Industrial control system (ICS) is a general term that encompasses several types of control systems and associated instrumentation used for industrial process control.
Such systems can range in size from a few modular panel-mounted controllers to large interconnected and interactive distributed control systems with many thousands of field connections. Systems receive data received from remote sensors measuring process variables (PVs), compare the collected data with desired setpoints (SPs), and derive command functions which are used to control a process through the final control elements (FCEs), such as control valves.
Larger systems are usually implemented by supervisory control and data acquisition (SCADA) systems, or distributed control systems (DCS), and programmable logic controllers (PLCs), though SCADA and PLC systems are scalable down to small systems with few control loops. Such systems are extensively used in industries such as chemical processing, pulp and paper manufacture, power generation, oil and gas processing, and telecommunications.
Discrete controllers
Panel
mounted controllers with integral displays. The process value (PV), and
setvalue (SV) or setpoint are on the same scale for easy comparison.
The controller output is shown as MV (manipulated variable) with range
0-100%.
A
control loop using a discrete controller. Field signals are flow rate
measurement from the sensor, and control output to the valve. A valve
positioner ensures correct valve operation.
The simplest control systems are based around small discrete controllers with a single control loop
each. These are usually panel mounted which allows direct viewing of
the front panel and provides means of manual intervention by the
operator, either to manually control the process or to change control
setpoints. Originally these would be pneumatic controllers, a few of
which are still in use, but nearly all are now electronic.
Quite complex systems can be created with networks of these
controllers communicating using industry standard protocols. Networking
allow the use of local or remote SCADA operator interfaces, and enables
the cascading and interlocking of controllers. However, as the number of
control loops increase for a system design there is a point where the
use of a programmable logic controller (PLC) or distributed control system (DCS) is more manageable or cost-effective.
Distributed control systems
Functional
manufacturing control levels. DCS (including PLCs or RTUs) operate on
level 1. Level 2 contains the SCADA software and computing platform.
A distributed control system (DCS) is a digital processor control
system for a process or plant, wherein controller functions and field
connection modules are distributed throughout the system. As the number
of control loops grows, DCS becomes more cost effective than discrete
controllers. Additionally a DCS provides supervisory viewing and
management over large industrial processes. In a DCS, a hierarchy of
controllers is connected by communication networks, allowing centralised control rooms and local on-plant monitoring and control.
A DCS enables easy configuration of plant controls such as cascaded loops and interlocks, and easy interfacing with other computer systems such as production control.
It also enables more sophisticated alarm handling, introduces automatic
event logging, removes the need for physical records such as chart
recorders and allows the control equipment to be networked and thereby
located locally to equipment being controlled to reduce cabling.
A DCS typically uses custom-designed processors as controllers,
and uses either proprietary interconnections or standard protocols for
communication. Input and output modules form the peripheral components
of the system.
The processors receive information from input modules, process
the information and decide control actions to be performed by the output
modules. The input modules receive information from sensing instruments
in the process (or field) and the output modules transmit instructions
to the final control elements, such as control valves.
The field inputs and outputs can either be continuously changing analog signals e.g. current loop or 2 state signals that switch either on or off, such as relay contacts or a semiconductor switch.
Distributed control systems can normally also support Foundation Fieldbus, PROFIBUS, HART, Modbus
and other digital communication buses that carry not only input and
output signals but also advanced messages such as error diagnostics and
status signals.
SCADA systems
Supervisory control and data acquisition (SCADA) is a control system architecture that uses computers, networked data communications and graphical user interfaces
for high-level process supervisory management. The operator interfaces
which enable monitoring and the issuing of process commands, such as
controller set point changes, are handled through the SCADA supervisory
computer system. However, the real-time control logic or controller
calculations are performed by networked modules which connect to other
peripheral devices such as programmable logic controllers and discrete PID controllers which interface to the process plant or machinery.
The SCADA concept was developed as a universal means of remote
access to a variety of local control modules, which could be from
different manufacturers allowing access through standard automation protocols. In practice, large SCADA systems have grown to become very similar to distributed control systems
in function, but using multiple means of interfacing with the plant.
They can control large-scale processes that can include multiple sites,
and work over large distances.
This is a commonly-used architecture industrial control systems,
however there are concerns about SCADA systems being vulnerable to cyberwarfare or cyberterrorism attacks.
The SCADA software operates on a supervisory level as control actions are performed automatically by RTUs
or PLCs. SCADA control functions are usually restricted to basic
overriding or supervisory level intervention. A feedback control loop is
directly controlled by the RTU or PLC, but the SCADA software monitors
the overall performance of the loop. For example, a PLC may control the
flow of cooling water through part of an industrial process to a set
point level, but the SCADA system software will allow operators to
change the set points for the flow. The SCADA also enables alarm
conditions, such as loss of flow or high temperature, to be displayed
and recorded.
