Industrial automation instruments come in a wide variety. Based on the process of acquiring, transmitting, reflecting, and processing information, they are categorized into five major types: (1) Detection instruments; (2) Display instruments; (3) Control instruments; (4) Actuators; (5) Centralized monitoring and control devices.
(I) Detection Instruments
During production processes, the temperature, pressure, flow rate, level, and other physical quantities of media within equipment and pipelines fluctuate constantly and are perpetually changing. Detection instruments are used to measure the instantaneous values of these physical quantities.
Based on the specific process parameters being measured, detection instruments can be categorized as follows.
1. Temperature Instruments: Common temperature measurement instruments include glass thermometers, bimetallic thermometers, pressure-type (bulb) thermometers, temperature switches, thermocouples, resistance temperature detectors (RTDs), as well as radiation thermometers such as radiation pyrometers, optical pyrometers, and photoelectric colorimetric pyrometers.
2. Pressure Instruments: Pressure measurement instruments detect pressure, vacuum, and differential pressure. Based on operating principles, they include: - Elastic pressure gauges (further classified by elastic elements: bourdon tube gauges, diaphragm gauges, capsule gauges, pressure switches, etc.); - Sensor-type pressure gauges (e.g., resistive, capacitive, inductive, Hall-effect pressure gauges); liquid column pressure gauges (e.g., U-tube, straight-tube, inclined-tube pressure gauges); and high-accuracy piston pressure gauges typically used for calibrating standard pressure gauges.
3. Flow Meters: Flow measurement instruments come in numerous varieties, with the most widely used currently being throttling devices paired with differential pressure flow transmitters. Common throttling devices include orifice plates, nozzles, and Venturi tubes. Other common flow meters include water meters, rotameters, oval gear flow meters, target flow meters, electromagnetic flow meters, vortex flow meters, Annubar flow meters, and mass flow meters.
4. Level Instruments: Level instruments primarily measure the liquid level of a medium within towers, tanks, or vessels; the interface between two liquids of different densities; or the level of solid materials. The most common liquid level gauges are glass tube level gauges and glass plate level gauges. Others include differential pressure level gauges and buoyancy-type level gauges (such as float level gauges, level switches, float chamber level gauges, buoy level gauges, steel tape level gauges, and tank level weighing instruments). For solid material level detection, instruments include resistive level gauges, capacitive level gauges, level switches, weight-type level detectors, tuning fork level gauges, ultrasonic level gauges, and radioactive level gauges.
5. Component Analysis Instruments: Component analysis instruments are used to verify the composition of process media and determine the content of specific components (or multiple components up to the full composition). Based on operating principles, they include electrochemical analyzers (e.g., conductivity meters, industrial pH meters, zirconia analyzers), thermal analyzers (e.g., thermal conductivity analyzers, thermochemical analyzers, infrared analyzers), magnetic analyzers, photoelectric colorimeters, mass spectrometers, and industrial gas chromatographs.
When installing online component analyzers, sample pretreatment is generally required to ensure parameters like sample state, temperature, pressure, and flow rate meet the instrument's operational conditions. This necessitates a piping system comprising components such as filters, dust collectors, drying vessels, coolers, rotameters, water seals, valves, and piping for general sample pretreatment. For special media (e.g., flue gas samples, high-temperature furnace gas samples, heavy oil analysis sampling, corrosive component sampling, and environmental monitoring sampling), the sampling pre-treatment system is more sophisticated. Such pre-fabricated sampling pre-treatment systems are termed sampling pre-treatment units.
Additionally, certain physical property measurement instruments-such as moisture meters, hygrometers, density meters, concentration meters, turbidity meters, and viscometers-are often classified under component analysis instruments.
6. Mechanical Quantity Instruments: Commonly used industrial mechanical quantity instruments include thickness gauges, thermal expansion detectors, tension detectors, deflection detectors, as well as devices for detecting shaft vibration, shaft displacement, and rotational speed in rotating machinery (such as large steam turbines and compressors). They also include weighing devices (e.g., electronic belt scales, belt deviation and slippage detectors, weighing indicators, and weighing bagging systems).
(II) Display Instruments
These instruments are designed to complement detection instruments by indicating or recording instantaneous values of measured parameters. Examples include moving-coil indicators such as ratio meters and millivolt meters for indication purposes, digital displays, and electronic potentiometers or electronic balance bridges for indicating or recording temperature (electronic potentiometers and balance bridges can also be combined with electric or pneumatic regulators to form composite instruments). Additionally, they include totalizing instruments with flow accumulation capabilities.
