18-09-2014, 11:59 AM
DISTRIBUTED CONTROL SYSTEMS
[attachment=68100]
Introduction
Generally, the concept of automatic control includes accomplishing two major
operations; the transmission of signals (information flow) back and forth and the
calculation of control actions (decision making). Carrying out these operations in real
plant requires a set of hardware and instrumentation that serve as the platform for these
tasks. Distributed control system (DCS) is the most modern control platform. It stands
as the infrastructure not only for all advanced control strategies but also for the lowliest
control system. The idea of control infrastructure is old. The next section discusses how
the control platform progressed through time to follow the advancement in control
algorithms and instrumentation technologies.
Historical Review
To fully appreciate and select the current status of affairs in industrial practice it
is of interest to understand the historical perspective on the evolution of control systems
implementation philosophy and hardware elements. The evolution concerns the heart of
any control system which is how information flow and decision making advanced.
Modes of Computer control
Computer control is usually carried out in two modes: supervisory control or
direct digital control. Both are shown in Figure 1. Supervisory control involves
resetting the set point for a local controller according to some computer calculation.
Direct digital control, by contrast, requires that all control actions be carried out by the
digital computer. Both modes are in wide use in industrial applications, and both allow
incorporating modern control technologies. Measurements are transmitted to computer
and control signals are sent from computer to control valves at specific time interval
known as sampling time. The latter should be chosen with care
1 Small Computer Network
In small processes such as laboratory prototype or pilot plants, the number of
control loops is relatively small. An inexpensive and straightforward way to deal with
the systems is to configure a network of personal computers for data acquisition and
control. An example configuration of a PC network control system is depicted in Figure
2. The network consists of a main computer linked directly to the process in two-way
channels. Other local computers are linked to the main computer and are also connected
to the process through one-way or two-way links. Some of these local computers can
be interconnected. Each of the local computers has a video display and a specific
function. For example, some local computers are dedicated for data acquisition only,
some for local control only and some other for both data acquisition and local control.
The main computer could have a multiple displays.
Conclusion
Digitally-based control instrumentation represents a revolutionary change in the
process control paradigm. With digital systems the control engineer has the opportunity
to go beyond the narrow limitation of standard analog control components to construct a
system that is optimum for the information processing and control requirements of large
processes or even of entire plants. This is why many industrial plants are updating
their hardware and instrumentation systems bearing in mind that the payout times for
installation and commissioning costs is as a low as three to four months.
[attachment=68100]
Introduction
Generally, the concept of automatic control includes accomplishing two major
operations; the transmission of signals (information flow) back and forth and the
calculation of control actions (decision making). Carrying out these operations in real
plant requires a set of hardware and instrumentation that serve as the platform for these
tasks. Distributed control system (DCS) is the most modern control platform. It stands
as the infrastructure not only for all advanced control strategies but also for the lowliest
control system. The idea of control infrastructure is old. The next section discusses how
the control platform progressed through time to follow the advancement in control
algorithms and instrumentation technologies.
Historical Review
To fully appreciate and select the current status of affairs in industrial practice it
is of interest to understand the historical perspective on the evolution of control systems
implementation philosophy and hardware elements. The evolution concerns the heart of
any control system which is how information flow and decision making advanced.
Modes of Computer control
Computer control is usually carried out in two modes: supervisory control or
direct digital control. Both are shown in Figure 1. Supervisory control involves
resetting the set point for a local controller according to some computer calculation.
Direct digital control, by contrast, requires that all control actions be carried out by the
digital computer. Both modes are in wide use in industrial applications, and both allow
incorporating modern control technologies. Measurements are transmitted to computer
and control signals are sent from computer to control valves at specific time interval
known as sampling time. The latter should be chosen with care
1 Small Computer Network
In small processes such as laboratory prototype or pilot plants, the number of
control loops is relatively small. An inexpensive and straightforward way to deal with
the systems is to configure a network of personal computers for data acquisition and
control. An example configuration of a PC network control system is depicted in Figure
2. The network consists of a main computer linked directly to the process in two-way
channels. Other local computers are linked to the main computer and are also connected
to the process through one-way or two-way links. Some of these local computers can
be interconnected. Each of the local computers has a video display and a specific
function. For example, some local computers are dedicated for data acquisition only,
some for local control only and some other for both data acquisition and local control.
The main computer could have a multiple displays.
Conclusion
Digitally-based control instrumentation represents a revolutionary change in the
process control paradigm. With digital systems the control engineer has the opportunity
to go beyond the narrow limitation of standard analog control components to construct a
system that is optimum for the information processing and control requirements of large
processes or even of entire plants. This is why many industrial plants are updating
their hardware and instrumentation systems bearing in mind that the payout times for
installation and commissioning costs is as a low as three to four months.