1. MECHANICAL SENSORS
BY
PROF. S. S. BABAR
MECHANICAL ENGINEERING
(INDIRA COLLEGE OF ENGINEERING &
MANAGEMENT, PUNE)
Shekhar_om @rediffmail.com,

2. MECHANICAL SENSORS
A sensor is a device that detects the state of
the environment such as
energy, heat, light, magnet, supersonic, etc.
and convert them to electric signals.

3. SENSOR , TRANSDUCER & ACTUATOR
 Transducer
 a device that converts a primary form of energy into a
corresponding signal with a different energy form
 Primary Energy Forms:
mechanical, thermal, electromagnetic, optical, chemical
, etc.
 take form of a sensor or an actuator
 Sensor (e.g., thermometer)
 a device that detects/measures a signal or stimulus
 acquires information from the “real world”
 Actuator (e.g., heater)
 a device that generates a signal or stimulus

4. SENSOR , TRANSDUCER & ACTUATOR
 Transducer: a device that converts energy from one
form to another
 Sensor: converts a physical parameter to an
electrical output (a type of transducer, e.g. a
microphone)
 Actuator: converts an electrical signal to a physical
output (opposite of a sensor, e.g. a speaker)

5. TYPES OF SENSORS
• Displacement Sensors:
 Resistance, inductance, capacitance, Edd
y current sensors, LVDT
• Proximity sensors
 Pneumatic proximity sensors
• Motion Sensors:
 Optical Encoder
 Incremental , Absolute Encoder

6. Displacement Measurements
 Used to measure directly and indirectly
the size, shape, and position of the object.
 Displacement measurements can be made
using sensors designed to exhibit a
resistive, inductive, capacitive or
piezoelectric change as a function of
changes in position

7. DISPLACETMENT SENSORS
[POTENTIOMETERS]
 Measure linear and angular position
 Resolution a function of the wire construction
 Measure velocity and acceleration

8. INDUCTIVE SENSORS
 Ampere’s Law: flow of electric current will create a
magnetic field
 Faraday’s Law: a magnetic field passing through an
electric circuit will create a voltage
i
v v1
v2
+
-
+
-
2
2
1
1 v
N
N
vN1
N2
dt
d
Nv

9. INDUCTIVE SENSORS
dt
di
Lv GnL 2
n = number of turns of coil
G = geometric form factor
m = effective magnetic permeability of the
medium

10. 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.

11. CAPACITIVE SENSOR (PRINCIPLE)
 If two metal plates are placed with a gap between
them and a voltage is applied to one of the
plates, an electric field will exist between the
plates. This electric field is the result of the
difference between electric charges that are
stored on the surfaces of the plates. Capacitance
refers to the “capacity” of the two plates to hold
this charge. A large capacitance has the capacity
to hold more charge than a small capacitance.
The amount of existing charge determines how
much current must be used to change the voltage
on the plate.

12. CAPACITIVE SENSOR (PRINCIPLE)
Size of the plates: capacitance increases as the plate size increases
Gap Size: capacitance decreases as the gap increases
Material between the plates (the dielectric):
Dielectric material will cause the capacitance to increase or decrease
depending on the material

13. CAPACITIVE SENSORS
x
A
C r0
0 = dielectric constant of free space
r = relative dielectric constant of the
insulator
A = area of each plate
x = distance between plates
Change output by changing r (substance flowing between plates), A
(slide plates relative to each other), or x.

14. CAPACITIVE SENSORS
When the capacitor is
stationary xo the voltage
v1=E.
A change in position
x = x1 -xo produces a
voltage
vo = v1 – E.
dt
dv
ci c
20
x
A
x
C
r

15. LVDT (LINEAR VARIABLE DIFFERENTIAL TRANSFORMER)
Inductive transducer
Translates liner motion into electrical signals
+
-
+
-
a) As x moves through the null position, the phase changes 180
, while the magnitude of vo is proportional to the magnitude of x.
a phase-sensitive demodulator is required.

