W08 Sensors and
Measurement (2/2)
Yrd. Doç. Dr. Aytaç Gören
ELK 2018 - Contents
W01 Basic Concepts in Electronics
W02 AC to DC Conversion
W03 Analysis of DC Circuits (self and condenser)
W04 Transistors and Applications (H-Bridge)
W05 Op Amps and Applications
W06 Midterm
W07 Sensors and Measurement (1/2)
W08 Sensors and Measurement (2/2)
W09 Basic Concepts in Digital Electronics (Boolean Algebra, Decimal to binary, gates)
W10 Digital Logic Circuits (Gates and Flip Flops)
W11 PLC’s
W12 Microprocessors
W13 Data Acquisition, D/A and A/D Converters.
2
Yrd. Doç. Dr. Aytaç Gören
ELK 2018 – W08 Contents
1.
2.
3.
4.
5.
6.
7.
Photoelectric Sensors
Thermal, Heat, Temperature Sensors
GPS
Flow, Fluid Velocity Sensors
Weather, Moisture, Humidity Sensors
Electric Current, Electric Potential
Magnetic Sensors
•
3
Yrd. Doç. Dr. Aytaç Gören
Connecting Sensors to Microcontrollers
*references 9
sensor
µC
sensor
signal timing
memory
keypad
display
Analog
many microcontrollers have a built-in A/D
8-bit to 12-bit common
many have multi-channel A/D inputs
Digital
serial I/O
use serial I/O port, store in memory to analyze
synchronous (with clock)
must match byte format, stop/start bits, parity check,
etc.
asynchronous (no clock): more common for comm. than
data
must match baud rate and bit width, transmission
protocol, etc.
frequency encoded
use timing port, measure pulse width or pulse frequency
Digital Sensors
As its name implies, Digital Sensors produce a discrete output
signal or voltage that is a digital representation of the quantity
being measured. Digital sensors produce a Binary output signal
in the form of a logic "1" or a logic "0", ("ON" or "OFF"). This
means then that a digital signal only produces discrete (noncontinuous) values which may be outputted as a single "bit",
(serial transmission) or by combining the bits to produce a single
"byte" output (parallel transmission).
Compared to analogue signals, digital signals or quantities have very
high accuracies and can be both measured and "sampled" at a very
high clock speed. The accuracy of the digital signal is proportional to
the number of bits used to represent the measured quantity. For
example, using a processor of 8 bits, will produce an accuracy of
0.195% (1 part in 512). While using a processor of 16 bits gives an
accuracy of 0.0015%, (1 part in 65,536) or 130 times more accurate.
This accuracy can be maintained as digital quantities are manipulated
and processed very rapidly, millions of times faster than analogue
signals.
Connecting Smart Sensors to PC/Network
“Smart sensor” = sensor with built-in signal processing & communication
Data Acquisition Cards (DAQ)
PC card with analog and digital I/O
interface through LabVIEW or user-generated code
Communication Links Common for Sensors
asynchronous serial comm.
universal asynchronous receive and transmit (UART)
1 receive line + 1 transmit line. nodes must match baud rate & protocol
RS232 Serial Port on PCs uses UART format (but at +/- 12V)
can buy a chip to convert from UART to RS232
synchronous serial comm.
serial peripheral interface (SPI)
*references 9
1 clock + 1 bidirectional data + 1 chip select/enable
I2C = Inter Integrated Circuit bus
designed by Philips for comm. inside TVs, used in several commercial
sensor systems
IEEE P1451: Sensor Comm. Standard
several different sensor comm. protocols for different applications
Sensor Calibration
Sensors can exhibit non-ideal effects
– offset: nominal output ≠ nominal parameter value
– nonlinearity: output not linear with parameter changes
– cross parameter sensitivity: secondary output variation with, e.g.,
temperature
Calibration = adjusting output to match parameter
6.000
T= temperature; V=sensor voltage;
a,b,c = calibration coefficients
*references 9
Compensation
T1
5.000
1001
1010
4.000
T2
offset
• T = a + bV
+cV2,
7.000
Frequency (MHz)
– analog signal conditioning
– look-up table
– digital calibration
3.000
1001
1101
1110
1111
2.000
T3
1.000
– remove secondary sensitivities
– must have sensitivities characterized
– can remove with polynomial evaluation
0.000
-30
-20
-10
0
10
20
30
40
50
60
70
Temperature (C)
• P = a + bV + cT + dVT + e V2, where P=pressure, T=temperature
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Photoelectric devices can be grouped into two main categories, those which
generate electricity when illuminated, such as Photo-voltaics or Photo-emissives
etc, and those which change their electrical properties in some way such as Photoresistors or Photo-conductors. This leads to the following classification of devices.
• Photo-emissive Cells - These are photodevices which release free electrons from
a light sensitive material such as caesium when struck by a photon of sufficient
energy. The amount of energy the photons have depends on the frequency of the
light and the higher the frequency, the more energy the photons have converting
light energy into electrical energy.
• Photo-conductive Cells - These photodevices vary their electrical resistance
when subjected to light. Photoconductivity results from light hitting a
semiconductor material which controls the current flow through it. Thus, more
light increase the current for a given applied voltage. The most common
photoconductive material is Cadmium Sulphide used in LDR photocells.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
• Photo-voltaic Cells - These photodevices generate an emf in proportion to the
radiant light energy received and is similar in effect to photoconductivity. Light
energy falls on to two semiconductor materials sandwiched together creating a
voltage of approximately 0.5V. The most common photovoltaic material is
Selenium used in solar cells.
