure in the system. The main problems, in the
implementation of control loops over NCSs, are:
•
Time Delays : distributed computing architectures introduce various forms of
delays in control loops: communication and computational delays.
Communication delays occur when processors (such as sensors, controllers
and actuators ) exchange data through the shared communication media.
Computational delays appear because processors take time in processing the
data.
•
Data Consistency: network packet drops occasionally happen on an NCS,
when there are node failures or message collisions.
There are two main approaches for accommodating all of these problematic issues in
the design of control loops over NCSs. An approach is to design the control system
regardless of delays or packets drops, but ensuring that the communication
mechanisms will minimize the likelihood of these events. The other approach is to
treat the network characteristics as design specifications for control applications.
That is, to take into account, delays and packet drops in the controller design.
6.3.1
Time delays /9/
12
When a control loop is closed over a network, data transfers between the controller
and the remote system will induce network delays in addition to the controller
processing delay.
Figure 7. General NCS configuration and network delays for NCS formulation /9/.
Figure 7 shows network delays in the control loop, where r is the setpoint, u is the
control signal, y is the output signal, k is the time index, and T is the sampling
period.
t se
t cs
t ce
Figure 8. Timing diagram of network delay propagations /9/.
13
t rs
Figure 8 shows the corresponding timing diagram of network delay propagations.
Network delays in a NCS can be categorized from the direction of data transfers as
the sensor-to-controller delay τ sc and the controller-to-actuator delay τ ca . The
delays are computed as
τ sc = t cs − t se ,
(1)
τ
(2)
ca
= t rs − t ce ,
where t se is the time instant that the remote system encapsulates the measurement to
a frame or a packet to be sent, t cs is the time instant that the controller starts
processing the measurement in the delivered frame or packet, t ce is the time instant
that the main controller encapsulates the control signal to a packet to be sent, and t rs
is the time instant that the remote system starts processing the control signal. The
controller processing delay τ c and both network delays can be lumped together as
the control delay τ for ease of analysis. Although the controller processing delay τ c
always exists, this delay is usually small compared to the network delays, and could
be neglected. The delays τ sc and τ ca are composed of at least the following parts:
•
Waiting time delay τ w . The waiting time delay is the delay, of which a
source ( the main controller or the remote system ) has to wait for queuing
and network availability before actually sending a frame or a packet out
•
Frame time delay τ F . The frame time delay is the delay during the moment
that the source is placing a frame or a packet on the network.
•
Propagation delay τ P . The propagation delay is the delay for a frame or a
packet traveling through a physical media. The propagation delay depends
on the speed of signal transmission and the distance between the source and
destination.
These three delay parts are fundamental delays that occur on a local area network.
When the control or sensory data travel across networks, there can be additional
delays such as the queuing delay at a switch or a router, and the propagation delay
between network hops. The delays τ sc and τ ca also depend on other factors such as
maximal bandwidths from protocol specifications, and frame or packet sizes.
6.4
Networked control methodologies /9/
The methodologies to control a NCS have to maintain the stability of the system in
addition to controlling and maintaining the system performance as much as possible.
To write these methodologies based on several types of network behaviors in
conjunction with different ways to treat the delay problems, some assumptions may
be required, as:
•
•
Network transmission are error-free.
Every frame or packet always has the same constant length.
14
•
The difference between the sampling times of the controller and of the sensor
is constant.
The computational delay τ c is constant and is much smaller than the
sampling period T.
The network traffic cannot be overloaded.
Every dimension of the output measurement or the control signal can be
packed into one single frame or packet.
•
•
•
Some methodologies are described below.
6.4.1
Robust control methodology /10/
Goktas (2000) designed a networked controller in the frequency domain using robust
control theory. This methodology is denoted as the robust control methodology. A
major advantage of this methodology is that it does not require a priori information
about the probability distributions of network delays. In the robust control
methodology, the network delays τ ca and τ sc are modeled as simultaneous
multiplicative pertubation. The network delay formulation is described as follows:
1
2
1
2
− 1 ≤ δ ≤ 1,
τ n = (τ max + τ min ) + (τ max − τ min )δ ,
= (1 − α )τ max + ατ max δ ,
0≤α ≤
1
,
2
(3)
where τ n can be τ sc and τ ca ,τ max is the upper bound of τ n , τ min is the lower
bound of τ n , α and δ are real numbers to be determined based on an application.
