Design and implementation of air conditioning control system for clean operating room

Abstract: The temperature and humidity control effect of the air conditioning system in the clean operating room has a direct impact on the surgery. This paper introduces a control system based on Siemens S7-200PLC and TD-400C to realize the control of temperature and humidity in the clean operating room. Due to the large hysteresis of temperature and humidity, it is difficult to obtain satisfactory results using the traditional PID adjustment method. By introducing the temperature change rate to adjust the dead zone, the control effect of the system is improved and overshoot is effectively avoided. After actual operation, its feasibility in the clean operating room control system is verified.
Keywords: temperature and humidity; PID; PLC; temperature change rate; dead zone

0 Introduction
As one of the units with the highest control requirements in the hospital, the clean operating room has constant control requirements for temperature and humidity. However, since the shadowless lamps and ordinary lighting lamps in the operating room will affect the temperature and humidity, it is difficult to meet the requirements of keeping the temperature and humidity constant during surgery. Therefore, the temperature and humidity indicators are the most important control indicators in the clean operating room. Temperature directly affects the comfort of patients and medical staff. When the humidity in the room is greater than 60%, the speed of bacterial reproduction will be greatly accelerated. From the perspective of controlling bacterial growth, humidity control is also extremely important. This requires the air conditioning system in the operating room to adjust the temperature and humidity according to a specific algorithm, and quickly respond to real-time control requirements according to the set values ​​on the corresponding operation panel. Here, the control system based on Siemens S7-224cn PLC and text display TD400C is used.

l System control requirements
1.1 Temperature control requirements
The return air temperature is detected by the integrated temperature and humidity sensor set in the air duct, and the measured temperature signal is sent to the analog input terminal of the PLC. The PLC compares the measured temperature with the temperature setting value of the operating room panel and performs PID calculation, and outputs the result to the corresponding analog output, such as the electric regulating valve of the refrigeration/hot water, and controls its opening to achieve the purpose of temperature control.
1.2 Humidity control requirements
The return air humidity is detected by the integrated temperature and humidity sensor set in the air duct, and the measured temperature signal is sent to the analog input terminal of the PLC. The PLC compares the measured humidity with the humidity setting value of the operating room panel and performs PID calculation, and outputs the result to the corresponding analog output. In winter mode, the purpose of humidity control is achieved by adjusting the opening of the humidifier electric regulating valve. In summer mode, humidity control is not achieved by a single control of the humidifier opening, but by comprehensive PID control of the water valve regulator and heater.
1.3 Air duct pressure control
According to process requirements, the pressure in the air duct needs to be maintained at a certain value to meet the medical requirements of the operating room. The system drives the blower through the inverter. The inverter frequency value that meets the process requirements is used as the set value in advance, and the pressure value measured by the return air pressure detection sensor set in the pipeline is used as the feedback value. The output frequency of the inverter is controlled by PID calculation to meet the wind pressure control requirements.
1.4 Human-machine interface requirements
According to the on-site process requirements, an operation panel is installed in the operating room to set the temperature, humidity and control the system start and stop. At the same time, the panel has a real-time temperature and humidity display function; similarly, the human-machine interface TD400C is installed on the control cabinet, and the temperature and humidity can also be set to start and stop the unit through TD400C. For the purpose of debugging the system, a forced mode is set on TD400C, and the return air temperature and humidity are manually set to shield the alarm signal.
The control priority of TD400C is higher than the operating room setting panel, and the setting signal from the operating room can be shielded.
1.5 Safety requirements
There are three types of fault signals in this system, namely, blower fault signal, air shortage protection fault signal, and medium-efficiency pressure difference signal. These three signals are all for ensuring air circulation in the air duct. The alarm should have sound and light displays to remind staff to take measures.

2 System hardware and process introduction
2.1 Hardware composition
The entire system hardware consists of detection components, control components and actuators.
The temperature and humidity of the return air are detected by the integrated temperature and humidity sensor installed in the air duct; the return air pressure is detected by the pressure sensor installed in the pipe air duct; the water temperature is detected by the temperature sensor installed on the water pipe and the working mode is determined by it; the control components mainly refer to S7-224 PLC, TD400C and operating room control panel; the actuators include the electric regulating valve of the water pipe, the humidifier regulating valve and the heater regulating valve, etc.

2.2 Process Introduction
After the system is started, if there is no alarm signal such as lack of air protection, the system will start to detect the water temperature working mode through the sensor installed on the water supply pipe. The working mode is divided into winter and summer. If the water temperature of the water supply pipe is greater than 30℃, it is winter mode, and if it is lower than 30℃, it is summer mode.
In winter mode, if the return air temperature is lower than the set temperature, the water valve actuator is opened to flow more hot water to increase the temperature; if the return air humidity is lower than the set humidity, the humidifier opening is opened to increase the humidity. In winter mode, if the return air temperature is higher than the set value, the output opening of the water valve actuator is reduced; usually, the return air humidity in winter mode is always lower than the set humidity, so the possibility of the return air humidity being higher than the set humidity is not considered. In summer mode, the control requirements are mainly dehumidification and dehumidification. The system requires humidity priority adjustment. The so-called humidity priority adjustment means that when the humidity is high in summer, the controller adjusts the opening of the water supply pipe (the pipe is filled with chilled water at about 7℃) after calculation to achieve the purpose of cooling and dehumidification, and the temperature difference caused by this is compensated by turning on the electric heater. When the return air humidity reaches the set requirement, the system automatically enters the temperature control state. The working mode selection flow chart is shown in Figure 1.

