Transcript www.geca.ac.in

Sensors and its Characteristics
7/17/2015
Dr. sanjay Chikalthankar
1

Transducer is a device which transforms energy from
one type to another, even if both energy types are in
the same domain.
 Typical energy domains are mechanical, electrical,
chemical, magnetic, optical and thermal.

Transducer can be further divided into Sensors,
which monitors a system and Actuators, which
impose an action on the system.
 Sensors are devices which monitor a parameter of a system,
hopefully without disturbing that parameter.
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A device which converts one form of energy to another
When input is a physical quantity and output electrical → Sensor
When input is electrical and output a physical quantity → Actuator
e.g. Piezoelectric:
Sensors
Force -> voltage
Actuators
Physical
parameter
Electrical
Electrical
Physical
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Output
Input
Output
Dr. sanjay Chikalthankar
Voltage-> Force
=> Ultrasound!
Microphone, Loud Speaker
3

Transducer
 a device that converts a primary form of energy into a corresponding
signal with a different energy form
▪ Primary Energy Forms: mechanical, thermal, electromagnetic, optical,
chemical, etc.
 take form of a sensor or an actuator

Sensor (e.g., thermometer)
 a device that detects/measures a signal or stimulus
 acquires information from the “real world”

Actuator (e.g., heater)
 a device that generates a signal or stimulus
real
world
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sensor
actuator
intelligent
feedback
system4

American National Standards Institute
 A device which provides a usable output in response to a specified measurand
Input Signal
Output Signal
Sensor



A sensor acquires a physical quantity and converts it into a signal
suitable for processing (e.g. optical, electrical, mechanical)
Nowadays common sensors convert measurement of physical
phenomena into an electrical signal
Active element of a sensor is called a transducer
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
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
▪ T= temperature; V=sensor voltage;
▪ a,b,c = calibration coefficients

Compensation
7.000
6.000
5.000
1001
1010
4.000
T2
3.000
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1001
1101
1110
1111
2.000
 remove secondary sensitivities
1.000
 must have sensitivities characterized
0.000
-30
-20
-10
0
 can remove with polynomial evaluation
▪ P = a + bV + cT + dVT + e V2, where P=pressure, T=temperature
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T1
offset
 analog signal conditioning
 look-up table
 digital calibration
▪ T = a + bV +cV2,
Frequency (MHz)

T3
10
20
30
40
50
60
70
Temperature (C)
6
Typically interested in electronic sensor
 convert desired parameter into electrically measurable signal

General Electronic Sensor
 primary transducer: changes “real world” parameter into electrical signal
 secondary transducer: converts electrical signal into analog or digital values
Primary
Transducer
Analog
Signal
Secondary
Transducer
sensor
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Input
Signal
Sensor Data
Microcontroller
Network
analog/digital
signal processing
communication
display
Sensor
(measured)
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
Components vary with application
 Digital sensor within an instrument
▪ microcontroller
sensor
µC
▪ signal timing
▪ data storage
sensor
signal timing
memory
keypad
display
handheld instrument
 Analog sensor analyzed by a PC
sensor interface
sensor
e.g., RS232
A/D, communication
signal processing
PC
comm. card
 Multiple sensors displayed over internet
internet
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sensor
sensor bus
processor
comm.
Dr. sanjay Chikalthankar
PC
comm. card
sensor bus
sensor
processor
comm.
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
Classification based on physical phenomena
 Mechanical: strain gage, displacement (LVDT), velocity (laser
vibrometer), accelerometer, tilt meter, viscometer, pressure, etc.
 Thermal: thermal couple
 Optical: camera, infrared sensor
 Others …

Classification based on measuring mechanism
 Resistance sensing, capacitance sensing, inductance sensing,
piezoelectricity, etc.

Materials capable of converting of one form of energy to
another are at the heart of many sensors.
 Invention of new materials, e.g., “smart” materials, would permit the
design of new types of sensors.
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
Sensors are pervasive. They are embedded in our
bodies, automobiles, airplanes, cellular telephones,
radios, chemical plants, industrial plants and
countless other applications.

Without the use of sensors, there would be no
automation !!
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Physical
phenomenon
Measurement
Output
Measurement output:
• interaction between a sensor and the environment surrounding the sensor
•compound response of multiple inputs
Measurement errors:
•System errors: imperfect design of the measurement setup and the
approximation, can be corrected by calibration
• Random errors: variations due to uncontrolled variables. Can be reduced
by averaging.
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Definition:
A device for sensing a physical variable of a
physical system or an environment
Classification of Sensors
Mechanical quantities: displacement, Strain, rotation velocity,
acceleration, pressure, force/torque, twisting, weight, flow

 Thermal
quantities: temperature, heat.
Electromagnetic/optical quantities: voltage, current, frequency
phase; visual/images, light; magnetism.


Chemical quantities: moisture, pH value
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
Accuracy: Error between the result of a
measurement and the true value being measured.

Resolution: The smallest increment of measure that
a device can make.

Sensitivity: The ratio between the change in the
output signal to a small change in input physical
signal. Slope of the input-output fit line.

Repeatability/Precision: The ability of the sensor to
output the same value for the same input over a
number of trials
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True value
measurement
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Precision without
accuracy
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Accuracy without
precision
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Precision and
accuracy
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
Dynamic Range: the ratio of maximum recordable input amplitude to
minimum input amplitude, i.e. D.R. = 20 log (Max. Input Ampl./Min. Input
Ampl.) dB

Linearity: the deviation of the output from a best-fit straight line for a given
range of the sensor

Transfer Function (Frequency Response): The relationship between physical
input signal and electrical output signal, which may constitute a complete
description of the sensor characteristics.