The first term in the equation (3) represents a constant delay, whereas the second
term represents the uncertain delay varying from the first term. The delay in the
equation (3) is converted for use in the frequency domain, and approximated by the
first-order Padè approximation as
e −τ
n
s
1 − sτ n / 2
1 + sτ n / 2
1 − s (1 − α )τ max / 2
1 − sατ max δ / 2
≈(
)*(
).
1 + s (1 − α )τ max / 2
1 + sατ max δ / 2
= e − s (1−α )τ max e − sατ maxδ ≈
(4)
The uncertain delay part is treated as the simultaneous multiplicative perturbation
expressed as follows:
1 − sατ max δ / 2
(
) = 1 + Wm ( s ) ∆,
1 + sατ max δ / 2
(5)
where ∆ is the perturbation function and
ατ max s
Wm ( s ) =
.
1 + ατ max s / 3.465
is a mutiplicative uncertainty weight which covers the uncertain delay. The factor
3.465 is selected based on a designer’s preference. The control loop in the robust
15
control methodology using this formulation is depicted in Figure 9, where R(s), U(s),
Y(s), and E(s)= R(s) – Y(s) are the reference, control, output and error signals in the
frequency domain, respectively.
Figure 9. Configuration of NCS in the robust control methodology /10/.
6.4.2
Fuzzy logic modulation methodology /11,12/
Fuzzy logic control is used for the control of a plant, where the plant modeling is
difficult and conventional control methods have shown limited successes. From the
viewpoint of the controller designer, the plant of NCS includes not only the plant to
be controlled but also the network that connects the system components. This
network part of the plant is quite difficult to handle because the network system is
stochastic in nature and there is no differential equation to describe it. In fact, the
network is a discrete event system of which state transition occurs at discrete instants
of time. Due to this reason, fuzzy logic control becomes a very attractive method for
controller design for NCS.
Almutairi (2001) proposed the fuzzy logic modulation methodology for a NCS with
a linear plant and a modulated PI controller to compensate the network delay effect
based on fuzzy logic. The PI controller needs not to be redesigned, modified, or
interrupted for use on a network enviroment. The system configuration of the fuzzy
logic modulator methodology is depicted in Fig.10.
Figure10. Configuration of NCS in the fuzzy logic modulation methodology /11/.
16
The output of the PI controller is defined as u PI (t ), and the modified PI controller
output by the fuzzy logic modulation methodology is defined as u C (t ) .
Rule base
Figure 11. Structure or fuzzy logic controller /12/.
Figure11 shows the structure of a fuzzy logic controller, which consists of three
parts: fuzzifier that converts the error and the change of error into linguistic values,
inference engine that creates the fuzzy output using fuzzy control rules generated
from expert experience, and defuzzifier that calculates control input to the plant from
the inferred results. The fuzzy logic modulation methodology can be implemented in
a unit called the fuzzy logic modulator, which modifies the control u PI (t ) by
t
u C (t ) = β u PI (t ) = β K I ∫ e(ξ )d (ξ ).
(6)
t0
The multiplicative factor β is used to externally adjust the controller gains at the
output without interrupting the original PI controller. The value of β is selected
from two fuzzy rules based on the network delay effects as follows:
If e(t) is SMALL, then β = β 1,
If e(t) is LARGE, then β = β 2,
where 0 < β 1 < β 2 < 1. The membership functions of e(t) are depicted in Fig. 12
where µ SMALL and µ LARGE are the membership functions representing the degrees of
memberships for the linguistic variable SMALL and LARGE, respectively; α 1 and
α 2 are factors to adjust the shapes of the membership functions.
17
Figure 12. Membership functions of e(t) /11/.
