The document discusses various types of sensors and transducers. It defines sensors as devices that produce an output signal in response to a physical input. Transducers are defined as devices that convert a signal from one form of energy to another. Common transducers include temperature sensors, pressure sensors, and position sensors. The document provides examples of different types of position sensors such as potentiometers, strain gauges, linear variable differential transformers (LVDTs), and optical encoders. It also discusses important specifications for sensors like sensitivity, accuracy, resolution, and hysteresis.

1. Professor Charlton S. Inao
Defence University
College of Engineering
Bishoftu, Ethiopia
PE-4030
Chapter 2/a

3. Sensors
Sensor is a device that produces an output signal
for the purpose of sensing of a physical
phenomenon
Sensor is a device that produces an output signal
for the purpose of sensing of a physical
phenomenon
Sensor is used for an input device that provides a
usable output in response to a specified physical
input. For example, a thermocouple is a sensor that
converts a temperature difference into an electrical
output.
Sensor is used for an input device that provides a
usable output in response to a specified physical
input. For example, a thermocouple is a sensor that
converts a temperature difference into an electrical
output.

4. Transducer
The term transducer is generally
used to refer to a device that
converts a signal from one form to
a different physical form. Thus
sensors are often transducers, but
also other devices can be
transducers, such as a motor that
converts an electrical input into
rotation
The term transducer is generally
used to refer to a device that
converts a signal from one form to
a different physical form. Thus
sensors are often transducers, but
also other devices can be
transducers, such as a motor that
converts an electrical input into
rotation

5. Transducers
Transducer is often used in place of the term
sensor. They are elements that when subject to
some physical change experience a related
change.
Sensors are transducers.
A measurement may use transducers, in addition
to the sensor, in other parts of the system to
convert signals in one form to another form.
Transducer is often used in place of the term
sensor. They are elements that when subject to
some physical change experience a related
change.
Sensors are transducers.
A measurement may use transducers, in addition
to the sensor, in other parts of the system to
convert signals in one form to another form.

6. Transducers
A transducer is a device that converts one type of
energy to another. Energy types include (but are not
limited to) electrical, mechanical, electromagnetic
(including light), chemical, acoustic or thermal energy.
While the term transducer commonly implies the use of
a sensor/detector, any device which converts energy can
be considered a transducer. Transducers are widely used
in measuring instruments.
Transduce means converts.
A transducer is a device that converts one type of
energy to another. Energy types include (but are not
limited to) electrical, mechanical, electromagnetic
(including light), chemical, acoustic or thermal energy.
While the term transducer commonly implies the use of
a sensor/detector, any device which converts energy can
be considered a transducer. Transducers are widely used
in measuring instruments.
Transduce means converts.

7. Transducers

8. Sensitivity
Resolution
Accuracy
Precision
Repeatability/Reproducibility
Linearity
Quality Parameters of an
Instrumentation System
Quality Parameters of an
Instrumentation System

9. Terminologies
Signal Conditioning - a front-end
preprocessing, which generally includes
functions such as signal amplification, filtering,
electrical isolation, and multiplexing.
In addition, many transducers require
excitation currents or voltages, bridge
completion, linearization, or high
amplification for proper and accurate
operation.

10. Signal Conditioning Devices
Charge amplifiers, lock-in amplifiers, power
amplifiers, switching amplifiers, linear
amplifiers, tracking filters, low-pass filters,
high-pass filters, and notch filters are some of
the signal-conditioning devices used in
mechatronic systems.

11. • Range- it is the limits between which the
input can vary. eg – load cell for the
measurement of forces might have a range of
0-50 kN.
• Error- the difference between the result of
the measurement and the true value of the
quantity being measured.
Error= measured value- true value

12. Terminology
• Accuracy- is the extent to which the value
indicated by a measurement system might be
wrong. It is the summation of all possible errors
that are likely to occur, as well as the accuracy to
which the transducer has been calibrated.
• Accuracy is often expressed as the full range
output or full scale deflection. Eg. A sensor might
be specified as having an accuracy of +
5% of full range output. Given: Temp Range 0-
200 deg Centigrade. Reading could be within +or
-10 deg centigrade of the true reading.

