Abstract: This paper presents a simple, practical, and high-precision automatic temperature characteristic detection system for temperature relays based on the principle of air measurement. The system uses a resistance heating furnace to simulate the ambient temperature around the temperature relay. By controlling the temperature changes within the furnace, the operating temperature of the temperature relay is measured. The system controls the temperature field according to a set temperature control curve and employs a fuzzy algorithm and least squares method for error calibration. The furnace wires are grouped by power, and an orifice plate is placed in the detection section of the test chamber to ensure uniform temperature in the test area, effectively improving the system's detection accuracy. Testing shows that the system's accuracy meets the requirements and it has good performance. Keywords: Temperature relay; Temperature characteristic; Resistance heating furnace; Fuzzy algorithm; Least squares method 0 Introduction Temperature relays are commonly used to automatically cut off circuits when temperatures are abnormal, realizing functions such as electrical overheat protection, temperature control, fire alarm, and automatic ignition, effectively preventing accidents. Therefore, they are widely used in various fields. The key process in the production of temperature relays is the detection of temperature characteristics (operating temperature during heating and recovery temperature during cooling). In the past, manual testing using mercury thermometers was the primary method, which was inefficient, labor-intensive, and inaccurate. With continuous technological advancements, automated testing methods have gradually replaced manual methods. Currently, there are three main methods for detecting temperature characteristics both domestically and internationally: the liquid method, the air method, and the test block method. The liquid method, while offering advantages such as constant temperature, good temperature uniformity, and high accuracy, has limited applicability due to the direct immersion of the sample in the liquid, and is often used for testing sealed relays. The test block method, conducted at room temperature, suffers from difficulty in temperature adjustment and poor accuracy, making it unsuitable for mass production. The air method utilizes temperature-settable air heating equipment with uniform stirring. While its accuracy is lower than the liquid method, it is simple to operate and does not require a sealed product structure, making it ideal for mass production. In summary, each of the three methods has its own characteristics. This research, based on the actual characteristics and technical requirements of the products being tested by an aviation factory, comprehensively analyzes and determines to conduct research and development based on the air method, focusing on temperature uniformity and temperature control algorithms. This will enable the testing of temperature relays. 1. Detection Principle [b]1.1 Working Principle of Temperature Relay[/b] The working principle of aviation temperature relays is basically the same as that of civil temperature relays. A disc-shaped bimetallic strip is usually used as the actuating element. Utilizing the working characteristics of bimetal, when the temperature rises, the bimetallic strip bends. When it bends to a certain extent, it drives the electrical contacts, thus connecting or disconnecting the load circuit. When the temperature drops, the bimetallic strip gradually returns to its original shape. When it returns to a certain extent, it reverses and drives the electrical contacts, thus disconnecting or connecting the load circuit. Its principle is shown in Figure 1. [align=left]1.2 Detection Principle of Temperature Relay The temperature relay operates according to changes in the ambient temperature. Therefore, the operating temperature and return temperature of the temperature relay are detected by controlling and detecting changes in the ambient temperature. 1.3 Detection of Temperature Field[/align][align=left] In this system, the sample to be tested is placed in a resistance heating furnace for heating or cooling. Since the uniformity of the temperature field (±2℃) and the temperature accuracy deviation (±2℃, temperature fluctuation ±1.5℃) in the heating furnace are not very high... The accuracy and repeatability of product testing parameters are affected by the location of the product in different positions within the heating furnace. Therefore, designing a high-performance testing temperature field is crucial for achieving the testing function and improving testing accuracy. This system designs a resistance heating furnace to simulate the testing temperature field. Research is conducted on temperature uniformity and temperature control algorithms to improve the testing performance of the temperature field. 1.3.1 Temperature Uniformity The testing temperature field in this design uses air as the heat transfer medium. During the testing process, although a blower is used to enhance heat convection, the heat radiation from the test chamber walls and the obstruction effect of the heat-conducting plate on the temperature relay fixture result in poor hot air circulation. Consequently, the testing temperature field decreases in height on both sides in the vertical direction, forming a temperature gradient field. Temperature non-uniformity is inevitable. Experiments show that the temperature difference between the measuring point and the tested object can reach more than 4℃. This directly reduces the testing accuracy. In this system... Two measures were taken to improve temperature uniformity: first, a longer preheating time was used to fully heat the air in the entire test chamber, reducing the temperature gradient; second, an orifice plate was placed in the detection section of the test chamber to ensure uniform airflow, thereby effectively guaranteeing the temperature stability of the detection point section. [align=left]1.3.2 Temperature Control Algorithm Based on theoretical analysis and repeated experiments, the ideal temperature control curve for the detection temperature field is shown in Figure 2. To improve testing efficiency, different temperature rise rates were adopted: At the start of operation, the temperature was rapidly increased by K; when the temperature rose to the relay operating temperature range, to ensure detection accuracy, the temperature field temperature was controlled to rise at a lower, constant rate while simultaneously detecting the relay operating temperature; when the temperature rose to the upper limit of the relay operating temperature range, the furnace heating power was gradually reduced or even stopped, and the temperature field temperature was controlled to cool down at a lower, constant rate K. Simultaneously, the relay recovery temperature was detected. Due to the characteristics of electric resistance furnaces such as large hysteresis, large inertia, severe nonlinearity, and time-varying parameters, this system uses a fuzzy algorithm to control the temperature field according to the set temperature control curve. That is, the temperature and its deviation are first fuzzified, and then fuzzy inference is performed according to the fuzzy control rules determined by experiments. Then, the precise value is obtained by fuzzy judgment through the centroid method. Finally, this value is converted by DA and then used to control the power regulator and voltage regulator, thereby controlling the temperature rise of the electric resistance furnace. In order to ensure a smooth transition from rapid heating to constant heating, a basic fuzzy PD controller with constant value setpoint is designed in the 0~t time segment. In order to ensure that the input of the ramp wave can be tracked, a fuzzy controller with integral action is used in the constant heating and cooling segments. In the temperature control process, in order to improve the detection accuracy and detection efficiency, the detection segment with a heating/cooling rate of (1~2)℃/ITlin is the best for detection effect. 