Zorman, C. A. | Search NASA

NASA NTRS2015

Wireless Capacitive Pressure Sensor With Directional RF Chip Antenna for High Temperature Environments

This paper presents the design, fabrication and characterization of a wireless capacitive pressure sensor with directional RF chip antenna that is envisioned for the health monitoring of aircraft engines operating in harsh environments. The sensing system is characterized from room temperature (25 C) to 300 C for a pressure range from 0 to 100 psi. The wireless pressure system consists of a Clapp-type oscillator design with a capacitive MEMS pressure sensor located in the LC-tank circuit of the oscillator. Therefore, as the pressure of the aircraft engine changes, so does the output resonant frequency of the sensing system. A chip antenna is integrated to transmit the system output to a receive antenna 10 m away.The design frequency of the wireless pressure sensor is 127 MHz and a 2 increase in resonant frequency over the temperature range of 25 to 300 C from 0 to 100 psi is observed. The phase noise is less than minus 30 dBcHz at the 1 kHz offset and decreases to less than minus 80 dBcHz at 10 kHz over the entire temperature range. The RF radiation patterns for two cuts of the wireless system have been measured and show that the system is highly directional and the MEMS pressure sensor is extremely linear from 0 to 100 psi.

Pressure sensor↗

NASA NTRS2000

Finite-Element Modeling of 3C-SiC Membranes

Finite-element modeling (FEM) of 3C-SiC thin-film membranes on Si substrates was used to determine the residual stress and center deflection with applied pressure. The anisotropic, three-dimensional model includes the entire 3C-SiC membrane and Si substrate with appropriate material properties and boundary conditions. Residual stress due to the thermal-expansion-coefficient mismatch between the3C-SiC film and Si substrate was included in the model. Both before-and after-etching, residual stresses were calculated. In-plane membrane stress and normal deflection with applied pressure were also calculated. FEM results predict a tensile residual stress fo 259 MPa in the 3C-SiC membrane before etching. This decreases to 247 MPa after etching the substrate below the membrane. The residual stress experimentally measured on sample made at Case Western Reserve University was 280 MPa on post-etched membranes. This is excellent agreement when an additional 30-40 MPa of residual stress to account for lattice mismatch is added to the FEM results.

DeAnna, R. G.↗