The transducer whose resistance varies because of the environmental effects such type of transducer is known as the resistive transducer. The change in resistance is measured by the ac or dc measuring devices. The resistive transducer is used for measuring the physical quantities like temperature, displacement, vibration etc.
The measurement of the physical quantity is quite difficult. The resistive transducer converts the physical quantities into variable resistance which is easily measured by the meters. The process of variation in resistance is widely used in the industrial applications.
The resistive transducer can work both as the primary as well as the secondary transducer. The primary transducer changes the physical quantities into a mechanical signal, and secondary transducer directly transforms it into an electrical signal.
Working Principle of Resistive Transducer
The resistive transducer element works on the principle that the resistance of the element is directly proportional to the length of the conductor and inversely proportional to the area of the conductor. equation-1
Where R – resistance in ohms.
A – cross-section area of the conductor in meter square.
L – Length of the conductor in meter square.
ρ – the resistivity of the conductor in materials in ohm meter.
The resistive transducer is designed by considering the variation of the length, area and resistivity of the metal.
Applications of Resistive Transducer
The following are the applications of the resistive transducer.
Potentiometer – The translation and rotatory potentiometer are the examples of the resistive transducers. The resistance of their conductor varies with the variation in their lengths which is used for the measurement of displacement.
Strain gauges – The resistance of their semiconductor material changes when the strain occurs on it. This property of metals is used for the measurement of the pressure, force-displacement etc.
Resistance Thermometer – The resistance of the metals changes because of changes in temperature. This property of conductor is used for measuring the temperature.
Thermistor – It works on the principle that the temperature coefficient of the thermistor material varies with the temperature. The thermistor has the negative temperature coefficient. The Negative temperature coefficient means the temperature is inversely proportional to resistance.
2. Transducer
An electrical transducer is a device which is capable of converting physical quantities
into a proportional electrical quantity such as voltage or electric current.
Hence it converts any quantity to be measured into a usable electrical signal.
This physical quantity which is to be measured can be pressure, level, temperature,
displacement etc.
8/30/2020 2
3. Contd..
The output which is obtained from the transducer is in the electrical
form and is equivalent to the measured quantity.
For example, a temperature transducer will convert temperature to an
equivalent electrical potential. This output signal can be used to control
the physical quantity or display it
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4. Resistive transducer
The resistive transducers are also known as resistive sensors or
variable resistance transducers.
These transducers are most frequently used for calculating
different physical quantities like pressure, vibration, temperature,
force, and displacement.
These transducers work in both primary as well as secondary.
8/30/2020 4
5. Contd..
But generally, these are used as secondary because the primary
transducer’s output can work as an input to the resistive transducer.
The output which is attained from it is adjusted against the amount of
input & it provides the input value directly.
8/30/2020 5
6. Definition
The resistive transducer can be defined as; the resistance of a transducer
can be changed due to the effects of the environment.
Here, the resistance change can be calculated with the help of
measuring devices like AC or DC.
The main purpose of this transducer is to measure physical quantities
such as vibration, displacement, temperature, etc.
8/30/2020 6
7. Contd..
This transducer works on both the
primary & the secondary.
The primary transducer converts
the physical quantities to a
mechanical signal whereas the
secondary transducer converts to
an electrical signal directly.
8/30/2020 7
8. Principle and working
The working of a resistive transducer is explained with the the conductor
rod.
These transducers work on the principle of the length of a conductor
which is directly proportional to the conductor’s resistance & it is
inversely proportional to the conductor’s area.
So, the denominated length of the conductor is ‘L’, the area is ‘A’ and
resistance is ‘R’ and the resistivity is ‘ρ’.
It is stable for every material which is used in conductor construction.
8/30/2020 8
9. Contd..
R = ρL/A
From the above equation,
‘R’ is the resistance of the conductor.
‘A’ is the side view part of the conductor.
‘L’is the conductor’s length.
‘ρ’ is the resistivity of the conductor.
8/30/2020 9
10. Contd..
The transducer’s resistance can be changed because of the exterior
environmental factors as well as the conductor’s physical properties.
The change in resistance can be measured using AC devices or DC
devices. This transducer acts like a primary as well as the secondary
transducer.
A primary transducer is used to change the physical quantity to the
mechanical signal whereas a secondary transducer is used to convert a
mechanical signal to an electrical signal
8/30/2020 10
11. Resistive Transducer Circuit
The best example of this circuit is the
sliding contact device.
The sliding contact of this transducer
mainly includes a long conductor whose
length can be changed.
