Sensors
Read Chapter 2 of Textbook
1.
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Displacement Sensors
Potentiometer (Already discussed)
Strain Gages
Inductive Sensors (LVDT)
Capacitive Sensors
Piezoelectric Sensors
=(A/V)/m = S/m
P and N doped Silicon Strain Gages
Gage Factor = G = (ΔR/R)/(ΔL/L)
---Stretching bar of N-type silicon crystal breaks electrons loose from impurity
sites, making resistance decrease, producing large negative G.
---Stretching bar of P-type silicon crystal inhibits holes from moving away from
their impurity sites, producing large positive G
Unbonded Strain Gage Pressure Sensor
This is a deflection mode
instrument, so it is
important to choose
Ri >> R1=R2=R3=R4
To avoid bridge loading
D.
BONDED Strain Gages
Silicon Integrated
Circuit Pressure
Sensor
Inductive Transducers
Coils must be
wound in
opposite
directions so
magnetic fluxes
oppose
<- L
<- L
+
Vo(t)
-
µ = magnetic permeability of core material
= µ R4π*10-7 F/m
Linear Variable Differential Transformer
Displacement Transducer
LVDT used to measure very small displacements in a seismometer that
measures movements in the earth’s crust due to earthquakes. It consists of a
middle primary coil and two outer secondary coil. The magnetic core moves
freely without touching bobbins, and at the null (zero) position, it extends
halfway into each secondary coil.
Example Medical Applications of Linear Variable
Displacement Transformer (LVDT)
NOTE: The
output voltage is
actually the
PEAK voltage of
an ac sine wave
whose
frequency is that
of the primary
winding
excitation sine
wave.
Advantages of LVDTs as
displacement sensors
Disadvantages of LVDTs
• All these advantages, in addition to their
reasonable cost, have made the LVDT an
attractive displacement measurement sensor.
However, LVDTs for use in medical applications
have the following disadvantages:
– They require a high frequency, constant-amplitude ac
sinusoidal excitation.
– They cannot be used in the vicinity of equipment that
creates strong magnetic fields.
– A somewhat complicated “phase sensitive” ac-to-dc
converter (detector) must be used if both positive and
negative displacements from the middle (null) position
needs to be measured.
LVDT showing AC Excitation and
Phase Sensitive Demodulator
Phase INSENSITIVE Detector (Diode Halfwave rectifier with capacitive filter.)
Commercial Diode
Ring Modulator (also
called “Double
Balanced Mixer”
LVDT Detector Circuit without need for a Phase-Sensitive Demodulator
LVDT Excitation Circuit (Power oscillator (1 kHz
Power Sine Wave Oscillator)
ic(t)
Does not effect circuit,
since output resistance of
op amp is nearly 0 ohms
C = q/v => v = q/C =>
kfL/(ε0εRA)
L = thickness
of piezo
transducer
Molecular Model of a Simple Piezoelectric Material: ZnS
How is the piezoelectric constant “k” (recall q = kF) related the constant “K”
above? Recall the definition of Young’s Modulus, Y:
L
Y
Therefore
f /A
x/L
=>
f 
A Y
x
L
A Y
qk f k
x  Kx
L
A
f
x
where
K k
A Y
L
Amplifier
Crystal
Cable
Rs
Cs
Cc
Ca
Ra
q = Kx
is = dq/dt = Kdx/dt
R  Rs // Ra
C  Cs  Cc  Ca
q  C  v0
dv0
iC  C 
dt
q  C  v0
iC  C 
dv0
dt
iC
iS
Eqn 2.19 is now written in phasor form:
iR
C  j  Vo  K  j  X 
Vo(1  j ) 
Vo
R
K
 j  RC  X
C
This transfer function is of the
same form as that of the RC
HPF 1st order filter and also
the capacitive displacement
sensor.
So, like the capacitive
transducer, it cannot
transduce constant (dc)
displacements!