Bandwidth: the frequency range between the lower and upper cutoff
frequencies, within which the sensor transfer function is constant gain or
linear.

Noise: random fluctuation in the value of input that causes random fluctuation
in the output value
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
Operating Principle: Embedded technologies that make sensors function, such
as electro-optics, electromagnetic, piezoelectricity, active and passive
ultraviolet.

Dimension of Variables: The number of dimensions of physical variables.

Size: The physical volume of sensors.

Data Format: The measuring feature of data in time; continuous or
discrete/analog or digital.

Intelligence: Capabilities of on-board data processing and decision-making.

Active versus Passive Sensors: Capability of generating vs. just receiving
signals.

Physical Contact: The way sensors observe the disturbance in environment.

Environmental durability: will the sensor robust enough for its operation
conditions
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
Resistance Temperature Detectors (RTDs)
 Platinum, Nickel, Copper metals are typically used
 positive temperature coefficients

Thermistors (“thermally sensitive resistor”)
 formed from semiconductor materials, not metals
▪ often composite of a ceramic and a metallic oxide (Mn, Co, Cu or Fe)
 typically have negative temperature coefficients

Thermocouples
 based on the Seebeck effect: dissimilar metals at diff. temps.  signal
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
A bimetallic coil from a thermometer reacts to the heat from a lighter, by uncoiling
and then coiling back up when the lighter is removed.

A bimetallic strip is used to convert a temperature change into mechanical
displacement.

The strip consists of two strips of different metals which expand at different rates as
they are heated, usually steel and copper, or in some cases brass instead of copper.

The
strips
are
joined
by riveting, brazing or welding.
together
throughout
their
length
The different expansions force the flat strip to bend one way if
heated, and in the opposite direction if cooled below its initial
temperature.

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Diagram of a bimetallic strip showing how the difference in thermal expansion in
the two metals leads to a much larger sideways displacement of the strip
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
Working Principle :If the two different metal strips are joined or welded together and heated,
the resultant strips having lower expansion rate gets bend. The deflecton of strip
is directly proportional to the squsre of the lenghth snd temperature and inversely
proportional to the thickness of the metal.
Advantages :
Disadvantages :
• Easy to install
• Calibration changes because of tough
handling
•Maintenance is not complex
•Mechanically rigid and tough
•Low cost
•Wide
operating temperature
range.
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Chikalthankar
• Accuracy is not as good because of glass
stem design.

Bimetallic Strip
L  L0[1   (T - T0)]

Metal A
δ
Application
 Thermostat (makes or
breaks electrical
connection with
deflection)
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Metal B

Curvature of a Bimetallic Beam:
Where
and
and
and
are the Young's Modulus and height of Material One
are the Young's Modulus and height of Material Two.
is the misfit
strain, calculated by:
Where
α1 is the Coefficient of Thermal Expansion of Material One and α2 is the
Coefficient of Thermal Expansion of Material Two.
ΔT is the current temperature minus the reference temperature (the
temperature where the beam has no flexure).
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
These elements nearly always require insulated leads attached.

At temperatures below about 250 °C PVC, silicon rubber or PTFE insulators are used.

Above this, glass fibre or ceramic are used.

The measuring point, and usually most of the leads, require a housing or protective sleeve,
often made of a metal alloy which is chemically inert to the process being monitored.

Selecting and designing protection sheaths can require more care than the actual sensor, as
the sheath must withstand chemical or physical attack and provide convenient attachment
points.
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
Common RTD sensing elements constructed of platinum, copper or nickel have a unique,
and repeatable and predictable resistance vs temperature relationship (R vs T) and operating
temperature range. The R vs T relationship is defined as the amount of resistance change of
the sensor per degree of temperature change.

Platinum is a noble metal and has the most stable resistance to temperature relationship over
the largest temperature range. Nickel elements have a limited temperature range because the
amount of change in resistance per degree of change in temperature becomes very non-linear
at temperatures over 572 °F (300 °C). Copper has a very linear resistance to temperature
relationship, however copper oxidizes at moderate temperatures and cannot be used over
302°F (150°C).

Platinum is the best metal for RTDs because it follows a very linear resistance to temperature
relationship and it follows the R vs T relationship in a highly repeatable manner over a wide
temperature range. The unique properties of platinum make it the material of choice for
temperature standards over the range of -272.5 °C to 961.78 °C, and is used in the sensors
that define the International Temperature Standard, ITS-90. It is made using platinum
because of its linear resistance-temperature relationship and its chemical inertness.
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
Resistance temperature device
(RTD)
R  R 0[1   (T - T0)]
R  R0 e
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1 1 

 T T0 
 
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
Resistance temperature detectors (RTDs), also called
resistance thermometers, are temperature sensors that
exploit the predictable change in electrical resistance of
some materials with changing temperature.

