Page 225 - MEMS Mechanical Sensors

214 Flow Sensors

9.1 Introduction to Microfluidics and Applications for Micro Flow
Sensors

Micromachining has numerous applications in fluidics, and its use in this area has
become even more important as people strive to create complete fluidic systems in
miniaturized formats. A broad range of devices and systems can be found in the
books Microfluidic Technology and Applications [4] and Micromachined Trans-
ducers Sourcebook [5], as well as in various review articles published [6–10]. A brief
introduction to microfluidics relevant for flow sensors and applications for micro
flow sensors is given in this chapter. The first and most obvious microfluidic devices
to integrate with a flow sensor were micropumps and/or valves, to form dosing sys-
tems or mass flow controllers [11–17]. Schematics of two typical dosing systems are
depicted in Figure 9.1.
Further integration took place including several pumps, valves, flow sensors,
and micromixers to form microanalysis systems (µTAS) [18–20]. As an example, a
microfluidic system using two pumps, two flow sensors, and a mixer is shown in Fig-
ure 9.2 [21]. A microsystem for measurement of flow rate, pressure, temperature,
conductivity, UV-absorption, and fluorescence on a single quartz glass chip was pre-
sented by Norlin et al. [22]. Another multisensor chip designed for catheter applica-
tions has been presented by Goosen et al. [23] and Tanase et al. [24]. It includes
blood flow, pressure, and oxygen saturation level sensing.
The automotive industry has been, and is still one of the major driving forces for
MEMS-based sensors. For example, in engine control applications, the number of
sensors used will increase from approximately 10 in 1995, to more than 30 in 2010
[25]. The micromachined flow sensor has already made the jump into the automo-
bile industry [25–27]. Electronic fuel injection systems need to know the mass flow
rate of air sucked into the cylinders to meter the correct amount of fuel. Other areas
of application are in pneumatics, bioanalysis [20], metrology (wind velocity and
direction [28, 29]), civil engineering (wind forces on building), the transport and
process industry (fluidic transport of media, combustion, vehicle performance),
environmental sciences (dispersion of pollution), medical technology (respiration
and blood flow, surgical tools [30]), indoor climate control (ventilation and air con-
ditioning [31]), and home appliances (vacuum cleaners, air dryers, fan heaters).
Flow sensors have even been used in space applications. The microinstrument for
life science research, developed at the University of Neuchatel, Switzerland, included

Figure 9.1 Schematics: (a) Monolithically assembled dosing system. (After: [12].) (b) Hybrid
dosing system. (After: [11].)