One side of the conductor is connected
whereas another side of the conductor
can be connected to a brush/slider
which moves through the conductor’s
full-length. 8/30/2020 11
12. Contd..
The displacement of the object can be calculated by connecting it to the
slider. Whenever energy is given to the object for moving them from its
first position, then the slider moves with the conductor’s length.
So the length of the conductor will change to reflect on modify within
the resistance of the conductor.
A transducer like a potentiometer works on the sliding contact type
principle which is used to calculate linear & angular displacement.
8/30/2020 12
13. Applications of Resistive Transducer
The applications of resistive transducer include potentiometer, resistance
thermometer, strain gauges, thermistor, etc.
These transducers are mainly used to calculate the temperature in
several applications.
The applications of resistive transducer include potentiometer, resistance
thermometer, strain gauges, thermistor, etc.
These transducers are used to measure displacement.
8/30/2020 13
14. Contd..
The best examples of this transducer are potentiometers like rotator & translation. The resistance
of these can be changed with the deviation within their lengths to measure the displacement.
The semiconductor material’s resistance can be changed when the strain happens on it. This
property can be used to measure force, displacement, and pressure, etc.
The metal’s resistance can be changed due to temperature change. So this property can be used
to calculate the temperature.
The working principle of this is the thermistor materials temperature coefficient can be changed
by the temperature. The temperature coefficient of the thermistor is negative which means this is
inversely proportional to resistance
8/30/2020 14
15. Advantages of Resistive Transducer
These transducers give quick responses.
These are available in different sizes and they have high resistance.
The voltage otherwise current for both the AC & DC is suitable for calculating
variable resistance.
They are low-cost.
8/30/2020 15
16. Contd..
The operation of these transducers is very easy and used in various
applications wherever the necessities are not mostly severe.
These are used to measure the huge amplitudes of displacement.
Its electrical efficiency is extremely high and gives adequate output to let
control operations.
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17. Disadvantages
When using these transducers, huge power is necessary to
move the sliding contacts.
The sliding contacts can exhaust, become uneven and
produce noise.
8/30/2020 17
18. Strain gauge
Strain Gauge or Strain Gage was invented in 1938 by Edward E. Simmons and Arthur C.
Ruge.
Strain gauge is a sensor whose resistance varies with applied force; It converts
force, pressure, tension, weight, etc., into a change in electrical resistance which
can then be measured.
When external forces are applied to a stationary object, stress and strain are the
result.
Stress is defined as the object's internal resisting forces, and strain is defined as
the displacement and deformation that occur. 8/30/2020 18
19. Contd..
The strain gauge is one of the most important sensor of the electrical
measurement technique applied to the measurement of mechanical
quantities.
As their name indicates, they are used for the measurement of strain.
As a technical term "strain" consists of tensile and compressive strain,
distinguished by a positive or negative sign. Thus, strain gauges can be
used to pick up expansion as well as contraction.
8/30/2020 19
20. Contd..
Any basic strain gauge consists of an insulating flexible backing that
supports a metallic foil pattern.
The gauge is attached to the object under stress using an adhesive. The
deformation in the object causes the foil to get distorted which
ultimately changes the electrical resistivity of the foil.
This change in resistivity is measured by a Wheatstone bridge which is
related to strain by a quantity called, Gauge Factor. 8/30/2020 20
21. Working of strain gauge
A strain gauge depends on the electrical resistivity of any conductor. The
resistance in any conducting device is dependent on its length as well as the
cross-section area.
Suppose L1 is the original length of wire and L2 is the new length after an
external force is applied on it, the strain (ε) is given by the formula:
ε = (L2-L1)/L1
Now, whenever an external force changes the physical parameters of an
object, its electrical resistivity also changes. A strain gauge measures this
deformity by using the Gauge Factor formula.
8/30/2020 21
22. Contd..
In the case of real-life monitoring, while constructing concrete structures
or monuments, the load is applied at the load application point of a load
cell that consists of a strain gauge underlying it.
As soon as the force is exerted, the strain gauge is deformed and, this
deformation causes a change in its electrical resistance which ultimately
changes the output voltage.
