Abstract: This paper proposes a method for cascade control of the liquid level of two linked tanks using an AI808 intelligent instrument. The MCGS configuration software is used to implement real-time configuration and monitoring of the field by a host computer. Keywords: two linked tanks; liquid level control; cascade control; intelligent instrument The Fluid Position Control System Design of Two Tanks Linked Based on Artificial Intelligence Measuring Appliance ZHU Guang et al (Henan Highway Development Limited Company, Xinxiang 453000, China) Abstract: The author proposes a method of cascade control to control two tanks linked using an AI808 artificial intelligence measuring appliance. Simultaneously, the position computer carries out real-time configuration, and the monitoring is realized by the MCGS configuration software. Key words: two tanks linked; level control; cascade control; artificial intelligence measuring appliance 1 Introduction Two linked tanks are widely used in industrial process control. In the control of the water level of the series dual-tank system, the water first enters the first tank and then flows out through the second tank. Compared with a single tank, the addition of a tank makes the response of the controlled variable lag behind in time, i.e., there is a volume delay, which makes the process difficult to control. Cascade control is an effective method to improve the dynamic performance of the regulation process. Due to its advanced control effect, it can greatly overcome the volume delay of the system [1]. The two-step tuning method is adopted, and the tuning process and curve are monitored in real time through MCGS configuration software until the optimal tuning parameters of the main and auxiliary loops are reached. 2 Flowchart of the series dual-tank system The process flow is shown in Figure 1. In Figure 1, a water pump is used as the delivery source to pump the water in the storage tank to the high-level tank. The water inflow can be adjusted by the action of the electric regulating valve. The water can be sent from the high-level tank to the low-level tank by the manual valve, so that the liquid level of the low-level tank is kept at a certain height. During the entire working process, there are corresponding instruments to detect and control the liquid level of the high-level tank and the low-level tank. [b]3 Control requirements and implementation of the control system 3.1 Control requirements for the level of the series dual-tank water tank[/b] According to the process requirements, in order to ensure control accuracy, the system uses the level of the low-level water tank as the main regulating parameter and the level of the high-level water tank as the secondary regulating parameter, forming a series dual-tank cascade control system. The level signal detected by the level sensor of the low-level water tank is compared with the given level value and sent to the main controller. After PID calculation, its output is used as the given value of the secondary controller. It is compared with the level signal detected by the level sensor of the high-level water tank and sent to the secondary controller. After PID calculation, its output controls the opening of the electric regulating valve, controls the size of the inlet flow, and thus controls the level port of the water tank [2]. The structure of the system is shown in Figure 2. 3.2 Structure and implementation of the control system According to the process requirements, considering that the system mainly processes analog signals such as level and flow, the intelligent instrument AI808 is used to process the signals and control the entire system; the configuration software MCGS is used to display and monitor the system [3]. As shown in Figure 2. 3.2.1 Signal Acquisition and Control The AI808 intelligent instrument adopts advanced microcomputer chips and technology, reducing its size and improving reliability and anti-interference performance. The instrument has a measurement accuracy of 0.2 class, a wide input power range of AC85～265V, and is available in various installation sizes. The input uses a digital correction system and self-calibration technology, ensuring accurate and stable measurements and eliminating measurement errors caused by temperature drift and time drift. Multiple alarm modes can be set. When connected to a resistance temperature detector (RTD) input, a three-wire connection is used to eliminate errors caused by lead wires; when connected to a thermocouple input, the instrument has an internal cold junction compensation component; when connected to voltage/current input, the corresponding physical range can be arbitrarily set. It is suitable for precise control of temperature, pressure, flow rate, liquid level, humidity, etc., and has functions such as control, artificial intelligence regulation, alarm, transmission, and communication. It also has manual adjustment, manual self-tuning, and position proportional output, making it very suitable for cascade control of liquid levels. The low-level water tank pressure signal detected by the low-level pressure sensor is converted into a standard signal by the level transmitter and then sent to the main controller AI808 (1). It is compared with the given value of the liquid level. After PID calculation, its output is used as the given value of the secondary controller AI808 (2). Its value is compared with the actual signal of the high-level water tank detected by the high-level water tank pressure sensor (which is also converted into a standard signal by the level transmitter). After PID calculation, its output drives the electric regulating valve to open or close the valve, thereby controlling the liquid level of the low-level water tank to be adjusted within its accuracy range. The parameter settings of each instrument are shown in Table 1. 3.2.2 Upper computer monitoring configuration software This system uses the MCGS5.5 industrial control configuration software of Beijing Kunlun Company. The PC communicates with the intelligent instrument AI808 through the RS232/RS485 converter. The MCGS5.5 configuration software can complete the functions of field data acquisition, real-time and historical data processing, alarm and safety mechanism, real-time monitoring of process, trend curves, etc. 4 Debugging of the control system In order to meet the accuracy requirements of the control system, a two-step tuning method [4] is adopted. First, the secondary loop is tuned. According to the 4:1 attenuation curve method, δs1=83 and Ts1=40s are obtained. The main loop is tuned in the same way to obtain δs2=6.5 and Ts2=30s. Thus, the parameters of the main controller and the secondary controller are obtained as follows: δ1=100, T11=20s, δ2=5, T12=10s, TD2=3s. When they are put into the system, the real-time control curve shown in Figure 3 can be obtained. 5 Conclusion The cascade control scheme adopted by the author has overcome the adverse effects of the capacity delay of the dual-capacity object on the liquid level control and achieved a good control effect. Through the design of the upper computer monitoring configuration interface, the operation and control status of the entire system are more intuitive. At present, the control system has been debugged and put into operation, and the system is running well. References : [1] Jin Yihui. Process Control [M]. Beijing: Tsinghua University Press, 2002. [2] Xie Jianying, Jia Qing. Microcomputer Control Technology [M]. Beijing: National Defense Industry Press, 2003. [3] Liu Leiting, Mao Weiqing. Design of Automatic Control System for Small Circulating Fluidized Bed Boiler [J]. Industrial Automation, 2003, (3): 39-41.