Page 247 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

8

Microsensors

8.1 INTRODUCTION

A sensor may be simply defined as a device that converts a nonelectrical input quantity E
into an electrical output signal E; conversely, an actuator may be defined as a device that
converts an electrical signal E into a nonelectrical quantity E (see Figure 8.1). In contrast,
a processor modifies an electrical signal (e.g. amplifies, conditions, and transforms) but
does not convert its primary form. A transducer is a device that can be either a sensor or
an actuator. Some devices can be operated both as a sensor and an actuator. For example,
a pair of interdigitated electrodes lying on the surface of a piezoelectric material can be
used to sense surface acoustic waves (SAWs) or to generate them. This device is referred
to as an interdigitated transducer (IDT). The importance of this device is such that we
have dedicated Chapter 13 to describing its applications as a microsensor and Chapter 14
to describing its use in microelectromechanical system (MEMS) devices.
It has been proposed by Middelhoek that a sensor or actuator can be classified according
to the energy domain of its primary input-output (I/O). There are six primary energy
domains and the associated symbols are as follows:

• Electrical E
• Thermal T
• Radiation R
• Mechanical Me
• Magnetic M
• Bio(chemical) C

For example, Figure 8.2 shows the six energy domains and the vectors that define the
conventional types of sensors and actuators, that is, A vector represents a thermal sensor,
whereas A represents a thermal actuator. In this way, all the different types of sensors
(and actuators) can be classified. In practice, the underlying principles of a sensor may
involve several stages; for example, the primary nonelectrical input (radiation) that first
transforms into the mechanical domain, then into the thermal domain, and finally into the
electrical domain.
Figure 8.3 shows the vectorial representation of this radiation sensor and the three
different stages of the conversion.
In theory, a transducer could have a large number of stages, but in practice, this is
usually between one and three. For example, an electromagnetic actuator has two: first,