Unit 2
Sensors parameters
• Digital or Analog devices such as mechanical switches, proximity switches,
photoelectric switches, encoders, temperature, pressure switches.
• Some of the more common terms used to define the performance of
sensors:
1. Accuracy: Is the extent to which the value indicated by a measurement
system or element might be wrong.
Ex: A temperature sensor might have an accuracy of + 0.1°C
Error : it is the measurement of difference between the result of the
measurement of true value of quality being measured.
• Non linearity error is used to describe the error that occurs as a result of
assuming a linear relationship between the input and output over the
working range.
• When the input value to a sensor changes, it will take some
time to reach and settle down to the steady state value.
• The response time is the time that elapses after the input to a
system or element is abruptly increased from zero to a
constant value, up to the point at which the system or
element gives an output.
• Sensitivity: ratio of output/input
• Ex: a thermocouple having a sensitivity of
20µV/°C, It gives an output of 20µV for each 1°
change in temperature.
• Stability: constant
• Repeatability: same value for repeated
measurement of the same variable.
• Reliability: probability that will operate to an
agreed level of a specified period.
Input devices
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Mechanical switches
Proximity switches
Photoelectric sensors and switches
Temperature sensors
Position sensors
Pressure sensors
Smart sensors
Mechanical switches
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ON/OFF signal
Switches are normally open, normally closed
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Disadvantage: switch debounce
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Overcome by including a software program by introducing
20ms delay.
Single Pole Double Throw(SPDT)
Single Pole Single Throw(SPST)
Limit switch
• Limit switch is used to detect the presence or
passage of a moving part. It can be actuated
by a cam, lever and roller
Proximity switches
• Proximity switch are used to detect the
presence of an item without making contact
with it.
• Types of proximity switch:
1. Eddy current
2. Reed switch
3. Capacitive switch
Eddy current
• A coil that is constantly energised by a AC and produces a constant
alternating magnetic field, when a metal object brought near to it
eddy currents are induced in it.
• The magnetic field due to these induces an EMF back in the coil.
• The voltage amplitude is thus an measure of the proximity of metallic
objects.
• The voltage can be used to activate an electronic switch circuit,
basically a transistor.
• Range is typically about 0.5 to 20mm
Reed switch
• It consists of two overlapping, but not touching
strips of a springy ferromagnetic material sealed in a
glass or plastic envelope. When a magnet or current
carrying coil is bought close to the switch the strip
becomes magnetized and attract each other.
• Application: burglar alarms to detect whether the
door is closed
Capacitive proximity switch
• A proximity switch that can be used with metallic
and non metallic objects is the capacitive
proximity switch.
• One plate of the capacitor is sensor, the other
being the metal object for which the proximity is
to be detect. Range (4mm to 60mm) from the
sensor head
• Inductive proximity switch
Photo electric sensor and switches
• The radiation emitter is generally a LED, detector might be a
phototransistor.
• Range typically less than 5mm.
• Photoconductive cell, the resistance often cadmium sulphide,
depends on the intensity of light falling on it.
• These sensors light is converted in to a current, voltage or
resistance.
Temperature sensor
Types of temperature sensors
1. Bimetallic strip
2. Resistive temperature detector(RTD)
3. NTC thermistors
4. LM35, LM3911N
5. Thermocouple
• It is used to turn ON/OFF a signal when a certain temperature is reached
using a bi-stable element.
• It consists of two metals Brass and Iron with different coefficient of
expansions.
• Resistive temperature detector(RTD): The electrical
resistance of metals or semiconductors changes with
temperature.
• Metals include platinum, nickel or nickel alloys.
• Bridge balance condition
P/Q=R/S
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Thermistors show a very large change in temperature with temperature, the
change is non-linear.
They have Negative temperature co-efficient, the resistance decreases with
increase in temperature.
Advantages: cheap and small, giving large change in resistance and having fast
reaction to temperature changes.
Disadvantage: non-linear, with limited temperature ranges.
