Abstract: This article introduces a dedicated stress sensor for intelligent tensioning equipment. This sensor directly acquires the changes in prestress during the tensioning process of prestressed steel bars, achieving real-time and digital measurement of prestress. The stress sensor was developed in conjunction with the QFZ600-25 tensioning jack manufactured by Shanghai Jack Factory, and its circuitry is universal. Calibration test results show that the stress sensor exhibits good linearity between input and output, high accuracy, and meets design requirements. Keywords: prestress measurement; stress sensor; sensor circuit [align=center]Study of One Kind of New Dedicated Stress Sensor QIAN Houliang, MEI Xue, LIN Jinguo (Nanjing University of Technology, Nanjing 210009, china)[/align] Abstract: In this paper, the authors present the idea of ​​one kind of dedicated stress sensor. With this sensor, we can directly obtain the change of the prestress in the process of tension, and make the tension process real-time and digital. The structure of this kind of stress sensor is developed according to the QFZ600-25 tension jack, and the circuit of the sensor is current. The experimental results show that there is a quality of linearity between the values ​​of the input and output of the stress sensor, and the qualified precision is achieved. Keywords : prestress measurement; stress sensor; sensor circuit 0 Introduction The design of this stress sensor is an important part of the development process of intelligent tensioning equipment. Prestressing tensioning involves many factors such as environment and wear, and is a non-linear transmission process. The accuracy of prestressing tensioning directly affects the safety and lifespan of prestressed components. Therefore, if the tensioning process is out of control, it can lead to longitudinal cracks at the anchorage end of the component or insufficient stress, resulting in deformation later on. In severe cases, it can cause cracks in the component or even breakage of the prestressing tendons, leading to major accidents. Accurate measurement of prestress is a crucial step during the tensioning process. In traditional prestressing measurement, prestress is obtained indirectly by measuring the oil pressure of the oil pump through a jack driven by an oil pump, followed by manual reading. This method has drawbacks such as slow reading, the need for conversion in prestressing acquisition, and losses during transmission. The new stress sensor is designed to address the shortcomings of traditional prestressing tensioning equipment. This sensor converts the tension force on the prestressing tendons into a voltage signal, which is then processed by a Siemens EM235 CN analog-to-digital converter. This sensor offers high accuracy and fast reading, minimizing measurement errors. 1. Sensor Mechanical Structure Design 1.1 Structural Description Figure 1 shows the structure of the elastic element of the new special-purpose stress sensor. This stress sensor is a through-hole sensor designed based on the QFZ600-25 tension jack, which works with the jack, tension pump, and matching anchor to complete the three functions of centering, anchoring, and force measurement. The core components of the sensor consist of an elastic element and a resistance strain gauge bridge. The elastic element is made of 40CrMnTi steel. The foil strain gauge needs to be protected with silicone during bonding to prevent it from falling off due to external factors such as vibration. The signal line uses a four-core shielded cable, which helps to reduce interference from external high-voltage signals. [align=center] Figure 1 Elastic Element of Stress Sensor[/align] 1.2 Basic Principle Figure 2 shows the working principle of the stress sensor. The jack performs anchoring under the drive of the oil pump, and the magnitude of its tension force is obtained by the stress sensor between the jack and the anchor. After being compressed axially, the stress sensor deforms, and the resistance of the resistance strain gauge attached to the surface of the elastic element changes, thereby causing the bridge to go from a self-balanced state to an unbalanced state. Depending on the magnitude of the force, the resistance value changes accordingly, and the bridge outputs a corresponding voltage value. The stability of the bridge and other factors are the focus of our research in designing the sensor circuit. [align=center] 1. Anchorage 2. Dedicated stress sensor 3. Tensioning jack 4. Prestressed steel bar Figure 2 Schematic diagram of stress sensor application[/align] The standard value of prestressed steel bar strength f[sub]pk[/sub] varies depending on the material of the prestressed steel bar, with a maximum value of 1860 MPa. In specific design, the tension control stress σ[sub]con[/sub] can be less than 0.75f[sub]pk[/sub] (or 0.9f[sub]pk[/sub]), but should not be less than 0.4f[sub]pk[/sub]. When designing the stress sensor, considering factors such as losses and over-tensioning, we set the maximum controllable tension stress of the sensor to 1860 MPa. The diameter of the through hole between the stress sensor and the QFZ600-25 jack is 43mm. The maximum diameter of the prestressed steel bar that can be tensioned is 20mm. The jack provides a maximum tension force of 600KN. The maximum pressure that the stress sensor should be able to withstand, f[sub]max[/sub], is given by formula (1): (1) r is the diameter of the prestressed steel bar that can be tensioned. 2 Sensor Circuit Design Figure 3 is the circuit diagram of the stress sensor. The resistance strain gauge forms a measuring bridge. When the elastic element is stressed, the bridge loses its balance. Since the output signal is a millivolt-level signal, if it is not converted, the signal is easily subject to serious interference from external signals during transmission. To solve this problem, we use a transmitter to convert the millivolt signal into a 4-20mA current signal. The chip of the transmitter is the XTR101 produced by BURR-BROWN. The output current signal is converted into a voltage signal using a 250-ohm load resistor, and then the converted voltage signal is used as the analog input signal of the Siemens analog-to-digital converter module EM235 CN. [align=center] Figure 3 Sensor circuit diagram[/align] Let e2 represent the voltage at pin4 and pin6, and e1 represent the voltage at pin3 and pin5. The input voltage is given by formula (2): Adjusting the value of resistor RS can compensate for the drift of the measuring bridge. R[sub]2[/sub] and R[sub]3[/sub] are zero-adjustment resistors. When the voltage input is zero and the output current is not 4mA, adjust R2 to make the current output 4mA. Capacitor C1 filters the power supply of the sensor. 3 Results Analysis We conducted four tension tests on the sensor using a pressure testing machine and recorded the corresponding output voltage VRL under different pressures (see Table 1). The average value of the output voltage under the same pressure value was taken. Based on the pressure values ​​and average output voltage values, we fitted a characteristic curve reflecting the pressure and voltage values. This curve provides an important basis for our tension control program development. [align=center]Table 1 Calibration of the pressure sensor Figure 4 Relationship curve between pressure and output voltage values[/align] Figure 4 shows the relationship curve between pressure and output voltage values. Although factors such as the nonlinearity of the bridge circuit, the nonlinearity of the resistance strain gauge, and the nonlinearity of the elastic element exist, we can see from the sensor's working characteristic curve that there is a good linear relationship between pressure and output voltage values ​​in the important working range (1～500KN). 4 Conclusion This new type of dedicated stress sensor is suitable for measuring prestress during tensioning. It has a simple structure, high measurement accuracy, and effectively solves the prestress measurement problem in intelligent internal stress measurement systems. It also features low cost and good stability. References [1] Huang Xianwu. 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Sensors and PLC Technology [M]. China Light Industry Press, 2006. Authors' profiles: Qian Houliang (1982-), male, from Taizhou, Jiangsu, Master's degree. Tel: 13655181114, E-mail: [email protected]. Mei Xue (1975-), female, from Nanjing, Jiangsu, PhD, Deputy Director of the Department of Intelligent Science, School of Automation, Nanjing University of Technology. Lin Jinguo (1957-), male, Professor, Dean of the School of Automation, Nanjing University of Technology.