Aeronautical intro.ppt

1
AERONAUTICAL
INSTRUMENTATION

Text Books:
• 1. Aircraft Instruments and Integrated Systems- EHJ Pallet,
Longman Scientific & Technical, 1992.
Reference Books:
• 1. Aircraft Instrumentation and Systems -S. Nagabhushana & L.K.
Sudha, IK International
• 2. Aircraft Systems: Mechanical, electrical, and avionics subsystems
integration - Ian Moir and Alla Seabridge, Third Edition, John Wiley &
Sons, Ltd., 2008.
3

What is an Airplane?
• Aircraft
– More general term
– Refers to any heavier-than-air object that is
• Supported by its own buoyancy
• Supported by the action of air on its structures
• Airplane
– Heavier-than-air craft propelled by an engine
– Uses aerodynamic surfaces (wings) to generate lift

What is an Airplane?
Every airplane is an aircraft, but not every
aircraft is an airplane.
– Space shuttle
– Gliders
– Helicopters

Why So Many Types?
Every modern aircraft is built for a specific
purpose.
– Different altitudes
– Different speeds
– Different weight-carrying capacities
– Different performance

Why So Many Types?
• Jet fighters
– Relatively lightweight
– Highly maneuverable and very fast
– Carry small amount of weight, including fuel
– Must refuel on long flights
• Passenger airplanes
– Larger, carry more weight, fly longer distances
– Less maneuverable and slower

TYPES OF AIRCRAFT
PURPOSE (military only) :
FIGHTER – used to fight other aircraft in the air
BOMBER – drops bombs
GROUND ATTACK – attacks targets on the ground
TRANSPORT – used to carry large quantities of
supplies or people
TRAINER – to learn student pilots to fly
TANKER – used to refuel other aircraft in the air
HELICOPTER – a vertical take-off aircraft

AIRCRAFT STRUCTURE
1. WINGS
2. FUSELAGE
3. POWER UNIT
4. UNDERCARRIAGE
5. CONTROL SURFACES

WINGS
The main distinguishing features of
an aircraft :
1. wing position
2. wing shape
3. wing number

WING POSITION
LOW WING
MID WING
HIGH WING

WING SHAPE
RECTANGULAR TAPERED
SWEPT DELTA

WING NUMBER
MONOPLANE BIPLANE
TRIPLANE

FUSELAGE
can contains :
– cockpit : flight and navigation
instruments, controls,
windshield
– passenger cabins
– galley
– toilets
– wardrobe
– technical compartement

FUSELAGE
NOSE
EMPENNAGE / TAIL UNIT

POWER UNIT
The main distinguishing features of
an aircraft :
1. engine position
2. engine number

ENGINE POSITION
ON THE WINGS ON THE WING PYLONS
CLOSE TO THE FUSELAGE REAR MOUNTED

ENGINE NUMBER
SINGLE ENGINE TWIN ENGINE
TRIPLE ENGINE
FOUR (MULTIPLE) ENGINE

KINDS of ENGINES
PISTON (PROPELLER)
TURBOPROP (PROPELLER TURBINE)
JET
TURBOFAN

UNDERCARRIAGE
Types of landing gear :
NOSE WHEEL
MAIN LANDING GEAR
fixed
retractable
TAIL WHEEL

CONTROL SURFACES
1. WING :
FLAPS
AILERONS
2. TAIL UNIT :
RUDDER
ELEVATORS

WING
FLAPS
extended for approach,
landing and take-off to
increase the lift of the wings at
low speed
AILERONS
move in opposite direction to
bank/roll the airplane (control
stick to R/L)

TAIL UNIT
RUDDER
hindged to the stationary FIN
to control the yaw (L/R) of an
airplane (pedals)
ELEVATORS
hinged to the HOROZONTAL
STABILIZER, move in same
direction to control the pitch
(up/down) of the airplane
(control stick push/pull)

Introduction to Sensors
And types of Sensors

Sensors?
• American National Standards Institute
– A device which provides a usable output in response to a specified measurand
• 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
Sensor
Input Signal Output Signal

Transducer?
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
Actuators
Sensors
Physical
parameter
Electrical
Output
Electrical
Input
Physical
Output
e.g. Piezoelectric:
Force -> voltage
Voltage-> Force
=> Ultrasound!
Microphone, Loud Speaker

Commonly Detectable Phenomena
•Biological
•Chemical
•Electric
•Electromagnetic
•Heat/Temperature
•Magnetic
•Mechanical motion (displacement, velocity, acceleration, etc.)
•Optical
•Radioactivity

