UNIT III FORCE, MAGNETIC AND HEADING SENSORS

Strain Gage, Load Cell, Magnetic Sensors –types, principle, requirement and advantages: Magneto resistive – Hall
Effect – Current sensor Heading Sensors – Compass, Gyroscope, Inclinometers

VARIABLE RESISTANCE TRANSDUCERS

Methods that involve measurement of change in resistance is preferred to those employing other principles
as both alternating as well as direct current and voltages are suitable for resistance measurements
Resistance of a metal conductor is expressed by
𝝆𝑳
𝑹=
𝑨
R - Resistance 𝜴
L - length of the conductor m
A - Cross sectional area of the conductor m2
ρ - Resistivity of conductor material 𝜴m
The design basis of the electrical resistive transducer involves the method of varying any one quantity involved in
above relationship

STRAIN GAUGES / PIEZORESISTIVE GAUGES

Principle:
If a metal conductor is stretched or compressed, its resistance changes i.e., both length and diameter of
conductor change. Also, there is a change in the value of resistivity of the conductor when it is strained and this
property is called piezo resistive effect. Therefore, resistance strain gauges are also known as piezoresistive gauges

Applications:
 The strain gauges are used for measurement of strain and associated stress in experimental stress analysis
 Construction of various transducer

Types of strain gauge

 Unbonded metal strain gauge
 Bonded metal wire strain gauge
 Bonded metal foil strain gauge
 Vacuum deposited thin metal film strain gauge
 Sputter deposited thin metal strain gauge
 Bonded semiconductor strain gauge
 Diffused metal strain gauge

1. Bonded wire strain gauge:

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  This type is used in both stress analysis and construction of transducer
 It consists of a grid of fine resistance wire of diameter 0.025 mm or less
 This grid is cemented to the base
 Base is made of thin paper sheet (to prevent it from mechanical damage) usually Bakelite, Teflon
 Base is bonded with an adhesive material to the specimen under study
 Wires cannot buckle as they are embedded in matrix of cement
 Spreading of wire permits uniform distribution of stress over the grid
Size varies with application (as small as 3mm by 3mm or larger up to 25mm long and 12.5mm wide)

Resistance wire strain Characteristics:

 Should possess high value of gauge factor – high value of GF indicates large change in R
 should have Low resistance temperature coefficients
 Should not have any hysteresis effect
 Should have linear characteristics - Maintain consistency of calibration over the entire range
Base (Carrier) Material
 Epoxy - 200°C to 150°C
 Bakelite cellulose or fiberglass materials - 200°0 to 300°C for continuous operation
 300°C for limited operation
Adhesive:
 Act as bonding materials
 Bonding depends upon surface preparation and usage of right bonding agent
 Quick drying type is preffered
 Should be insensitive to moisture
 Ethyl cellulose cement, Nitrocellulose cement, Bakelite cement and Epoxy cement
 The temperature ranges up to which they can be used is usually below 175 °C
Leads:
 Should be made up of materials which have low and stable resistivity and low resistance temperature
coefficient

2. Unbonded metal strain gauge

Construction:
 It consists of a wire stretched between two points in an insulating medium such as air
 It made of various copper nickel chrome nickel or nickel iron alloys
 They are about 0.003 mm diameter
 It has a Gauge factor of 2 to 4 and Sustain a force upto 2mN
 Length of wire is 25mm or less
 Flexure element is connected to a diaphragm via rod which is used for sensing pressure

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 Wires are fixed with some initial tension between two frames which can move relative to each other
 This initial tension or preload is necessary, to avoid buckling under compression or negative displacement
Working:
 The preloaded resistance wires are connected in a wheat stones bridge
 At initial preload, the strain and resistance of 4 arms are nominally equal
 The output voltage of the bridge is eo=0
 Application of pressure produce small displacement (0.004mm)
 This displacement increases tension in two wires and decrease in other two
 Thereby increase resistance of two wires which are in tension and decrease the resistance of the remaining two
wires
 This causes unbalance in bridge producing an output voltage which is proportional to the input displacement
and hence to applied pressure
 Electrical Resistance of each arm: 120 to 1000 ohm, Input voltage: 5 to 10V, Output: 20 to 50mV

