PLC_ DCS_ FCS and the characteristics of three different control systems

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					PLC, DCS, FCS and the characteristics of three different control systems
PLC, DCS, FCS and the characteristics of three different control systems
Abstract: PLC, DCS, FCS and the characteristics of three different control systems are
analyzed, pointing out that the three control systems of origin and development.
Keywords: programmable logic controller (PLC) distributed control system (DCS)
field bus control system (FCS)

1. Foreword
Nineties of the last century into practical field bus control s ystem, being the
momentum of rapid development of rapid, is the world's latest control
system. Fieldbus control system in automation technology is a hot, being at home and
abroad automation equipment manufacturers and users increasingly strong concern.
The emergence of fieldbus control systems, automation will bring about another
revolution, the depth and breadth of its history, any more than once, thereby creating a
new era of automation.
In some industries, FCS is evolved by the PLC; in other sectors, FCS in turn, evolved
from DCS, PLC and DCS so between FCS and inextricably linked, they are
essentially and difference. This article tries to PLC, DCS, FCS three control system
for an analysis of the characteristics and differences, that between them the origin and

2. PLC, DCS, FCS basic characteristics of the three control systems
At present, continuous process manufacturing automation (PA) or industrial process
control used to call it that, there are three main control system, tha t is, PLC, DCS and
FCS. Their respective basic features are as follows:

2.1 PLC
(1) from the switching control developed to the sequence control, delivery address, is
from the bottom up.
(2) Continuous PID control function, PID in the break station.
(3) can be a PC-based stations, more than the same type of PLC from the station.
(4) can also be a PLC-based stations, more than the same type of PLC from the station,
constitutes a PLC network. This PC, as master than convenience is that: a user
programming, do not know the communication protocol, as long as the format,
according to the instructions written on the line.
(5) PLC grid can be used as an independent DCS / TDCS, also as DCS / TDCS
(6) large-scale system with the DCS / TDCS, as TDC3000, CENTUMCS, WDPFI,
(7) PLC networks such as the Siemens company SINEC-L1, SINEC-H1, S4, S5, S6,
S7 and so on, GE's GENET, Mitsubishi's MELSEC-NET,
(8) is mainly used for industrial process sequence control, the new PLC is also both
closed- loop control functions.
(9) Manufacturer: GOULD (United States), AB (U.S.), GE (U.S.), OMRON (Japan),
MITSUBISHI (Japan), Siemens (Germany) and so on.

2.2 DCS or TDCS
(1) Distributed Control System DCS Distributed Contro l System and the TDCS is a
4C (Communication, Computer,
Control, CRT) technology in a monitoring technology.
(2) large-scale systems from top to bottom of the tree topology, in which
communication (Communication) is the key.
(3) PID in the break points, the relay station connected computers and on-site
instrumentation and control devices.
(4) is the tree topology and the link structure of parallel continuous, there are a large
number of parallel cable from the relay station to the site instrumentation.
(5) analog signals, A / D-D / A, mixed with a microprocessor.
(6) one meter a pair of wire to I / O, by the controller linked to the local area network
(7) DCS is the control (engineering station), operations (operator station), field
instruments (on-site monitoring stations) in the three structures.
(8) The disadvantage is high cost, the company's products are not
interchangeable, not interoperable, large DCS systems are various different.
(9) for large-scale continuous process control, such as petrochemicals.
(10) Manufacturer: Bailey (U.S.), Westinghous (U.S.), HITACH (Japan), LEEDS
NORTHRMP (U.S.), SIEMENS (Germany), Foxboro (United States), ABB
(Switzerland), Hartmann &
Braun (Germany), Yokogawa (Japan), Honewell (United States), Taylor (U.S.) and so

