As production scales continue to expand and production technologies advance, there is an increasing demand for higher levels of automation in production processes. Consequently, industrial instruments have undergone a development process from nonexistence to existence, from simplicity to complexity, and from single-function to multi-function capabilities. From the earliest on-site measurement and display instruments for temperature (e.g., glass thermometers), pressure (e.g., U-tube pressure gauges), flow (e.g., glass rotor flow meters), and liquid level (e.g., glass tube level gauges), as well as simple on-site controllers, the industry has gradually evolved toward remote transmission, centralized display, and remote control capabilities. In addition to the increasing variety of detection elements and instruments for measuring various parameters, the development of process control instruments has been rapid, evolving from pneumatic unit combination instruments, electric unit combination instruments, electronic integrated control devices to industrial computer control systems.
Industrial automation instruments are diverse, and based on the process of information acquisition, transmission, reflection, and processing, they are categorized into five major types: (1) Detection instruments; (2) Display instruments; (3) Control instruments; (4) Actuators; (5) Centralized monitoring and control devices.
Detection instruments
During production processes, the temperature, pressure, flow rate, level, and other physical quantities of the medium at different locations in equipment and pipelines change rapidly and are constantly in flux. Detection instruments are used to measure the values of these physical quantities at each instant.
Based on the different process parameters being measured, detection instruments can be classified into the following types:
1. Temperature instruments: Commonly used temperature measurement instruments include glass thermometers, bimetallic thermometers, pressure-type (thermowell) thermometers, temperature switches, thermocouples, thermistors, as well as radiation-type high-temperature meters such as optical high-temperature meters and photoelectric colorimetric high-temperature meters.
2. Pressure instruments: Pressure measurement instruments are used to detect pressure, vacuum, and pressure differential. Based on their working principles, they can be classified into: elastic pressure gauges (further divided into bourdon tube pressure gauges, diaphragm pressure gauges, capsule pressure gauges, pressure switches, etc.); sensor-type pressure gauges (such as resistive, capacitive, inductive, and Hall-effect pressure gauges); liquid column pressure gauges (such as U-tube, straight tube, and inclined tube pressure gauges); and piston pressure gauges, which are highly accurate and typically used for calibrating standard pressure gauges.
3. Flow meters: Flow measurement instruments come in a wide variety, with the most widely used currently being those consisting of a throttling device and a differential pressure flow transmitter. Commonly used throttling devices include orifice plates, nozzles, and Venturi tubes. Other commonly used flow meters include water meters, rotor flow meters, elliptical gear flow meters, target flow meters, electromagnetic flow meters, vortex flow meters, Annubar flow meters, and mass flow meters.
4. Level meters: Level meters primarily measure the liquid level of a specific medium or the interface between two liquids of different densities within towers, tanks, or containers, as well as the material level of solid materials. The most common liquid level gauges are glass tube liquid level gauges and glass plate liquid level gauges. Other types include differential pressure liquid level gauges and buoyancy-type liquid level gauges (such as float ball liquid level gauges, liquid level switches, float drum liquid level gauges, float buoy liquid level gauges, steel tape liquid level gauges, and tank liquid level weighing instruments). For solid material level detection, there are resistive level gauges, capacitive level gauges, level switches, weight-type level gauges, fork-type level gauges, ultrasonic level gauges, and radioactive level gauges.
5. Component analysis instruments: Component analysis instruments are used to determine the composition of process media and measure the concentration of a specific component (or multiple components up to the entire composition). Based on their working principles, they can be classified into electrochemical analyzers (such as conductivity meters, industrial pH meters, zirconia analyzers, etc.), thermal analyzers (such as thermal conductivity analyzers, thermal chemical analyzers, infrared analyzers), magnetic analyzers, photometric colorimeters, mass spectrometers, and industrial gas chromatographs.