3 System software design
According to the process requirements and hardware configuration, Siemens' STEP7 MICRO/WIN32 software is used for system configuration and TD400C human-machine interface design.
The software design mainly includes temperature and humidity control, forced mode selection, working mode selection and fault alarm processing.
3.1 Mode selection
The control mode setting refers to the system control is divided into TD400C setting (manual) and operating room control panel setting (automatic). In manual mode, there are forced and non-forced modes. In manual mode, the set temperature, humidity and system start and stop signals of the operating room panel can be shielded. For the purpose of debugging the system, the system also sets a forced mode. In forced mode, the sensor signals such as return air temperature, return air humidity, return air pressure, etc. can be manually set, and various fault alarm signals can be manually reset. The control mode selection flow chart is shown in Figure 2.

3.2 Fault alarm processing
According to the design requirements, if there is a blower fault signal, air shortage protection fault signal, or medium-efficiency pressure difference signal, there will be an audible and visual display to remind the staff to take measures. The flow chart is shown in Figure 3.

3.3 Temperature and humidity control
The core part of the software design is temperature and humidity control, which uses PID control to adjust temperature and humidity. PID control refers to the proportional integral differential control of a closed-loop control system.
The block diagram of a conventional PID control system is shown in Figure 4.

PID is a linear controller that forms a control deviation based on the given value r(t) and the actual output value y(t):

e(t)=r(t)-y(t)
controls the controlled object by combining the deviation proportion (P), integral (I) and differential (D) into a control quantity u(t) through a certain linear combination. Its control law is:

Where: KP is the proportional coefficient; TI is the integral time constant, and TD is the differential time constant.
When there is an error (or control deviation) between the target value and the detected value of the controlled quantity, the smaller the error, the smaller the operation quantity, and the larger the error, the larger the operation quantity. Therefore, the control algorithm contains a deviation proportional term, referred to as P action. When proportional control is applied to a self-balancing control object, its step change will leave a certain error, which is called steady-state error or offset. The control algorithm contains an error integral proportional term to eliminate the steady-state error, referred to as I action. The increase or decrease of the deviation is reflected in the operation quantity. In order to improve the control characteristics, the control algorithm contains a deviation differential proportional term, referred to as D action, which is a pre-action. The control algorithm that includes the above three actions is PID control.
Formula (1) can also be written as:

In the formula: KI = KP / TI is the integral coefficient; KD = KP · TD is the differential coefficient.
Considering that the controlled object has a large hysteresis and the PLC processes digital quantities, the formula (2) is discretized to obtain:

Where: θ represents the sampling period, e(k) represents the error at this moment, and e(k-1) represents the error of the last sampling period. If θ is small enough, this approximation can be quite accurate, and the controlled process is very close to the continuous control process. This algorithm is called a position algorithm. Since the position algorithm outputs the entire amount, each output is related to the past state. When calculating, e(k) must be accumulated, and the computer calculation workload is large. Moreover, because the u(k) output by the computer corresponds to the actual position of the actuator, if the computer fails, a large change in u(k) will cause a large change in the position of the actuator, which is often not allowed in production practice. Considering that the control objects of this system are all valves, it is appropriate to use an incremental PID control algorithm.

According to the recursive principle, we can deduce formula (3):

Formula (5) is called the incremental PID algorithm.
Since the determination of the control increment △u(k) is only related to the sampling values ​​of the most recent k times, it is easier to obtain a better control effect through weighted processing.
According to the on-site control requirements, the controlled process cannot have overshoot, that is, overshoot. However, combined with the on-site construction conditions and the large hysteresis characteristics of temperature and humidity, overshoot is inevitable.
The decisive link of temperature control quality lies in the adjustment of the PID control parameters of temperature, and the quality of PID parameter adjustment determines the quality of temperature control. In actual applications, the hospital hopes that the temperature adjustment time of the operating room is as fast as possible. However, the rapid response of temperature will cause overshoot and is likely to form a long-term temperature oscillation. The temperature adjustment speed and temperature control accuracy are often irreconcilable. If the adjustment speed is accelerated under the premise of accuracy, it means to reduce the oscillation as much as possible. The usual practice is to introduce a dead zone in the PID adjustment process to adjust the control effect. However, even if the dead zone protection is added, it is subject to the on-site conditions and its effect is not universal.
If Er is set as the temperature error value, Es is the temperature set value, Eb is the actual temperature value, and Et is the temperature dead zone, then:

The consequence of this is that even if the dead zone is introduced, the improvement of the PID adjustment effect is limited. For this reason, the temperature change rate is introduced, and the dead zone is artificially changed by changing the temperature change rate, so that the error value calculated by PID will change, and the adjustment speed will also change accordingly.
Let T1 be the temperature one minute ago, T2 be the temperature at this moment, and Tc be the temperature change rate, then Tc=T2-T1. Through Tc, we can not only judge the rise and fall of temperature, but also know how fast the temperature changes.
Therefore, the temperature dead zone is set using Tc and the current actual temperature error Es-Eb.
If Es-Eb>0, that is, the temperature has not reached the set value, and Tc>1, then let Et=|Es-Eb|, that is, for temperature PID adjustment, the error Er input is zero, then the temperature PID stops adjusting, and the temperature automatically rises to reach the set value. Vice versa.

4 Conclusion
Aiming at the control requirements of clean operating rooms, a control system with S7-224PLC as the control core and TD400C as the human-machine interface was established. This system meets the temperature and humidity control requirements of the operating room, avoids overshoot to the greatest extent, and improves the controllability and stability of the entire system.