This document provides an overview of sensor networks and wireless sensor network architectures. It begins with an introduction to wireless sensor networks and their components. It then discusses the topics, challenges, and enabling technologies for WSNs. The document outlines the architecture of a sensor node and its goals. It provides examples of WSN applications and discusses sensor network deployment considerations. Finally, it addresses the design challenges, operational challenges, and required mechanisms for WSNs to meet their requirements.

1. Unit II
Sensor Networks –
Introduction & Architectures
Dr.S.Periyanayagi
Professor & Head
Ramco Institute ofTechnology
20.08.2020

2. Topics
• Challenges forWireless Sensor Networks
• EnablingTechnologies forWireless Sensor Networks
• WSN application examples
• Single-NodeArchitecture - Hardware Components
• Energy Consumption of Sensor Nodes
• NetworkArchitecture - Sensor Network Scenarios
• Transceiver Design Considerations
• Optimization Goals and Figures of Merit
2

3. WSN - Introduction
• A Wireless Sensor Network (WSN) is a wireless network
consisting of a large number of spatially distributed sensor
nodes
• very sensitive to the environment
• capable of communication with each other through
wireless channels
PictureTaken from: https://microcontrollerslab.com/wireless-sensor-networks-wsn-applications/
3

4. Sensor & Sensor Nodes
• Sensor Network is an infrastructure comprised of sensing,
computing and communication elements that give the
ability of observing and reacting to events in a specified
environment to an administrator.
• The Administrator typically is a civil, governmental,
commercial or industrial entity.
• The environment can be a physical world, a biological
system, or an information technology framework.
4

5. Contd…
• Sensing is a Technique used to gather information about
physical object or Process, Including occurrence of the
event.
• An Object performing such a sensing task is called a
sensor.
• For eg: Remote Sensors
• The human body is equipped with sensors that are able to
capture optical information from the environment
• Acoustic information such as sounds (ears) and smell (Nose)
• A sensor is a device that translates parameters or events in
the physical world into signals that can be measured and
analyzed.
5

6. • A sensor is a electronic device that measures a physical
quantity and converts it into a signal which can be read
by an observer or by an instrument.
• Sensor Node : Basic unit in Sensor Network
Picture taken from : https://www.researchgate.net/publication/312332362_Application aware _ Energy _
Efficient_Centralized_Clustering_Routing_Protocol_for_Wireless_Sensor_Networks/figures?lo=1
6

7. Architecture of Sensor Node
7

8. Goal of a Sensor Node
• The goal from the sensor node is
• to collect the data at regular intervals
• then transform the data into an electrical signal
• finally send the signal to the sink or the base node
8

9. Early, wireless sensor networks functioned mainly with two important
application domains namely monitoring and tracking.
9

10. One Minute Paper
• List fewApplications ofWSNs – you are familiar with
10

11. Wireless Sensor Networks
Applications
• Forest fire detection
• Air pollution monitoring
• Water quality monitoring
• Land slide detection
• Automotive application
• Military application
• Animal Habitat Monitoring &Tracking
• Agriculture
• Health Care Monitoring
11

12. Contd…
• Disaster relief applications
– Sensor nodes are equipped with thermometers and can
determine their own location
– Drop sensor nodes from an aircraft over a wildfire
– Each node measures temperature
– Derive a “temperature map” of the area
• Biodiversity mapping
– Use sensor nodes to observe wildlife
Picture taken from: https://www.slideshare.net/shikhathegreat/ppt-on-low-power-wireless-sensor-network-
5th-sem 12

13. Forest fire detection
•A network of Sensor Nodes can be installed in a forest to
detect when a fire has started.
•The nodes can be equipped with sensors to measure
temperature, humidity and gases which are produced by
fire in the trees or vegetation.
• If the node detects fire,it sends an alarm message(along
with its location) to the base station
13

14. Air Quality monitoring
• Traditional air quality monitoring methods, such as
building air quality monitoring stations, are typically
expensive.
• The solution to these is air quality monitoring system
based on the technology of wireless sensor networks
(WSNs).
• Wireless sensor networks have been deployed in several
cities to monitor the concentration of dangerous gases for
citizens.
14

