Page 261 - MEMS Mechanical Sensors

250 Flow Sensors

ultrasonic waves and it has the potential to measure liquid flow rates. However, the
idea of this novel MEMS flow sensing principle has not been picked up by future
researchers, and thus, it was not discussed above. Ultrasonic wave sensors to meas-
ure liquid density and viscosity have also been described [129, 130]. Ultrasound
enables instruments to be noninvasive because the acoustic wave can often penetrate
the walls of channels. Other sensing principles that have gained little attention so far
are the fluid flow detection via a pyroelectric element [131] and resonant flow sens-
ing mechanism [132, 133].
Turbulent flow studies, with considerable impact from micromachining, may
open a new area. In the future an aircraft wing could be covered with wall shear
stress sensors and actuators to actively influence the flow profile. A first step in this
direction is the micromachined flexible shear stress sensor skin applied to an
unmanned aerial vehicle presented recently by Xu et al. [134]. Here, an array of 36
shear stress sensors was mounted over the 180° surface of the leading edge of a wing,
and data during flight was collected for an aerodynamic study.
Flow sensors have made the jump from the MEMS into the NEMS world
(nanoelectro mechanical systems). Ghosh et al. [36] measured flow rates for various
liquids using carbon nanotubes. They reported that the flow of a liquid on single
walled carbon nanotube bundles induces a voltage in the sample along the direction
of the flow. The magnitude of the voltage depends on the ionic conductivity and on
the polar nature of the liquid. Nanotube bundles with an average tube diameter of
1.5 nm were densely packed between two metal electrodes. The dimensions of the
3
sensor were 1 × 0.2 × 2mm . Flow rates between micrometers per second and milli-
meters per second were measured. This approach using carbon nanotubes may have
the potential to measure extremely low flow rates.
Lerch et al. [135] writes, “The research is often technology driven and does not
necessarily fit industrial or market requirements. Beyond scientific and technical
interest, the market finally decides if the developed devices are of practical signifi-
cance.” As for all MEMS sensors, this is also true for micro flow sensors. Without
the basic research, however, there would be a lack of variety of principles. Not all
applications are in the automotive field.

References

[1] van Putten, A. F. P., and S. Middelhoek, “Integrated Silicon Anemometer,” IEE Electronics
Letters, Vol. 10, 1974, pp. 425–426.
[2] van Riet, R. W. M., and J. H. Huysing, “Integrated Direction-Sensitive Flow-Meter,” IEE
Electronics Letters, Vol. 12, 1976, pp. 647–648.
[3] Weast, R. C., (ed.), Handbook of Chemistry and Physics, 69th ed., Boca Raton, FL: CRC
Press, 1989.
[4] Koch, M., A. G. R. Evans, and A. Brunnschweiler, Microfluidic Technology and Applica-
tions, Baldock, England: Research Studies Press Ltd., 2000.
[5] Kovacs, G. T. A., Micromachined Transducers—Sourcebook , New York: McGraw-Hill,
1998.
[6] Dario, P., et al., “Micro-Systems in Biomedical Applications,” Journal of Micromechanics
and Microengineering, Vol. 10, 2000, pp. 235–244.
[7] Harrison, D. J., et al., “From Micro-Motors to Micro-Fluidics: The Blossoming of
Micromachining Technologies in Chemistry, Biochemistry and Biology,” Proc. Transduc-
ers, No. 1A2.2, Sendai, Japan, 1999.