Page 249 - MEMS Mechanical Sensors

238 Flow Sensors

full wafer thickness resulting in a free-hanging silicon tube system with six-edged
1-mm-high tube cross-sections and a wall thickness of 100 m [Figure 9.30(c)].
Measurements show that the device is a true mass-flow sensor with direction
sensitivity and high linearity in the investigated flow range. The micromachined sili-
con tube structure has measured Q factors of 600 to 1,500, depending on their
vibration mode (antiphase and in-phase bending, antiphase and in-phase torsion),
with water filling and operation in air. Data for the sensor is shown in Table 9.7.
The sensor can also be used for measuring the fluid density since the resonance fre-
quency of the sensor is a function of the fluid density.
The major disadvantage of Coriolis mass-flow sensors is that they require
rather complex drive and detection electronics. It is quite difficult to measure the
very small Coriolis force when the twisting amplitude is in the nanometer range.
These amplitudes, however, are sufficient for capacitive detection and make it pos-
sible to produce a more compact sensor structure, for instance, by anodic bonding
of glass lids with integrated electrodes for electrostatic excitation and capacitive
detection [96].
A sensor using a U-shaped resonant silicon microtube measuring fluid flow also
with the Coriolis force is proposed by Sparks et al. [97]. So far, the resonant micro-
tube is used to sense chemical concentration, but experimental results for flow meas-
uring are proposed for an upcoming publication.

9.4.4 Static Turbine Flow Meter
A silicon micromachined torque sensor is used to measure the volume flow con-
verted by a static turbine wheel (the wheel does not rotate) [98]. The flow sensor has
been developed for monitoring respiratory flow of ventilated patients. The applica-
tion requires a bidirectional flow sensor with a low pressure drop, resistance to
humidity, and temperature variations of the respiratory gas. The sensor setup con-
sists of a wheel, which is fixed to the torque sensor and, in turn, is connected to the
pipe wall. A schematic is shown in Figure 9.31. The flow is deflected as it passes the
turbine wheel blades, providing a change in momentum [Figure 9.31(a, b)], which
excerpts forces on the blade generating a torque, which is measured by the torque
sensor. The torque depends on the flow velocity, the fluid density, the length of the
blade, and the blade angle. The flow passing the wheel is distributed over the cir-
cumference of the wheel, thus levelling out effects of nonuniform flow profiles and
leading to a profile-independent volumetric flow measurement. The torque-sensing
element has been DRIE etched to form three different parts: the mounting part, the
supporting part, and two stiffness reduction beams, as shown in Figure 9.31(c). The
wheel is fixed to the mounting part just above the stiffness reduction beams. On each
side of the stiffness reduction beams are boron doped polysilicon resistors connected
to a Wheatstone bridge. When a flow passes the turbine wheel, the strain gauges
(polysilicon resistors) on one side are tensed and on the other side compressed,

Table 9.7 Data for Coriolis Force Type Flow Sensor
Author; Year Flow Range Sensitivity Q-Factors Fluid Chip Size
Enoksson et al. 0–0.5 g/s 2.95 (mV/V)/(g/s) 600–1,500 Water 12 × 21 × 1
[96]; 1997 mm 3