3. Bonded Metal foil Strain Gauges

 It is an extension of bonded metal wire strain gauge
 Used for Stress analysis and construction of transducers
 Have high Heat dissipation capacity as their surface area is larger
 Can be used for high temperature range
 Large surface area leads to better bonding
 Sensing elements are formed from sheets of less than 0.005mm thick by photo etching process which allows
greater flexibility with regard to shape
 They are mounted on flexible insulating Carrier film
o Thickness: 0.025mm
o Material: polyamide, glass phenolic
 Gauge factor range is 2 to 4
 Smallest gauge size is 0.38mm long\
 Many Adhesives are available
 Gauges and fastening methods are available to cover temperature range -269°C to 816°C

4. Evaporation Deposited Thin Metal Strain Gauges

 Used for the fabrication of transducers
 They are of sputter deposited variety
 Elastic metal element converts the physical quantity into a strain
 All the strain gauge elements are directly attached on the strain surface
Working:
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  The diaphragm is placed in a vacuum chamber with some Insulating material
 Heat is applied until the insulating material vaporises and then condenses, forming a thin dielectric film on
the diaphragm
 Suitably shaped templates are placed over the diaphragm, and the evaporation and condensation processes
are repeated with the metallic- gauge material, forming the desired strain gauge pattern on top of the
insulating substrate

5. Sputter deposited thin metal strain gauge

 Thin dielectric layer is deposited in vacuum over the entire diaphragm surface
 The complete layer of metallic gauge is sputtered on the top of the dielectric material without using any
substrate
 Diaphragms are now removed from the vacuum chamber, and microimaging techniques using photo masking
materials are used ·to form the gauge pattern
 The diaphragms are then returned to the vacuum –chamber
 Sputter etching techniques are used to remove all unmasked metal layer, leaving behind the desired gauge
pattern
 Resistance and gauge factors of film gauges are identical to those of foil gauges
 Since no organic-cementing materials are used, thin-film gauges exhibit a better time and temperature stability
6. Semiconductor strain gauges
 Used where a very high gauge factor and a small envelope are required
 The resistance of the semiconductor changes with change in applied strain
 Unlike in the case of metallic gauges where the change in resistance is mainly due to change in dimensions
when strained, the semiconductor strain gauge depends for their action upon
piezo-resistive effect.
 Semi conducting materials such as silicon and germanium are used as resistive materials
 A typical strain gauge consists of a strain sensitive crystal material and leads are sandwiched in a protective-
matrix
 The production of these gauge employs conventional semiconductor technology using semi conducting wafer
(or) filaments which-have a thickness of 0.05 mm and bonding them on a suitable insulating substrate, such as
Teflon
 Gold leads are generally employed for making the contacts
 These strain gauges can be fabricated along with integrated circuit operational amplifiers which can act as a
pressure sensitive transducers
Advantages
• Have High gauge factor of ±130
• Hysteresis characteristics are excellent
• High fatigue life
• Frequency response is 1012 Hz
• Very small in size
Disadvantages
• Very sensitive to changes in temperature
• Linearity is poor
• Expensive
• Difficult to attach to the object under study

7. Diffused strain gauges

 Primarily used in transducers
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 E.g: In pressure transducer, the diaphragm is of silicon rather than metal and the strain gauge effect would be
realized by depositing impurities in the diaphragm to form an intrinsic strain gauge
 This type of construction may allow lower manufacturing costs in some designs, as a large number of
diaphragms can be made on a single silicon wafer

Rosettes
 combination of strain gauge is called "Rosettes"
 they are available in many combinations for specific stress analysis (or) transducer application
 In general element may be subjected to stress in any direction
 It’s not easy to locate the direction of stress
 this problem was overcome by rosette as it measures strain and stress without knowing the direction

 LOAD CELL:
 LOAD CELL are also called as FORCE METER
 A load cell is a device that converts force into pressure. If the area on which the load is placed is
known then the calibration of force can be done directly.
 Pressure is measured which is proportional to force.
Pα F
P = x.F where x – constant.