Common Conversion Methods
•Physical
–thermo-electric, thermo-elastic, thermo-magnetic, thermo-optic
–photo-electric, photo-elastic, photo-magnetic,
–electro-elastic, electro-magnetic
–magneto-electric
•Chemical
–chemical transport, physical transformation, electro-chemical
•Biological
–biological transformation, physical transformation

Commonly Measured Quantities
Stimulus Quantity
Acoustic Wave (amplitude, phase, polarization), Spectrum, Wave
Velocity
Biological & Chemical Fluid Concentrations (Gas or Liquid)
Electric Charge, Voltage, Current, Electric Field (amplitude, phase,
polarization), Conductivity, Permittivity
Magnetic Magnetic Field (amplitude, phase, polarization), Flux,
Permeability
Optical Refractive Index, Reflectivity, Absorption
Thermal Temperature, Flux, Specific Heat, Thermal Conductivity
Mechanical Position, Velocity, Acceleration, Force, Strain, Stress,
Pressure, Torque

Physical Principles: Examples
• Amperes’s Law
– A current carrying conductor in a magnetic field experiences a force (e.g.
galvanometer)
• Curie-Weiss Law
– There is a transition temperature at which ferromagnetic materials exhibit
paramagnetic behavior
• Faraday’s Law of Induction
– A coil resist a change in magnetic field by generating an opposing
voltage/current (e.g. transformer)
• Photoconductive Effect
– When light strikes certain semiconductor materials, the resistance of the
material decreases (e.g. photoresistor)

Need for Sensors
• 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 !!
– Imagine having to manually fill Poland Spring
bottles

Motion Sensors
• 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.
Primary Secondary
LVDT Displacement Sensor
Optoisolator
Potentiometer

Strain Gauge: Motion, Stress, Pressure
Strain gauge is used to measure deflection, stress, pressure, etc.
The resistance of the sensing element changes with applied strain
A Wheatstone bridge is used to measure small changes in the strain gauge resistance

Temperature Sensor: Bimetallic Strip
• Bimetallic Strip
• Application
– Thermostat (makes or
breaks electrical
connection with
deflection)
Metal A
Metal B
δ
L= L0[1+ β(T-T0)]

Temperature Sensor: RTD
• Resistance temperature device
(RTD)
R= R0[1+ α(T-T0)]
R= R0e
γ
[1
T
−
1
T0 ]

Light Sensor
• Light sensors are used in
cameras, infrared detectors,
and ambient lighting
applications
• Sensor is composed of
photoconductor such as a
photoresistor, photodiode, or
phototransistor
p n
I
+ V -

Photoresistors
• 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.
Symbol
1
0 1
0
l
o
g l
o
g
R P


R
101 103
102
101
104
102
103
104
Relative illumination (P)

Magnetic Field Sensor
• Magnetic Field sensors are
used for power steering,
security, and current
measurements on
transmission lines
• Hall voltage is proportional
to magnetic field x x x x x x
x x x x x x
x x x x x x
+ + + + + + + + + + + + + + +
- - - - - - - - - - - - - - -
I (protons) +
VH
-
B
V H=
I⋅ B
n⋅q⋅t

Ultrasonic Sensor
• Ultrasonic sensors are used
for position measurements
• Sound waves emitted are
in the range of 2-13 MHz
• Sound Navigation And
Ranging (SONAR)
• Radio Dection And Ranging
(RADAR) –
ELECTROMAGNETIC WAVES
!!
15° - 20°

Photogate
• Photogates are used in
counting applications (e.g.
finding period of period
motion)
• Infrared transmitter and
receiver at opposite ends of
the sensor
• Time at which light is broken
is recorded

Position Sensor
• Linear Variable Differential Transformer (LVDT)
• Magnetostrictive Linear Position Sensor
• Eddy Current Sensor
• Fiber-Optic Position Sensor

LDVT-Configuration
• An alternating
current is driven
through the primary,
causing a voltage to
be induced in each
secondary
proportional to its
mutual inductance
with the primary.
The frequency is
usually in the range
1 to 10 kHz.

LDVT-Parameter
• Range: 0.01-24 in.
• Noncontact
• Nonlinearity: 0.10%-0.25%
• Resolution: 1uin.
• Cost: medium
• Lifetime: high

Magnetostrictive Linear Position
Sensors
• Magnetostriction is a property of
ferromagnetic materials such as iron,
nickel, and cobalt. When placed in a
magnetic field, these materials change
size and/or shape
• the reverse is also true: applying stress to
a magnetostrictive material changes its
magnetic properties (e.g., magnetic
permeability). This is called the Villari
effect.
• Normal magnetostriction and the Villari
effect are both used in producing a
magnetostrictive position sensor.

