外文翻译基于的锅炉水位控制新版系统.doc

Boiler level control system based on ControlLogix5550 PLC
Abstract-This paper is a research design based on EFPT process control device. In the design，actual industry field has been simulated and corresponding modeling has been carried on for the boiler level system. Then the appropriate PID parameter has been sorted out and ControlLogia5550PLC has been used to control the entire boiler level system.
At last， a corresponding control interface has been established and the boiler level has been under a safe and accurate control

Keywords：EFPT，PID，Modeling，Boiler level;

1 Introduction
The task of the industrial boiler level control is to maintain a dynamic balance by controlling the water flow and evaporation，so that the drum level can be maintained in the technological level，which is a necessity for ensuring safe operation and also one of the main indicators of the boiler's normal operation. Water level which is too high will affect the effect of the steam-water separation，but too low it is will break ring cycle or even cause boiler explosion. To ensure a safe and efficient production，the boiler level must be strictly controlled in maintaining constant or changing only according to a certain rule.
Using Logix5550 PLC with analogy I/O modules，launched by Rockwell Automation Company as controllers，and EFPT process control experimental device as control object，this system have brought the boiler water level under an accurate control in a mini boiler system with sensors and actuators that used in industrial production.
Fig. 1 Boiler level setting value adjustment system
2 System Overview
This system is composed of an EFPT process control device，an inverter，a Logix5550 PLC and a computer. EFPT process control device is a simulated heating and water supply and drainage system for a micro-small boiler. It realizes process control in a mini boiler system with sensors and actuators used in industrial production. The actuator includes not only measuring appliance，but also AC inverter，heating controller，heater and so on. The system simulates industry scene through a mini-boiler heating，water supply and drainage system，which is reliable and visual.
In the design，boiler Level was selected as the controlled variable. The controlled object is composed of the water trough，the force pump，the boiler and the pipe-line valve. Micro Master 6SE9214-ODA40 inverter is taken as the actuator and the boiler level is controlled by Logix5550. Configuration software RSView32 and touch screen PanelView1000 are combined to realize the real-time monitoring. In the design，a simple design of single-loop boiler liquid level value adjustment is selected for the study. The composition of the system is shown in Fig. 1.

In the design，the inverter as an actuator directly receives PLC analogy I/O port output，and converters into frequency of inverter so as to drive the 3-phase motor in the lift pump，change the inlet，and adjust the boiler level to the dynamic balance at last. And the configuration software is used to design monitoring picture to realize the computer and the touch screen to the boiler level long-distance and the scene monitoring.
[1]. Pump started，pid controller pressure transmitter to the control point to provide the pressure signal，when the pressure point is not equal to set value，pid controller by adjusting the inverter output frequency to change the pump speed to regulate the pipeline pressure constitutes a closed-loop feedback control system，water pressure regulator so that users timely，small fluctuations in results. And in the process control system designed to take full account of the principle of load balancing，to take "first-in-first out" strategy of lining up，the way the implementation of the rotating frequency to ensure that the basic balance of the water usage.
1 Frequency Control Constant Pressure Water Supply
Principle
   Frequency Control Constant Pressure Water Supply System by the programmable logic controller (plc)，converter (built-pid)，pressure transmitter，low-voltage electrical and pump components，such as Figure
1. Map m1 ~ m3 for water pump motors，p1 ~ p3 for pumps，km1 ~ km6 for motor starting and stopping each of the AC contactor switch. Basic working principle of the system：by the plc a signal to the inverter to control the water cycle work，when the large volume of water consumed by the plc control of all water line installed in the outlet pipe of the pressure transmitter，pressure standard signals into dc 0 v ~ 10 v into the analog signal converter with pid regulator，pressure settings and pressure feedback values after the implementation of pid calculated values of the output of a frequency converter to set the value for the pump by the inverter to provide variable frequency electrical power to realize stepless speed regulation of electric pumps to regulate the exit of the water supply pipe network pressure，constant pressure water supply to achieve the purpose. Pressure feedback signal in order to ensure an accurate value，yet the value of feedback signal to set the filter time constant. The scope of the power converter，Figure 1 can increase the number of the motor to achieve "a multi-trailers," Optimization of the program.
3 Establishing Mathematics model for the charged object
One of the main tasks of establishing control system mathematical model is to determine the mathematical model of the controlled object. Generally，there are two kinds of basic methods for establishing process control mathematical model：mechanism analysis and experimental method. However，for controlled object whose structure and internal process is very complex，it is very difficult to determine the object just by its own internal physical process and to solve out the differential equations systematically. Besides，considering the nonlinear factor，mechanism analysis used some approximation and hypothesis for mathematical deduction. Although these approximation and assumptions have practical basis，but not fully reflect actual situation，and even cause incalculable effects.
Therefore，in this design，the experimental method is chosen to establish a mathematical model for controlled object. This kind of modeling is based on the input and output in the actual production process，that is to say，establishing mathematical model for the controlled object through process identification and parameter estimation. In this design，step response curve method is used to identify mathematical models of the process. A 20Hz step disturbance input signal is applied to the charged object，and the response curve of the output that changes with time can be mapped. After the analysis， the transfer function of the controlled object can be defined. In the process of experiment，the object was conducted several tests. Using RSLogix5000 trend monitoring function curve，more than 10 charged object step response curve have been recorded. To all the parameters for average，steady time：ts≈821.525s，steady value：h(∞)=58.5，peak time：tp=394.4s overshoot：a%}29%. According to the theoretical analysis，the controlled object is the most likely second-order object.
However，the difference is very apparent between the ideal second-order controlled object step response curve and the actual curves. So the ideal curve can't response to its actual characteristics. It is inferred that the controlled object may be the second-order controlled object that includes zero. The try and error method and MATLAB simulation tools are used to get a curve whose parameters are close to the average dynamic parameters of the controlled object's response curve. It is shown in Fig. 2.

