SCADA

Typical simulation of a SCADA for a process plant, based on a pipe and instrumentation diagram.

SCADA, an acronym for Ssupervisory Control And Data Acquisition (Supervisory Control and Data Acquisition) is a concept used to create software for computers that allows remote control and supervision of industrial processes. It provides real-time feedback with field devices (sensors and actuators), and controls the process automatically. It provides all the information that is generated in the production process (supervision, quality control, production control, data storage, etc.) and allows its management and intervention.

In control theory, feedback is a process whereby a certain proportion of a system's output signal is redirected back to the input. This is often used to control the dynamic behavior of the system. Examples of feedback can be found in most complex systems, such as engineering, architecture, economics, sociology, and biology.

Open and closed loop

There are mainly two types of systems: open-loop or non-feedback systems and closed-loop or feedback systems. Closed-loop systems work in such a way that they make the output go back to the beginning so that the difference with a reference value is analyzed and in a second option the output adjusts until the error is zero or below. a previously defined threshold. Any system whose purpose is to control a quantity such as temperature, speed, pressure, flow, force, position, among other variables, are normally closed-loop. Open loop systems do not compare the controlled variable with a reference input. Each input setting determines a fixed operating position on the control elements (for example with timers).

So, feedback is a mechanism or process whose signal moves within a system and returns to the beginning of the system as in a loop, which is called a "feedback loop". In a control system (which has inputs and outputs), part of the output signal returns back to the system as part of its input; this is called "feedback" or feedback.

Feedback includes all those application solutions that refer to the capture of information from a process or plant, not necessarily industrial, so that, with this information, it is possible to carry out a series of analyzes or studies with which Obtain valuable indicators that allow feedback on an operator or on the process itself, such as:

• Indicators without inherent feedback (do not affect the process, only the operator):
  • Current status of the process. Instant values;
  • Deviation or derivation of the process. Historical and cumulative developments;
    • Measuring parameters that are created necessary

• Inherent feedback indicators (attain the process, then the operator):
  • Generation of alarms;
  • HMI Human Machine Interface (Man-Machine Interface);
  • Decision-making:
    • Through human operation;
    • Automatic (using knowledge-based systems or expert systems).

Schematic of a typical system

This diagram is an example of the application of the SCADA system in industrial areas. These areas can be:

• Monitoring chemical, physical or transport processes in water supply systems, to control the generation and distribution of electricity, gas or pipelines and other distribution processes.
• Production management (facilitates manufacturing programming).
• Maintenance (provides such amounts of interest to evaluate and determine fault modes, MTBF, reliability rates, etc.).
• Quality Control (provides automated data to calculate CP and CPk production stability indices, tolerances, NOK/OK part index, etc.
• Administration (now these SCADA data can be linked to an ERP server (Enterprise Resource Planning or enterprise resource planning system), and integrated as one more module).
• Historical processing of information (through its incorporation into databases).

System Settings

Supervision: The act of observing the work or tasks of another (individual or machine) who may not know the subject in depth, supervising does not mean controlling the other, but guiding him in a context work, professional or personal, that is, for corrective and/or modification purposes.

Automatic: technological science that seeks to incorporate elements of autonomous execution that emulate human behavior or even better.

Main families: automata, robots, motion controls, data acquisition, artificial vision, etc.

PLC: Programmable Logic Controller, Programmable Logic Controller.

PAC: Programmable Automation Controller.

A SCADA system includes input and output signal hardware, controllers, Human Machine Interface (HMI), networks, communications, database, and software.

The term SCADA usually refers to a central system that supervises and controls a complete site or a part of a site that we are interested in controlling (the control can be over machines in general, tanks, pumps, etc.) or finally a system that extends over a long distance (kilometers/miles). Most of the site control is actually performed automatically by a Remote Terminal Unit (RTU), by a Programmable Logic Controller (PLC) and more currently by a Programmable Automation Controller (PAC). Server control functions are almost always restricted to basic site reset or supervisory level capabilities. For example, a PLC can control the flow of chilled water through a process, but a SCADA system can allow an operator to change the control set point for the flow, and allow any alarm conditions to be recorded and displayed. such as loss of flow or high temperature. The control loop feedback is closed via the RTU or PLC; the SCADA system monitors the overall performance of that loop. The SCADA system can also display historical graphs, trends, tables with alarms and events, among other functions. It can and should be subject to permissions and access by users and developers according to their hierarchical level in the organization and the function they fulfill within it. Process monitoring needs:

• Limitations of the visualization of the acquisition and control systems.
• Control software. Closure of control loop.
• Collect, store and view the information.

