Application of sensor automatic identification device in air conditioning energy saving

Building energy conservation is an emerging field that is being researched and invested heavily in at home and abroad. Central air conditioning is one of the important equipment that cannot be ignored in modern buildings. While providing people with a comfortable environment, central air conditioning also brings huge energy consumption. According to statistics, in large and medium-sized hotels and guesthouses, the energy consumed by central air conditioning accounts for more than 60% of the energy consumption of buildings. Therefore, the development and application of central air conditioning energy-saving technology is an important field of building energy conservation. Actively developing and rationally utilizing energy-saving control technology is the fundamental way to reduce air conditioning energy consumption. In the central air conditioning energy-saving system, the temperature is detected by sensors installed on the outlet pipe and the return pipe, and the actual temperature is sent to the automatic control device for comparison with the set temperature and operation control. This article mainly introduces the application of sensor automatic identification device in central air conditioning energy-saving control.

1 Central air conditioning variable flow energy saving principle

Variable flow refers to the flow of fluid (gas or liquid) changing according to a certain law when it flows through a specific pipe or air duct. The purpose of using variable flow to transport fluid is to achieve energy saving in the transportation system while meeting the terminal load requirements. The central air-conditioning system is mainly composed of the air-conditioning host, cold and warm water control equipment, cooling water control equipment and cooling tower fan control equipment. In the central air-conditioning system, the capacity of the cold and warm water pump, cooling water pump and cooling tower fan is selected according to the maximum design heat load of the building, and a 10%-15% margin is left. Throughout the year, the system operates at a fixed maximum water flow for a long time.

Due to the changes in seasons, day and night, and user load, the actual heat load of the air conditioner is much lower than the design load most of the time. The number of operating hours with a load rate below 50% in a year accounts for more than 50% of the total operating time. When the air conditioning cooling load changes, the required air conditioning circulating water volume should also change accordingly with the load. Therefore, the use of variable flow control technology can achieve energy saving in the central air conditioning system, including variable flow control of the central air conditioning primary pump system for the cold and warm water pumps, cooling water pumps and cooling tower fans; variable flow control of the fresh air system, variable air volume system, primary return air system and secondary return air system in the central air conditioning system. These variable flow control systems generally use automatic control technology and variable frequency speed regulation technology to achieve energy saving by controlling the flow.

After the system is started normally, the temperature sensor and transmitter, flow sensor and transmitter, differential pressure sensor and transmitter, smart meter and other devices installed on site transmit the detected data to the smart I/O module. By comparing the measured value with the set value, the deviation and deviation change rate of the value are calculated. Through fuzzy control operation, the operation frequency of the inverter is adjusted in real time to meet the needs of the system load. Among them, the detected return temperature of cold and warm water is used to control the speed of the cold and warm water pump; the detected cooling water outlet temperature is used to control the speed of the cooling water pump; the detected cooling water inlet temperature is used to control the speed of the cooling tower fan. The power consumption of the water pump and fan during operation is proportional to the cube of the speed. Controlling the speed can achieve significant energy saving. The pressure difference of the supply and return water main pipe detected by the differential pressure sensor is mainly used for protection function. When the pressure difference exceeds a certain value, the opening of the bypass valve is controlled to ensure the safe operation of the system; the value detected by the smart meter is used for system management, calculating the power consumed by the system, drawing energy saving curves and energy saving history records.

2 Composition of the sensor automatic identification device

The air conditioner host is usually installed in the basement or a special room. The working environment is bad and it is far away from the room that needs heating (or cooling). During the operation of the air conditioner, we need to collect the supply and return water temperature of the chilled water and the inlet and outlet water temperature of the cooling water. In the traditional air conditioning system, the sensor and the control computer are connected by cable wiring. When debugging, monitoring and repairing the system, it is often necessary to disconnect the connecting cable at the output end of the sensor for some test operations. When the operation is completed and the connection is restored, it is easy to cause connection errors. At the engineering site, the sensor number identification is unclear or even disappears. The engineering technical documents are difficult to obtain or even lost. In this way, it is necessary to spend considerable manpower, material resources and time to identify and confirm the connection relationship between each sensor and each input end. Once the connection between the sensor and the input port is wrong, it will cause system operation failure. In view of this situation, we study a sensor automatic identification device, which uses the sensor automatic identification technology to automatically identify the category and purpose of each sensor online, without manual number identification and number connection. It can ensure that the system automatically operates according to the input-output relationship agreed in the design, which can reduce the system construction, debugging and maintenance costs and avoid accidental failures caused by connection errors. This is a device that can automatically identify the type and purpose of each sensor without the need for interface numbering, ensuring that the automatic control device always operates according to the agreed input-output relationship.

