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Introduction to SCADA
The term SCADA stands for Supervisory Control And Data Acquisition. A SCADA system is an automated system which collects data from sensors and instruments placed at strategically chosen remote locations. The data collected is then sent to a central site, and uses a Human Machine Interface (HMI) in order to display on a SCADA host computer. The data is then used to control or monitor the system as required. SCADA systems found in industry can monitor and control up to thousands of I/O points. (Bentek Systems)
Outline of SCADA Procedure and DCS
A SCADA system is not a full control system, but focuses on the supervisory level as it is a software package that is interfaced with the hardware of the system. The SCADA system interfaces with the hardware through the use of Programmable Logic Controllers (PLCs). The control site of the SCADA system performs centralized monitoring and control for field sites through communications networks, including monitoring alarms and processing status data. The information obtained from remote stations is used to drive the remote station control devices, through automated or operator-driven supervisory commands. These control devices, also known as field devices, control local operations such as opening and closing valves and breakers, collecting data from sensors, and monitoring the local environment for alarm conditions. (K. Stouffer, 2006)
Distributed Control Systems (DCSs) are used for the control of various industrial processes such as power generation, water and wastewater treatment and automotive production.
Distributed Control Systems (DCSs) are used to control industrial processes such as electric power generation, oil and gas refineries, water and wastewater treatment, and chemical, food, and automotive production. DCSs are integrated as a control architecture containing a supervisory level of control overseeing multiple, integrated sub-systems that are responsible for controlling the details of a localized process. Product and process control are usually achieved by deploying feedback or feed forward control loops whereby key product and/or process conditions are automatically maintained around a desired set point. To accomplish the desired product and/or process tolerance around a specified set point, specific programmable controllers (PLC) are employed in the field and proportional, integral, and/or differential settings on the PLC are tuned to provide the desired tolerance as well as the rate of self-correction during process upsets. DCSs are used extensively in process-based industries.
Fundamental principles of modern SCADA systems
In modern manufacturing and industrial processes, telemetry is often needed to connect equipment and systems separated by large distances. Such distances can range from a few meters to thousands of kilometers. Telemetry is used to send commands, programs to other remote locations and in turn, it also receives monitoring information.
As remarked in the introduction, SCADA uses telemetry and data acquisition to collect information, transfer it back to the central site, carry out any necessary analysis and control, and then display 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, production and plant systems were controlled using relay. With the introduction of the CPU and other electronic devices, manufacturers incorporated digital electronics into relay logic equipment. One of the most widely used control systems in industry is the PLC or programmable logic controller. With the advancement of technology and the 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.
Figure : PC to PLC or DCS with a fieldbus and sensor
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 systems' requirements grew and became more complex, the need for smaller and smarter systems increased, and therefore 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.
Figure : PC to IED using a fieldbus
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
A SCADA system consists of a number of remote terminal units (RTUs) which use a communication system to collect field data and send that data back to a master station. The master station displays the acquired data and allows the operator to perform remote control tasks. The data, which is very precise and accurate, 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
The master station(s)
The commercial data processing department computer system
An interface is provided by the RTU to the field analog and digital sensors located at each remote site. The remote sites are connected to the master station via a communication. This communication system can be wire, fiber optic, radio, telephone line, microwave and possibly even satellite. Specific protocols and error detection techniques are used for efficient and optimum transfer of data.
The master station (or sub-masters) gather data from the various RTUs and 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.
SCADA software can be divided into two types, proprietary or open. Proprietary software is developed by companies to communicate to their hardware. These systems are sold as 'turn key' solutions. The main disadvantage with this system is that it entirely reliable on the supplier of the system. On the other hand, 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.
Figure : Typical SCADA system
Key features of SCADA software are:
RTU (and PLC) interface
Access to data
Fault tolerance and redundancy
Client/server distributed processing
The twelve golden rules
A few rules in specifying and implementing a SCADA system are listed below:
Apply the 'KISS' principle and ensure that the implementation of the SCADA system is simple.
Ensure that the response times of the total system (including the future expansion) are within the correct levels (typically less than one-second operator response time).
Evaluate redundancy requirements carefully and assess the impact of failure of any component of the system on the total system.
Apply the open systems approach to hardware selected and protocols communication standards implemented. Confirm that these are indeed TRUE open standards.
Ensure that the whole system including the individual components provide a scalable architecture (which can expand with increasing system requirements).
Assess the total system from the point of view of the maximum traffic loading on the RTU, communication links and master stations and the subsequent impact on hardware, firmware and software subsystems.
Ensure that the functional specification for the system is clearly defined as far as number of points are concerned, response rates and functionality required of the system.
Perform a thorough testing of the system and confirm accuracy of all data transferred back, control actions and failure of individual components of system and recovery from failures.
Confirm operators of individual components of the system in the (industrial) environment to which they would be exposed (including grounding and isolation of the system).
Ensure that all configuration and testing activities are well documented.
Ensure that the operational staffs are involved with the configuration and implementation of the system and they receive thorough training on the system.
Finally, although the temptation is there with a sophisticated system, do not overwhelm the operator with alarm and operational data and crowded operator screens. Keep the information of loading to the operator clear, concise and simple.
User Interface (HMI)
A SCADA system includes a user interface, usually called Human Machine Interface (HMI). The HMI of a SCADA system is where data is processed and presented to be viewed and monitored by a human operator. This interface usually includes controls where the individual can interface with the SCADA system.
HMI's are an easy way to standardize the facilitation of monitoring multiple RTU's or PLC's (programmable logic controllers). Usually RTU's or PLC's will run a pre programmed process, but monitoring each of them individually can be difficult, usually because they are spread out over the system. Because RTU's and PLC's historically had no standardized method to display or present data to an operator, the SCADA system communicates with PLC's throughout the system network and processes information that is easily disseminated by the HMI.
HMI's can also be linked to a database, which can use data gathered from PLC's or RTU's to provide graphs on trends, logistic info, schematics for a specific sensor or machine or even make troubleshooting guides accessible. In the last decade, practically all SCADA systems include an integrated HMI and PLC device making it extremely easy to run and monitor a SCADA system.
A human machine interface uses information flow in two directions; from the machine to the human and vice versa. The flows are independent, since their content can be on different levels, but linked, because the automation system interprets an operator action on a control interface as a specifically defined action and, in return, produces information depending on whether the action was successful or not. The HMI interface contains all the necessary functions to control and supervise the operation of the SCADA system. The operator of the HMI must be ready to:
Stop and start processes and procedures manually or set up the system to perform actions automatically
Manage the controls and make changes to needed for regular process run
Monitor the system continually to deal with unforeseen events, such as irregular situations or system failure.
The quality of the design of the human machine interface can be measured by the effort required to notice and understand an event, and by how efficient his response is. This includes:
Employing flashing lights, colour changes and sound alarms to alert the operator when any abnormal changes occur.
Displaying any information in an intelligible and clear manner
Placing and displaying any control buttons or keys in the suitable locations to allow the operator to act swiftly.