What are supervisory systems?

1) Introduction

Supervisory systems allow information from production processes or physical installations to be monitored and tracked. Such information is collected via data acquisition devices, which handles them, analyses them, store them, and finally present them to the user. These systems are called SCADA (Supervisory Control and Data Acquisition).

The first SCADA systems, basically telemetric ones, allowed informing periodically the current status of the industrial process by monitoring representative signs of devices’ statuses and measurements via a panel with light bulbs and indexes, with no application interface with the operator.

Nowadays, industrial automation systems use computing and communication technologies to automatize both monitoring and control of industrial processes, collecting data in complex environments (occasionally in different geographical locations) and presenting them to the operator in a friendly way, with elaborate graphic resources (human-machine interfaces) and multimedia content.

To do so, SCADA systems identify tags, which are all numeric or alphanumeric variables involved in the application that can execute computing functions (mathematical, logic, vector, or string operations) or represent input/output points in the process being controlled. In this case, they correspond to the variables in the real process (e.g., temperature, level, flow, etc.), behaving as a link between controller and system.  It is based on the tags’ values that the collected data are presented to the user.

SCADA systems can also check alarm conditions, identified whenever tag value surpasses a predefined range or condition; you can program the recording of values in databases, sound activation, messages, color changes, SMSs, e-mails, etc.

2) Physical components of a supervisory system

A supervisory system comprise the following physical components: sensors and actuators; communication network; and remote (acquisition/control) and central monitoring (SCADA computing system) stations.

Sensors are devices connected to the controlled equipment and monitored by SCADA systems that convert physical parameters, such as speed, water, and temperature levels into analog and digital signals that can be read by the remote station.  Actuators are devices that actuate on the system, connecting and disconnecting the equipment.

The process of data control and acquisition starts in remote stations, PLCs (Programmable Logic Controllers), and RTUs (Remote Terminal Units), by reading the current values of the devices liked to it and their respective controls.  PLCs and RTUs are specific computing units, and are used in several types of installation to read input, perform calculations and controls, an update output. The difference between PLCs and RTUs is that the former’s programming language and input/output control is more flexible, while the latter’s architecture is more distributed between its central process unit and the input/output cards, with higher precision and events sequencing.

The communication network is the platform through which information flows from the PLCs/RTUs to the SCADA system; considering the system’s requirements and the distance to be covered, it can be implemented via Ethernet cables, optical fibers, dial-up and dedicated lines, modem radios, etc.

The central monitoring stations are the main units in SCADA systems, being responsible for gathering information generated by remote stations and acting according to the detected events; they can be centralized into a single computer or distributed to a computer network, in order to allow the collected information to be shared.

Supervisory control system

3) Logical components of a SCADA system

Internally, SCADA systems usually divide their main tasks into blocks or modules, which will allow more or less flexibility and robustness, according to the desired solution.

In general terms, these tasks can be divided into:

• Processing core.
• Communication with PLCs/RTUs.
• Alarm management.
• Historics and databases.
• Internal programming logic (scripts) or controls.
• Graphic interface.
• Reports.
• Communication with other SCADA stations.
• Communication with external/corporate systems.
• Others.

The normal operation of a SCADA system begins in the processes of communication with field devices, whose data is sent to the software’s main core. This core is responsible for distributing and coordinating the flow of this data to the other modules, until they reach the expected format for the system’s operator, in the graphic interface or in the operation console with the process, usually with charts, animations, reports, etc., so that the evolution of statuses of devices and controlled processes is displayed, allowing anomalies reports, action courses, or automatic reactions.

The computing technologies used for developing SCADA systems have evolved greatly in the past years, so that their reliability, flexibility, and connectivity can improve continuously, in addition to including new tools to allow diminishing the time spent setting up and adapting the system to each installation’s needs.

4) Communication modes

The main functionality in any SCADA system is information exchange, which can be:

• Communication with PLCs/RTUs.
• Communication with other SCADA stations.
• Communication with other systems.

Communication with field devices, performed via a shared protocol whose methodology can be ither public domain or restricted, can usually happen via polling or interruption,  and is commonly designated by Report by Exception.

Communication via polling (Master/Slave) causes the central station (Master) to have absolute control over communication, polling each remote station (Slave) sequentially; those remote stations only answer to the central station after the request has been received, i.e., in half duplex. This makes data collection simple, with no collision on the network traffic, and non-smart remote stations. On the other hand, this makes it impossible for remote stations to communicate situations to the central station.

Communication via interruption, on the other hand, occurs when there is a PLC or RTU monitoring its input values and, when detecting significant changes or values that surpass the defined limits, it is responsible for sending information to the central station.  This helps to avoid any unnecessary data transfer (which decreases network traffic); it also allows quick detection of important information, as well as the communication between remote stations (slave-to-slave). One disadvantage in this type of communication is that the central station can only detect connection failures after a certain given period (that is, when system polling occurs), and other methods (or even direct interference from the operator) become necessary to obtain the desired values.

Communication with other SCADA stations can occur via protocols developed by the SCADA system manufactures, or even via known protocols via Ethernet TCP/IP networks, and private or dialed lines.

The Internet has been used more and more as a means of communication for SCADA systems. With the use of web-related technologies, and standards such as Ethernet, TCP/IP, HTTP, and HTML, it is possible to access and to share data both in production areas and in supervision and control areas in different plants.  By using an Internet browser, you can control in real time any machine, located anywhere in the world. The browser communicates with the web server via http protocol and, after the request regarding the intended operation has been sent, it receives the response in an HTML page.

Some of the advantages of using the Internet and its browser as the SCADA’s visualization interface are its simple interaction mode, to which many people are already used, and its low-maintenance system, which only happens at server level.

Communication with other system, such as corporate order, or other data collectors or suppliers, can happen by implementing specific modules, via Databases or other technologies, like XML and OPC.