A Sawtooth MEMS Capacitive Strain Sensor for Passive Telemetry in Bearings

The development of economical and high-sensitive strain sensor is crucial for precise measurement of strain in industrial applications, such as bearings. During operation, bearing itself is a harsh environment, which usually has large temperature variations and large centrifugal forces. This paper presents a sawtooth MEMS capacitive strain sensor for passive telemetry in bearings. The interdigital fingers are designed into sawtooth shape to decrease the influence of centrifugal acceleration. The strain sensor chip is 4mm
$\times4$
mm in size, and is fabricated by a simple SOI process. Connecting the sensitive capacitor with an inductor, an LC resonant tank is formed, and the strain signal can be read out wirelessly and passively through a readout coil. Measurement results show that, in the range of
$0\sim 1000~\mu \varepsilon $
, the sensitivity of the LC strain sensor reaches 34kHz/
$\mu \varepsilon $
, and the gauge factor (GF) of the strain sensor reaches 430. The influence of thickness of adhesive layer on the strain sensitivity is investigated. When the thickness of the adhesive layer reaches
$500~\mu \text{m}$
, the applied stress cannot be transferred at all. Finally, the LC strain sensor is tested on a rotating platform. The measured resonant frequency shows only 0.4% deviation under 1000m/s
²
centrifugal acceleration.