Wiedemann effect
When an axial magnetic field is
applied to a magnetostrictive wire,
and a current is passed through
the wire, a twisting occurs at the
location of the axial magnetic field.
Since the current is applied as a
pulse, the mechanical twisting
travels in the wire as an ultrasonic
wave. The wave travels at the speed
of sound in the waveguide material,
~ 3O00 m/s.

Magnetostriction-Parameter
• Range: 0.5-90 in.
• Noncontact
• Nonlinearity: 0.02%
• Resolution: 80 uin.
• Cost: high
• Lifetime: high

Eddy Current Sensor
• Eddy current: caused when a
conductor is exposed to a
changing magnetic field due to
relative motion of the field source
and conductor; or due to
variations of the field with time.
• The eddy current generates a
opposite magnet field, which
superimposes with the exciting
magnet field. As consequence, the
impedance Z of the sensor coil
changes.

Eddy Current Sensor-Configuration
• An eddy current
sensor consists of
four components:
the sensor coil, the
target, the sensor
drive electronics,
and a signal
processing block.

For a defined measuring
target the change of coil
impedance is a function
of the distance a.
Therefore, the distance
can be derived by
measuring impedance
change.

Eddy current
sensors work most
efficently at high-
oscillation
frequencies nearby
their resonance
frequencies. The
resonance
frequency of an
eddy current sensor
depends on the

Fiber-Optic position sensor
• immunity to EMI and an inability to create
sparks in a potentially explosive environment.
Noncontact.
• suitable for measurement ranges varying from
centimeters to many meters and for which
extremely high resolution is not needed.

• Fluorescence followed by absorption is at the
heart of this sensor.
• The logarithm of the ratio of the two signals S1
and S2 is linear in x and independent of the
strength of the pump source.

Although insensitivity to pump strength or coupling of pump light to the
fluorescent fiber is a distinct advantage of this sensor, signal-to-noise
problems will arise if the individual signals S1 and S2 are too low.

Level is another common process variable that is measured in many
industries. The method used will vary widely depending on the nature
of the industry, the process, and the application.
Inventory:
-- a constant supply or storage of material
Control:
-- continuous, batch, blending, and mixing control
-- stabilize flow to the next process
Alarming:
-- hi/lo limits, safety shut down
Data Logging:
-- material quantities for inventory and billing purposes and
where regulatory requirements are necessary
Level Measurement

What is measured?
The measured medium can be liquid, gas or solid and stored
in vessels (open/closed tanks), silos, bins and hoppers.
Units of level can be expressed in:
 feet (meters)
 gallons (liters)
 pounds (kilograms)
 cubic volume (ft3, m3)

 Hydrostatic Head
 Float
 Load Cells
 Magnetic Level Gauge
 Capacitance
Transmitters
 Magnetostrictive
 Ultrasonic
 Microwave
 Laser
 Radar
 Guided Wave Radar
 Dip Stick
 Vibration
Methods ---- Direct or Indirect (inferential)

Direct Methods
Direct methods sense the surface or interface of the
liquid and is not affected by changes in material
density (Specific Gravity)
Examples:
 Dip Stick
 Resistance Tapes
 Sight Glass
 Floats
 Ultrasonic

Indirect Methods (Inferential)
Indirect methods “infer” liquid level by measuring some other
physical parameter such as pressure, weight, or temperature.
Changing materials means a corrective factor must be used or
recalibrating the instrument.
Examples:
 Hydrostatic head methods
 Load Cells
 Capacitance
 Conductivity

When determining the type of level sensor that should be used for
a given application, there are a series of questions that must be
answered:
 Open tank or closed tank?
 Can the level sensor be inserted into the tank or should it be
completely external? Contact or non-contact?
 Continuous measurement or point measurement?
 Direct or Indirect measurement?
 What type of material is being measured? Liquid or Solid?
Clean or Slurry?
Selection Criteria

For all liquids you will need:
 The system operating temperature with max. and min.
excursions?
two wide range – expensive the sensor
 The system operating pressure?
 Check that system ‘T’ and ‘P’ do not conflict with the
materials of construction?
Selection Criteria

For Solids:
 Bulk density
Be careful with very large silos as compaction at the bottom
can greatly change assume bulk densities
 Flow characteristics?
 Expected particle size distribution?
 Is solid abrasive and/or corrosive and what is the
moisture/solvent content?
Selection Criteria