13. • Sensitivity- the sensitivity is the relationship
indicating how much output you get per unit
input, ie. Output/input.
Example: A resistance thermometer may have
sensitivity of 0.5 ohms/deg Centigrade.
-A transducer for the measurement of pressure
might be quoted as having a temperature
sensitivity of + 0.1% of the reading per degree
Centigrade change in temperature.

14. • The sensitivity is defined as the ratio between
output signal and measured property. For
example, if a sensor measures temperature
and has a voltage output, the sensitivity is a
constant with the unit [V/K]; this sensor is
linear because the ratio is constant at all
points of measurement.
Sensitivity is the ability of the measuring instrument
to respond to changes in the measured quantity.
It is also the ration of the change of output to change
of input.
Sensitivity is the ability of the measuring instrument
to respond to changes in the measured quantity.
It is also the ration of the change of output to change
of input.

15. • Hysteresis error- Transducers can give
different outputs from the same value of
quantity being measured according to
whether that value has been reached by a
continuously increasing change or a
continuously decreasing change. The
hysteresis error is the maximum difference in
output for increasing and decreasing values.

16. Hysteresis
• Hysteresis is an error
caused by when the
measured property reverses
direction, but there is some
finite lag in time for the
sensor to respond, creating
a different offset error in
one direction than in the
other.
Hysteresis Error

17. Nonlinearity and hysteresis

18. • The error of a measurement is the difference between the result of
the measurement and the true value of the quantity being
measured.
• Nonlinearity error is used to describe the error that occurs as a
result of assuming a linear relationship between the input and
output over the working range, that is, a graph of output plotted
against input is assumed to give a straight line.
• Non linearity error- The error associated in the deviation from
linearity between the input and the output. The error is quoted as
the percentage of the full range output.
• Three methods: 1) Draw the straight line joining the output values
at the end points of the range 2) Find the straight line by using the
method of least squares to determine the best fit line. 3) Find the
straight line by using the method of least squares to determine the
best fit line which passes through zero point.

19. Non Linearity Error

20. • Repeatability- The repeatabiity of the transducer is its
ability to give the same output for repeated
applications of the same input value. Example:
Angular velocity => repeatability + 0.01% of the full
range at a particular angular velocity.
• Reproducibility- the ability to give the same output
when used to measure a constant and is measured on
a number of occasions.
• Between the measurements the transducer is
disconnected and reinstalled. The error is usually
expressed as a percentage of the full range output.

21. • Stability- The stability of a transducer is its
ability to give the same output when used to
measure a constant input over a period of
time. The term drift is often used to describe
the change in output that occurs over time.
• The drift may be expressed a s a percentage of
the full range output.
• The term zero drift is used for the changes
that occur in output when there is zero input.

22. • Deadband- the deadband or dead space of a
transducer is the range of input values for
which there is no output. For example bearing
friction in a flowmeter using a rotor might
mean that there is no output until the input
has reached a particular velocity threshold.

23. • Resolution- The resolution of a sensor is the
smallest change it can detect in the quantity that
it is measuring.
• Often in a digital display, the least significant digit
will fluctuate, indicating that changes of that
magnitude are only just resolved. The resolution
is related to the precision with which the
measurement is made.
• For example, a scanning tunneling probe (a fine
tip near a surface collects an electron tunnelling
current) can resolve atoms and molecules.

