In modern industrial production and numerous technological application scenarios, control instruments play an extremely crucial role. They are like precise commanders, regulating various complex systems in an orderly manner. Essentially, a control instrument is a device that automatically controls the controlled variables. It accurately compares the measured signals with the set values, calculates based on specific control laws for the deviation signals, and finally outputs the calculation results as specified signals to achieve precise control over the entire process.
Control instruments come in a wide variety of classifications. In terms of energy sources, there are electric, pneumatic, and hydraulic control instruments. Electric control instruments are widely used in the industrial field due to their high efficiency, precision, and ease of integration with modern electrical systems. Pneumatic control instruments, on the other hand, hold a certain market share because of their safety in environments with high explosion – proof requirements and their adaptability to certain special processes. Hydraulic control instruments play an important role in situations where high torque and high – power output are required. Structurally, they can be divided into base – mounted and unit – combined types. All components of base – mounted control instruments are closely connected through an inseparable mechanical structure and are installed in a single enclosure. Just one instrument can handle a series of tasks such as measurement, recording, and control in simple automation systems. Common examples include temperature controllers and pressure controllers. They are compact in structure and concentrated in function, suitable for relatively independent small – scale systems with less complex control requirements. Unit – combined controllers, however, consist of multiple units such as 变送 (transmitter), regulation, operation, display, and execution. Standard signals are used for communication between these units. This modular design allows for flexible combination of various units according to different needs, enabling the construction of control systems of different complexities. As a result, it greatly improves the versatility and flexibility of control instruments, and is widely applied in large – scale industrial automation production lines and complex industrial process control systems.
There are various sophisticated control laws embedded within control instruments, which are the core of achieving precise control. In the field of chemical automation, common basic control laws include proportional action (P), integral action (I), and derivative action (D). In proportional action, the change in the controller’s output is in strict proportion to the change in the deviation. The larger the deviation, the stronger the control action.
In situations such as liquid – level control where strict elimination of residual deviation is not required, proportional action alone can achieve good control results. In integral action, the rate of change of the controller’s output is proportional to the deviation. The longer the deviation exists, the stronger the control action. It is mainly used in situations where high requirements for eliminating residual deviation are imposed. However, it is rarely used alone in the chemical industry. In derivative action, the controller’s output is proportional to the rate of change of the deviation. It can excel in situations where accelerating the regulation process is necessary, and is also seldom used alone in the chemical industry. In practical applications, control laws are often a clever combination of proportional action and other actions. For example, for the control of general parameters such as flow rate, proportional – integral action (PI) is commonly used. It combines the advantages of proportional action’s rapid response to deviation and integral action’s elimination of residual deviation. For objects with large inertia, such as temperature and composition control, proportional – integral – derivative action (PID) is usually adopted. This combination can more comprehensively deal with complex control situations and achieve more accurate and efficient control.
Control instruments are widely applied in industrial production and many other industries. They can cooperate seamlessly with various sensors and transmitters to achieve accurate measurement and clear display of physical quantities such as temperature, pressure, liquid level, volume, and force. Moreover, they can coordinate with various actuators to perform PID regulation and control on electric heating equipment, electromagnetic, and electric valves. They also have alarm control and data acquisition functions. In the petrochemical industry, the precise control of key parameters such as reaction temperature and pressure by control instruments is directly related to product quality and the safety and stability of the production process. In the pharmaceutical industry, they ensure the accuracy of raw material ratios during drug production and the precision of finished product packaging dosages, safeguarding drug quality and patient safety. In the food processing industry, they are used for accurate metering of ingredients and finished products, ensuring strict compliance with food quality and safety standards. From large – scale industrial production to fine – scale pharmaceutical and food processing, control instruments are everywhere, providing solid support for the efficient and stable operation of various industries.
