DCS Basic Question

1. What is DCS?

DCS is the English abbreviation of Distributed Control System, which is also called Distributed Control System in the automatic control industry.

2. What are the characteristics of DCS?

DCS is a highly integrated product of computer technology, control technology, and network technology. DCS usually uses several controllers (process stations) to control many control points in a production process, and each controller is connected through a network and can exchange data. The operation adopts a computer operating station, which is connected to the controller through the network to collect production data and deliver operating instructions.

3. What is the structure of DCS?

From the structural division, DCS includes process level, operation level, and management level. The process level is mainly composed of process control stations, I/O units, and field instruments, which are the main part of system control functions. The operation level includes operator station and engineer station to complete system operation and configuration. The management level mainly refers to the factory management information system (MIS system).

4. Who executes the control procedures of DCS?

The DCS control decision is completed by the process control station, so the control program is executed by the process control station.

5. What is the composition of the process control station?

The DCS process control station is a complete computer system, mainly composed of power supply, CPU, network interface, and I/O hardware.

6. What is I/O?

The control system needs to establish signal input and output channels, which is I/O (input-output). An I/O module card has one or more I/O channels for connecting sensors, actuators, transmitters, etc. That use for whether deliver command and/or receive feedback from field instrument.

7. What is an I/O unit?

Usually, a process control station is composed of several racks/cabinets (depend on the manufacture's design), and each rack/cabinet can be placed with a certain number of modules. The rack/cabinets where the CPU is located is called the DPU (distributed processing unit). In general, there can only be one (or 2, the other for backup) CPU unit in the same process station.

8. How is the I/O unit and the CPU unit connected?

In general, the I/O unit and the CPU are connected through a field bus. 

9. What is a Fieldbus?

Fieldbus is a digital network applied to the process control field. It not only includes the exchange of process control information, but also the exchange of equipment management information. Through the field bus, various intelligent devices (smart transmitters, adjustment methods, analyzers, and distributed I/O units) can easily exchange data, and the process control strategy can be completely implemented at the field device level. At present, the most commonly used Fieldbus is Profibus. The application of Fieldbus technology can conveniently connect various related intelligent devices and I/O units distributed in the control field to form a control system.

11. What is the unit for expressing network transmission speed?

The network transmission speed is generally expressed in baud rate (bps) as the unit, which means the number of digits of binary numbers transmitted per second. Different networks generally have different baud rates, and the same network can achieve different baud rates using different network cables. Using optical fiber has higher baud rates compare with twisted pair cable. So, optical fiber sends the signal faster than twisted pair cable. In addition, the length of the transmission distance will affect the transmission speed. The longer the distance, the slower the speed.

12. What is configuration?

The process of defining a system through dedicated software called configuration. The process of defining the arrangement position and type of each module of the process station is called process station hardware configuration. The process of defining process station control strategies and procedures is called process control configuration. The process of defining operator station monitoring procedures is called operator station configuration. The process of defining the system's network connection mode and each station address is called network configuration.

13. What is PLC?

PLC is a programmable logic controller.

14. Can PLC be used for process control?

The early PLC was only used to complete some electrical logic control and switching values. The current PLC has greatly improved its performance, especially its ability to process analog signals, so now PLC can be used for process control.

15. What are analog and digital quantities?

Analog quantity refers to a continuously changing signal (such as 4~20mA, 0~5v); digital quantity is a signal with only switch status.

16. How many I/O signals are classified?

Commonly used I/O signals are generally divided into AI, AO, DI, and DO. AI - analog input signal; AO - analog output signal; DI - digital input signal; DO - digital output signal.

17. What is the openness of DCS?

The openness of DCS refers to the ability of DCS to easily connect with third-party systems or equipment through different interfaces and obtain its information. This kind of connection is mainly realized through the network. The use of universal and open network protocols and standard software interfaces is the guarantee of the openness of DCS.

18. What is system redundancy?

In some applications that require high system reliability, the design of DCS needs to consider hot backup (standby mode), that is system redundancy. This means that some key modules or networks in the system have one or more backups in the design. When some problems occur, the system can automatically switch to backup through special software or hardware, thus ensuring uninterrupted operation of the system. Commonly designed redundancy methods include CPU redundancy, network redundancy, and power supply redundancy. In extreme cases, some systems will consider full system redundancy, which also includes I/O redundancy.

19. What is I/O spare?

Different from redundancy, I/O spare is just that the number of I/O in the system is greater than the amount of application needed. This spare is only quantitative, and the main purpose is to make the system have the possibility of adding control signals in the future.