Fundamentals of
Measurement Technology
(11)
Prof,Wang Boxiong
4.12 Infrared radiation detection
Inf rar ed rad iatio n,also kn ow n as inf rar ed ligh t,is
the inv isib le light ou ts id e the red l i gh t w ithin t he
sun ’ s light spect rum,A ny ob j ect,w hen its
tem perature is hig her th an the abso l ute zero gr a de
(i.e,
C 16.2 7 3
),is s aid to be i n a,ther m al
status,,T he m olecules a nd atom s of a sub stance in
therm al statu s are in a con tinuo us m ov e m ent and
rota tion,an d an elec t ron tr ansi tio n occu rs to
prod uce electrom agn etic wav es,
4.12 Infrared radiation detection
S inc e the w av elen g th o f the el ect ro m ag n etic w av e is
o u tsid e of tha t o f the v isi b le (red ) lig h t,so the lig h t is
call ed inf rar ed ray,W h en an o b ject los es it s the rm al
eq u il ibriu m w ith the am b ien t tem p er atu re,i t em its o r
ab so rbs inf r ared ray s,T h is p roc e ss is cal led th e rm al
radia tion o r inf rared rad i atio n,T h e i n f rared sp ec tru m
g en era lly is d ef ine d a s the rang e f rom
76.0
to
ab o u t
m?1000
,
Fig,4.106 Electromagnetic radiation spectrum
4.12 Infrared radiation detection
The relationship between an object’s temperature and
radiation power are described by Stefan-Boltzmann’s law:
4.12 Infrared radiation detection
4
TW ( 4,1 2 3 )
w h e r e
W n e t r a d ia n t h e a t tr a n sf e r o r h e a t f lux in
2
mW,
the Ste f a n - B o ltz ma n n c o n sta n t,
428
106 6 9 7.5
KmW,
T the r mo d y n a mic te m p e r a tur e in K,
the sp e c if ic r a d i a n c e o f o b je c t ( r a tio o f n o n - b la c k - b o d y
r a d ia n c e to b la c k - b o d y r a d ia n c e )
T h e r a d i a n t h e a t f lux o f a n o b je c t,
W
,is p r o p o r ti o n a l to the 4
th
p o w e r o f its a b so lute te mp e r a tur e
T
,
4.12 Infrared radiation detection
O bject,w hi ch abso rb s com pletely rad iatio n s of any
w aveleng ths f alling on i t in any tem peratu res,is called a
blackb od y,that is,its em is sivity 1,
C o m m on obje cts oth er th an a b lackb od y h ave the s pecif ic
radian ce
1
,in oth er w ords,they cann ot abs orb
tota lly th e radia n t p ow er on t heir su rfac es,T heir
em issivitie s are also s m aller than a blackb od y,and are
theref ore called,gray b od ies”,
The Planck’s Law describes the radiation intensity
distribution for an ideal radiator (blackbody) under
different temperatures:
4.12 Infrared radiation detection
1
2
5
1
T
C
e
C
W
（ 4.131 ）
in which
W — the e ner g y e m it te d by wave le ngth
,in
12
mWm? ；
1
C
— the 1
st
r adiation coe f f ic ie nt,
24
1
/15.37 4 mmMWC ；
2
C
— the 2
nd
r adi at ion coe f f ic ie nt,
mKC?14388
2；
T — the a bsol ute t em per at ur e,in K ；
— the wave le ngth,in
m?
。
If we should heat an ideal radiator to various
temperatures and determine the relative intensities
at each wavelength,we would obtain characteristic
energy-distribution curves such as those shown in
Fig,4.107,Not only is the radiation intensity of
the higher-temperature body increased,but the
wavelength of maximum emission is also shifted
toward shorter waves (from red toward blue).
4.12 Infrared radiation detection
Fig,4.107 Graphical representation illustrating the basis for Wien’s
displacement law
4.12 Infrared radiation detection
4.12 Infrared radiation detection
For a nonideal body,the inte nsity dist ri bution
m ust be m ulti plie d by the em issi vity,?,The
w avele ngth of peak int ensit y is given by the
W ien’ s displa cem ent l aw,
mT /2 89 8
m a x
（ 4.132 ）
Devices converting infrared radiations into electric
quantities are called infrared detectors.