100 kΩ and
100k Ω) = 16 nF
8
VCC = 1 5 V
3
+
1
4
2
Piezo
Xducer
R3
3.2 MEG
C1
1uF
VEE = -15 V
R2
11.1k
R1
10k
100k
C2
160 nF
16 nF
Vout
xo
Kx0
C
Piezoelectric Displacement Sensor has a high pass filter step response
Predicting Piezo Transducer Step Response Via Laplace Transforms
Replacing “jω” in Text Eqn 2.20 by the Laplace complex frequency variable “s”
Note that for a step input of amplitude
Vo( s ) K S  s 
H (s) 

xo, x(t) = xo u(t) => X(s) = x0/s
X (s)
s   1
K  s 
K  s  xo xo  K S 
Vo( s )  S
 X (s)  S
 
s   1
s   1 s
s   1
vo(t )  xo  K S e t / Note this is response only to the leading
edge of the input pulse, x(t) = x0 u(t)
xo
But in reality, x(t) = x0 {u(t) – u(t-Td)}
0
x(t)
xoKS
xoKSe-1 = 0.37xoKS
t=Td
vo(t)
t=0
t = τ = RC
- xoKS
+
vout
Useable Operating
Frequency Range
for “wideband”
signal transducer,
such as audio
microphone
Operating frequency for
very sensitive, narrowband signal transducer,
such as 40 kHz resonant
ultrasonic pulsed
distance measuring
application
General Block Diagram of a Feedback Oscillator
1. Magnitude of
voltage gain
around feedback
loop “BA” must
be > 1 at
frequency of
oscillation.
2. Phase shift
around feedback
loop must be an
integer multiple
of 360 degrees
so oscillations
can build up at
frequency of
oscillation.
Pierce Crystal Oscillator Circuit Made from Digital Inverter Gate
Biasing Resistor R1
R1 acts as a feedback resistor, biasing the inverter
into its linear amplifying region of operation, and
effectively causing it to function as a high gain
inverting amplifier. To see this, assume the inverter
is ideal, with very high input impedance and very
low output resistance; this resistor forces the
inverter’s input and output voltages to be equal.
Hence the inverter will neither be fully on nor off, but
in the transition region where it has gain.
Piezoelectric Crystal Resonator
The crystal in combination with C1 and C2 forms
a Hi-Q “Pi network” bandpass filter, which provides
a 180 degree phase shift and also a voltage gain
from the output to input at approximately the
resonant frequency of the crystal. To understand the
operation of this, it can be noted that at the
frequency of oscillation, the crystal appears
inductive; thus it can be considered a large inductor
with a very high Q. The combination of the 180
degree phase shift (i.e. inverting gain) from the pi
network and the negative gain from the inverter
results in a positive loop gain, making the bias point
set by R1 unstable and leading to oscillation.
Vi
Vo
Vo
Vo=Vi
Load line
established by
biasing
resistor R1,
biases
inverter into
its analog
amplifying
region
Vi
8 MHz Crystal Oscillator
RFC (RF Choke) is a 10 uH inductor
that has a high enough reactance at
the parallel resonant frequency of the
XTAL (8 MHz) to guarantee a loop
gain > 1 at 8 MHz. Note XRFC = 500
ohms at 8 MHz. Oscillator will have
high harmonic content with RFC, so
RFC is sometimes replaced with
parallel resonant circuit to encourage
oscillation at only the harmonic
frequency the parallel resonant
circuit has been tuned to resonate at.
R1, R2, RE bias BJT CE amplifier
into the middle of its amplifying
region.
Typical Values:
Vcc = 9 V, R1 = R2 = 560 ohms,
RE = 1 k ohm, Cb = 0.1 uF
C1 = C2 = 30 pF (XTAL often cut to
resonate at desired frequency with
these external values of C1 and C2.)
AM (Amplitude Modulation)
We can amplitude modulate
the 5 MHz carrier wave
produced by the crystal
oscillator by varying the 5 V
dc power supply up and down
at an audio rate. This is done
by placing an ac source in
series with the dc power
supply. In this demo, I will
simply add “dc offset” to a 1
kHz sine wave produced by
the function generator, and
replace the dc 5V supply with
the function generator output.
DLP (Digital Light
Processor ) IC
How DLP Technology Works
1. The semiconductor that continues to
reinvent projection
At the heart of every DLP® projection
system is an optical semiconductor known
as the DLP® chip, which was invented by
Dr. Larry Hornbeck of Texas Instruments in
1987.