15. Water Quality Monitoring
• Water quality monitoring involves analyzing water
properties in dams, rivers, lakes & oceans, as well as
underground water reserves.
• Parameters considered include – temperature, turbidity
and pH
Picture taken from: Paper by Jungsu Park et al.,‘Recent Advances in Information and Communications
Technology (ICT) and SensorTechnology for MonitoringWater Quality’
15

16. Military Surveillance
• Enemy tracking, battlefield surveillance
• Target detection
• Monitoring, tracking and surveillance of borders
• Nuclear, biological and chemical attack detection
Picture taken from: https://www.researchgate.net/figure/WSNs-used-in-Military-
Applications_fig1_4365726 16

17. Landslide detection system
• A landslide detection system makes use of a wireless
sensor network to detect the slight movements of soil and
changes in various parameters that may occur before or
during a landslide.
• Through the data gathered it may be possible to know the
occurrence of landslides long before it actually happens.
17

18. Eruption
PictureTaken from: https://www.researchgate.net/figure/Monitoring-volcanic-eruptions-with-
a-WSN-24_fig3_230660610
18

19. Precision Agriculture
PictureTaken from:Article by uferah safri et al,‘PrecisionAgricultureTechniques and
Practices: From Considerations toApplications’
– Bring out fertilizer/pesticides/irrigation only where needed
19

20. Medical & Health Care
Monitoring
Picture taken from: https://www.slideshare.net/DeeptimanMallick/using-tiny-os-in-
wireless-sensor-network
– Post-operative or intensive care
– Long-term surveillance of chronically ill patients or the elderly
20

21. • Intelligent buildings (or bridges)
– Reduce energy wastage by proper humidity, ventilation, air
conditioning (HVAC) control
– Needs measurements about room occupancy,
temperature, air flow, …
– Monitor mechanical stress after earthquakes
• Facility management
– Intrusion detection into industrial sites
– Control of leakages in chemical plants, …
• Machine surveillance and preventive maintenance
– Embed sensing/control functions into places no cable has gone
before
– E.g., tire pressure monitoring
21

22. • Logistics
– Equip goods (parcels, containers) with a sensor node
– Track their whereabouts –total asset management
– Note: passive readout might sufficient –compare RF IDs
• Telematics
– Provide better traffic control by obtaining finer-grained
information about traffic conditions
– Intelligent roadside
– Cars as the sensor nodes
Picture taken from : https://www.semanticscholar.org/paper/Industry%3A-using-dynamic-WSNs-in-
smart-logistics-for-Bijwaard-Kleunen / 377f4ffcece496334f65255263b942f3509bbe7c /figure/0
22

23. A general work process of WSN
23

24. How are sensor nodes deployed in their environment?
• Dropped from aircraft ! Random deployment
– Usually uniform random distribution for nodes over finite area
is assumed
– Is that a likely proposition?
• Well planned, fixed ! Regular deployment
– E.g., in preventive maintenance or similar
– Not necessarily geometric structure, but that is often a
convenient assumption
Deployment options for WSN
24

25. • Mobile sensor nodes
– Can move to compensate for deployment shortcomings
– Can be passively moved around by some external force (wind,
water)
– Can actively seek out “interesting” areas
Maintenance options
• Feasible and/or practical to maintain sensor nodes?
– E.g., to replace batteries?
– Or: unattended operation?
– Impossible but not relevant? Mission lifetime might be very small
• Energy supply?
– Limited from point of deployment?
– Some form of recharging, energy scavenging from environment?
– E.g., solar cells
25

26. Assignment
• Explain any one application ofWSN (Agriculture,
Medical, Military, Under water,Animal Habitat,
IOT, IIOT etc..) in Detail
– What isWSN?
– Type of Sensor Used
– Application in Detail
– Working
– Refernces
26

27. Challenges for Wireless Sensor
Networks
21.08.2020

28. Design Challenges in WSN
• Heterogeneity
– The devices deployed may be of various types and need to
collaborate with each other.
• Distributed Processing
– The algorithms need to be centralized as the processing is
carried out on different nodes.
• Low Bandwidth Communication
– The data should be transferred efficiently between sensors
• Large Scale Coordination
– The sensors need to coordinate with each other to produce
required results.
28