 Simple and cheap
 Can be used with any wet
material and not affected by
density.
 Can not be used with pressurized
tanks
 Visual indication only (electronic
versions are available)
RodGauge - similar to a dipstick found in a car, it has weighted line markings to
indicate depth or volume
For Liquids
Dip Stick

Another simple direct
method of measuring
liquids.
Can be used in
pressurized tanks (as
long as the glass or
plastic tube can
handle the pressure)
Good for applications where non-contact measurement is
needed (like beverages)
Sight Glass
For Liquids

Float rides the surface level to provide the measurement. Many
different styles are available. Usually used for pump control,
high/low level alarms and emergency shut-off
Liquid density does not affect measurement
Floats
For Liquids

Point Level
Measurement
Continuous Level
Measurement
Advantages and disadvantages
Low Cost
Conductive, non-coating liquids only
Insulating coatings can cause problems
Conductivity Level Measurement
For Liquids

The pressure of the fluid in the tank causes the tape to short-
circuit, thus changing the total resistance of the measuring
tape. An electronic circuit measures the resistance; it's
directly related to the liquid level in the tank.
Resistance Tape
For Liquids

Bubblers allow the
indicator to be
located anywhere.
The air pressure in the
tube varies with the
head pressure of the
height of the liquid.
Bottom of
tube
determines
reference
point
P
Regulated
purge
system
(air or
nitrogen)
Instrument
input does
not matter
Can’t be used in closed tanks or where purging a liquid is not allowed (soap). Very popular in the paper
industry because the air purge keeps the tube from plugging.
Bubblers
For Liquids

Advantages:
-- Easy installation
-- Continuous reading providing
analogue or digital signal
-- No moving parts
-- Good accuracy and
repeatability
Bottom of
tube
determines
reference
point
P
Regulated
purge
system
(air or
nitrogen)
Instrument
input does
not matter
Bubblers
For Liquids

Limitations:
-- Not suitable for pressurized
tanks
-- Sediments may block tube or
probe
-- Tanks must be freely vented
Bottom of
tube
determines
reference
point
P
Regulated
purge
system
(air or
nitrogen)
Instrument
input does
not matter
Bubblers
For Liquids

 These methods infer level by measuring the
hydrostatic head produced by the liquid column.
 A pressure sensing element is installed at the
bottom of the tank and pressure is converted to
level.
 Different liquid densities or closed tank
applications must be accounted for.
Hydrostatic Head Level Sensors

General Theory for Head Measurement
The Pressure exerted by the
Height of the liquid is:
P = H x Density*
If the Density of the liquid is
known then
H = Pressure
Density*
Height (H)
Pressu
re PSI
Liquid
Density
(D)
*Note: For liquids other than water, use the density of water
0.0361 lb/in3 as a reference and multiply by the SG of the
Hydrostatic Head Level Sensors

Example
Height
(H)
Tan
k 1
PSI
Water
Densit
y (D)
Height
(H)
Tan
k 2
PSI
Oil
Densit
y (D)
A dip stick measurement of the level of these 2 tanks
indicates 30 feet of liquid in both tanks. Calculate the
pressure that each gauge will read if tank 1 contains water
(S.G. = 1) and tank 2 contains oil (S.G. = 0.85)
P = H x Density
= 30 ft x 0.0361 lbs/in3
= (30 x 12) x 0.0361
= 13 psi
P = ? psi

 Non-Contact direct level sensor
 Level is a function of the time it
takes an ultrasonic pulse to hit the
surface and return
Limitations include:
• Surface foam absorbs signal, agitation create reflections
• High Pressure & High Temperatures affect the signal speed
• Vapour and condensate create false echo’s
UltraSonic Level Measurement

Similar to ultrasonic but at a much higher frequency (6.3 GHz)
Various designs
-- Frequency Modulated
Continuous Wave
-- Pulsed Wave
-- Guided Wave
These sensors have better performance in applications where vapour, dust or uneven surfaces exist.
Radar Level Sensors (Microwave)

Summary
• Level is measured by locating the boundary
between two media, called the interface
• Level can be measured directly or indirectly
• Noninvasive devices are preferred when the
material is corrosive, hazardous, sterile, or at
a high temperature or pressure

1. Pressure = Force / Area
1. Pressure can be used inferentially to measure other
variables such as Flow and Level
1. Pressure plays a major role in determining the Boiling
Point of Liquids
1. Fluids exerts pressure on the containing vessel
equally and in all directions
Pressure Measurement

