Introduction - Energy Harvesting Autonomous Sensor Systems - page 11

Radio
Communication
(Tx/Rx)

Physical
Parameters/
Quantities
(Temperature,
Vibration, etc)

Sensor/
Transducer

Data
Acquisition

Micro-
processor

Energy Source

Energy Harvester Power Management Unit

FIGURE 1.9
Block diagram of a wireless sensor node.

sensor nodes in the network can use it. As illustrated in Section 1.1.1, network
nodes share this information via a wireless communication link. Because of
the great potential of WSNs, many groups around the world have invested
research efforts and time in the design of sensor nodes for their specific ap-
plications. These include Berkeley's Mica motes [16], PicoRadio projects [17],
MIT's Amps [18], as well as many others. In addition, the TinyOS project [19]

provides a framework for designing flexible distributed applications for data
collection and processing across the sensor network. All of these sensor nodes
have similar goals, such as small physical size, low power consumption, and
rich sensing capabilities. A block diagram of the wireless sensor node of a
WSN is shown in Figure 1.9 . The sensor node typically consists of four sub-
units: the sensor itself, data acquisition circuit, local microcontroller, and radio
communication block. Some examples of a wireless sensor node are given in
Figure 1.10 .
Referring to Figure 1.9 , the sensor/transducer converts an environmen-
tal parameter such as temperature, vibration, humidity, and the like to an
electrical signal. A data acquisition circuit is incorporated in the sensor node
to realize amplification and preprocessing of the output signals from sen-
sors, for example, conversion from analog to digital form and filtering. The
conditioned signals are then processed and stored in the embedded micro-
controller for other sensor nodes in the network to use them. Other than data
processing, the microcontroller also provides some level of intelligence such
as time scheduling to the sensor node. To enable the sensor node to com-
municate with its neighbour node or the base station in a wireless manner,
a radio communication block such as shown in Figure 1.9 is included. All
four subunits of the sensor node are power sink modules, and they need to
consume electrical energy from the power source in order to operate. Because
of that, the wireless sensor node would consume all the energy stored in the
battery after some time, and the sensor node would then go into an idle state.
Once the percentage of nodes that have not terminated their residual energy
falls below a specific threshold, which is set according to the type of applica-
tion (it can be either 100% or less) [20], the operational lifetime of the WSN
ends.