Wireless SAW Strain Sensor Using Orthogonal Frequency Coding

Wireless sensors offer a unique solution for many applications where wired designs may not be feasible. This paper presents the design and experimental results of a passive, wireless surface acoustic wave (SAW) strain sensor using orthogonal frequency coding (OFC) for the device identification. The sensor operates at a frequency of 915 MHz with a bandwidth of 68 MHz. A vector network analyzer is used to interrogate the sensor from a range of 60 cm (2 ft), with further range possible by increased transmitter output power or averaging. The sensor is bonded directly to a steel cantilever test structure and could be attached directly to other structures. An external connection site facilitates an antenna, with bond wires joining the connection site to the SAW die. A 3-D printed package protects the sensor while allowing the antenna to be mounted externally from the sensor package. Experimental results show that the wireless SAW strain sensor performs comparably to a wired, commercial strain gage. In addition, the strain sensitivity is extracted for this SAW OFC strain sensor embodiment which describes the frequency shift with the applied strain (-1.80 ppm/με).