Application of PLC in fire alarm and processing system

1 Introduction

PLC (Programmable Controller) is a control device specially designed for industrial automatic control. It has been widely used in the field of industrial automation. Its characteristics are strong anti-interference ability, high reliability, small size, easy design, use and maintenance. With the development of computer technology, network technology and communication technology, automatic control has gradually developed towards intelligence, networking and remote control. In order to meet the needs of the market, PLC manufacturers have successively launched PLCs with network communication capabilities and related products.

SIEMENS s7-200 series PLC is widely used in various industries, and fire alarm and its processing system are of great significance to ensure the safety of life and property in case of fire. However, the use of PLC to realize the control of fire alarm and processing system is not very widespread in China. This article focuses on the design and application of using PLC to realize fire alarm and processing system in combination with reality.

2 Hardware composition of fire alarm and processing system

The structure diagram of the system is shown below:

Figure 1 Structure diagram of fire alarm and processing system

The main components are:

1 Upper water tank. 2 Lower water tank. 3 Upper water tank water level sensor. 4 Lower water tank water level sensor. 5 Upper water tank water supply pump A. 6 Upper water tank water supply pump B. 7 Lower water tank water supply valve. 8 Sound and light alarm. 9 Smoke exhaust fan. 10 Smoke and temperature sensors. 11 Sprinkler pump. 12 Sprinkler head.

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This example uses Siemens' SIEMENS s7-200 series PLC, which has the characteristics of compact structure, modularization, strong scalability, and rich instruction set. The CPU model selected is CPU 224, which can expand 7 modules, up to 94DI/74DO, 16AI/16AO (analog input/analog output) and provides 14 digital inputs and 10 digital outputs. The input/output interface circuits all use optical coupling circuits, which have strong adaptability to external interfaces. Because an electric control valve is used, an EM 232 analog output module is also expanded. The module has 2 analog outputs, the current output range is 0~20mA, the current full range resolution is 11 bits, the accuracy at 25°C is ±0.25%, and the stabilization time is 2ms. It can meet the requirements of more complex control systems.

3 The functions to be achieved by the system are described as follows:

When the system is powered on and the smoke alarm or temperature alarm sends a signal, the system enters the operating state.

(1) The PLC controls the spray pump to turn on and the timer is set to 10 seconds. If there is no water flow in the spray pipe after the 10 seconds (water flow signal from the water flow sensor), turn off the spray pump and turn on the spray pump fault light, waiting for maintenance.

(2) The PLC controls the smoke exhaust fan to turn on. When the high temperature sensor sends a high temperature signal, it means that it is impossible for anyone to survive in the fire building. If the exhaust fan is kept on, it will only increase the firepower, so the smoke exhaust fan needs to be turned off.

(3) When the upper water tank is at a low water level, it means that water needs to be replenished, so the spray pump is turned on. The spray pump is turned on in the following rules: A and B pumps are turned on alternately. When the upper water tank reaches a high water level, the spray pump is turned off. During this period, when any of the A and B pumps is turned on, a 10-second timing is performed. If the timing time is up and there is still no water flow in the water replenishment pipe (water flow signal from the water flow sensor), it means that the water pump is faulty. At this time, the fault indicator of pump A (or pump B) is turned on, and the switch is made to pump B (pump A). ​​At the same time, the timing is performed. If there is no water flow after 10 seconds, the fault indicator of the water pump is turned on again, and the switch is made to another pump, and the cycle is repeated.

(4) When the water level in the lower water tank is low, open the lower water tank water supply valve and count for 10 seconds. If the timing is up and there is no signal in the lower water tank water supply flow indicator, open the lower water tank water supply valve fault indicator. When the water level in the lower water tank is high, stop water supply.

The above four steps are performed in parallel. When the water levels of the upper and lower water tanks are full, press the reset button and the system returns to the initial state.

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4 The SFC diagram of the system is shown below:

Figure 2 SFC diagram of fire alarm and processing system

The control quantities corresponding to PLC input/output are shown in the following table:

5 PLC Programming

The program is in the form of a ladder diagram, which uses symbols such as relay contacts and coils, and the graphical representation is easy to understand.

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Part of the ladder diagram program is as follows:

The I/O and internal variables involved in the program are shown in the following table:

The control process is as follows:

I0.6 is on → Q0.6, Q0.7, Q1.0 is on (after 10 seconds) → Q1.0 is off → Q1.1 is on → Q0.6, Q0.7 is on If I0.4 is on → Q0.6 is on → Q0.7 is on → Q1.1 is on. If I0.6, I0.1 is on → Q0.6 is on → Q0.7 is on → Q0.0 is on (after 10 seconds) → Q0.0 is off → Q0.1 is on → Q0.2 is on (after 10 seconds) → Q0.1 is off → Q0.2 is off → Q0.1 is on → Q0.3 is on (after 10 seconds) → Q0.1 is off → Q0.3 is off (and so on). If I0.6, I0.3 is on → Q0.6 is on → Q0.7 is on → Q0.4 is on (after 10 seconds) → Q0.6 is on → Q0.7 is off → Q0.5 is on → Q0.4 is off. If I0.6 is on → Q0.6 is on → Q0.7 is on → Q1.0 is on; if I0.7 is on → Q0.7 is off.

Because the fire alarm and processing system is related to the safety of the company's personnel and property, the PLC control system has adopted a series of hardware and software anti-interference measures. For example, the anti-interference design of the power supply uses an isolation transformer, which has a good effect on suppressing interference signals in the power grid. In terms of software anti-interference, economical and effective methods such as digital filtering and software fault-tolerant technology are used to further improve the reliability of this system.

6 Conclusion

The author's innovation is to apply PLC to the control system of fire alarm and processing. This system has been applied in a district-level municipal company and achieved good practical results, generating economic benefits of more than 800,000 yuan, and has good safety performance. Practice has proved that constructing a fire alarm and processing control system with PLC as the core is a simple, effective and low-cost solution with high reliability, flexibility and economic applicability.