24. Example: Strain Gauge Transducer
•Thermal Sensitivity :0.03 % full range output /deg Centigrade

25. Interpretation for Strain Gauge
Transducer Specs
• The range indicates that the transducer can be
used to measure pressures between 70 and
1000 kPa or 2000 and 70000kPa.
• It requires a supply of 10 Vdc or ac rms for its
operation
• It will give an output of 40mV when the
pressure on the lower range is 1000 kPa and
on the upper range of 70 000kPa

26. Interpretation for Strain Gauge
Transducer Specs
• Nonlinearity and hysteresis will lead to errors of +
.5% of 1000, i.e + 5kPa on the lower range and +
.5% of 70 000 , that is i.e + 350kPa on the upper
range.
• The transducer can be used between the
temperature range -54 deg C and +120 deg C.
• When the temperature changes by 1 deg C the
output of the transducer for zero input will
change by 0.03% of 1000=0.3kPa on the lower
range and 0.03% of 70 000=21 kPa on the upper
range.

27. Interpretation for Strain Gauge
Transducer Specs
• When the temperature changes by 1deg C,
the sensitivity of the transducer will change by
0.3 kPa on the lower range and 21kPa on the
upper range.
• This means that readings will change by these
amounts when such temperature change
occurs.

28. Example : MX100AP Pressure Sensor
Specs
• Supply voltage: 3 V (6 V max)
• Supply current: 6 mA
• Full-scale span: 60 mV
• Range: 0 to 100 kPa
• Sensitivity: 0.6 mV/kPa
• Nonlinearity error: 0.05% of full
range
• Temperature hysteresis: 0.5% of full
scale
• Input resistance: 400 to 550 O
• Response time: 1 ms (10% to 90%)

29. Displacement, Position and Proximity SensorsDisplacement, Position and Proximity Sensors

30. Displacement/Position Sensors
The term position sensor is used for a sensor
that gives a measure of the distance
between a reference point and the current
location of the target, while a displacement
sensor gives a measure of the distance
between the present position of the target
and the previously recorded position.

31. POSITION SENSORS
Types of Position Sensors
Every commonly applied position sensing technology has its own
characteristic benefits and limitations. some of these technologies provide a
better fit than others in different applications. The goal is to find the most
cost-effective solution for the performance parameters that are important in
your specific application and environment.
The types of position sensors include:
■ Contact devices
• Limit switches
• Resistive position transducers
■ Non-contact devices
• Magnetic sensors, including Hall effect and magneto-resistive sensors
• Ultrasonic sensors
• Proximity sensors
• Photoelectric sensors

32. Limit Switches
Limit switches are electromechanical contact
devices. Easy to understand and apply,
they are the cost-effective switches of choice for
detecting objects that can be touched. These
rugged, dependable switches are offered in a
variety of sizes with different seals,
enclosures, actuators, circuitries and electrical
ratings.
CONTACT TYPECONTACT TYPE

33. Various limit switches provide years of reliable
operation even in the most demanding
environmental conditions. They are appropriate for:
■ Material handling
■ Breweries
■ Packaging machinery
■ Wood products
■ Special machinery
■ Garbage compactors/trucks
■ Valves
■ Foundry equipment

34. Resistive Position Sensors
Resistive position sensors, also called potentiometers or simply position
transducers, were originally developed for military applications.
They were widely used as pane lmounted adjustment knobs on radios and
televisions in the years before pushbuttons and remote controls.
 Today, potentiometers are most commonly found in industrial applications
that range from forklift throttles to machine slide sensing.
Potentiometers are passive devices, meaning they require no power supply
or additional circuitry to perform their basic linear or rotary position sensing
function.
They are typically operated in one of two basic modes: rheostat and voltage
divider (true potentiometric operation).
As resistance varies with motion, rheostat applications make use of the
varying resistance between a fixed terminal and the sliding contact wiper.
 In voltage divider applications, a reference voltage signal is applied across
the resistive element track so that the voltage “picked up” by the movable
contact wiper can be used to determine the wiper’s position.

36. NON-CONTACT TYPENON-CONTACT TYPE

37. Applications

38. Ultrasonic Position Sensors

39. Application

40. Proximity Sensors

41. Capacitive Sensors

42. Inductive Sensors

43. Position Sensors
• Two Groups
– Linear
– Angular
 Linear displacement sensors might be used to
monitor the thickness or other dimensions of
sheet materials, separation of rollers, the position
or presence of a part, the size of a part.
 Angular displacement methods might be used to
monitor the angular displacement of shafts.

