Page 103 - An Introduction to Microelectromechanical Systems Engineering

82 MEM Structures and Systems in Industrial and Automotive Applications

designs. Measuring the change in resistance and amplifying the corresponding out-
put signal tend to be rather simple, requiring a basic knowledge of analog circuit
design. A drawback of silicon piezoresistivity is its strong dependence on tempera-
ture which must be compensated for with external electronics.
By contrast, capacitive sensing relies on an external physical parameter changing
either the spacing or the relative dielectric constant between the two plates of a
capacitor. For instance, an applied acceleration pushes one plate closer to the other.
Or in the example of relative humidity sensors, the dielectric is an organic material
whose permittivity is function of moisture content [1]. The advantages of capacitive
sensing are very low power consumption and relative stability with temperature.
Additionally, the approach offers the possibility of electrostatic actuation to perform
closed-loop feedback. The following section on actuation methods explains this
point further. Naturally, capacitive sensing requires external electronics to convert
minute changes in capacitance into an output voltage. Unlike measuring resistance,
these circuits can be substantially intricate if the change in capacitance is too small.
This is frequently the case in MEMS where capacitance values are on the order of 1
pF (10 −12 F) and changes in capacitance can be as small as a few fF (10 −15 F).
Yet another sensing approach utilizes electromagnetic signals to detect and
measure a physical parameter. Magnetoresistive sensors on the read heads of high-
density computer disk drives measure the change in conductivity of a material slab in
response to the magnetic field of the storage bit. In Hall-effect devices, a magnetic
field induces a voltage in a direction orthogonal to current flow [2]. Hall-effect sen-
sors are extremely inexpensive to manufacture. They are used in high-reliability
computer keyboards and make excellent candidates to measure wheel velocity in
vehicles. Another form of electromagnetic transducing uses Faraday’s law to detect
the motion of a current-carrying conductor through a magnetic field. Two yaw-rate
sensors described later in this chapter make use of this phenomenon. The control
electronics for magnetic sensors can be readily implemented using modern CMOS
technology, but generating magnetic fields often necessitates the presence of a per-
manent magnet or a solenoid.

Common Actuation Methods
A complete shift in paradigm becomes necessary to think of actuation on a miniature
scale—a four-stroke engine is not scalable. The next five schemes illustrate the diver-
sity and the myriad of actuation options available in MEMS. They are electrostatic,
piezoelectric, thermal, magnetic, and phase recovery using shape-memory alloys.
The choice of actuation depends on the nature of the application, ease of integration
with the fabrication process, the specifics of the system around it, and economic jus-
tification (see Table 4.2). Examples of each actuation method will arise throughout
this chapter and the next.

Electrostatic actuation
Electrostatic actuation relies on the attractive force between two conductive plates
or elements carrying opposite charges. A moment of thought quickly reveals that the
charges on two objects with an externally applied potential between them can only
be of opposite polarities. Therefore, an applied voltage, regardless of its polarity,