Automatic constant-pressure water supply control system based on PLC

Abstract

The invention discloses an automatic constant-pressure water supply control system based on a PLC, and belongs to a control system in the field of industrial automation. The system mainly comprises a motor, a frequency converter, a PLC controller, a soft starter, and a motor protector data acquisition and auxiliary device. The system can automatically adjust the rotating speed of a certain water pump and the input and output of a plurality of water pumps according to the instantaneous pressure change of a pipe network, so that the outlet end of a main pipe of the pipe network is kept at a constant set pressure value, the system is not limited by use places and application environments of users, and the system can be expanded to other similar water supply application occasions through the combination of software function blocks; the flexibility is high; the automatic constant-pressure control water supply system combining PLC control and frequency converter control can effectively solve the problem of water consumption of urban buildings, and the water supply pressure is kept constant.

Description

Technical Field

[0001] The present invention relates to the field of control systems in the industrial automation field, and specifically to an automatic constant pressure water supply control system based on a PLC. Background Art

[0002] With the increasingly prominent problem of urban building water supply, it is quite important to maintain a constant water supply pressure and improve the water supply quality; at the same time, the reliability and safety of water supply are required. The designed water supply method and control system for the above problems consist of a main circuit, a standby circuit, a clear water tank and a pump house. Among them, the pump house is equipped with 3 pump units, and there are also multiple electric gate valves or butterfly valves to control each water supply circuit and water flow. The control system adopts a multi-functional and highly reliable control system with a PLC having rich functions as the core.

[0003] As a general machinery in water supply projects, pumps consume a large amount of energy, and the electricity consumption often accounts for more than 60% of the water production cost. In China, the annual electricity consumption of pumps accounts for 21% of the total electricity consumption. In order to save energy and reduce consumption, adjustment measures must be taken to make the pump station adapt to the operation of load changes. Therefore, an automatic constant pressure water supply control system based on a PLC is designed to meet the above water supply problems. This control system adopts a "one-driving-multiple" control method. Compared with the "one-control-one" method, the hardware investment cost is less, and it has a higher price competitiveness; the system is composed of general equipment units with relatively independent functions, which is convenient for fault judgment, daily maintenance and replacement of functional units; the system is basically not restricted by the user's use place and application environment, and can be extended to other similar water supply application occasions through the combination of software function blocks, with strong flexibility; the system can, in the case of unattended operation, through the detection and judgment of faults, automatically process according to the "graceful degradation use" scheme, reducing the system shutdown and water cut-off phenomenon; in addition to the conventional protection functions such as overload, overcurrent, overvoltage, etc., the system also has additional functional protections such as shutdown without water and upper limit pressure protection of the pipe network.

[0004] However, the traditional constant pressure water supply system lacks an automatic control device and has many disadvantages. During the process of a certain motor switching from frequency conversion operation to power frequency operation, since the motor is in a coasting operation mode after the frequency conversion drive is cut off, at this time, the motor is in a state of induction power generation, and there is a possibility that the phase of induction power generation is inconsistent with the phase of the power frequency power supply. When switching to the power frequency, due to the motor being in a coasting operation mode, a current impact phenomenon may occur, which may lead to electromagnetic interference problems of the power grid or motor equipment. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the present invention aims to provide an automatic constant pressure water supply control system based on a PLC to solve the problems raised in the above background technology. To achieve the above object, the present invention provides the following technical solutions: An automatic constant pressure control system based on a PLC, characterized in that the functions of the control system include: industrial frequency manual function, variable frequency automatic function, semi-automatic operation, manual operation, PID control function, water pump system function, and booster pump system function.

[0006] Preferably, the industrial frequency manual function is specifically to switch the transfer switch to the "industrial frequency" gear, and the operator can decide to start or stop the operation of any pump according to needs. In this operation mode, the PLC, PID, frequency converter, etc. do not participate in the control.

[0007] Preferably, the variable frequency automatic function is specifically to switch the transfer switch to the "variable frequency" gear and press the variable frequency start button, and the system will automatically judge and select the starting variable frequency operation pump inlet and enter the automatic operation.

[0008] Preferably, the semi-automatic operation is specifically that when a problem occurs in the PLC system and the automatic control system fails, the system works in a semi-automatic state at this time, that is, one pump has the variable frequency automatic constant pressure control function. When the water consumption is insufficient, one or several industrial frequency pumps can be manually put into operation.

[0009] Preferably, the manual operation is specifically that when the pressure sensor fails or the frequency converter fails, to ensure water use, the six pumps can operate in a manual industrial frequency mode respectively. The actual operation of the implementation effect proves that this control system constitutes the most economical structure for the automatic control of multiple deep well pumps. In the software design, the instantaneous pressure and current impact during the switching between variable frequency and industrial frequency are fully considered. Using soft start for each pump is the key to solving this problem.

