Compared to other control systems, SCADA systems offer a wider range of equipment options and greater flexibility. Before selecting equipment, the first step is to determine the type of system to be chosen. Due to the diversity of SCADA system solutions, a thorough analysis is necessary to select a system with a high performance-to-price ratio that meets user needs and ultimately satisfies the end user.
1. Determining the System Type
Generally, SCADA systems typically use commercial computers or industrial control computers as the host computer, which is then configured with a server. The main differences lie in the slave computers and the communication network. Several main types of slave computers include:
(1) PLC or PAC - suitable for applications with relatively few analog quantities and a large number of digital quantities.
(2) Various RTUs - suitable for applications where monitoring points are extremely dispersed and each monitoring point has few I/O points.
(3) Instruments with communication interfaces are suitable for metering-based applications, such as metering and monitoring of thermal power plant heat supply.
(4) PLC and distributed analog acquisition module hybrid system - suitable for analog and digital hybrid systems, where users are sensitive to system price and have low requirements for analog control.
(5) Other various dedicated lower-level controllers.
Of course, for smaller systems, a centralized monitoring approach can be adopted. This involves using commercial or industrial PCs as hardware, configured with various data acquisition boards or remote data acquisition modules. The application software uses general-purpose software such as Visual Basic or Visual C++. Most of these lower-level systems have a serialized, modular, and standardized structure, allowing system designers to freely choose according to requirements and assemble the system like building blocks. This approach can improve system development speed, enhance system technical level and performance, increase reliability, and facilitate system maintenance.
Compared with general computer control systems, SCADA systems have the most diverse and complex communication methods, and contain more communication networks and layers, especially for large-scale SCADA systems.
2. Equipment Selection
The selection of equipment for a SCADA system includes the following parts.
1) Selection of host computer system
The selection of the host computer system mainly involves choosing the monitoring host, operating computer, server, and corresponding network, printing, UPS, and other equipment. There are many computer brands available; you can choose a brand that meets your requirements in terms of CPU frequency, memory, hard drive, graphics card, and monitor. Of course, if higher reliability is required, an industrial PC can be chosen. Generally speaking, industrial PCs have lower configurations than commercial PCs (for the same configuration, industrial PCs are more recent to the market). Designers can make a reasonable selection based on their requirements. Computers in monitoring centers are often equipped with large-screen monitors. Many large-scale SCADA system monitoring and dispatch centers are generally equipped with large-screen display systems or analog screens to facilitate system monitoring and dispatching; however, these devices must be designed and manufactured by specialized manufacturers.
When selecting a host computer, configuration software, databases, and other application software should also be considered to meet the requirements of production monitoring and plant-wide information management for data storage, querying, analysis, and printing.
2) Selection of lower-level machine
Select the appropriate product based on the determined type of lower-level device. The range of lower-level device products is extremely wide, and the vast majority of existing products can meet the functional requirements of general SCADA systems. It is recommended to choose mainstream products from mainstream manufacturers. This ensures reliable maintenance, upgrades, and after-sales service, and provides sufficient technical support and reference materials during system development. Furthermore, these products are widely used and have a large user base, guaranteeing their performance.
When selecting a lower-level device, special attention should be paid to its controller module's memory capacity, operating frequency (scan time), programming method and language support, communication interface, and networking capabilities to ensure sufficient data processing power, control precision, and speed for easy program development and debugging. The selection of the lower-level device should also consider whether the chosen configuration software supports the device.
When selecting I/O devices, pay attention to the number of channels, channel isolation, signal type, and signal level. For analog modules, also consider the conversion rate and accuracy. When selecting digital I/O devices for lower-level systems, for output modules, choose relay modules, transistor modules, or thyristors based on the characteristics of the control device, and pay attention to the voltage level and load requirements for contact current; for input modules, consider whether to select source-type or sink-type devices (if applicable). Additionally, pay attention to the selection of special function modules and communication modules.
In lower-level systems, it is important to ensure isolation between I/O devices and field detection and actuators, especially in chemical and petrochemical environments, where safety barriers or similar devices should be used. For digital inputs and outputs, relays can be used for electrical isolation.
3) Communication network equipment
The selection of communication network equipment in SCADA systems is complex. Firstly, SCADA systems utilize fieldbuses or device-level buses for lower-level machines; fieldbuses for networking lower-level machines; and wired or wireless communications connecting various lower-level machines to the upper-level machine. Especially for large-scale, long-distance communication, data transmission typically relies on fixed-line telephone networks or wireless networks from mobile communication companies, which introduces factors beyond the user's control. For example, communication quality is dependent on the service level of these service providers. Therefore, when selecting communication methods, it is crucial to choose methods and media that are controllable by the user. The selection of communication equipment and media for the communication system must primarily meet the requirements for bandwidth, real-time performance, and reliability in data transmission. For applications with high reliability requirements, redundancy using different communication methods can be considered. For example, redundancy between wired and wireless communication, with wired communication as the primary method and wireless communication as a backup.
4) Instrumentation and control equipment
Instrumentation and control equipment mainly involves the selection of sensors, transmitters, and actuators. The selection of these devices is a crucial factor affecting control accuracy. Based on the characteristics of the controlled object, the type of actuator should be determined. Multiple options should be compared, and the optimal choice should be made considering factors such as the working environment, performance, and price. Measurement instruments can convert signals such as flow rate, speed, acceleration, displacement, and humidity into standard electrical signals. For the same measured signal, various measuring instruments can meet the requirements. Designers can select measurement instruments based on the accuracy requirements of the measured parameters, the range, the type and characteristics of the measured medium, and the operating environment. To reduce maintenance workload, non-contact measuring instruments should be used whenever possible, which is a current trend in instrument selection. For some detection points where only qualitative information is needed, switch-type detection devices, such as level switches, flow switches, and pressure switches, can be used to reduce hardware costs.
The actuator is an essential component of the control system. Its function is to receive control signals from the computer and convert them into the actions of the regulating mechanism, so that the production process can operate normally according to the pre-defined requirements.
Actuators are classified into three types: pneumatic, electric, and hydraulic. Pneumatic actuators are characterized by their simple structure, low price, and fire and explosion resistance; electric actuators are characterized by their small size, wide variety, and ease of use; and hydraulic actuators are characterized by their high thrust and high precision. In addition, various contact and non-contact switches also function as actuators, performing switching actions. When selecting an actuator, it is important to consider whether the controlled system has requirements regarding the actuator's response speed and frequency.