16. LVDT
 Variable inductance sensors for linear displacement
measurement
 Three symmetrically spaced coils wound
 Series opposing circuit
 A single primary winding
 Two secondary windings wound on a former –
equal turns – placed on either side of primary
winding
 Primary winding – connected to AC
 Movable soft iron core is placed inside the former

17. A single primary winding
Two secondary windings wound on a former –
equal turns – placed on either side of primary
winding
Primary winding – connected to AC
Movable soft iron core is placed inside the former

18. WORKING
 Core Made of high permeability, annealed nickel
hydrogen – gives high sensitivity, low null voltage-
 slotted longitudinally – reduces eddy I losses
 Entire assembly – in stainless steel housing
 At normal (NULL) position, flux in both coils equal-
so E =0
 If core is moved left E1 > E2 ( depends on flux
linkage). E in phase with primary Voltage
 If core is moved right E1 < E2 , E –out of phase

19. LVDT( ADVANTAGES & DISADAVANTAGES)
 High range (from 1.25mm
to 250 mm)
 Low power consumption
(<1 w)
 High sensitivity
 Frictionless device
 Tolerant to shocks &
vibrations
 Immunized to external
effects
 Large displacement
required for small o/p
 Sensitive to stray
magnet
 Performance affected
by temp
 Limited dynamic
response
ADVANTAGES DISADVANTAGES

20. APPLICATIONS OF LVDT
 LVDT is used to measure displacement ranging
from fraction millimeter to centimeter.
 Acting as a secondary transducer, LVDT can be
used as a device to measure force,weight and
pressure, etc..

21. ENCODER
 What is an encoder?
An encoder is a sensor for converting rotary
motion or position to a series of electronic
pulses

22. ENCODERS
 A rotary encoder, also called a shaft encoder, is an
electro-mechanical device that converts the angular
position or motion of a shaft or axle to an analog or
digital code.
 The output of absolute encoders indicates the
current position of the shaft, making them angle
transducers.
 The output of incremental encoders provides
information about the motion of the shaft, which is
typically further processed elsewhere into
information such as speed, distance, and position.

23. OPTICAL ENCODER
 An encoder is a sensor for converting rotary
motion or position to a series of electronic
pulses
 Incremental Optical Encoders : Optical
incremental encoders are linear/angular
position sensors that use light and optics to
sense motion.

24. ABSOLUTE ENCODERS
•Absolute encoders have a unique code that can be
detected for each angular position
•Absolute encoders are much more complex and
expensive than incremental encoders

25. ABSOLUTE ENCODERS
 Every position of an absolute encoder
is unique. Unlike an incremental
encoder, where position is determined
by counting pulses from a zero mark
or home base, the absolute encoder
reads a system of coded tracks to
establish position information. No two
positions are alike..
 Since each position is unique, true
position verification is available as
soon as power is up. It is not
necessary to initialize the system by
returning to home base.

26. ABSOLUTE ENCODERS
 An absolute encoder disk features several concentric
tracks, each consisting of a pattern of transparent and
opaque segments. These independent tracks provide a
unique combination of absolute values for each position. The
coded format is a variation of Binary code called Gray code.

27. INCREMENTAL ENCODER
 Measures instantaneous angular position to some arbitrary
datum but unable to give any indication about the absolute
position of shaft
 Pulses from LEDS are counted to provide rotary position. Two
detectors are used to determine direction (quadrature. Index
pulse used to denote start point Otherwise pulses are not
unique

28. OPTICAL INCREMENTAL ENCODER

29. OPTICAL INCREMENTAL ENCODER

30. INCREMENTAL ENCODER WITH TWO SENSORS

31. APPLICATIONS
Encoders are wildly used in industry
 machine tools
 textile machinery
 printing presses
 wood working machines
 handling technology
 conveying and storage technology
 robotics

32. APPLICATIONS
 While a lead screw or rack-and-pinion converts rotary
motion to linear motion, an encoder converts the same
motion into electronic pulses. The pulses typically are
used as input signals for counters, PLCs, or numerical-
control equipment
 Roll or sheet materials need to be measured during
transport through converting or cut-to-length machinery.
An encoder, when combined with a measuring wheel or
coupled to a roller, will produce electronic pulses equal to
units of length. Since fractional units may be
measured, very precise operation is possible