[0010] Preferably, the PID control function is specifically that when the pressure set value is greater than the actual pressure value, the PID instrument will increase the analog output value to accelerate the operation of the water pump motor; when the pressure set value is less than the actual pressure value, the PID instrument will decrease the analog output value to reduce the operation of the water pump motor.

[0011] Preferably, the water pump function is specifically that the control system designs six pumps into two groups, and the cumulative running time of each pump motor can be displayed, and it rotates once every 24 hours, which not only ensures that there is a standby pump in the water supply system, but also ensures that the pumps in the system have the same running time, ensuring the reliable life of the pumps.

[0012] Preferably, the function of the pressure pump system is specifically that the pressure pump is equipped with a soft starter and a motor protector to ensure the long-term reliable operation of the pressure pump. At the same time, a water level sensor, a digital display instrument and a water shortage sensor for the elevated water tank are equipped. To ensure the constant pressure water supply of the entire main water supply network, when the elevated water tank is full and there is water in the main water pipe, the pressure pump stops. At this time, the main pipe pressure will be "pressurized", ultimately causing the main pipe pressure to rise and transmitting this pressure to the water pumping station.

[0013] Compared with the prior art, the multi-pump circulating variable frequency constant pressure water supply system based on PLC control in the present invention uses the digital input / output mode of PLC to control functions such as the start and stop of the motor, state migration, maintenance and fault handling. The speed regulation of the variable frequency drive motor pump is achieved through a PID instrument and a pressure transmitter, so as to achieve the purpose of constant pressure water supply. When designing the control system program, the principle of load balance is fully considered, and the queuing strategy of "first start, first stop" is adopted to execute the rotation of the variable frequency mode, ensuring that the utilization rates of each pump are basically balanced. The variable frequency constant pressure water supply has changed the previous fixed supply mode and realized the principle of "distribution according to demand" for the first time. Therefore, compared with the industrial frequency control mode, the variable frequency constant pressure water supply mode not only has the functional advantages described above, but also has very good energy-saving effects. In previous projects, the power saving rate usually reached more than 20%. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic diagram of an automatic constant pressure water supply control system based on PLC proposed by the present invention.

[0015] Figure 2 It is a schematic diagram of the main circuit of an automatic constant pressure water supply control system based on PLC proposed by the present invention.

[0016] Figure 3 It is a schematic diagram of the control circuit of an automatic constant pressure water supply control system based on PLC proposed by the present invention.

[0017] Figure 4 It is the part of Network 1-13 of the schematic diagram of the ladder program of an automatic constant pressure water supply control system based on PLC proposed by the present invention.

[0018] Figure 5 It is the part of Network 14-27 of the schematic diagram of the ladder program of an automatic constant pressure water supply control system based on PLC proposed by the present invention.

[0019] Figure 6 It is the part of Network 28-30 of the schematic diagram of the ladder program of an automatic constant pressure water supply control system based on PLC proposed by the present invention.

[0020] Figure 7 It is the PID control circuit diagram and logic schematic diagram of an automatic constant pressure water supply proposed by the present invention.

[0021] In the attached figure, the meanings of the numbers are as follows: 1. ABB frequency converter; 2. Programmable Logic Controller (PLC); 3. Water pool; 4. Pressure transmitter; 5. PID instrument; 6. Power supply; 7. Small water pump; 8. Fire water pump; 9. Domestic water pump; 10. Water supply valve; 11. Pump 1; 12. Pump 2; 13. Pump 3; 14. KM1, KM2, KM3 are thermal relays for overload protection of motors M1, M2, M3; 15. Main circuit air switch; 16. QF2, QF3, QF4, QF5 frequency converters and three pumps’ power frequency operation air switches; 17. KM2, KM4, KM6 realize manual functions; 18. CPU; 19. Water pool water shortage switch; 20. Manual / automatic conversion switch; 21. Manual / automatic intermediate relay; 22. Normally open contact; DETAILED DESCRIPTION

[0022] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] The present invention is an automatic constant pressure water supply control system based on PLC, which specifically includes the following contents:

[0024] Example 1: When a motor fails or a motor water pump needs to be repaired, the control system will be reduced to 3-pump circulation mode for automatic operation; when the frequency converter fails, the control system will use the industrial frequency drive mode to control the operation and stop of the pump to ensure that the water supply pressure is within a certain range, but the system cannot achieve a constant pressure value, and an alarm buzzer will sound to notify the operator to handle it; when a waterless contact signal comes, the PLC will shut down all variable frequency and industrial frequency outputs until the waterless contact signal disappears, and the PLC will automatically restore the control output; when a fire signal comes, the PLC control will enter the subroutine segment for execution, shut off domestic water, open the fire water supply valve 10, and achieve the purpose of supplementing water supply to the fire pipeline. The system will automatically complete the constant pressure fire water supply according to the fire water supply pressure setting value set in the PLC program. When the fire signal is released, the system automatically returns to the variable frequency constant pressure water supply working state;

