First, the classification of control devices and instruments
There are a wide variety of control devices and instrument functions, and there are many classification methods. Different classifications can be performed according to different principles. For example, according to the energy classification used by the instrument, it can be divided into pneumatic instrument, electric instrument and hydraulic instrument (less application); according to the instrument combination, it can be divided into the base instrument, unit instrument and comprehensive control device; Installation forms can be divided into field instruments, panel mounted instruments and rack mounted instruments; with the development of microprocessors, whether or not microprocessors have been introduced into the instrument can be classified into smart meters and non-intelligent meters; according to the form of the meter signals, Divided into analog instruments and digital instruments.
1. According to the instrument use function classification
According to the different role of the instrument in the process of information transmission, it can be divided into the following five categories:
(1) Test Instruments
The main function of the instrumentation is to obtain on-site information and make appropriate conversions. During the production process, the measuring instrument measures certain process parameters, such as temperature, pressure, flow, level, and material composition, and converts them into electrical signals (voltage, current, frequency, etc.).
(2) Display instrument
The function of the display instrument is to display the information obtained by the instrumentation, including various analog and digital quantities of electricity, pneumatic indicators, recorders and integrators, as well as industrial televisions, image displays, and so on.
(3) Centralized control device
Including a variety of tour detectors, tour control devices, data processors, electronic computers, instrumentation control panels and consoles.
(4) Control instruments
The control instrument can perform various operations on the input signal as needed, such as proportional, integral, differential, and the like. Control instruments include various electric and pneumatic controllers as well as digital control instruments.
(5) Actuators
The actuator can accept the output signal of the control instrument or a direct instruction to operate or control the production process. Actuators include various pneumatic, electric, hydraulic actuators or control valves.
2. According to the instrument work energy classification
According to the energy used, the instrument can be divided into pneumatic instruments, electric meters and hydraulic instruments. At present, the first two are commonly used.
(1) Pneumatic instruments
This type of instrument uses air as its energy source, has a simple structure, works reliably, and has strong anti-interference ability against temperature, humidity, electromagnetic field, and radioactivity, and can be used for fire protection and explosion protection.
(2) Electric meters
This type of instrument uses electricity as an energy source, and the signals are easily connected. It is suitable for long-distance transmission and control, and is easy to connect with a computer. Electric meters have made great progress in fire prevention and explosion protection, which is more conducive to the safe use of electric meters.
3. Classification according to instrument structure
According to the structure of the instrument, it can be divided into the base type control instrument, the unit combined control instrument, the component assembly control instrument, and the distributed/decentralized instrument.
(1) Base Control Instrument
The base-type control instrument is mainly composed of indicating and recording instruments, and some control mechanisms are added, that is, the detection, control, and display functions are designed within a single entity. The general structure of this type of instrument is relatively simple and inexpensive. It can not only indicate or record certain process variables, but also has control functions. Therefore, it is simple to install and easy to use, but it is generally poor in generality and is only suitable for small-scale, simple control systems. The currently used XCT series of dynamic coil control instruments and TA series of simple regulators are such instruments.
(2) Unit combined control instrument
The unit combined control instrument divides the whole set of instruments into several units that can independently realize certain functions, and each unit uses a unified signal to contact each other. When used, the units can be selected and combined according to the needs of the control system, so as to form a variety of automatic detection and control systems with different degrees of complexity. Its characteristics are flexible use, strong universality, and ease of use and maintenance. It is suitable for automatic control of various enterprises. The widely used unit combined control instruments include electric unit combination meter (DDZ type) and pneumatic unit combination meter ((2 type).
The types of units include a transmission unit, an execution unit, a control unit, a conversion unit, an operation unit, a display unit, a given unit, and an auxiliary unit.
1) Transmitter unit. It can convert various measured parameters such as temperature, pressure, flow, liquid level and other physical quantities into the corresponding standard unified signals (4~20mA, 0~10mA or 20~100kPa) and transmit them to the receiving instrument or device for indication. , record or control. The transmission unit includes a temperature transmitter, a pressure transmitter, a differential pressure transmitter, a flow transmitter, and a liquid level transmitter.
2) Conversion unit. The conversion unit converts electrical signals such as voltage and frequency into standard unified signals, or converts between standard unified signals so that different signals can be used in the same control system. The conversion unit includes a DC millivolt converter, a frequency converter, an electric/gas converter, a gas/electrical converter, and the like.
3) Control unit. The control unit compares the measurement signal from the transmission unit with the given signal and gives the control signal according to the deviation to control the actuator action. Control units include: proportional-integral-derivative controllers, proportional-integral controllers, differential controllers, and controllers with special features.
4) Operation unit. The operation unit adds, subtracts, multiplies, divides, squares, squares, etc., the !TL standard unified signals, and is suitable for various parameters such as integrated control, ratio control, and temperature and pressure compensation calculation of flow signals. The operation unit includes an adder, a multiplier and a divider, a squarer, and the like.
