Industrial control system

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An industrial control system (ICS) is an electronic control system and

associated instrumentation used for industrial process control. Control systems can range in
size from a few modular panel-mounted controllers to large interconnected and
interactive distributed control systems (DCSs) with many thousands of field connections.
Control systems receive data from remote sensors measuring process variables (PVs),
compare the collected data with desired setpoints (SPs), and derive command functions
that 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 DCSs, and programmable logic controllers (PLCs), though

SCADA and PLC systems are scalable down to small systems with few control loops.
[1] Such systems are extensively used in industries such as chemical processing, pulp and
paper manufactunange 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.

Main article: Distributed control system

A distributed control system (DCS) is a digital process control system (PCS) 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,[further explanation needed] 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 the
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 setpoint 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. [2] This is a commonly-used architecture
industrial control systems, however there are concerns about SCADA systems being
vulnerable to cyberwarfare or cyberterrorism attacks.[3]
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.