8/30/2020 22
23. Gauge factor
The Gauge Factor is the sensitivity coefficient of strain gauges and, is given by the formula:
GF = [ΔR / (RG * ε)]
Where,
ΔR = Change in the resistance caused due to strain
RG = resistance of the undeformed gauge
ε = Strain
The gauge factor for common metallic foil is usually a little over 2. The output voltage of the
Wheatstone Bridge, SV is given by the formula:
SV = {EV x [(GF x ε)/4]}
Where,
EV is the bridge excitation voltage
8/30/2020 23
24. Gauge factor
Metal foil strain gauge - 2–5
Thin-film metal (e.g. constantan)- 2
Single crystal silicon -125 to + 200
Polysilicon ±30
p-type Ge 102
Thick Film Resistors 100
8/30/2020 24
25. Contd..
Strain gauges work on the principle of the conductor’s resistance which
gives you the value of Gauge Factor by the formula:
GF = [ΔR / (RG * ε)]
The change in the strain of an object is a very small quantity which can
only be measured using a Wheatstone Bridge
8/30/2020 25
26. Contd..
A Wheatstone Bridge is a network of
four resistors with an excitation voltage,
Vex that is applied across the bridge.
The Wheatstone Bridge is the electrical
equivalent of two parallel voltage divider
circuits with R1 and R2 as one of them
and R3and R4as the other one.
The output of the Wheatstone circuit is
given by:
Vo = [(R3/R3+R4) — (R2/R1+2)] * Vex
8/30/2020 26
27. Contd..
Whenever R1/ R2 = R4/ R3, the output voltage Vo is zero and the bridge is said
to be balanced.
Any change in the values of R1, R2, R3, and R4 will, therefore, change the
output voltage.
Replacing the R4 resistor with a strain gauge, even a minor change in its
resistance will change the output voltage Vex which is a function of strain
8/30/2020 27
28. Strain gauge configuration
The three types of strain gage configurations quarter- half-, and full-
bridge, are determined by the number of active elements in the
Wheatstone bridge, the orientation of the strain gages, and the type of
strain being measured.
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29. Quarter-Bridge Strain Gage-Configuration Type I
Measures axial or bending strain
Requires a passive quarter-bridge
completion resistor known as a
dummy resistor
Requires half-bridge completion
resistors to complete the
Wheatstone bridge
R4 is an active strain gage
measuring the tensile strain (+ε)
8/30/2020 29
30. Quarter-Bridge Strain Gage-Configuration Type II
Ideally, the resistance of the strain
gage should change only in response
to applied strain. However, strain gage
material, as well as the specimen
material to which the gage is applied,
also responds to changes in
temperature.
The quarter-bridge strain gage
configuration type II helps further
minimize the effect of temperature by
using two strain gages in the bridge.
8/30/2020 30
31. Contd..
Typically one strain gage (R4) is active and a second strain gage(R3) is mounted
in close thermal contact, but not bonded to the specimen and placed
transverse to the principal axis of strain.
Therefore the strain has little effect on this dummy gage, but any temperature
changes affect both gages in the same way.
Because the temperature changes are identical in the two strain gages, the ratio
of their resistance does not change, the output voltage (Vo) does not change,
and the effects of temperature are minimized
8/30/2020 31
32. Half-Bridge Strain Gage-Configuration Type I
Measures axial or bending strain
Requires half-bridge completion resistors
to complete the Wheatstone bridge
R4 is an active strain gage measuring the
tensile strain (+ε)
R3 is an active strain gage compensating
for Poisson’s effect (-νε)
This configuration is commonly confused
with the quarter-bridge type II
configuration, but type I has an active R3
element that is bonded to the strain
specimen. 8/30/2020 32
33. Half bridge strain gauge –Configuration Type II
Measures bending strain only
Requires half-bridge completion resistors
to complete the Wheatstone bridge
R4 is an active strain gage measuring the
tensile strain (+ε)
R3 is an active strain gage measuring the
compressive strain
(-ε)
8/30/2020 33
34. Full bridge strain gauge
A full-bridge strain gage configuration has four active strain gages and is
available in three different types.
Types 1 and 2 measure bending strain and type 3 measures axial strain.
Only types 2 and 3 compensate for the Poisson effect, but all three
types minimize the effects of temperature.
8/30/2020 34
35. Configuration Type I
Highly sensitive to bending strain only
R1 and R3 are active strain gages
measuring compressive strain (–e)
R2 and R4 are active strain gages
measuring tensile strain (+e)
8/30/2020 35
36. Configuration type II
Sensitive to bending strain only
R1 is an active strain gage measuring the
compressive Poisson effect (–νe)
R2 is an active strain gage measuring the tensile
Poisson effect (+νe)
R3 is an active strain gage measuring the
compressive strain (–e)
R4 is an active strain gage measuring the tensile
strain (+e)
8/30/2020 36
37. Configuration type III
Measures axial strain
R1 and R3 are active strain gages
measuring the compressive
Poisson effect
(–νe)
R2 and R4 are active strain gages
measuring the tensile strain (+e)
8/30/2020 37
38. Characteristics of strain gauges
They are highly precise and don’t get influenced due to temperature changes.
