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The Variable Frequency and Speed Regulation Constant Pressure Water Supply

2009 International Conference on Measuring Technology and Mechatronics Automation

The Variable Frequency and Speed Regulation Constant Pressure Water Supply System Based on PLC and Fuzzy Control

Peng Xiaohong, Xiao Laisheng
College of Information Technology Guangdong Ocean University Zhanjiang, P.R China e-mail: lgdpxh@126.com

Mo Zhi
Technology Department Zhanjiang New Zhongmei Chemical Industries Co.,Ltd. Zhanjiang, P.R China

Liu Guodong
Maoming Branch China Construction Bank Maoming, P.R China

Abstract—For solving the problem of poor quality and waste of electric energy coursed by water supply mode of a high water tower and a direct water pump, introduce a constant pressure supply water system, which adopts embedded fuzzy control technology, using PLC and frequency convener as its core. When the system runs, water pressure of the pipe is inputted into the PLC from terminal X0 through pressure transmitter. Comparing the value of measured pressure with its set value, PLC controls the work frequency of the frequency converter by the output signal on fuzzy calculation in order to control rotation speed of water pump and adjust water pressure of the pipe. The real time control performance to the water Pressure of water supply system designed in this paper was proved to be all-right according to the experimentation validated. The hard ware module is steady-going and reliable, and the fuzzy controller is proper and valid and can guarantee the better stability of water pressure. Keywords-fuzzy control; PLC; variable frequency and speed regulation

I.

INTRODUCTION

With the high-speed development of economy and society, energy shortage is becoming more and more obvious and the people’s requirements on both quality and reliability of water supply is also increasing. Therefore it has become an inevitable trend to use advanced automation technology to design the constant pressure water supply system with high energy conservation and high reliability. Because the users’ requirement of water supply is often unsteady, a closed-loop regulation is needed to use outside a frequency converter in order to keep a constant water pressure and it is generally realized by PID in the past. However, the water supply system is a lag one with great inertia and the users’ requirement for water supply has large randomness and it may lead to water pressure on the pipeline without changing law, so it is a hard job to describe it with precise mathematical model. Therefore, it is difficult to

achieve satisfactory control effect by using the traditional PID control. The fuzzy control technology is a good way to use fuzzy inference to realize system control and it does not need to build mathematical models [1].In recent years, many researchers added fuzzy control strategy to a variety of control schemes and achieved satisfactory results. Li Zhong and others designed a kind of fuzzy water supply control system controlled by PLC that can overcome the issues to adjust the parameters difficultly in the traditional PID control [2]. Zhao Bao-yong has designed a fuzzy-PID controller with self-adjusting factor based on a single chip, which has played an important role to control the water supply system with non-linear, lag characteristics and also solved the problem to adjust parameters difficultly in the traditional PID control [3]. LI Xin-chun designed a constant pressure water supply controller based on AT89C51 and applied fuzzy self-tuning PID algorithm to it [4]. Qiao Weide puts forward a design with adaptive closed-loop control functions on pipeline pressure through the combination of neural networks and fuzzy logic and it has been robust and reliability [5]. Wang Cai-xia has designed a fuzzy water supply system for one frequency converter to control three pumps, which greatly reduces the cost of both design and operation [6] Among the schemes mentioned above, most of them are based on the PID control, which have greater frequency fluctuations and easily lead to water pressure in the pipeline unsteady. To solve those problems, after having systemically analyzed the work characteristic and the technical requirement for a water supply control system, we have taken fuzzy control method to build a constant pressure water supply system based on PLC and frequency converter. We make use of fuzzy logic control theory to make pressure sensor signal to input and control variable from output fuzzy and achieve fuzzy logic control table by fuzzy reasoning according to the rule table made by operating experience. In

