Sensors(part2)
Instructor: Shuvra Das
Mechanical Engineering Dept.
University of Detroit Mercy
Flowchart of Mechatronic
Systems
Some Sensors: Strain gages
• Bonded to surface of object.
• Resistance varies as a function of the
surface strain experienced by the object
• Hence subjected to same strain as the object
when the object elongates or compresses
• Can be used to measure stress, force,
torque, pressure, etc.
Some Sensors: Strain gages
• R = (rL)/A = ( ) W, r = resistivity
• Stretching/compression changes Resistance
A
L
– G= (DR/R0)/(DL/L) = (DR/R0)/e
• G is called gage factor
Some Sensors: Strain gages
• G = gage factor ~ 2 for metal foil strain gages
• Maximum strain foil can measure =
0.005=0.5%
• DR= R0G e, R0~120W, DR=120 X 2 X 0.005 =
1.2W , for a strain of 0.005
• Strain gage is placed in a wheatstone bridge
circuit and the change in resistance is
measured
Some Sensors: Strain gages
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Wheatstone bridge is used
Voltage change is measured
Voltage change is related to DR
Example:
– cantilever beam
– 4 strain gages, 2 on top, 2 bottom
– R1&R4 are in tension
– R2&R3 are in compression
Strain gages
• Can be used in quarter
bridge mode, half bridge
mode and full bridge
mode.
• All resistances are chosen
as equal initially.
• The bridge is used to
measure the change in
resistance and that leads to
strain.
Strain gages
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Full bridge mode: R1 and R4 stretches
R2 and R3 compresses
R1=R4 = R0+ DR;
R2 = R3 = R0 - DR;
V0 = va-vb = iaR1-ibR3
V0=VsR1/(R1+R2) - VsR3/(R3+R4)
V0=Vs(R0+ DR)/(R0+ DR+ R0-DR) Vs(R0-DR)/(R0-DR+ R0+ DR)
Strain gages
• V0=Vs(R0+ DR)/(R0+ DR+ R0-DR) Vs(R0-DR)/(R0-DR+ R0+ DR)
• V0 = Vs (2DR)/2R0=Vs DR/R0
• V0 =Vs DR/R0=VsGe
• V0 =VsGe = VsGf(F)
• relation will change for 1/4 bridge or half bridge
Some sensors: Strain gages
• Strain gages are typically mounted in the
cantilever mode or on surfaces of tubes to
form load cells. Load cells are used to
measure forces/loads
Some sensors: Thermocouples
• A thermocouple is formed by the junction of two
dissimilar metal
Metal A
Metal B
• The junction is called a thermoelectric junction
and produces a voltage proportional to the
temperature of the junction
• the above effect is called SEEBECK effect (1821).
Some sensors: Thermocouples
• To form an electrical loop the thermocouple
junctions should be in pairs. The voltage
difference between the two junctions is related to
the temperature difference.
T1
T2
V
Thermocouples: examples
Some Sensors:Thermocouples
• As the previous table shows the temperature
is non-linearly related to the measured
voltage difference and can be expressed as
n
T =  aiV i
i =0
• in the previous table V is measured in
VOLTS
Thermocouples
• Correspondingly the voltage can be
represented in terms of the temperature as
the third table shows and in this case:
n
V =  ciT i
i =0
• In the table on the following page the
voltage is shown in millivolts
Thermocouples
• As the previous table indicates voltage has a
non-linear dependence on temperature.
• This dependence is obtained by polynomial
curve fitting of empirical data
• All the data in these tables are with
reference to 0 degrees
• if the reference is a non-zero value one
needs to convert the voltage reading (see a
little later)
Thermocouple: connections
• To measure voltage in a thermocouple
circuit additional leads may be introduced
which work as additional dissimilar-metal
junctions.
• Special connections are needed to handle
these situations.
Thermocouple: connections
• Need to know these
junction temperatures
• another option is to
use a cold-junction
compensation (this
works as a reference)
Thermocouples: Cold junction
compensation
• Measured voltage now
depends on T1-Tref
• Isothermal block
keeps external
junctions at the same
temperature
Thermocouples: laws
• Law of intermediate temperatures: the voltage due
to two junctions in a circuit is only dependent on
the temperatures of the junctions and is
independent of any intermediate temperature.
• E t/0 = E t/i+E i/0
• the tabulated emf values are all E t/0
t
i
i
0
Thermocouples: Laws
• Law of intermediate metals: A third metal in a circuit has
no effect on the resulting voltage if the new junctions
created are all at the same temperatures. This is why
copper connectors used for measurements has no effect on
measured temp.
T3
T3
T1
T2
V
Thermocouples:Example
• A standard two-junction thermocouple
configurations being used to measure temperatures
in a wind tunnel. The reference junction is held at
a constant temp of 10C. We have only standard
thermocouple tables that are referenced to 0C. We
want to determine the output voltage when the
measuring junction is exposed to an air temp. of
100C.