Temperature

The resistance ideally varies linearly with temperature.
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Metal Resistance
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Straight line equation
R(T )  R(To )[1  o T ] T1  T  T2
R(T)
R(T0)
αo
ΔT
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= Approximation of resistance at
temperature T
= Resistance at temperature T0
= Fractional change in resistance per
degree of temperature at T0
= T - T0
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
Sensitivity is shown by the value αo
 Platinum – 0.004/ °C
 Nickel – 0.005/ °C

Thus, for a 100Ω platinum RTD, a change of
only 0.4 Ω would be expected if the
temperature is changed by 1°C
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
Resistive thermometers
 typical devices use platinum wire (such a device is
called a platinum resistance thermometers or PRT)
 linear but has poor sensitivity
A typical PRT element
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A sheathed PRT
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
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
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
• Thermocouple:
Thermistor
Therm istor
Thermal
Resistor
 Eg 
R  exp 

2
kT
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

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Seeback effect to transform a
temperature difference to a voltage
difference
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
Working Principle :When the two dissimilar metals A and B are
welded or joined together to form a closed and the
junctions are kept at two different temperatures T1
and T2, then the generated resulting flow of current
in the circcuit or loop.
One of the two junctions in thermocouple is
reference or cold junction which is generally kept at
0 degree centigrade and the other is the measuring
junction at which the temperature is to be measured.
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




Type K
Type J
Type E
Type N
Type T

It is important to note that thermocouples measure the
temperature difference between two points, not absolute
temperature.
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: Chromel - Alumel
: Iron - Constantan
: Chromel - Constantan
: Nicros - Nisil
: Copper - Constantan
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
Thermocouples are most suitable for measuring over a large temperature
range, up to 1800 K.
Example:
Type K : Chromel - Alumel
(-190⁰C to 1260⁰C)
Type J : Iron - Constantan
(-190⁰C to 760⁰C)
Type E : Chromel - Constantan (-100⁰C to 1260⁰C)
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ADVANTAGES

Construction is mechanically
strong and rigid.

It is suitable for reading of
rapidly varying temperatures.

Low cost

No need of bridge circuit

Installation and calibration is
easy

Suitable for temperature range of
-270 to 2800 degree centigrade.
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DISADVANTAGES

Requires a protective wall or
sheath

Needs compensating
arrangements

Amplifier circuit is necessary to
increase the output voltage level

For long distance temperature
measurement , compensating
wires are necessary.
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 Steel industry:- Type B, S, R and K thermocouples are used extensively in
the steel and iron industries to monitor temperatures and chemistry throughout the
steel making process.
 Heating appliance safety:- Many gas-fed heating appliances such
as ovens and water heaters make use of a pilot flame to ignite the main gas burner
when required.
 Thermopile radiation sensors:- Thermopiles are used for measuring the intensity
of incident radiation, typically visible or infrared light, which heats the hot junctions,
while the cold junctions are on a heat sink.
 Manufacturing:- Thermocouples can generally be used in the testing of prototype
electrical and mechanical apparatus.
 Process plants :-Chemical production and petroleum refineries will usually employ
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computers for logging and limit testing the many temperatures associated with a
process, typicallyDr.numbering
in the hundreds.
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• use materials with a high thermal coefficient of
resistance
• sensitive but highly non-linear
A typical disc thermistor
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A threaded thermistor
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
Thermistors are widely used for temperature sensing purposes (sensitivity,
accuracy, reliability)

Thermistors are temperature dependent resistors

Most common: Negative-Temperature Coefficient (NTC) thermistors

NTC themistors have nonlinear R-T characteristics

Steinhart-Hart equation is widely used to model the R-T relationship.
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Symbol of Thermistor
45

Assuming, as a first-order approximation, that the relationship between resistance and temperature
islinear, then:
where
= change in resistance
= change in temperature
= first-order temperature coefficient of resistance

Thermistors can be classified into two types, depending on the sign of .
If is positive, the resistance increases with increasing temperature, and the device is called
a positive temperature coefficient (PTC) thermistor, or posistor.
If is negative, the resistance decreases with increasing temperature, and the device is called
a negative temperature coefficient (NTC) thermistor.

Resistors that are not thermistors are designed to have a as close to zero as possible, so that
their resistance remains nearly constant over a wide temperature range.
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
Instead of the temperature coefficient k,
sometimes the temperature coefficient of
resistance (alpha sub T) is used. It is defined
as
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
Photodiode Circuits

Thermistor Half-Bridge
 voltage divider
 one element varies

Wheatstone Bridge
 R3 = resistive sensor
 R4 is matched to nominal value of R3
 If R1 = R2, Vout-nominal = 0
 Vout varies as R3 changes
VCC
R1+R4
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
Air conditioning and seat temperature controls.

Electronic fuel injection, in which air-inlet, air/fuel
mixture and cooling water temperatures are monitored to
help determine the fuel concentration for optimum
injection.

Warning indicators such as oil and fluid temperatures, oil
level and turbo-charger switch off.

