Page 102 - An Introduction to Microelectromechanical Systems Engineering

Techniques for Sensing and Actuation 81

of previously developed individual process steps, or to design a custom process spe-
cific to the device or system. If the production unit volume is not sufficiently large, it
may be challenging to identify reputable manufacturing facilities willing to develop
and implement custom processes.

Techniques for Sensing and Actuation

Common Sensing Methods
Sensing is by no means a modern invention. There are numerous historical accounts
describing the measurement of physical parameters—most notably, distance,
weight, time, and temperature. Early Chinese attempts at making compasses date
back to the twelfth century with the use of lodestone, a naturally occurring magnetic
ore. Modern sensing methods derive their utility from the wealth of scientific
knowledge accumulated over the past two centuries. We owe our intimate familiar-
ity with electrostatics and capacitance to the work of Charles Augustin de Coulomb
of France and John Priestly of England in the late eighteenth century and observe
that Lord Kelvin’s discovery of piezoresistivity in 1856 is recent in historical terms.
What distinguishes these modern techniques is the ability to sense with greater
accuracy and stability; what makes them suitable for MEMS is their scalable
functionality.
The objective of modern sensing is the transducing of a specific physical
parameter, to the exclusion of other interfering parameters, into electrical energy.
Occasionally, an intermediate conversion step takes place. For example, pressure or
acceleration are converted into mechanical stress, which is then converted to elec-
tricity. Infrared radiation in image sensors is often converted into heat and then
sensed as an electrical voltage or a change in electrical resistance. Perhaps the most
common of all modern sensing techniques is temperature measurement using the
dependence of various material properties on temperature. This effect is pro-
nounced in the electrical resistance of metals. The rate at which the resistance rises
with temperature—TCR—of most metals ranges between 10 and 100 parts per mil-
lion per degree centigrade.
Piezoresistivity and piezoelectricity are two sensing techniques described in
greater detail in Chapter 2 (see Table 4.1). Impurity-doped silicon exhibits a pie-
zoresistive behavior that lies at the core of many pressure and acceleration sensor

Table 4.1 The Relative Merits of Piezoresistive, Capacitive, and Electromagnetic Sensing Methods
Piezoresistive Capacitive Electromagnetic
Simple fabrication Simple mechanical structure Structural complexity varies
Low cost Low cost Complex packaging
Voltage or current drive Voltage drive Current drive
Simple measurement circuits Requires electronic circuits Simple control circuits
Low-output impedance Susceptible to EMI Susceptible to EMI
High-temperature dependence Low-temperature dependence Low temperature dependence
Small sensitivity Large dynamic range Sensitivity ∝ magnetic field
Insensitive to parasitic resistance Sensitive to parasitic capacitance Insensitive to parasitic inductance
Open loop Open or closed loop Open or closed loop
Medium power consumption Low power consumption Medium power consumption