## 1 Introduction  
Torque sensors are devices used to measure axial torque in rotating systems such as motors, engines, robot joints, and so on. A torque applied to a cylinder can be calculated by measuring the torsional deformation of the cylinder surface, just like measuring the applied axial force using a strain gauge on a component. The performance of the torque sensors can be largely evaluated by measurement range and sensitivity, but it is difficult for them to be compatible together. Increasing the sensor sensitivity reduces the measurement range, whereas a sensor with a wider measurement range must be less sensitive. When using conventional torque sensors, an appropriate compromise between the required measurement range and sensitivity suitable for the purpose is essential at the purchasing stage of the sensor. Recently, force/torque sensors are being used in a variety of fields to improve systems with high performance control of robots, beyond simple measurement [1]. [PERSON] et al. developed a multi-axial force/torque sensor that can measure six-axial force and torque for robotic application [2], and [PERSON] et al., also developed a 6-DOF sensor for micro manipulation [3]. Some studies have developed methods to improve the accuracy of torque measurements [4; 5; 6], while others have developed torque sensors that can have wide measurement range [7; 8]. Meanwhile, some torque sensors have been developed for biomedical purposes [9; 10]. Minimally invasive surgical robots to which force/torquesensors are applied for safe operation are still being studied actively [11; 12]. Recently, research has been conducted by [PERSON] et al., to apply a force/torque sensor to a system that enables catheterization at a distance [13]. [PERSON] et al., tried to secure the safety of minimally invasive robotic surgery (MIRS) by using an optical fiber Bragg grating (FBG) force sensor [14]. For these various purposes of force and torque measurement, different types of force/torque sensors have been developed. The most representative strain gauge type torque sensors are widely used due to their high accuracy. Force/torque measurement using non-contact displacement sensors such as optical, capacitive and inductive sensors requires a large displacement to obtain high sensitivity, which inevitably lowers the stiffness of the sensor. The FBG sensor has the advantage of accurate measurement of strain on an object like the traditional electronic strain gauge, and it is also sterilizable with a small size. However, its application is limited due to the complexity and high cost of the sensor system. Therefore, so far, electronic strain gauge-based force/torque sensors or simple strain gauge load cells are mainly used in robotic applications with high measurement accuracy and relatively low price [15]. Torque sensors for robot joints must simultaneously satisfy torsional stiffness and sensitivity in a compact size. [PERSON] et al. developed a square-cut torque sensor (SCTS) by modifying a conventional planar hub-sprocket type torque sensor, and they applied it to a hydraulic quadruped robot [16; 17]. Similarly, [PERSON] et al. have developed a torque sensor that can be manufactured inexpensively using a computerized numerical control (CNC) milling machine [18]. Along with these, torque sensors with two stacked hollow disks with wing pairs are also commonly used. In particular, [PERSON] et al. proposed a hexaform torque sensor composed of an increased number of wing pairs to satisfy both stiffness and sensitivity [19]. It is also possible to measure multi-axial force/torque by applying a similar structure such as the Stewart platform [20; 21]. However, the high-priced multi-axial sensors are not always better than single axis sensors, because excessive measurable dimensions that are not suitable for a given situation can cause inaccurate output force/torque or inaccurate motion due to low sensor stiffness. Torque sensors are required in a variety of industries, but in many cases, these industries cannot afford expensive products.