However, if they do get affected by temperature changes, a thermistor is
available for temperature corrections.
They are ideal for long distance communication as the output is an electrical
signal.
Strain Gauges require easy maintenance and have a long operating life.
The production of strain gauges is easy because of the simple operating
principle and a small number of components.
8/30/2020 38
39. Contd..
The strain gauges are suitable for long-term installation. However,
they require certain precautions while installing.
All the strain gauges produced by Encardio-Rite are hermetically
sealed and made up of stainless steel thus, waterproof.
They are fully encapsulated for protection against handling and
installation damage.
The remote digital readout for strain gauges is also possible. 8/30/2020 39
40. Strain gauge based displacement transducer
There are various transducers for displacement measurement like Linear
variable differential transformer (LVDT), capacitive transducer,
potentiometric transducer, resistive transducer, optical transducers etc.
The LVDT is most common among these due to its high output for small
displacement
8/30/2020 40
41. Linear Variable Differential Transformer
Linear Displacement Measurement
Linear displacement is movement in one
direction along a single axis.
A position or linear displacement sensor is
a device whose output signal represents the
distance an object has traveled from a
reference point.
A displacement measurement also
indicates the direction of motion.
A linear displacement typically has units of
millimeters (mm) or inches (in.) and a
negative or positive direction associated
with it 8/30/2020 41
42. Principle of LVDT
Linear variable differential transformers (LVDT)
are used to measure displacement. LVDTs
operate on the principle of a transformer.
LVDT consists of a coil assembly and a core.
The coil assembly is typically mounted to a
stationary form, while the core is secured to the
object whose position is being measured.
The coil assembly consists of three coils of wire
wound on the hollow form.
8/30/2020 42
43. Contd..
A core of permeable material can slide freely through the center of the
form.
The inner coil is the primary, which is excited by an AC source as shown.
Magnetic flux produced by the primary is coupled to the two secondary
coils, inducing an AC voltage in each coil.
8/30/2020 43
44. LVDT Measurement
An LVDT measures displacement by associating a
specific signal value for any given position of the
core.
This association of a signal value to a position
occurs through electromagnetic coupling of an AC
excitation signal on the primary winding to the core
and back to the secondary windings.
The position of the core determines how tightly the
signal of the primary coil is coupled to each of the
secondary coils.
8/30/2020 44
45. Contd..
The two secondary coils are series-opposed, which means wound in series but in
opposite directions.
This results in the two signals on each secondary being 180 deg out of phase.
Therefore phase of the output signal determines direction and its amplitude,
distance.
The core causes the magnetic field generated by the primary winding to be
coupled to the secondaries
8/30/2020 45
46. Contd..
When the core is centered perfectly between both secondaries and the
primary, the voltage induced in each secondary is equal in amplitude and 180
deg out of phase.
Thus the LVDT output (for the series-opposed connection shown in this case) is
zero because the voltages cancel each other.
8/30/2020 46
47. Contd..
Displacing the core to the left causes
the first secondary to be more
strongly coupled to the primary than
the second secondary.
The resulting higher voltage of the
first secondary in relation to the
second secondary causes an output
voltage that is in phase with the
primary voltage.
8/30/2020 47
48. Contd..
Displacing the core to the right causes
the second secondary to be more
strongly coupled to the primary than the
first secondary.
The greater voltage of the second
secondary causes an output voltage to
be out of phase with the primary
voltage.
8/30/2020 48
49. Contd..
The LVDT closely models an ideal zeroth-order
displacement sensor structure at low frequency,
where the output is a direct and linear function
of the input.
It is a variable-reluctance device, where a
primary center coil establishes a magnetic flux
that is coupled through a center core (mobile
armature) to a symmetrically wound secondary
coil on either side of the primary.
Thus, by measurement of the voltage amplitude
and phase, one can determine the extent of the
core motion and the direction, that is, the
displacement.
8/30/2020 49
50. Contd..
The linearity of the device was shown within a range of core displacement.
The output is not linear as the core travels near the boundaries of its range.
This is because less magnetic flux is coupled to the core from the primary.
However, because LVDTs have excellent repeatability, nonlinearity near the
boundaries of the range of the device can be predicted by a table or
polynomial curve-fitting function, thus extending the range of the device.