• Photo-junction Devices - These photodevices are mainly true semiconductor
devices such as the photodiode or phototransistor which use light to control the
flow of electrons and holes across their PN-junction. Photojunction devices are
specifically designed for detector application and light penetration with their
spectral response tuned to the wavelength of incident light.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Infrared - Active
Sensor type = Reflective IR
IR detector = Panasonic PNA4602M
IR LED type = Narrow focus 10º
I/O required = 3 digital lines: 2
outputs, 1 input
Range = Approximately 4“ to 26"
Input voltage = 5vdc regulated @ 8mA
PC board size = 2.3" x .75"
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Photovoltaic
• Photovoltaic
– light falling on a pn-junction can
be used to generate electricity
from light energy
(as in a solar cell)
– small devices used as sensors are
called photodiodes
– fast acting, but the voltage
produced is not linearly related to
light intensity
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
LDR
• Photoconductive
– such devices do not produce electricity, but
simply change their resistance
– photodiode (as described earlier) can be
used in this way to produce a linear device
– phototransistors act like photodiodes but
with greater sensitivity
– light-dependent resistors (LDRs) are slow,
but respond like the human eye
The Light Dependent Resistor (LDR) is made from a piece of exposed
semiconductor material such as cadmium sulphide that changes its electrical
resistance from several thousand Ohms in the dark to only a few hundred Ohms
when light falls upon it by creating hole-electron pairs in the material. The net
effect is an improvement in its conductivity with a decrease in resistance for an
increase in illumination. Also, photoresistive cells have a long response time
requiring many seconds to respond to a change in the light intensity.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
LDR
The most commonly used
photoresistive light sensor
is the ORP12 Cadmium
Sulphide photoconductive
cell. This light dependent
resistor has a spectral
response of about 610nm
in the yellow to orange
region of light.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
LDR
The resistance of the cell when
unilluminated (dark resistance) is very
high at about 10MΩ's which falls to about
100Ω's when fully illuminated (lit
resistance). To increase the dark
resistance and therefore reduce the dark
current, the resistive path forms a zigzag
pattern across the ceramic substrate. The
CdS photocell is a very low cost device
often used in auto dimming, darkness or
twilight detection for turning the street
lights "ON" and "OFF", and for
photographic exposure meter type
applications.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
LDR
This basic light sensor circuit is of a relay output
light activated switch. A potential divider circuit is
formed between the photoresistor, LDR and the
resistor R1. When no light is present ie in darkness,
the resistance of the LDR is very high in the
Megaohms range so zero base bias is applied to the
transistor TR1 and the relay is de-energised or
"OFF".
As the light level increases the resistance of the LDR
starts to decrease causing the base bias voltage at
V1 to rise. At some point determined by the
potential divider network formed with resistor R1,
the base bias voltage is high enough to turn the
transistor TR1 "ON" and thus activate the relay
which inturn is used to control some external
One simple use of a Light
circuitry. As the light level falls back to darkness
Dependent Resistor, is as a light again the resistance of the LDR increases causing
sensitive switch.
the base voltage of the transistor to decrease,
turning the transistor and relay "OFF" at a fixed light
level determined again by the potential divider
network.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
LDR
By replacing the fixed resistor R1 with a potentiometer VR1, the point at which
the relay turns "ON" or "OFF" can be pre-set to a particular light level. This type
of simple circuit shown above has a fairly low sensitivity and its switching point
may not be consistent due to variations in either temperature or the supply
voltage. A more sensitive precision light activated circuit can be easily made by
incorporating the LDR into a "Wheatstone Bridge" arrangement and replacing the
transistor with an Operational Amplifier.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
LDR
In this basic dark sensing circuit, the light dependent resistor LDR1 and the potentiometer
VR1 form one adjustable arm of a simple resistance bridge network, also known commomly
as a Wheatstone bridge, while the two fixed resistors R1 and R2 form the other arm. Both
sides of the bridge form potential divider networks across the supply voltage whose outputs
V1 and V2 are connected to the non-inverting and inverting voltage inputs respectively of
the operational amplifier.
The operational amplifier is configured as a Differential Amplifier also known as a voltage
comparator with feedback whose output voltage condition is determined by the difference
between the two input signals or voltages, V1 and V2. The resistor combination R1 and R2
form a fixed voltage reference at input V2, set by the ratio of the two resistors. The LDR VR1 combination provides a variable voltage input V1 proportional to the light level being
detected by the photoresistor.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
LDR
As with the previous circuit the output from the operational amplifier is used to control a
relay, which is protected by a free wheel diode, D1. When the light level sensed by the LDR
and its output voltage falls below the reference voltage set at V2 the output from the opamp changes state activating the relay and switching the connected load. Likewise as the
light level increases the output will switch back turning "OFF" the relay. The hysteresis of
the two switching points is set by the feedback resistor Rf can be chosen to give any
suitable voltage gain of the amplifier.