Thermocouple: Example
Junction temp(C )
0
10
20
30
40
50
60
70
80
90
100
Output Voltage (mV)
0
0.507
1.019
1.536
2.058
2.585
3.115
3.649
4.186
4.725
5.268
Thermocouple: Example
• E 100/0 = E 100/10+E 10/0
• 5.268 = E 100/10 +0.507
• E 100/10 = 5.268 - 0.507=4.761 mV
Some Sensors: Capacitive sensor
• Also used to measure displacements
• Parallel plate capacitor with a fixed end
• One plate fixed and the other moves in
harmony
• Displacements => change in
capacitance=>change in voltage in a circuit
• e.g. condenser microphone
Ultrasonics
• Sonar was developed in
WWII
• Two designs:
• * Passive sonar: Detects
the noise emitted from
submarines
• * Active sonar: Transmits
a signal and listens to the
reflected signal. Distance
can be determined from
the time it takes the pulse
to travel back and forth.
• Typical transmission
and receiving pattern
for a sonar onboard a
ship
To find distance
• To determine the range of an object that has reflected a
signal back to the sonar’s receiver, we can use:
• d=0.5vt, Where
– d = distance (m)
– v = propagation velocity of sound in the water (m/s)
– t = time (s)
Three types
• Magnetorestrictive (high power)
• piezoelectric (high frequency, <600 khz)
• electrostatic (low frequency, 20-50 khz)
Ultrasonic transducer
• Piezoelectric element
resonates at a specific
frequency.
• Pulses supplied through
the connector and the
impedance matcher.
• Damping chamber
absorbs residual energy to
prevent “ringing”.
• Acoustic lens focuses
acoustic energy.
The Electrostatic Microphone
a.k.a Capacitor Microphone
• Diaphragm and the fixed plate,
together, form a parallel plate
capacitor.
• As the diaphragm vibrates, due
to sound pressure, the distance
between the two capacitor
plates varies, producing a
variable capacitance output.
• The variable output impedance
can be used to measure the
acoustic pressure.
Implementation Issues
• With an oblique surface the reflection may not go back
towards the sensor.
• A diffuse surface will scatter the sound pulse instead of
reflecting it back to the sensor. On the Polaroid sensor, the
surface cannot be more than 25 degrees off perpendicular
with the sensor.
Implementation Issues
• With a wide beam sensor it has more of a chance of hitting
something normal but directionality is lost. With a narrow
beam, the object must reflect the pulse close to normal but
location measurement is very accurate.
Applications
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distance measurement
density measurement
crack/failure detection
flow rate measurement (doppler)
fluid level sensing
speed measurements (doppler)
Active Noise Control (ANC)Systems
• Active Noise Control
(concept): use a
microphone as a sensor
and a loudspeaker as an
actuator to minimize the
effect of a primary noise
source at a specific
location.
Outside noise signal processing resulting wave
ULTRASONIC DETECTION
• Detection of sonic energy that is beyond 20 KHz (up to 5
MHz).
• Used extensively in medicine for diagnostic purposes. In
some cases replacing x-rays.
• Used in industry and in consumer electronics for the
measurement of distance, velocity, and detection of small
objects.
• Very high frequencies are used (as opposed to sonar
detection), since shorter wavelengths bounce off smaller
surface areas far more efficiently.
Detection of flaws in workpiece
• Because of the known
geometry of the
workpiece the echo
pattern is known
ahead of time.
• Any flaws in the
workpiece will add
additional echos to this
pattern. Also referred
to as pulse echo
ultrasonic method.
Application: Measuring Velocity
• A continuous signal of a specific frequency is transmitted.
• Frequency of the echo signal is compared with that of the
original signal.
• the change in frequency indicates the velocity of the target
(Doppler Effect).
Application (Acoustic Imaging)
• The precise distance measuring
capabilities of ultrasonic
transducers is utilized to
generate an acoustical image of
an object.
• The target is scanned by a
highly collimated ultrasonic
beam. An accurate
representation of the target’s
relief features can then be
developed.
• The resultant image is produced
on a screen
Mems sensors: Airbag
• Almost all the collisions occur within 0.125 seconds after
car crash according to investigation.
• The airbag is designed to inflate in less than 0.04 second.
• In a collision, the airbag begins to fill within 0.03 second.
• By 0.06 second, the airbag is fully inflated and cushions
the occupant from impact.
• The airbag then deflates (de powers) 0.12 second after
absorbing the forward force.
• The entire event, takes about 55 milliseconds --- about half
the time to blink an eye
Mems sensors
Capacitive Sensing & Actuation
• A large number of MEMS
devices operate based on
capacitive sensing and
actuation techniques.
