Sensors for Engineering Applications

1. Sensors for Engineering Applications
UNIT-2
Prepared by
Dr. Jami Venkata Suman
Department of ECE

2. Unit II
Position and weight sensors
Position, direction, distance measurement-
large scale, distance travelled, and rotation.
Force, Load and Weight Sensors: Quartz
Sensors-Charge Mode High-Impedance
Piezoelectric Force Sensor, Voltage Mode Low-
Impedance Piezoelectric Force Sensor,
Piezoelectric Force Sensor Construction, types
of Strain Gage Sensors

3. Position
Position, as applied in measurement, invariably
means position relative to some point, the starting
point of the motion of an object, or any other
convenient reference point.

4. POSITION SENSOR

5. Methods of determining position make use of distance and
direction (angle) information, so that a position can be specified
either by using rectangular (Cartesian) co-ordinates (Figure 1) or
by polar co-ordinates (Figure 2).
Position on flat surfaces, or even on the surface of the Earth, can be
specified using two dimensions, but for air navigational purposes
three dimensional coordinates are required.

6. For industrial purposes, positions are usually
confined within a small space (for example, the
position of a robot tug) and it may be possible to
specify position with a single number, such as the
distance travelled along a rail.

7. Direction
The sensing of direction on the Earth’s surface can be
achieved by observing and measuring the apparent
direction of distant stars, by using the Earth’s magnetic
field, by making use of the properties of gyroscopes, or by
radio methods, the most modern of which are satellite
direction-finders.
The most ancient method, observation of stars, otherwise
known as Celestial navigation, depends on making precise
angle measurements.
The simplest form of celestial navigation is the observation
of local noon.
The sextant is used to measure the angle of the sun above
the horizon at local noon, and the Almanac will find the
latitude corresponding to this angle value.

8. The Hall effect is an example of the action of a
magnetic effect on moving charged particles, such as
electrons or holes.
The principle is a comparatively simple one, but for
most materials, detecting the effect requires very
precise measurements.

14. Load Sensor

15. Weight Sensor

16. Distance Measurement – Large Scale
The predominant method of measuring
distance to a target point on a large scale is
based on wave reflection of the type used in
radar or sonar.

17. The principle is that a pulse of a few waves is
sent out from a transmitter, reflected back
from some distant object and detected by a
receiver when it returns.
Since the speed of the waves is known, the
distance of the reflector can be calculated
from the time that elapses between sending
and receiving. This time can be very short, of
the order of microseconds or less, so that the
duration of the wave pulse must also be very
short, a small fraction of the time that is to be
measured.

18. Figure shows a block diagram of a radar system for distance
measurement. Radar or Sonar is used to provide target
movement indications, the time measurements will be used to
provide a display on a cathode ray tube.

19. Distance Travelled
The sensing of distance travelled, as distinct from
distance from a fixed reference point, can make
use of a variety of sensors.
Sensors for small distances can make use of
1. Resistive transducers
2. capacitive transducers
3. inductive transducers and
4. Interferometers.
The methods that are described here are all
applicable to distances in the range of a few
millimeters to a few centimeters.

20. A simple system of distance sensing is the use of a linear
potentiometer.
The moving object is connected to the slider of the potentiometer, so
that each position along the axis will correspond to a different output
from the slider contact. The output can be displayed on a meter,
converted to digital signals to operate a counter, or used in
conjunction with voltage level sensing circuits to trigger some action
when the object reaches some set position.
The main objections to this potentiometric method are that
1. the range of movement is limited by the size of potentiometers, and
2. the friction of the potentiometer is an obstacle to the movement. The
precision that can be obtained depends on how linear the winding
can be made, and 0.1% should be obtainable with reasonable ease.

21. capacitive sensor, this can take the form of a
metal plate located on the moving object and
moving between two fixed plates that are
electrically isolated from it. Capacitance between
plates is inversely proportional to plate spacing,
this method is practicable only for very short
distances, and is at its most useful for distances
of a millimeter or less.

22. An alternative physical arrangement of the plates is shown in
Figure in which the spacing of the fixed plates relative to the
moving plate is small and constant, but the movement of the
moving plate alters the area that is common to the moving plate
and a fixed plate.
This method has the advantage that an insulator can be used
between the moving plate and the fixed plates, and that the
measurable distances can be greater, since the sensitivity depends
on the plate areas rather than on variable spacing.