When installing online component analyzers, it is generally necessary to pre-treat the samples to ensure that their state, temperature, pressure, flow rate, and other parameters meet the operational requirements of the analyzers. Therefore, a piping system composed of components such as filters, dust collectors, drying vessels, coolers, rotameters, water seals, valves, and piping must be configured to perform general pre-treatment of the samples. For certain special media (such as flue gas samples, high-temperature gas samples like furnace gas, heavy oil analysis samples, corrosive component samples, and environmental monitoring samples), the sampling pre-treatment system is more comprehensive. Such pre-treatment systems in finished form are referred to as sampling pre-treatment devices.
Additionally, some physical property measurement instruments, such as moisture meters, humidity meters, density meters, concentration meters, turbidity meters, and viscosity meters, are often classified under component analysis instruments.
6. Mechanical quantity instruments: Commonly used mechanical quantity instruments in industry include thickness gauges, thermal expansion detectors, tension detectors, deflection detectors, and devices for detecting shaft vibration, shaft displacement, and rotational speed in rotating machinery (such as large steam turbine compressors), as well as weighing devices (such as electronic belt scales, belt deviation and slippage detection devices, weighing display instruments, and weighing bagging devices).
Display instruments
These instruments are used in conjunction with detection instruments to indicate or record the instantaneous values of measured parameters. Examples include moving-coil indicators such as ratio meters and millivolt meters, digital display instruments, and electronic potentiometers and electronic balance bridges (which can be combined with electric or pneumatic regulators to form composite instruments) for indicating or recording temperature, as well as cumulative-type instruments with flow accumulation functionality.
Control Instruments
Control instruments not only receive measurement signals from process detection instruments and transmitters for display but also issue control signals to regulate the operation of actuators (actuator mechanisms and control valves), thereby forming a closed-loop control system.
Control instruments can be broadly categorized into two types based on signal type: analog control instruments and digital control instruments.
1. Analog control instruments include base-mounted instruments, unit combination instruments (pneumatic, electric), and assembled instruments.
(1) Unit combination instruments are divided into different units based on their functions in the control system. Each unit instrument exists independently and can be arbitrarily combined into different detection and regulation systems as needed, offering flexible and convenient system configuration. Signal transmission between units uses a unified standard signal (also known as an analog signal). Unit combination instruments were widely used from the 1950s to the early 1970s and represent truly functionally distributed instruments, meaning that a single instrument is used to perform a specific required function.
It should be noted that the transmitter units in unit combination instruments (except for temperature transmitters) functionally belong to the category of detection instruments.
Unit combination instruments are further classified into pneumatic unit combination instruments and electric unit combination instruments based on their working energy source:
Pneumatic unit combination instruments: Pneumatic unit combination instruments evolved from the original pneumatic instruments. These instruments use compressed air at 0.14 MPa as their working energy source and employ compressed air at a pressure of 0.02 to 0.1 MPa as the unified signal. Since both their working energy and signal transmission utilize compressed air, pneumatic unit instruments inherently possess explosion-proof properties when applied in petroleum refining and chemical production facilities. However, their drawback is that pneumatic signal transmission distances are generally limited to within 150 meters; when transmission distances exceed this limit, signal propagation delays occur, affecting the sensitivity of display and regulation. Pneumatic unit combination instruments include the following unit instruments:
a. Transmitter units (i.e., transmitters) include pressure transmitters, differential pressure transmitters, target-type flow transmitters, built-in orifice plate flow transmitters, single (or dual) flange differential pressure (level) transmitters, internal (or external) float level transmitters, and temperature transmitters, among others.
b. Display unit instruments such as color tape indicators, bar indicators, multi-needle indicators, indicator recorders, and totalizers.
c. Control unit instruments include indicator controllers, record controllers, cascade controllers, and proportional (integral, derivative) controllers.
d. Calculation unit instruments such as adders, multipliers, and ratio calculators.
e. Setpoint unit instruments, such as setpoint controllers and time program setpoint controllers.
f. Auxiliary unit instruments, such as pneumatic (Q-type) actuators, manual/automatic switch actuators, high (low) value selectors, relays, switches, limiters, ratio controllers, load distributors, and high-flow filter-regulator valves.