• Thermo diodes and thermo transistors are used as
temperature sensors, in which electrons and holes
diffuse across semiconductor region
• LM35 which gives an output of 10mV/C when the
supply voltage is 5V.
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Thermocouple consists of two wires A and B forming a junction, when the junction is heated so that it is at a
higher temperature than the other junctions in the circuit, which remains at a constant cold temperature.
An EMF is produced that is related to the hot junction temperature.
The thermocouple voltage is small and needs amplification before it can be fed to the analog channel input of the
PLC.
Advantage: able to sense the temperature at almost any point , ruggedness and being to operate over a large
temperature range.
Disadvantage: non linear response.
Position displacement sensor
• Position sensor: It is used to measure the
distance between a reference point and the
current location of the target.
• Displacement sensor: It gives a measure of
distance between the present position of the
target and the previously recorded position.
• Resistive linear and angular position sensors are
widely used and relatively inexpensive
Position sensor
• A DC voltage is provided across the full length of a track and
the voltage signal between the slides over the resistance track
and one end of the track is related to the position of the
sliding contact between the ends of the potentiometer. Thus
it behaves as an angular position sensor.
Displacement sensor
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LVDT: Linear Variable Differential Transformer, it gives an voltage output related to
the position of a ferrous road.
The LVDT consists of three symmetrically placed coils through which the ferrous rod
moves
When an alternating current is applied to the primary coil , alternating voltages V1
and V2 are induced in the two secondary coils.
When the ferrous rod core is cantered between the two secondary coils, the voltage
induced in them are equal so no output voltage.
When the rod is displaced from its central position there is more of the rod in one
secondary coil than the other. The difference between the two gives the output
voltage.
Pressure sensor
• They are designed to give outputs that are proportional to the difference in
pressure between two input ports.
• If one of the ports is left open to the atmosphere, the gauge measures the
pressure changes with respect to the atmosphere and the pressure
measured is known as gauge pressure.
• Absolute pressure: when it is measured with respect to vacuum.
• Piezoelectric pressure sensor
• The output is a voltage with a sensitivity of 0.6mV/kpa.
Pressure sensor
• Pressure switch are designed to switch on or off at a particular pressure.
• Diaphragm moves under the action of the pressure and operates a
mechanical switch.
• Diaphragm are less sensitive than bellows but can withstand greater
pressure.
Smart sensor
• Sensors generally need signal amplifying circuits to convert from
Analog to Digital and vice versa.
• To get the sensors signal in Wright form take an account of
nonlinearities and calibrate it. A gradual change in the properties of
sensors overtime.
• Some sensors have all these elements take care of in a single package
called smart sensors.
• Smart sensors usually consists of data convertors, a processors,
firmware's and some form of non volatile electrically erasable
programmable read only memory(EEPROM).
• Smart sensors can have their elements produced on a single silicon
chip.
• the IEEE1451.4 standard interface for smart sensor and actuators is
based on electronic data sheet(TEDS)
• The standard requires the non-volatile EEPROM embedded memory
to hold and communicate data, which will allow a plug and play
capability.
Sensors, Actuators, and Smart Objects
• Active or passive:
• Sensors can be categorized based on whether
they produce an energy output and typically
require an external power supply (active) or
whether they simply receive energy and
typically require no external power supply
(passive)
Classification of sensors
• Invasive or non-invasive:
• Sensors can be categorized based on whether
a sensor is part of the environment it is
measuring (invasive) or external to it (noninvasive).
Classification of sensors
• Contact or no-contact:
• Sensors can be categorized based on whether
they require physical contact with what they
are measuring (contact) or not (no-contact).
Classification of sensors
• Absolute or relative:
• Sensors can be categorized based on whether
they measure on an absolute scale (absolute)
or based on a difference with a fixed or
variable reference value (relative).
Classification of sensors
• Area of application: Sensors can be categorized
based on the specific industry or vertical where
they are being used.
• How sensors measure: Sensors can be
categorized based on the physical mechanism
used to measure sensory input (for example,
thermoelectric, electrochemical, piezoresistive,
optic, electric, fluid mechanic, photoelastic).
