Process control instruments

Author: Chief Editor: Xu Chunshan
Publisher:
Publish Date: 2004-01-01
Features:
Section: Chapter Introduction Process control instruments are important tools for industrial production process automation. These instruments are divided into two major categories: analog process control instruments and digital process control instruments. Although their technical standards and design methods differ, due to the same purpose, certain fundamental concepts are consistent, and the design philosophy is similar. Digital process control instruments have evolved from analog process control instruments, so currently, both digital process control instruments and digital automatic control systems are hybrid analog-digital systems. Therefore, this book first introduces analog process control instruments and then digital process control instruments.
Section: Process Control Instrument Development and Classification
I. Development Process of Process Control Instruments
The development of process control instruments and industrial automation are synchronized, and their development process can be roughly divided into the following three stages:
(1) 1930s-1940s: Applying classical control theory primarily focused on single-variable control, using large-sized base-mounted instruments to achieve decentralized local control, thereby stabilizing product quality, improving labor conditions, and reducing raw material and energy consumption.
(2) 1950s-1960s: Applying modern control theory primarily focused on multi-variable control, optimal control, and adaptive control, using unit-mounted instruments to form centralized-decentralized control systems. Combining unit-mounted instruments, loop detection instruments, and industrial computers to form control systems to adapt to the large-scale and continuous operation of industrial equipment, achieving centralized and optimal control, improving equipment efficiency, and ensuring production safety.
(3) Since the 1970s: Applying large-system theory, using large, medium, and small microcomputers to form hierarchical control systems—briefly referred to as distributed control systems (DCS)—linking single-machine control, coordinated control, optimal control, and management scheduling to achieve comprehensive automation.
The stages of process control instrument development do not have clear time boundaries in different countries. Since the founding of the country, guided by the policy of self-reliance and with the efforts of all industrial automation professionals, China's process control instruments, like other industrial automation instruments, have developed from nothing to something, from basic to advanced, and have basically kept up with international development levels. In the late 1950s to the mid-1960s, China independently designed and produced relatively complete series of base-mounted display and control instruments. During this period, pneumatic unit-mounted instruments and Type I and II electric unit-mounted instruments were also developed, followed later by more advanced Type III electric unit-mounted instruments. China's early DJSK industrial computers were successfully used in industrial control and are direct control instruments with 64 loops. Since the reform and opening-up, many advanced foreign technologies have been introduced, including control technology and process control instruments. On the basis of assimilation and absorption, China has moved from scattered and decentralized development of microcomputer process control instruments to independently designed series of microcomputer process control instruments suitable for China's conditions—DDDZ-S series control instruments. Due to the successive emergence of large-scale, high-efficiency, and critical-parameter new production equipment in many industrial sectors, new requirements have been raised for process control instruments. The main requirements are as follows:
(1) Diversified control functions. According to equipment operation requirements, not only various feedback control functions and multiple adjustment laws but also program control and various interlock protections are needed.
(2) The system should be easy to expand in functionality. Automatic control systems can be improved from simple to complex or changed conveniently with process changes. These requirements demand that control instruments can flexibly form various control systems.
(3) Solving the compatibility issue between analog instruments and computers. With technological advancements, people have found ways to use computers for industrial control, and computer control inevitably involves analog instruments. Therefore, comprehensively considering the compatibility between industrial computers and analog instruments is an important issue.
(4) Improving the reliability of instruments and systems. Many modern large-scale industrial installations operate in critical states, so extremely stringent requirements are placed on the reliability of automatic control. This not only requires high quality and reliability in instruments themselves but also calls for monitoring and protection measures in the system.
(5) Simplifying operation. With the emergence of large-scale, high-efficiency, and critical-process installations, automatic control systems are becoming increasingly large and complex, and the number of control instruments used is also increasing. To address this, the latest achievements in various fields of automation instruments should be comprehensively utilized, such as using program control technology, digital logic technology, character/image technology, and data communication technology to automate the start and stop of main equipment. Digital control technology can significantly reduce the control panel size, facilitating operation and management.
(6) Facilitating systematic installation for engineering. If the entire automatic control system is pre-assembled at the instrument factory, the workload for design and installation units can be greatly reduced, the construction cycle can be accelerated, and installation costs can be minimized.
II. Classification of Process Control Instruments
1. Classification of Analog Instruments
(1) By power source, analog instruments are divided into pneumatic control instruments and electric control instruments. The characteristics of pneumatic control instruments are stable performance, high reliability, and inherent explosion-proof properties, making them particularly suitable for hazardous environments such as petrochemical plants. Due to the development of electronic technology, the explosion-proof issue of electric control instruments has been resolved, enabling their rapid development and widespread application. The rapid development of electric control instruments is also due to the following advantages over pneumatic control instruments:
- Electric signals have no delay in transmission;
- It is easier to implement complex adjustment laws;
- Remote centralized display and operation, making them suitable for large-scale control systems;
- Convenient power supply, making them more suitable for medium and small-scale production.
Although electric control instruments have developed rapidly, pneumatic control instruments still hold their position and development potential, accounting for about one-third of the global sales volume. In particular, pneumatic actuators offer advantages such as safety, reliability, smooth operation, and high power, making them widely used.
(2) By structural form, analog instruments are divided into base-mounted control instruments, unit-mounted instruments, and modular assembly control instruments.
1) Base-mounted Control Instruments
The structural feature of base-mounted control instruments is that they center on indicating and recording instruments, with pneumatic components or electronic circuits added to complete control tasks. Due to their low cost and versatility, they are suitable for small and medium-sized enterprises and single-machine automatic control systems. The XCT series control instruments and TA series electronic regulators produced in China belong to base-mounted control instruments.
2) Unit-mounted Instruments
The structural feature of unit-mounted instruments is that the entire instrument is divided into several units that can independently achieve certain functions, with standardized signals used between units. A small number of these unit instruments can be combined to form various types of automatic detection and control systems with different levels of complexity. Electric unit-mounted instruments are represented by DDZ, while pneumatic unit-mounted instruments are represented by QDZ.
3) Modular Assembly Control Instruments
The structural feature of modular assembly control instruments is that they are composed of complete instrument installations made up of functionally separated modules. They are generally divided into control boxes and operation panels. Several module boxes are inserted into the control box, and several module boards are inserted into the module boxes, with high-density installation to make full use of space. The display and operation panel occupies only a small area. If an electronic screen (image) is used for centralized display, it improves man-machine interaction. The information exchange between modules in the control box is achieved through matrix terminal wiring. Wiring work is done in the matrix terminal wiring box. Modular assembly instruments also incorporate analog and digital technologies and can be used in conjunction with new tools such as industrial control computers, program control devices, and image displays. Therefore, they are particularly suitable for the automation of high-efficiency large-scale equipment. The TF-type control devices and MZ-III-type control devices produced in China belong to modular assembly control devices.