23. The most commonly used methods for sensing distance
travelled on the small scale, however, depend on induction.
The basic principle of induction methods is illustrated in
Figure, in which two fixed coils enclose a moving
ferromagnetic core. If one coil is supplied with an AC
signal, then the amplitude and phase of a signal from the
second coil depends on the position of the ferromagnetic
core relative to the coils. The amplitude of signal, plotted
against distance from one coil, varies as shown in Figure.

24. Development of the simple inductive sensor is
the linear variable differential transformer
(LVDT), which is now the most commonly used
sensor for distance in the range of millimeters to
centimeters.
The device consists basically of three fixed coils,
one of which is connected to an AC supply. The
other two coils are connected to a phase-
sensitive detector, and as a core of ferromagnetic
material moves in the coil axis, the output from
the detector will be proportional to the distance
of the core from one end of the coils. The output
from the phase-sensitive detector will be fairly
linearly proportional to distance.

25. Inductive (LVDT) displacement and
position sensors
https://www.eddylab.com/products/inductive-transducers~c-20

26. The device consists basically of three fixed coils, one of
which is connected to an AC supply. The other two coils
are connected to a phase-sensitive detector, and as a core
of ferromagnetic material moves in the coil axis, the
output from the detector will be proportional to the
distance of the core from one end of the coils. The output
from the phase-sensitive detector will be fairly linearly
proportional to distance.

27. Advantages of LVDT
1. Virtually zero friction, since the core need not be in
contact with the coils, and so no wear.
2. Linear output.
3. Very high resolution, depending mainly on the
detector.
4. Good electrical isolation between the core and the
coils.
5. A large output signal from the coils so that the phase-
sensitive detector needs little or no amplification.
6. No risk of damage if the core movement is excessive.
7. Strong construction that is resistant to shock and
vibration.

28. Distance travelled
The sensing of distance travelled can make use of a variety of
sensors.
Sensors for small distances can make use of resistive, capacitive or
inductive transducers in addition to the use of interferometers
The methods that are described above are all applicable to
distances in the range of a few millimetres to a few centimetres.
Beyond this range the use of radar methods becomes much more
attractive.

29. A simple system of distance sensing is the use
of a linear (in the mechanical sense)
potentiometer.
The moving object is connected to the slider of the
potentiometer, so that each position along the axis will
correspond to a different output from the slider contact –
either AC or DC can be used since only amplitude needs to
be measured.

30. The main objections to this potentiometric
method are: that the range of movement is
limited by the size of potentiometers that are
available(although purpose-built potentiometers
can be used), and that the friction of the
potentiometer is an obstacle to the movement.
The precision that can be obtained depends on
how linear (in the electrical sense) the winding
can be made, and 0.1% should be obtainable with
reasonable ease.

31. An alternative that is sometimes more attractive, but often less
practical, is the use of a capacitive sensor. This can take the form of a
metal plate located on the moving object and moving between two
fixed plates that are electrically isolated from it. The type of circuit
arrangement is illustrated in Figure, showing that the fixed plates
are connected to a transformer winding so that AC signals in
opposite phase can be applied. The signal at the moveable plate will
then have a phase and amplitude that depends on its position, and
this signal can be processed by a phase-sensitive detector to give a
DC voltage that is proportional to the distance from one fixed plate

32. Because the capacitance between plates is
inversely proportional to plate spacing, this
method is practicable only for very short
distances, and is at its most useful for
distances of a millimetre or less.

33. An alternative physical arrangement of the plates is
shown in Figure, in which the spacing of the fixed plates
relative to the moving plate is small and constant, but
the movement of the moving plate alters the area that is
common to the moving plate and a fixed plate. This
method has the advantage that an insulator can be used
between the moving plate and the fixed plates, and that
the measurable distances can be greater, since the
sensitivity depends on the plate areas rather than on
variable spacing.

34. Rotation
The sensing and measurement of rotational
movement is an important in many
applications.
The quantity that corresponds to distance for
a rotation is the angle rotated.
For a one complete rotation of a shaft is 360
degrees, the total angle turned by a shaft is
360 degrees X the number of complete turns.

35. Angular velocity sensor is the AC or DC
generator, which can also act as a transducer.
For sensing and measurement purposes only a
minimum of power must be used, so that a
miniature AC generator called the tacho-
generator is normally used.
The construction of the tacho-generator, is a
more precision built version of an AC
generator and usually has rotating magnets
with output from stator coils so as to avoid the
need for slip-rings.