(III) Control Instruments
Control instruments both receive measurement signals from process detection instruments and transmitters for display, and issue regulating signals to control the operation of actuators (actuators and control valves), forming a closed-loop control system.
Control instruments can be broadly categorized into analog control instruments and digital control instruments based on signal type.
1. Analog Control Instruments include base-mounted instruments, unit combination instruments (pneumatic/electric), and modular instruments.
(1) Unit combination instruments are divided into distinct units based on their functions within the control system. Each unit exists independently and can be freely combined to form various detection and regulation systems, offering flexible and convenient system configuration. Signal transmission between units employs standardized analog signals. Unit combination instruments were widely used from the 1950s to the early 1970s. They represent truly functionally distributed instruments, where a single instrument performs a specific required function.
It should be noted that transmitter units within unit combination instruments (except temperature transmitters) functionally belong to the category of detection instruments.
Unit combination instruments are further classified into pneumatic and electric types based on their operating energy source.
Pneumatic unit combination instruments: These evolved from earlier pneumatic instruments. They utilize 0.14 MPa compressed air as the operating energy source and employ compressed air at 0.02–0.1 MPa as the standardized signal. Precisely because both their operating energy and signal transmission rely on compressed air, pneumatic unit instruments inherently provide explosion-proof protection when used in refining and chemical production facilities. Their drawback is that pneumatic signal transmission is generally limited to distances under 150 meters. When transmission distances exceed this limit, signal propagation lags occur, affecting the sensitivity of display and regulation.
Pneumatic unit combination instruments comprise 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.
b. Indicating units such as color-band indicators, bar indicators, multi-needle indicators, indicating recorders, and totalizers.
c. Control units including indicating controllers, recording controllers, cascade controllers, and proportional (integral, derivative) controllers.
d. Calculating units such as adders, multipliers, dividers, and ratio calculators.
e. Setpoint unit instruments: Setpoint controllers, time-programmed setpoint controllers, etc.
f. Auxiliary unit instruments: Pneumatic (Q-type) actuators, manual/automatic changeover actuators, high (low)-value selectors, relays, changeover switches, limiters, ratio controllers, load distributors, high-flow filter-regulator valves, etc.
Electric Unit Combination Instruments: Electric unit combination instruments operate on DC power. Due to updates in their basic electronic components, these instruments have undergone three developmental stages: 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 instruments remain widely applied in petroleum refining and chemical production facilities. The electric unit combination instruments discussed here refer exclusively to Type III. Type III instruments operate on a DC 24V power supply. Signal transmission between individual instruments within the control room employs DC 1–5V voltage signals, while communication between control room instruments and field-installed transmitters, control valves, and actuators utilizes DC 4–20mA current signals. To meet diverse explosion-proof requirements, field-installed transmitters and their associated control room input/output units (such as safety keepers and safety barriers) are categorized as either flameproof or intrinsically safe types. Additionally, driven by the advancement of industrial computer control technology, microprocessor-based intelligent unit instruments have emerged in recent years, representing a new generation of electric unit instruments.
Electric unit combination instruments comprise 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 differential temperature) transmitters, intelligent pressure transmitters, and intelligent differential pressure transmitters.
b. Display unit instruments such as single (dual) pointer indicators, color band indicators, single (dual) pointer alarms, single (dual) pen recorders, multi-point indicator recorders, proportional (square root) totalizers, etc.
c. Control unit instruments include indicating controllers, SPC/DDC backup controllers, multi-channel valve position tracking controllers, special function controllers, integrators, differentiators, etc.
d. Calculation unit instruments such as adders/subtractors, multipliers/dividers, 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.
f. Setpoint unit instruments include constant current setters, ratio setters, percentage setters, alarm setters, parameter program setters, and time program setters.
g. Auxiliary unit instruments include electric (D-type) actuators, DDC actuators, safety holders, safety barriers, power distributors, voltage boxes, signal selectors, isolators, inverters, elevators, signal dampers, signal reversers, signal limiters, and rate-of-change selectors.