Fan motor control, based on cooling water temperature

Frost sensors, for outside temperature measurement
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Operating temperature range
(2) Zero power resistance of thermistor
R=R0expB(1/T-1/T0), T, T0 are ambient temperatures, R, R0 are
corresponding resistances and B is the B-constant (or β constant )
of the thermistor
Or
B=ln(R/R0)/(1/T-1/T0)
(3) Since thermistor is a resistor, power dissipation
P=C(T2-T1), where C is the thermal dissipation constant (mW/ºC).
This causes self-heating.
(4) Thermal time constant
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A common 10kOhm NTC thermistor
• It is nonlinear!!
• Temperature goes up more
charges in semiconductor
resistance goes down! (NTC)
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1
 C1  C2  ln( R)  C3  ln( R)3
T
3 term form:
2 term form:
T is measured in Kevin.
1
 C1 'C2 ' ln(R)
T
NoteC1 '  C1 , C2 '  C2
Measure 3 resistances and 3 temperatures, you can solve three
unknowns C1, C2 and C3.
Matrix inversion (linear algebra)
Minimize (least square) error in curve fitting
Once C1, C2 and C3 are known, S-H equation (for your sensor) can
be used to predict T based on R measurement.
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1
 C1  C2  ln( R)  C3  ln( R)3
T
1
 
3
T

 1 ln R1
1


C

C
ln
R

C
ln
R
 
1
2
1
3
1
 1   C  C ln R  C ln R 3   1 ln R
2
2
3
2 
2

 T2   1
3
 1  C1  C2 ln R3  C3 ln R3   1 ln R3
 
 T3 
ln R1 3   C1 
3 
ln R2    C2 
ln R3 3  C3 
A  B  X (A, B are known,solve X)
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1
 C1  C2  ln( R)  C3  ln( R)3
T
1
 
3
T

 1 ln R1
1


C

C
ln
R

C
ln
R
 
1
2
1
3
1
 1   C  C ln R  C ln R 3   1 ln R
2
2
3
2 
2

 T2   1
3
 1  C1  C2 ln R3  C3 ln R3   1 ln R3
 
 T3 
ln R1 3   C1 

ln R2 3   C2 
ln R3 3  C3 
A  B  X (A, B are known,solve X)
B 1  A  B 1  B  X
1
where B 1 
bij
B
 
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T
 
1

b ji
B
Matrix inversion
Matrix determinant
Matrix transpose
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(1) Voltage divider circuit
 Relating Vout to RT
(2) Wheatstone bridge circuit*
 Balancing the Bridge circuit
 Relating Vout to RT
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Vs


R1
RT Vout
Vout
RT
 VS
RT  R1
Where RT varies with T
Design considerations:
 Vout voltage range (signal conditioning in order to
interface with ADC)
 Vout sensitivity varies at different temperature range
(R-T characteristics curve)
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Vs


R2
RT
+
R1
Vout
R3
 R1
R3 

Vout  VS 

 RT  R1 R2  R3 
R
R
if T  2 (bridge is balanced)
R1 R3
Vout  0
if R2  R3
& R1  RT
 VS  2Vout
T hen: RT  R1 
 VS  2Vout




Design considerations:
 More sensitive to small changes
 V voltageDr. sanjay
range
(to interface with ADC)
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buffer
REF195
+5 V
reference
10k
-
To ADC
+
Thermistor
nominal at 10k
Vout
1/4
AD8606
(AD8605)
Voltage divider and a unity gain buffer is required!
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
The ECT responds to change in Engine Coolant Temperature. By measuring engine coolant

temperature, the ECM knows the average temperature of the engine. The ECT is usually

located in a coolant passage just before the thermostat. The ECT is connected to the THW terminal on
the ECM.
The ECT sensor is critical to many
ECM functions such as fuel injection,
ignition timing, variable

valve timing, transmission shifting, etc.
Always check to see if the engine is at
operating

temperature and that the ECT
is accurately reporting the temperature
to the ECM.

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
The IAT detects the temperature of the incoming air stream. On vehicles equipped with a MAP sensor, the
IAT is located in an intake air passage.

On Mass Air Flow sensor equipped vehicles, the IAT is part of the MAF sensor. The IAT is connected to
the THA terminal on the ECM.
The IAT is used for detecting
ambient temperature on a cold start
and intake air temperature as the
engine heats up the incoming air.

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
The EGR Temperature Sensor is located in the EGR passage and measures the temperature
of the exhaust gases.
The EGR Temp sensor
is connected to the THG
terminal on the ECM.

When the EGR valve
opens, temperature
increases.

From the increase in
temperature, the ECM
knows the EGR valve is
open and that exhaust
gases are flowing.

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
Pressure sensors are used to measure intake
manifold pressure, atmospheric pressure,
vapour pressure in the fuel tank etc.

Though the locations is different and the
pressure being measured vary, the operating
principles are similar.
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Manifold
Absolute
Pressure
Sensor
Intake
Manifold
Pressure
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Barometric
Pressure
Sensor
Atmospheric
Pressure
Dr. sanjay Chikalthankar
Vapour
Pressure
Sensor
Evaporative
Pressure
Sensor
Turbocharger
Pressure
Sensor
Intake
Manifold
Pressure
65

Barometric pressure changes vs. altitude and temperature, so
we can use pressure sensor data to indicate the altitude change
in the rockets during their launch.