6.4.3
Novel auto-tuning robust PID controller for networked control system
/13/
S. Li et al. proposed the methodology to design a novel dual PID controller for NCS
after making some reasonable simplification of the complex structure of NCS. One
of advantages of the novel PID controller is that it improves dynamic performance
and disturbance rejection capability. To effect the simplification of NCS, we need
some definition and proposition:
Definition 1. Delay τ sc - Communication delay between the sensor and the
controller.
Definition 2. Delay τ ca - Communication delay between the controller and the
actuator.
Proposition 1. The time-varying delay τ sc and τ ca in the Fig.13 can be lumped
together as a single delay τ = τ (τ sc ,τ ca ) in the as Fig.14 shows. It is equivalently
provided that
A1. The sensor and controller have identical sampling periods
A2. No message rejection at the sensor and controller terminals
A3. τ (t ) ≥ 0, ∀t ;
A4. τ (kT + τ k ) = τ k ;
A5. τ (kT + δ ) ≤ δ , ∀δ ∈ [τ k , T + τ k +1 ).
where τ k = τ ksc + τ kca .
18
Actuator
u(t)
y(t)
Plant
τ ca
Sensor
τ sc
Network
uk
Controller
R
Figure13. A generic diagram for an NCS /13/.
R +
Controller
Z.O.H.
y(t) -
Lumped delay
τ = τ (τ sc ,τ ca )
Plant
T0
Figure 14. Reasonable simplification diagram for an NC /13/.
According to proposition 1, NCS can be simplified as Figure 14 shows. From the
process o f designing controllers we know that PID controller can be easily designed
if the model of the system is known. The authors search for parameters with the
method of relay auto-tuning. Its basic idea is to set up two modes in the control
system: tuning mode and modulation mode. In the tuning mode, the nonlinear relay
part tests the oscillation frequency and the gain of the plant. In the modulation mode
PID controller is first obtained according to feature parameters of the plant, then the
controller modulates its dynamic performance. If the parameters of the plant are
changed, the mode needs to be switched to the tuning one, which again returns to the
modulation one after tuning is finished. The switching of the modes is realized
through a digital switch as shown in Figure 15.
Figure 15. A schematic diagram for relay auto-tuning PID control system /13/.
19
Figure 16. The output wave of system and relay /13/.
e −τs
. The pure time delay τ can
Ts + 1
be directly obtained by oscillation as Figure 16. The computing equation of the gain
k is
Let the transfer function of the plant be G(s)= k
Ra = k (h0 − h + 2h∆T1 / Tc ) ,
(7)
where Ra is the mean value of oscillation wave. After approximating the system
output illustrated as Fig. 16 as the step response of one-order inertia, the computing
equation of time constant T can be obtained:
T=
τ
kh − ε
ln
kh − a
.
(8)
As τ → 0,
T=
Tc
,
[ R − ε − k (h0 + h)][ R + ε − k (h0 − h)]
ln
[ R − ε − k (h0 − h)][ R + ε − k (h0 + h)]
(9)
where relay characteristic amplitude h and hysteresis loop width ε need to be set
before the system begins to oscillate. Fig. 17 presents the system structure of dual
PID controllers, where ϑ (s) is a deterministic network disturbance. Ge −τ m s and
Ge −τs are transfer functions of the plant model and real-life plant respectively.
20
ϑ ( s)
+
-
+
+
Gh ( s)
Dl 1
G ( s )e −τs
-
+
Gm e −τ m s
T
Gh (s )
T0
Dl 2
Figure 17. The system structure with dual PID controller /13/.
Whenever the mismatch of model occurs, the digital switch is on the tuning mode.
Whenever the mismatch of model disappears, the digital switch is on the modulation
mode. In this case the controller Dl 2 of loop l 2 act on ϑ ( s ) independently and is
irrelevant with controller Dl1 . The robust PID design method needs other definitions
and propositions, which are:
Definition 3. z Definition: let z be equivalent to the complex factor z −1 in the ztransform.
Proposition
2.