2 Automatic Detection System 2.1 Detection Process During the detection, the system automatically controls the temperature field and displays the relay's operating temperature in real time. The system automatically and gradually increases the temperature when it reaches the lower limit of the relay's nominal operating point. When the relay operates, the operating temperature is displayed in real time with a prompt. Then, the temperature is automatically and gradually decreased. When the relay returns to its original position, the return temperature is displayed in real time with a prompt, thus completing one full testing process. If the product does not operate when the temperature rises to the upper limit of the relay's operating point or drops to the lower limit of the return point throughout the entire testing process, the system issues an over-limit alarm and interrupts the testing process. In this case, the tested temperature relay can be determined to be a defective product. 2.2 System Structure The system consists of a control console and a resistance heating furnace, as shown in Figure 3. 2.2.1 Hardware Platform The basic block diagram of the hardware platform is shown in Figure 4. Due to the severe electrical interference, complex and variable environment, and long continuous working time in industrial settings, the host computer uses the IEI-61O industrial control computer, which has strong anti-interference capabilities and high reliability. The I/O interface uses the general-purpose 1-7000 series distributed data acquisition module, which greatly simplifies the hardware design and ensures system reliability. The temperature measurement section uses a JWB integrated temperature transmitter, and a transmitter module for linearization and amplification is installed in the junction box, thereby improving the sensor's measurement accuracy. 2.2.2 Software Platform The software platform of this system is developed using Microsoft Visual Basic 6.0 under the Windows 2000 operating system. It consists of three basic modules: a detection module, a user interface, and a printing and data backup module. The detection module, as the control core of the entire detection system, realizes basic detection functions such as automatic temperature rise and fall control, real-time acquisition of temperature signals and temperature relay status signals, detection and recording of action temperature and recovery temperature, and over-alarm. As a pioneering development tool in visual programming, it has unparalleled advantages in user interface development compared to other development environments, offering powerful functionality while remaining easy to use. The user interface of this system mainly includes three parts: command buttons and menus, real-time temperature curves, a real-time display window for detection status, and a real-time parameter modification window, thus providing a user-friendly human-machine interface. 2.2.3 The resistance heating furnace is self-made according to commonly used industrial standards, using nickel-chromium alloy as the heating wire. An orifice plate is placed in the furnace test chamber to improve the airflow uniformity in the detection section, and water cooling is used to dissipate heat from the blower. 2.3 Technical Features This system successfully achieves temperature detection with a wide measurement range from tens to hundreds of degrees Celsius; it achieves fitting of measured temperature curves, automatic temperature control, and completes technical challenges such as overall system error calibration, thus improving the overall testing performance of the system. The main technical key adopted by this system is: A power-based grouping strategy for the resistance heating furnace heating wires. Because the three-phase power regulator indirectly adjusts the heating power by regulating the on/off time of the heating wires to control the heating and cooling of the resistance furnace, and high-power heating wires have large adjustment inertia, making it difficult to accurately control the heating and cooling rate, a grouping strategy is adopted where low-power heating wires are used in the low-temperature section and high-power heating wires are used in the high-temperature section. Practice has proven that this improves the performance of the resistance heating furnace and prevents temperature overshoot. **3. Overall Least Squares Error Calibration** This system employs overall least squares error calibration. First, a high-level temperature probe is used to measure the temperature at the system's detection point, while simultaneously reading the analog current value sent to the computer. A set of data is measured within the temperature range. Then, this set of data is fitted using MATLAB 6.1 with a fourth-order least squares method to obtain the coefficients of each term in the least squares polynomial. Finally, these coefficients are substituted into the program to fit the measured temperature curve, completing the overall system error calibration. **System Metrology Experiment** Based on the temperature deviation range (±5℃) of the test sample, the deviation between the measured temperature value and the standard temperature value in the test field is required to be within ±1.5℃. Five locations were randomly selected in the test field for temperature metrology testing. The results are shown in Table 1. The temperature metrology results show that the deviations are all less than 0.5℃, within the specified deviation range, meeting the accuracy test requirements. **4. Conclusion** This automatic detection system has passed temperature metrology and testing acceptance. The deviation between the measured temperature value and the standard temperature value is less than 1.5℃, within the specified range, and the system temperature uniformity meets the requirements. This detection system has been put into production, performs well, and has significant economic benefits and application prospects. Click to download: Research on Automatic Temperature Characteristics Detection System for Temperature Relays. Editor: Chen Dong