Each sensor has slightly different characteristics, so we need
to calibrate them individually.
Voltage
Pressure sensors
on R-DAS or IMU
Analog Signal
voltageconditioning
Environment with
varying pressures
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Analog
0-5V
ADC
on R-DAS
Raw
data
0-1024
Computer
LabVIEW
66

A barometer is a scientific instrument used in meteorology to
measure atmospheric pressure. Pressure tendency can forecast short term changes
in the weather. Numerous measurements of air pressure are used within surface
weather analysis to help find surfacetroughs, high pressure systems, and frontal
boundaries.
Schematic drawing of a simple mercury barometer
with vertical mercury columnDr.and
reservoir
at base
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sanjay
Chikalthankar
Old barometers
67


A barograph is a recording aneroid barometer.A barograph is used to monitor pressure.The
pointer in an aneroid barometer is replaced with a pen. It produces a paper or foil chart called
a barogram that records the barometric pressure over time.
Barographs use one or more aneroid cells acting through a gear or lever train to drive a
recording arm that has at its extreme end either a scribe or a pen. A scribe records on smoked
foil while a pen records on paper using ink, held in a knib. The recording material is mounted
on a cylindrical drum which is rotated slowly by clockwork. Commonly, the drum makes one
revolution per day, per week, or per month and the rotation rate can often be selected by the
user.
ASI's next generation, solid state,
precision digital barograph.
7/17/2015
Capsule pile and linkage. This
barograph can be seen to have five
aneroid capsules stacked in series.
Dr. sanjay Chikalthankar
68



Many techniques have been developed for the measurement
of pressure and vacuum. Instruments used to measure pressure are called pressure
gauges or vacuum gauges.
A manometer could also refer to a pressure measuring instrument, usually limited
to measuring pressures near to atmospheric. The term manometer is often used to
refer specifically to liquid column hydrostatic instruments.
A vacuum gauge is used to measure the pressure in a vacuum—which is further
divided into two subcategories, high and low vacuum (and sometimes ultra-high
vacuum). The applicable pressure range of many of the techniques used to measure
vacuums have an overlap. Hence, by combining several different types of gauge, it
is possible to measure system pressure continuously from 10 mbar down to
10−11 mbar.
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Membrane-type manometer
 The Bourdon pressure gauge uses the principle that a flattened tube tends to change
to be straightened or larger circular cross-section when pressurized. Although this
change in cross-section may be hardly noticeable, and thus involving
moderate stresses within the elastic range of easily workable materials, the strain of
the material of the tube is magnified by forming the tube into a C shape or even a
helix, such that the entire tube tends to straighten out or uncoil, elastically, as it is
pressurized.

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Membrane-type
Dr. sanjay Chikalthankar Mechanical side with Bourdon tube
manometer
70

A time pressure gauge is an instrument that digitally displays pressure data into time
intervals. It translates complex pressure data (pounds per square inch or psi) into
simplified duration format (time) creating greater precision and efficiency.[1] While
a pressure gauge indicates a general unit amount, only a time pressure gauge accounts
for varying consumption and capacity in relation to time remaining.

Applications
1.
Time pressure gauge applications are universal. Welders using oxygen and acetylene can
plan more efficiently if they know the energy duration due to varying consumption in
cutting techniques. A nurse concerned that a patient may run out of oxygen can monitor
the workload more efficiently by knowing how much time is remaining rather that how
much pressure is left.
2.
Scuba divers could determine the length of time they could remain submerged. A pilot
could manage supplemental oxygen flow rates of an aircraft to determine
possible altitudes for maximizing fuel efficiency.[2] Ultimately any activity that uses
pressurized contents is applicable.
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
Thin walled cylindrical sheets with deep convolutions sealed at one end

Sealed end moves axially when pressure is applied

No. of convolution s – vary from- 2 to 50 – depends on range, operating temp

Used for low pressure measurement
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Displacement
y = 2.n. A q P Rx 2 /( Et 2 )
where
n – no. of convolutions
A q- effective area
Et - young’s modulus of elasticity
Rx – radius of diaphragm
P – pressure
ie, Y  P
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
Pressure units


Pascal (Pa)=N/m2: standard atmosphere P0=101325 Pa=101.325kPa
Bar: 1 bar=100 kPa
 Psi= (Force) pound per square inch: 1 Psi=6.89465 KPa

MPX4115A measures pressure in the range: 15-115 kPa

Sensitivity: 45.9mV/kPa (pressure range 100kPa voltage range 4.59V)

Typical supply voltage 5.1V

Output analog voltage
 Offset voltage (Voff) is the output voltage measured at minimum rated pressure
(Typical@ 0.204V)
 Full scale output (Vfso) measured at maximum rated pressure (Typical@ 4.794
V)
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4.794 V
y=ax+b
Calibration!
0.204 V
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Digital reading
Comparison with
Manufacture specifications
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Digital
 5  Analogvoltagefromsensor
1024
1 Psi  6.89465kPa
Calibration curve to find pressure in
field test
Pressure (Psi)
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76

Voltage Error=Pressure Error x Temperature
Error Factor x0.009 x Vs

Temperature Error Factor=1 (0oC-85oC),
otherwise higher

Pressure Error: +/- 1.5KPa
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


Collect data sets (x1,y1) (x2, y2)……(xn, yn), n>2
Best fit (regression or least square) line
Excel, Matlab or KlaidaGraph, of course LabView……
Excel Example
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Believe it or not you can actually do it by hand:
 n
  n  n 
n    xi yi     xi   yi 
  i 1  i 1 
Slope a   i 1
2
n
n


2 
n    xi     xi 
 i 1   i 1 
 n 
yi  a  xi 

 i 1 
Interceptb  i 1
n
n
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Dr. sanjay Chikalthankar

Assume constant temperature gradient dT/dh, the altitude h is
a function of pressure P given by:
 dT  R 

dh
 P g 
T0
h
 1   

dT
P0 




dh




where






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
h = altitude (above sea level) (Units in feet)
P0 = standard atmosphere pressure= 101325Pa
T0 = 288.15K (+15ºC)
dT/dh=-0.0065 K/m: thermal gradient or standard temperature lapse rate
R = for air 287.052 m2/s2/K
g = (9.80665 m/s²)
Dr. sanjay Chikalthankar
80
Plug in all the constants
0.1902


P
(
kPa)


5 

h  1.454410  1  

  101.325kPa 



•
h is measured in feet.
•
This equation is calibrated up to 36,090 feet (11,000m).
•
A more general equation can be used to calculate the
Relationship
for
different
layers
of
atmosphere
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
81
Pressure
Voltage
Calibration curve
Time (second)
Altitude
Time (second)
Equation (1)
Time (second)
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
Position/speed sensors provide information to the ECM about the
position of a component, the speed of a component, and the
change in speed of a component.