5) Display unit. The display unit is used to indicate, record, alarm and accumulate various measured parameters for use by the operating personnel to monitor the operating conditions of the system. The display unit includes a pointer, a recorder, a proportional totalizer, an alarm, and the like.
6) Given unit. It will output the standard unified signal, which is sent to the control unit as a given value of the controlled variable to achieve the fixed control. The given unit includes: constant current setter, customizer, ratio setter, and time program setter.
7) Execution unit. The execution unit changes the control amount according to the control signal output by the control unit. Execution units include: quarter-turn actuators, linear actuators, and pneumatic diaphragm regulators.
8) Auxiliary unit. Auxiliary units are instruments that are added to meet certain requirements of automatic control systems, such as sending, switching, remote control, and so on. Auxiliary units include: manipulators, dampers, limiters, safety barriers, etc.
(3) Component-assembled control instruments
A component-integrated control instrument is a set of instruments (or devices) that are functionally separated and structurally modularized. It is mainly based on analog devices, using both analog and digital technologies. The entire instrument (or device) is composed of a control cabinet and a console. The control panel is equipped with component boards with various functions. It is mounted with high density and has a compact structure. This type of control instrument (or device) is particularly suitable for automatic control systems of large and medium-sized enterprises that require various complicated controls and centralized display operations. The domestically produced TF-type, MZ-m-type, and SPEC200 assembly meters are all such control instruments.
(4) Distributed/Distributed Instruments
Distributed/decentralized instruments use computers or microprocessors as their core components. They have undergone three stages of centralized computer control, distributed computer control, and fieldbus-based distributed computer control. In the first two stages, the measurement transmission and execution unit still uses analog meters, except that the adjustment unit uses digital meters, ie, digital regulators, programmable logic controllers, or industrial control machines. Therefore, it is an analog and digital hybrid meter. . In fieldbus-based distributed computer process control, digital communications have been extended to the field due to the adoption of a fully digital, two-way transmission, multi-branch structure of the communication network. The communication protocol is designed to be standardized, standardized, and open. The various control systems not only realize interconnection, interchange, and interoperability through the field bus, but also facilitate centralized management and information integration.
4. Classification according to use
According to the use of the instrument, it can be roughly divided into indoor and outdoor meters. In the process of use, according to the characteristics of the production process, the protection of the instrument, fire prevention, explosion protection, lightning protection, and anti-corrosion issues will generally be involved. For example, the explosion-proof problem is to choose the right explosion protection measure, ie, Intrinsic Safety (Intrinsically Safe) or Explosion-proof. Display instruments can be divided into recording instruments and indicating instruments, analog instruments and digital instruments. Record instruments can be divided into single-point records and multi-point records (indicating that there can be single-point and multi-point). Adjusting the instrument can be divided into the base type adjusting instrument and the unit combined regulation instrument. Due to the introduction of microprocessors, there are also programmable regulators and fixed program regulators. The actuator consists of an actuator and a regulating valve. The actuators are divided into pneumatic actuators, electric actuators and hydraulic actuators according to energy sources.
Second, the development of control devices and instruments
As early as the mechanization era, people began to have automated instruments by replacing mechanical labor with machinery. The boiler water level regulator and steam turbine governor that was invented at that time had promoted and promoted the first industrial revolution. Later, in order to reduce physical labor, people gradually developed a variety of detection and control instruments as "eyes" and "hands and feet" to help people observe and manipulate production equipment, and gradually automate the production process, thus entering the era of automation. With the rapid development of electronic and computer technologies, people have also developed “smart†meters that can imitate “heads†and have the ability to memorize and analyze. This not only extends and expands the functions of “eyes†and “hands and feetâ€, but also Reducing and replacing part of human brain work, and thus entering the era of intelligence.
Control devices and instruments have experienced the development stages of pneumatic instruments, electric meters and digital instruments in the actual industrial production process applications. The structure of the control system has undergone analog electrodynamic instrumentation control, direct digital control, digital regulator control, decentralized control systems, and fieldbus control systems. The operational form of the control system is from the initial base-type on-site operation to the dashboard operation of the central control room, and then to a graphically centralized operation.
With the continuous improvement of the degree of automation, people have also put forward higher requirements for the functions of automated instruments. Therefore, for the development of automated instruments, digitization, intelligence, and systematization are the main directions for future development. Specific performance in the following areas:
1) The signals of the instrument and system change from analog to digital.
2) The instrument design adopts functional components and the structure tends to be modular, which facilitates the combination of instrument control systems and the development of diversified products.
3) Actively apply electronic technology and microcomputer technology to replace the traditional mechanical structure and operation control in the instrument and develop intelligent new instruments with excellent performance.
4) Using new materials and new sensing elements, develop a new generation of physical sensors, digital sensors and smart transmitters.
5) Improve the traditional structural design of various types of actuators, improve the performance of the regulating valve, and achieve compactness and light weight. Development of multi-function, regulators, valve positioners and converters for special applications, and development of digital actuators.
6) Develop an integrated control management system that develops an integrated control and management system that integrates process control, electrical control, supervisory control, optimization decision making, and production management.
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