Given
real
coefficient
polynomials
as
f = bz d + Ma(1 − z ), b = b0 + b1 z + ... + bm z m , a = a 0 + a1 z + ... + a n z n
The general forms of digital PID controller given are
Dl 1 =
k p (c 0 + c1 z + c 2 z 2 ) + k1
k
k
and Dl 2 = 1
=
1− z
1− z
1− z
(10)
According to Figure 17, the transfer function of a closed loop system is
y(z)= y r ( z ) r + yϑ ( z )ϑ ,
(11)
where
yr ( z) =
Dl1 (GG h ) z
d
1 + Dl1 (GG h ) z d
, yϑ ( z ) =
1
Gz d
1 + Dl 2 (GGh ) z d
1 + Dl1 (GGh ) z d
.
(12)
The dynamic performance and disturbance rejection ability of the system can be
easily changed through the change of parameter ξ . The controller parameter is
chosen as:
21
2
⎧
⎫
k p = ν sgn ⎨a (1)b(1)∑ ci ⎬,ν ∈ (0,ν 0 ]
i =0
⎩
⎭
where ν = ξ
a (1)
2
b(1)∑ ci (1 + ξ )
(13)
, d ≤ d0 = [
d +1
τ max
T0
]
i =0
Dl1 designs under different control plants are given in Table 1.
Table 1. Dl1 design under different control plants /13/.
22
(14)
7 Wireless network
The term wireless refers to telecommunication technology, in which radio waves,
infrared waves and microwaves, instead of cables or wires, are used to carry a signal
to connect communication devices. Many fields today such as industrial plants,
healthcare and emergency care require mobility of works. Wireless networking
makes it possible to place portable computers in their hands. It is very useful when
employees must process information on the spot, directly in front of customers and
patients, or share real-time information. There are a wide-range of wireless devices
that implement radio frequency (RF) to carry the communication signal. Some
wireless systems operate at infrared frequencies, whose electromagnetic wavelengths
are shorter than those of RF fields. Wireless systems can be devided into fixed,
portable, and IR wireless systems.
A fixed wireless system uses radio frequency requiring a line of sight for connection.
Unlike cellular and other mobile wireless systems, they use fixed antennas with
narrowly focused beams.
A portable wireless system is a device or system, usually battery-powered, that is
used outside the office or vehicle.
An IR wireless system uses infrared radiation to send signals within a limited-range
of communication.
7.1 Comparison between wireless and wired systems /14/
•
Simplicity and Flexibility: Broadband wireless links can be deployed faster
than wire line links. As well, broadband wireless network can be quickly,
easily, and inexpensively modified to meet changing connectivity needs. A
broadband wireless network gives flexibility to painlessly add or eliminate
locations, or secure additional bandwidth, users can access date while on the
move. Promotes new ways of working.
•
Speed and reach: A wireless building-to-building link provides throughput
rates several times faster than those offered by wire line alternatives. And
unlike wire line high-speed circuits that are typically available only in urban
centers, broadband wireless links can be deployed virtually anywhere, urban,
suburban, and rural locations alike.
•
Reliability and security: Wireless suffers a few more reliability problems
than wire. Wireless signals are subject to interference, with careful
installation, the likelihood of interference can be minimized. In theory,
wireless are less secure than wires, because wireless communication signals
travel through the air operating in the 2.4 GHz frequency range, potential for
interference.
•
Economy and Pricing: Connecting LANs together using wireless links is
significantly less expensive than upgrading existing wires or laying new
ones. As well, wireless infrastructure is less prone to required maintenance
than wire line infrastructure.
•
23
7.2
Topology of wireless /14/
Two topologies of wireless networks are possible. They differ on how wireless
devices communicate to each other. Wireless network operates either in ad-hoc or
infrastructure mode. The mode of infrastructure requires the use of a Basic Service
Set (BSS) which is a wireless access point (WAP). The access point is required to
allow for wireless computers to connect not only to each other but also to a wired
network as shown in Figure18.
Figure 18. Infrastructure of wireless networking.
Ad-hoc is also known as peer-to-peer wireless networking as shown in Figure 19.
There are a number of wireless computers that need to transmit files to each other.
This mode of operation is known as Independent Basic Service Set (IBSS).
Figure 19. Ad-hoc wireless networking.