Pressure is commonly quoted as being Absolute or Gauge
Easiest way of thinking
Some Fluid = Some Pressure = Some absolute pressure
No Fluid = No Pressure = Zero absolute pressure
Whereas
Fluid Pressure + Atmospheric Pressure = Some Gauge Pressure
No Fluid + Atmospheric Pressure = Zero Gauge Pressure
Which follows
Gauge Pressure – Atmospheric Pressure = Pressure due to fluid itself = Absolute fluid
pressure
Pressure Measurement

1. Mechanical Methods
1. Electrical Methods
Pressure Measurement Methods

1. Elastic pressure transducers
1. Manometer method
1. Pressure measurement by measuring vacuum
1. Electric pressure transducers
1. Pressure measurement by balancing forces produced on a
known area by a measured force
Pressure Measurement Methods

1. Bourdon tube pressure gauge
1. Diaphragm pressure transducers
1. Bellows
Uses flexible element as sensor. As pressure changed
,the flexible element moved, and this motion was
used to rotate a pointer in front of dail.
Elastic Pressure Transducers

Bourdon tubes are generally are of
three types;
1. C-type
2. Helical type
3. Spiral type
Bourdon Tube Pressure Gauge

Diaphragm are popular because they required less space
and the motion they produce is sufficient for operating
electronic transducers
Diaphragm and Bellows Pressure Gauge

They are used to measure gauge pressures over very low ranges.
 Two types of diaphragm pressure gauges are:
1. Metallic diaphragms gauge
(brass or bronze)
2. Slack diaphragms gauge (Rubber)
Diaphragm Pressure Gauge

Why Electrical Pressure Transducers?
 Transmission requirements for remote display as electric signal
transmission can be through cable or cordless.
 Electric signals give quicker responses and high accuracy in digital
measurements.
 The linearity property of the electric signal produced to pressure
applied favors simplicity.
 They can be used for extreme pressure applications, i.e. high
vacuum and pressure measurements.
 EPTs are immune to hysteresis, shock and mechanical vibrations.
Electric Pressure Transducers

1. Pressure sensing element such as a bellow , a diaphragm or a bourdon tube
1. Primary conversion element e.g. resistance or voltage
1. Secondary conversion element
Electric Pressure Transducers

 Strain gauge pressure transducers
 Capacitive pressure transducers
 Potentiometer pressure transducers
 Resonant Wire pressure transducers
 Piezeoelectric pressure transducers
Types of Electric Pressure Transducers

A strain gauge is a passive type resistance pressure transducer whose electrical
resistance changes when it is stretched or compressed
The wire filament is attached to a structure under strain and the resistance in
the strained wire is measured
Strain Gauge Pressure Transducer

Capacitive Pressure Transducer
C=ε0 εr A/d
Where,
C = the capacitance of a capacitor in farad
A = area of each plate in m2
d = distance between two plates in m
εr= dielectric constant
ε0 = 8.854*10^-12 farad/m2
Thus, capacitance can be varied by changing distance
between the plates, area of the plate or value of the
dielectric medium between the plates. Any change in
these factors cause change in capacitance.
In capacitive transducers, pressure is utilized to vary any of the above mentioned
factors which will cause change in capacitance and that is a measureable by any
suitable electric bridge circuit and is proportional to the pressure.

-- Originally developed for use in low vacuum research
-- Wide rangeability from high vacuum in the micron range to 10,000 psig
-- Differential pressure as low as 0.01 inch can be readable
-- Accurate within 0.1 % of reading or 0.01 % of full scale
-- More Corrosion resistant
Capacitive Pressure Transducer

Potentiometer Pressure Transducer
-- Extremely small and installed in very tight quarters such inside the
housing of 4.5 in dial pressure gauge
-- Provide strong output so no need of additional amplifier
-- Range 5 to 10,000 psig
-- Accurate within 0.5 % and 1 % of full scale

Resonant Wire Pressure Transducer

Resonant Wire Pressure Transducer
-- Used for low differential pressure applications
-- Generates inherently digital signal
-- Sensitive to shock and variation
-- Range :
From Absolute pressure 10 mm Hg
Up to Differential pressure 750 in Water
or Gauge pressure 6000 psig
-- Accuracy 0.1 % of Calibrated Spam

Piezoelectric Pressure Transducer

Piezoelectric Pressure Transducer
-- Signals generated by crystals decays rapidly so unsuitable for static force
or pressure measurements
-- measure rapidly changing pressure resulting from blasts, explosions or
pulsation pressures
-- Range : 5,000 to 10,000 psir
-- Rugged construction, small size and high speed

Where and How have EPTs failed?
 EPTs require a constant supply of electricity for them to
function. They do not come with built-in power supply.
 High performance comes at a cost. Installation of auxiliary
display modules and electrical circuitry increases capital cost.
 Physical properties, like temperature, which can affect electrical
constants may affect the consistency of EPTs.
 For this reason, temperature compensation is always required
with EPTs.
 Some electrical phenomena, like piezolectric energy, have
limited applicability. This limits their use in industry.
 Electricity exposes personnel to potential hazards.