29. Contd…
• Utilization of Sensors
– The sensors should be utilized in a ways that produce the
maximum performance and use less energy.
• RealTime Computation
– The computation should be done quickly as new data is
always being generated.
29

30. Challenges for WSNs
• Type of service ofWSN
– Not simply moving bits like another network
– Rather: provide answers(not just numbers)
– Issues like geographic scoping are natural requirements, absent
from other networks
• Quality of service
– Traditional QoS metrics do not apply
– Still, service of WSN must be “good”: Right answers at the right
time
• Fault tolerance
– Be robust against node failure (running out of energy, physical
destruction, …)
30

31. Contd..
• Lifetime
– The network should fulfill its task as long as possible –
definition depends on application
– Lifetime of individual nodes relatively unimportant
– But often treated equivalently
• Scalability
– Support large number of nodes
• Wide range of densities
– Vast or small number of nodes per unit area, very
application-dependent
31

32. Contd..
• Programmability
– Re-programming of nodes in the field might be
necessary, improve flexibility
• Maintainability
– WSN has to adapt to changes, self-monitoring, adapt
operation
– Incorporate possible additional resources, e.g., newly
deployed nodes
32

33. Operational Challenges of Wireless Sensor
Networks
• Energy Efficiency
• Limited storage and computation
• Low bandwidth and high error rates
• Errors are common
– Wireless communication
– Noisy measurements
– Node failure are expected
• Scalability to a large number of sensor nodes
• Survivability in harsh environments
• Experiments are time- and space-intensive
33

34. Required mechanisms to meet
requirements
• Multi-hop wireless communication
• Energy-efficient operation
– Both for communication and computation, sensing, actuating
• Auto-configuration
– Manual configuration just not an option
• Collaboration & in-network processing
– Nodes in the network collaborate towards a joint goal
– Pre-processing data in network (as opposed to at the edge) can
greatly improve efficiency
34

35. Contd..
• Data centric networking
– Focusing network design on data, not on node identifies(id-
centric networking)
– To improve efficiency
• Locality
– Do things locally (on node or among nearby neighbors) as far
as possible
• Exploit tradeoffs
– E.g., between invested energy and accuracy
35

36. Enabling technologies for WSN
• Cost reduction
– For wireless communication, simple microcontroller,
sensing, batteries
• Miniaturization
– Some applications demand small size
– “Smart dust” as the most extreme vision
• Energy scavenging
– Recharge batteries from ambient energy (light,
vibration, …)
36

37. Single Node Architecture
24.08.2020

38. Single-node Architecture
Goals
• Survey the main components of the composition of a node for a
wireless sensor network
– Controller, radio modem, sensors, batteries
• Understand energy consumption aspects for these components
– Putting into perspective different operational modes and
what different energy/power consumption means for
protocol design
38

39. Main components of a WSN node
• Controller - A controller to process all the relevant data, capable of
executing arbitrary code.
• Memory - Some memory to store programs and intermediate data;
usually, different types of memory are used for programs and data.
• Communication device(s) - Turning nodes into a network requires a
device for sending and receiving information over a wireless channel
• Sensors/actuators - The actual interface to the physical world: devices
that can observe or control physical parameters of the environment
• Power supply - As usually no tethered power supply is available, some
form of batteries are necessary to provide energy. Sometimes, some form of
recharging by obtaining energy from the environment is available as well
(e.g. solar cells).
39

40. Single-node Architecture
• Each of these components has to operate balancing the trade-off
between as small an energy consumption as possible on the one
hand and the need to fulfill their tasks on the other hand.
40

41. Controller
Main options:
• General purpose processor
– Used in Desktop Computers
– Highly over powered
– Energy Consumption is excessive
• Micro controller
– optimized for embedded applications
– Flexibility in connecting other devices
– low power consumption
– Build in Memory
– Freely programmable and flexible
– Going to Sleep State
41

42. Controller
Main options:
• DSPs
– optimized for signal processing tasks
– Advantages are not suitable here
• FPGAs (Field –Programmable GateArrays)
– may be good for testing
– Reprogrammed
• ASICs
– Specialized processor
– Custom Design for application
– only when peak performance is needed, no flexibility
42