24
7.3
Types of wireless networks /15/
Major types of wireless networks include:
CDPD ( Cellular Digital Packet Data ): CDPD is a specification for supporting
wireless access to the Internet and other public packet-switched networks over
cellular telephone networks. it supports TCP/IP and Connectionless Network
Protocol, the max bandwidth is 19.2 Kb.
HSCSD ( High Speed Circuit Switched Data ): HSCSD is a specification for data
transfer over GSM ( Global System for Mobile communication ) networks. HSCSD
utilizes up to four 9.6 Kb or 14.4 Kb time for a total bandwidth of 38.4 Kb or 57.6
Kb.
PDC.P ( Packet Data Cellular ): PDC.P is a packet switching message system.
PDC.P utilizes up to three 9.6 Kb TDMA channels, for a total maximum bandwidth
of 28.8 Kb.
GPRS ( General Packet Radio Service ): GPRS ia a specification for data transfer on
TDMA and GSM networks. GPRS utilizes up to eight 9.05 Kb or 13.4 Kb TDMA
timeslots, for a total bandwidth of 72.4 Kb or 107.2 Kb.
Bluetooth: Bluetooth provides three types of power classes, although class 3 devices
are not in general availability.
Type
Class 3 devices
Class 2 devices
Class 1 devices
Power level
100 mW
10 mW
1 mW
Operating range
Up to 100 meters
Up to 10 meters
0.1 – 10 meters
Table 2. Bluetooth power classes /15/.
IrDA: IrDA defines a standard for an interoperable universal two way cordless
infrared light transmission data port. IrDA is utilized for high speed short range, line
of sight, digital cameras, handheld data collection devices. The max bandwidth is 16
Mb.
MMDS ( Multichannel Multipoint Distribution Service ): MMDS is a broadband
wireless point-to-multipoint specification utilizing UHF ( Ultra High Frequency )
communications. The max bandwidth is 10 Mb.
802.11: 802.11 ( Wi-Fi ) is a suit of specifications for wireless Ethernet. The 802.11
standards are defined by the IEEE.
25
Standard
Speed
Frequency
Modulation
802.11
2 Mb
2.4 GHz
Phase-Shift
Keying
802.11a
54 Mb
5 GHz
802.11b
11 Mb
2.4 GHz
Orthogonal
Frequency
Division
Multiplexing
Complementary
Code Keying
802.11g
54 Mb
2.4 GHz
802.11n
100 Mb
20-40 MHz
Orthogonal
Frequency
Division
Multiplexing
Orthogonal
Frequency
Division
MultiplexingMultiple input,
multiple
output
many
antennae
technique
Table 3. IEEE 802.11 standards /15/.
7.4
Wireless network control system for chemical plants
The manufacture and processing of chemicals is a highly capital and resourceintensive, as well as potentially dangerous business. There is very little room for
product differentiation or innovation: production methods are well established and
widely known. The way to make a profit is to maximise efficiency and continuity,
while paying close attention to safety, and professional on-site wireless
communications can make a valuable contribution in all of these areas. The chemical
industry is a classic commodity business, in the areas like basic organic and
inorganic compounds, fertilisers, gasoline and plastics. The supply chain is typically
long and complex, and involves many transactions,with extremely tight margins. The
chemical industry is also very transparent. Most of the technology and processes
have been around for 50 years or more old, and there are virtually no commercial
secret and very few patents. While chemical manufacturing has one of the best safety
records of any of the process industries, and industry in general, in terms of
accidents per man-hour, fire explosions and even sabotage are always present.
Identifying and acting on problems early on is key to preventing small faults
snowballing into major incidents.
