Introduction of Process Control Instruments

As the key equipment of normal production and operation, automation instruments have a direct impact on the efficiency and safety of industrial production. With the gradual improvement of industrial production automation requirements and the gradual expansion of the scope of application of automation instruments, the configuration of automation instruments and the depth of application of process control technology in the industrial production process determine the level of enterprise production technology. With the support of an automatic instrument and process control system, it can realize real-time control of the operation status of various devices and equipment, and real-time diagnosis of the abnormal status of devices through data analysis, thus improving industrial production efficiency.

Classification and application characteristics of automatic instruments

There are many types of automatic instruments, which can be divided into temperature instruments, pressure instruments, level instruments, flow instruments, etc. according to the measured objects. Automation instrument mainly includes the following parts: sensor, transmitter, display screen, and actuator. In the industrial production process, the original data of the measured object is collected in real-time by the sensor. The transmitter converts the original data into standard signals for access to the control system and displays the measured values on the display screen. For the regulating instrument, the actuator can respond to the control command to adjust the production conditions.

Temperature instrument

In the process of industrial production, it is necessary to monitor the working temperature of each device and equipment in real-time to ensure that the device and equipment operate within the design temperature range and avoid safety accidents. For production links with high-temperature control requirements, temperature control within the optimal range is conducive to reducing production costs and improving product quality. Therefore, the selected temperature-measuring instrument must be reliable and accurate. At present, temperature instruments commonly used in industrial production include thermal resistance and thermocouple. The working principle of thermal resistance is that under the action of temperature, the resistance value of the temperature sensing element changes with the temperature change. The temperature and the resistance value have a linear relationship with high accuracy. 

The thermal resistance is mainly composed of the temperature sensing element, the external protective tube, the junction box, and the fixed device. The working principle of the thermocouple is that the two ends of two conductors with different components are welded to form a loop. When there is a temperature difference between the two ends, the electronic flow in the loop generates an electromotive force, which is only related to the conductor material and temperature difference, so the temperature value can be measured. In practical application, when the on-site temperature is lower than 500 ℃, thermal resistance measurement can ensure high measurement accuracy, and a thermocouple is recommended for temperature measurement in high-temperature areas.

Pressure instrument

In the industrial production process, the importance of pressure monitoring is no less than that of temperature. Pressure instruments can be classified into differential pressure instruments, gauge pressure instruments, and absolute pressure instruments. The pressure instrument is mainly composed of the measuring element body, process connection, isolation diaphragm, pressure sensor, gauge head, etc. The pressure of the measured medium is transferred to the measuring element through the process connection, and then acts on the pressure sensor through the isolation diaphragm and filling fluid, so as to measure the pressure value. 

In the selection of pressure instruments, in addition to instrument accuracy and measuring pressure range, the range ratio is also an important parameter of pressure instruments, which determines the actual range that can be set and the actual application accuracy. At the same time, the influence of seasonal changes in local atmospheric pressure on the zero drift of gauge pressure instruments should be considered in specific applications. The pressure instrument with the Hart protocol is selected, that is, the Hart signal is superimposed on the 4~20mA signal, which is convenient for on-site debugging and maintenance through the Hart manual operator.

Level instrument

In the process of industrial production, compared with temperature and pressure measurement, the measurement of material level is more complex. Its measurement is closely related to the physical properties, state, and storage environment of the measured medium, so the types of material-level instruments are also complex. From the application type, the material level instrument can be divided into the material level switch and a continuous material level meter. According to the measurement principle, the material level switch includes a vibrating rod type material level switch, tuning fork type material level switch, anti-rotation type material level switch, RF admittance material level switch, etc. Continuous level meter includes radar level meter, ultrasonic level meter, heavy hammer level meter, etc.

Taking the vibrating rod level switch as an example, it is mainly composed of probe rods, vibration elements, and electronic sensors. The vibration element is driven by a circuit. When the measured medium contacts the probe rod, the amplitude changes, and the electronic sensor analyzes the amplitude change and converts it into a switch signal. In practical application, the resonant frequency and minimum dielectric density parameters should be focused on. 

At the same time, the sensitivity can be adjusted, the delay alarm and other functions can adapt to the frequently changing conditions of material types, and eliminate false alarms. The radar level meter transmits microwave from the antenna system to the measured medium and is received by the antenna system after being reflected by the surface of the measured medium. The distance between the material and the level meter is calculated through the time difference between microwave transmission and reception, and then the material height is obtained. With the development of measurement technology, the measurement accuracy of radar level meter antenna systems has been significantly improved from horn antenna to plane antenna, microwave frequency from 20GHz low frequency to 80GHz or even 100GHz high frequency, and from pulse wave to continuous wave, and its application scope has been greatly expanded. 

In practical application, in addition to measurement accuracy, attention should be paid to whether the wave velocity angle matches the material bin and whether the minimum dielectric constant matches the material property. At the same time, for the working conditions with dust, the increase of external purging can avoid material adhesion and prolong the maintenance period of the level meter.