43. Controller
Example microcontrollers
• Intel strongARM
– High end Processor with PDAs
– SA-1100 model has 32 bit reduced Instruction Set Computer
(RISC) core, running at up to 206 MHz
• Texas Instruments MSP430
– 16-bit RISC core, up to 4 MHz, versions with 2-10 kbytes RAM,
several DACs, RT clock, prices start at 0.49 US$
• AtmelATMega
– 8-bit controller
– Usage in embedded application with external interfaces.
43

44. 25.08.2020

45. Memory
• The memory component is fairly straightforward.
– Need for RandomAccess Memory (RAM) to store
intermediate sensor readings, packets from other nodes.
– While RAM is fast, disadvantage - loses its content if power
supply is interrupted.
• Program code can be stored in
– Read-Only Memory (ROM)
– Electrically Erasable Programmable Read-Only Memory
(EEPROM) or
– flash memory
45

46. Contd…
• Flash memory serve as intermediate storage of data in case
RAM is insufficient or when the power supply of RAM shut
down for some time.
• The long read and write access delays of flash memory need
high energy.
• Manufacturing costs and power consumption.
• Memory requirements are very much application
dependent.
46

47. Communication Devices
• Choice of transmission medium
• Transceivers
• Transceivers tasks and characteristics
• Transceiver structure
• Transceiver operational states
• Advanced Radio Concepts
• Nonradio frequency wireless communication
• Examples of radio transceivers
47

48. Choice of transmission medium
• The communication device is used to exchange data between
individual nodes.
• wired communication can actually be the method of choice and is
frequently applied in many sensor network like settings (using field
buses like Profibus, LON, CAN, or others).
• The first choice to make is that of the transmission medium
– Radio frequencies
– Optical communication
– Ultrasound
– other media like magnetic inductance are only used in very
specific cases
48

49. Contd..
• Radio Frequency (RF)-based communication - best fits the
requirements of mostWSN applications
– It provides relatively long range and high data rates
– acceptable error rates at reasonable energy expenditure
– does not require line of sight between sender and receiver
• Wireless sensor networks typically use communication frequencies
between about 433 MHz and 2.4 GHz.
49
Picture taken from: https://www.britannica.com/science/radio-frequency-spectrum

50. Transceivers
• For communication, both a transmitter and a receiver are
required in a sensor node.
• The essential task is to convert a bit stream coming from a
microcontroller (or a sequence of bytes or frames) and
convert them to and from radio waves.
• Device that combines these two tasks in a single entity -
transceivers.
50

51. Contd…
• Half-duplex operation is realized
• A range of low-cost transceivers is commercially available
that incorporate all the circuitry required for transmitting
and receiving – modulation, demodulation, amplifiers,
filters, mixers etc
51

52. Transceiver tasks and characteristics
• Service to upper layer
– A receiver has to offer certain services to the upper layers, most
notably to the MediumAccess Control (MAC) layer.
– This service is packet oriented; sometimes,
– Transceiver only provides a byte interface or even only a bit
interface to the microcontroller.
• Power consumption and energy efficiency
– The simplest interpretation of energy efficiency is the energy
required to transmit and receive a single bit.
– Transceivers should be switchable between different states - active
and sleeping.
52

53. Contd…
• Carrier frequency and multiple channels
– Transceivers are available for different carrier frequencies - match
application requirements and regulatory restrictions.
– Channels helps to alleviate some congestion problems in dense
networks.
– Such channels or “subbands” are relevant, for example, for certain
MAC protocols (FDMA or multichannel CSMA/ALOHA techniques)
• State change times and energy
– A transceiver can operate in different modes:
• sending or receiving
• use different channels
• different power-safe states
53

54. Contd…
– In any case, the time and the energy required to change between
two such states are important figures of merit.
– The turnaround time between sending and receiving, for example,
is important for various medium access protocols
• Data rates
– Carrier frequency and used bandwidth together with modulation
and coding determine the gross data rate.
– Typical values are a few tens of kilobits per second
– Different data rates can be achieved - by using different
modulations or changing the symbol rate.
54

55. Contd…
• Modulations
– The transceivers typically support one or several of on/off-keying,
ASK, FSK, or similar modulations.
• Coding
– Some transceivers allow various coding schemes to be selected
• Transmission power control
– Some transceivers can directly provide control over the
transmission power to be used;
– some require some external circuitry.
– Maximum output power is usually determined by regulations.
55

