-
1. Briefly define the following terms
1)
Transducer
A
transducer
is
a
device
that
converts
a
signal
from
one
physical
form to a corresponding signal having a
different physical form .
2)
Sensor
A
sensor
converts
a
physical
signal
into
an
electrical
signal
(i.e.,
a
microphone).
3)
Actuator
An actuator is a
device that converts electrical energy into
physical energy (i.e.,
a loudspeaker).
4)
Linearity
The linearity describes the closeness
between the calibration curve and a specified
straight
line.
5)
Sensitivity
The sensitivity is
defined
in
terms
of the relationship
between input physical
signal
and
output
electrical
signal.
It
is
generally
the
ratio
between
a
small
change in
electrical signal to a small change in physical
signal.
The sensitivity is
the slope of the calibration
curve.
6)
Hysteresis
The hysteresis
refers to the difference between two output values
that
correspond to the same input,
depending on the direction (increasing or
decreasing) of successive input values.
That is, similarly to the magnetization
in ferromagnetic materials, it can
happen that the output corresponding to a
given input depends on whether the
previous input was higher or lower than the
present one.
Some sensors do
not return to the same output value when the input
stimulus is
cycled up or down. The
width of the expected error in terms of the
measured
quantity is defined as the
hysteresis.
7)
Repeatability
The repeatability is the
closeness of agreement between successive results
obtained with the same method under the
same conditions and in a short time
interval.
δ
R
?
(
2
~
3
)
σ
?
100
%
y
FS
δ—
sample
standard deviation
8)
Strain
(mechanical)
Fractional change in
length ΔL/L.
9)
Gage factor
The gage factor
is defined as the fractional change in resistance
divided by the
strain.
10)
Piezoresistive effect
The change in resistivity
as a result of a mechanical stress is called the
piezoresistive effect.
11)
direct piezoelectric effect.
the
phenomenon of generation of a voltage under
mechanical stress is referred to as the
piezoelectric effect.
converse piezoelectric effect.
The mechanical strain
produced in the crystal under electric stress is
called the converse
piezoelectric
effect.
Numerical Aperture
The
ultimately how much
remains in the fiber, and is referred to as the
numerical aperture.
Extrinsic sensor
The optical
fiber plays no part in achieving the modulating
but simply acts as a
transmission
medium these are extrinsic sensors.
Intrinsic sensors (fiber optic sensor)
The optical fiber plays a major role in
modulating the energy from the source; these are
referred to as intrinsic sensors.
Humidity
12)
13)
14)
15)
16)
a quantity
representing the amount of water vapor in the
atmosphere or a gas
17)
Absolute
humidity
Absolute humidity is the mass
of water vapor per unit volume of air.
18)
Relative
humidity
The ratio of the actual vapor
density
to the theoretical
maximum (saturation) vapor
density at
the same temperature, expressed as a percentage.
The relative humidity is the
ratio of
the actual vapor pressure to the saturation vapor
pressure at given temperature.
19)
Peltier
effect
When two dissimilar metals are
connected together, a small voltage called a
thermojunction voltage is generated at
the junction. This is called the Peltier effect.
20)
Law of
Homogeneous Conductors
For
a
given
pair
of homogeneous conductors
forming
a
closed
loop,
the
Seebeck
emf
depends only on the
temperatures of the junctions, and not on the
temperature distribution
along the
length of the conductors.
21)
Law of
intermediate metals
A third
(intermediate) metal wire can be inserted in
series with one of the wires without
changing the voltage reading (provided
that the two new junctions are at the same
temperature).
If there is a
third metal introduced into the thermocouple
circuit , it will not adversely
effect
the reading, if and only if the two junctions of
the third metal are at the same
temperatures .
22)
Bernoulli’s
theorem
Bernoulli’s
equation states
that energy is approximately conserved across a
constriction in
a pipe.
Bernoulli’s equation
:
P/(ρ?g) + ?v2/g + y =
constant
(
ρ
=density;
g=acceleration
of gravity
v=fluid
velocity;
y=elevation )
2.
Describe the following devices and how they work
1)
Strain gage
The strain gauge usually consists of
wire, baking, thinpaper,
and lead welded. The wire
is arranged
in the form of a grid in order to obtain higher
resistances.
2)
Parallel plate
Capacitive
Sensor
The
parallel
plate
Capacitive
Sensor
is
a
function
of
the
distance
d
(cm)
between
the
electrodes of a structure, the surface
area A (cm2) of the electrodes, and the
permittivity
ε0
?
12
(
8
.
8
5
?
10
F/m
for
air)
of
the
dielectric
between
the
electrodes;
therefore:
C
?
?
A
d
?
?
r
?
0
< br>A
d
3)
Differential
Capacitive Sensor
A differential
capacitor consists of two variable capacitors so
arranged that they undergo
the same
change, but in opposite directions. The amplifier
circuit, depending on its
configuration, can generate a voltage
proportional to C1 - C2 or C1/C2 or (C1 - C2)/(C1
+
C2).
4)
Variable Reluctance Sensors
A typical single-coil variable-
reluctance displacement sensor is illustrated in
the Figure
below. The sensor consists
of three elements: a ferromagnetic core, a
variable air gap, and
a ferromagnetic
plate.
Based on change in
the reluctance of a magnetic flux path. Self-
inductance L of the coil is:
2
L
?
W
R
m
l
R
?
Relucta
nce can be given as:
m
??
0
S
5)
Variable-
Reluctance Tachogenerators
It
consists
of
a
ferromagnetic,
toothed
wheel
attached
to
a
rotating
shaft,
a
coil
and
a
magnet. The wheel rotates
in close proximity to the pole piece, thus causing
the flux linked
by thecoil to change.
The sensors output depends on the speed of the
rotation of the wheel
and the number of
teeth.
6)
LVDT
An LVDT consists of three coils, a form
and a core. The coils are wound on a hollow form.
The primary is excited by some ac
source. Flux formed by the primary is linked to
the two
secondary
coils,
inducing
an
ac
voltage
in each
coil. A
core
is
inside
the
former.
It
can
slide freely through the center of the
form.
In
many
applications,
the
two
secondary
coils
are
connected
in
series
opposition.
Then the two voltages will subtract;
that is, the differential voltage is formed. When
the
core is centrally located, the net
voltage is zero. When the core is moved to one
side, the
net voltage will increase.
7)
Compression
Mode Piezoelectric Accelerometers
Upright compression designs sandwich
the piezoelectric crystal between a seismic mass