Download Slide # 2

Displacement sensor
LVDT sensor
LVDT stands for Linear Variable Differential Transformer
• For operation, the LVDT's primary winding is energized by
alternating current of appropriate
• amplitude and frequency, known as the primary excitation.
• The LVDT's electrical output signal is the differential AC
voltage between the two secondary windings, which varies
with the axial position of the core within the LVDT coil.
• Usually this AC output voltage is converted by suitable
electronic circuitry to high level DC voltage or current that is
more convenient to use.
Slide # 1
Displacement sensor
LVDT sensor
Advantages:
• Friction-Free Operation
One of the most important features of an LVDT is its
friction-free operation. In normal use, there is no
mechanical contact between the LVDT's core and coil
assembly, so there is no rubbing, dragging or other source
of friction. This feature is particularly useful in materials
testing, vibration displacement measurements, and high
resolution dimensional gaging systems.
• Infinite Resolution
Since an LVDT operates on electromagnetic coupling
principles in a friction-free structure, it can measure
infinitesimally small changes in core position. This infinite
resolution capability is limited only by the noise in an LVDT
signal conditioner and the output display's resolution.
• Unlimited Mechanical Life
Because there is normally no contact between the LVDT's
core and coil structure, no parts can rub together or wear
out. This means that an LVDT features unlimited
mechanical life. This factor is especially important in high
reliability applications such as aircraft, satellites and space
vehicles, and nuclear installations.
• Low output impedance and noise/interference
Slide # 2
Hall Effect sensor: Basics
Lorentz force and Hall effect
F  qv  B
Jx
q
B
nq
nq e E x
q
B
nq
VH
 qE y  q
w
VH
LVH
VH
e 


wE x B wRx IB BI  sheet
VH  I sheet e B
nsheet
 sheet
1


q
q sheet 
Slide # 3
Hall Effect sensor: Chip and Spec
ANALOG DEVICES - AD22151YRZ IC, HALL EFFECT SENSOR, LINEAR,
8-SOIC
Slide # 4
Hall Effect sensor: Analog and Digital o/p
Analog Output circuit
Digital Output circuit
Slide # 5
The Schmitt trigger waveforms
A small positive vi would make the o/p
voltage positive (saturated to supply
voltage usually), or a small negative
voltage would saturate the o/p voltage
to negative supply voltage.
Suppose Vo = +5 V. Then as long as we have Vin > -5 V, we have the vi > 0, and the o/p voltage
remains at +5 V.
However, if Vin < -5 V, then vi < 0, and the o/p will switch to -5V. Then vo will remain at -5 V, unless vin
exceeds +5 V.
6
Slide #
Hall Effect sensor: Applications II
• To sense displacement or rotation, Hall Effect
sensors will have to use a magnet mounted to a
moving part
• The magnet can be brought closer or further, or
interrupted to produce a square wave signal
• In some applications, 4 Hall Sensors are arranged
in a quad cell (like a wheatstone bridge) to sense
rotary signal
Slide # 7
Optical sensors: Fiber Optic sensors
Vibration and displacement sensor
• Fiber optic sensors are great because they sense by
non-contact means, which at the very basic level is
simply based on the changes in light intensity
• A common application is to couple light from one
fiber to another by means of a movable mirror, which
based on changes such as pressure, would couple
different intensity of light to the receiving fiber.
• Another applications is in sensing the level of liquid
as shown in the adjacent figure. When the liquid is in
touch with the bent section of the tube, more light
scatter into water, and less light is detected at the
other end.
Liquid level
sensor
Vibration and
displacement sensor
utilizing FO cable with
microbends
FO strain sensors
Slide # 8
Linear Optical sensors I
• Used for precision position sensing over the
short and long range
• An IR LED is typically used to send out the
signal, which is then sensed from a object
using a position sensitive detector
• The detector consists of p and n+ doped Si
layers positioned on opposite side of the bar.
Wherever the light beam falls, it creates a low
resistance path connecting the top and the
bottom layers
• The movement of the object can be calculated
from the movement of the spot on the detector
Mathematical Analysis:
If the beam hits at a distance x from the electrode A,
and the corresponding resistance is Rx, then the
overall photoelectric current I0 will be split into IA and
IB as
I A  I0
RD  Rx
RD
I B  I0
Rx
RD
RD is the resistance between
the terminals A and B
Slide # 9
Linear Optical sensors II
Assuming linear variation of resistance
with distance, IA and IB can be written as:
Dx
I A  I0
D
I B  I0
x
D
Then the ratio of the current P is given as:
P
IA D
 1
IB x
or
x
D
P 1
Applying similar triangle principles from Fig. 7.38 we have:
Thus we have:
L0  f
LB x
L
  L0  f B
L0 f
x
LB
P  1
D
Slide # 10