Introduction And 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 information 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 control 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