Home Work
INDUCTIVE/RELUCTIVE PRESSURE
TRANSDUCERS

High Pressure and Vacuum Measurement
High pressure designs
-- Can detect pressure up to
10,000 psig and operate up to
8000 degree F
-- The pressure of the output air
signal follows the process
pressure in inverse ratio to the
areas of the two diaphragms.
If the diaphragm area ratio is
200:1, a 1,000-psig increase in
process pressure will raise the air
output signal by 5 psig.

High pressure designs
-- May include as many as
twenty coils
-- can measure pressures well
in excess of 10,000 psig
-- standard element material
is heavy-duty stainless steel
-- measurement error is around
1% of span
-- Suitable for fluctuating
pressure service

Very High pressure
The bulk modulus cell consists of a hollow cylindrical steel probe closed at the inner end with a projecting
stem on the outer end . When exposed to a process pressure, the probe is compressed, the probe tip is
moved to the right by the isotropic contraction, and the stem moves further outward. This stem motion is
then converted into a pressure reading.
detect pressures up to 200,000
psig with 1% to 2% full span
error

-- A basic manometer can consist of a reservoir filled with a
liquid and a vertical tube .
-When detecting vacuums, the top of the column is sealed evacuated.
-- A manometer without a reservoir is simply a U-shaped tube, with
one leg sealed and evacuated and the other connected to the
unknown process pressure
-- The difference in the two column heights indicates the process vacuum.
-- An inclined manometer can consist of a well and transparent
tube mounted at an angle. A small change in vacuum pressure will
cause a relatively large movement of the liquid.
--Manometers are simple, low cost, and can detect vacuums
down to 1 millitorr.

A capacitance sensor operates by measuring the
change in electrical capacitance that results from the
movement of a sensing diaphragm
relative to some fixed capacitance electrodes
Accuracy is typically 0.25 to 0.5% of reading. Thin diaphragms can measure down to 10-5 torr, while
thicker diaphragms can measure in the low vacuum to atmospheric range.

Force, Torque and Tactile
Sensors

Sensor Types
A. Based on power requirement:
1. Active: require external power, called
excitation signal, for the operation
2. Passive: directly generate electrical signal in
response to the external stimulus
B. Based on sensor placement:
1. Contact sensors
2. Non-contact sensors

Force Sensors
 The fundamental operating principles of force,
acceleration, and torque instrumentation are
closely allied to the piezoelectric and strain gage
devices used to measure static and dynamic
pressures.

Force sensors contd…
 Piezoelectric sensor produces a voltage when it is
"squeezed" by a force that is proportional to the
force applied.
 Difference between these devices and static force
detection devices such as strain gages is that the
electrical signal generated by the crystal decays
rapidly after the application of force.
 The high impedance electrical signal generated by
the piezoelectric crystal is converted to a low
impedance signal suitable for such an instrument
as a digital storage oscilloscope.

Force sensors Contd...
 Depending on the application requirements,
dynamic force can be measured as either
compression, tensile, or torque force.
 Applications may include the measurement of
spring or sliding friction forces, chain tensions,
clutch release forces.

Torque Sensors
 Torque is measured by either sensing the actual
shaft deflection caused by a twisting force, or by
detecting the effects of this deflection.
 The surface of a shaft under torque will experience
compression and tension, as shown in Figure.

Torque sensor Contd...
 To measure torque, strain gage elements usually
are mounted in pairs on the shaft, one gauge
measuring the increase in length (in the direction
in which the surface is under tension), the other
measuring the decrease in length in the other
direction.

Force/Torque Measurement
 Force and torque measurement finds
application in many practical and
experimental studies as well as in control
applications.
 Force-motion causality. When measuring
force, it can be critical to understand whether
force is the input or output to the sensor.
 Design of a force sensors relies on deflection,
so measurement of motion or displacement
can be used to measure force, and in this
way the two are intimately related.

Design of a Force Sensor
 Consider a simple sensor that is to be developed to
measure a reaction force at the base of a spring, as
shown below.