Electric unit combination instruments: Electric unit combination instruments use DC power as their operating energy source. These instruments have undergone three developmental stages due to the updating of their basic electronic components: Type I (vacuum tube circuits), Type II (transistor circuits), and Type III (linear integrated circuits). Currently, Types I and II have been phased out and are no longer in use. Type III is still widely applied in petroleum refining and chemical production facilities. The electric unit combination instruments discussed here refer exclusively to Type III. Type III electric instruments are powered by a DC 24V supply. Signal transmission between individual instruments in the control room uses DC 1–5V voltage signals, while communication between control room instruments and field-installed transmitters, control valves, and actuators employs DC 4–20mA current signals. To meet different explosion-proof requirements, field-installed transmitters and their connected control room input/output units (safety keepers, safety barriers) are further classified into explosion-proof type and intrinsically safe type. Additionally, due to the development needs of industrial computer control technology, intelligent unit instruments based on microprocessors have been developed in recent years, becoming a new category of electric unit instruments.
Electric unit combination instruments include the following units:
a. Transmitter units (i.e., transmitters) include pressure transmitters, differential pressure transmitters, target-type flow transmitters, built-in orifice plate flow transmitters, single (or dual) flange differential pressure (level) transmitters, internal (or external) float level transmitters, temperature (or temperature difference) transmitters, intelligent pressure transmitters, and intelligent differential pressure transmitters, among others.
b. Display unit instruments include single (or dual) needle indicators, color tape indicators, single (or dual) needle alarms, single (or dual) pen recorders, multi-point indicator recorders, proportional (or square root) integrators, etc.
c. Control unit instruments include indicator controllers, SPC/DDC backup controllers, multi-channel valve position tracking controllers, special-function controllers, integrators, and differentiators, etc.
d. Calculation unit instruments include adders, multipliers, dividers, and square root calculators, etc.
e. Conversion unit instruments include current signal converters, pulse/voltage converters, frequency/current converters, impedance converters, function converters, electrical/pneumatic converters, and pneumatic/electrical converters, etc.
f. Setpoint unit instruments include constant current setpoint controllers, ratio setpoint controllers, rate setpoint controllers, alarm setpoint controllers, parameter program setpoint controllers, and time program setpoint controllers, etc.
g. Auxiliary unit instruments include electric (D-type) actuators, DDC actuators, safety holders, safety barriers, distributors, voltage boxes, signal selectors, isolators, inverters, elevators, signal dampers, signal reversers, signal limiters, and rate-of-change selectors, among others.
(2) Modular Integrated Control Instrument
This is a new series in the development of process control instruments, also known as a modular integrated control device. It adopts a modular assembly structure, enabling flexible and convenient configuration of process control systems. The system internally uses a 0-10V DC voltage signal system and can receive various pneumatic and electric signals (including current, voltage, contacts, pulses, frequency, and encoding) from field detection instruments and detection elements.
Modular integrated control devices include the following instruments and components:
a. Input/output components: input conversion components, output conversion components, pulse conversion components, mV/V conversion components, P/E conversion components, cumulative power driver components, etc.
b. Signal processing components: Signal buffering components, relay buffering components, signal generation components (slope generation components, timing components, etc.), analog calculation components (multiplication/division components, square root components, addition components, function components, limiting components, signal selection components, etc.), accumulation components, alarm components, and logic components.
c. Regulation components: PID components (proportional, integral, derivative components), dynamic compensation components, tracking components, multi-output interface components, and audio-visual control components.
d. Auxiliary components and other components: power distribution components, signal distribution components, switching components, setpoint components, relay components, and monitoring components.
e. Display and operation instruments: Single (dual) pointer indicators, single (dual) pen recorders, three (four) pen recorders, trend recorders, handheld controllers, control display and operation units.
