Page 121 - MEMS Mechanical Sensors

110 Mechanical Transduction Techniques

Original position
of actuator Direction of applied
magnetic field

Plane of
substrate Electroplated permalloy

Polysilicon
Figure 5.19 An example of an in-plane magnetic actuator. (After: [14].)

sensor and the instrument may not be apparent. Many of today’s commercial
devices have some form of electronic processing within the main sensor housing;
perhaps simple electronic filtering or more sophisticated digital signal processing.
The terms intelligent and smart sensor have been used, almost interchangeably, over
the past 20 years or so to refer to sensors having additional functionality provided
by the integration of microprocessors, microcontrollers, or application specific inte-
grated circuits (ASICs) with the sensing element itself. The interested reader is
encouraged to read the texts by Brignell and White [15], Gardner et al. [16], and
Frank [17], for a deeper insight into the field of smart sensor technologies. For con-
sistency in this text, we will adopt the term smart sensor to refer to a microsensor
with integrated microelectronic circuitry.
Smart sensors offer a number of advantages for sensor system designers. The
integration of sensor and electronics allows it to be treated as a module, or
black-box, where the internal complexities of the sensor are kept remote from the
host system. Smart sensors may also have additional integrated sensors to monitor,
say, localized temperature changes. This is sometimes referred to as the sensor-
within-a-sensor approach and is an important feature of smart sensor technology.
An example of a smart sensor system is depicted in Figure 5.20.
Many physical realizations of smart sensors may contain some or all of these ele-
ments. Each of the main subsystems will now be described in more detail.
The sensing element is the primary source of information into the system. Exam-
ples of typical sensing techniques have already been outlined in this chapter. The
smart sensor may also have the ability to stimulate the sensing element to provide a
self-test facility, whereby a reference voltage, for example, can be applied to the
sensor in order to monitor its response. Some primary sensors, such as those based
on piezoelectrics, convert energy directly from one domain into another and there-
fore do not require a power supply. Others, such as resistive-based sensors, may
need stable dc sources, which may benefit from additional functionality like pulsed
excitation for power-saving reasons. So excitation control is another distinguishing
feature found in smart sensors.