56. Contd…
• Noise Figure
NF of an element is defined as the ratio of the Signal-to-Noise Ratio
(SNR) ratio SNRi at the input of the element to the SNR ratio SNRO at
the element’s output:
NF=
𝑆𝑁𝑅𝑖
𝑆𝑁𝑅𝑜
The degradation of SNR due to the element’s operation and is typically
given in dB: NF dB = SNRi dB − SNRO dB
56

57. Contd…
• Gain
– The gain is the ratio of the output signal power to the input signal power and is
typically given in dB.
– Amplifiers with high gain are desirable to achieve good energy efficiency.
• Power efficiency
– The efficiency of the radio front end is given as the ratio of the radiated power
to the overall power consumed by the front end
– power amplifier, the efficiency describes the ratio of the output signal’s power
to the power consumed by the overall power amplifier.
• Receiver sensitivity
– The receiver sensitivity (given in dBm) is the minimum signal power at the
receiver needed to achieve a prescribed Eb/N0
– Better sensitivity levels extend the possible range of a system.
57

58. Contd…
• Range
– The range is considered in absence of interference; it evidently
depends on the maximum transmission power, on the antenna
characteristics, on the attenuation caused by the environment,
which in turn depends on the used carrier frequency, on the
modulation/coding scheme that is used, and on the bit error rate
that one is willing to accept at the receiver.
– It also depends on the quality of the receiver – based on
sensitivity.
– The products with ranges between a few meters and several
hundreds of meters are available
58

59. Contd…
• Blocking performance
– The blocking performance of a receiver is its achieved bit error
rate in the presence of an interferer.
– Blocking performance can be improved by interposing a filter
between antenna and transceiver.
– An important special case is an adjacent channel interferer that
transmits on neighboring frequencies.
– The adjacent channel suppression describes a transceiver’s
capability to filter out signals from adjacent frequency bands (and
thus to reduce adjacent channel interference) has a direct impact
on the observed Signal to Interference and Noise Ratio (SINR).
59

60. Contd…
• Out of band emission
– The inverse to adjacent channel suppression is the out of band
emission of a transmitter.
– To limit disturbance of other systems, or of the WSN itself in a
multichannel setup, the transmitter should produce little
transmission power
• Carrier sense and RSSI
– The precise semantics of this carrier sense signal depends on the
implementation.
– For example, the IEEE 802.15.4 standard [468] distinguishes the
following modes:
60

61. Contd..
• A carrier has been detected, that is, some signal which
complies with the modulation.
• Carrier detected and energy is present.
• The signal strength at which an incoming data packet has
been received can provide useful information (e.g. a rough
estimate about the distance from the transmitter assuming
the transmission power is known);
• A receiver has to provide this information in the Received
Signal Strength Indicator (RSSI).
61

62. Contd..
• Frequency Stability
– The frequency stability denotes the degree of variation
from nominal center frequencies when environmental
conditions of oscillators like temperature or pressure
change
– Poor frequency stability can break down communication
links
• Voltage range
– Transceivers should operate reliably over a range of
supply voltages
– Inefficient voltage stabilization circuitry is required
62

63. Transceiver structures
• Radio frequency front end
– Performs Analog signal processing in the actual radio
frequency band
• Baseband Processor
– Performs all Signal Processing in digital domain
– Communicates with a sensor node processor or other
digital circuitry
Between these two parts a frequency conversion
constituted by DACs and ADCs
63

64. • The Power Amplifier (PA) accepts upconverted signals from the IF or
baseband part and amplifies them for transmission over the antenna.
The Low Noise Amplifier (LNA) amplifies incoming signals up to
levels suitable for further processing without significantly reducing
the SNR [470].
64

65. • The range of powers of the incoming signals varies from very
weak signals from nodes close to the reception boundary to
strong signals from nearby nodes; this range can be up to 100
dB.
• LNA is active all the time and can consume a significant fraction
of the transceiver’s energy.
• Elements like local oscillators or voltage-controlled oscillators
and mixers are used for frequency conversion from the RF
spectrum to IF or to the baseband.
• The incoming signal at RF frequencies fRF is multiplied in a
mixer with a fixed-frequency signal from the local oscillator
(frequency fLO).
• IF = fLO − fRF.
65

