The Application of PLC in the Air Conditioning Temperature and Humidity Control System of Clean Operating Room

In the temperature and humidity control system of the clean operating room, the temperature and humidity control is a complex process with nonlinearity, large lag, and time variation. This article introduces the S7-200 PLC as the main controller to realize PID control of temperature and humidity under different working conditions. The operating results show that the temperature and humidity control system has high anti-interference ability, high reliability, stable operation, accurate temperature and humidity control, and can fully meet the application requirements of clean operating rooms.

In the air conditioning control system of the clean operating room, the heat and humidity exchange process of the air is mainly completed, so that it reaches the ventilation state point to meet the indoor temperature and humidity requirements. Its control object-temperature and relative humidity are two related parameters. While adjusting the temperature, the relative humidity will also change accordingly.

In the past, meters and DDC controllers often choose temperature priority or humidity priority for control according to needs, which makes the adjustment time of temperature and humidity control too long, low control accuracy, and it is difficult to achieve simultaneous temperature and humidity control.

Combining the temperature and humidity control of the clean operating room, this article introduces the use of PLC's powerful digital processing function to realize the temperature and humidity control of the clean air conditioner, and selectively mix, cool, dehumidify, and add air in the functional section of the modular air conditioning unit. Temperature, humidification and other processing to achieve the best control of temperature and humidity, this solution temperature and humidity control system has fast response speed and good stability, and can also be used in the temperature and humidity control of other air conditioners.

System overview and hardware composition

The room temperature and humidity control of the clean operation is mainly composed of three parts: surface cooling, electric heating, and electric humidification. They are placed in a combined air-conditioning unit. In summer, condensate is precipitated through the surface cooler and the water baffle, and the air is cooled and dehumidified. Electric heating heats up the air. In winter, the surface cooler heats up the air, and electric humidification is used to increase the relative humidity of the air, and finally meet the control requirements of the air supply temperature and humidity. Its structure is shown in Figure 1.

The clean operating room control system is mainly composed of PLC system, temperature and humidity sensor, surface cooling system, electric heating equipment, electric humidification equipment and operating room control panel. The system structure block diagram is shown as in Fig. 2.

In this system, the PLC controller uses S7-200 series CPU224 and extended analog input module EM231 and analog output module EM232. EM231 is a four analog input module, and EM232 is a two analog output module. The temperature and humidity sensor is selected from the SIEMENS company QFM2160 air duct temperature and humidity sensor. The QFM2160 sensor outputs a 0~10V standard voltage signal corresponding to the temperature and humidity, which is sent to the input interface of the EM231 and sent to the OMCS-5000C for display.

The analog output signal controls the meter cold valve, electric humidifier and electric heater module through the EM232 output interface, and controls the valve opening, humidification and heating respectively. The operating room control panel adopts OMCS-5000C, the surface cooling controller adopts SIEMENS SQX62 actuator and VVF31 two-way valve for control, and the electric heating adopts STY-380D three-phase voltage regulating module to control the heating of resistance wire.

Set the temperature and humidity through the operation control panel. The operation control panel converts the temperature and humidity setting value into a 0-10V voltage signal, which is connected to the EM231 module. CPU224 is used to realize logic control and analog operation, control the start and stop of fan, electric humidification and electric heating through I/O port, and control the opening degree of meter cold valve, electric humidification and electric heating through analog output, and finally control the indoor Temperature and humidity are controlled.

System software design

This control system mainly includes three parts: fan control, temperature control and humidity control. The room temperature and humidity setting for clean operation is generally t=21~27℃, φ=40%~65%. [[4]] According to the northern climate, the surface cooler is hot water in winter (the water temperature is generally greater than 45°C), and it is cold water in other seasons (the water temperature is generally less than 9°C). According to the relationship between the set temperature and humidity and the ambient temperature and humidity, the working conditions of the control system are divided into three types:

1) Ambient temperature>set temperature, ambient relative humidity>set relative humidity. After the fresh air and the return air are mixed, the temperature is reduced by the surface cooler to make the humidity equal to the set humidity, and the electric heating works to raise the temperature to the set temperature, and then air is sent to the room. The working process is shown in Figure 3(a).

2) Ambient temperature>set temperature, relative humidity<set relative humidity. After the fresh air and the return air are mixed, the surface cooler is cooled to the set temperature so that the temperature is equal to the set temperature. The electric humidification work raises the humidity to the set humidity, and then air is sent to the room. The working process is shown in Figure 3(b).

3) Ambient temperature <set temperature, ambient relative humidity <set relative humidity. After the fresh air and the return air are mixed, the surface cooler is heated to the set temperature so that the temperature is equal to the set temperature. The electric humidification work raises the humidity to the set humidity, and then sends air to the room. The working process is shown in Figure 3(b).

When the system is working, first start the fan to run. Then PLC measures the temperature and humidity of the return air through the temperature and humidity sensor and compares it with the set value, and performs calculations according to different working conditions. The PLC controls the fan, heating, and humidification power supply through the digital I/O port, and controls the valve opening, electric heating, and electric humidification through the analog output interface. Set the PLC timer interrupt 0 interrupt time to 250ms, and complete the PID calculation in the interrupt program.

In automatic control systems, PID control algorithm has been widely used as a classic control algorithm. S7-224 provides standard PID loop instructions, making the programming and implementation of PID algorithms very easy. The output result of PID operation, the control deviation e tends to zero, making the system reach a stable state.

The PID loop instructions (proportional, integral, and differential loop) provided by the S7-200CPU perform PID calculations based on formulas. PID loop operation depends on 9 parameters stored in the 36-byte loop table. Adjusting the size of the parameter value can easily change the temperature and humidity deviation and the deviation change rate, so as to quickly realize the temperature and humidity control.

、Conclusion

Using PLC has the characteristics of high stability and flexible structure to realize the intelligent control of indoor temperature and humidity. In the above-mentioned temperature and humidity control system, corresponding strategies are adopted for control according to different working conditions. The temperature and humidity control system has been put into operation in 4 hospitals in Tianjin and Shanxi. The control accuracy is: temperature ±0.5℃, relative humidity ±3%.

The operating results show that the temperature and humidity control system has high anti-interference ability, high reliability, stable operation, and can fully meet the application requirements of temperature and humidity control in clean operating rooms.