Human-machine interface

A Human Machine Interface or HMI ("Human Machine Interface") is the device that presents data to an operator (human) and through which he controls the process.

HMI systems can be thought of as a "window of a process". This window can be on special devices such as operator panels or on a computer. HMI systems on computers are also known as HMI or supervisory monitoring and control software (or application). The signals of the process are conducted to the HMI by means of devices such as input/output cards in the computer, PLC's (Programmable Logic Controllers), PACs (Programmable Automation Controller), RTUs (Remote I/O Units) o DRIVER's (motor speed variators). All these devices must have a communication that the HMI understands.

The HMI industry was essentially born out of the need to standardize the way to monitor and control multiple remote systems, PLCs and other control mechanisms. Although a PLC automatically performs pre-programmed control over a process, it is normally distributed throughout the entire plant, making it difficult to collect data manually, SCADA systems do it automatically. Historically PLCs do not have a standard way of presenting information to the operator. The data obtained by the SCADA system starts from the PLC or from other controllers and is done through some type of network, later this information is combined and formatted. An HMI may also have links to a database to provide trends, diagnostic data and management information as well as a schedule of maintenance procedures, logistics information, detailed schematics for a particular sensor or machine, even expert systems. with troubleshooting guide. Since about 1998, virtually all major PLC manufacturers have offered integration with HMI/SCADA systems, many of which use open, non-proprietary communications protocols. Many third-party HMI/SCADA packages offer built-in compatibility with most PLCs.

SCADA is popular because of this compatibility and security. This is used from small scale applications, such as temperature controllers in a space, to very large applications such as the control of nuclear plants.

Hardware Solutions

The SCADA solution often has components of distributed control systems, DCS (Distributed Control System). The use of RTUs or PLCs or lately PACs without involving master computers is increasing, which are autonomous executing simple logic processes. A functional programming language is frequently used to create programs that run on these RTUs and PLCs, always following the standards of the IEC 61131-3 standard. The complexity and nature of this type of programming means that programmers need some specialization and knowledge about the actuators they are going to program. Although the programming of these elements is slightly different from traditional programming, languages that establish procedures are also used, such as FORTRAN, C or Ada95. This allows SCADA system engineers to implement programs to be executed on RTUs. .

System components

The three components of a SCADA system are:

1. Multiple Remote Terminal Units (also known as UTR, RTU or External Stations).
2. Master and Computer Station with HMI.
3. Communication infrastructure.

Remote Terminal Unit (RTU)

The RTU connects to equipment physically and reads status data such as open/closed states from a valve or switch, reads measurements such as pressure, flow, voltage, or current. By the equipment the RTU can send signals that can control it: open it, close it, exchange the valve or set the speed of the pump, start it, stop it.

The RTU can read the status of digital data or analog data measurements and send output digital commands or analog setpoints.

One of the most important parts of a SCADA implementation are alarms. An alarm is a digital status point that has either NORMAL or ALARM value. The alarm can be created at each step that the requirements require. An example of an alarm is the car's 'empty fuel' light. The SCADA operator pays attention to the part of the system that requires it, due to the alarm. They can be sent by email or text messages with the activation of an alarm, alerting the administrator or even the SCADA operator.

Master Station

The term "Master Station" refers to the servers and the software responsible for communicating with the field equipment (RTUs, PLCs, etc.) in these is the HMI software running for the workstations in the control room, or anywhere else. In a small SCADA system, the master station may be on a single computer. In large scale SCADA systems the master station may include many servers, distributed software applications, and disaster recovery sites.

The SCADA system usually presents the information to the operating personnel graphically, in the form of a representation diagram. This means that the operator can see a schematic that represents the plant that is being controlled. For example, a drawing of a pump connected to the pipeline can show the operator how much fluid is being pumped from the pump through the pipeline at any given time, or the liquid level in a tank, or whether the valve is open or closed. Rendering diagrams may consist of line graphs and schematic symbols to represent the elements of the process, or they may consist of digital photographs of the equipment on which the sequences are animated.

The software blocks of a SCADA (modules) allow acquisition, supervision and control activities.

Features

• Settings: allows you to define the working environment of the SCADA, adapting it to the particular application you want to develop.
• Operator's graphic interface: provides the operator with the control and supervision functions of the plant. The process is represented by synoptic graphics stored on the process computer and generated from the editor incorporated into the SCADA or imported from another application during the package configuration.
• Process Module: executes pre-programmed command actions from current values of read variables.
• Data management and file: data storage and processing, so that another application or device can access them.
• Communications: transfer of information between the plant and the hardware architecture that supports the SCADA, and also between it and the rest of management computer elements.