Programmable logic controllers
Siemens
Simatic S7-400 system in a rack, left-to-right: power supply unit
(PSU), CPU, interface module (IM) and communication processor (CP).
PLCs can range from small modular devices with tens of inputs and
outputs (I/O) in a housing integral with the processor, to large
rack-mounted modular devices with a count of thousands of I/O, and which
are often networked to other PLC and SCADA systems. They can be
designed for multiple arrangements of digital and analog inputs and
outputs, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory.
History
A
pre-DCS era central control room. Whilst the controls are centralised
in one place, they are still discrete and not integrated into one
system.
A
DCS control room where plant information and controls are displayed on
computer graphics screens. The operators are seated as they can view and
control any part of the process from their screens, whilst retaining a
plant overview.
Process control
of large industrial plants has evolved through many stages. Initially,
control was from panels local to the process plant. However this
required personnel to attend to these dispersed panels, and there was no
overall view of the process. The next logical development was the
transmission of all plant measurements to a permanently-manned central
control room. Often the controllers were behind the control room panels,
and all automatic and manual control outputs were individually
transmitted back to plant in the form of pneumatic or electrical
signals. Effectively this was the centralisation of all the localised
panels, with the advantages of reduced manpower requirements and
consolidated overview of the process.
However, whilst providing a central control focus, this
arrangement was inflexible as each control loop had its own controller
hardware so system changes required reconfiguration of signals by
re-piping or re-wiring. It also required continual operator movement
within a large control room in order to monitor the whole process. With
the coming of electronic processors, high speed electronic signalling
networks and electronic graphic displays it became possible to replace
these discrete controllers with computer-based algorithms, hosted on a
network of input/output racks with their own control processors. These
could be distributed around the plant and would communicate with the
graphic displays in the control room. The concept of distributed control was realised.
The introduction of distributed control allowed flexible
interconnection and re-configuration of plant controls such as cascaded
loops and interlocks, and interfacing with other production computer
systems. It enabled sophisticated alarm handling, introduced automatic
event logging, removed the need for physical records such as chart
recorders, allowed the control racks to be networked and thereby located
locally to plant to reduce cabling runs, and provided high-level
overviews of plant status and production levels. For large control
systems, the general commercial name distributed control system
(DCS) was coined to refer to proprietary modular systems from many
manufacturers which integrated high speed networking and a full suite of
displays and control racks.
While the DCS was tailored to meet the needs of large continuous
industrial processes, in industries where combinatorial and sequential
logic was the primary requirement, the PLC evolved out of a need to
replace racks of relays and timers used for event-driven control. The
old controls were difficult to re-configure and debug, and PLC control
enabled networking of signals to a central control area with electronic
displays. PLC were first developed for the automotive industry on
vehicle production lines, where sequential logic was becoming very
complex.
It was soon adopted in a large number of other event-driven
applications as varied as printing presses and water treatment plants.
SCADA's history is rooted in distribution applications, such as
power, natural gas, and water pipelines, where there is a need to gather
remote data through potentially unreliable or intermittent
low-bandwidth and high-latency links. SCADA systems use open-loop control with sites that are widely separated geographically. A SCADA system uses remote terminal units
(RTUs) to send supervisory data back to a control center. Most RTU
systems always had some capacity to handle local control while the
master station is not available. However, over the years RTU systems
have grown more and more capable of handling local control.
The boundaries between DCS and SCADA/PLC systems are blurring as time goes on.
The technical limits that drove the designs of these various systems
are no longer as much of an issue. Many PLC platforms can now perform
quite well as a small DCS, using remote I/O and are sufficiently
reliable that some SCADA systems actually manage closed loop control
over long distances. With the increasing speed of today's processors,
many DCS products have a full line of PLC-like subsystems that weren't
offered when they were initially developed.
In 1993, with the release of IEC-1131, later to become IEC-61131-3,
the industry moved towards increased code standardization with
reusable, hardware-independent control software. For the first time, object-oriented programming
(OOP) became possible within industrial control systems. This led to
the development of both programmable automation controllers (PAC) and
industrial PCs (IPC). These are platforms programmed in the five
standardized IEC languages: ladder logic, structured text, function
block, instruction list and sequential function chart. They can also be
programmed in modern high-level languages such as C or C++.
Additionally, they accept models developed in analytical tools such as MATLAB and Simulink. Unlike traditional PLCs, which use proprietary operating systems, IPCs utilize Windows IoT.
IPC's have the advantage of powerful multi-core processors with much
lower hardware costs than traditional PLCs and fit well into multiple
form factors such as DIN rail mount, combined with a touch-screen as a panel PC,
or as an embedded PC. New hardware platforms and technology have
contributed significantly to the evolution of DCS and SCADA systems,
further blurring the boundaries and changing definitions.