• Actuation: Applying a
voltage exerts force on
parallel plates.
• Sensing: Movement of
plates, changes the size of
the gap, and hence the
voltage sensed in the
capacitor.
Airbag sensor element
• Upper flat plate of capacitor
is asymmetrically shaped
and is balanced by the
torsion bar.
• An acceleration produces
moment about the torsion bar
that makes the plate rotate
about torsion bar, thus
altering the capacitance.
• Typical dimensions are 1000
X 600 X 5 microns
Displacement, proximity,
position, (linear and angular)
• Proximity/limit switch (mechanical)
– Binary output
• Potentiometer (rotary or linear)
Microswitch™ Limit Switch
– Analog output
• LVDT: Linear Variable differential Transformer
Differential Transformer
LVDT
Joystick (2 pots)
Non-Contact Sensors
• Ultrasonic
Ultrasonic
Fluid Level Meter
• Optical
• Magnetic (Inductive, Reed, Hall Effect)
• Laser vibrometer
• Capacitive or Eddy current
– measuring vibration of rotating shafts
Rotational Position/Velocity
• Optical Encoder –absolute or incremental
angle, direction
4 Bit Absolute Encoder
50 pulses/rev Incremental
500 pulses/rev
Incremental, Quadrature
Rotational Position/Velocity
•Tachometer – voltage  shaft velocity,
•typically a small PM DC motor used as a
generator
•Toothed wheel (gear) + magnetic pickup +
counter
Vibration
• Accelerometer
– Piezo-electric (AC)
– IC, Strain gage (DC)
Force/Torque
• Strain gage
Resistance  Strain
FORCE
• Piezo-electric (AC coupled)
• Piezo-resistive, piezo-ceramic
Pressure
• Microphone
(AC coupled)
• Diaphragm
(for static measurement)
• Tube, Bellows
• Manometer
Flow measurement
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Orifice plate, venturi
Turbine meter
Float
Rotameter
Hot-wire anemometer
Flow measurement
•Laser interferometer
•Pitot tube
•Positive displacement meter (rotary vane)
Temperature
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Thermometer
Thermocouple
Thermistor
RTD (platinum)
Solid state sensor (thermodiodes and
transistors)
• Pyro-electric sensor
• Bimetallic strip
• Optical pyrometer
Optical
• Photo-voltaic cell
• CdS sensor (R output)
• Phototransistor
Selection Criteria
• What is to be measured?
• Magnitude, range, dynamics of measured quantity
(fast, slow?)
• Required resolution, accuracy
• Cost
• Environment (hot, dirty?)
• Interface Requirements:
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Output quantity (voltage, current, resistance,…)
Sensitivity
Signal conditioning
A/D requirements (#bits, data rate)
Sensor Specifications
• Accuracy/Error: Conformity of the
measurement to the true value
• Precision: Ability to reproduce the reading
repeatedly
• Resolution: Smallest measurable increment
• Span: Linear operating range; min to max
value
• Range: Range of measurable values
Other related terms
• Error = actual value – measured value
• Accuracy = extent the measured value might be
wrong
ex. ± 2°C or ± 2% of full scale
• Sensitivity = (gain) linear output/unit input e.g. 5
mv/psi, 0.5W/ °C
• Hysteresis error – output value depends on
whether input is rising or falling
• Non-linearity error – error resulting when
assuming that the output is linearly related to
input
Other related terms
• Repeatability/reproducibility – same output for
repeated same input?
• Stability – drift of output over time for constant
input
• Dead band/time- range of input for no
measurable output
• Resolution (least count)– output steps, smallest
measurable change in input
• Output impedance – how sensor output is
effected by the electrical characteristics of what it
is connected to
Static and Dynamic
Characteristics
• Response time – time to
95% of final value for step
input
• Time constant – time to
63.2% (1-e-1) of final value
• Rise time – time to rise
some specified percentage
of s.s. output
• Settling time – time to get to
Sensor Specification and
meaning
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Strain gage pressure sensor specifications:
Range: 70 to 1000 kPa
supply voltage: 10V dc or ac rms
full range output: 40mV
non-linearity and hysterisis: +/- 0.5% full range
output
• Temperature range: -54C to 120C when operating
• Thermal zero shift: 0.030% full range output/
degree C
Sensor Specification and
meaning
• Range: Can measure 70 to 1000 kPa
• supply voltage: requires supply voltage of 10V dc
or ac rms
• full range output: 40mV will be output for
1000kPa pressure
• non-linearity and hysterisis will lead to an error of
+/- 0.5% (I.e. +/- 5kPa).
• Can be used between -54C to 120C
• when temp changes by 1C the output for zero
input will change by 0.030% of 1000 = 0.3kPa.