The operation of this type of light sensor circuit can also be reversed to switch the relay
"ON" when the light level exceeds the reference voltage level and vice versa by reversing
the positions of the light sensor LDR and the potentiometer VR1. The potentiometer can be
used to "pre-set" the switching point of the differential amplifier to any particular light level
making it ideal as a simple light sensor project circuit.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
opto-switch
• Opto-switches
– consist of a light source and a light sensor within a single
unit. Two common forms are the reflective and slotted types
A reflective opto-switch
A slotted opto-switch
Opto-switch sensor
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
In example above, the speed of the rotating shaft is measured by using a digital LED/Optodetector sensor. The disc which is fixed to a rotating shaft (for example, from a motor or
wheels), has a number of transparent slots within its design. As the disc rotates with the
speed of the shaft, each slot passes by the sensor inturn producing an output pulse
representing a logic level "1". These pulses are sent to a register of counter and finally to an
output display to show the speed or revolutions of the shaft. By increasing the number of
slots or "windows" within the disc more output pulses can be produced giving a greater
resolution and accuracy as fractions of a revolution can be detected. Then this type of sensor
arrangement could be used for positional control.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Photodiode Circuits
*references 10
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Photodiode Circuits
Photodiodes are very versatile light sensors that can turn its current flow both
"ON" and "OFF" in nanoseconds and are commonly used in cameras, light meters,
CD and DVD-ROM drives, TV remote controls, scanners, fax machines and copiers
etc, and when integrated into operational amplifier circuits as infrared spectrum
detectors for fibre optic communications, burglar alarm motion detection circuits
and numerous imaging, laser scanning and positioning systems etc.
*references 6
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Photodiodes
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Light Sensor
– photoconductor
• light  R
Photodiode
• light  I
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Phototransistors
Semiconductor light sensors include: photodiodes, phototransistors,
photodarlingtons. Like diodes, all transistors are light-sensitive. Phototransistors are
designed specifically to take advantage of this fact. The most-common variant is an
NPN bipolar transistor with an exposed base region. Here, light striking the base
replaces what would ordinarily be voltage applied to the base so, a phototransistor
amplifies variations in the light striking it. Note that phototransistors may or may not
have a base lead (if they do, the base lead allows you to bias the phototransistor's
light response.
All of these have similar noise performance, but phototransistors and darlingtons
have better sensitivity (more current for given light input).
Phototransistor:
1 mA @ 1000 lux
Photodarlingtons
up to 100x this
sensitivity.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Basically, a phototransistor
can be any bipolar transistor
with a transparent case. There
are some variations provide
advantages. For example, a
focusing lens can be built into
the case for directional
sensitivity. Coatings can be
applied to block some higher
or lower wavelengths. The
transistor itself may provide
higher gain, or higher
frequency, or lower
capacitance, etc.
Phototransistors
The diagram above illustrates the
frequency response of silicon
phototransistor junctions, along with the
spectral output of an infrared LED.
Yrd. Doç. Dr. Aytaç Gören20
Photoelectric Sensors
Phototransistors
Phototransistor
is
basically
a
photodiode with amplification. The
Phototransistor light sensor has its
collector-base PN-junction reverse
biased exposing it to the radiant light
source. Phototransistors operate the
same as the photodiode except that
they can provide current gain and are
much more sensitive than the
photodiode with currents are 50 to
100 times greater than that of the
standard photodiode and any normal
transistor can be easily converted into
a phototransistor light sensor by
connecting a photodiode between
the collector and base.
a photo-darlington
Yrd. Doç. Dr. Aytaç Gören20
Photoelectric Sensors
Phototransistors
Phototransistors consist mainly of a bipolar NPN Transistor with its large base
region electrically unconnected, although some phototransistors allow a base
connection to control the sensitivity, and which uses photons of light to
generate a base current which inturn causes a collector to emitter current to
flow. Most phototransistors are NPN types whose outer casing is either
transparent or has a clear lens to focus the light onto the base junction for
increased sensitivity.
Yrd. Doç. Dr. Aytaç Gören20
Photoelectric Sensors
Photovoltaic Cells
Photovoltaic cells are made from single crystal silicon PN junctions, the same
as photodiodes with a very large light sensitive region but are used without the
reverse bias. They have the same characteristics as a very large photodiode
when in the dark. When illuminated the light energy causes electrons to flow
through the PN junction and an individual solar cell can generate an open
circuit voltage of about 0.58v (580mV). Solar cells have a "Positive" and a
"Negative" side just like a battery.
Yrd. Doç. Dr. Aytaç Gören
Photoelectric Sensors
Cameras
Two solid-state camera types: CCD and CMOS.
A charge-coupled device (CCD) is a device for the movement of electrical
charge, usually from within the device to an area where the charge can be
manipulated, for example conversion into a digital value.
A CCD image sensor is an analog device. When light strikes the chip it is held
as a small electrical charge in each photo sensor. The charges are converted
to voltage one pixel at a time as they are read from the chip. Additional
circuitry in the camera converts the voltage into digital information.
CCD is the more mature technology, and has the widest performance range.
8 Mpixel size for cameras
Low noise/ high efficiency for astronomy etc.
Good sensitivity (low as 0.0003 lux, starlight)
CCDs require several chips,but are still cheap ($50 +)
Most CCDs work in near infrared
and can be used for night vision
if an IR light source is used.
(*ref.12, 13)
Yrd. Doç. Dr. Aytaç Gören20
Cameras
Photoelectric Sensors
CMOS (Complementary Metal-Oxide Semiconductor) is a technology used in
fabricating integrated circuit chips.
A CMOS imaging chip is a type of active pixel sensor made using the CMOS
semiconductor process. Extra circuitry next to each photo sensor converts
the light energy to a voltage. Additional circuitry on the chip may be included
to convert the voltage to digital data.
CMOS cameras are very compact and inexpensive, but haven’t matched
CCDs in most performance dimensions.
Start from $20(!)
Custom CMOS cameras
integrate image processing
right on the camera.
Allow special functions like
motion detection, recognition.