2.3 FCS
(1) The basic tasks are: the nature (intrinsic) security, hazardous area, variable course,
very difficult to deal with the environment.
(2) fully digital, intelligent, multi- function to replace single- function analog
instruments, meters, control devices.
(3) connected with two lines scattered field instruments, control devices, PID and
control center to replace the two lines on each instrument.
(4) on the bus PID and instruments, meters, control devices are equal.
(5) multi-variable, multi- node, serial, digital communication system instead of a
single variable, single point, parallel simulation system.
(6) is connected, two-way, open to replace a one-way, closed.
(7) replaced with a distributed virtual centralized control station controller.
(8) from on-site computer control, can also be linked to the host computer, take the
same bus on a computer.
(9) local area network, then connected with internet.
(10) to change the traditional signal standards, communications standards and systems
standards into the enterprise management network.
(11) Manufacturer: American Honeywell, Smar, Fisher-Rosemount,
AB / Rockwell, Elsag-Bailey, Foxboro, Yamatake, Japan Yokogawa, Europe
Siemens, GEC-Alsthom, Schneider, proces-Data, ABB, etc..
(12) 3 categories of typical FCS
1) continuous process control such as the petrochemical industry, including the
"safe explosion-proof" technology is absolutely essential,
typical products are FF, World
FIP, Profibus-PA;
2) The process of separation of robot motion control, such as automobile
manufacturing, automobile, typical products are Profibus-DP, CANbus;
3) multi-point control, such as building automation, the typical product is LON Work,
From the above description of the basic points, we note that the three major systems
for process control, no power station and Kaifa against, or that in their development of
early, are not to make the system of Shouxuan control station object. The instructions
in these systems is also not the Power Station as the preferred application, and some
simply do not mention the scope of application in power stations. Now surprisingly,
these three control systems, in particular DCS, PLC, are widely used in power s tations,
and results are very good.

3. The difference between the three control systems
We already know, FCS is evolved by the DCS and PLC, FCS not only have the
characteristics of DCS and PLC, and taken a revolutionary step. At present, the new
DCS with the new PLC, has a tendency to each other * rope. DCS has a strong new
order of control; the new PLC, in dealing with closed- loop control is not bad, and
both can form large networks, DCS and PLC's application has been
considerably intersections. The next section will only compare the DCS and FCS. In
the previous chapters, in fact, related to differences between DCS and FCS, will the
following architecture, investment, design, use and other aspects of narrative.
3.1 points difference
DCS system, the key is communication. Data can also be said highway is the
backbone of distributed control system DCS. Because of its mission is to provide
communication between all system components the network, therefore, data highway,
the design itself determines the overall flexibility and security. Data highway, the
media can be: a pair of conductors, coaxial cable or fiber optic cable.
The design parameters through the data highway, basically about a specific DCS
system, the relative strengths and weaknesses.
(1) system can handle the number of I / O information.
(2) the system can handle the number of control loops and control of information.
(3) to adapt the number of users and devices (CRT, control stations, etc.).
(4) data integrity is how thorough examination.
(5) Data maximum allowable length of the road is.
(6) data to support the number of slip road.
(7) data highways to support from the other manufacturer's hardware
(programmable logic controller, computer, data recording devices, etc.).
To ensure the integrity of communications, most DCS manufacturers can provide
redundant data highway.
To ensure the security of the system, using a complex communication protocol and
error detection techniques. The so-called communication protocol is a set of rules to
ensure that the transmitted data is received and understood the data obtained and sent
the same.
DCS systems currently in general use two types of means of communication, that is,
synchronous and asynchronous, synchronous communication according to * a clock
signal to adjust the data transmission and reception of asynchronous network
reporting system with no clock.