(2) Modular Integrated Control Instruments
This represents a new series in the evolution of process control instruments, also known as modular integrated control units. Featuring a modular assembly structure, it facilitates flexible configuration of process control systems. Internally, it employs a 0-10V DC voltage signal system and can receive various pneumatic and electric signals (including current, voltage, contacts, pulses, frequency, encoders, etc.) from field detection instruments and sensing elements.
Modular integrated control units comprise the following instruments and components:
a. Input/Output Components: Input conversion modules, output conversion modules, pulse conversion modules, mV/V conversion modules, P/E conversion modules, cumulative power driver modules, 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.), totalization components, alarm components, logic components.
c. Control Components: PID components (proportional, integral, derivative components), dynamic compensation components, tracking components, multi-output interface components, audible-visual control components.
d. Auxiliary and Other Components: Power distribution components, signal distribution components, switching components, setpoint components, relay components, monitoring components.
e. Display and Operation Instruments: Single (dual) pointer indicators, single (dual) pen recorders, triple (quad) pen recorders, trend recorders, handheld controllers, control display operators.
(3) Base-mounted Control Instruments
During the evolution of industrial automation instruments from local detection and display to centralized control, a type of instrument integrating 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-recording regulators with pneumatic actuators and certain local regulators with single-function regulation (e.g., temperature regulators, pressure regulators, differential pressure regulators, flow regulators). Base-mounted control instruments are further categorized by their power source into pneumatic and electric types.
Self-operated regulators are a type of local control instrument. They derive their name from relying on the measured medium as their power source and are thus also called direct-acting regulators. Additionally, because they are integrated with the control valve, self-operated regulators are also referred to as self-operated 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 encompass distributed control systems (DCS), programmable logic controllers (PLC), industrial control computers (IPC), and safety control systems (FSC).
In the 1960s, as industrial production processes became increasingly large-scale and complex, industrial automation control systems were required to handle massive data volumes, perform advanced computational control, facilitate information communication, enable centralized display and operation, achieve higher-level control, and enhance control precision. Conventional analog instruments alone could no longer meet these demands, leading to the adoption of computer control systems that significantly elevated the level of integrated process control. However, the high concentration of control functions also concentrated the risk of accidents. Should the computer control system fail, control, monitoring, and operation would cease, severely impacting production and potentially causing catastrophic system-wide failures.
After the 1970s, the advent of large-scale integrated circuits and microprocessors, coupled with further advancements in control technology, display technology, computer technology, and communication technology, led to the development of new process control systems based on microprocessors and microcomputers, such as the Distributed Control System (DCS). DCS inherits the advantages of conventional analog instruments and computerized control systems. While retaining centralized display and operation along with centralized management, it decentralizes control authority, thereby enhancing the safety and reliability of the control system. This is achieved by distributing microprocessors according to control functions or zones. Each microprocessor-equipped control station can manage several to dozens of loops. Multiple control stations are combined to oversee the entire production process, thus realizing decentralized control and dispersing risks. Building upon this foundation, vast amounts of information are transmitted via data communication cables to a microprocessor-based CRT display and operation station in the central control room. This station centrally displays or records the information. Simultaneously, it coordinates with higher-level computers (process management computers and production management computers) to implement centralized monitoring and management of the production process.
Distributed control systems enable continuous control, batch (intermittent) control, sequential control, data acquisition and processing, as well as advanced control, closely integrating operational management with the production process. These systems also feature self-diagnostic capabilities, capable of inspecting hardware and software components. Upon detecting a fault, they trigger audible and visual alarms while displaying the exact location of the malfunction.
A typical distributed control system comprises field control stations, CRT display operator stations, communication networks, and peripheral devices such as printers.
During subsequent development, the control and communication capabilities of distributed control systems became increasingly refined and standardized. Based on their control functionalities, programmable logic controllers (PLCs) emerged from DCS systems-which primarily focused on loop control-to specialize in sequential control. Originally designed to replace traditional relay-based interlocking alarm systems, PLCs handle both input and output signals as switch signals. They execute logic, sequencing, timing, counting, and arithmetic functions through software programming, making them suitable for complex interlocks. The defining feature of PLCs is their "programmability"-control schemes can be altered simply by modifying the program. Their reliability, flexibility, operational speed, and capacity for complex control schemes far surpass those achievable with relay circuits.