Two types,
Thermo-sensitive detectors
Photosensitive detectors
4.11.2 Infrared detectors
Nonselective infrared detectors,
Thermo-sensitive detectors take advantage of the
heat effect produced by semiconductor films when
exposed to infrared radiation.
The thermo-sensitive device has a relatively long
response time of the order of 10-3 s.
Basically,thermo-sensitive detectors have the same
response to radiations with different wavelengths,
They have a flat optical spectrum,and a nearly
constant sensitivity over the whole wavelength
range of measurement.
4.11.2 Infrared detectors
4.11.2 Infrared detectors
A ph oto electr ical detector i s a sem icond uctive o ne,
W hen ph oto ns are striking o n th e ph oto sensitive
com po nen t,elect ron - ho le pairs ar e deco m po sed to
gen erat e electr ical signals,A s the p ho toelectr ical
ef f ect appears v ery f ast,respon se of a ph oto electr ical
detector to inf rar ed radiation is m uch f aster t han that
of a therm o - sensitive d etector,usu ally in the o rder of
ms
or even
s?
,
T h e d etecto r ha s an u n ch an g ed w av elen g th rang e,an d a
p eak v alu e at th e w av elen g th,
p
,T h e resp o n se c o m es
rapid ly to sto p w h en th e w av elen g th ex ceed s
p
(see
cu rve 2 in F ig,4,1 0 8 ),T h e reaso n is th at th e am o u n t o f
p h o ton s is ins u f f icien t to activ ate th e releas e o f electro n s
w ithin a ran g e o f w av elen g ths lar g er tha n a ce rtain
w av elen g th,
4.11.2 Infrared detectors
Photoelectrical detectors operate only in low
temperatures
4.11.2 Infrared detectors
Fig,4.108 Optical spectrum response of infrared detector
1,Radiation pyrometers
4.11.3 Applications
Fig,4.109 Operating principle for radiation pyrometer
The Stefan-Boltzmann’s law can be used in radiation
pyrometers.
4.11.3 Applications
T he m easured ob ject i s usu ally a gr ay bo dy w ith its
1,U sing the blackb od y radia tion as a c riter ion
f or calib ration,it is po ssible to evalu ate the
tem perature of the m easured ob j ect by m eans of E q,
(4.13 0) and the def init ion of?,if the? of the
ob ject i s kn ow n,
Assuming that all energy radiated by the gray body is
absorbed by the blackbody,their total energies must
be identical,that is,
4.11.3 Applications
4
00
4
TT
w h ere
= sp ecif ic rad ian ce o f th e m easu red o b ject
0
= sp ecif ic rad ian ce o f b lack b o d y,1
0
T
= ab so lute tem p eratu re o f o b ject
= th e S tef an - B o ltzm an n co n stan t
T h u s w e h av e
4
o
T
T? （ 4,1 2 6 ）
2,Infrared thermometry
4.11.3 Applications
Radiati on py rom ete rs ar e usuall y used in m easur em ent
of te m pera ture s higher than C?800,Ho w ever,the
inf ra r ed ther m om etr y w e ar e now deal ing w it h
concer ns w it h m easur em ents i n low te m pera ture and
inf ra re d light r ange,
Fig,4.110 Block diagram of an infrared thermometering device
4.11.3 Applications
3,Infrared thermal imaging
As the infrared light is invisible to human eyes,it is
impossible to use common cameras to take infrared
pictures,Infrared thermal imaging technique can be
used to convert infrared radiation into visible light
for display.
4.11.3 Applications
Fig,4.111 Active infrared thermal imaging
1-infrared light source
2-infrared camera
3-monitor
There are two types of thermal imaging:
Active thermal imaging,utilizes an infrared
radiation source to irradiate the measured object,
The reflected radiation from the measured object is
received by an infrared camera,
Passive thermal imaging,acquires the thermal
images directly from the infrared radiation of the
object itself.