Some adjustments can be made according to the following rules:
1) When the zero is closer to the imaginary axis，settling time will be longer and the overshoot will be bigger and peak time will be smaller. With the zero closing to the imaginary axis，the effect is more obvious.
2) The effect which the closed loop dominant apices have on dynamic performance is increasing the peak time，reducing the overshoot and adjusting time. Nonparametric model is used to describe the controlled object. In other words，step response curve which approximately describe the controlled object is used because of the controlled object's complexity and uncertainty.
To a constant water supply，landing in the water pressure to increase the inverter output frequency，when the maximum frequency converter and a constant pressure pump can not meet the requirements，while running the pump frequency to switch to the running frequency，At the same time need to start a second frequency or third pump until the water supply to meet the requirements. Need to determine criteria for the launch of a new water pump is the inverter output frequency is set to reach the upper limit. In order to determine the frequency of the inverter limits the accuracy of least squares by chance should be the frequency of the frequency of fluctuations in the ceiling from the situation. Inverter control motor pumps are activated each time the soft-start，and requires the use of water pumps must turn to serve as a continuous variable frequency pump operation shall not exceed 3 h，each time when the need to launch a new water pump，the pump will run from the existing removal of the inverter，and replace the power-frequency operation，reset the inverter and run the pump for a new start. Three pumps to control the duty cycle is the use of pump No. 1 way to achieve (to 3 in the plus 1 equal to zero)，or combined with the total number of pump pump pump rotation to achieve its work.
4 The installation of controller parameter
4.1 The selection of control algorithm
After establishing the approximate mathematical model of the controlled object，a complete feedback control system can be formed to improve the performance of the open-loop control system. PID is an ideal control law in that integral is introduced basing on the proportion，which can eliminate the residual error，plus the derivative action，which can also improve the stability of the system. According to the characteristics of the controlled object and laboratory conditions，a single-loop feedback control loop for the controlled object is established，and PID algorithm is used to realize boiler level control. The schematic diagram of level control is shown in Fig. 3.

Open the outlet valve to a certain degree，and make the hydraulic discharge invariable. Comparing the process variables of the water level in feedback with the given volume，the deviation can be obtained. PID instruction does PID operation on the deviation，and the results is a control variable，so the frequency of the inverter can be changed to control the rotate speed of the pump. If the liquid level is on the high side，the results make the control variable smaller，and
reduce the rate of inflow，make liquid level lower；if the level is on the low side，the results make the control variable larger，and increase the rate of inflow，make liquid level higher.
4.2 The Parameter Tuning of PID
Because the transfer function of the controlled object includes a zero second-order link， the computation work load is quite big regardless of using the root-locus method or the frequency characteristic law among theory methods when tuning PID parameter. And the process mathematical model can only reflect dynamic parameter approximately，so the reliability of the parameter value which is obtained by the theoretical calculation is not very accurate and it will be adjusted constantly in the scene. Therefore，engineering parameter tuning is chosen to seek the PID parameter in the design. The common method of engineering tuning are dynamic characteristic parameters，the stable boundary law，the decay curve law and field experience setting method，etc. In the process of PID parameters，the 4:1 decay curve law is
adopted. The steps are:
1) In the closed system，regulator's integral time is set the largest (Ti≈∞) and differential time Td is set zero (Td=0). The proportion is taken the great value to perform the given value perturbation experiment repeatedly，and the proportion is reduced gradually until the record curve presents up to 4:1 weaken. Then the proportion is called 4:1 weaken proportion s s
and the distances between two neighboring wave ridge's are called 4:1 damped cycle Ts. In the experiment，the level quantitative test is set for the 200mm， and then the system response curve is obtained and reorganized 4:1 decay curve (thick red line is shown in Fig. 4
'Thus measuring：δs≈8，Ts≈2.2;
2) According to the following formula，each parameter of the regulators is
δ=0.8，δs≈6.4;
Ti=0.3，Ts≈6.6;
Td=0.1，Ts≈2.2
3) According to these results，regulator parameters are set. Then the dynamic process of system is observed and the parameters are made adjustment to determine the optimum parameters.
5 Monitoring Design
Frequency according to the principle of constant pressure water supply system the design water level control system of the water tower block diagram shown in Figure 2，the city water network eq with the water level controller to control the injection valve yv1，as long as the water level below the high water mark，then to the pool water automatically. Pool of high / low water level signal directly to the plc，as a high / low level alarm. In order to ensure continuity of supply，the water level of the smaller distance between the upper and lower limits. Domestic water and fire water pump three shared [3]，usually yv2 solenoid valve in a state of loss，the closure of the fire pipe network，when a fire occurs，the solenoid valve was yv2 electricity，water distribution network with the closure of life，three water pumps for fire use，and maintain high-pressure fire water value. After the lifting of the fire，three use domestic water pump replaced and the cycle of operation，to maintain constant low water value. Each pump motor can run frequency，and frequency can be run. When the small volume of water consumed，a water pump under the control of the converter stable operation of large quantities of water to the pump when running at full speed there is no guarantee that the pressure on network stability，plc pressure given the lower limit of the inverter signal and high-speed signal at the same time plc was detected，plc automatically work in the original state of the pump frequency into frequency operation，in order to maintain the continuity of pressure at the same time use the next pump put into operation after start-up converter in order to increase the supply pipe network pressure of water to ensure stability；if running two pumps can not meet the requirements of the pressure，the frequency of work will be followed by a state into the pump-frequency operation，and then pump into a variable frequency operation；when the water consumption reduction，the first perfo