The HMI package for the SCADA system typically includes a drawing program with which operators or system maintenance personnel can change the appearance of the interface. These representations can be as simple as a few on-screen traffic lights representing the current state of a field in current traffic, or as complex as a multi-projector screen representing the positions of all the elevators in a skyscraper or all the commuter trains. a railway. Open platforms like GNU/Linux that were not widely used initially are used because of the highly dynamic development environment and because a client has the ability to accommodate in the field of hardware and mechanisms to be controlled that are usually sold UNIX or with licenses. OpenVMS. Today all the big systems are used in the master station servers as well as in the HMI workstations.

Operational Philosophy

Instead of relying on operator intervention or master station automation RTUs can now be required to operate themselves, performing their own control mostly for security reasons. The master station software requires further analysis of the data before it is presented to operators, including historical analysis and analysis associated with particular industry requirements. Security requirements are being applied to the systems as a whole and even the master station software must implement the strongest security standards in certain markets.

For some installations, the costs that can result from a control system failure is extremely high, it is even possible that there is a risk of injuring people. SCADA system hardware is generally robust enough to withstand extremes of temperature, humidity, vibration, and voltages, but in these installations it is common to increase reliability through redundant hardware and multiple communication channels. A failing part can be easily identified and its functionality can be automatically developed by backup hardware. A failed part can be replaced without interrupting the process. The confidence in each system can be calculated statistically and this state is the meaning of mean time between failures, which is a variable that accumulates times between failures. The calculated result means that the mean time between failures of highly reliable systems can be decades.

Infrastructure and Communication Methods

SCADA systems traditionally have a combination of radios and direct serial signals or modem connections to meet communication requirements, including Ethernet and IP over SONET (fiber optics) is also often used in very large sites such as railways and bus stations. electric power. What's more, the connection methods between systems may even be through wireless communication (for example if we want to send the signal to a PDA, to a mobile phone) and thus not have to use cables.

For the installation of a SCADA to be perfectly used, it must meet several objectives:

1. They must be open architecture systems (capable of adapting according to the needs of the company).
2. They must communicate easily to the user with the plant and rest of the company (local and management networks).
3. They must be simple programs to install, without excessive hardware requirements. They also have to be easy to use.

SCADA Applications

To develop a SCADA system you need an IDE in which to design, among other things:

• the appearance of the SCADA
• the functions and events you need to perform when interacting with your HMI interface
• the operations and calculations to be performed with the acquired data

It also works with the logic controllers established by National Instrument such as LABVIEW and MULTISIM PROTEUS among others, it even establishes connection with microcontrollers such as the ARDUINO, making SCADA a very useful tool for automated control systems. Thus, one of the solutions in SCADA control is to use the application created together with a program to monitor, control and automate analog and digital signals, captured through data acquisition cards. One of the most widely used programs for this purpose is LabView (National Instruments).

• pvBrowser - GPL application for SCADA monitoring with web interface.
• FreeSCADA - Open source application for SCADA projects
• Likindoy Professional free GPL Scada system - Centrologic
• SCADA - Yokogawa FAST/TOOLS SCADA
• Acimut Scada Monitors - Creation of functional SCADA projects by "painting and dragging"
• Scada Argos - SCADA Project for Linux
• Scada Factory Talk View SE - FactoryTalk View SE by Rockwell Automation.
• AppVision de Prysm Software - Platform based on.NET with an advanced development environment.

Practical example of a SCADA system for beginners on the subject

A SCADA is used to supervise and its main objective is to measure in order to correct.

We have a chemical process, which can be from a gelatin factory, to an antibiotic factory, that we want to supervise. What we will put in the production plant will be PLCs, HMIs, etc., which is called Level I, or basic level of automation. The data obtained by these industrial hardware are transported through a bus or several buses to a server (server), which is the supervisor, the one that controls, through the aforementioned SCADA. This sending of data can be done through ethernet, for example.

The server, in turn, sends the data to a database in order to store the information (to work with it, create error or alarm histories). This database can be integrated into the server's own hard drive. It is also possible that the server sends the information to another PC, PDA, Telephone, Internet, that is, transmits the information to other operating systems, in which clients, shareholders, bosses, supervisors, can access the information.

Information in transit can be concentrated in Remote Terminal Units, and travels in space through satellite communication that is regularly processed by a Control and Monitoring Center, and operational movements can be made with confirmations from specific instruments (In place). At higher levels of SCADA, there is cooperation with management systems such as ERP SAP, in which data can be recorded to analyze and predict the behavior of a production line.