Flow instrument

In the process of industrial production, the flow measurement objects include liquid, gas, and steam. Different types of flow meters are used according to the nature, process conditions, and accuracy requirements of the measurement objects. Typical liquid flow meters include electromagnetic flowmeters, vortex flowmeters, etc; Gas and steam flow instruments include orifice flowmeters, rotameter, bar flowmeters, etc. For liquid flow measurement of medium and small pipe diameters, the electromagnetic flowmeter is a commonly used instrument. There are no blocking and moving parts in the pipe, so it will not cause additional energy loss and blockage. It is especially suitable for liquid-solid two-phase fluids, but not for liquids with low conductivity, such as demineralized water.

Integrated application of automatic instrument and process control system

Industrial production is a process operation in which a series of production equipment work together. There is a highly coupled phenomenon in process control, which requires highly technical personnel. With the refinement and large scale of modern industry, the introduction of process control systems to achieve semi-automatic or fully automatic generation has become an inevitable choice. As the "eye" of the process control system, the automation instrument provides all the raw data needed for control. Therefore, the integrated application of automation instruments and process control systems is one of the most effective ways to improve the stability and safety of industrial production, which is mainly reflected in the following aspects.

Data transmission and processing

The IEC international standard signal transmission is adopted between the automation instrument and the process control system to ensure the real-time and integrity of the field data. The signal types include digital signals and analog signals. The copper core cable with a shielding layer is usually used as the signal transmission medium. In a site with strong electromagnetic interference, the computer cable of the split screen+collective screen should be considered to ensure the stability of data transmission. On the control system side, the modules used for data exchange with automatic instruments are IO cards, including AI cards, AO cards, DI cards, DO cards, and special communication cards. 

In order to increase system safety and reliability, each channel of the analog card has its own A/D or D/A converter, and all inputs and outputs are configured with optical or electrical isolation and have an anti-jitter filtering processing function. The digital measuring card adopts relay isolation output to directly drive the field positioner, solenoid valve, actuator, and other automatic instruments, and the relay contact capacity shall adapt to the field instruments. SCADA in the process control system can store all data generated by field automation instruments for a long time, and provide historical data required for production data analysis and accident tracing.

Control logic programming

The integrated application of automatic instrument and process control systems is also reflected in the control logic programming. Computer hardware and software programming technology have been developed and popularized in industrial instruments, providing assistance for the optimization and upgrading of distributed control systems. The process control system takes the programmable controller as the center, and programs various equipment and instruments in a graphical way through the supporting software, so that the complex process control can be realized. For the relatively important process system, the programmable controller is usually configured as one for use and one for standby redundancy, and the failure of any controller will not affect the safety of the processing system and the operation of field instruments at all. Through control logic programming, the operation mode of field equipment and automatic instruments is further optimized to make control more accurate and process parameters more reasonable.

Advanced control functions

The basic process control mainly takes a single instrument or equipment as the control object, that is, to realize the single operation function of instruments and equipment. With large-scale and complex industrial applications, the linkage operation with process subsystems or group equipment as the controlled object has gradually become the mainstream demand, that is, the advanced control function of the process control system. The advanced control functions include feedforward control, feedback control, fuzzy control, and user-defined advanced function algorithm with a specific process as the control object, among which the closed-loop feedback control represented by PID is the most widely used. The three parameters of PID represent proportion, integral, and differential respectively.

In the control mechanism, the larger the proportional band, the slower the adjustment speed, and the greater the deviation, the stronger the adjustment effect; The smaller the integration is, the more sensitive it will be, and the longer the deviation time is, the greater the integration adjustment effect will be; Differential is mainly used to solve the problem of control delay. The greater the differential value or the greater the deviation change rate, the stronger the regulation effect. In different applications, the PID setting parameters need to be adjusted one by one according to the on-site working conditions to achieve a good control effect and improve production efficiency.

Quality Assurance Measures of Automation Instruments in Industrial Process Control

Improve the accuracy of automatic instrument design and selection

As one of the most important links in industrial production, the design and selection of automation instruments should fully understand the working conditions in which they are measured. First, determine the measurement principle and instrument type according to the measurement requirements, and then determine the specification requirements of the instrument according to the dielectric property. Finally, compare the advantages and disadvantages of the instruments that meet the specification requirements of each brand, and finally determine the instrument brand and model. At the same time, the appropriate allowance shall be considered for design and type selection to adapt to process adjustment and upgrading in the later period.

Strengthen routine maintenance of automatic instruments

In the process of industrial production, in order to improve the reliability of automatic instruments and their control systems, eliminate hidden dangers and reduce sudden failures, daily maintenance work should be strengthened, especially for instruments in key processes. Routine maintenance mainly includes routine inspection, regular calibration, and regular evaluation of instrument reliability. At the same time, strengthen the management of instrument spare parts to minimize the loss of sudden failure.

Pay attention to the upgrading and transformation of automation instruments

In the information age led by scientific and technological innovation, electronic technology is changing with each passing day, and the cycle of upgrading automation instruments is getting shorter and shorter. Mastering and reasonably applying the latest measurement technology is one of the important means to improve production efficiency. Therefore, when conditions are met, the upgrading and transformation of automation instruments should be carried out in stages, with objectives and plans.

Conclusion

The application of automatic instruments in industrial process control further improves the reliability and stability of the processing system. With the support of the process control system, real-time control of the operation status of various devices and equipment is realized, and real-time diagnosis of the abnormal status of devices is conducted through data analysis, thus improving industrial production efficiency.