4 Automation and Control Terms You Need To Be Familiar With SCADA, DCS, PLC, and RTU

 SCADA (Supervisory Control and Data Acquisition)

SCADA a system of software and hardware that allows industrial organization to industrial process locally and remotely.

The basic SCADA architecture begins with programmable logic controllers (PLCs) or remote terminal units (RTU). PLCs and RTUs are microcomputers that communicate with an array of objects such as machines, electric motor, HMIs, sensors, and actuators and then route the information from those objects to computers with SCADA software.

The SCADA software processes, distributes and displays the data, helping operators and other employees analyze the data and make important decisions.

In most cases a SCADA system will consist of a PLC (serving as the master unit) and several RTUs (connecting to field devices) in remote and local location via several communication protocols and links.

Hence, SCADA system are evaluated and judged noth only by the smooth performance of the RTUs, communication links, and the master station (all falling under the umbrella of SCADA system) but also the field devices (both transducers and control devices).

Typical SCADA system (source: electricalengineering portal - https://electrical-engineering-portal.com/scada-dcs-plc-rtu-smart-instrument)

On a more complex SCADA there are essentially five levels - 

1. Field level instrumentation and control devices
2. Marshalling terminals and RTUs
3. Communication systems
4. Master stations
5. The computer processing unit 

The RTU provides an interface to the field devices (analog and digital signals) situated at each remote/local site.

The communication system provides pathway for communication between the master station and the RTU sites.

Example of communication medium RTUs used in communicating with SCADA master station include - radio communication, telephone line, microwave, satellite, and fiber.

The master station and submasters gather data from the various RTUs and generally provide an operator interface for display of information and control of the remote sites.

SCADA System Consideration

- Sequence logic
- Analog loop control
- Ratio and number of anlog to digital points
- Speed of control and data acquisition
- Type of display required
- Expansion capacity
- Application software and modelling
- System redundancy
- Historical archiving requirements
- System consideration

Benefits of SCADA system

- Improved operation of the plant or process resulting in savings due to optimization of the system
- Increased productivity of the personnel
- Improved safety of the system due to better information and improved control
-  Protection of the plant equipment
- Safeguarding the environment from a failure of the system
- Improved energy savings due to optimization of the plant
- Improved and quicker receipt of data so that clients can be invoiced more quickly and accurately

2. Distributed Control System (DCS)

A distributed control system (DCS) is a platform for automated control and operation of a plant or industrial process. A DCS combines the following into a single automated system:  human machine interface (HMI), logic solvers, historian, common database, alarm management, and a common engineering suite.

 There is always a DCS controller(which is the brain of the system) connected to all other smart devices.

The data highway is normally capable of fairly high speeds (typically 1Mbps up to 10Mbps)

DCS Architecture (by Daniele Pugliesi)

The DCS is a system of sensors, controllers, and associated computers that are distributed throughout a plant. Each of these elements serves a unique purpose of data acquisition, process control as well as data storage and graphical display. These individual elements communicate with a centralized computer through the plant's local network. As the brain of the plant, the DCS makes automated decisions based on production trends it sees them in real-time throughout a plant.

Distributed control system (DCS) -(source: electricalengineering portal - https://electrical-engineering-portal.com/scada-dcs-plc-rtu-smart-instrument)

As an example the DCS at a power plant might automatically increase steam generation capacity of multiple turbines in order to keep up with changing demand for electricity during hot summer days and then decrease it as outdoor temperatures cool overnight and demand subsides. Wheres a PLCS could adjust a single unit operation, the DCS can make adjustments to each of a plant's many interacting unit operations.

Importance of DCS System

A comparison of two plants may help to illustrate the importance of the DCS. First, imagine a small municipal waste water treatment facility that employs a dozen control loops. The plant’s engineering staff can easily keep a mindful eye on the performance of such a limited number of controllers. Next, imagine a large refinery that operates 10,000 highly dynamic and interacting control loops. While coordinating control at the treatment facility is relatively easy, the task of orchestrating control at the refinery can be overwhelming without the use of a DCS. Over the years, the line between DCS and PLC has blurred. While the DCS has traditionally been the only solution for achieving safe and efficient plant-wide control, advancement in processing capabilities has allowed the PLC to rapidly take on greater responsibility. In the years to come it can be expected that the PLC and the DCS will become more and more interchangeable.

3. PLC System

A PLC, or programmable language controller, is a compact industrial computer designed to control system processes from one location. You may have heard about PLCs but haven’t fully grasped the concept. PLCs are like the brains of industrial automation equipment. Instead of having several machines or workers managing different systems and equipment, PLC controllers can complete various tasks at once with a built-in I/O module (inputs and outputs). Programmable logic controllers are everywhere and heavily used across many industries. This post will dive into the main components of a PLC, programming languages, types of PLCs, and PLC systems in the future.

They are often used in the implementation of a SCADA RTU as they offer a standard hardware solution, which is very economical.

logic software and solid state electronic input and output modules.

Programmable logic controller (PLC) system

Traffic light signals are a great example of a PLC application. Instead of having someone reading data and pushing buttons, there is now a PLC system in place to control and monitor this function within one housing unit. If programmable logic controllers are relatively a new concept for you, then it might be hard to imagine the many types of possible PLC applications around you. 

4. Remote Unit Terminals

A RTU is a microprocessor based device that monitors and controls field devices, that then connects to plant control or SCADA (supervisory control and data acquisition) systems.

As far as process control, the RTU is probably on equal ground with the PLC. Both controllers can have multiple different types of I/O, different communications modules, and programming of processes that may require little to no operator intervention.
Where the RTU is considered to have some advantages is in environment tolerances, backup power options, and autonomy.

It generally also has the facility for having its configuration and control programs dynamically downloaded from some central station. There is also a facility to be configured locally by some RTU programming unit.

Typical RTU hardware structure 

Programming Advantange

A PLC requires specific software and specific skills and knowledge of ladder logic, structured text, structured text, function block, etc

An RTU can sometimes be programmed through a simple web interface. In other instances, the RTU comes with setup software that can help you to configure input streams to output streams as well as communications.

There are also many RTU's that have preprogrammed modules that can just be applied for the desired function.

This advantage can quickly be turned into a disadvantage.

Some RTUs may be programmed with languages such as Basic, Visual Basic and C#. Of course, these languages require a special skill set just as the PLCs do.

Some RTUs are even programmed in the same languages that PLCs are programmed with such as Ladder Logic and Structured Text.

Environmental Tolerance

A definite advantage is in the environmental tolerances of a RTU.

RTU's are widely used in environments with extreme temperatures and located in remote locations.

And when I say remote, I don't mean within a few meter or few hundred meters, I mean kilometers down dirt roads, on top of mountains, on off shore oil rigs, that kind of remote.

Some RTU have a backup battery and charging circuit, such as solar power, that will allow the RTU to continue to operate even when AC power is lost.

Of course, PLC can have a UPS backup which adds the same benefit but the charging circuit is a definite plus.