The DLP chip is perhaps the world's most
sophisticated light switch. It contains a
rectangular array of up to 2 million hingemounted microscopic mirrors; each of
these micromirrors measures less than onefifth the width of a human hair.
When a DLP chip is coordinated with a
digital video or graphic signal, a light source,
and a projection lens, its mirrors can reflect
a digital image onto a screen or other
surface. The DLP chip combined with the
advanced electronics that surround it
produce stunning images and video that
have redefined picture quality.
2. The grayscale image
A DLP chip's micromirrors tilt either toward the light source in a DLP
projection system (ON) or away from it (OFF). This creates a light or dark
pixel on the projection surface.
The bit-streamed image code entering the semiconductor directseach
mirror to switch on and off up to several thousand times per
second. When a mirror is switched on more frequently than off, it reflects a
light gray pixel; a mirror that's switched off more frequently reflects a darker
gray pixel.
In this way, the mirrors in a DLP projection system can reflect pixels in up
to 1,024 shades of gray to convert the video or graphic signal entering the
DLP chip into a highly detailed grayscale image.
3. Adding color
The white light generated by the lamp in a DLP projection system passes through
a color filter as it travels to the surface of the DLP chip. This filters the light into a
minimum of red, green, and blue, from which a single-chip DLP projection system
can create at least 16.7 million colors.
With BrilliantColor™ Technology, additional colors are added including Cyan,
Magenta and Yellow to expand the color pallet for even more vibrant color
performance.
Some DLP projectors offer solid-state illumination which replaces the traditional
white lamp. As a result, the light source emits the necessary colors eliminating the
color filter.
In some DLP systems, a 3-chip architecture is used, particularly for high
brightness projectors required for large venue applications such as concerts and
movie theaters. These systems are capable of producing no fewer than 35 trillion
colors.
The on and off states of each micromirror are coordinated with these basic
building blocks of color. For example, a mirror responsible for projecting a purple
pixel will only reflect red and blue light to the projection surface; those colors are
then blended to see the intended hue in a projected image.
“Single Chip” DLP Technology
Many data projectors and HDTVS using DLP technology
rely on a single chip configuration like the one described
above.
White light passes through a color filter, causing red,
green, blue and even additional primary colors such as
yellow cyan, magenta and more to be shone in
sequence on the surface of the DLP chip.
The switching of the mirrors, and the proportion of time
they are 'on' or 'off' is coordinated according to the color
shining on them. Then the sequential colors blend to
create a full-color image you see on the screen.
“PainGone” –
A Drug Free, Battery Free Pain
Relief Piezoelectric Device
Paingone is a pocket sized pain relief device that
works by delivering a controlled electronic
frequency through the nerve pathways to the
brain. This stimulates endorphins, the body's
natural painkillers for natural pain relief wherever
and whenever you need it. It has been
successfully clinically tested by people suffering
from a number of painful conditions such as
arthritis, back pain, osteoporosis, sciatica and
inflammatory conditions. Many NHS Hospitals and
GPs use PainGone in their Pain Clinics and
recommend it to their patients as a safe, drug free
therapy. PainGone’s effectiveness has been
clinically confirmed, as independent tests show, it
stops or relieves pain quickly in up to 87% of
cases on which it is used, making it a reliable
alternative to medication.
How does it work?
PainGone works by pressing the button on
top of the device to deliver a low frequency,
gentle electrical charge produced by crystals,
straight to the point of pain. Each click sends
a pulse that will activate endorphins, the
body's natural painkillers to free you from
pain. This stimulating frequency can thus
provide prolonged and often instant relief.
This means that anywhere, at anytime, pain
relief is but a click away.
•Used on the point of pain
•No leads, pads or batteries
•Small and lightweight
•Use as often as required
•NHS recommended
•CE registered Class IIa medical
device
•Money Back Guarantee
•Estimated life time of over 100,000
clicks
•Over 3 million worldwide users
•Clinically tested
•Drug free and safe
•Simple to use and no side effects
•30 second treatment
•Works through clothing