Wireless communications supports fast, accurate processes for dealing with faults
and emergency situations. The wireless communications solution can be integrated
26
with plant monitoring and alarm systems for example, so that critical alarms
immediately trigger an alert to the most appropriate person regardless of where they
are on the plant. Plants are also typically very large sites, with relatively few people
working on them. Together, these factors mean reliable on-site communications is
vital in ensuring day-to-day operations run smoothly. In a typical large processing
plant, wireless communication has an essential role in ensuring that plant engineers
and operators are able to communicate with each other and with the control room,
wherever they happen to be. With well-integrated wireless communications, plant
supervisors do not need to stay in the control room in order to be kept fully up-todate on operations. Wireless communications solutions can be integrated with plant
monitoring and control systems to pull together information from a variety of
sources and present it on screen to plant managers and engineers wherever they are
working. On-site wireless communications can make a significant contribution,
enabling quicker response to changing conditions, enhanced efficiency and
continuity, and improved health and safety.
27
8 Comparison of experimental results between wireless
and wired networks
For examining the performance of the remote process control across the wireless and
wired networks two test series with the laboratory process presented in the reference
/16/ have been made. In the first test series the process and the controller are in the
same room at the university and in the second test series the controller is in main
library of Oulu City and the process at the university. In this case the distance
between the controller and the process is about seven kilometres. Delay times and
loss packets are compared in different test series.
The behaviour of the delay time, where both the controller and the process locate in
the university is presented in Figure 20. The network is wireless. In Figure 21 there
are the same conditions, but it is used the wired network.
Time [s]
Delays Time [1Hz]
0,05
0,048
0,046
0,044
0,042
0,04
0,038
0,036
0,034
0,032
0,03
0,028
0,026
0,024
0,022
0,02
0,018
0,016
0,014
0,012
0,01
0,008
0,006
0,004
0,002
0
1
26 51 76 101 126 151 176 201 226 251 276 301 326 351 376 401 426 451 476 501 526 551 576 601 626 651 676 701 726 751 776 801 826
Packets number
Figure 20. Delay time in the first test series with the sample frequency 1Hz via the
wireless network
28
Delays Time [1Hz]
0,02000
Time [s]
0,01500
0,01000
0,00500
0,00000
1
14 27 40 53 66 79 92 105 118 131 144 157 170 183 196 209 222 235 248 261 274 287 300 313 326 339 352 365 378 391
Packets number
Figure 21. Delay time in the first test series with sample frequency 1Hz via the
wired network /16/.
Figures 20 and 21 show that the delay time is in the wireless network three times
more than that in the wired network. The average time in the wireless network is 3
ms and in the wired network 1 ms. In the wired network the delay time of the
packets over 20 ms is 1,8 % of the all 391 sampled packets, while in the wireless
network the corresponding number is 5,2 % of the all 826 sampled packets. Also in
this case the delay times are more distributed.
29
Delays time [s]
Delays Time [2Hz]
0,05
0,048
0,046
0,044
0,042
0,04
0,038
0,036
0,034
0,032
0,03
0,028
0,026
0,024
0,022
0,02
0,018
0,016
0,014
0,012
0,01
0,008
0,006
0,004
0,002
0
1
42
83
124 165 206 247 288 329 370 411 452 493 534 575 616 657 698 739 780 821 862 903 944 985 1026 1067
Packets number
Figure 22. Delay time in the first test series with sample frequency 2Hz via the
wireless network
Delay time [2Hz]
0,01200
0,01000
0,00800
Time [s]
0,00600
0,00400
0,00200
0,00000
1
48
95
142
189
236
283
330
377
424
471
518
565
612
659
706
753
800
847
894
941
988 1035 1082 1129 1176
Packet number
Figure 23. Delay time in the first test series t with sample frequency 2Hz via the
wired network /16/.
Figures 22 and 23 show the delay time with the sample frequency 2 Hz, where the
controller and process are in the same room is quite similar as with sample frequency
1 Hz.
30
Delays Time [s]
Delays Time [5Hz]
0,05
0,048
0,046
0,044
0,042
0,04
0,038
0,036
0,034
0,032
0,03
0,028
0,026
0,024
0,022
0,02
0,018
0,016
0,014
0,012
0,01
0,008
0,006
0,004
0,002
0
1
122 243 364 485 606 727 848 969 1090 1211 1332 1453 1574 1695 1816 1937 2058 2179 2300 2421 2542 2663 2784 2905 3026 3147
Packets number
Figure 24. Delay times in the first test series with sample frequency 5Hz via the
wireless network.
