Monday, September 10, 2018

How Digital Communication Technology is applied in Measurement and Control Systems

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 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.
Magnetrol Level Transmitter Configuration using Pactware

Examples of digital communication standards include:
  • Modbus
  •  HART
  •  FOUNDATION Fieldbus
  • Profibus PA
  • Profibus DP
  • Profibus FMS
  • AS-I
  •  ControlNET
  •  DeviceNet
  • BACnet
  • LonWorks
  • CANbus
Some of the digital communication instruments find common use in distributed control systems (DCS) applications.
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.
The process data in a SCADA system is sensed by various measurement devices (transmitters) converted to digital form by an RTU (Remote Terminal Unit), and communicated to one or more MTUs (Master Terminal Units) at a central location where we have human operators monitoring the data and at the same time make command decisions.
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

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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