PLCs have evolved rapidly, incorporating analog control capabilities, computational functions, and even features like CRT dynamic graphics display, database management, and file generation. Meanwhile, DCS systems have adopted PLC technical characteristics while enhancing batch processing and sequential control functions. This functional convergence is narrowing the distinction between DCS and PLC, blurring their boundaries. As distributed control systems continue to evolve-particularly through system miniaturization, intelligent field transmitters, standardized fieldbuses, standardized communication networks, mutual integration of DCS and PLC, incorporation of monitoring computers and PCs into DCS systems, and further refinement of system software-they will better adapt to diverse process control requirements and achieve superior technical and economic benefits.
Fieldbus (FCS) is a digital, serial, multipoint, bidirectional data bus installed between field devices and control room automation equipment. Its fundamental concept involves eliminating direct one-to-one connections between control room DCS/PLC stations, intelligent controllers, and field instruments (such as transmitters, control valves, switches) via dedicated I/O channels. Instead, these devices connect to the H2 high-speed channel of the fieldbus through their serial interfaces. They then link to the H1 fieldbus via H2/H1 conversion bridges, enabling communication between H1 and H2 field instruments for process monitoring and detection.
As the fieldbus constitutes the lowest-level communication network interconnecting field devices (including field equipment and field instruments), integrating both field control and field communication functions, its nodes comprise intelligent transmitters (covering temperature, pressure, flow, level, process analyzers, etc.) and intelligent actuators.
Industrial computers are categorized by their control and management functions into basic automation control devices and management computers. Basic automation devices constitute the first level of multi-level control systems, including Distributed Control Systems (DCS), Programmable Logic Controllers (PLC), Direct Digital Control (DDC) devices, and Fieldbus Control Systems (FCS). Process management computers serve as the upper-level machines for basic automation devices, representing the second level of multi-level control. Production management computers are applicable to the third through fifth levels of multi-level control systems.
(IV) Actuators
Actuators, also known as control valves, consist of two parts: the actuator mechanism and the valve body. Based on the power source of the actuator mechanism, they are classified into four major types: pneumatic control valves, electric control valves, hydraulic control valves, and hybrid control valves. Pneumatic control valves are further categorized by their actuator mechanism type into diaphragm-type control valves, piston-type control valves, and long-stroke control valves.
(V) Centralized Monitoring and Control Devices
Centralized monitoring devices utilize sensing elements or detectors to centrally display measured variables or alarm contact signals. Centralized control devices manage actuators according to preset programs by integrating a series of measured variable signals. These systems encompass various data acquisition units, patrol monitoring systems, signal alarm devices, safety detection equipment, industrial television and remote monitoring systems, as well as sequence control units. Centralized monitoring and control devices are generally categorized as follows:
1. Safety Monitoring Devices: These include combustible gas detection and alarm systems, toxic gas detection and alarm systems, flame monitors, automatic ignition systems, combustion safety protection devices, oil leak detection systems, and high-resistance detection devices.
2. Industrial television systems comprise cameras and ancillary equipment (lighting, purging, cooling devices, motorized turntables), displays, and auxiliary equipment (operators, distributors, compensators, switchers).
3. Remote control devices receive input variable signals, process information, display alarms visually, and output control signals to the control terminal.
4. Signal alarm devices encompass various types including flashing signal alarms, intelligent flashing alarm devices, and relay circuit alarm systems.
5. Sequential control devices include relay interlock protection systems, logic monitoring devices, sequential controllers, and intelligent sequential controllers.
6. Data acquisition and loop detection alarm devices include data acquisition units and loop detection alarm instruments.
(6) Other Automatic Control Equipment
This category primarily includes various types of instrument panels (channel-type, cabinet-type, frame-type, panel-type), instrument boxes, operator consoles, insulation (protective) boxes, power supply boxes, etc.
(7) Automation Materials
Automation materials refer to components required for instrument installation, encompassing diverse types such as pressure piping (seamless steel pipes, stainless steel pipes, high-pressure pipes), air supply piping (galvanized steel pipes, brass pipes), pneumatic signal piping (copper pipes, copper tubing cables, nylon tubing cables, connection boxes), electrical conduit materials (welded steel pipes, galvanized steel pipes), valves, flanges, and fittings for various piping systems, electrical equipment materials for automation (cables, wires, junction boxes, electrical equipment and components), instrument cable trays, structural steel materials like angle iron and channel steel for fabricating instrument equipment brackets and supports, heat tracing insulation materials, and anti-corrosion coating materials.