There are three key elements of FCS
(1) FCS is the core of the system bus protocol, the bus standard
Has been described in previous chapters, a type of bus, as long as the bus protocol has
been established, the related key technologies and related equipment also been
identified. Agreement on the basic principles of their bus, the bus is the same all have
to address the bi-directional serial digital transmission as the basis for communication.
However, for various reasons, various types of bus-bus protocol there is a big
In order to fulfill an fieldbus interoperability requirements to become a true ope n
system, in the IEC international standard, field bus communication protocol Moxing
the user Ceng in, on clearly defined layers with Zhuangzhimiaoshu Gong Neng Yong
Hu. In order to achieve interoperability, each field bus devices are used to describe the
device described in DD. DD can be considered a drive device, which includes all the
necessary parameters required to describe and master the steps. As the communication
device DD including a description of all the information needed, and nothing to do
with the main station, so you can make on-site installation of true interoperability.
The same situation as above, the answer is no. The current through the field bus with
8 types of international standards, international standards, the original IEO one of
only 8 types, and the other 7 types of bus status is equal. Other 7 bus, regardless of
how much market share, each bus protocol has a set of software and hardware support.
They can form a system, the formation of products, while the original international
standard IEC Fieldbus is a neither software nor hardware support supported basket
case. Therefore, to achieve these bus compatibility and interoperability, in terms of the
current state is almost impossible.
Through this, we can draw such an image: an open fieldbus control system
interoperability, on a specific type of field bus, as long as follow the type of field bus
bus protocol, their products are open of, and interoperability. In other words, no
matter what the manufacturers of the product, not a field bus of the
company's products, as long as the bus protocol to follow the bus between
products is open and interoperable, we can form the bus network.
(2) FCS system is based on the number of intelligent field devices
FCS digital intelligent field device is the hardware support is the foundation, the
reason is very simple, FCS system to perform the automatic control devices and
on-site two-way digital communication between devices field bus signal system. If the
field device does not follow a unified bus protocol, that is related to the
communication protocol, does not have digital communication capabilities, so-called
two-way digital communications merely empty words, it can not be called field bus
control system. Another point, one of the major characteristics of the fieldbus is to
increase the field- level control functions. If the site is not a multi- functional
intelligent device product, the characteristics of fieldbus control system would not
exist, the so-called simplified system to facilitate the design is conducive to
maintenance of the superiority of virtual.
(3) FCS system is the essence of the scene of information processing
For a control system, whether it is using DCS or field bus, the system needs to deal
with the amount of information is at least as much. In fact, the use of field bus, you
can get more information from the scene. Field bus system has not reduced the
amount of information, and even increased, while the cable transmission of
information has been greatly reduced. This req uires the cable to transmit information
to greatly improve the ability to make a lot of information on the other hand,
completed on site, reducing on-site with information on travel between the control
room. It can be said is the nature of Fieldbus field o f information processing.
Reduce the information to and from network design and system configuration is an
important principle. Reduce the information from often can bring the benefits of
improved system response time. Therefore, the network design should give priority to
the large amount of information exchange between nodes, on the inside with a slip.
Reduce the information from the cable system and reduce the sometimes conflicting.
At this time should do to save the principle of investment choices. If you choose the
response time allows, the program should be selected to save cable. If the selected
system response time is rather tight, a little bit of information to reduce the
transmission enough to use, and it should be selected to reduce transmission of
information programs.
Now, some with live instrumentation fieldbus function block itself has a lot, although
the same functional blocks of different products in the performance will be slightly
different, but a network of support on the road has many features similar function
block is an objective reality. Choosing a site meter on the function block is a system
configuration problem to be solved.
Principle to consider this question: to minimize the information from the bus. General
can select information about the output of the function that sets up the instrument on
the function block.
3.2 Comparison of typical system
Through the use of field bus, the user can significantly reduce field wiring, field
instrument can be realized with a single multi- variable communication between
devices produced by different manufacturers can be fully interoperable, increase
site- level control functions, greatly simplifying system integration, and maintenance
is very Simple. A typical field bus system box icon in Figure 1. Can be seen from
Figure 1, the traditional process control instrumentation systems for each field device
to the control room are required to use a pair of dedicated twisted pair to send 4 ~
20mA signal, Figure 2 shows the field bus system, each field devices to the junction
box of the twisted pair can still be used, but the junction box from the field to a
central control room in just a twisted pair to complete digital communications.
Through the use of field bus control system, in the end save much cable, editors have
not done the calculation. However, we can not use the power plant DCS systems and
automatic control system used in cable-related mileage see, cable's share of
investment in infrastructure.
A power plant, 2 × 300MW coal- fired units. Thermal system as a unit system. Each
unit set up a central control building, used machine, oven, electric unit centralized
control. Unit control room of the elevation of 12.6 meters, consis tent with the
Operating Floor Elevation. DCS uses WDPF-Ⅱ, each unit designed I / O points 4500
Cable laying by EC Software, 8 people with 1.5 months to complete the design of
cable laying mission.
Each 300MW unit within the main factory automation cables root number 4038.
Each 300MW unit within the main factory automation cable length of 350 km.