978-0-7695-3583-8/09 $25.00 ? 2009 IEEE DOI 10.1109/ICMTMA.2009.392

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the course of real-time control, the system transforms input signal of pressure detected in real-time into output by seeking fuzzy logic control table, and then we obtain a precise output by fuzzy decision-making. At last, the system completes the whole process of fuzzy logic control for constant water pressure by controlling the frequency of motor according to the precise output. After a lot of test, we have proved that the constant pressure water supply system designed in this paper has a good performance of real-time control and its hardware modules work stably and reliably. The adaptive fuzzy controller designed here is also correct and effective for guaranteeing the stability of the water pressure on pipeline and it is really a better control system. II. SYSTEM CONTROL SCHEME

Power 380V AC Starter Alarm signal Convert er SB1 Start and stop PLC fuzzy controller 220V AC Alarm Control panel

SB3

SB2

SB4

SB5

SB6

Transmitter sensors group

24V DC

Users Switch information onsite

Pump M2 Water entrance

Pump M1

Pump M3

Figure 1. Control system schematic

A. The principle of system control Three pumps run in parallel in the system. PLC has a periodic real-time detection on a given pressure and actual pressure on pipe network, and their values are put into PLC. Fuzzy controller computes their deviation E and changes in the rate of deviation E. Output control signals from the fuzzy control algorithm are gained, which are sent to converter through analog output module to adjust the pump motor power supply voltage and frequency. When there is the small amount of water, one pump has a stable operation under the control of a frequency converter. When the amount of water is so much that water pump which runs at full speed cannot guarantee the stable pressure on the pipe network, lower limit pressure signal on PLC and the high-speed signal on converter was detected by PLC at the same time. PLC transforms the first pump from variable frequency state to work frequency state in order to keep pressure continuity, while another pump which is started by frequency converter begins to run in order to keep stable pressure by increasing the volume of the water supply network. If the operation of the two pumps cannot meet the requirements of the pressure, the pump which is working in variable frequency state is transformed to work frequency state, and another pump will be started. When the water consumption reduces, the signal which shows that the frequency converter works at the minimum speed is effective. At this time if the pressure upper limit signal is still being, PLC will stop the first pump which works at work frequency state to reduce the amount of water. When the two signals are still being, PLC stops the second pump which works at work frequency state, until the last pump supplies water by frequency converter. B. System hardware System selects the Siemens S7-214PLC[7], with I/O expansion modules. The main detection components are optical switches, pressure detection switches, totaling 12 input signals. The Implementation components are motor, frequency converter, sound and light alarm, and so on, a total of 3 output points. Control system schematic is shown in Fig. 1. The PLC mainly completes on-site data acquisition, conversion, storage, alarm, controlling frequency converter

to complete pressure adjustments. Three pumps are driven directly by frequency converter to keep constant pressure.The start and stop of inverter is divided into manual control and PLC control. On the control panel there is a manual/automatic conversion switch to which PLC has a real-time detection of states. When the manual function is selected, the start and stop and their switch of the pump operation is manually controlled through the panel buttons and switches. When the automatic function is selected, all control and alarm are completed by PLC. III. DESIGN OF FUZZY CONTROLLER Systems uses two-dimensional fuzzy controller. The input is the deviation E between the given water pressure and measured water pressure in the water pipe network and the changes in the rate of deviation which is computed from the deviation and previous deviation. The output of fuzzy controller is control system increment U. The fuzzy control rules are summed up according to the real situation of water supply in a residential area and the practical experience of operators. The corresponding control decisionmaking can be gained based on fuzzy control rules and fuzzy reasoning synthesis rules and the corresponding amount of control will be gained from the principle of maximum membership. After computer computes Off- line and has debugging and amendment time after time, the fuzzy control table will be gotten which can be applied in practice and is put in PLC. When the system runs, it computes E, E from input data and computes domain value of fuzzy amount. Then it searches fuzzy control table to gain controlled quantity and the output of controlling frequency converter will be gotten from the controlled quantity multiplied by the ratio coefficient[8,9]. A. Making input and output variables fuzzy The universes of discourse of input variable E and E are {-3,-2,-1,0,1,2,3}. Its Fuzzy language values are {NB(negative big), NM(negative middle), NS(negative small), ZO(zero), PS(positive small), PM (positive middle) and PB(positive big)}. Membership function uses triangle , shows in Fig. 2.

