Tension detection was initially used in safety protection applications, such as crane over-limit detection, elevator over-limit detection, and cable tension detection. With the increasing speed of automated production and the increasing requirements for product quality, accurate tension detection and control have played an increasingly important role in automated production. For example, industries such as steel, nonferrous metals, papermaking, printing, plastic film, and textiles all require accurate and reliable tension detection and control to meet the goals of increasing product processing speed, ensuring stable and high-quality product quality, improving equipment production efficiency, and ultimately realizing intelligent production processes. According to different working principles, tension detection sensors can be divided into several types: capacitive [1], piezomagnetic [2], optical fiber [3], photoelectric [4], surface acoustic wave [5], image [6], and resistance strain, etc. Compared with other technical solutions, tension detection sensors based on the resistance strain principle have the advantages of maturity, stability, and accurate measurement, and are therefore the most widely used at present [7]. The tension detection sensor based on this principle is similar to the common force sensor (or weighing sensor) in structure and principle, but is developed for specific application requirements of tension detection. Therefore, this paper classifies and summarizes the application and design of force sensors in tension detection systems according to the characteristics of the structure, operating conditions and installation methods of the equipment under test in tension detection. Finally, combined with market demand, the importance of tension detection is briefly explained, and the future development direction of force sensors in tension detection is prospected. 1  Overview of tension measurement The objects of tension detection are mainly wires and strips, such as yarns, steel ropes, cables, films, steel strips and metal foils. In engineering mechanics, this type of constraint formed by soft ropes, chains or strips is called flexible constraint, also known as "flexible rope". Its characteristic is that it can only be pulled but not compressed, so when it is under load, only the axial force is not zero. The total internal force of the flexible cable in any cross section is tangent to the curve of the flexible cable in the ideal catenary state, so in general engineering applications, it is collectively referred to as the tension of the flexible cable. In engineering applications, the purpose of measuring the tension of the flexible cable can be roughly divided into two categories: safety monitoring and process (quality) control: 1.1 Safety monitoring Flexible cable structures are widely used in equipment and projects such as lifting and transportation, oil and mineral exploration. Using wire ropes or ropes as flexible cables can achieve lifting, hoisting and other operations. In this process, the tension control of the wire rope is very critical, and it is necessary to ensure the smooth and accurate transportation of heavy objects and the safe operation of the hoisting equipment. As a typical flexible cable, the wire rope is an ideal device for transmitting tensile force. When it passes through a pulley or a pulley block, the direction and size of the tension can be changed. If the friction of the pulley block is not considered, it can be considered that the tension it is subjected to is in a certain proportional relationship with the load. Since the steel wire rope has such characteristics as a flexible cable device, various lifting machinery must be weighed, overload alarmed and torque alarmed according to laws and regulations, and most of them look for solutions on the steel wire rope. Nowadays, the tension measuring device that supports the bridge-type force sensor at both ends of the fixed pulley of the overhead crane can make the weighing system reach an accuracy of more than 0.1%FS. 1.2 Process (quality) control In the production process of products such as silk, fiber, fabric, cable and steel wire rope, it is necessary to use a constant force to wind them on the drum. However, as the diameter of the drum increases during the winding process, the linear speed of the drum will gradually increase, so a tension measuring device (more commonly a beam-type force sensor) is needed to provide tension information feedback to the controller to control the speed of the drum so that it has a constant winding force. In the papermaking and printing industries, papermaking equipment such as rewinders, sizing machines, calenders, coating machines, and slitting machines require precise tension control during the production process, because the tension of the paper machine affects the quality of the finished paper, papermaking efficiency, and production costs. Generally, a tension detection device is installed on the paper guide rollers of each transmission part of the paper machine for tension control. For example, a pin-type or beam-type force sensor can be installed on both sides of the running roller for tension detection. In the steel and metallurgical industries, in order to control the quality of thin strip steel on the rolling production line, it is necessary to continuously measure and control the dynamic tension of the running thin plate during the annealing, cleaning, galvanizing and other processes. At this time, a large-capacity force sensor is generally used for tension detection. In the automotive industry, the quality of automobile tires is directly related to the dimensional stability and tension control of their extruded parts. In the structure of the all-steel radial tire, the inner liner, sidewall, tread, cushion rubber and other components are all produced by the extruder linkage line, among which the tension control of the linkage line is the key point of process control. Usually, the tension is directly detected by pairing two force sensors and assembling them at both ends of the detection guide roller. When the extruded material passes through the detection guide roller, the force is applied to the force sensor for measurement.
2  Tension measurement method and design analysis of force sensors
As mentioned above, tension detection based on force sensors has a wide range of applications. By measuring and controlling the tension, the safe operation of related equipment and structures can be ensured, and the production process can be optimized and improved. Although force sensors are used in tension detection in many industries, according to the type of equipment and installation method, this article divides their application methods in tension detection into the following types, and briefly analyzes their design requirements.
2.1  Tension series detection
The force sensor is directly connected in series with the flexible cable on a line, which is the simplest and most effective way to detect the tension of the flexible cable. The following three typical application examples illustrate the design technology of force sensors and their detection devices in this way.
Figure 1 is a structural diagram of a tension detection device installed at the fixed end of a wire rope on a certain lifting equipment and a schematic diagram of the appearance of a pull-type force sensor designed for this purpose. In the design of this device, the wire rope is not directly threaded into the ring hole of the sensor. The reasons are mainly based on the following two points: ① When the wire rope is directly threaded into the ring hole of the sensor, its bending radius is too small, which will cause the outer layer of the wire rope to break due to the small winding curvature when the wire rope is frequently subjected to the rated load. ② As a detection element, the force sensor has a relatively high working stress (generally higher than 200MPa) at its rated load in order to achieve a certain output sensitivity. There is a stress concentration area inside. If it is directly used in the load device of the lifting equipment, there is a certain risk. Therefore, as shown in Figure 1, protective plates with sufficient overload capacity are attached to both sides of the force sensor to ensure the safety of the detection device itself. In addition, according to the requirements of this type of tension detection, the force sensor structure has been optimized: not only is it uniquely designed to match the installation space in terms of appearance and connection structure, but it is also different from the general plate ring sensor in internal structure. Instead, it adopts a structure with a plate ring as the frame and a shear blind hole in the strain area. This structure has two main advantages: one is that it improves the overload capacity of the sensor; the other is that the sensor will not produce obvious interference signals to the torsion of the wire rope.