Contactless torque sensors based on optical methods: A review

Torque is one of the mighty core parameters of rotating machinery, and it has profound implications for the success of many engineering and industrial manufacturing activities. To date, more and more torque sensors are being researched and manufactured to meet the requirements of torque measurements in all of the engineering sectors that rely on accurate measurement of torque parameters for their success. A direct implication of these constantly growing developments is the recent development of various torque sensors, which operate based on more than ten different principles. Although several measurement principles have been created, torque sensors themselves can only be divided into two categories based on whether they come into contact with the rotating objects being measured (contact sensors) or do not have any point of contact with these objects (contactless sensors). In this paper, the concept of contactless sensors for torque measurement is redefined and the major classes of torque sensors (i.e., contactless sensors based on the laser Doppler effect, laser speckle effect, and optical coherence vibrometer, etc.) are introduced outlining the main progress, challenges, and future directions involving torque measurement applications. As the key to the success of torque measurement systems, some advanced signal processing techniques that are commonly used to enhance the accuracy of the torque measurement data are also introduced.