 In the force sensor design given, no specific
sensing mechanism was implied. The constraint
placed on the stiffness exists for any type of force
sensor.
 It is clear, however, that the force sensor will have
to respond to a force and provide an output
voltage. This can be done in different ways.
Sensor Mechanisms for Force

Sensing Mechanisms
 To measure force, it is usually necessary to
design a mechanical structure that determines
the stiffness. This structure may itself be a
sensing material.
 Force will induce stress, leading to strain which
can be
detected, most commonly, by
– strain gages (via piezoresistive effect)
– some crystals or ceramics (via piezoelectric
effect)
 Force can also be detected using a
displacement sensor, such as an LVDT.

Strain-gage Force Sensor
Design
 Let’s consider now the force sensor studied
earlier, and consider a design that will use
one strain gage on an axially loaded material.

Strain guages
 Many types of forcetorque sensors are based on
strain gage measurements.
 The measurements can be directly related to stress
and force and may be used to measure other types of
variables including displacement and acceleration

What’s a strain gauge?
 The electrical resistance of a length of wire varies in
direct proportion to the change in any strain applied
to it. That’s the principle upon which the strain gauge
works.
 The most accurate way to measure this change in
resistance is by using the wheatstone bridge.
 The majority of strain gauges are foil types, available
in a wide choice of shapes and sizes to suit a variety
of applications.
 They consist of a pattern of resistive foil which is
mounted on a backing material.

Strain gauge contd..
 They operate on the principle that as the foil is
subjected to stress, the resistance of the foil
changes in a defined way.

Strain gauge Configuration
 The strain gauge is
connected into a
wheatstone Bridge circuit
with a combination of four
active gauges(full
bridge),two guages (half
bridge) or,less commonly, a
single gauge (quarter
bridge).

Guage factor
 A fundamental parameter of the strain guage is its
sensitivity to strain, expressed quantitatively as the
guage factor (GF).
 Guage factor is defined as the ratio of fractional
change in electrical resistance to the fractional change
in length (strain).

Strain guage contd..
 The complete wheatstone brigde is excited with a
stabilized DC supply.
 As stress is applied to the bonded strain guage, a
resistive change takes place and unbalances the
wheatstone bridge which results in signal output with
respect to stress value.
 As the signal value is small the signal conditioning
electronics provides amplification to increase the
signal.

Torque Sensor
 Torque is a measure of the forces that causes an
object to rotate.
 Reaction torque sensors measure static and
dynamic torque with a stationary or non-rotating
transducer.
 Rotary torque sensors use rotary transducers to
measure torque.

Technology
 Magnetoelastic : A magnetoelastic torque sensor
detects changes in permeability by measuring
changes in its own magnetic field.
 Piezoelectric : A piezoelectric material is
compressed and generates a charge, which is
measured by a charge amplifier.
 Strain guage : To measure torque,strain guage
elements usually are mounted in pairs on the
shaft,one guage measuring the increase in length the
other measuring the decrease in the other direction.

Figures showing Torque sensors

Torque Measurement
 The need for torque measurements has led to
several methods of acquiring reliable data from
objects moving. A torque sensor, or transducer,
converts torque into an electrical signal.
 The most common transducer is a strain guage that
converts torque into a change in electrical
resistance.
 The strain guage is bonded to a beam or structural
member that deforms when a torque or force is
applied.

Torque measurement contd..
 Deflection induces a stress that changes its resistance.
A wheatstone bridge converts the resistance change
into a calibrated output signal.
 The design of a reaction torque cell seeks to eliminate
side loading (bending) and axial loading, and is
sensitive only to torque loading.
 The sensor’s output is a function of force and
distance, and is usually expressed in inch-pounds,
foot-pounds or Newton-meters.

Contact/Non-contact methods
 Contact: slip rings are used in contact-type torque
sensors to apply power to and retrive the signal from
strain gages mounted on the rotating shaft.
 Non-contact: the rotary transformer couples the strain
gages for power and signal return. The rotary
transformer works on the same principle as any
conventional transformer except either the primary or
secondary coils rotate.

Applications of force/torque sensors
 In robotic tactile and manufacturing applications
 In control systems when motion feedback is
employed.
 In process testing, monitoring and diagnostics
applications.
 In measurement of power transmitted through a
rotating device.
 In controlling complex non-linear mechanical
systems.

Tactile sensors
Introduction
 Tactile and touch sensor are devices which
measures the parameters of a contact between
the sensor and an object.
 Def: This is the detection and measurement of
the spatial distribution of forces perpendicular
to a predetermined sensory area, and the
subsequent interpretation of the spatial
information.
 used to sense a diverse range of stimulus
ranging from detecting the presence or absence
of a grasped object to a complete tactile image.