66. Transceiver Operational States
• Transmit State:
– In the transmit state, the transmit part of the transceiver is active
and the antenna radiates energy.
• Receive State:
– the receive part is active.
• Idle State:
– A transceiver that is ready to receive but is not currently receiving
anything is said to be in an idle state.
– many parts of the receive circuitry are active, and others can be
switched off.
66

67. Contd..
– For example, in the synchronization circuitry, some elements
concerned with acquisition are active, while those concerned with
tracking can be switched off and activated only when the
acquisition has found something.
– A major source of power dissipation is leakage.
• Sleep State:
– significant parts of the transceiver are switched off.
– There are transceivers offering several different sleep states.
– These sleep states differ in the amount of circuitry switched off
– associated recovery times and startup energy
67

68. Activity: Brainstorming
68

69. Advanced Radio Concepts
– Wake up radio
• One of the most power-intensive operations is waiting for a
transmission to come in, ready to receive it.
• During this time, the receiver circuit must be powered up - to
observe wireless channel needs spending energy without any
immediate benefit.
• A receiver structure is necessary that does not need power but
can detect when a packet starts to arrive.
• To keep this specialized receiver simple, it should raise an
event to notify other components of an incoming packet; upon
such an event, the main receiver can be turned on and perform
the actual reception of the packet.
• Such receiver concepts are called wakeup receivers
• Each packet – power consumption is 1 microwatt
69

70. Contd…
– Spread spectrum transceivers
• ASK, FSK has limited Performance when lot of interference.
• To overcome Spread spectrum transceivers – DSSS (Direct
Sequence spread spectrum, Frequency Hopping Spread
Spectrum
• Complex hardware and costly
– Ultraband communication
• Using such a large bandwidth, an ultra wideband
communication will overlap with the spectrum of a
conventional radio system.
• But, because of the large spreading of the signal, a very small
transmission power suffices UWB transmitter is actually
relatively simple since it does not need oscillators or related
circuitry found in transmitters for a carrier-frequency-based
transmitter.
• The receivers require complex timing synchronization. 70

71. Non radio frequency wireless
communication
– Optical
– Optical link between sensors
– Advantage – very small energy per bit
– LEDs – High efficiency senders
– Disadvantages: Strongly influenced by whether condition
– Line of Sight
– Ultra sound
– For underwater communication: Ultra sound communication
is suitable
– Travels for long distances
– Different propagation speed
71

72. Examples of radio Transceiver
– RFMTR1000 family
– Hardware accelerators (Mica motes)
– Chipcon CC100 and CC2420 family
– InfineonTDA 525x family
– IEEE802.15.4/Ember EM2420 RFTransceiver
– National Semiconductor LMX3162
– Conexant RDSSS9M
72

73. Contd..
– RFMTR1000 family
• The TR1000 family of radio transceivers from RF
Monolithics2 is available for the 916 MHz and 868 MHz
frequency range.
• It works in a 400 kHz wide band centered at, for
example, 916.50 MHz.
• It is intended for short-range radio communication with
up to 115.2 kbps.
• Low-power consumption in both send and receive modes
and especially in sleep mode.
73

74. Contd..
– Hardware accelerators (Mica motes)
• The Mica motes use the RFM TR1000 transceiver and
contain also a set of hardware accelerators.
• The transceiver offers a very low-level interface, giving
the microcontroller tight control over frame formats,
MAC protocols, and so forth.
• On the other hand, framing and MAC can be very
computation intensive, for example, for computing
checksums, for making bytes out of serially received bits
or for detecting Start Frame Delimiters (SFDs) in a
stream of symbols
74

75. • ChipconCC1000
– Range 300 to 1000 MHz, programmable in 250 Hz steps
– FSK modulation
– Provides RSSI
• ChipconCC 2400
– Implements 802.15.4
– 2.4 GHz, DSSS modem
– 250 kbps
– low power consumption than above transceivers