[0025] When only a single pump operates with variable frequency, and it is in the lowest output frequency state and there are no upper and lower pressure limits for a long time, the control system will operate at variable frequency of 50 Hz for 30 s or until the network pressure reaches about 1.2 times the set value, and then immediately stop running and enter the sleep state until the actual network pressure is about 80% of the pressure set value, at which point the control system will automatically resume variable-frequency operation again, that is, wake up from sleep. Of course, if there is a pneumatic tank in the network, the system should use the upper and lower limit contacts of the pressure controller of the pneumatic tank as the conditions for sleep and wake-up control.

[0026] Combined with Figure 2 , the main circuit mainly consists of three motors 11, 12, and 13, and AC contactors 14 and 17 control the operation of the three motors. 14 is a thermal relay for overload protection of motors M1, M2, and M3. 15 and 16 are the air switches for the power frequency operation of the main circuit, frequency converter, and three pumps respectively. KM1, KM3, and KM5 implement the automatic function, and KM2, KM4, and KM6 implement the manual function.

[0027] Combined with Figure 3 , the sensor power supply of CPU224, 24V (DC), can output a current of 600 mA, which can easily meet the requirements in this example. The output relay contact capacity of CPU224 is 2A, and the voltage range is 5 - 30V (DC) or 5 - 250V (AC). Figure 3 I01 - I06 in are connected to the corresponding control lines within the dotted box in the control circuit diagram. 201 is connected to DCOM1 of the frequency converter, 202 - 203 are connected to DI1 - DI2 of the frequency converter. The normally open point of RO1 of the frequency converter is connected to I0.0 of the PLC, and the normally open contact of R02 is connected to I0.1 of the PLC.

[0028] Combined with Figure 4 , Figure 5 , Figure 6 , 20 is a manual / automatic changeover switch, and KA is a manual / automatic intermediate relay. When it is turned to position 1, it is in the manual state; when it is turned to position 2, it is in the automatic state, and at the same time KA is energized. In the manual state, SB1 - SB6 can be pressed to control the start and stop of the three pumps. In the automatic state, the system operates according to the PLC program and automatically controls the start and stop of the pumps. HL1 - HL8 are various operation indicator lights. The normally open contact of the intermediate relay KA is connected to I03 to control the start in the automatic state. The three normally closed contacts of the intermediate relay KA are connected to the manual control circuits of the three pumps to control the manual operation of the three pumps. In the automatic state, the three pumps can switch and operate orderly and smoothly under the control of the PLC.

[0029] The functions of the pressure pump system are specifically implemented as follows: Since the pumping station is far from the elevated water tank, the head of the water pump directly supplying water to the elevated water tank is insufficient. Therefore, a pressure pump house is designed at a place with a 36-meter drop from the elevated water tank, equipped with two vertical centrifugal pumps with a motor power of 75KW and a head of 36 meters. The control system of the pressure pump needs to consider that if the water level in the elevated water tank is low and there is water in the main pipe, the inlet electric butterfly valve is opened and the pressure pump is started to supply water to the elevated water tank. If the elevated water tank is full and there is water in the main pipe, an alarm signal is given and the pressure pump and the inlet electric butterfly valve are closed. If there is no water in the main pipe, it indicates that the water consumption has increased or the pumping station has stopped supplying water, and the outlet electric butterfly valve must be opened to supplement water from the elevated water tank to the main pipe. To ensure the long-term reliable operation of the pressure pump, a water level sensor, a digital display meter, and a water shortage sensor for the elevated water tank are equipped. To ensure the constant-pressure water supply of the entire main water supply network, when the elevated water tank is full and there is water in the main pipe, the pressure pump stops, which ultimately causes the pressure in the main pipe to rise and this pressure is transmitted to the pumping station. The control system of the water pump detects this pressure for constant-pressure variable-frequency control, thereby achieving the constant-pressure water supply of the entire main water supply network.

[0030] Combined with Figure 3 、 Figure 4 、 Figure 5 、 Figure 6 , the I / O allocation of the PLC is specifically as follows:

[0031]

[0032]

[0033] Example 2: The PID control function described above has the following specific automatic constant-pressure water supply PID circuit formula. The input impedance of the linear integrated amplifier is:

[0034]

[0035] The feedback impedance is:

[0036]

[0037] If the linear integrated element is in an ideal situation, its output is:

[0038]

[0039] Taking the Laplace transform of equations (1), (2), and (3) gives:

[0040]

[0041] The specific automatic constant-pressure water supply PID control schematic diagram is Figure 7As shown, when the pressure set value is greater than the actual pressure value, the PID instrument will accelerate the operation of the water pump motor by increasing the analog output value; when the pressure set value is less than the actual pressure value, the PID instrument will reduce the operation of the water pump motor by decreasing the analog output value; the PID instrument will continuously adjust the speed of the water pump motor by changing the magnitude of the analog output value.