Yrd. Doç. Dr. Aytaç Gören20
Temperature Sensors
The most commonly used type of all the sensors are those which detect
Temperature or heat. These types of temperature sensor vary from simple
ON/OFF thermostatic devices which control a domestic hot water system to
highly sensitive semiconductor types that can control complex process control
plants. We remember from our school science classes that the movement of
molecules and atoms produces heat (kinetic energy) and the more movement,
the more heat is generated. Temperature Sensors measure the amount of heat
energy or even coldness that is generated by an object or system, and can
"sense" or detect any physical change to that temperature producing either an
analogue or digital output.
Yrd. Doç. Dr. Aytaç Gören
Temperature Sensors
Temperature sensors consist of two basic physical types:
Contact Temperature Sensor Types - These types of temperature sensor are
required to be in physical contact with the object being sensed and use
conduction to monitor changes in temperature. They can be used to detect
solids, liquids or gases over a wide range of temperatures.
Non-contact Temperature Sensor Types - These types of temperature sensor
use convection and radiation to monitor changes in temperature. They can be
used to detect liquids and gases that emit radiant energy as heat rises and
cold settles to the bottom in convection currents or detect the radiant energy
being transmitted from an object in the form of infra-red radiation (the sun).
The two basic types of contact or even non-contact temperature sensors can
also be sub-divided into the following three groups of sensors, Electromechanical, Resistive and Electronic.
Yrd. Doç. Dr. Aytaç Gören
Sensors
• Sensor operation
– small prism-shaped sample of single-crystal undoped GaAs attached to
ends of two optical fibers
– light energy absorbed by the GaAs crystal depends on temperature
– percentage of received vs. transmitted energy is a function of temperature
• Can be made small enough for biological implantation
Yrd. Doç. Dr. Aytaç Gören
Thermal Sensors
The Thermostat
The Thermostat is a contact type electro-mechanical temperature sensor or switch, that
basically consists of two different metals such as nickel, copper, tungsten or aluminium etc,
that are bonded together to form a Bi-metallic strip. The different linear expansion rates of
the two dissimilar metals produces a mechanical bending movement when the strip is
subjected to heat. The bi-metallic strip is used as a switch in the thermostat and are used
extensively to control hot water heating elements in boilers, furnaces, hot water storage
tanks as well as in vehicle radiator cooling systems.
The Bi-metallic Thermostat
The thermostat consists of two
thermally different metals stuck
together back to back. When it is cold
the contacts are closed and current
passes through the thermostat. When
it gets hot, one metal expands more
than the other and the bonded bimetallic strip bends up (or down)
opening the contacts preventing the
current from flowing.
Yrd. Doç. Dr. Aytaç Gören
Thermal Sensors
The Thermostat
There are two main types of bi-metallic strips based mainly upon their movement
when subjected to temperature changes, "snap-action" types that produce an
instantaneous "ON/OFF" or "OFF/ON" type action on the electrical contacts and
the slower "creep-action" types that gradually change their position as the
temperature changes. Snap-action thermostats are commonly used in homes for
controlling the temperature of ovens, irons, immersion hot water tanks and on
walls to control the domestic heating system.
Creeper types generally consist of a bi-metallic coil or spiral that slowly unwinds or
coils-up as the temperature changes. Generally, creeper type bi-metallic strips are
more sensitive to temperature changes than the standard snap ON/OFF types as
the strip is longer and thinner making them ideal for use in temperature gauges
and dials etc.
One main disadvantage of the standard snap-action type thermostats when used
as a temperature sensor, is that they have a large hysteresis range from when the
electrical contacts open until when they close for example, set to 20oC but may not
open until 22oC or close again until 18oC. So the range of temperature swing can be
quite high. Commercially available bi-metallic thermostats for home use do have
temperature adjustment screws that allow for a desired set-point and even its
hysteresis level to be pre-set and are available over a wide operating range.
Yrd. Doç. Dr. Aytaç Gören
Thermal Sensors
The Thermistor
The Thermistor is another type of temperature sensor, whose name is a
combination of the words THERM-ally sensitive res-ISTOR. A thermistor
is a type of resistor which changes its physical resistance with changes in
temperature.
Thermistors are generally made from ceramic type semiconductor materials
such as oxides of nickel, manganese or cobalt coated in glass which makes them
easily damaged. Most types of thermistor's have a Negative Temperature
Coefficient of resistance or (NTC), that is their resistance value goes DOWN with
an increase in the temperature but some with a Positive Temperature
Coefficient, (PTC), their resistance value goes UP with an increase in
temperature are also available. Their main advantage is their speed of response
to any changes in temperature, accuracy and repeatability.
Thermistors are passive resistive devices which means we need to pass a
current through it to produce a measurable voltage output. Then thermistors
are generally connected in series with a suitable biasing resistor to form a
potential divider network and the choice of resistor gives a voltage output at
some pre-determined temperature point or value.
Yrd. Doç. Dr. Aytaç Gören
The Thermistor
Thermal Sensors
The following thermistor has a resistance value of 10KΩ at 25oC and a
resistance value of 100Ω at 100oC. Calculate the voltage drop across the
thermistor and hence its output voltage (Vout) for both temperatures
when connected in series with a 1kΩ resistor across a 12v power supply.
At 25oC
At 100oC
by changing the fixed resistor value of R2 (in our example 1kΩ) to a potentiometer or
preset, a voltage output can be obtained at a predetermined temperature set point for
example, 5v output at 60oC and by varying the potentiometer a particular output voltage
level can be obtained over a wider temperature range.