76. • InfineonTDA 525x family
– provides flexible, single-chip, energy-efficient transceivers
– E.g.,TDA5250: 868 -870 MHz transceiver
– ASK or FSK modulation
– RSSI, highly efficient power amplifier
– Intelligent power down,“self-polling” mechanism(define
data rate)
– Excellent blocking performance (quite resistant to
interference)

77. Example radio transceivers for ad hoc networks
• Ad hoc networks: Usually, higher data rates are required
• Typical: IEEE 802.11 b/g/a is considered
– Up to 54 MBit/s
– Relatively long distance (100s of meters possible, typical 10s of
meters at higher data rates)
– Works reasonably well (but certainly not perfect) in mobile
environments
– Problem: expensive equipment, quite power hungry

78. Sensors and actuators
01.09.2020

79. Contd…
• Sensors
– Sensors can be roughly categorized into three categories
• Passive, omnidirectional sensors
– These sensors can measure a physical quantity at the point of the
sensor node without actually manipulating the environment by
active probing – in this sense, they are passive.
– Moreover, some of these sensors actually are self-powered in the
sense that they obtain the energy they need from the environment
– energy is only needed to amplify their analog signal. There is no
notion of “direction” involved in these measurements.

80. Contd…
– Typical examples for such sensors include thermometer, light
sensors, vibration, microphones, humidity, mechanical stress
or tension in materials, chemical sensors sensitive for given
substances, smoke detectors, air pressure, and so on.
• Passive, narrow-beam sensors
– These sensors are passive as well, but have a well-defined
notion of direction of measurement.
– A typical example is a camera, which can “take
measurements” in a given direction, but has to be rotated if
need be.

81. Contd…
• Active sensors
– This last group of sensors actively probes the
environment, for example, a sonar or radar sensor or
some types of seismic sensors, which generate shock
waves by small explosions.
• Obvious trade-offs include accuracy, dependability, energy
consumption, cost, size, and so on – all this would make a
detailed discussion of individual sensors quite ineffective.
• Overall, most of the theoretical work on WSNs considers
passive, omnidirectional sensors.

82. Contd…
• Narrow-beam-type sensors like cameras are used in some practical
testbeds, but there is no real systematic investigation on how to
control and schedule the movement of such 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.

83. Actuators
• In principle, all that a sensor node can do is to open or close a
switch or a relay or to set a value in some way.
• Whether this controls a motor, a light bulb, or some other
physical object is not really of concern to the way
communication protocols are designed.
• In a real network, however, care has to be taken to properly
account for the idiosyncrasies of different actuators.
• Also, it is good design practice in most embedded system
applications to pair any actuator with a controlling sensor –
following the principle to “never trust an actuator”

84. Power supply of sensor nodes
• Goal: to provide as much energy as possible at smallest cost/ volume/
weight/ recharge time/longevity
– InWSN, recharging may or may not be an option
• Options
– Primary batteries –not rechargeable
– Secondary batteries –rechargeable, only makes sense in
combination with some form of energy harvesting
• Storing power is conventionally done using batteries.
• As a rough orientation, a normal AA battery stores about 2.2–2.5 Ah
at 1.5V.
• Battery design is a science and industry in itself, and energy
scavenging has attracted a lot of attention in research.

85. Contd…
Storing energy: Batteries
• Traditional batteries
– The power source of a sensor node is a battery, either
nonrechargeable (“primary batteries”) or, if an energy scavenging
device is present on the node, also rechargeable (“secondary
batteries”).
– In some form or other, batteries are electro-chemical stores for
energy – the chemicals being the main determining factor of
battery technology.

86. Contd…
Battery examples
• Energy per volume (Joule per cubic centimeter):

87. Contd…
Upon these batteries, very tough requirements are imposed:
Capacity
• They should have high capacity at a small weight, small
volume, and low price.
• The main metric is energy per volume, J/cm3.Above table
shows some typical values of energy densities, using
traditional, macroscale battery technologies.

88. Contd…
Capacity under load
• They should withstand various usage patterns as a sensor
node can consume quite different levels of power over time
and actually draw high current in certain operation modes.
• In most technologies, the larger the battery, the more
power can be delivered instantaneously.
• In addition, the rated battery capacity specified by a
manufacturer is only valid as long as maximum discharge
currents are not exceeded, lest capacity drops or even
premature battery failure occurs

89. Contd…
Self-discharge
• Their self-discharge should be low; they might also have to
last for a long time (using certain technologies, batteries are
operational only for a few months, irrespective of whether
power is drawn from them or not).
• Zinc-air batteries, for example, have only a very short
lifetime (on the order of weeks), which offsets their
attractively high energy density.

