Packaging the SAW torque sensor with Teflon

Abstract

The packaging of surface acoustic wave (SAW) sensors has a significant influence on the characteristics of the sensor. It is necessary to find a way to improve the interface bonding quality between SAW torque sensor and tested shaft. In order to hold the SAW sensor, the cross-section of the measured shaft becomes rectangular. First, a perturbation analysis is performed from which the torque sensitivity is obtained. Since Teflon (Polytetrafluoroetylene) is hydro-phobic and has one of the lowest coefficients of friction against any other solid, we propose a method to bond the SAW torque sensor to the measured shaft using Teflon which protects the SAW resonator (SAWR) from being damaged. Different from sticking skills of resistance strain gauge, the surface of SAWR can't be touched directly. Therefore, in a vacuum environment, a Teflon film is covered on the upper surface of SAWR to avoid the glue from sticking to the interdigital electrodes. Then, the SAWR and Teflon form a unit and the sticking skills of resistance strain gauge are adopted to mount the SAWR to the shaft. Finally, the Teflon is removed and a metal cap is employed to seal and isolate the SAW torque sensor from the environment outside. A comparison is done among three different packaged sensors, which are packaged under the same conditions except for the bonding method. The experiment verifies the validity of the proposed packaging method. The experimental torque sensitivity 3.2 KHz/Nm, which is close to the theoretical calculation 4.013 KHz/Nm, is improved compared with other two methods 1.671 KHz/Nm and 0.824 KHz/Nm, respectively, as well as the reliability.