Delays Time [5Hz]
Time [s]
0,06000
0,04000
0,02000
0,00000
1
68
135 202 269 336 403 470 537 604 671 738 805 872 939 1006 1073 1140 1207 1274 1341 1408 1475 1542 1609 1676
Packets number
Figure 25. Delay time in the first test series with sample frequency 5Hz via the
wired network /16/.
31
Also in the case of the sample frequency 5 Hz the average delay times are the same
as in the experiments with the sample frequencies 1 and 2 Hz i.e. 1 ms in the wired
network and 3 ms in the wireless network. In the wired network it can be found that
the initial time is unstable in the range of the sampled packets between 68 and 135,
but after this range it is very constant.
Figures 26 and 27 present the delay times in the wired and wireless networks, when
the controller locates about from seven kilometres from the process.
Delay time [1Hz]
0,05
0,048
0,046
0,044
0,042
0,04
0,038
0,036
0,034
0,032
0,03
Time0,028
[s]
0,026
0,024
0,022
0,02
0,018
0,016
0,014
0,012
0,01
0,008
0,006
0,004
0,002
0
1
22
43
64
85 106 127 148 169 190 211 232 253 274 295 316 337 358 379 400 421 442 463 484 505 526 547 568 589 610 631 652 673
Packet number
Figure 26. Delay time in the second test series t where the controller and process
locate seven kilometres from each other. The sample frequency is 1Hz via the
wireless network
32
Delay tTime [1Hz]
0,350
0,300
Time [s]
0,250
0,200
0,150
19
0,100
25
31
37
43
49
55
61
67
73
79
85
91
97 103 109 115 121 127 133 139 145 151 157 163 169 175 181
Packet number
Figure 27. Delay time in the second experiment where the controller and process
locate seven kilometres from each other. The sample frequency is 1Hz via the wired
network /16/.
The above figures show that the delay time is longer in the wired network than in the
wireless network, even the difference is minimal. The average delay time is 30 ms
for the wired network and 25 ms for the wireless network. In both cases they are
constant in the whole range of the sampled packets.
Delays Time [2Hz]
0,08
0,076
0,072
0,068
0,064
0,06
0,056
0,052
Time [s]
0,048
0,044
0,04
0,036
0,032
0,028
0,024
0,02
0,016
0,012
0,008
0,004
0
1
58
115 172 229 286 343 400 457 514 571 628 685 742 799 856 913 970 1027 1084 1141 1198 1255 1312 1369 1426 1483
Packets number
Figure 28. Delay time in the second test series with the sample frequency 2Hz via
the wireless network.
33
Delay time [2Hz]
0,500
0,400
0,300
Time [s]
0,200
0,100
0,000
1
43
85
127
169
211
253
295
337
379
421
463
505
547
589
631
673
715
757
799
841
883
925
967 1009 1051
Packetnumber
Figure 29. Delay time in the second test series with the sample frequency 2Hz via
the wired network /16/.
Figures 28 and 29 show that with the sample frequency 2 Hz the difference between
the values of delay times in these two networks is not big, but in the wired network it
is unstable compared with the wired network. In the wireless network the average
time is 25 ms in the whole range of sampled packets and in the wired network the
average delay time is 30 ms, but it is unstable. The number of packets with the delay
time over 0,1 s is 32,3 % of the total sampled packets.
34
Delay time [5Hz]
0,05
0,048
0,046
0,044
0,042
0,04
0,038
0,036
0,034
0,032
0,03
Time0,028
[s]
0,026
0,024
0,022
0,02
0,018
0,016
0,014
0,012
0,01
0,008
0,006
0,004
0,002
0
1
170
339
508
677
846 1015 1184 1353 1522 1691 1860 2029 2198 2367 2536 2705 2874 3043 3212 3381 3550 3719 3888 4057 4226
Packet number
Figure 30. Delay time in the second experiment with the sample frequency 5Hz via
the wireless network.