4.11.3 Applications
The image thus formed is called the thermal
image,and the device for acquiring thermal
images is called the thermal imaging system,
A thermal imaging system doesn’t need any
extra infrared light source,is easy to manipulate,
and can acquire accurately the thermal images
representing object’s temperature field,
4.11.3 Applications
Fig,4.112 Arrangement of optical system of
an infrared thermal imaging system
4.11.3 Applications
Infrared thermal imaging systems have been
widely used in industry,in measurement of
temperatures for different environmental
conditions,such as thermal deformation of
machine parts due to temperature rising.
4.11.3 Applications
Fig,4.113 Isothermal curve plot of the section of a lathe spindle bearing
4.11.3 Applications
Fig,4.114 Thermal imaging detection for supersonic cavitation
4.11.3 Applications
1-nozzle
2-model
3-thermal camera
4-observation window
Thermal imaging systems are used to detect
temperature rising of electrical equipment,especially
switches and power lines,to prevent possible faults.
In non-destructive detection of fissures,air pores,
foreign substances,and sectional area abberances
of sectional areas of different materials such as
metal,ceramics,plastics,multi-layer fiber plates,
etc
In power industry
Thermal imaging systems are used for safety control,
in petrochemical and metallurgical industries
4.11.3 Applications
In public security and fire control,thermal imaging
systems are used to detect in-site the damages by fire
and the situations of the persons remaining in the
building in fire,
Another very useful application of thermal imaging is
in the medical diagnosis.
4.11.3 Applications
Fig,4.115(a) Thermal image (breast cancer) acquired by thermal
imaging system; Female; sustained high-temperature in left breast;
4.11.3 Applications
N i p p l e s t e m p e r a t u r e d i f f e r e n c e,1,3 C? ; D i a g n o s i s,l e f t b r e a s t c a n c e r
4.11.3 Applications
Fig,4.115(b) Periarthritis of shoulder (scapulohumeral periarthritis);
Male; Symptom,sustained high-temperature in right shoulder
Solid-state image sensor is a solid-state integrated
element,whose kernel part is a charge-coupled
device (CCD),
A CCD device is composed of a number of metal-
oxide-semiconductor (MOS) capacitors arranged
on a substrate,which has the functions of
generating photo-electric charges,storing and
transferring the charges,
4.13 Solid-state image sensors
The basic principle for CCD camera:
Usually a silicon wafer has hundreds or thousands of
mutually-independent,regularly-lined MOS
photosensitive elements known as the photosensitive
element array,If a positive bias is applied to metal
electrodes,hundreds or thousands of mutually-
independent potential traps will be generated in silicon
wafer,If an image with dark-bright variations is
exposed to the photosensitive elements,they will
produce a photoelectrical image corresponding to the
light intensities,This is the basic principle for CCD
camera,
4.13 Solid-state image sensors
Line-array and area-array cameras based on CCD
devices can transmit image information,and have
been widely used in areas like TV,fax,
photography,image transmission and processing,
Especially for such advantages as small size,high
speed,high sensitivity,high stability,and non-
contact measurement,CCD devices are also used
in size and position measurements,event counting,
image recognition,and automatic monitoring and
control,etc,
4.13 Solid-state image sensors
Fig,4.116 Self-scanning diode arrays and cameras.
(a) MOS photo-sensitive unit
4.13 Solid-state image sensors
Fig,4.116 Self-scanning diode arrays and cameras,
(b) 1024 element array
4.13 Solid-state image sensors
Fig,4.116 Self-scanning diode arrays and cameras.
(c) line-scan camera
4.13 Solid-state image sensors
Fig,4.117 Application example,two-dimensional measurand acquisition
with CCD line-array camera
4.13 Solid-state image sensors
Fig,4.118 shows an example of object position
detection by use of a laser-optical triangulation sensor.