The sensor automatic identification device consists of a sensor, an analog input unit, an operation unit, a sensor automatic identification rule base, a sensor failure judgment rule base, a sensor identification unit and a sensor information display unit. Its principle block diagram is as follows:

(1) The number of sensors is 1 to 10, the analog level of each sensor output is the same, the actual effective value range of the physical quantity collected by each sensor is relatively small compared to the full range, or the characteristic quantities of the physical quantity collected by each sensor, such as time domain, frequency domain or correlation characteristic quantities, are significantly different. Most commonly used sensor groups in automatic devices can meet the requirements.

(2) The operation unit performs feature extraction operation on the output signals of each sensor. The extracted feature quantities may include arithmetic mean, variance, range, differential value, fluctuation period, etc. The number of operation samples may be 10 to 20. The feature quantities and the number of samples that need to be extracted during operation are determined by the automatic recognition rule base.

(3) The automatic identification rule base and the failure judgment rule base are preset offline according to the types and characteristics of the sensors used in the system. The built-in rules of the two rule bases come from professional knowledge, common sense knowledge, and experiential knowledge.

(4) The identification unit automatically identifies the characteristic quantities of each sensor output by the operation unit according to the rules of the automatic identification rule base and the failure judgment rule base. The output of the identification unit is the type, purpose, detection point location of each sensor used in the system, and whether it is absolutely failed. The output of the identification unit is sent to the computer controller in the form of a digital signal. After the controller confirms that all sensors are normal, it enters the control operation program and ensures that the control operation is carried out according to the input-output relationship agreed in the design, and displays it accordingly on the display.

(5) The automatic identification and failure judgment program is executed after the central air-conditioning system has been running for 15 minutes.

3 Working principle of sensor automatic identification device

(1) Collect the sampling values ​​of each sensor. The number of samples is 10 to 20 times and the sampling period is 0.5 to 5 seconds.

(2) The sampling values ​​of the sensor are respectively subjected to feature extraction operations by the operation unit. The extracted feature quantities include arithmetic mean, variance, variation range, 1st, 2nd, 3rd, and 4th order derivatives of the data with respect to time, fluctuation period of the data, frequency domain characteristics, and correlation. The number and types of feature quantities to be extracted vary depending on the automatic control device and are determined by the built-in automatic recognition rule library.

(3) The characteristic quantities of each sensor are compared and judged by the sensor identification unit. The input information of the comparison and judgment operation is the characteristic quantity, type and purpose of each sensor; the basis of the comparison and judgment operation is the sensor automatic identification rule base; the decision of the comparison and judgment operation is the type, purpose and input port of each sensor for the automatic control device.

(4) After the automatic control device operates normally, the data collected by each sensor is monitored online for a long time. When the data collected by the sensor deviates from the range, it is judged as an absolute failure, and an alarm signal is provided in the sensor information display unit; when the data collected by the sensor is within the range but deviates from the normal range, it is judged as a relative failure (relative failure occurs in the early stage after the system is started and has not yet entered stable operation), and a hold operation signal is provided (hold time T can be set). After the hold time T, the data collected by the sensor is within the range but still deviates from the normal range, which is judged as a connection relationship error, and the sensor information display unit provides a connection error alarm signal. The judgment basis for long-term monitoring is the failure identification rule base.

The hardware and software for realizing the automatic identification method mainly involve the CPU unit, memory, and operation interface. The configuration of the hardware and software can be considered together when configuring the hardware and software of the automatic control device, and does not need to be set separately.

4 Conclusion

The key to the sensor automatic identification device is to correctly establish the sensor automatic identification rule base and the sensor failure judgment rule base. In addition, the sensor operation unit must also establish various operation rules. We can select the operation results of different operation models to obtain optimized control of the controlled system. This device is different from the method of adding wired or wireless channels and encoding and identifying sensors. Instead, it uses the characteristic quantity of the sensor's own transmission signal for identification, that is, using software technology to replace hardware configuration. The use of this device can avoid errors in the interface connection relationship between the sensor and the control device, and ensure that the automatic control device always operates according to the agreed input-output relationship, so as to reduce construction and debugging costs and improve work efficiency. The use of the sensor automatic identification device in the central air-conditioning energy-saving system is an application case, and this technology can also be used in other motor-driven automatic control systems.