44. Application: Location and position of
object on a conveyor

46. • Displacement Sensors
– Potentiometer
– Strain Gauge element
– Capacitive Element
– Differential Transformer
– Optical Encoders
• Absolute
• Incremental

47. Potentiometer
• A potentiometer
consists of a resistance
element with a sliding
contact which can be
moved over the length
of the element. Such
element can be used for
linear or rotary
displacements.

48. Potentiometer

49. Strain Gauge
• The electrical resistance strain gauge is a metal wire,
metal foil strip, or a strip of semiconductor material
which is waferlike and can be stuck into surfaces like a
postage stamp.
• When subject to strain, its resistance R changes, the
fractional change in resistance delta R/R being
proportional to the strain E, that is delta R/R=GE
Where G, the constant of proportionality, is termed a s
the gauged factor. The resistance change of a strain
gauge is a measurement of the change in length of the
element to which the strain gauge is attached

50. Example: An electrical resistance of
100 ohms and a gauge factor of 2.0.
What is the change of resistance ΔR of
the gauge when it is subject to a strain
of 0.001.
Answer=Fractional Change in R= G x
strain x R=2.0X0.001x100=0.2 ohms
Example: An electrical resistance of
100 ohms and a gauge factor of 2.0.
What is the change of resistance ΔR of
the gauge when it is subject to a strain
of 0.001.
Answer=Fractional Change in R= G x
strain x R=2.0X0.001x100=0.2 ohms

52. Linear Variable Differential
Transformer.
Linear variable differential transformer is a mechanical displacement
transducer.
It gives an a.c. voltage output proportional to the distance of the transformer
core to the windings. The LVDT is a mutual-inductance device with three
coils and a core An external a.c. power source energizes the central coil
and the two- identical secondary coils connected in seriesin such a way
that their outputs oppose each other.
The net result is zero output.
A magnetic core is moved through the central tube as a result of
displacement being monitored. However when the core is displaced from
the central position there is a greater amount of magnetic core in one coil
than the other. A greater displacement means even more core in one coil
than the other, the output, the difference between the emf increases, the
greater the displacement being monitored.

53. LVDT
A greater displacement means even more core in one coil than the
other, the output, the difference between the emf increases, the
greater the displacement being monitored.

54. LVDT

55. Optical Encoder
• An encoder is a device that provides digital
output as a result of linear and angular
displacement. Position encoders are of two
types: Incremental and absolute. Incremental
encoders detect changes in rotation from
some datum while the absolute encoders give
the actual angular position.

56. Absolute
encoder
Incremental
encoder

57. Optical Encoder

58. Optical Encoder

59. 1.2 Proximity Sensors
Proximity switches are used to detect the presence of an item without making
contact with it.
Proximity Sensors
-eddy
-reed
-capacitive
-inductive
There are a number of forms of such switches, some being suitable only for metallic
objects.
The eddy current type of proximity switch has a coil that is energized by a constant
alternating current and produces a constant alternating magnetic field. When a
metallic object
is close to it, eddy currents are induced in it .

60. Proximity Sensors

61. Photoelectric Sensors

62. Capacitive
A proximity switch that can be used with metallic
and nonmetallic objects is the capacitive
proximity switch. The capacitance of a pair of
plates separated by some distance depends
onthe separation; the smaller the separation, the
higher the capacitance. The sensor of ecapacitive
proximity switch is just one of the plates of the
capacitor, the other plate being themetal object
for which the proximity is to be detected Thus
the proximity of the object is detected by a
change in capacitance.

