Sensors and transducers

Measurement is an important subsystem of a mechatronics system. Its main function is

to collect the information on system status and to feed it to the micro-processor(s) for
controlling the whole system.

Measurement system consists of sensors, transducers and signal processing devices. A

wide variety of these elements and devices are available in the market. For a mechatronics
system designer it is quite difficult to choose suitable sensors/transducers for the desired
application(s). It is therefore essential to learn the principle of working of commonly used
sensors/transducers.

Sensor technology has the following important advantages.

a. Sensors alarm the system operators about the failure of any of the subunits of the
manufacturing system. It helps operators to reduce the downtime of complete
manufacturing systems by carrying out the preventative measures.
b. Reduces requirement of skilled and experienced laborers.
c. Ultra-precision in product quality can be achieved.

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Displacement sensors

Potentiometer Sensors

Applications of potentiometer

These sensors are primarily used in the control systems with a feedback loop to ensure
that the moving member or component reaches its commanded position. These are typically
used on machine-tool controls, elevators, liquid-level assemblies, forklift trucks, automobile
throttle controls. In manufacturing, these are used in control of injection molding machines,
woodworking machinery, printing, spraying, robotics, etc. These are also used in
computer-controlled monitoring of sports equipment.

Capacitive element based sensor

Applications of capacitive element sensors

Feed hopper level monitoring
Small vessel pump control
Grease level monitoring
Level control of liquids

Metrology applications
to measure shape errors in the part being produced
to analyze and optimize the rotation of spindles in various machine tools such as surface
grinders, lathes, milling machines, and air bearing spindles by measuring errors in the
machine tools themselves

Assembly line testing

to test assembled parts for uniformity, thickness or other design features
to detect the presence or absence of a certain component, such as glue etc.

Linear variable differential transformer (LVDT)

Applications of LVDT sensors

Measurement of spool position in a wide range of servo valve applications
To provide displacement feedback for hydraulic cylinders
To control weight and thickness of medicinal products viz. tablets or pills
For automatic inspection of final dimensions of products being packed for dispatch
To measure distance between the approaching metals during Friction welding process
To continuously monitor fluid level as part of leak detection system
To detect the number of currency bills dispensed by an ATM

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Proximity sensors
1. Eddy current proximity sensors

Eddy current proximity sensors are used to detect non-magnetic but conductive
materials. They comprise of a coil, an oscillator, a detector and a triggering circuit. Figure shows
the construction of eddy current proximity switch. When an alternating current is passed thru
this coil, an alternative magnetic field is generated. If a metal object comes in the close proximity
of the coil, then eddy currents are induced in the object due to the magnetic field. These eddy
currents create their own magnetic field which distorts the magnetic field responsible for their
generation. As a result, impedance of the coil changes and so the amplitude of alternating
current. This can be used to trigger a switch at some pre-determined level of change in current.

Figure: Schematic of Inductive Proximity Sensor

Eddy current sensors are relatively inexpensive, available in small in size, highly reliable
and have high sensitivity for small displacements.

Applications of eddy current proximity sensors

● Automation requiring precise location
● Machine tool monitoring
● Final assembly of precision equipment such as disk drives
● Measuring the dynamics of a continuously moving target, such as a vibrating element,
● Drive shaft monitoring
● Vibration measurements

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2. Inductive proximity switch
Inductive proximity switches are basically used for detection of metallic objects. Figure
shows the construction of inductive proximity switch. An inductive proximity sensor has four
components; the coil, oscillator, detection circuit and output circuit. An alternating current is
supplied to the coil which generates a magnetic field. When, a metal object comes closer to the
end of the coil, inductance of the coil changes. This is continuously monitored by a circuit which
triggers a switch when a preset value of inductance change is occurred.

Figure: Schematic of Inductive Proximity Switch

Applications of inductive proximity switches

• Industrial automation: counting of products during production or transfer

• Security: detection of metal objects, arms, land mines

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Tachogenerator

Figure: Principle of working of Techogenerator

Tachogenerator works on the principle of variable reluctance. It consists of an assembly

of a toothed wheel and a magnetic circuit as shown in figure. Toothed wheel is mounted on the
shaft or the element of which angular motion is to be measured. Magnetic circuit comprising of a
coil wound on a ferromagnetic material core. As the wheel rotates, the air gap between wheel
tooth and magnetic core changes which results in cyclic change in flux linked with the coil. The
alternating emf generated is the measure of angular motion. A pulse shaping signal conditioner
is used to transform the output into a number of pulses which can be counted by a counter.

Figure: Construction and working of AC generator

An alternating current (AC) generator can also be used as a techognerator. It comprises

of rotor coil which rotates with the shaft. Figure: shows the schematic of AC generator. The rotor
rotates in the magnetic field produced by a stationary permanent magnet or electromagnet.
During this process, an alternating emf is produced which is the measure of the angular velocity
of the rotor. In general, these sensors exhibit nonlinearity error of about ± 0.15% and are
employed for the rotations up to about 10000 rev/min.

