Design and application of control system for Chinese herbal medicine extraction process

A key process in the production of traditional Chinese medicine is the extraction of effective ingredients. It is necessary to ensure the quality of the extracted drug ingredients and improve the efficiency and economy of the extraction process. For different Chinese medicinal materials, it is necessary to select and ensure appropriate extraction process conditions and strictly and automatically extract Chinese medicine according to the correct extraction process procedures. This is a crucial link in the production and process research of traditional Chinese medicine. Our school cooperated with Tianjin Daming Pharmaceutical Equipment Factory to establish the "Chinese Medicine Equipment R&D Center". During the project construction, according to the actual needs of the process, in line with the principles of advancement, reliability and practicality, the design and development of computer process control systems for each unit of the natural medicine pilot production process were completed, and the computer automatic detection and control of various important process parameters in the natural medicine pilot production process were successfully realized. Among them, the extraction unit realized the full process computer control of multiple extraction methods, thus providing a reliable and effective means for researchers to study natural medicine extraction technology and extraction equipment in the future. [1]

2 Main process flow and automatic control requirements of the extraction process

2.1 Extraction process control

(1) Feeding control

The medicinal materials are put into the extraction tank, and 5-10 times of the solvent is added. The amount of solvent added is controlled by the liquid level control method, and the flow chart is shown in Figure 1.

Figure 1 Liquid level control flow chart

(2) Dynamic extraction

Dynamic extraction is the core of the Chinese medicine extraction process. It requires controlling the temperature of the medicine liquid in the tank, keeping the pressure in the tank at normal pressure, and managing the opening and closing of the electromagnetic valve and manual valve in the medicine circulation channel, as well as the start and stop of the medicine circulation pump and hot oil pump. The control structure diagram of the extraction tank is shown in Figure 2.

Figure 2 Extraction tank control structure diagram

The solvents (solvents) commonly used in the extraction process of traditional Chinese medicine include water or ethanol. Different extraction solvents have different control requirements. When water is used as a solvent, the vent valve must always be in an open state during the extraction process, that is, the pressure in the tank is a constant value, and only an alarm is required when an accident occurs (such as blockage of the exhaust channel). When alcohols are used as solvents, the vent valve must always be in a closed state to maintain the tank pressure at a constant value. The adjustment is different from when water is used as a solvent.

2.2 Measurement and control parameters

Detection parameters: temperature in the extraction tank, pressure in the extraction tank, liquid level in the extraction tank, cooling water inlet and outlet temperatures of the cooler, oil outlet and inlet temperatures of the hot oil pump, etc. Control parameters: extraction tank temperature, extraction tank pressure, feed control, discharge control, start and stop control of the circulation pump, hot oil pump, self-priming pump and liquid discharge pump, etc.

2.3 Self-control requirements

According to the characteristics of the uncertainty of the pilot process of this project, the automatic control design of the extraction part requires that the whole process of different extraction modes can be conveniently and flexibly realized under the conditions of limited and fixed equipment and pipelines, that is, it is required to be able to perform both "water extraction" and "alcohol extraction"; both "single tank extraction" and "double tank extraction"; both "circulation extraction" and "non-circulation extraction". The whole process of adding liquid to the extraction tank, heating and maintaining the temperature of the extraction tank, drug leaching, circulation extraction, extraction liquid discharge, until the end of extraction or re-extraction is completed reliably and automatically.

The automatic control circuit of the extraction part mainly includes automatic control of the temperature of the extraction tank; automatic control of the quantitative and liquid addition of the extraction tank; automatic control of the material discharge of the extraction tank; automatic detection of the temperature of the extraction tank and the extraction liquid storage tank; and automatic detection of the liquid level of the extraction liquid storage tank. The number of extractions, extraction temperature and cycle time of a batch of medicinal materials are determined by the process. This scheme uses hot oil to heat the extraction tank with a jacket, boil the medicinal materials in the decoction tank, and circulate the medicinal liquid with a circulation pump.

3 System Features

The overall design and selection of the system are kept in sync with domestic and foreign systems, so as to ensure that the computer measurement and control system used is technologically advanced and in line with international standards. After considering relevant domestic and foreign systems such as Honeywell control system, Yokogawa µXLDCS and SIEMES control system, it was decided to choose SIEMENS control system, thus ensuring the maturity of the system. Another reason for choosing Siemens system is that this system is easy to maintain and expand. In the production process of traditional Chinese medicine, the phenomenon of "foam-induced liquid leakage" often occurs. The CTN-XM series dual-level fully automatic foam elimination control device is used. The foam detection electrode based on the resistance response principle has a strong anti-pollution ability. In the case of scaling due to severe pollution, it can still detect the liquid level position, thereby achieving reliable and effective elimination of foam and achieving the purpose of controlling "liquid leakage". [2]

4 System Structure and Configuration

This paper uses PROFIBUS-DP to build the underlying network of the Chinese medicine extraction process monitoring system. The new fieldbus control system breaks through the defects caused by the closed system of the dedicated network in the DCS system, and turns the closed and dedicated solution into an open and standardized solution. There are dozens of fieldbus standards. Each bus standard has its own characteristics and shows its own advantages in specific application fields. As an international, open, and equipment manufacturer-independent fieldbus standard, the process fieldbus PROFIBUS has been widely used in process control and process industries as a fieldbus solution for fully integrated process and factory automation. PROFIBUS adopts the physical layer and data link layer of the OSI model. When the transmission rate is 9.6Kbps~12Mbps, its transmission distance can reach 100m. When the transmission rate is 1.5Mbps, its transmission distance can reach 400m. The transmission medium of PROFIBUS can be twisted pair or optical cable. It can connect up to 127 sub-stations. PROFIBUS adopts functional modular design, and different application systems are designed using different modules. The underlying system structure is shown in Figure 3. At this level, the system is divided into two layers, namely the equipment control layer and the monitoring layer.