Yrd. Doç. Dr. Aytaç Gören
Thermal Sensors
Resistive Temperature Detectors (RTD)
Another type of electrical resistance temperature sensor is the Resistance
Temperature Detector or RTD. RTD's are precision temperature sensors made from
high-purity conducting metals such as platinum, copper or nickel wound into a coil
and whose electrical resistance changes as a function of temperature, similar to that
of the thermistor. Also available are thin-film RTD's. These devices have a thin film of
platinum paste is deposited onto a white ceramic substrate.
RTD
Resistive temperature detectors have positive temperature coefficients (PTC) but
unlike the thermistor their output is extremely linear producing very accurate
measurements of temperature. However, they have poor sensitivity, that is a change
in temperature only produces a very small output change for example, 1Ω/oC. The
more common types of RTD's are made from platinum and are called Platinum
Resistance Thermometer or PRT's with the most commonly available of them all the
Pt100 sensor, which has a standard resistance value of 100Ω at 0oC. However,
Platinum is expensive and one of the main disadvantages of this type of device is its
cost.
Yrd. Doç. Dr. Aytaç Gören
Resistive Temperature Detectors (RTD)
Thermal Sensors
Like the thermistor, RTD's are passive resistive devices and by passing a constant
current through the temperature sensor it is possible to obtain an output voltage
that increases linearly with temperature. A typical RTD has a base resistance of
about 100Ω at 0oC, increasing to about 140Ω at 100oC with an operating
temperature range of between -200 to +600oC.
Thin Film Elements
Wire-wound Elements
These elements work with
temperatures to 660 °C.
These
Elements
works with
temperatures
to 300 °C.
Coiled elements
These Elements works with temperatures to 850 °C.
Yrd. Doç. Dr. Aytaç Gören
Thermal Sensors
Thermocouples
The Thermocouple is by far the most commonly used type of all the temperature
sensing devices due to its simplicity, ease of use and their speed of response to
changes in temperature, due mainly to their small size. Thermocouples also have
the widest temperature range of all the temperature sensors from below -200oC
to well over 2000oC.
Thermocouples are thermoelectric sensors
that basically consists of two junctions of
dissimilar metals, such as copper and
constantan that are welded or crimped
together. One junction is kept at a constant
temperature called the reference (Cold)
junction, while the other the measuring
(Hot) junction. When the two junctions are
at different temperatures, a voltage is
developed across the junction which is used
to measure the temperature sensor.
http://www.efunda.com/designstandards/sensors/thermocouples
/thmcple_theory.cfm
Yrd. Doç. Dr. Aytaç Gören
Thermal Sensors
Thermocouples
The principle of operation is that the junction of the two dissimilar metals such as copper
and constantan, produces a "thermo-electric" effect that produces a constant potential
difference of only a few millivolts (mV) between them. The voltage difference between the
two junctions is called the "Seebeck effect" as a temperature gradient is generated along
the conducting wires producing an emf. Then the output voltage from a thermocouple is a
function of the temperature changes. If both the junctions are at the same temperature the
potential difference across the two junctions is zero in other words, no voltage output as
V1 = V2. However, when the junctions are connected within a circuit and are both at
different temperatures a voltage output will be detected relative to the difference in
temperature between the two junctions, V1 - V2. This difference in voltage will increase with
temperature until the junctions peak voltage level is reached and this is determined by the
characteristics of the two dissimilar metals used.
Yrd. Doç. Dr. Aytaç Gören
Thermal Sensors
Thermocouples
Thermocouples can be made from a variety of different materials enabling extreme
temperatures of between -200oC to over +2000oC to be measured. With such a large
choice of materials and temperature range, internationally recognized standards have
been developed complete with thermocouple color codes to allow the user to choose
the correct thermocouple sensor for a particular application.
Yrd. Doç. Dr. Aytaç Gören
Thermal Sensors
Thermocouples
The three most common thermocouple materials used above for general
temperature measurement are Iron-Constantan (Type J), Copper-Constantan
(Type T), and Nickel-Chromium (Type K). The output voltage from a
thermocouple is very small, only a few millivolts (mV) for a 10oC change in
temperature difference and because of this small voltage output some form of
amplification is generally required.
The type of amplifier, either discrete or in the form of an Operational
Amplifier needs to be carefully selected, because good drift stability is
required to prevent recalibration of the thermocouple at frequent intervals.
This makes the chopper and instrumentation type of amplifier preferable for
most temperature sensing applications.
Yrd. Doç. Dr. Aytaç Gören
Thermal Cameras
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Flow, Fluid Velocity Sensors
A flowmeter is an instrument used to
measure linear, nonlinear, mass or
volumetric flow rate of a liquid or a gas.
http://www.omega.com/prodinfo/flowmeters.html
Yrd. Doç. Dr. Aytaç Gören
Flow, Fluid Velocity Sensors
Types of Flowmeters
•
Turbine
•
Differential Pressure
•
Coriolis Mass
•
Ultrasonic
•
Electromagnetic
•
Thermal
http://www.omega.com/prodinfo/flowmeters.html
Yrd. Doç. Dr. Aytaç Gören
Flow, Fluid Velocity Sensors
Turbine flowmeters
Turbine flowmeters use the
mechanical energy of the fluid to
rotate a “pinwheel” (rotor) in the flow
stream. Blades on the rotor are
angled to transform energy from the
flow stream into rotational energy.
The rotor shaft spins on bearings.
When the fluid moves faster, the
rotor spins proportionally faster.
Ref: reference 14
Yrd. Doç. Dr. Aytaç Gören
Flow, Fluid Velocity Sensors
Differential Pressure
Differential pressure flowmeters use
Bernoulli's equation to measure the
flow of fluid in a pipe. Differential
pressure flowmeters introduce a
constriction in the pipe that creates a
pressure drop across the flowmeter.