8/30/2020 50
51. Advantages of LVDT
The main advantage of the LVDT transducer over other types of displacement transducer is
the high degree of robustness.
Because there is no physical contact across the sensing element, there is no wear in the
sensing element.
Because the device relies on the coupling of magnetic flux, an LVDT can have infinite
resolution.
Therefore the smallest fraction of movement can be detected by suitable signal
conditioning hardware, and the resolution of the transducer is solely determined by the
resolution of the data acquisition system.
8/30/2020 51
52. KFGT-Strain gauge as temperature sensor
The KFGT gages are foil strain gages incorporating a T-type thermocouple for
simultaneous measurement of strain and temperature.
They ensure not only efficient strain measurement under environments where
temperature change or temperature gradient requires simultaneous
measurement of strain and temperature but also highly precise compensation of
thermally-induced apparent strain.
It is recommended to use KYOWA data logger UCAM-60B as a mating
instrument.
8/30/2020 52
53. Biomedical Applications of Strain Gauge
CT Scan Machines This technology has highly
consistent table positioning, accurate movement of
the CT scan imaging device and equal patient weight
distribution.
High accuracy is needed to perform imaging functions
while preventing over-travel of the patient placed
within the scanning tube.
A multi-axis strain gauged sub-assembly is very
effective in ensuring smooth reliable movement and
table positioning while adjusting for weight
distribution 8/30/2020 53
54. Mammography Machines
These are commonly used for the detection of breast
tumors and related conditions.
For this kind of application, a medical equipment OEM
needed to monitor the amount of physical force that is
applied to the patient by the machine itself when attempting
to take an image.
For this triaxial and dual strain gauge sensors were used
along with a redundant multi-axis sensor.
Other successful medical machinery position and motion
applications of HBM strain gauge technology include
operating table adjustments, chiropractic beds and dental
surgery chairs.
The use of dual and triaxial strain gauge force sensors can
help achieve the highest possible image resolution and
patient comfort during a mammogram. 8/30/2020 54
55. Patient Lift Systems
Motorized lift systems are commonly used to move or transfer patients
from their beds to gurneys or wheelchairs as well as to turn patients to
avoid pressure ulcers or reduce pneumonia.
A major medical equipment OEM incorporated a custom strain gauge
force sensing assembly within the lift system bed handle that helped
them achieve better control over system rate of movement.
8/30/2020 55
56. Medical Weighing
A recent example of a successful OEM medical weighing application
developed at HBM is the incorporation of a load cell into a metal plate
on a baby scale.
The subassembly was manufactured by HBM and the customer simply
supplied the top plate to form an entire scale.
8/30/2020 56
57. Remote Robotics Surgeries
Robotic methodologies are being used for orthopedic surgery.
This is major medical equipment OEM required to be able to precisely
measure both depth of force and drill bit rotational force during remote
hip surgeries.
This highly challenging application requirement caused HBM to design
and manufacture an array of multi-axis custom strain gauge sensor
subassemblies in both tension and compression modes to measure
downward and upward force and motion.
Another strain gauge sensor was mounted at right angles to measure full
deflection and drilling motion consistency ensuring reliable patient
positioning on the operating table. 8/30/2020 57
58. Insulin Pump Fluid Flow Monitoring
HBM was approached by a medical device OEM to develop a strain
gauge to monitor and control the fluid output of an insulin pump.
For this a highly rugged, precise and lightweight strain gauge sensing
technology was required.
An innovative subminiature strain gauge sensor assembly was designed
by HBM, positioned and strategically weakened to enable both manual
and automatic control of insulin delivery
8/30/2020 58
59. Medical Infusion Pumps/Syringe Pumps
A series of strain gauges were developed by HBM for fluid flow
monitoring of a medical infusion pump to monitor and control fluid flow
of intravenous medication that was to be received via the tubing clamp.
For this application, HBM designed and developed a unique 1.5 lbf strain
gauge sensor assembly, positioned and strategically weakened to form a
blade-shaped configuration.
8/30/2020 59
60. Kidney Dialysis Machines
Strain gauge sensing technology is used in kidney dialysis machines to
ensure uniform fluid flow and circulation of proper rate, proportion and
frequency according to the parameters set by its accompanying
electronic controller device.
HBM has designed a variety of custom strain gauge sensor assemblies,
with accompanying load cell technologies for canister weight and
measurement.
Reference:Hottinger Baldwin Messtechnik GmbH
8/30/2020 60
