Wireless Sensor Networks:
Principles and Applications
Chris Townsend, Steven Arms, MicroStrain, Inc.
22.1 Introduction to Wireless Sensor Networks
Sensors integrated into structures, machinery, and the environment, coupled with the efﬁcient delivery of sensed information, could provide tremendous beneﬁts to society. Potential beneﬁts include: fewer catastrophic failures, conservation of natural resources, improved manufacturing productivity, improved emergency response, and enhanced homeland security . However, barriers to the widespread use of sensors in structures and machines remain. Bundles of lead wires and ﬁber optic “tails” are subject to breakage and connector failures. Long wire bundles represent a signiﬁcant installation and long term maintenance cost, limiting the number of sensors that may be deployed, and therefore reducing the overall quality of the data reported. Wireless sensing networks can eliminate these costs, easing installation and eliminating connectors.
The ideal wireless sensor is networked and scaleable, consumes very little power, is smart and software programmable, capable of fast data acquisition, reliable and accurate over the long term, costs little to purchase and install, and requires no real maintenance.
Selecting the optimum sensors and wireless communications link requires knowledge of the application and problem deﬁnition. Battery life, sensor update rates, and size are all major design considerations. Examples of low data rate sensors include temperature, humidity, and peak strain captured passively. Examples of high data rate sensors include strain, acceleration, and vibration.
Recent advances have resulted in the ability to integrate sensors, radio communications, and digital electronics into a single integrated circuit (IC) package. This capability is enabling networks of very low cost sensors that are able to communi-
cate with each other using low power wireless data routing protocols. A wireless sensor network (WSN) generally consists of a basestation (or “gateway”) that can communicate with a number of wireless sensors via a radio link. Data is collected at the wireless sensor node, compressed, and transmitted to the gateway directly or, if required, uses other wireless sensor nodes to forward data to the gateway. The transmitted data is then presented to the system by the gateway connection. The purpose of this chapter is to provide a brief technical introduction to wireless sensor networks and present a few applications in which wireless sensor networks are enabling.
22.2 Individual Wireless Sensor Node Architecture
A functional block diagram of a versatile wireless sensing node is provided in Figure 22.2.1. A modular design approach provides a ﬂexible and versatile platform to address the needs of a wide variety of applications . For example, depending on the sensors to be deployed, the signal conditioning block can be re-programmed or replaced. This allows for a wide variety of different sensors to be used with the wireless sensing node. Similarly, the radio link may be swapped out as required for a given applications’ wireless range requirement and the need for bidirectional communications. The use of ﬂash memory allows the remote nodes to acquire data on command from a basestation, or by an event sensed by one or more inputs to the node. Furthermore, the embedded ﬁrmware can be upgraded through the wireless network in the ﬁeld. The microprocessor has a number of functions including:
1) managing data collection from the sensors
2) performing power management functions
3) interfacing the sensor data to the physical radio layer
4) managing the radio network protocol
A key feature of any wireless sensing node is to minimize the power consumed by the system. Generally, the radio subsystem requires the largest amount of power. Therefore, it is advantageous to send data over the radio network only when required....
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