 A load cell is a transducer that is used to convert a force into electrical signal. This conversion is
indirect and happens in two stages.
o Stage-1: The force being sensed deforms a strain gauge.
o Stage-2: The strain gauge converts the deformation (strain) to electrical signals

Electronic balance
1. TYPES OF LOAD CELL:

The types of load cells are as follows

I. HYDRAULIC LOAD CELL
II. PNEUMATIC LOAD CELL
III. MAGNETO – ELASTIC LOAD CELL
IV. PIEZO ELECTRIC LOAD CELL
V. STRAIN GAUGE LOAD CELL

4. iv. PIEZO ELECTRIC LOAD CELL

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PRINCIPLE:
 “If dimension of crystal is altered, an electronic potential appears across certain surface of crystal
material due to displacement of charges that induces a output voltage and induces a voltage
proportional to the force applied”.

CONSTRUCTION & WORKING:

 Piezoelectric materials used:
o Lithium sulphate,
o Dipotassium titrate,
o Rochelle salt,
o Ammounium dihydrogen phosphate.
 In a typical quartz-based force sensor, a charge-collection electrode is sandwiched between quartz-
crystal element. The quartz elements are oriented to supply the same polarity voltage to the
electrode when compressed, while the opposite polarity is applied to the sensor housing.
 Any force applied to the piezoelectric sensing element produces a separation of charges within the
atomic structure of the material, generating an electrostatic output voltage. The polarity of the
voltage generated depends on the atomic structure of the material and the direction in which the
force is applied.
 When a force is applied to surface of electrode, the quartz elements generate an output voltage
which can be routed directly to a charge amplifier.
 The magnitude and polarity of induced surface charges are proportional to magnitude and
direction of applied force.
 Piezoelectric force sensors are mostly used for dynamic- force measurements such as oscillation,
impact, or high speed compression or tension.

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STRAIN GAUGE LOAD CELL
PRINCIPLE:

 Strain gauge load cell is also called electromechanical transducer.

 “Change in applied force is converted into change in voltage”.
 When a steel cylinder is subjected to a force, it tends to change in
dimension. If the strain gauges are bounded on the cylinder, when force is
applied strain gauge is stretched or compressed causing a change in
strain gauge dimension along length and diameter. If dimension of strain
gauge is changed, its resistance also changes. The change in resistance is
a measure of applied force.
CONSTRUCTION & WORKING:
 Strain gauge load cell is shown below.
 A cylinder made of steel on which 4 identical strain gauge are mounted on
sides of the cylinder.
 The gauges are connected to a wheat stone bridge circuit.
 The 4 strain gauge are arranged at right angles to each other.
 Under no load condition on cylinder, all 4 gauge will have same
resistance. Hence the wheat stone bridge is balanced and the output
voltage will be zero.
 When force is applied, the gauges along axis Y1 & Y2 will be compressed
and resistance will decrease and the gauges along axis X1 & X2 will be
stretched and resistance will increase.
 When a steel cylinder is subjected to a force, it tends to change in
dimension. If the strain gauges are bounded on the cylinder, when force is
applied strain gauge is stretched or compressed causing a change in
strain gauge dimension along length and diameter. If dimension of strain
gauge is changed, its resistance also changes. The change in resistance is
a measure of applied force.
 Hence variation in resistance of strain gauge unbalance the wheatstone
bridge. The change in resistance is proportional to applied force when
calibrated in terms of force.
 Application:
o Used in vehicle load measurement.
o Used in force dynamometers.

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Used to measure force when the load

HALL EFFECT TRANSDUCER:

Definition: The hall effect element is a type of transducer used

for measuring the magnetic field by converting it into an emf. The direct measurement
of the magnetic field is not possible. Thus, the Hall Effect Transducer is used.
The transducer converts the magnetic field into an electric quantity which is
easily measured by the analogue and digital meters.

Principle of Hall Effect Transducer

• The principle of hall effect transducer is that if the current carrying strip of the conductor is
placed in a transverse magnetic field, then the EMF develops on the edge of the conductor.
The magnitude of the developed voltage depends on the density of flux, and this property of a
conductor is called the Hall effect.
• The Hall effect element is mainly used for magnetic measurement and for sensing the
current.
• The metal and the semiconductor have the property of hall effect which depends on the
densities and the mobility of the electrons.