Yrd. Doç. Dr. Aytaç Gören
Flow, Fluid Velocity Sensors
Coriolis mass flowmeters
Coriolis mass flowmeters measure
the force resulting from the
acceleration caused by mass
moving toward (or away from) a
center of rotation
Yrd. Doç. Dr. Aytaç Gören
Flow, Fluid Velocity Sensors
Ultrasonic flowmeters
Ultrasonic flowmeters use sound
waves to determine the velocity of a
fluid flowing in a pipe with uses
Doppler Effect.
When the fluid moves faster, the
frequency shift increases linearly. The
transmitter processes signals from the
transmitted wave and its reflections to
determine the flow rate.
Yrd. Doç. Dr. Aytaç Gören
Flow, Fluid Velocity Sensors
Magnetic flowmeters
Magnetic flowmeters use
Faraday’s Law of
Electromagnetic Induction
to determine the flow of
liquid in a pipe.
Yrd. Doç. Dr. Aytaç Gören
Flow, Fluid Velocity Sensors
Thermal flowmeters
Thermal flowmeters use the
thermal properties of the
fluid to measure the flow of
a fluid flowing in a pipe or
duct.
Yrd. Doç. Dr. Aytaç Gören
Global Positioning System (GPS)
The Global Positioning System (GPS) is a spacebased satellite navigation system that provides
location and time information in all weather,
anywhere on or near the Earth, where there is an
unobstructed line of sight to four or more GPS
satellites.
A GPS receiver calculates its position by precisely timing the signals sent by GPS
satellites high above the Earth. Each satellite continually transmits messages
that include
-the time the message was transmitted
-satellite position at time of message transmission
The receiver uses the messages it receives to determine the transit time of each
message and computes the distance to each satellite. These distances along
with the satellites' locations are used with the possible aid of trilateration,
depending on which algorithm is used, to compute the position of the receiver.
This position is then displayed, perhaps with a moving map display or latitude
and longitude; elevation information may be included.
Yrd. Doç. Dr. Aytaç Gören
Global Positioning System (GPS)
All satellites broadcast at the same two
frequencies, 1.57542 GHz (L1 signal) and
1.2276 GHz (L2 signal). The satellite
network uses a CDMA spread-spectrum
technique where the low-bitrate message
data is encoded with a high-rate pseudorandom (PRN) sequence that is different
for each satellite. The receiver must be
aware of the PRN codes for each satellite
to reconstruct the actual message data.
The C/A code, for civilian use, transmits
data at 1.023 million chips per second,
whereas the P code, for U.S. military use,
transmits at 10.23 million chips per
second.
Subframes
Description
1
Satellite clock,
GPS time relationship
2–3
Ephemeris
(precise satellite orbit)
4–5
Almanac component
(satellite network synopsis,
error correction)
GPS message format
Yrd. Doç. Dr. Aytaç Gören
Global Positioning System (GPS)
The receiver uses messages received from satellites to determine the satellite
positions and time sent. The x, y, and z components of satellite position and the
time sent are designated as [xi, yi, zi, ti] where the subscript i denotes the satellite
and has the value 1, 2, ..., n, where
Knowing when the message was received
, the receiver computes the message's transit time as
Note that the receiver
indeed knows the reception time indicated by its on-board clock, rather than .
Assuming the message traveled at the speed of light (c) the distance traveled is
(tr − ti)c. Knowing the distance from receiver to satellite and the satellite's
position implies that the receiver is on the surface of a sphere centered at the
satellite's position. Thus the receiver is at or near the intersection of the
surfaces of the spheres. In the ideal case of no errors, the receiver is at the
intersection of the surfaces of the spheres.
Let b denote the clock error or bias, the amount that the receiver's clock is off.
The receiver has four unknowns, the three components of GPS receiver position
and the clock bias [x, y, z, b]. The equations of the sphere surfaces are given by:
Yrd. Doç. Dr. Aytaç Gören
Global Positioning System (GPS)
or in terms of pseudoranges,
, as
These equations can be solved by algebraic or numerical methods.
Yrd. Doç. Dr. Aytaç Gören
Weather, Moisture, Humidity Sensors
A hygrometer is an instrument used for measuring the moisture content in the
environmental air, or humidity. Most measurement devices usually rely on
measurements of some other quantity such as temperature, pressure, mass or a
mechanical or electrical change in a substance as moisture is absorbed. From
calculations based on physical principles, or especially by calibration with a
reference standard, these measured quantities can lead to a measurement of
humidity. Modern electronic devices use temperature of condensation, or changes
in electrical capacitance or resistance to measure humidity changes.
http://www.epluse.com/en/products/humidity-instruments/humidity-sensor/hct01/
http://www.sensirion.com/en/01_humidity_sensors/00_humidity_sensors.htm
Yrd. Doç. Dr. Aytaç Gören
Weather, Moisture, Humidity Sensors
•Energy consumption:
•RH operating range:
•T operating range:
•RH response time:
•Output:
80uW (at 12bit, 3V, 1 measurement / s)
0 – 100% RH
-40 – +125°C (-40 – +257°F)
8 sec (tau63%)
digital (2-wire interface)
•Maximal accuracy limits for relative humdity and temperature:
http://www.epluse.com/en/products/humidity-instruments/humidity-sensor/hct01/
http://www.sensirion.com/en/01_humidity_sensors/00_humidity_sensors.htm
Yrd. Doç. Dr. Aytaç Gören
Pyranometer
A pyranometer is a type of actinometer used to measure broadband solar
irradiance on a planar surface and is a sensor that is designed to measure the
solar radiation flux density (in watts per metre square) from a field of view of
180 degrees.