Above the root of the number and length of cable does not include the whole plant fire
alarm in the plant for the factory the auxiliary cable and cable manufacturing plant.
Cable tray of the column, the bridge and use a small slot Galvanized steel box full,
about 95 tons per unit.
Other cable tray, including straight, elbow, tee, cross, cover, terminal head, width
modulated films, direct films such as aluminum alloy material used, about 55 tons
each 300MW unit. Annex with the bridge (such as the bolts, nuts).
A power plant, 4 × MW gas power station fuel. Thermal system as a unit system. DCS
uses TELEPERM-XP. Design of each unit I / O points to 5804 points.
Cable laying by EC Software, 12 people with 2.5 months to complete the design of
cable laying mission.
Each 325MW unit within the main factory automation cables root number 4413.
Each 235MW unit within the main factory automation cable length of 360 km.
All galvanized steel used per unit cable bridge, its weight is about 200 tons.
Power cables can be divided into six categories: high-voltage power cables, low
voltage power cables, control cables, thermal control cables, weak lines (mainly
computer cable), other cables. If the two 300MW units at the same time to do cable
laying, the number of cables Automation about 8500 or so. The thermal control cables
and weak cable will be greater than 5000, or about 60% (measured by number of
3.3 design, investment and use of
The above comparison is more emphasis on purely technical, economic factors, the
following comparison to be added.
The former question is more typical DCS system, the ideal comparison FCS system.
Why do such an assumption. As a DCS system developed to the early development of
the technical requirements already met and has been perfected to further improve the
current situation is therefore not the typical, ideal view. As FCS system, just 90 years
into the practical, as the early development of the technical requirements: compatible,
open, two-way digital communications, digital intelligent field devices, high-speed
bus and so far not to be perfect. This state and Fieldbus international standards can not
be said it was not. Over the past ten years, the bus organizations are busy setting
standards, product development, and occupy more of the market, is aimed at a niche
in the international standards, legal occupation of a larger market. Now the war has
come to an end to international standards, companies organizations have realized that
to really capture the market, you have to improve systems and related products. We
can do this kind of prediction, in the near future, complete field bus systems and
related products have become the mainstream of fieldbus technology.
The comparison:
(1) DCS system is a big system, its powerful controller functions but also in the
important role of the system, data highway, is the key to the system, it must be the
overall investment in one step, subsequent expansion difficult. The more radical
devolution of FCS capabilities, information processing site, Digital widespread
adoption of intelligent field devices, making the controller function and importance of
the relatively weakened. Therefore, FCS system investment from a low, you can side
with, side extension, side put into operation.
(2) DCS system is a closed system, the basic product is not compatible. The FCS
system is an open system, the user can select different vendors, different brands of
equipment connected to the bus, to achieve the best system integration.
(3) DCS system, all information is the formation of binary or analog signal, there
must be D / A and A / D conversion. The FCS system is fully digital, it eliminates the
D / A and A / D conversion, high-performance integrated, so that accuracy can be
increased from ± 0.5% to ± 0.1%.
(4) FCS system closed- loop PID control function can be loaded into the transmitter or
actuator, the shortening of the control cycle, the current from the DCS 2 to 5 times per
second, up to FCS 10 to 20 times per second, which to improve regulation
(5) DCS technology which can control and monitor the whole process of their own
diagnosis, maintenance and configuration. However, due to its fatal weakness, its I / O
signals using traditional analog signals, so that it can not stand on the DCS engineers
on-site instrumentation (including transducers, actuators, etc.) for remote diagnosis,
maintenance and configuration . FCS uses all-digital technology, digital
multi- variable intelligent field devices to send information, not just a single variable
information, and also has the function of error detection information. FCS uses a
two-way digital communication system of field bus signal. Therefore, it can be on-site
installation (including transducers, actuators, etc.) for remote diagnosis, maintenance
and configuration. FCS is the DCS advantages of this point can not be compared.
(6) FCS field of information processing technology, compared with the DCS can save
a considerable number of isolation devices, terminal cabinets, I / O terminal, I / O
card pieces, I / O files and I / O cabinet, but also save the I / O devices and equipment
room space and area. Some experts believe can save 60%.
(7) and (6) for the same reason, FCS can reduce the large number of cables and laying
cables in the bridge, etc., but also save the design, installation and maintenance costs.
Some experts believe can save 66%.
For (6), (7) two points should add that using the FCS system, the effect of savings in
investment is no doubt, but whether, if any, experts say 60 ~ 66%. These figures
appear in many articles, the editor that this is the result of mutual selections, has yet to
find the original source of these figures, therefore reference to these figures, the
reader should be careful.
(8) FCS relative to the DCS configuration is simple, because the structure,
performance, standardization, ease of installation, operation and maintenance.
(9) for FCS design and development process control points. This is not a comparison
with the DCS, simply stated for process control or process used to simulate the
continuous development of the class FCS should be focused in the design
1) require the bus explosion of the security function, and is of primary importance.
2) basic monitoring such as flow, material level, temperature, pressure and the change
is slow, but also the lag effect, therefore, we quickly Bingbu Monitoring Nodes in the
response time of electronics, but requires complex analog Handling in capacity. The
physical characteristics of the system is basically the decision to use more
concentrated among the main one from the polling system, which is technically
justified, economically beneficial.
3) flow, level, temperature, pressure and other parameters measured, the physical
principle is classical, but the sensors, transmitters and controllers should be the
development of digital intelligence.
4) as a continuous process for the class and its instrumentation and the development
of FCS, should focus on improving the design of low-speed bus H1.

