Page 527 - The Mechatronics Handbook

0066_frame_C19 Page 149 Wednesday, January 9, 2002 5:32 PM

FIGURE 19.123 Schematic diagram of the microfabrication process for realizing a tactile sensor.

FIGURE 19.124 Perspective diagram of a thermal-
transfer based flow sensor (anemometer).

Flow Sensors Based on Heat Transfer Principles
For sensors based on thermal transfer principles, a heated element is used with temperature slightly
above the temperature of the ambient fluid (Fig. 19.124). The heat is generally created by passing
current through a resistive element. An ideal element to serve as the heating element is doped
polysilicon resistor. The resistivity is generally lower than what can be achieved using metal resistors
of the same dimension, hence the resistance value is greater and the heating element can be made
smaller.
The movement of the fluid creates velocity-dependant forced convection of heat, thereby reducing the
temperature of the heated element accordingly. The temperature of the element is therefore used to
provide information about the flow rate and direction. Such sensors are commonly referred to as hot-
wire anemometers. Micromachined hot-wire anemometers have been demonstrated by several groups
[11,12].

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