• What sensors measure: Sensors can be
categorized based on their applications or what
physical variables they measure.
Actuators
• Type of motion: Actuators can be classified based on
the type of motion they produce (for example, linear,
rotary, one/two/three-axes).
• Power: Actuators can be classified based on their
power output (for example, high power, low power,
micro power)
• Binary or continuous: Actuators can be classified based
on the number of stable-state outputs.
• Area of application: Actuators can be classified based
on the specific industry or vertical where they are
used.
• Type of energy: Actuators can be classified based on
their energy type.
Smart object
Stepper Motor
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Stepper motor or stepping motor that produces rotation through equal angles, the so
called steps.
If one input pulse produces a rotation of 1.8° then for a rotation of 360°, 200 such
digital pulses are required. The motor thus can be used for angular position.
Application: If stepping motor is used to drive a continuous belt, it can be used to give
accurate linear positioning. Such a motor is used with computers, printers, robots,
machine tools
Conveyor Belt
• Conveyor belt is used to transport goods from a loading
machine to a packing area.
• When an item is loaded onto the conveyor belt, a contact
switch might be used to indicate that an item is on the
belt and to start the conveyor motor.
• Motor remains off until the next item is loaded
Input / Output Signal Conditioning
• When connecting sensors that generate digital or discrete signals
to an input, care has to be taken to ensure that voltage levels
match.
• A standard form of analog is used to convert analog signal to a
current in the range 4 to 20mA by passing it through a 250Ώ
resistance to give a 1 to 5V input signal.
• Where 0 level is indicated by 4mA
• 1 is indicated by 20mA
Changing Voltage Levels
• A potential divider can be used to reduce the voltage from a
sensor to the required level Vout
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Vout= R2*Vin/(R1+R2)
• Amplifiers are used to increase the Voltage level
1. Inverting amplifier
2. Non-inverting amplifier
Inverting Amplifier
• Input is applied to inverting terminal of the op-amp.
Non-Inverting Amplifier
• Input is applied to non inverting
terminal of the op-amp.
Differential Amplifier
• It is used to amplify the difference between two
input voltages.
Signal conditioning with a strain
gauge sensor
• The arrangement that might be used for a strain gauge sensor .
• Sensor is connected in a wheat stone bridge and the out of potential is
amplified by a differential amplifier before being fed via an anlog-to-digital
converter unit
Op-amp Comparator
• The output of an op-amp saturates at about + 12V.
• Operational amplifiers are widely used to give on/off signals
based on the relative values of the two input signals.
• One is connected to the inverting and the other is connected to
the non inverting terminal of the Op-amp.
Directional Control Valves
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Solenoid as an actuator is a solenoid operated valve. They are used to control the directions of
pressurized air or oil and used to operate devices, such as piston moving in a cylinder.
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Pressurized air hydraulic fluid is input from port P which is connected to the power supply
from a pump or compressor and port T is connected to hydraulic fluid return to the supply
tank or in the case of a pneumatic system .
The hydraulic fluid or pressurized air is fed to the right of the piston and exhausted from the
left, the result being the movement of the piston to the left.
When a current is passed through a solenoid, the spool valve switches the hydraulic fluid or
pressurized air to left of the piston and exhaust from the right.
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Directional Control Valves
• The valve has four ports – A,B,P and T and two control positions.
4/2 valve.
• Basic symbol for a valve is a square, with one square used to
describe each of the control positions.
• With each square the switching positions are then described by
arrows to indicate a flow direction, or a terminated line to
indicate no flow path .
Directional Control Valves
• Direction control valves are used to control the direction of
motion of position in cylinders.
1. 2 ports and 2 control outputs
2. 3 ports and 2 control outputs B is absent.