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Consider the hall effect element shown in the figure above.
The current supply through the lead 1 and 2 and the output is obtained from the strip 3 and 4. The lead 3 and 4
are at same potential when no field is applied across the strip.
When the magnetic field is applied to the strip, the output voltage develops across the output leads 3 and 4. The
develops voltage is directly proportional to the strength of the material.

The output voltage is,

I is the current in ampere and the B is the flux densities. The current and magnetic field
strength both can be measured with the help of the output voltages. The hall effect EMF is
very small in conductors because of which it is difficult to measure. But semiconductors
like germanium produces large EMF which is easily measured by the moving coil
instruments.

Applications of Hall Effect Transducer

1. Magnetic to Electric Transducer – The Hall effect element is used for converting the magnetic flux into an
electric transducer. The magnetic fields are measured by placing the semiconductor material in the measurand
magnetic field. The voltage develops at the end of the semiconductor strips, and this voltage is directly proportional
to the magnetic field density. The Hall Effect transducer requires small space and also gives the continuous signal
concerning the magnetic field strength. The only disadvantage of the transducer is that it is highly sensitive to
temperature and thus calibration requires in each case.
2. Measurement of displacement:

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The Hall effect element measures the displacement of the structural element. For example –
Consider the ferromagnetic structure which has a permanent magnet.
The hall effect transducer placed between the poles of the permanent magnet. The magnetic
field strength across the hall effect element changes by changing the position of the
ferromagnetic field.
. Measurement of Current – The hall effect transducer is also used for measuring the
current without any physical connection between the conductor circuit and meter.

The AC or DC is applied across the conductor for developing the magnetic field. The strength
of the magnetic field is directly proportional to the applied current. The magnetic field
develops the emf across the strips. And this EMF depends on the strength of the conductor.

4. Measurement of Power – The hall effect transducer is used for measuring the power of
the conductor. The current is applied across the conductor, which develops the magnetic field.
The intensity of the field depends on the current. The magnetic field induces the voltage
across the strip. The output voltage of the multiplier is proportional to the power of the
transducer.

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Working principle of Magnetic compass:
The magnetic compass is a device containing a magnetic needle pivoted in a horizontal plane
that indicates North when held on the earth's surface. Use: Magnetic compass is used to find
directions The magnetic compass works with the Earth’s magnetic field principle and shows
the magnetic North and South. The magnetic compass has a magnetised needle, that
can freely rotate in a horizontal plane. Such a magnetic needle tends to settle in the magnetic
meridian.
.

Compasses are mainly used in navigation to find direction on the earth. This works because
the Earth itself has a magnetic field which is similar to that of a bar magnet (see the picture
below). The compass needle aligns with the Earth's magnetic field direction and points north-
south.

Magnets are the objects having a north and south pole at opposite ends. A magnet contains
electrons that have both uneven orbits and uneven spins. Inside each domain, magnetic atoms
align in straight rows. Moreover, all these domains line up in the same direction. We can
easily notice about magnets that opposite poles brought near each other attract, and like poles
repel. Magnets produce forces also called as Magnetic force.

Earth’s magnetic field is very similar to the magnetic field of a giant bar magnet. The Earth
has two magnetic poles, a north, and a south pole similar to a bar magnet. Also with magnetic
poles, the Earth also has two geographic poles. These geographic poles are the points on the
Earth’s surface where the line of the Earth’s axis of rotation meets the surface. We call the
geographic north pole as the true north.

A magnetic compass works because the Earth is like a very big magnet and surrounded by a
huge magnetic field. The Earth has two magnetic poles near the North and South poles. This
magnetic field of the Earth causes a magnetized ‘needle’ of iron or steel to swing freely into a
north-south position. A compass works easily by utilizing the Earth’s magnetism in order to
find directions. After its invention, people become unable to perform navigation over long
distances across the sea. A compass points towards the north because all magnets have two
poles, a north pole, and a south pole.

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The north pole of one magnet is attracted towards the south pole of another magnet. As we
know that the Earth is a magnet which can interact with other magnets in this way. Thus, the
north end of a compass magnet is drawn to align with the Earth’s magnetic field.

Since the Earth’s magnetic North pole attracts the “north” ends of other magnets, so it is
technically the “South Pole” of our planet’s magnetic field.

GYROSCOPE

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CLINOMETER

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