Figure 3 PROFIBUS underlying network structure

3.1 Device Control Layer

Siemens PROFIBUS-DP is used in the equipment control layer to connect various solenoid valves, centrifugal pumps, motors, temperature sensors, pressure sensors, ET200 substations and other functional units into a whole. The temperature sensor uses Pt1O0 platinum resistance temperature sensor , which outputs 4-20mA current signal. It has strong anti-interference ability and is easy to transmit signals remotely. The pressure sensor uses isolated sensor components and integrated circuit component technology to produce steam high temperature resistant pressure sensors, which have the advantages of good linearity, small temperature drift, and good time stability.

The field equipment involved in this layer include: water extraction tank, alcohol extraction tank, liquid storage tank, solvent recovery tank, concentration tank, distillation tower, filter, condenser, heater, etc. The lower actuators that control the above equipment include control switch valve, PID regulating valve, electromagnetic flowmeter, liquid level meter, temperature transmitter, pressure transmitter, self-priming pump, liquid discharge pump and hot oil pump, etc. The underlying PLC hardware configuration: Siemens S7-300 series PLC, CPU model CPU315-2DP, ET200M module 6ES7 l53-2AA02-0XB0, power module 6ES7 307-1KA00-0AA0, 32-point 24V digital input module 6ES7 321-1BL00-0AA0, 16-point relay output module 6ES7 322-1HH00-0AA0, 8-way analog input module 6ES7 331-7KF01-0AB0, 8-way thermal resistor input module 6ES7 331-7PF00-0AB0, 4-way PID module 6ES7 355-0VH10-0AE0, etc.

In this system, an electric furnace is used for heating. Since the heating and heat preservation of the electric furnace are heated by resistance wires, and the cooling is cooled by the natural environment, once the temperature overshoots, it is impossible to use control means to cool it down. It has the characteristics of unidirectional heating, nonlinearity, uneven gain and large hysteresis, and it is difficult to establish a mathematical model. Since the system has large hysteresis and unidirectional heating characteristics, it is easy to cause integral saturation using conventional PID control methods, so it is difficult to obtain satisfactory control effects. To this end, the preliminary mathematical model of the heating furnace is first established using the step response curve method to obtain its approximate amplification factor K, time constant T and lag time r; then the Ziegler-Nichols method is used to adjust the PID parameters, and the segmented variable coefficient incremental PID control strategy is used to achieve control of the heating furnace. The specific method is shown in Figure 4. Before starting to point A, the maximum control amount can be given to make the system heat up quickly; PD control is used between points AB; in the critical steady-state area, in order to eliminate the steady-state error of the system, it is necessary to add integral action and use PID control; when the overshoot is greater than 5%, the output is 0 to stop heating. In this system, the function module FM-355-2C of Siemens' S7-300 series is used to implement PID control.

Figure 4 Schematic diagram of step response curve

3.2 Monitoring Layer

The upper computer part of the monitoring layer: The server uses the Windows 2000 Server operating system, and the monitoring software uses Siemens SIMATIC WINCC6.0 SP1. WINCC (Windows Control Center) configuration software is jointly developed by Siemens and Microsoft. It undertakes data management and collection, alarm, historical trend, data recording and reporting. [3-5] Its multi-level authority management and electronic records meet the requirements of FDA 21 CFR PART 11. The screen of the extraction part includes: static flow chart of the extraction process, dynamic flow chart, control instrument loop, heating part, cooling part, cycle control part, event alarm, parameter setting, report printing, real-time curve and historical curve display, user management, OPC server, remote monitoring management and help part, etc. WINCC can call MicroSoft SQL Server to establish a database through 0DBC technology. The database has powerful functions such as query, deletion, modification, backup, import and export. WINCC can also call Excel to establish data reports through DDE technology.

Lower computer part of the monitoring layer: The lower computer programming software adopts Siemens' STEP 7 software, and realizes modularization of equipment and process through function block FC programming, so that each section of the control program is relatively independent and the process is clear. The orderly classification of data blocks DB makes the data structure more reasonable and data reading and writing safer. The whole system is set up with two sets of automatic and manual operation plans. In the program design, the system action is divided into multiple action segments, so that the system can be "paused" at any time during operation and maintained in the current state; and the system can continue to run in the previous state through the "continue" function. In this way, the system can respond to some emergencies that may occur during operation at any time, reduce program interruptions caused by unexpected situations, and improve the safety and reliability of system operation. [6, 7]

5 Conclusion

The Chinese medicine extraction monitoring system introduced in this article uses PROFIBUS technology to build the underlying network, monitor and control the data of each key process, and implement the tracking of the entire process. The system can not only perform unit operations, but also find the best working conditions. The system has become an effective means for the center's scientific researchers to conduct natural medicine production process research and pilot development, and has achieved good results in practical applications.

The author's innovation is to introduce PROFIBUS technology into the laboratory project construction of the "Chinese Medicine Equipment R&D Center" and use it to build the underlying network of the Chinese medicine extraction process control system, so that laboratory researchers can more conveniently and efficiently engage in the research and pilot development of Chinese medicine production technology.