3. 4 ports and 2 control outputs
Directional Control Valves
• Actuation symbols
a) Solenoid
b) push button
c) Spring operated
d) Spring and solenoid operated 4/2 valve
Directional Control Valves
• Single acting cylinder
• Double acting cylinder
Control of single
acting cylinder
Control of double
acting cylinder
Communications Criteria
• The various technologies used for connecting sensors can
differ greatly depending on the criteria used to analyze them
• Range: This section examines the importance of signal
propagation and distance.
• Frequency Bands: This section describes licensed and
unlicensed spectrum, including sub-GHz frequencies.
• Power Consumption: This section discusses the
considerations required for devices connected to a stable
power source compared to those that are battery powered.
• Topology: This section highlights the various layouts that may
be supported for connecting multiple smart objects.
• Constrained Devices: This section details the limitations of
certain smart objects from a connectivity perspective.
• Constrained-Node Networks: This section highlights the
challenges that are often encountered with networks
connecting smart objects
Communication technologies for
connecting smart objects
• IEEE 802.15.4: This section highlights IEEE 802.15.4, an older but
foundational wireless protocol for connecting smart objects.
• IEEE 802.15.4g and IEEE 802.15.4e: This section discusses
improvements to 802.15.4 that are targeted to utilities and smart cities
deployments.
• IEEE 1901.2a: This section discusses IEEE 1901.2a, which is a
technology for connecting smart objects over power lines.
• IEEE 802.11ah: This section discusses IEEE 802.11ah, a technology built
on the well-known 802.11 Wi-Fi standards that is specifically for smart
objects.
• LoRaWAN: This section discusses LoRaWAN, a scalable technology
designed for longer distances with low power requirements in the
unlicensed spectrum.
• NB-IoT and Other LTE Variations: This section discusses NB-IoT and
other LTE variations, which are often the choice of mobile service
providers looking to connect smart objects over longer distances in the
licensed spectrum
IoT access technologies
• Standardization and alliances: The standards bodies
that maintain the protocols for a technology.
• Physical layer: The wired or wireless methods and
relevant frequencies.
• MAC layer: Considerations at the Media Access Control
(MAC) layer, which bridges the physical layer with data
link control.
• Topology: The topologies supported by the technology
• Security: Security aspects of the technology
• Competitive technologies: Other technologies that are
similar and may be suitable alternatives to the given
technology
Message Queue Telemetry Transport
MQTT
• MQTT is machine-to-machine (M2M)/"Internet of
Things" connectivity protocol.
• originally developed and released by IBM
• It was designed as an extremely lightweight
publish/subscribe messaging transport.
• It is useful for connections with remote locations
where a small code footprint is required and/or
network bandwidth is at a premium.
• MQTT-enabled smart phone applications have
taken off on a strong note.
Design principles and aims of MQTT
• It provides publish-and-subscribe messaging
• specifically designed for resource-constrained
devices.
• low bandwidth and high latency networks
such as dial-up lines and satellite links.
• Facebook are using it as part of their mobile
applications because it has such a low power
footprint and is extremely light on network
bandwidth.
MQTT-enabled device connectivity.
MQTT
• MQTT enables resource-constrained IoT
devices to send, or publish, information about
a given topic to a server that functions as an
MQTT message broker.
• The broker then pushes the information out to
those clients that have previously subscribed to
the client's topic.
MQTT
• Many open source implementations of clients and
brokers are available
• Really small message broker (RSMB)
• Mosquitto
• Micro broker: Java based for PDAs, notebooks
• Quality of Service Levels: Three levels:
• 0 = At most once (Best effort, No Ack)
• 1 = At least once (Acked, retransmitted if ack not
received)
• 2 = Exactly once [Request to send (Publish), Clear-tosend (Pubrec), message (Pubrel), ack (Pubcomp)]
QOS Zero
• This service level guarantees a best-effort delivery. There is no guarantee
of delivery.
• The recipient does not acknowledge receipt of the message and the
message is not stored and re-transmitted by the sender.
• QoS level 0 is often called “fire and forget” and provides the same
guarantee as the underlying TCP protocol.
QoS 1 – at least once
• QoS level 1 guarantees that a message is delivered at least one time to the
receiver.
