SCADA for Industry

 Introduction and a brief history of SCADA

This manual is designed to provide a thorough understanding of the fundamental concepts and the practical issues of SCADA systems. Particular emphasis has been placed on the practical aspects of SCADA systems with a view to the future. Formulae and details that can be found in specialized manufacturer manuals have been purposely omitted in favor of concepts and definitions.

This chapter provides an introduction to the fundamental principles and terminology used in the field of SCADA. It is a summary of the main subjects to be covered throughout the manual.

SCADA (supervisory control and data acquisition) has been around as long as there have been controlling systems. The first ‘SCADA’ systems utilized data acquisition by means of panels of meters, lights, and strip chart recorders. The operator manually operating various control knobs exercised supervisory control. These devices were and still are used to do supervisory control and data acquisition on plants, factories, and power generating facilities. The following figure shows a sensor to panel system.

The sensor to panel type of SCADA system has the following advantages:

• It is simple, no CPUs, RAM, ROM or software programming needed

• The sensors are connected directly to the meters, switches and lights on the

panel

• It could be (in most circumstances) easy and cheap to add a simple device like

a switch or indicator

The disadvantages of a direct panel to sensor system are:

• The amount of wire becomes unmanageable after the installation of hundreds

of sensors

• The quantity and type of data are minimal and rudimentary

• Installation of additional sensors becomes progressively harder as the system

grows

• Re-configuration of the system becomes extremely difficult

• Simulation using real data is not possible

• Storage of data is minimal and difficult to manage

• No off site monitoring of data or alarms

• Someone has to watch the dials and meters 24 hours a day

Fundamental principles of modern SCADA systems

In modern manufacturing and industrial processes, mining industries, public and private utilities, leisure and security industries telemetry is often needed to connect equipment and systems separated by large distances. This can range from a few meters to thousands of kilometers. Telemetry is used to send commands, programs and receives monitoring information from these remote locations.

SCADA refers to the combination of telemetry and data acquisition. SCADA encompasses the collecting of the information, transferring it back to the central site, carrying out any necessary analysis and control and then displaying that information on a number of operator screens or displays. The required control actions are then conveyed back to the process.

In the early days of data acquisition, relay logic was used to control production and plant systems. With the advent of the CPU and other electronic devices, manufacturers incorporated digital electronics into relay logic equipment. The PLC or programmable logic controller is still one of the most widely used control systems in industry. As need to monitor and control more devices in the plant grew, the PLCs were distributed and the systems became more intelligent and smaller in size. PLCs and DCS (distributed control systems) are used as shown below.

The advantages of the PLC / DCS SCADA system are:

• The computer can record and store a very large amount of data

• The data can be displayed in any way the user requires

• Thousands of sensors over a wide area can be connected to the system • The operator can incorporate real data simulations into the system

• Many types of data can be collected from the RTUs

• The data can be viewed from anywhere, not just on site

The disadvantages are:

• The system is more complicated than the sensor to panel type

•Different operating skills are required, such as system analysts and

programmer

• With thousands of sensors there is still a lot of wire to deal with

• The operator can see only as far as the PLC

As the requirement for smaller and smarter systems grew, sensors were designed with the intelligence of PLCs and DCSs. These devices are known as IEDs (intelligent electronic devices). The IEDs are connected on a fieldbus, such as Profibus, Devicenet or Foundation Fieldbus to the PC. They include enough intelligence to acquire data, communicate to other devices, and hold their part of the overall program. Each of these super smart sensors can have more than one sensor on-board. Typically, an IED could combine an analog input sensor, analog output, PID control, communication system and program memory in one device.

The advantages of the PC to IED fieldbus system are: • Minimal wiring is needed

• The operator can see down to the sensor level

• The data received from the device can include information such as serial

numbers, model numbers, when it was installed and by whom

• All devices are plug and play, so installation and replacement is easy

• Smaller devices means less physical space for the data acquisition system

The disadvantages of a PC to IED system are:

• More sophisticated system requires better trained employees

• Sensor prices are higher (but this is offset somewhat by the lack of PLCs) • The IEDs rely more on the communication system

1.3 SCADA hardware

A SCADA system consists of a number of remote terminal units (RTUs) collecting field data and sending that data back to a master station, via a communication system. The master station displays the acquired data and allows the operator to perform remote control tasks.

The accurate and timely data allows for optimization of the plant operation and process. Other benefits include more efficient, reliable and most importantly, safer operations. This results in a lower cost of operation compared to earlier non-automated systems.

On a more complex SCADA system there are essentially five levels or hierarchies: • Field level instrumentation and control devices

• Marshalling terminals and RTUs

• Communications system

• The master station(s)

• The commercial data processing department computer system

The RTU provides an interface to the field analog and digital sensors situated at each remote site.

The communications system provides the pathway for communication between the master station and the remote sites. This communication system can be wire, fiber optic, radio, telephone line, microwave and possibly even satellite. Specific protocols and error detection philosophies are used for efficient and optimum transfer of data.

The master station (or sub-masters) gather data from the various RTUs and generally provide an operator interface for display of information and control of the remote sites. In large telemetry systems, sub-master sites gather information from remote sites and act as a relay back to the control master station.

1.4 SCADAsoftware

SCADA software can be divided into two types, proprietary or open. Companies develop proprietary software to communicate to their hardware. These systems are sold as ‘turn key’ solutions. The main problem with this system is the overwhelming reliance on the supplier of the system. Open software systems have gained popularity because of the interoperability they bring to the system. Interoperability is the ability to mix different manufacturers’ equipment on the same system.

Citect and WonderWare are just two of the open software packages available in the market for SCADA systems. Some packages are now including asset management integrated within the SCADA system. The typical components of a SCADA system are indicated in the next diagram.

Key features of SCADA software are: • User interface

• Graphics displays

• Alarms

• Trends

• RTU (and PLC) interface • Scalability

• Access to data

• Database

• Networking

• Fault tolerance and redundancy

• Client/server distributed processing

References:

Practical SCADA for Industry

David Bailey BEng, Bailey and Associates, Perth, Australia

Ct“y„ aˆywx MIPENZ, BSc(Hons), BSc(Elec Eng), IDC Technologies, Perth, Australia