ME3K Micro Project – Basic Electrical and Electronics Engineering BEE (312020)-Prepare report on Introduction to Sensors and Transducers.

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CLASSIFICATION OF SENSORS

Sensors can be classified into various
groups according to the factors such as measure and, application fields,
conversion principle, energy domain of the measure and thermodynamic considerations.
These general classifications of sensors are well described in the references. Detail
classification of sensors in view
of their applications in manufacturing is as follows.

A.
Displacement, position and proximity sensors

·     Potentiometer

·    
Strain-gauged
element

·    
Capacitive
element

·    
Differential
transformers

·     Eddy current proximity sensors

·     Inductive proximity switch

·     Optical encoders

·    
Pneumatic
sensors

·     Proximity switches(magnetic)

·   Hall effect sensors

B. Velocity and motion

·   Incremental encoder

·   Tacho generator

distance
of a transducer. 1.
Potentiometer Sensors Figure shows the
construction of a rotary type potentiometer sensor employed

to measure the linear displacement. The potentiometer can he of linear or angular type. It works on the principle of conversion of
mechanical

displacement
into an electrical signal. The sensor has a resistive element and a sliding contact (wiper). The slider moves along this
conductive body, acting as a movable electric contact.

The object of whose displacement is to be
measured is connected to the slider by using • a rotating shaft (for angular displacement) •
a moving rod (for linear displacement) • a cable that is kept stretched during
operation There is five element is a wire wound track or conductive plastic .The
track comprises of large number of closely
packed turns of a resistive wire. Conductive plastic is made up of plastic resin embedded with the
carbon powder. Wire wound track has a resolution of the order of ± 0.01 % while the
conductive plastic mav have the
resolution of about 0.1 pm. During the sensing operation, a voltage Vs is applied across the resistive element. A voltage divider circuit is formed
when

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, wood working machinery, printing, spraying,
robotics, etc.

2. Strain Gauges

The strain in an element is a ratio of change in knoll iti   direction of applied

load to the original length ()fan element. The strain changes the resistance R ut the element. Therefore, we can
say, AR/R a E; AR/R = G z where G is the constant of proportionality and is called as
gauge factor. In general, the value of G is considered in between 2 to 4 and the resistances are taken of
the order of 100 Q. Resistance strain gauge follows the principle of change
in resistance as per the equation. It comprises of a pattern of resistive foil
arranged as shown in Figure 5.These foils are made of Constant an alloy
(copper-nicke155-45%alloy)and are bonded to a backing material plastic (polyimide), epoxy
or glass fiber reinforced epoxy. The strain gauges are secured to the work piece by
using epoxy or Cyanoacrylate
cement Eastman910 SL. As the work piece undergoes change in its shape due to external loading, the resistance of
strain gauge element changes. This change in
resistance can be detected by a using a Wheat stones resistance bridge as shown in Figure 6. In the balanced bridge
we can have a relation, R2/ R1 = Rx /
R3 where Rx is resistance of strain gauge element, R2 is balancing/adjustable resistor, RI and R3 are known
constant value resistors. The
measured deformation or displacement by the stain gauge is calibrated against
change in resistance of adjustable resistor R2 which makes the voltage across nodes A and B equal to zero.

Applications
of strain gauges

Strain gauges are widely used in
experimental stress analysis and diagnosis on machines and failure analysis. They are
basically used for multi-axial stress fatigue testing, proof testing, residual stress and
vibration measurement, torque measurement, bending and deflection measurement,
compression and tension measurement and strain measurement. Strain gauges are
primarily used as sensors
for machine tools and safety in auto motives. In particular, they are

employed for force measurement in
machine tools, hydraulic or pneumatic press

and as
impact sensors in aerospace vehicles. 3. Capacitive element based sensor

Capacitive
sensor is of non-contact type sensor and is primarily used to measure

the linear displacements from few
milli meters to hundreds of milli meters. It

comprises of three plates, with
the upper pair forming one capacitor and the

lower pair another. The linear
displacement might take in two forms; a. one of

the plates is moved by the
displacement so that the plate separation changes b.

Area of overlap changes due to the displacement.

The
capacitance C of a parallel plate capacitor is given by,

C = Er co A / d Where Er is the relative
permittivity of the dielectric between the plates, co permittivity of free space, a area
of overlap between two plates and d the plate separation. As the central plate
moves near to top plate or bottom one due to the movement of the element/work piece of which
displacement is to be measured, separation in between the plate changes. This
can be given as,

C I= (Er co A) / (d + x)

C2– (Er co A) / (d x)

When Cl and C2 are connected to a Wheat
stone” s bridge, then the resulting out-of balance voltage would be in
proportional to displacement x. Capacitive element canal so be used as proximity sensor
.The approach of the object towards the sensor plate is used for induction of
change in plate separation. This changes the capacitance which is used to detect the
object.

Applications of capacitive element sensors