A typical pyranometer does not require any power to operate.
Yrd. Doç. Dr. Aytaç Gören
Pyranometer
The solar radiation spectrum extends approximately from 300 to 2,800 nm. Pyranometers
usually cover that spectrum with a spectral sensitivity that is as “flat” as possible.
To make a measurement of irradiance, it is required by definition that the response to
“beam” radiation varies with the cosine of the angle of incidence, so that there will be a
full response when the solar radiation hits the sensor perpendicularly (normal to the
surface, sun at zenith, 0 degrees angle of incidence), zero response when the sun is at the
horizon (90 degrees angle of incidence, 90 degrees zenith angle), and 0.5 at 60 degrees
angle of incidence. It follows that a pyranometer should have a so-called “directional
response” or “cosine response” that is close to the ideal cosine characteristic.
(1) sensor, (2, 3) glass domes, (5) cable,
standard length 5 m, (9) desiccant.
Yrd. Doç. Dr. Aytaç Gören
Pressure Sensor
A pressure sensor measures pressure, typically of gases or liquids. Pressure
is an expression of the force required to stop a fluid from expanding, and is
usually stated in terms of force per unit area. A pressure sensor usually acts
as a transducer; it generates a signal as a function of the pressure imposed.
For the purposes of this article, such a signal is electrical.
Diaphragm (Upper electrode)
Lower electrode
Types
Absolute pressure sensor
This sensor measures the pressure relative to perfect vacuum.
Gauge pressure sensor
This sensor measures the pressure relative to atmospheric pressure. A tire
pressure gauge is an example of gauge pressure measurement; when it
indicates zero, then the pressure it is measuring is the same as the ambient
pressure.
Yrd. Doç. Dr. Aytaç Gören
Pressure Sensor
Vacuum pressure sensor
This term can cause confusion. It may be used to describe a sensor that measures
pressures below atmospheric pressure, showing the difference between that low
pressure and atmospheric pressure (i.e. negative gauge pressure), but it may also be
used to describe a sensor that measures low pressure relative to perfect vacuum (i.e.
absolute pressure).
Differential pressure sensor
This sensor measures the difference between two pressures, one connected to each side
of the sensor. Differential pressure sensors are used to measure many properties, such
as pressure drops across oil filters or air filters, fluid levels (by comparing the pressure
above and below the liquid) or flow rates (by measuring the change in pressure across a
restriction). Technically speaking, most pressure sensors are really differential pressure
sensors; for example a gauge pressure sensor is merely a differential pressure sensor in
which one side is open to the ambient atmosphere.
Sealed pressure sensor
This sensor is similar to a gauge pressure sensor except that it measures pressure
relative to some fixed pressure rather than the ambient atmospheric pressure (which
varies according to the location and the weather).
Yrd. Doç. Dr. Aytaç Gören
Pressure Sensor
Pressure-sensing technology
There are two basic categories of analog pressure sensors.
Force collector types These types of electronic pressure sensors generally use a force
collector (such a diaphragm, piston, bourdon tube, or bellows) to measure strain (or
deflection) due to applied force (pressure) over an area.
•Piezoresistive strain gauge
Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due
to applied pressure. Common technology types are Silicon (Monocrystalline),
Polysilicon Thin Film, Bonded Metal Foil, Thick Film, and Sputtered Thin Film.
Generally, the strain gauges are connected to form a Wheatstone bridge circuit to
maximize the output of the sensor. This is the most commonly employed sensing
technology for general purpose pressure measurement. Generally, these
technologies are suited to measure absolute, gauge, vacuum, and differential
pressures.
•Capacitive
Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain
due to applied pressure. Common technologies use metal, ceramic, and silicon
diaphragms. Generally, these technologies are most applied to low pressures
(Absolute, Differential and Gauge)
Yrd. Doç. Dr. Aytaç Gören
Pressure Sensor
•Electromagnetic
Measures the displacement of a diaphragm by means of changes in inductance
(reluctance), LVDT, Hall Effect, or by eddy current principle.
•Piezoelectric
Uses the piezoelectric effect in certain materials such as quartz to measure the strain
upon the sensing mechanism due to pressure. This technology is commonly
employed for the measurement of highly dynamic pressures.
•Optical
Techniques include the use of the physical change of an optical fiber to detect strain
due to applied pressure. A common example of this type utilizes Fiber Bragg Gratings.
This technology is employed in challenging applications where the measurement may
be highly remote, under high temperature, or may benefit from technologies
inherently immune to electromagnetic interference. Another analogous technique
utilizes an elastic film constructed in layers that can change reflected wavelengths
according to the applied pressure (strain).[1].
•Potentiometric
Uses the motion of a wiper along a resistive mechanism to detect the strain caused
by applied pressure.
Yrd. Doç. Dr. Aytaç Gören
Pressure Sensor – Where they are used
Pressure sensing
This is where the measurement of interest is pressure, expressed as a force per unit area
Altitude sensing
This is useful in aircraft, rockets, satellites, weather balloons, and many other applications.
All these applications make use of the relationship between changes in pressure relative to
the altitude. p = 101325 (1 - 2.25577 10-5 h)5.25588 ; p[Pa], h[m]
Flow sensing
This is the use of pressure sensors in conjunction with the venturi effect to measure flow.
Differential pressure is measured between two segments of a venturi tube that have a
different aperture. The pressure difference between the two segments is directly
proportional to the flow rate through the venturi tube. A low pressure sensor is almost
always required as the pressure difference is relatively small.
