Analysis of Input and Output Signals of Automatic Control System

In the instrument automation engineering design, in order to make the instrument and control equipment selected for the design work normally, in addition to the factors of the equipment itself, there is another factor that is easily overlooked, that is, the signal matching and transmission between the control system and the instrument. The following editor will share with you what the consequences will be if the input and output signals of the automatic control system do not match. For the convenience of description, we refer to all instrument control equipment that receives and/or outputs signals as centralized control equipment: control systems such as DCS, PLC, SIS, etc., secondary instruments (such as regulators, indicators, recorders, flash alarms, etc.) device, etc.) etc. The centralized control equipment should receive on-site instrument signals, such as temperature sensors, flow, pressure, liquid level transmitters and other signals; the centralized control equipment should output control signals to the field actuators, such as regulating valves, frequency converters, etc.; between the centralized control equipment There are also mutual transmission of signals, such as between the safety barrier and the input card of the control system or the panel mounted instrument. The signal input to the centralized control device is called the input signal, and the signal output from the centralized control device is called the output signal. When the output signal of one instrument or device is used as the input signal of another instrument or device, the output and input signals must match each other. For example, there is a 2088 diffused silicon pressure transmitter on site that outputs 4~20mA to the DCS system, so the 4~20mA analog input card of the DSC system should be selected to match. What happens if the signals don't match? (1) The centralized control equipment cannot receive the signal, or the received signal causes the centralized control equipment to display incorrect information, so that the control system or the control loop cannot operate normally; (2) The worst case may damage the centralized control equipment The components (components) of the control system, such as the switch input card of the control system, receive dry contact signals, and send it 220VAC active contact signals, which will cause a certain channel or even the entire card to be damaged. The signal is the link between the control devices, and it can also be said to be the language of the dialogue between the control devices. If the language is different, they cannot communicate with each other, which means that the entire control system or control loop cannot operate normally. Therefore, when designing and selecting field instruments and centralized control equipment, it is very important to match the type of signal input to the centralized control equipment and the type of signal output from the centralized control equipment to the next-level control equipment. There are many types of signals in the automatic control system. , people are very easy to confuse the concepts of these signal types in practical applications. Let's talk about the commonly used signal types and selection principles. 1. Common signal types and meanings 1. Analog signal An analog is a physical quantity that is continuous in time and quantity, and the signal it represents is an analog signal. Any value of the analog quantity in the continuous change process is a specific and meaningful physical quantity, such as temperature, pressure, current and so on. Commonly used analog signals are: thermal resistance, thermocouple, 4~20mA, 1~5VDC, mV, etc. Analog signals usually come from field instruments, such as platinum thermal resistance, pressure, flow, liquid level transmitters, etc.; centralized control equipment will also send analog signals to control valves, frequency converters, etc.; there are also analog signals between centralized control equipment Communication, such as signals between panel mounted instruments. 2. The switch signal generally refers to the 'on' and 'off' states of the contacts. In computer equipment, '0' or '1' is also used to represent the state of the switch. The switch quantity is divided into active switch quantity signal and passive switch quantity signal. The active switch quantity signal refers to the state of 'on' and 'off' with power supply, usually the power supply is 220VAC, 24VDC; passive switch quantity signal refers to When the switch is in the 'on' and 'off' states, there is no power supply, which is commonly known as a dry contact in the jargon. For the control system, usually the switch input card receives passive switch signal, and the switch output signal can be active or passive according to the actual application. On-off signals are usually sent by on-site instruments, such as temperature switches, pressure, flow, level switches, position switches, etc.; centralized control equipment sends on-off signals to actuators, such as on-off valves, solenoid valves, etc. Switch signal exchange, such as panel-mounted instrument sends signal to flash signal alarm, etc. 3. Digital signal The digital quantity, that is, the discrete quantity, refers to the quantity that is scattered and has no intermediate value. For example, the value that a switch can take is discrete, it can only be on or off, and there is no intermediate situation. Therefore, digital quantities are discrete physical quantities in time and quantity, and the signals represented by them are digital signals. Digital quantities are signals composed of 0 and 1, such as cyclic binary codes. 4. Pulse quantity Signal pulse quantity is the signal quantity in which the instantaneous voltage or current jumps from a certain value to another value. After quantification, its constant and regular changes are digital quantities. In industrial applications, some flow meters can output pulse signals. For example, oval gear meters usually use the output pulse signals. If it changes from 0 to a fixed value and remains unchanged, it is a switch value 5. Communication is the transmission of information, which refers to the transmission and exchange of information from one place to another, the purpose of which is to transmit messages, According to different communication protocols, there are HART, RS-485, etc.
come in many forms, like customized level indicator and customized level indicator, and they all provide efficient solution for our customized level indicator needs.
Guangdong Kaidi Energy Technology Co., Ltd. also maintains a friendly, fair, and creative work environment, which respects diversity, new ideas, and hard work.
level gauge is sold in oversees market and has high reputation. Besides, our products are sold with reasonable prices.