(3) Base-mounted regulating instruments
During the development of industrial automation instruments from local detection and display to centralized control, a type of instrument that integrates measurement, display, and regulation functions emerged. We refer to this as a base-mounted regulating instrument or simply a base-mounted instrument. Examples include indicating and recording regulators with pneumatic regulators and some local regulators with single regulation functions (such as temperature regulators, pressure regulators, differential pressure regulators, and flow regulators). Base-mounted regulating instruments are further classified into pneumatic and electric types based on their power source.
Self-acting regulators are also a type of local regulating instrument. They are named for their reliance on the measured medium as their power source and are therefore also called direct-acting regulators. Additionally, since they are integrated with their control valves, self-acting regulators are also referred to as self-acting control valves. Common self-operated regulators include self-operated temperature regulators, self-operated pressure regulators, and self-operated flow regulators.
2. Digital control instruments
Digital control instruments include distributed control systems (DCS), programmable logic controllers (PLC), industrial control computers (IPC), and safety control systems (FSC).
In the 1960s, with the large-scale and complex nature of industrial production processes, industrial automation control systems were required to handle large amounts of data, perform advanced computational control, facilitate information communication, achieve centralized display and operation, and enhance control precision. Conventional analog instruments could no longer meet these requirements, leading to the adoption of computer control systems, which further improved the comprehensive control level of production processes. However, as control functions became highly centralized, the risk of accidents also became highly concentrated. If the computer control system malfunctioned, control, monitoring, and operation would become impossible, causing significant disruptions to production and potentially leading to major accidents.
After the 1970s, with the advent of large-scale integrated circuits and microprocessors, and further developments in control technology, display technology, computer technology, and communication technology, new process control systems based on microprocessors and microcomputers were developed, such as the Distributed Control System (DCS). DCS inherits the advantages of conventional analog instruments and computer-based control systems. While retaining centralized display and operation, as well as centralized management, it decentralizes control authority, thereby further enhancing the safety and reliability of the control system. This is because DCS distributes microprocessors according to control functions or control areas. Each control station equipped with a microprocessor can control several to dozens of loops, and by combining multiple control stations, the entire production process can be controlled, thereby achieving decentralized control and dispersing risks. Based on this, a large amount of information is transmitted via data communication cables to the central control room's microprocessor-based CRT display and operation station, where this information is concentrated for display or recording. Simultaneously, in conjunction with upper-level computers (process management computers and production management computers), the production process is subject to centralized monitoring and management.
Distributed control systems can achieve continuous control, batch (intermittent) control, sequential control, data acquisition and processing, and advanced control, closely integrating operational management with the production process. Distributed control systems also have self-diagnostic functions, enabling inspection of system hardware and software. Upon detecting a fault, they issue audible and visual alarms and display the location of the fault.
A distributed control system typically consists of field control stations, CRT display and operation stations, communication networks, and peripheral devices such as printers.
In its subsequent development, the control communication functions of distributed control systems became increasingly refined and standardized. Based on the emphasis of their control functions, programmable logic controllers (PLCs) were separated from distributed control systems (DCSs), which primarily focused on loop control. The original purpose of PLCs was to replace traditional relay-based interlocking alarm systems. Their input/output signals are all switch signals, and they use software programming to execute functions such as logic, sequence, timing, counting, and calculation, making them suitable for more complex interlocking systems. The key feature of PLCs is their "programmability"; simply changing the program can alter the control scheme. Its reliability, flexibility, operational speed, and the complexity of its control schemes far surpass those of relay circuits.
PLCs have developed rapidly, enhancing their analog control functions, computational capabilities, and even incorporating CRT dynamic graphic displays, database management, and file generation. Meanwhile, DCS systems have adopted PLC's technical features, strengthening batch processing and sequential control functions. This functional overlap between the two systems is narrowing the distinction between DCS and PLC, making their boundaries increasingly blurred. As distributed control systems continue to evolve, particularly in terms of system miniaturization and micro-sized designs, intelligent field transmitters, standardized field buses, standardized communication networks, mutual integration between DCS and PLC, the incorporation of monitoring computers and PCs into DCS systems, and further refinement of system software, distributed control systems will become increasingly adaptable to various process control requirements and achieve better technical and economic benefits.