63. Inductive
• The inductive proximity switch, consists of a coil wound a
round a ferrous metallic core. When one end of this core is
placed near a ferrous metal object, there is effectively a
change in the amount of metallic core associated with the
coil and so a change in its inductance.
• This change can be monitored using a resonant circuit, the
presence of the ferrous metal object thus changing the
current in that circuit.
• The current can be used to activate an electronic switch
circuit and so create an on/off device. The range over
which such objects can be detected is typically about 2 mm
to 15 mm. An example of the use of such a sensor is to
detect whether bottles passing along a conveyor belt have
metal caps on.

64. Velocity and
Motion Sensors

65. Example Velocity Sensor Specs

67. Velocity Sensor
• Incremental Encoder-This can be used for measuring
angular velocity, number of pulses produced per
second being determined.
• Tachogenerator -Used to measure angular velocity. It
is essentially a small electric generator, consisting of
coil mounted in magnetic field .when the coil rotates
an alternating emf is induced in the coil, the size of the
maximum emf being a measure of the angular velocity.
when used with a commutator a dc output can be
aobtained which is a measure of the angular velocity.

68. Motion Sensor
By motion, we mean the four kinematic variables:
• Displacement (including position, distance, proximity,
and size or gage)
• Velocity
• Acceleration
• Jerk
Note that each variable is the time derivative
of the preceding one. Motion measurements are
extremely useful in controlling mechanical
responses and interactions in mechatronic
systems.

69. The rotating speed of a work piece and the feed rate of
a tool are measured in controlling machining
operations.
Displacements and speeds (both angular and
translatory) at joints (revolute and prismatic) of
robotic manipulators or kinematic linkages are used
in controlling manipulator trajectory In high-speed
ground transit vehicles, acceleration and jerk
measurements can be used for active suspension
control to obtain improved ride
quality.

70. • Angular speed is a crucial measurement that is used in the control
of rotating machinery, such as turbines, pumps, compressors,
motors, and generators in power-generating plants. Proximity
sensors (to measure displacement) and accelerometers (to
measure acceleration) are the two most common types of
measuring devices used in machine protection systems for
condition monitoring, fault detection, diagnostic, and on-line (often
real-time) control of large and complex machinery .
• The accelerometer is often the only measuring device used in
controlling dynamic test rigs.
• Displacement measurements are used for valve control in process
applications. Plate thickness (or gage) is continuously monitored by
the automatic gage control (AGC) system in steel rolling mills.

71. Force and
Fluid Pressure
Sensors
Force and
Fluid Pressure
Sensors

72. Force, Load and Weight Sensors
• While other technologies exist, the most
commonly used sensors are generally based
on either piezoelectric quartz crystal or
strain gage sensing elements
• Force: The measurement of the interaction
between bodies.
• Load: The measurement of the force exerted
on a body.
• Weight: The measurement of gravitational
forces acting on a body.

73. Quartz Sensors
• Technology Fundamentals
Quartz force sensors are ideally designed and
suited for the measurement of dynamic
oscillating forces, impact, or high speed
compression/tension forces. The basic design
utilizes the piezoelectric principle, where
applied mechanical stresses are converted
into an electrostatic charge that accumulates
on the surface of the crystal.
Quartz force sensors are ideally designed and
suited for the measurement of dynamic
oscillating forces, impact, or high speed
compression/tension forces. The basic design
utilizes the piezoelectric principle, where
applied mechanical stresses are converted
into an electrostatic charge that accumulates
on the surface of the crystal.

74. The quartz crystals of a
piezoelectric force sensor
generate an electrostatic
charge only when force is
applied to or removed
from them. In other words,
if you apply a static force
to a piezoelectric force
sensor, the electrostatic
charge output initially
generated will eventually
leak away and the sensor
output ultimately will
return to zero.
The quartz crystals of a
piezoelectric force sensor
generate an electrostatic
charge only when force is
applied to or removed
from them. In other words,
if you apply a static force
to a piezoelectric force
sensor, the electrostatic
charge output initially
generated will eventually
leak away and the sensor
output ultimately will
return to zero.