Delay time [5Hz]
0,120
0,100
0,080
Time [s]
0,060
0,040
0,020
0,000
1
46
91
136 181
226 271 316 361
406 451 496 541 586
631 676 721 766
811 856 901 946
991 1036 1081 1126 1171
Packet number
Figure 31. Delay time in the second experiment with the sample frequency 5Hz via
the wired network /16/.
35
Figures 30 and 31 show the results are quite similar compared with the experiments
with sample frequency 1Hz. The average delay times for wired and wireless
networks are respectively 30 and 25 ms, and they are constant in the whole range of
sampled packets.
The experiments show that the delay times in the wired and wireless networks have
some differences. When the controller and process are close to each other, the delay
time in the wired network are smaller than in the wireless network, even if this
difference is very small. Instead, when the controller and process are far, the wireless
network is faster than the wired network. Also delay time in the wired network is
more unstable than in the wireless network.
The data given in Tables 3 and 4 presents the packet loss in the first test series via
the wireless and wired networks.
Measurement
Controller
Packet loss
packages
packages
[%]
Freq.[Hz] Bytes sent Bytes received Bytes sent
Bytes received Measurement Controller
1
2
5
10
50
6552
8520
25200
61640
230088
6560
8528
25256
61656
231168
6552
8504
25200
61640
229896
6552
8520
25200
61640
230088
0.12
0.09
0.22
0.03
0.47
0
0.19
0
0
0.08
Table 3. Percentage of the packet loss in the first test series via the wireless network.
Measurement
Controller
Packet loss
packages
packages
[%]
Freq.[Hz] Bytes sent Bytes received Bytes sent
Bytes received Measurement Controller
1
2
5
10
50
771
1406
2443
9390
22168
771
1406
2443
5014
22168
771
1406
3443
5014
22168
771
1406
2443
9390
22168
0
0
0
0
0
0
0
0
0
0
Table 4. Percentage of the packet loss in the first test series via the wired network /16/.
The difference in the packet loss between the wireless and wired networks is evident.
This confirms the problem of the reliability of the wireless network, even if the
percentage of the packet loss in every sample frequency is very low. The values show
that in the wired network there is no packet loss, while in the wireless network the packet
36
loss can be found due to the presence of possible obstacles or interferences of devices
which operate in the same frequency.
Freq.[Hz]
1
2
5
10
50
Measurement
Controller
Packet loss
packages
packages
[%]
Bytes sent
4240
7144
16560
33208
160616
Bytes received
4224
7144
16560
33056
160216
Bytes sent
4240
7144
16560
33056
160216
Bytes received
4240
7144
16560
33056
160216
Measurement
0
0
0
0.46
0.25
Controller
0.38
0
0
0
0
Table 5. Percentage of the packet loss in the second test series via the wireless
network.
Measurement
Controller
Packet loss
packages
packages
[%]
Freq.[Hz] Bytes sent
Bytes received Bytes sent
Bytes received Measurement
Controller
1
2
5
10
20
50
909
1987
2858
7111
13491
25696
909
1987
2848
7099
13474
25685
0
0
0.35
0.24
0.17
0.47
909
1987
2848
7099
13474
25685
909
1987
2848
7094
13468
25684
0
0
0
0
0
0
Table 6. Percentage of the packet loss in the second test series via the wired network
/16/.
The tables 5 and 6 show the percentage of the packet loss when the controller locates
in Oulu City and the process in the university. In this case it can be found small
packet loss also in the wired network and it concerns control packages. The average
percentage is 0,20 %. This average percentage is the same as in the wireless network
but in the inverse run, because it concerns measurement packages. The packet loss is
due to the packet collision, i.e. the traffic of data in the network. However, the
packet loss in these two networks is very low
37
Conclusions
The main focus in this research has been to examine delay times and packet losses in
the wireless and wired networks in the control of a laboratory-scaled process.
Therefore two different test series have been done where the controller and the
process itself locate in the same room or they are about seven kilometres from each
other.
The results show that the process control across the wireless network is good and
feels little the effects of distance between the process and controller. The packet
losses and delay times are minimal, and almost the same both for the wired and
wireless network. These experiments show that wireless technology applies well for
process control taking into the delay time and packet loss.
38
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