4. The prospect of PLC & DCS
We already know that some of FCS is evolved by the PLC, while others developed
from FCS is the DCS, then the FCS has to practical today, PLC and DCS How will
the prospects.
PLC in the late 60's first appeared in the United States, aims to replace the
relay, the implementation of logic, timing, counting and other control functions in
order to establish flexible process control system. Officially named in 1976, and given
the definition: PLC is a dedicated digital control computer, which uses a
programmable memory store instruction, the implementation of such logic, sequence,
timing, counting and calculation functions, and through analog and digital inputs,
output, components, control of various mechanical or work procedures. After 30 years
of development, PLC has a very mature and complete, and the development of the
analog closed- loop control functions. PLC status in the FCS system seems to have
been determined that no amount of argument. See Figure 3 : IEC fieldbus control
system recommended architecture. PLC as a stand hung in high-speed bus. PLC full
volume in dealing with the advantages of switching. In addition, the thermal power
plant auxiliary plant, such as water treatment plant, recycled water plant, ash and slag
handling plant, coal plant, etc., in the workshop process with sequential control over
the main. PLC control for the sequence has its unique advantages. Editor thought,
auxiliary plant control system to comply with the PLC fieldbus communication
protocols or to communicate with the FCS PLC is an optimal exchange of information
Since 1973, made the first microprocessor-based controller, since it improved
gradually, and eventually the formation of functional, safe and can be * digital
distributed control system DCS. Its performance is much better than to live any kind
of control system. Meet the thermal power plant DAS, MCS, SCS and APS the
various requirements of the system, now also through the establishment of the
management of industrial Ethernet networks to meet increasing demand for thermal
power plants to strengthen the management requirements. It can be said, DCS control
system can cover a large thermal power process the entire process.
However, since with FCS, and was 90 to practical use has been the following
argument in open publications and the publications, namely: "From now
on, the new FCS field bus control system will gradually replace the traditional
DCS"; "when regulatory function delegated to the scene to the
future, the traditional DCS need not exist and would Zidong 消 失
";" the next decade, the traditional 4 ~ 20mA analo g Xin Hao
Zhi Qiangzhubupi two-way digital communication signals Zongxian site instead of
the system, simulation and digital distributed control system DCS will be upgrading
to all-digital field bus control system FCS "... .... These arguments
summarized in one sentence: FCS will replace the DCS, DCS will die from this.
From the above arguments are authoritative experts in the mouth, does justified.
Digital communications is a trend, it represents a technological advancement, and no
one can stop. Fieldbus bidirectional digital communication system and signal
generated by it a great impetus to accelerate the field devices and control instruments
of change, to develop more and more full- featured digital intelligent field devices.
These are not available in DCS systems, and the resulting advantages as well as to the
power plant design, configuration, operation, maintenance and management, the
benefits are less than the DCS system. Furthermore, FCS was evolved by the DCS
and the PLC, it retains the characteristics of DCS, or DCS FCS absorbed years of
research and development and on-site practical experience, of course, including
lessons learned. Thus concluded, "FCS will replace the DCS",
seems logical things.
Meanwhile, we should see, DCS systems development for nearly 30 years, the Power
Plant so widely. Its design, configuration configuration, function is perfect matching
has reached the extent (of course, DCS there are further developments, such as
high- level software development, to meet the requirements of information integration),
has infiltrated into the power plant control system various fields, and also some FCS
system embodiment. From this perspective, DCS can not say from this system seems
to die. Furthermore, from the preceding chapters have spoken narrative, for those FCS
system can not give full play to its own characteristics and advantages of the area,
DCS system is still useless.

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