Sensor node: Difference between revisions

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{{primary sources|date=February 2015}}

{{Cleanup|date=September 2009}}

[[Image:Sensornode.svg|thumb|300px|The typical [[Computer architecture|architecture]] of the sensor node.]]

A '''sensor node''', (also known as a '''mote''' (chiefly in [[North America]]), isconsists aof an individual [[Node (networking)|node]] infrom a [[sensor network]] that is capable of performing somea processing,desired action such as gathering, sensoryprocessing information andor communicating information with other connected nodes in thea network. A mote is a node but a node is not always a mote. {{citation needed|reason=This statement is not motivated|date=September 2015}}

==History==

<!-- Image with unknown copyright status removed: [[Image:MICAz.jpg|thumb|200px|MicaZ Commerical Sensor node from CrossBow Technologies]] -->Although [[Wireless powerlineSensor sensorNetworks|wireless sensor nodesnetworks]] have existed for decades and used for diverse applications as diversesuch as earthquake measurements toor warfare, the modern development of small sensor nodes dates back to the 1998 [[Smartdust]] project<ref>[http://robotics.eecs.berkeley.edu/~pister/SmartDust/ Smart Dust<!-- Bot generated title -->]</ref> and the NASA. [[Sensor Web]]s Project<ref>[http://www.techbriefs.com/content/view/2227/32/ NASA Tech Brief]</ref> One of the objectives of the [[Smartdust]] project was to create autonomous sensing and communication within a cubic millimeter of space., Thoughthough this project ended early on, it led to many more research projects. They includeand major research centres insuch as The Berkeley NEST<ref>[http://webs.cs.berkeley.edu/ Home<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20011110031435/http://webs.cs.berkeley.edu/ |date=2001-11-10 }}</ref> and CENS.<ref>[http://research.cens.ucla.edu/ CENS: Center for Embedded Networked Sensing<!-- Bot generated title -->] {{webarchive|url=http://webarchive.loc.gov/all/20090407035705/http://research.cens.ucla.edu/ |date=2009-04-07 }}</ref> The researchers involved in these projects coined the term ''mote'' to refer to a sensor node. The equivalent term in the NASA Sensor Webs Project for a physical sensor node is ''pod'', although the sensor node in a Sensor Web can be another Sensor Web itself. Physical sensor nodes have been able to increase their effectiveness and its capability in conjunction with [[Moore's Law]].
The chip footprint contains more complex and lower powered [[microcontrollers]]. Thus, for the same node footprint, more silicon capability can be packed into it. Nowadays, motes focus on providing the longest wireless range (dozens of km), the lowest energy consumption (a few uA) and the easiest development process for the user.<ref>[http://www.sensor-networks.org/index.php?page=0932832814 "Waspmote: a modern mote"]</ref>

==Components==

The main components of a sensor node areusually involve a [[microcontroller]], [[transceiver]], external [[memory]], [[Electric power|power source]] and one or more [[sensors]].

===Sensors===

[[Sensor]]s are used by wireless sensor nodes to capture data from their environment. They are hardware devices that produce a measurable response to a change in a physical condition like temperature or pressure. Sensors measure physical data of the parameter to be monitored and have specific characteristics such as accuracy, sensitivity etc. The continual [[analog signal]] produced by the sensors is digitized by an [[analog-to-digital converter]] and sent to controllers for further processing. Some sensors contain the necessary electronics to convert the raw signals into readings which can be retrieved via a digital link (e.g. I2C, SPI) and many convert to units such as °C. Most sensor nodes are small in size, consume little energy, operate in high volumetric densities, be autonomous and operate unattended, and be adaptive to the environment. As wireless sensor nodes are typically very small electronic devices, they can only be equipped with a limited power source of less than 0.5-2 ampere-hour and 1.2-3.7 volts.

Sensors are classified into three categories: passive, omnidirectional sensors; passive, narrow-beam sensors; and active sensors. Passive sensors sense the data without actually manipulating the environment by active probing. They are self powered; that is, energy is needed only to amplify their analog signal. Active sensors actively probe the environment, for example, a sonar or radar sensor, and they require continuous energy from a power source. Narrow-beam sensors have a well-defined notion of direction of measurement, similar to a camera. Omnidirectional sensors have no notion of direction involved in their measurements.

Most theoretical work on WSNs assumes the use of passive, omnidirectional sensors. Each sensor node has a certain area of coverage for which it can reliably and accurately report the particular quantity that it is observing. Several sources of power consumption in sensors are: signal sampling and conversion of physical signals to electrical ones, signal conditioning, and analog-to-digital conversion. Spatial density of sensor nodes in the field may be as high as 20 nodes per cubic meter.