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Temperature sensors

Temperature conveys the state of a mechanical system in terms of expansion or

contraction of solids, liquids or gases, change in electrical resistance of conductors,
semiconductors and thermoelectric emfs. Temperature sensors such as bimetallic strips,
thermocouples, thermistors are widely used in monitoring of manufacturing processes such as
casting, molding, metal cutting etc. The construction details and principle of working of some of
the temperature sensors are discussed in following sections.

Resistance temperature detectors (RTDs)

RTDs work on the principle that the electric resistance of a metal changes due to change in its
temperature. On heating up metals, their resistance increases and follows a linear relationship
as shown in Figure. The correlation is Rt = R0 (1 + αT)

where Rt is the resistance at temperature T (⁰C) and R0 is the temperature at 0⁰C and α is the
constant for the metal termed as temperature coefficient of resistance. The sensor is usually
made to have a resistance of 100 Ω at 0 °C

Figure Behavior of RTD materials [1]

Figure Construction of a Resistance temperature detector (RTD)

Figure shows the construction of a RTD. It has a resistor element connected to a

Wheatstone bridge. The element and the connection leads are insulated and protected by a
sheath. A small amount of current is continuously passing through the coil. As the temperature
changes the resistance of the coil changes which is detected at the Wheatstone bridge.

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RTDs are used in the form of thin films, wire wound or coil. They are generally made of metals
such as platinum, nickel or nickel-copper alloys. Platinum wire held by a high-temperature glass
adhesive in a ceramic tube is used to measure the temperature in a metal furnace. Other
applications are:

● Air conditioning and refrigeration servicing

● Food Processing
● Stoves and grills
● Textile production
● Plastics processing
● Petrochemical processing
● Micro electronics
● Air, gas and liquid temperature measurement in pipes and tanks
● Exhaust gas temperature measurement

Thermistors

Thermistors follow the principle of decrease in resistance with increasing temperature.

The material used in thermistor is generally a semiconductor material such as a sintered metal
oxide (mixtures of metal oxides, chromium, cobalt, iron, manganese and nickel) or doped
polycrystalline ceramic containing barium titanate (BaTiO3) and other compounds. As the
temperature of semiconductor material increases the number of electrons able to move about
increases which results in more current in the material and reduced resistance. Thermistors are
rugged and small in dimensions. They exhibit nonlinear response characteristics.

Figure Schematic of a thermistor

Thermistors are available in the form of a bead (pressed disc), probe or chip. Figure
shows the construction of a bead type thermistor. It has a small bead of dimension from 0.5 mm
to 5 mm coated with ceramic or glass material. The bead is connected to an electric circuit
through two leads. To protect from the environment, the leads are contained in a stainless steel
tube.

Applications of Thermistors

● To monitor the coolant temperature and/or oil temperature inside the engine
● To monitor the temperature of an incubator
● Thermistors are used in modern digital thermostats
● To monitor the temperature of battery packs while charging
● To monitor temperature of hot ends of 3D printers
● To maintain correct temperature in the food handling and processing industry
equipments

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 ● To control the operations of consumer appliances such as toasters, coffee makers,
refrigerators, freezers, hair dryers, etc.

Thermocouple

Thermocouple works on the fact that when a junction of dissimilar metals heated, it
produces an electric potential related to temperature. As per Thomas Seebeck (1821), when two
wires composed of dissimilar metals are joined at both ends and one of the ends is heated, then
there is a continuous current which flows in the thermoelectric circuit.

Figure shows the schematic of the thermocouple circuit. The net open circuit voltage
(the Seebeck voltage) is a function of junction temperature and composition of two metals. It is
given by, ∆VAB = α ∆T

where α, the Seebeck coefficient, is the constant of proportionality.

Generally, Chromel (90% nickel and 10% chromium)–Alumel (95% nickel, 2%

manganese, 2% aluminium and 1% silicon) are used in the manufacture of a thermocouple.

Figure: Schematic of thermocouple circuit

Table shows the various other materials, their combinations and application
temperature ranges.

Table- Thermocouple materials and temperature ranges

Materials Range (ºC) (μV/ºC)
Platinum 30% rhodium/platinum 6% 0 to 1800 3
Chromel/constantan -200 to 1000 63
Iron/constantan -200 to 900 53
Chromel/alumel -200 to 1300 41
Nirosil/nisil -200 to 1300 28
Platinum/platinum 13% rhodium 0 to 1400 6
Platinum/platinum 10% rhodium 0 to 1400 6
Copper/constantan -200 to 400 43

Applications of Thermocouples
● To monitor temperatures and chemistry throughout the steel making process

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● Testing temperatures associated with process plants e.g. chemical production and
petroleum refineries
● Testing of heating appliance safety
● Temperature profiling in ovens, furnaces and kilns
● Temperature measurement of gas turbine and engine exhausts
● Monitoring of temperatures throughout the production and smelting process in the steel,
iron and aluminum industry

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