[0042] When the analog output value of the PID instrument reaches the maximum and the network pressure is still at the lower limit of pressure, the PLC program starts timing. After the timing time reaches, the PLC will issue a series of action commands to realize the migration from the current state to the next state. It must be pointed out that during each state migration, first, the frequency converter start signal should be disconnected, and then the frequency conversion mode contactor between the frequency converter and the motor should be disconnected. And the frequency converter stop mode parameter is preferably set to the "free parking" mode. If the action execution sequence is reversed, it will cause the frequency converter to malfunction or be damaged. During the conversion from the frequency conversion working mode to the power frequency working mode, after disconnecting the frequency conversion contactor, the power frequency working mode should be engaged within the shortest possible time. Otherwise, when the motor speed drops to a certain level, starting impact will occur. Similarly, when the analog output value of the PID instrument reaches the minimum and the network pressure is still at the upper limit of pressure, the PLC program starts timing. After the timing time reaches, the PLC will complete the migration from the current state to the previous state. Usually, a frequency converter parameter of the lowest output frequency lower limit needs to be set, generally taking 15Hz - 20Hz. Doing so is on the one hand to avoid possible stall phenomena of the motor in the low-frequency state and the temperature rise problem of the motor running at low frequency for a long time, and on the other hand to reduce the system oscillation caused by frequent state migrations.

[0043] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An automatic constant pressure water supply control system based on PLC, characterized in that, The functions of the control system include: power frequency manual function, variable frequency automatic function, semi-automatic operation, manual operation, PID control function, water extraction pump system function, and pressurization pump system function.

2. The automatic constant pressure water supply control system of a PLC according to claim 1, characterized in that: The so-called power frequency manual function is specifically to turn the transfer switch to the "power frequency" position. The operator can decide to start or stop the operation of any pump according to needs. In this operation mode, the PLC, PID, frequency converter, etc. do not participate in the control.

3. An automatic constant pressure water supply control system for a PLC as claimed in claim 1, characterized in that: The so-called variable frequency automatic function is specifically to turn the transfer switch to the "variable frequency" position and press the variable frequency start button. The system will automatically judge and select the inlet of the starting variable frequency operation pump and enter the automatic operation.

4. An automatic constant pressure water supply control system for a PLC according to claim 1 of the patent claim, characterized in that: The so-called semi-automatic operation is specifically that when there is a problem with the PLC system and the automatic control system fails, the system works in a semi-automatic state at this time, that is, one pump has the variable frequency automatic constant pressure control function. When the water consumption is insufficient, one or several power frequency pumps can be manually put into operation.

5. An automatic constant pressure water supply control system for a PLC as claimed in claim 1 of the patent, characterized in that: The so-called manual operation is specifically that when the pressure sensor fails or the frequency converter fails, to ensure water supply, the six pumps can operate in the manual power frequency mode respectively. The actual operation of the implementation effect proves that this control system constitutes the most economical structure for the automatic control of multiple deep well pumps. In the software design, the instantaneous pressure and current impact during the switching between variable frequency and power frequency are fully considered. The soft start of each pump is the key to solving this problem.

6. An automatic constant pressure water supply control system for a PLC according to claim 1, characterized in that: The so-called PID control is specifically that when the pressure set value is greater than the actual pressure value, the PID instrument will increase the analog output value to accelerate the operation of the water pump motor; when the pressure set value is less than the actual pressure value, the PID instrument will reduce the analog output value to reduce the operation of the water pump motor.

7. An automatic constant pressure water supply control system for a PLC as described in claim 1 of the patent claim, characterized in that: The so-called water extraction pump system function is specifically that the control system is designed with six pumps in two groups. The cumulative running time of each pump motor can be displayed and rotated every 24 hours, which not only ensures that there is a standby pump in the water supply system, but also ensures that the pumps in the system have the same running time and ensures the reliable life of the pumps.

8. An automatic constant pressure water supply control system for a PLC according to claim 1 of the claims, characterized in that: The so-called pressurization pump system function is specifically that the pressurization pump is equipped with a soft starter and a motor protector to ensure the long-term and reliable operation of the pressurization pump. At the same time, it is equipped with a water level sensor, a digital display instrument and a water shortage sensor for the high-level water tank. To ensure the constant pressure water supply of the entire main water pipe network, when the high-level water tank is full and there is water in the main water pipe, the pressurization pump stops. At this time, the main pipe pressure will "hold pressure", which will eventually cause the main pipe pressure to rise and transmit this pressure to the water extraction pump house.

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