75. Discharge Time Constant (DTC).
the charge signal decays according to the
equation:
q = Qe–t/RC
where:
q = instantaneous charge (Coulomb)
Q = initial quantity of charge (Coulomb)
R = resistance prior to amplifier (ohm)
C = total capacitance prior to amplifier (Farad)
e = base of natural log (2.718)
t = time elapsed after time zero (Second)
the charge signal decays according to the
equation:
q = Qe–t/RC
where:
q = instantaneous charge (Coulomb)
Q = initial quantity of charge (Coulomb)
R = resistance prior to amplifier (ohm)
C = total capacitance prior to amplifier (Farad)
e = base of natural log (2.718)
t = time elapsed after time zero (Second)

76. Piezoelectric Force Sensor
Construction
The basic mechanical construction of general purpose quartz forces sensors consist
of thin quartz discs that are “sandwiched” between upper and lower base plates. A
relatively elastic, beryllium-copper stud (or sometimes a sleeve) holds the upper and
lower plates together and preloads the crystals. Preloading of the crystals is required
to assure that the upper and lower plates are in intimate contact with the quartz
crystals, ensuring good linearity and the capability for tension as well as compression
measurements. This “sensing element” configuration is then packaged into a rigid,
stainless steel housing and welded to provide hermetic sealing of the internal
components against contamination. The Figure depicts the
typical construction of a general-purpose quartz force sensor.

77. Type of Piezoelectric Force Sensor

85. Applicable Standards
The basic design of quartz-based force sensors is
not governed by a specific standard.
However, applicable standards do exist for
calibration and certification. Most
manufacturers comply or conform to
standards such as ISO 10012-1 (former MIL–
STD-45662A), ISO 9001 and ISO/IEC 17025.

86. Major Manufacturers
PCB Piezotronics, Inc. – 3425 Walden
Avenue, Depew, NY 14043
Kistler Instrument Corporation – 75 John
Glenn Drive, Amherst, NY 14228
Dytran Instruments, Inc. – 21592 Marilla St.,
Chatsworth, CA 91311
Endevco Corporation – 30700 Rancho Viejo
Rd., San Juan Capistrano, CA 926

87. Strain Gauge Force Sensor
Technology Fundamentals
Sensors based on foil strain gage technology
are ideally designed for the precise
measurement of a static weight or a quasi-
dynamic load or force. The design of strain
gage-based sensors consists of specially
designed structures that perform in a
predictable and repeatable manner when a
force, load or weight is applied. The applied
input is translated into a voltage by the
resistance change in the strain gages, which
are intimately bonded to the transducer
structure. The amount of change in resistance
indicates the magnitude of deformation in the
transducer structure and hence the load that is
applied.

88. The strain gages are connected in a four-arm Wheatstone bridge
configuration, which acts as an adding and subtracting electrical network
and allows for compensation of temperature effects as well as cancellation
of signals caused by extraneous forces.
A regulated 5 to 20 volt DC or AC rms excitation is required and is
applied between A and D of the bridge. When a force is applied to the
transducer structure, the Wheatstone Bridge is unbalanced, causing an
output voltage between B and C proportional to the applied load. Most
load cells follow a wiring code established by the Western Regional Strain
Gage
Committee as revised in May 1960. The code is as follows:

89. Sensor Types
Sensor Types
The most critical mechanical component in
any strain gage-based sensor is the “spring
element.” In general terms, the spring
element serves as the reaction mechanism to
the applied force, load or weight. It also
focuses it into a uniform, calculated strain
path for precise measurement by the bonded
strain gage.

90. 3 Common Designs of Strain
Gauge
• Three common structure designs
used in the industry are
1) bending beam,
2) column and
3) shear.

94. Classification
General Purpose
General-purpose load cells are designed for a multitude of applications in the
automotive, aerospace, and industrial markets. The general-purpose load cell,
as the name implies, is designed to be utilitarian in nature. Within the general-
purpose load cell market there are several distinct categories: precision,
universal, weigh scale, and special application. Universal load cells are the
most common in industry.

