Gas viscosity sensing based on the electrostatic pull-in time of microactuators

• We evaluate how the pull-in time of an actuator is affected by viscosity.
• The squeeze-film damping coefficient is directly proportional do viscosity.
• Increasing viscosity increases pull-in time.
• We performed measurements on gases with viscosities ranging from 9 to 18 μPa s.
• We report a sensitivity of 2 ms/(μPa s) for this structure.

A new principle for gas viscosity sensing using electrostatic pull-in and its implementation using a microstructure are presented in this paper. The sensor is based on viscosity-dependent pull-in time measurement. A nonlinear dynamic analysis of pull-in demonstrates the influence of damping conditions on the pull-in time of devices that are operated at meta-stability (requiring specific damping and electrostatic actuation conditions) with a squeeze-film damping coefficient at low frequencies directly proportional to viscosity. Therefore, the fundamentals of pull-in behavior suggest that pull-in can be used for the implementation in a gas viscosity sensor.Capacitive parallel-plates MEMS structures with squeeze-film dampers have been fabricated and pull-in time measurements have been performed for different gas media. Both pure gases (H2, CH4, CO2, CO and N2) and mixtures (H2N2, CH4N2 and CH4N2CO2) have been tested, with viscosity values in the range between 9 and 18 μPa s. The results show a sensitivity of 2 ms/(μPa s), which can be further increased by manipulating the actuation voltage. Further efforts are necessary to reduce the device sensitivity to external vibration, which translated to a significant amount of noise in the measurements.