• The sender stores the message until it gets a PUBACK packet from the
receiver that acknowledges receipt of the message.
• It is possible for a message to be sent or delivered multiple times.
QoS 2 – exactly once
• QoS 2 is the highest level of service in MQTT.
• This level guarantees that each message is received only once by
the intended recipients.
• QoS 2 is the safest and slowest quality of service level.
• The guarantee is provided by at least two request/response flows (a
four-part handshake) between the sender and the receiver.
• The sender and receiver use the packet identifier of the original
PUBLISH message to coordinate delivery of the message.
MQTT
MQTT Application
• Home pacemaker monitoring solution
• Sensors on patient
• Collected by a monitoring equipment in home (broker)
using MQTT
• Subscribed by a computer in the hospital
• Alerts the doctor if anything is out-of-order
• VerneMQ is a high-performance distributed MQTT
message broker.
• low latency and fault tolerance, connects virtually
everything
• It is being positioned as the one for enabling connected
cars, smarter lighting, public transportation, and
smarter homes.
• ThingMQ: is a carrier-grade multiprotocol
message broker for IoT that runs on the cloud.
Any device can be easily connected to their
cloud using MQTT, CoAP, REST, or WebSockets
and ThingMQ handles the rest.
Constrained Application Protocol
(COAP)
• It is an application layer protocol for Machine to
Machine (M2M) applications.
• Meant for constrained applications.
• COAP uses a client server architecture. Clients
communicate with servers using connectionless
diagrams.
• COAP supports methods such as GET, PUT,
POST and DELETE
User Datagram Protocol
UDP is an alternative communications protocol to Transmission
Control Protocol (TCP) used primarily for establishing low-latency and
loss-tolerating connections between applications on the internet.
XMPP Extensible Messaging and
Presence Protocol
• Protocol for real time applications and streaming XML
data between network entities.
• XMPP powers wide range of applications including
messaging, presence, data syndication, gaming,
voice/video calls.
• XMPP also sending small chunks of XML data from one
network entity to another in near real time.
• XMPP supports both client to server and server to
servers communication paths.
• XMPP allows real time communications between IOT
devices.
IEEE 802.15.4
• IEEE 802.15.4 is a wireless access technology
for low-cost and low-data-rate devices that
are powered or run on batteries.
Applications:
• Home and building automation
• Automotive networks
• Industrial wireless sensor networks
• Interactive toys and remote controls
IEEE 802.15.4
• The negatives around reliability and latency often have to
do with the Collision Sense Multiple Access/Collision
Avoidance (CSMA/CA) algorithm.
• CSMA/CA is an access method in which a device “listens” to
make sure no other devices are transmitting before starting
its own transmission.
• If another device is transmitting, a wait time (which is
usually random) occurs before “listening” occurs again.
• Interference and multipath fading occur with IEEE 802.15.4
because it lacks a frequency-hopping technique.
• Later variants of 802.15.4 from the IEEE start to address
these issues
Protocol Stacks Utilizing IEEE 802.15.4
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ZigBee
6LoWPAN
ZigBee IP
ISA100.11a
WirelessHART
Thread
Zigbee- 802.15.4
• ZigBee protocol was framed by the ZigBee
alliance. Following features of ZigBee make it
very suitable for IoT applications:
• Low power consumption
• Low cost
• Support for large number of network nodes
(<=65K nodes)
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ZigBee specification uses lower layers of IEEE
802.15.4 protocol stack and defines its own
upper layers from network to application
including application profiles.
The ZigBee network and security layer provides
mechanisms for network startup, configuration,
routing, and securing communications.
This includes calculating routing paths in what is
often a changing topology, discovering
neighbors, and managing the routing tables as
devices join for the first time.
The network layer is also responsible for
forming the appropriate topology, which is
often a mesh but could be a star or tree as well.
From a security perspective, ZigBee utilizes
802.15.4 for security at the MAC layer, using the
Advanced Encryption Standard (AES) with a 128bit key and also provides security at the network
and application layers.
Applications of Zigbee