90. Contd…
Efficient recharging
• Recharging should be efficient even at low and
intermittently available recharge power; consequently, the
battery should also not exhibit any “memory effect”.
• Some of the energy-scavenging techniques are only able to
produce current in the μA region (but possibly sustained) at
only a few volts at best.
• Current battery technology would basically not recharge at
such values.

91. Contd…
Relaxation
• Their relaxation effect – the seeming self-recharging of an
empty or almost empty battery when no current is drawn from
it, based on chemical diffusion processes within the cell – should
be clearly understood.
• Battery lifetime and usable capacity is considerably extended if
this effect is leveraged.
• example, it is possible to use multiple batteries in parallel and
“schedule” the discharge from one battery to another, depending
on relaxation properties and power requirements of the
operations to be supported

92. Contd…
Energy scavenging
• Some of the unconventional energy stores– fuel cells, micro heat
engines, radioactivity – convert energy from some stored, secondary
form into electricity in a less direct and easy to use way than a normal
battery would do.
• The entire energy supply is stored on the node itself – once the fuel
supply is exhausted, the node fails.
• To ensure truly long-lasting nodes and wireless sensor networks, such
a limited energy store is unacceptable.
• Rather, energy from a node’s environment must be tapped into and
made available to the node – energy scavenging should take place.
Several approaches exist

93. Photovoltaics
• The well-known solar cells can be used to power sensor nodes.
• The available power depends on whether nodes are used outdoors or
indoors, and on time of day and whether for outdoor usage.
• Different technologies are best suited for either outdoor or indoor
usage.
• The resulting power is somewhere between 10 μW/cm2 indoors and
15 mW/cm2 outdoors.
• Single cells achieve a fairly stable output voltage of about 0.6 V (and
have therefore to be used in series) as long as the drawn current does
not exceed a critical threshold, which depends, among other factors,
on the light intensity.
• Hence, solar cells are usually used to recharge secondary batteries.

94. Temperature gradients
• Differences in temperature can be directly converted to electrical
energy.
• Theoretically, even small difference of, for example, 5 Kelvin can
produce considerable power, but practical devices fall very short of
theoretical upper limits (given by the Carnot efficiency).
• Seebeck effect-based thermoelectric generators are commonly
considered; one example is a generator, which will be commercially
available soon, that achieves about 80 μW/cm2 at about 1V from a 5
Kelvin temperature difference

95. Vibrations
• One almost pervasive form of mechanical energy is vibrations:
• walls or windows in buildings are resonating with cars or trucks
passing in the streets, machinery often has low frequency vibrations,
ventilations also cause it, and so on.
• The available energy depends on both amplitude and frequency of the
vibration and ranges from about 0.1 μW/cm3 up to 10,000 μW/cm3
for some extreme cases (typical upper limits are lower).
• Converting vibrations to electrical energy can be undertaken by various
means, based on electromagnetic, electrostatic, or piezoelectric
principles.

96. Pressure variations
• Somewhat similar to vibrations, a variation of pressure can
also be used as a power source. Such piezoelectric
generators are in fact used already.
• One well-known example is the inclusion of a piezoelectric
generator in the heel of a shoe, to generate power as a
human walks about.
• This device can produce, on average, 330 μW/cm2. It is,
however, not clear how such technologies can be applied to
WSNs.

97. Flow of air/liquid
• Another often-used power source is the flow of air or liquid
in wind mills or turbines.
• The challenge here is again the miniaturization, but some of
the work on millimeter scale MEMS gas turbines might be
reusable.
• However, this has so far not produced any notable results.

99. Comparison of energy sources

100. • As these examples show, energy scavenging usually has to be
combined with secondary batteries as the actual power
sources are not able to provide power consistently,
uninterruptedly, at a required level; rather, they tend to
fluctuate over time.
• This requires additional circuitry for recharging of batteries,
possibly converting to higher power levels, and a battery
technology that can be recharged at low currents