The following sensors provide this data:
• Camshaft Position Sensor (also called G sensor).
• Crankshaft Position Sensor (also called NE sensor).
• Vehicle Speed Sensor.
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
The distance between the rotor and pickup coil is critical. The further apart they are, the
weaker the signal.

These sensors generate AC voltage, and do not need an external power supply.

Another common characteristic is that they have two wires to carry the AC voltage.

The wires are twisted and shielded to prevent electrical interference from disrupting the
signal.

By knowing the position of the camshaft, the ECM can determine when cylinder No. I is on
the compression stroke.

The ECM uses this information for fuel injection timing, for direct ignition systems and for
variable valve timing systems.

This sensor is located near one of the camshafts. With variable timing V-type engines, there
is one sensor for each cylinder bank.
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
Camshaft Position Sensor (G Sensor)

The terminal on the ECM is designated with a letter G, and on some
models a G and a number, such as G22 is used.

Variable Valve Position Sensor

Some variable valve timing systems call the Camshaft Position Sensor the
Variable Valve

Position Sensor. See section on variable valve timing systems for more
information.
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89

The ECM uses crankshaft position signal to determine engine RPM, crankshaft
position, and engine misfire.

This signal is referred to as the NE signal.

The NE signal combined with the G signal indicates the cylinder that is on
compression and the ECM can determine from its programming the engine
firing order.
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
The MIRE is driven by the output shaft on a transmission or output
gear on a transaxle.

This sensor uses a magnetic ring that revolves when the output shaft
is turning.

The MIRE senses the changing magnetic field. This signal is
conditioned inside the VSS to a digital wave.

This digital wave signal is received by the Combination meter, and
then sent to the ECM. The MIRE requires an external power supply
to operate.
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
The reed switch type is driven by the speedometer cable.

The main components are a magnet, reed switch, and the speedometer cable.

As the magnet revolves the reed switch contacts open and close four times per revolution.

This action produces 4 pulses per revolution.

From the number of pulses put out by the VSS, the combination meter/ECM is able to determine vehicle
speed.
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96

The ECM uses the Vehicle Speed Sensor (VSS) signal to modify engine
functions and initiate diagnostic routines.

The VSS signal originates from a sensor measuring transmission/ transaxle
output speed or wheel speed.

Different types of sensors have been used depending on models and
applications.

On some vehicles, the vehicle speed sensor signal is processed in the
combination meter and then sent to the ECM.

On some anti-lock brake system (ABS) equipped vehicles, the ABS computer
processes the wheel speed sensor signals and sends a speed sensor signal to the
combination meter and then to the ECM.
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97

Scanning Laser Vibrometry
 No physical contact with the test object; facilitate remote, mass-
loading-free vibration measurements on targets
 measuring velocity (translational or angular)
 automated scanning measurements with fast scanning speed
 However, very expensive (> $120K)
Photo courtesy of Bruel & Kjaer Dr. sanjay Chikalthankar
7/17/2015
Photo courtesy of Polytec
98

Shock Pressure Sensor
 Measurement range up to 69 MPa (10 ksi)
 High response speed (rise time < 2  sec.)
 High frequency bandwidth (resonant frequency up to
> 500 kHz)
 Operating temperature: -70 to 130 C
 Light (typically weighs ~ 10 g)
Photo courtesy of PCB Piezotronics

Shock Accelerometer
 Measurement range up to +/- 70,000 g
 Frequency bandwidth typically from 0.5 – 30 kHz at
-3 dB
 Operating temperature: -40 to 80 C
 Light (weighs ~ 5 g)
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
Inertial Gyroscope (e.g., http://www.xbow.com)
 used to measure angular rates and X, Y, and Z acceleration.

Tilt Sensor/Inclinometer (e.g., http://www.microstrain.com)
 Tilt sensors and inclinometers generate an artificial horizon and measure
angular tilt with respect to this horizon.

Rotary Position Sensor (e.g., http://www.msiusa.com)
 includes potentiometers and a variety of magnetic and capacitive
technologies. Sensors are designed for angular displacement less than one
turn or for multi-turn displacement.
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101

Measurements of size, shape, and position utilize
displacement sensors

Examples
 diameter of part under stress (direct)
 movement of a microphone diaphragm to quantify liquid movement
through the heart (indirect)

Primary Transducer Types





Resistive Sensors (Potentiometers & Strain Gages)
Inductive Sensors
Capacitive Sensors
Piezoelectric Sensors
Secondary Transducers
 Wheatstone Bridge
 Amplifiers
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
Light Sensor
 photoconductor
▪ light  R
 photodiode
▪ light  I
 membrane pressure sensor
▪ resistive (pressure   R)
▪ capacitive (pressure  C)
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
A piezoelectric accelerometer that utilizes the piezoelectric
effect of certain materials to measure dynamic changes in
mechanical variables. (e.g. acceleration, vibration, and
mechanical shock)

As with all transducers, piezoelectric accelerometers convert
one form of energy into another and provide an electrical
signal in response to a quantity, property, or condition that is
being measured.

