Page 237 - MEMS Mechanical Sensors

226 Flow Sensors

conventional hot film design, and reverse flow detection; thus, they are able to meas-
ure the net air intake. Nitride membranes of 150-nm thickness can withstand more
than 1 bar and are therefore stable enough to be used as sensor membranes [41]. Par-
ticles, even after air filters, reach the surface of the sensor chip. This can result in
changes of the calibrated sensor signal as the particles slowly remove the surface
protection layer above the heater/sensor resistors creating shortcuts or even damag-
ing the resistors themselves. Therefore, the sensor can be placed within an aerody-
namic bypass as the one developed by Robert Bosch GmbH [73] (Figure 9.14).
A silicon-based bidirectional, thermal air flow sensor is produced by Robert
Bosch GmbH, Germany, for the automotive industry and is used by most car manu-
facturers worldwide within the air intake module (Figure 9.15). Under operating
conditions of the car engine, strong oscillations lead to temporal reverse flow. The
bypass mentioned above not only prevents particle damage, but leads to a reduction
of the pulsation amplification near the sensor element and a correction of the mean
value of the flow passing the sensor element. The production of the micromachined
sensor started in 1996 and more than 20 million sensors have been sold so far.

Aerodynamic
bypass
Flow
sensor

Flow

Figure 9.14 Photograph of an aerodynamic bypass for automotive applications. (Courtesy Robert
Bosch GmbH, Germany.)

Measurement
electronics
Flow sensor
Aerodynamic
bypass

Figure 9.15 Photograph of a mass air flow meter for automotive applications developed by
Konzelmann et al. [74]. (Courtesy Robert Bosch GmbH, Germany.)