The
continued advancement in digital technology has revolutionized how data is
acquired in measurement and control instruments. Data acquisition in
instrumentation systems encompasses the measurement and recording of process
data.
The bottom line
The ability
of being able to relay or communicate large amounts of data over a limited
number of channels gives digital technology an upper hand over Analog
technology. For example in Analogy technology where 4-20 mA or 3-15 PSI signals are used, each
pair of wires can communicate only one variable whereas in digital networks,
one pair of wires can communicate a limitless number of variables, with the
only limit to it being the speed of that data communication.
From the
above, 4-20 mA analog signals can be expensive to utilize in especially for an
instrument generating multiple variables of measurement like Coriolis mass
flowmeters that measures Flow rate, density, and temperature at the same time.
In this case you will need dedicated wire pair for each process variable. Digital
Technology overcomes these shortcomings of 4-20 mA analog signals.
One solution
to this problem is using HART digital signals superimposed on 4-20 mA signals;
we normally call this 4-20 mA plus HART. With this, you retain the analog
signals while at the same time enjoying the multi-variable communication
benefits that comes with digital technology however wired-HART communication is
rather slow by any standard, and this restricts its use to maintenance {range
changes, diagnostic data polling} and used only for slow process control
processes.
Examples of
digital communication standards include:
- Modbus
- HART
- FOUNDATION Fieldbus
- Profibus PA
- Profibus DP
- Profibus FMS
- AS-I
- ControlNET
- DeviceNet
- BACnet
- LonWorks
- CANbus
SCADA
(Supervisory, Control and Data Acquisition) systems use digital communication
technology. SCADA is similar to DCS, but it is spread over a large
geographical area whereas DCS may cover only a plant floor. You will find
SCADA systems applied in areas like:
- Gas and oil exploration and distribution (pipeline) systems
- Electric power generation and distribution (power line, substation) systems.
- Water and wastewater treatment and distribution lines (water line, pumping stations) systems.
- Large irrigation or harvesting systems.
In a system
where we have the flow of information just in one way (simplex) from the
measurement devices to human operators, the system may be referred to as a
Telemetry system rather than a SCADA system.
SCADA
implies two-way (Duplex) information flow, where human operators not only
monitor the process data but also issue commands back to the remote terminal
units to effect change.
Actually the
need of remote monitoring and control of electric power distribution systems
lead to the development of power line carrier analog telemetry systems. These
systems superimposed high-frequency (50 kHz to 150 kHz) carrier signals on a
low-frequency (50 Hz and 60 Hz power line conductors to communicate basic
information like human voice (telephone network dedicated to power system
operators), power flow (MVAR meter, Wattmeter) monitoring and protective relay
(automatic trip) controls. These are examples of telemetry systems that were
among the first to benefit from digital technology.
Large scale
power systems cannot be operated safely and with efficiency without the use of
remote data monitoring and control systems.
You can also read: How to integrate PLC into a Control System
You can also read: How to integrate PLC into a Control System
The bottom line
From the
above discussion, you can see that digital communication technology forms an essential
part of any modern measurement and control system, and as more research and
development is being done in the digital field, industries will continue to use
this technology to improve efficiency in their industrial production and processing systems.
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