Detection of Strain Magnitude and Direction Based on an RFID Sensor Array

Radio frequency identification (RFID) antenna sensors have become one of the most interesting areas of passive strain sensing owing to the advantages of battery-less and wireless interrogation. However, existing RFID methods could mainly detect strain magnitude with its direction along the length or width of the radiant element. To meet the practical purpose of strain detection, a magnitude and direction detection method based on an RFID sensor array is proposed. First, a single strain sensor, i.e., a short stub feed patch antenna, is designed. Specifically, taking resonant frequency shift (RFS) as the feature of strain amplitude, the sensor has different sensitivities in the electrical length direction and width direction. Then, based on this characteristic, a sensor array consisting of four sensors with a mutual angle of 45° is proposed to achieve the quantitative detection of strain magnitude and direction. Finally, the feasibility of the proposed method is validated by simulations and experiments. Experimental results show that the sensitivities of the single strain sensor in the longitude direction and transverse direction are −873.91 and 57.28 Hz/
$\mu \varepsilon $
, respectively. The strain magnitude detection accuracy of the proposed sensor array is within 10% except for individual abnormal values, and the direction detection error of the sensor array is within 3°.