===Controller===

The controller performs tasks, processes data and controls the functionality of other components in the sensor node. While the most common controller is a [[microcontroller]], other alternatives that can be used as a controller are: a general purpose [[Desktop computer|desktop]] [[microprocessor]], [[digital signal processors]], [[Field Programmable Gate Array|FPGAs]] and [[Application-specific integrated circuit|ASICs]]. A microcontroller is often used in many [[embedded systems]] such as |sensor nodes]] becausedue ofto its low cost, flexibility to connect to other devices (or nodes in a network), ease of programming, and low power consumption. A general purpose microprocessor generally has a higher power consumption than a microcontroller, thereforemaking it is often not considered aan suitableundesirable choice for a sensor node.{{Citation needed|date=September 2010}} Digital Signal Processors may be chosen for broadband [[wireless communication]] applications, but in [[Wireless Sensor Networks]] the wireless communication is often modest: i.e., simpler, easier to process [[modulation]] and the [[signal processing]] tasks of actual sensing of data is less complicated. Therefore, the advantages of DSPs are not usually of much importance to wireless sensor nodes. FPGAs can be reprogrammed and reconfigured according to requirements, but this takes more time and energy than desired.{{Citation needed|date=September 2010}}

'''Transceiver'''

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===Power source===

A wireless sensor node is a popular solution when it is difficult or impossible to run a mains supply to the sensor node. However, since the wireless sensor node is often placed in a hard-to-reach ___location, changing the battery regularly can be costly and inconvenient. An important aspect in the development of a wireless sensor node is ensuring that there is always adequate energy available to power the system.

The sensor node [[Power consumption|consumes power]] for sensing, communicating and data processing. More energy is required for [[data communication]] than any other process. The energy cost of transmitting 1 Kb a distance of {{convert|100|m|ft}} is approximately the same as that used for the execution of 3 million instructions by a 100 million instructions per second/W processor.{{citation needed|date=March 2020}} Power is stored either in batteries or capacitors. Batteries, both rechargeable and non-rechargeable, are the main source of power supply for sensor nodes. They are also classified according to electrochemical material used for the electrodes such as [[Nickel-cadmium battery|NiCd]] (nickel-cadmium), [[Nickel-zinc battery|NiZn]] (nickel-zinc), [[Nickel-metal hydride battery|NiMH]] (nickel-metal hydride), and [[Lithium-ion battery|lithium-ion]].

Current sensors are able to renew their energy from [[Solar energy|solar]] sources, Radio Frequency(RF), [[Thermogenerator|temperature]] differences, or [[Vibration powered generator|vibration]]. Two power saving policies used are [[Dynamic Power Management]] (DPM) and [[Dynamic Voltage Scaling]] (DVS).<ref>Dynamic Power Management in Wireless Sensor Networks, Amit Sinha and Anantha Chandrakasan, IEEE Design & Test of Computers, Vol. 18, No. 2, March–April 2001</ref> DPM conserves power by shutting down parts of the sensor node which are not currently used or active. A DVS scheme varies the power levels within the sensor node depending on the non-deterministic workload. By varying the voltage along with the frequency, it is possible to obtain quadratic reduction in power consumption.

===Sensors===

[[Sensor]]s are used by wireless sensor nodes to capture data from their environment. They are hardware devices that produce a measurable response to a change in a physical condition like temperature or pressure. Sensors measure physical data of the parameter to be monitored and have specific characteristics such as accuracy, sensitivity etc. The continual [[analog signal]] produced by the sensors is digitized by an [[analog-to-digital converter]] and sent to controllers for further processing. Some sensors contain the necessary electronics to convert the raw signals into readings which can be retrieved via a digital link (e.g. I2C, SPI) and many convert to units such as °C. Most sensor nodes are small in size, consume little energy, operate in high volumetric densities, be autonomous and operate unattended, and be adaptive to the environment. As wireless sensor nodes are typically very small electronic devices, they can only be equipped with a limited power source of less than 0.5-2 ampere-hour and 1.2-3.7 volts.

Sensors are classified into three categories: passive, omnidirectional sensors; passive, narrow-beam sensors; and active sensors. Passive sensors sense the data without actually manipulating the environment by active probing. They are self powered; that is, energy is needed only to amplify their analog signal. Active sensors actively probe the environment, for example, a sonar or radar sensor, and they require continuous energy from a power source. Narrow-beam sensors have a well-defined notion of direction of measurement, similar to a camera. Omnidirectional sensors have no notion of direction involved in their measurements.

Most theoretical work on WSNs assumes the use of passive, omnidirectional sensors. Each sensor node has a certain area of coverage for which it can reliably and accurately report the particular quantity that it is observing. Several sources of power consumption in sensors are: signal sampling and conversion of physical signals to electrical ones, signal conditioning, and analog-to-digital conversion. Spatial density of sensor nodes in the field may be as high as 20 nodes per cubic meter.

==See also==

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{{Wireless Sensor Network}}

{{Sensors}}

{{DEFAULTSORT:Sensor Node}}

[[Category:Wireless sensor network]]