36. The frequency of the output signal is proportional to
the revolutions per second of the shaft that is
coupled to the tacho, so that a frequency-sensitive
detector can be used to give a DC output
proportional to angular frequency.
The drawback for some applications is the need to
make a mechanical coupling between the Tacho and
the revolving shaft.

37. Much more versatile method is the magnetic disc type.
A magnet on the end of the shaft will cause a rotating
field as the shaft turns, and if a metal disc (which need
not be magnetic) is held close to this magnet then the
torque on the disc (caused by the interaction of the
magnet and the eddy currents that are induced in the
disc) will be proportional to the angular speed of the
shaft.
Advantage is that no contact is needed, though there
must not be any metal between the magnet and the
disc.

38. In some other method, a signal that is sent out for
each revolution of a wheel or shaft is sufficient
for angular velocity sensing, and this can be
achieved by the use of piezoelectric or magnetic
pulsing.
A piezoelectric pulse can be operated, as
indicated in Figure , by a cam on a shaft that
will cause the piezoelectric crystal to be
compressed on each rotation of the shaft . Since
the signal from the piezoelectric crystal can be
of several volts amplitude, this type of sensor
often needs no amplification, but the output is
at a high impedance. In addition, the friction on
the shaft is fairly large compared to the
alternative system of magnetic pulsing.

39. Torque measurement on rotating shafts, as
distinct from static measurement, is much more
difficult. The conventional method of measuring
torque on the shaft of a rotating motor uses a
load with frictional coupling that can be adjusted
so that the load remains still, but exerts torque on
strain gauges. The measurement reliability can
be improved by digitally processing the signals,
but the method is not really useful for transient
torque changes.

40. Force, Load and Weight Sensors
Measurement of a force, load, or a weight can
be accomplished by different sensors.
The most commonly used sensors are
generally based on either piezoelectric quartz
crystal or strain gage sensing elements

41. It is important to recognize the difference
between force, load, and weight.
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.

42. Quartz Sensors
The basic design of sensors utilizes the
piezoelectric principle, where applied
mechanical stresses are converted into an
electrostatic charge that accumulates on the
surface of the crystal (shown in figure).

43. 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 rate at which the charge leaks back to zero is
exponential and based on the sensor’s discharge
time constant (DTC).

44. DTC is defined as the time required for a sensor or
measuring system to discharge its signal to 37% of the
original value from a step change of measurand.
This DTC (seconds) is generally known and is
determined by multiplying the lowest insulation
resistance path (ohms) by the total capacitance
(farads) of the system prior to the amplifier circuit.

45. The DTC of a system directly relates to the low
frequency monitoring capabilities of a system.
It is because of this characteristic that
piezoelectric force sensors can only be used for
“quasi-static” measurements and are not
generally used for weighing applications.

46. A weight is placed on top of a piezoelectric force
sensor. Initially, the piezoelectric sensing crystals
will generate a charge (Q), which is immediately
seen at the input to the built-in (or external)
amplifier. However, after this initial step input, the
charge signal decays according to the equation
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)

47. It can be inferred that the longer the DTC, the more
accurately the sensor will be able to track longer
duration events. Generally, piezoelectric force
sensors with built-in electronics can have time
constants that vary from just a few seconds to >2000
seconds. Special time constants can be supplied by
altering the resistor value, R, in the sensor’s built-in
circuitry.

48. Sensor Types
Types of force Sensors
1. Charge Mode, High-Impedance, Piezoelectric Force Sensor
2. Voltage Mode, Low-Impedance, Piezoelectric Force Sensor

49. Charge Mode, High-Impedance, Piezoelectric Force Sensor
A charge mode piezoelectric force sensor,
when stressed, generates an electrostatic
charge from the crystals. For accurate analysis
or recording purposes, this high impedance
charge must be routed through low noise
cable to an impedance converting amplifier.

50. Figure shows a typical charge mode sensor
system schematic including sensor, low noise
cable, and charge amplifier.
The primary function of the
external amplifier is to convert
the high impedance charge
output to a usable low impedance
voltage signal for analysis or
recording purposes.

51. In a charge mode system, the sensors do not
contain built-in amplifiers. Therefore, the
sensor range and DTC are determined by the
settings on an external charge amplifier.
A feedback resistor working together with a
capacitor on the operational amplifier
determines these characteristics.