Tactile sensors Contd...
 A tactile sensor consists of an array of touch
sensitive sites, the sites may be capable of
measuring more than one property.
 The contact forces measured by a sensor are able
to convey a large amount of information about
the state of a grip.
 Texture, slip, impact and other contact conditions
generate force and position signatures, that can
be used to identify the state of a manipulation.
 This information can be determined by
examination of the frequency domain .

Desirable characteristics of a tactile
sensor
 A touch sensor should ideally be a single-point
contact, though the sensory area can be any
size. In practice, an area of 1-2 mm2 is
considered a satisfactory.
 The sensitivity of the touch sensor is dependent
on a number of variables determined by the
sensor's basic physical characteristic.
 A sensitivity within the range 0.4 to 10N, is
considered satisfactory for most industrial
applications.
 A minimum sensor bandwidth is of 100 Hz.

Characteristics Contd….
 The sensor’s characteristics must be stable and
repeatable with low hysteresis. A linear response is
not absolutely necessary, as information processing
techniques can be used to compensate for any
moderate non-linearities.
 As the touch sensor will be used in an industrial
application, it will need to be robust and protected
from environmental damage.
 If a tactile array is being considered, the majority of
application can be undertaken by an array 10-20
sensors square, with a spatial resolution of 1-2 mm.

Tactile sensor technology
 Many physical principles have been
exploited in the development of tactile
sensors. As the technologies involved are
very diverse, in most cases, the developments
in tactile sensing technologies are application
driven.
 Conventional sensors can be modified to
operate with non-rigid materials.
• Mechanically based sensors
• Resistive based sensors
• Force sensing resistor

Contd…
• Capacitive based sensors
• Magnetic based sensor
• Optical Sensors
• Optical fibre based sensors
• Piezoelectric sensors
• Strain gauges in tactile sensors
• Silicon based sensors
• Multi-stimuli Touch Sensors

Mechanically based sensors
 The simplest form of touch sensor is one where the
applied force is applied to a conventional mechanical
micro-switch to form a binary touch sensor.
 The force required to operate the switch will be
determined by its actuating characteristics and any
external constraints.
 Other approaches are based on a mechanical
movement activating a secondary device such as a
potentiometer or displacement transducer.

Resistive based sensors
 The majority of industrial analogue touch or tactile
sensors that have been used are based on the principle
of resistive sensing. This is due to the simplicity of
their design and interface to the robotic system.
 The use of compliant materials that have a defined
force-resistance characteristics have received
considerable attention in touch and tactile sensor
research.
 The basic principle of this type of sensor is the
measurement of the resistance of a conductive
elastomer or foam between two points.
 The majority of the sensors use an elastomer that
consists of a carbon doped rubber.

Contd…
 In adjacent sensor the
resistance of the
elastomer changes with
the application of force,
resulting from the
deformation of the
elastomer altering the
particle density.

Resistive sensors contd..
 If the resistance measurement is taken between
opposing surfaces of the elastomer, the upper contacts
have to be made using a flexible printed circuit to
allow movement under the applied force.
 Measurement from one side can easily be achieved by
using a dot-and-ring arrangement on the substrate.
 Resistive sensors have also been developed using
elastomer cords laid in a grid pattern, with the
resistance measurements being taken at the points of
intersection.
 Arrays with 256-elements have been constructed.
This type of sensor easily allows the construction of a
tactile image of good resolution.

Disadvantages of The conductive elastomer or foam
based sensor :
 An elastomer has a long nonlinear time constant. In addition the
time constant of the elastomer, when force is applied, is
different from the time constant when the applied force is
removed.
 The force-resistance characteristic of elastomer based sensors
are highly nonlinear, requiring the use of signal processing
algorithms.
 Due to the cyclic application of forces experience by a tactile
sensor, the resistive medium within the elastomer will migrates
over a period of time.
 Additionally, the elastomer will become permanently deformed
and fatigue leading to permanent deformation of the sensor.
This will give the sensor a poor long-term stability and will
require replacement after an extended period of use.

Conclusion
 From these, we can estimate object properties such as
geometry, stiffness, and surface condition.
 This information can then be used to control grasping or
manipulation, to detect slip, and also to create or improve
object models.
 Thus Tactile sensors occupy a primary position in the present
industry to increase the efficiency of the mechanical work
being done.
 Performance monitoring and evaluation, failure detection,
diagnosis, testing depend heavily on measurement of
associated forces and torques.
 These forces and torques present in dynamic systems are
generally functions of time.

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