Distributed control system (DCS) is a
computerized control system used to control production line in most
industries.
Importance of DCS systems
Importance of DCS systems
In most cases, products go through many stages in
the factory before they reach their final stage. During their passage through
those stages, it requires a kind of control in order to adjust the quality of
the each product. To adjust quality, you need to control many physical
quantities such as Pressure, Temperature, Level, Flow etc. In petrochemical
factories and nuclear reactors the control of these parameters is very critical, in fact losing control may lead to an explosion of the whole plant.
So what are the key components that make up a DCS system?
A DCS system consists of:
Programmable Logic Controllers (PLC): These includes input or output modules, CPU, Communication bus, interface
module, PS etc.
Operator Station: It is basically
human interface machine (HMI) with a monitor, the operator man can view the
process in the plant and check if an alarm is present, change any setting, or
print reports etc.
Engineering Station: It is used to
configure all input & output module, make drawing of anything required and
also, monitors an operator station.
The following diagram shows a Basic Block Diagram of a DCS
system:
The Working of a DCS system
The DCS system receives input signals from other
devices, these signals will be processed and analysed by the CPU and based on
the result, an action will be taken. There are many sensors and transducers
inside the plant which converts physical quantities such as temperature,
pressure, level, flow to an electrical quantity and also transmits the
electrical signals representation to the DCS system.
The electrical signals could either be analog or
digital signals, they could be in either of the two forms: 4-20 mA or 1-5 VDC
The digital signals could either be logic 0 or
logic 1
When the PLC receives the analog signals from
different transmitters it will convert it to digital signals via input module
then pass it to the CPU for processing. The CPU will compare the process
measured value (PV) with the set point value (SV). If both the values are not
matched, then the DCS will generate an output called manipulated value (MV) to
the field via output modules in order to adjust the physical quantity by for example
controlling the opening or closing of a valve.
On the face plate, the operator can see both
parameters reading SV and PV. The SV can be set to the required level by the
operator. DCS system will operate the pump by sending MV signal via output
module to fill the tank and continue as long as PV signal received from the
transmitter is less than the SV. Once the water level reaches the required
point i.e. PV=SV, the DCS will stop the pump and this happen if face-plate mode
is AUTO, however in MANUAL mode, the operator can control MV directly and SV
pointer will be disabled. This is a simple working of a DCS system. You may
find different physical quantities being controlled, it could be the flow rate
or pressure in the system.
You can also read:
Some of the leading manufacturers of DCS systems
include:
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