P = pressure, ρ = density of the fluid, g = standard gravity,
h = height of fluid column above pressure sensor
Level / depth sensing
A pressure sensor may also be used to calculate the level of a fluid. This technique is
commonly employed to measure the depth of a submerged body (such as a diver or
submarine), or level of contents in a tank (such as in a water tower). For most practical
purposes, fluid level is directly proportional to pressure
Yrd. Doç. Dr. Aytaç Gören
Pressure Sensor – Where they are used
Leak testing
A pressure sensor may be used to sense the decay of pressure due to a system leak.
•Ratiometric Correction of Transducer Output
Piezoresistive transducers configured as Wheatstone bridges often exhibit
ratiometric behavior with respect not only to the measured pressure, but also the
transducer supply voltage.
where:
is the output voltage of the transducer.
is the actual measured pressure.
is the nominal transducer scale factor (given an ideal transducer supply
voltage) in units of voltage per pressure.
is the actual transducer supply voltage.
is the ideal transducer supply voltage.
Correcting measurements from transducers exhibiting this behavior requires
measuring the actual transducer supply voltage as well as the output voltage and
applying the inverse transform of this behavior to the output signal:
Yrd. Doç. Dr. Aytaç Gören
Electric Current, Electric Potential
Hall effect sensor
A Hall effect sensor is a transducer that varies its output voltage in response to a
magnetic field. Hall effect sensors are used for proximity switching, positioning,
speed detection, and current sensing applications.
In its simplest form, the sensor operates as an analogue transducer, directly
returning a voltage. With a known magnetic field, its distance from the Hall plate
can be determined. Using groups of sensors, the relative position of the magnet
can be deduced.
Electricity carried through a conductor will produce a magnetic field that varies
with current, and a Hall sensor can be used to measure the current without
interrupting the circuit. Typically, the sensor is integrated with a wound core or
permanent magnet that surrounds the conductor to be measured.
Frequently, a Hall sensor is combined with circuitry that allows the device to act
in a digital (on/off) mode, and may be called a switch in this configuration.
Commonly seen in industrial applications such as the pictured pneumatic
cylinder, they are also used in consumer equipment; for example some computer
printers use them to detect missing paper and open covers. When high reliability
is required, they are used in keyboards.
Yrd. Doç. Dr. Aytaç Gören
Electric Current, Electric Potential
Hall effect sensor
The Hall effect comes about due to the nature of the
current in a conductor.
For a simple metal where there is only one type of charge
carrier (electrons) the Hall voltage VH is given by
where I is the current across the plate length, B is the magnetic
field, d is the depth (thickness) of the plate, e is the electron
charge, and n is the charge carrier density of the carrier
electrons. The Hall coefficient is defined as
where j is the current density of the carrier electrons, and
the induced electric field. In SI units, this becomes
is
Yrd. Doç. Dr. Aytaç Gören
Magnetic Sensors
Magnetic Sensor
Active (emitting)
Metal detectors
Follows metallic strips on or under the floor
Magnetometer
Magnetic Resonance Imaging (MRI)
Passive (sensors only)
Compass
Magnetic field sensor (→oscillating current)
Wheatstone bridge configuration that converts magnetic fields into a
millivolt output. These wheatstone bridges are passive components that
don’t emit any fields or broadband noise.
Resolution: The magnetic sensors feature very low noise floors for their
size. Typical resolution ranges from 27 to 120 microgauss (for HMC).
*reference 15
Yrd. Doç. Dr. Aytaç Gören
MRI
Magnetic Sensors
Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging
(NMRI), or magnetic resonance tomography (MRT) is a medical imaging
technique used in radiology to visualize detailed internal structures. MRI
makes use of the property of nuclear magnetic resonance (NMR) to image
nuclei of atoms inside the body.
An MRI machine uses a powerful magnetic field to align the magnetization of
some atomic nuclei in the body, and radio frequency fields to systematically
alter the alignment of this magnetization. This causes the nuclei to produce a
rotating magnetic field detectable by the scanner—and this information is
recorded to construct an image of the scanned area of the body. Magnetic
field gradients cause nuclei at different locations to rotate at different
speeds. By using gradients in different directions 2D images or 3D volumes
can be obtained in any arbitrary orientation.
Yrd. Doç. Dr. Aytaç Gören
Magnetic and Radio Sensors
MRI
Yrd. Doç. Dr. Aytaç Gören
Thanks,
Assist. Prof. Dr. Aytaç Gören.
References for this week
1. Practical Applications of Statistical Methods in the Clinical Laboratory, Roger L.
Bertholf, Ph.D., DABCC,
2. http://www.ladyada.net/learn/sensors/pir.html
3. Mechatronics Principles and Applications, Godfrey C. Onwubolu
4. THE MECHATRONICS H A N D B O O K, E d i t o r - i n - C h i e f, Robert H. Bishop
5. http://www.transtekinc.com/products/LVDT.html
6. http://www.electronics-tutorials.ws
7. www.ab.com/catalogs
8. http://www.doc.ic.ac.uk/~nd/surprise_97/journal/vol4/jmd/
9. Sensors, Prof. A. Mason
10. Electrical and Electronic Systems – Pearson Educaton Ltd.
11. http://www.coilgun.info/theory/phototransistors.htm
12. Design Realization lecture 18, John Canny.
13. Wikipedia
14. http://www.flowmeters.com/ufm/index.cfm?task=magnetic
15. http://www.magneticsensors.com/magnetic-sensor-components.php
•
Yrd. Doç. Dr. Aytaç76Gören