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The universes of discourse of output variable U are {4,-3,-2,-1,0,1,2,3,4}. Its Fuzzy language values are { NB(negative big), NM(negative middle), NS (negative small), ZO(zero), PS(positive small), PM (positive middle) and PB(positive big)}. Membership function uses triangle , shows in Fig. 3. B. Fuzzy rule table Fuzzy rule table is shown in TABLE I, which is summed up from the experience of the on-site operators to manually adjust water pressure. C. Seeking a fuzzy relationship matrix and fuzzy control query table Two-input & single-output language control strategy shown in TABLE I is composed of 49 fuzzy conditional statements. We obtain the precise amount by removing fuzzy U in accordance with the largest membership law[10]. For all the combination of the elements on the domain for E and E, we can work out precise amount of emergency degree U corresponding to each combination. So we can get fuzzy control query table.

IV.

SYSTEM SOFTWARE

For the convenience of programming and debugging, system controller uses modular programming. The main module consists of manual operation module, automatic operation module, fault diagnosis and alarm modules. A. Manual operation module When the system runs at manual state, PLC only receives the circuit protection and the sensor signals and judges the running state of the pumps. In the event of failure, the alarm signal outputs. The start, stop and their switch of the pumps operates by hand through the buttons and switches on the panel. B. Automatic operation module Automatic operation module includes system initialization, testing of the boot order, data acquisition subroutine, fuzzy control Subroutine, initial value subroutine, motor control subroutine, and so on. Module flow chart is shown in Fig. 4. Data acquisition subroutine: completing the acquisition of data from the main water pipe pressure. Motor control subroutine: controlling the operation and stop of 3 water pumps. The output frequency of frequency converter is related with the speed of pumps. As a result of it, the more the water consumption is, the higher the output frequency of the converter is, the larger the speed of the pumps is. Whereas the little the water consumption is, the lower the output frequency of the converter is, the smaller the speed of the pumps is. When frequency is up to 50Hz, i.e. pumps running at full speed, and it cannot meet the requirement of water supply, PLC automatically transforms the first pump from variable frequency state to work frequency state and the second pump which is started by frequency converter will run. If the water supply cannot meet the requirements although the second pump runs at full speed, PLC will automatically switch second pump to work frequency operation and the third pump which is started by frequency converter will run. According to the low the pumps will be put into operation one by one. When two pumps run at full speed and at work frequency state and the third pump operates at variable frequency state, water consumption reduces and the output frequency comes down. If frequency reaches the lowest limit and water supply is also greater than the water consumption, the system automatically stop the third pump. Similarly, if water supply is still greater than the water consumption after the third pump stops, the system automatically stop the second pump, and so on.. Fuzzy control subroutine: it is called by a timing interrupting. Its function is to keep constant output water pressure by adjusting the speed of the pumps. When the main program initializes the system, we firstly get the present actual water pressure. We can also get the present error volume by using this pressure subtracting the pressure settings and error changes will be gotten by using present error volume subtracting the previous error volume. And

NB

NM

NS

ZO

PS

PM

PB

-3

-2

-1

0

1

2

3

Figure 2. The membership degree of input var. E&

NB

NM

NS

ZO

PS

PM

PB

-4

-3

-2

-1

0

1

2

3

4

Figure 3. The membership degree of output var. TABLE I. FUZZY CONTROL RULE TABLE

E NB NM NS ZO PS PM PB

E NB PB PB PM PM PS PS ZO NM PB PM PM PM PS ZO ZO NS PM PM PS PS ZO NS NS ZO PM PM NS ZO NS NS NM PS PM PS ZO NS NM NM NM PM ZO ZO NS NM NM NM NB PB ZO NS NS NM NM NM NB