95. Fatigue Rated
Fatigue rated load cells are specially designed and manufactured to withstand
millions of cycles. Many manufacturers utilize premium fatigue resistant steel
and special processing to insure mechanical and electrical integrity as well as
accuracy. Fatigue rated load cells typically are guaranteed to last 100 million
fully reversed cycles (full tension through zero to full compression). An added
benefit of fatigue rated load cells is that they are extremely resistant to
extraneous bending and side loading forces

96. Special Application
Special application load cells are load cells that have
been designed for a specific unique force
measurement task. Special application load cells
can be single axis or multiple axes. They include but
are not limited to:
• Pedal Effort • Seat Belt • Steering Column
• Crash Barrier • Hand Brake • Road Simulator
• Tow Ball • Femur • Skid Trailer
• Bumper Impact • Tire Test • Gear Shift

100. • Strain Gauge Load Cell –use of electrical
resistance to monitor the strain produced in
some member when stretched, compressed
or bent.

101. Strain Gauge
Strain Gauges a)metal wire b)Metal foil c)semiconductor

102. Strain Gauge

103. Sample Calculations –Strain Gauge
Wheatstone bridge. Calculate the value of the output
voltage(Vo) of the Wheatstone bridge below if
the value of R1 varies from 10 kΩ to 15 kΩ and the
Value of Supply Voltage, Vs= 1 V

104. Vo=R1/(R1 + R2) – R3/(R3 + R4)
R1 R1/(R1 + R2) R3/(R3 + R4) Output
Voltage(Vo)
10kΩ 10/(10 + 10) 10/(10 + 10) 0 V
11kΩ
12kΩ
13kΩ
14kΩ
15kΩ

105. Pressure Sensors
Common methods of pressure sensing are the following:
1.Balance the pressure with an opposing force (or head) and measure
this force. Examples are liquid manometers and pistons.
2. Subject the pressure to a flexible front-end (auxiliary) member and
measure the resulting deflection.
Examples are Bourdon tube, bellows, and helical tube.
3. Subject the pressure to a front-end auxiliary member and measure
the resulting strain (or stress). Examples are diaphragms and capsules.
The liquid column of height h and density r provides a counterbalancing
pressure head to support the measured pressure p with respect to the
reference (ambient) pressure pref

107. The pressure is determined by measuring F
using a force sensor. The Bourdon tube shown
in Figure 6.77(c) deflects with a straightening
motion as a result of internal pressure. This
deflection can be measured using a
displacement sensor (typically, arotatory sensor)
or indicated by a moving pointer.

108. The bellows deflect with internal pressure,
causing a linear motion, as shown in Figure
6.77(d). The deflection can be measured using
a sensor such as LVDT or a capacitive sensor,
and can be calibrated to indicate pressure.
 The helical tube shown in Figure 6.77(e)
undergoes a twisting (rotational) motion when
deflected by internal pressure. This deflection
can be measured by an angular displacement
sensor (RVDT, resolver, potentiometer, etc.), to
provide pressure reading through proper
calibration.

109. Figure 6.77(f) illustrates the use of a
diaphragm to measure pressure. The
membrane (typically metal) will be strained
due to pressure. The pressure can be
measured by means of strain gauges
(piezoresistive sensors) mounted on the
diaphragm. MEMS pressure sensors that use
this principle are available. In one such device,
the diaphragm has a silicon wafer substrate
integral with it.

110. Through proper doping (using boron,
phosphorous, etc.) a microminiature
semiconductor strain gauge can be formed. In
fact more than one piezoresistive sensor can
be etched on the diaphragm, and used in a
bridge circuit to provide the pressure reading,
through proper calibration. The most sensitive
locations for the piezoresitive sensors are
closer to the edge of the diaphragm, where
the strains reach the maximum.

111. Fluid Pressure Sensors

112. Bellows and Orifice

113. The End