Using the general sensing method upon which all
accelerometers are based, acceleration acts upon a seismic
mass that is restrained by a spring or suspended on a
cantilever beam, and converts a physical force into an
electrical signal.

Before the acceleration can be converted into an electrical
quantity it must first be converted into either
a force or displacement. This conversion is done via the mass
spring system shown in the figure to the right.
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104

Potentiometers
 Resistive potentiometers are one of the most
widely used forms of position sensor
 Can be angular or linear
 Consists of a length of resistive material with a
sliding contact onto the resistive track
 When used as a position transducer a potential is
placed across the two end terminals, the voltage on
the sliding contact is then proportional to its
position
 An inexpensive
and easy to use sensor
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105

Monitor location of various parts in a system
 absolute/relative position
 angular/relative displacement
 proximity
 Acceleration

Principle of operation
 Magnetic, resistive, capacitance, inductive, eddy current, etc.
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Potentiometer
Distance
Electrical signal
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107
R2
Vo 
VT
R1  R2
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
LVDT (Linear Variable Differential Transformer):
 Inductance-based ctromechanical sensor
 “Infinite” resolution
▪ limited by external electronics
 Limited frequency bandwidth (250 Hz typical for
DC-LVDT, 500 Hz for AC-LVDT)
 No contact between the moving core and coil
structure
▪ no friction, no wear, very long operating lifetime
 Accuracy limited mostly by linearity
▪ 0.1%-1% typical
sanjay
Chikalthankar
7/17/2015
Models with strokes Dr.
from
mm’s
to 1 m available
110
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
Current is driven through the primary coil at A, causing an induction
current to be generated through the secondary coils at B.

The linear variable differential transformer (LVDT) (also called just
a differential transformer) is a type of electrical transformer used for
measuring linear displacement (position). A counterpart to this device that
is used for measuring rotary displacement is called a rotary variable
differential transformer (RVDT).
Cutaway view of an LVDT
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
The linear variable differential transformer has three solenoidal coils placed end-to-end around a tube.

The center coil is the primary, and the two outer coils are the top and bottom secondaries.

A cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides along the axis
of the tube.

An alternating current drives the primary and causes a voltage to be induced in each secondary proportional to
the length of the core linking to the secondary.

The frequency is usually in the range 1 to 10 kHz.

As the core moves, the primary's linkage to the two secondary coils changes and causes the induced voltages to
change.

The coils are connected so that the output voltage is the difference (hence "differential") between the top
secondary voltage and the bottom secondary voltage.

The LVDT can be used as an absolute position sensor.

Even if the power is switched off, on restarting it, the LVDT shows the same measurement, and no
positional information is lost.

Its biggest advantages are repeatability and reproducibility once it is properly configured.
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Primary
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Secondary
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
When the core is in the center, the voltage induced in the two
secondaries is equal.

When the core is moved in one direction from the center, the
voltage induced in one winding is increased and that in the
others is decreased.

Movement in the opposite direction reverse the effect.
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Core at the center
V1 = V2
Vo = 0
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119
Core moves
towards S1
V1 > V2
Vo increase
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Core moves
towards S2
V2 > V1
Vo decrease
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122

Characteristics:
1. Due to no moving contact (non-contact) hence very low noise level. Resolution is
excellent.
2. The frequency response is limited mechanically by the mass of the core and
electrically by the frequency of the applied primary voltage (carrier), the frequency
of this carrier should be at last ten times that of the highest frequency component to
be measured.

Demerits:
1. Quite expensive.
2. The operation can be severely affected by stray magnetic A.C. fields or by the
presence of large mass of metal near by.
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Strain gauge is used to measure deflection, stress, pressure, etc.
The resistance of the sensing element changes with applied strain
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Chikalthankar small changes in the strain gauge resistance125
A Wheatstone
bridge is used
to measure

Remember: for a strained thin wire

DR/R = DL/L – DA/A + Dr/r
▪ A = p (D/2)2, for circular wire

DD/D = - m DL/L
Thus


L
Poisson’s ratio, m: relates change in diameter D to change in length
L


D
dimensional effect
piezoresistive effect
DR/R = (1+2m) DL/L + Dr/r
Gage Factor, G, used to compare strain-gate materials
 G = DR/R = (1+2m) + Dr/r
DL/L
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DL/L
Dr. sanjay Chikalthankar
126
 Resolution (infinite), depends on?
 High frequency bandwidth (> 10 kHz)
 Fast response speed
 Velocity (up to 2.5 m/s)
 Low cost
 Finite operating life (2 million cycles) due to contact wear
 Accuracy: +/- 0.01 % - 3 % FSO
 Operating temperature: -55 ~ 125 C
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127