52. Voltage Mode, Low-Impedance, Piezoelectric Force Sensor
Voltage mode sensor system is shown schematically in
figure.
Voltage mode or low impedance force sensors share the
same basic design used in charge mode force sensors,
but incorporate a built-in microelectronic amplifier.
This amplifier serves to
convert the high impedance
charge output from the quartz
crystals into a low impedance
voltage signal for analysis or
recording.

53. This type of sensor, powered by separate constant
current source, operates over long ordinary
coaxial or ribbon cables without signal
degradation.
The low impedance voltage signal is not affected
by triboelectric cable noise (noise caused by
vibration or movement of the cable) or
environmental contaminants.
The sensor range and DTC are fixed by the
components in the voltage mode sensor’s internal
amplifier.

54. Piezoelectric Force Sensor Construction
Figure depicts the typical
construction of a general-
purpose quartz force sensor.
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.

55. 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.

56. Selecting and Specifying
Selecting and specifying a quartz based force
sensor is primarily application driven.
Application considerations typically include
the magnitude, frequency and the direction of
the force or forces to be measured.
Additional considerations may include size
constraints, environmental conditions and
mechanical integration requirements.

57. General Purpose – Internally preloaded for
measurement of compression and/or tension
forces.
impact testing
punching and forming
drop testing
materials testing
fatigue testing
material fracture
machinery studies
modal analysis force input and biomechanics

58. Penetration – Penetration style sensors are
specifically designed for compression and
impact force measurements in materials
testing such as helmet testing.

59. Miniature – The miniature sensor
configuration permits low-amplitude,
dynamic compression, tension, and impact
force measurements.

60. Impact – Impact style sensors are specifically
designed for impact force measurements.
crash testing
wire crimping and metal forming,
Machinery studies
impact testing
drop testing
laboratory shock test machines

61. Rings – Ring sensor configurations measure
dynamic compression.
tablet presses
stamping
punching
and forming operations
balancing
machinery studies
and force-controlled vibration testing

62. Links – Link style sensors measure dynamic
compression and tension.
tablet presses
tensile testing
stamping
punching and forming operations
balancing, machinery studies
force-controlled vibration testing

63. Multi-component – Multi-component sensors
permit simultaneous measurement of
dynamic force vector components in three
orthogonal directions.
machine tool cutting forces
stamping

64. 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.
i. ISO 10012-1 (former MIL– STD-45662A)
ii. ISO 9001
iii. ISO/IEC 17025

65. Strain Gage Sensors
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.

66. Strain Gage Sensors
A strain gauge is a resistor used to measure strain on an
object. When an external force is applied on an object, due
to which there is a deformation occurs in the shape of the
object. This deformation in the shape is both compressive
or tensile is called strain, and it is measured by the strain
gauge. When an object deforms within the limit of
elasticity, either it becomes narrower and longer or it
become shorter and broadens. As a result of it, there is a
change in resistance
The strain gages are connected
in a four-arm Wheatstone
bridge configuration.

67. 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.

68. 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.

69. Sensor Types
The most critical mechanical component in
any strain gage-based sensor is the “spring
element.”
The spring element serves as the reaction
mechanism to the applied force, load or
weight.

70. Three common structure designs used in the
industry are
1. bending beam
2. column
3. shear

71. Bending Beam
Sensor spring elements that employ the bending
beam structure design are the most common.
This is because the bending beam is typically a
high-strain, low force structural member that
offers two equal and opposite surfaces for strain
gage placement. The bending beam design is
typically used in lower capacity load cells.

72. Column
The column type load cell is the earliest type of strain gage transducer. Although
simple in its design, the column spring element requires a number of design and
application considerations. The column should be long enough with respect to its
cross section so that a uniform strain path will be applied to the strain gage. In
application, the end user must beware of second-order effects, as the column load cell
is susceptible to the effects of off-axis loading.

73. Shear-Web
The principle of a shear-web load cell typically
takes the form of a cantilever beam that has been
designed with a cross section larger than normal
with respect to the rated load to be carried in
order to minimize structure deflection. Under
this condition, the surface strain along the top of
the beam would be too low to produce an
adequate electrical output from the strain gage.
However, if the strain gages are placed on the
sides of the beam at the neutral axis, where the
bending stress is zero, the state of stress on the
beam side is one of pure shear, acting in the
vertical and horizontal direction.