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then we can obtain fuzzy control volume through fuzzy quantitative to error volume and its changes and seeking the fuzzy control table. At last, the real control volume which comes from fuzzy control volume multiplied by the scale factor controls the frequency of the converter by PLC and its communication in order to keep constant pressure water supply. Its flow chart is shown in Fig. 5. C. fault diagnosis and alarm modules Converter has the function of avoiding short circuit and overload. When there is short circuit, overload fault in water pump motor, converter automatically cuts off a power supply circuit, accessing to the state of protection and outputting alarm signals. System links auxiliary nodes of the contactors, circuit breakers corresponding to the point of failure to PLC and PLC scans the state of these contacts. These state data are stored in the storage area through PLC program. According to logical analysis to control program and the state of settings, the system can determine whether there is a failure in the equipment or components. If there is a failure, the contactors of water pumps will be cut off and Inverter will reset. And then the contactors of back-up pump turn on and back-up pump will start. At the same time, the system outputs alarm signals of the failure pump, such as lighting indicators.
Start
Start

V.

CONCLUSION

Fuzzy control theory is used to transform the system's input into output in order to enrich people’s experience during the course of manual control, which is reflected during the course of automatic control and provides a guarantee for the reliable operation of the system. PLC is used as a controller. Its hardware structure is simple and low cost. Water pump motor of the system works at stepless speed regulation. Its operating parameters are automatically adjusted, which can keep constant water pressure to meet the requirements while water consumption changes. In addition, there is a RS-485 interface in the basic unit of S7-214 PLC by which it can communicate with building monitoring center to achieve unattended remote control. After six months of test the system has stable performance, reliable operation and energy-saving effect. REFERENCES
[1] Zadeh L.A, “Fuzzy sets information and control”,1965,(8),pp.338353. [2] Zhong Li, Yan Li, Pingliang Yu, “Constant pressure water supply system with PLC and inverter”, Huadong Chuanbo Gongye Xueyuan Xuebao/Journal of East China Shipbuilding Institute,1998,(12),pp.6973. [3] Baoyong Zhao, “Application research of the fuzzy control technology in the variable-frequency speed-governing constant-pressure water supply system”, Electric Drive Automation, 2003,25(6),pp.16-17. [4] Xinchun Li, Fengshu Li, Shun Yang, “The application of single-chip computer in the fuzzy control of water-supply system with variable frequency and constant pressure” , Journal of Liaoning Technical University(Natural Science),2002,03,pp.354-355. [5] Weide Qiao, “Application of fuzzy neural network in PLC water supplycontrol system with constant voltage”, Electric Drive Automation, 2007,29(2),pp.48-51. [6] Caixia Wang, “Study of fuzzy variable frequency control system for constant pressure”,Journal of Changchun University of Science and Technology, Vol.26,No.3,2003, pp.53-54. [7] Zhaofang Wen, SIMATIC S7-214 Programmable Controller Tutorial, Beijing institue of technology publisher, 2002. 9. [8] Qishou Peng, Weiling Huang,, “single_chip fuzzy control of a constant pressure water supply”, Techniques of Automation & Applications,2006,pp.77-78. [9] Guozhong Xu, Jing Zhu, “Single-chip fuzzy control constant pressure water supply system”, Microelectronics and Computer, 1998,pp. 812. [10] Junpu Wang, “Intelligent control”, Press of University of Science and Technology of China ,1996,pp.90-135.

Data collection subroutine

Read A/D out and pressure settings

Initial Value? Y Initial value subroutine

N

Comoute error and its changes and save

Trans form to fuzzy volume

Fuzzy control subroutine

Comoute address to seek table

Op. switch? Y Motor control subroutine

N

Seek fuzzy control table to obtain control volume Comoute real control volume to control converter

Figure 4. Automatic operation module flow chart

Figure 5. Fuzzy control subroutine

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