The capacitance of a capacitor can be changed by
varying its area, gap length or dielectric constant.
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• Recall, capacitance of a parallel plate capacitor is:
C
 r 0 A
d
– A: overlapping area of plates (m2)
Air escape hole
– d: distance between the two plates of the capacitor (m)
0
– : permittivity of air or free space 8.85pF/m
r :
–
air
dielectric constant
Parallel plate
capacitor
Fuel tank
•The following variations can be utilized to make capacitance-based sensors.
–Change distance between the parallel electrodes.
–Change the overlapping area of the parallel electrodes.
–Change
the dielectric constant.
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 Exceptional performance for long stroke position measurement up to 3 m
 Operation is based on accurately measuring the distance from a
predetermined point to a magnetic field produced by a movable permanent
magnet.
 Repeatability up to 0.002% of the measurement range.
 Relatively low frequency bandwidth (-3dB at 100 Hz)
 Resolution up to 0.002% of full scale range (FSR)
 Very expensive
 Operating temperature: 0 – 70 C
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 Relatively short stroke
 High resolution
 Non-contact between the measured object and sensor
Type of Construction
Standard tubular
Fixing Mode
by 8mm diameter
Total Measuring Range
2(+/-1)mm
Pneumatic Retraction
No
Repeatability
0.1um
Operating Temperature
Limits
-10 to +65 degrees
C
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
All the methods used in linear measurement can be applied
to angular measurement.

Analog Methods:
Resistive, Inductive, and Capacitive

Digital Methods:
Absolute angular encoder, Use of maximal length, and
Incremental
angular
encoder
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
If a coil is moving inside a magnetic field is moved to cut directly
across the lines of flux, then a voltage is induced in the coil :
e=Banv
where

B is the flux density
a is area of the coil
n is number of turn of the coil
v is relative velocity between the coil and the field
Therefore e is proportional to v, when other parameters are
constant. There are mainly two types --- moving coil and moving
magnet.
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– coil inductance is greatly
affected by the presence of
ferromagnetic materials
– here the proximity of a
ferromagnetic plate is
determined by measuring
the inductance of a coil
– we will look at inductance in
later lectures
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Inductive proximity sensors
135
 Simplest form of digital displacement sensor
▪ Many forms: lever or push-rod operated microswitches;
float switches; pressure switches; etc.
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A limit switch
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A float switch
136
 Consist of a light source and a light sensor within a
single unit
▪ 2 common forms are the reflective and slotted types
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A reflective
opto-switch
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A slotted opto-switch
137
 A pattern of light and dark strips is printed on to a
strip and is detected by a sensor that moves along it
▪ the pattern takes the form of a series of lines as shown
below
▪ it is arranged so that the combination is unique at each
point
▪ sensor is an array of photodiodes
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 uses a single line that alternates black/white
▪ two slightly offset sensors produce outputs as shown
below
▪ detects motion in either direction, pulses are counted to
determine absolute position (which must be initially
reset)
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 several methods use counting to determine position
▪ two examples are given below
Inductive sensor
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Opto-switch sensor
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

Light sensors are used in
cameras, infrared detectors, and
ambient lighting applications
Sensor is composed of
photoconductor such as a
photoresistor, photodiode, or
phototransistor
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I
p
+
n
V
-
141
• Light sensitive variable resistors.
• Its resistance depends on the intensity of light incident upon it.
– Under dark condition, resistance is quite high (M: called dark resistance).
– Under bright condition, resistance is lowered (few hundred ).
• Response time:
– When a photoresistor is exposed to light, it takes a few milliseconds, before it
lowers its resistance.
– When a photoresistor experiences removal of light, it may take a few seconds
to return to its dark resistance.
• Photoresisotrs exhibit a nonlinear characteristics for incident optical illumination
versus the resulting resistance.
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log10 R     log10 P
104
R
Symbol
103
102
101
101
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103
104
Relative illumination (P)
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 Properties
 open-loop gain: ideally infinite: practical values 20k-200k
▪ high open-loop gain  virtual short between + and - inputs
 input impedance: ideally infinite: CMOS opamps are close to ideal
 output impedance: ideally zero: practical values 20-100
 zero output offset: ideally zero: practical value <1mV
 gain-bandwidth product (GB): practical values ~MHz
▪ frequency where open-loop gain drops to 1 V/V
 Commercial opamps provide many different properties
 low noise
 low input current
 low power
 high bandwidth
 low/high supply voltage
 special purpose: comparator, instrumentation amplifier
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
Voltage Comparator
 digitize input

Voltage Follower
 buffer

Non-Inverting Amp
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• Inverting Amp
145

Summing Amp

Differential Amp

Integrating Amp

Differentiating Amp
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
Current-to-Voltage

Voltage-to-Current
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gain stage

Robust differential
gain amplifier
input stage

Input stage
 high input impedance
▪ buffers gain stage
 no common mode gain
 can have differential gain

Gain stage
 differential gain, low input impedance

total differential gain
2 R2  R1  R4 
 
Gd 
R1  R3 
Overall amplifier
 amplifies only the differential component
▪ high common mode rejection ratio
 high input impedance suitable for biopotential electrodes with high output
impedance
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instrumentation amplifier
HPF
non-inverting amp
With 776 op amps, the circuit was found to have a CMRR of 86 dB at 100 Hz and a noise level of 40 mV
peak to peak at the output. The frequency response was 0.04 to 150 Hz for ±3 dB and was flat over 4 to 40
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Chikalthankar
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Hz. The
total gain is 25 (instrument
amp)
x 32 (non-inverting amp) = 800.
Dr. Sanjay Chikalthankar
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150