Fieldbus (FCS) is a digital, serial, multi-point, bidirectional communication data bus installed between production site devices and automatic control devices in the control room. Its basic concept is that control stations, intelligent regulators, and other devices in the control room no longer need to connect to field instruments (such as transmitters, control valves, switches) via their respective input/output (I/O) channels, but instead connect them to the H2 high-speed channel of the fieldbus via their respective serial interfaces, and then connect them to the H1 fieldbus via an H2/H1 bridge, enabling communication between H1 and H2 field instruments for monitoring and detecting the production process.
Since the fieldbus is the lowest-level communication network interconnecting field devices (field equipment and field instruments), integrating field control and field communication functions, the nodes of the fieldbus communication network are intelligent transmitters (including temperature, pressure, flow, level, process analyzers, etc.) and intelligent actuators.
Industrial computers are classified into basic automation control devices and management computers based on their control and management functions. Among these, basic automation devices constitute the first level of multi-level control, including distributed control systems (DCS), programmable logic controllers (PLC), direct digital control devices (DDC), and fieldbus control systems (FCS). Process management computers serve as the upper-level machines of basic automation devices, belonging to the second level of multi-level control; production management computers are applicable to the third to fifth levels of multi-level control.
Actuators
Actuators, also known as control valves, consist of two parts: the actuator mechanism and the valve. Based on the power source of the actuator mechanism, they are classified into four major categories: pneumatic control valves, electric control valves, hydraulic control valves, and hybrid control valves. Pneumatic control valves are further divided into diaphragm-type control valves, piston-type control valves, and long-stroke control valves based on the form of their actuator mechanism.
Centralized Monitoring and Control Devices
Centralized monitoring devices utilize detection elements or sensors to centrally display measured variables or alarm contact signals; centralized control devices control actuators according to pre-set programs using a series of measured variable signals. Centralized monitoring and control devices include various data acquisition devices,巡回检测装置, signal alarm devices, safety detection devices, industrial television and remote control devices, and sequential control devices. Centralized monitoring and control devices are generally classified into the following categories:
1. Safety monitoring devices include flammable gas detection and alarm devices, toxic gas detection and alarm devices, flame monitors, automatic ignition devices, combustion safety protection devices, oil leakage detection devices, and high-resistance detection devices, etc.
2. Industrial television systems consist of cameras and their auxiliary equipment (such as lighting, purging, cooling devices, and motorized turntables), displays, and auxiliary equipment (such as controllers, distributors, compensators, and switches).
3. Remote control devices receive input variable signals, process the information, display alarms on screens, and output control signals to the control end.
4. Signal alarm devices include flashing signal alarms, intelligent flashing alarm devices, relay circuit alarm systems, and other types of signal alarm devices.
5. Sequential control devices include relay interlock protection systems, logic monitoring devices, sequential control devices, and intelligent sequential controllers.
6. Data collection and patrol detection alarm devices include data collection devices and patrol detection alarm instruments.
Other automatic control equipment
This category of equipment primarily includes various types of instrument panels (channel-type, cabinet-type, frame-type, panel-type), instrument boxes, control consoles, insulation (protection) boxes, power supply boxes, etc.
Automation materials
Automation materials refer to the materials required for instrument installation, which are diverse in type, such as pressure-conducting piping (seamless steel pipes, stainless steel pipes, high-pressure pipes), air supply piping (galvanized steel pipes, brass pipes), and air signal piping (copper pipes, copper cable pipes, nylon cable pipes, connection boxes), electrical piping materials (welded steel pipes, galvanized steel pipes), valves, flanges, and fittings in various piping systems, electrical equipment materials for automation (cables, wires, junction boxes, electrical equipment, and components), instrument cable trays, angle steel, channel steel, and other structural steel materials used to manufacture instrument equipment brackets and supports, heat tracing insulation materials, and corrosion-resistant coating materials, etc.