4.13 Solid-state image sensors
Fig,4.118 Displacement and profile measurement using
triangulation method
An example of triangulation measurement of
workpiece profile,
4.13 Solid-state image sensors
Fig,4.119 3-D profile measurement by use of triangulation method
An example of the measurement of object’s 3-D
profile using line-projection method
4.13 Solid-state image sensors
Fig,4.120 Measuring principle of line-projection method
The moiré fringes of a measured object
4.13 Solid-state image sensors
Fig,4.121 Measuring principle of moiré method
Fig,4.122 Example of a moiré structure
4.13 Solid-state image sensors
Chapter 5 Signal conversion and
conditioning
Prof,Wang Boxiong
Chapter 5 Signal conversion and
conditioning
5.1 Introduction
5.2 Bridges
5.3 Modulation and demodulation
5.4 Filters
5.1 Introduction
A mechanical quantity,after it has been detected and
transduced into an electrical form,needs to be further
converted,amplified,filtered and analyzed to remove
unwanted noise and to improve signal-to-noise ratio
before it is used in a suitable form for driving an
indicator or recorder as well as for further possible
processing with a computer,
The signal conversion and conditioning covers a very
wide area,our study will focus on some techniques in
common use such as bridges,signal modulation and
demodulation (MODEM),signal filtering,analog-to-
digital and digital-to-analog conversions,
5.2 Bridges
A bridge is a measuring circuit,which converts
a variation in resistance,capacitance and
inductance into an equivalent voltage or
current output.
According to the types of excitation power
supply,bridges are usually divided into dc
bridges and ac bridges; according to their
operating principles,they are divided into the
null and the deflection types.
5.2.1 DC Bridges
Of all the possible configurations,the
Wheatstone resistance bridge is undoubtedly
used to the greatest extent,
In applications,one or more of the arms is a
resistance transducer whose resistance is to be
determined,Typical resistances used with a
circuit of this kind include resistance
thermometers,thermistors,or resistance-type
strain gages,
5.2.1 DC Bridges
Fig,5.1 DC bridge circuit
Requirements for bridge
balance,When the output
terminals are connected with
meter or amplifying circuit
having large input impedance,
the bridge circuit can be
considered as an open circuit,
and the output current is zero,
hence:
5.2.1 DC Bridges
43
0
2
21
0
1
KR
e
I
KR
e
I
0
21
1
41 eRR
RRIU
ab
0
43
4
42 eRR
RRIU
ad
(5.1)
(5.2)
(5.3)
The output voltage:
5.2.1 DC Bridges
04321
4231
0
43
4
0
41
1
e
RRRR
RRRR
e
RR
R
e
RR
R
UUe adaby
(5.4)
In balance,the voltage across the meter is zero,
then,
4231 RRRR?
is the condition for the balance of a dc bridge,
Any change in one or more of the four
resistances will cause the bridge unbalanced,
resulting in a voltage output,
(5.5)
5.2.1 DC Bridges
Commonly-used bridges are usually connected
into forms of single-arm-half-bridges,double-
arm-half-bridges,and full-bridges.
Single-arm-half-bridge circuit,Letting the
resistance be R1 and the variation ΔR,
0
43
4
21
1 e
RR
R
RRR
RRe
y
(5.6)
When R1=R2=R0,R3=R4=R0’,Further,we
assume R0=R0’,Eq.(5.6) becomes
5.2.1 DC Bridges
0
0 24
eRR Re y
(5.7)
Usually ΔR<<R0,so
0
04
eRRe y
(5.8)
The output voltage ey is proportional to the
exciting voltage e0,and has pure linear
relationship with the resistance change of the
varied arm,if e0 and R0 are fixed.
5.2.1 DC Bridges
Fig,5.2 Configurations of DC bridges
(a) single-arm-half-bridge circuit;
(b) double-arm-half-bridge circuit;
(c) full-bridge circuit
5.2.1 DC BridgesDouble-arm-half-bridge connection,
two bridge arms (usually two adjacent arms)
change with the measurand:
11 RR 22 RR
,
When:
021 RRR,'43 RRR?＝,RRR 21
and '
00 RR?
Then,the output voltage
0
02
eRRe y
(5.9)
5.2.1 DC BridgesFull-bridge connection,
four bridge arm resistances vary with the
measurand:
11 RR 22 RR
,
When:
021 RRR,'43 RRR?＝,RRR 21
and '
00 RR?
Then,the output voltage
(5.10)
,33 RR and 44 RR
0
0
eR Re y
5.2.1 DC Bridges
When the resistance changes of the four arms are
of the same sign,that is:
11 RR 22 RR 33 RR 44 RR
04321 RRRRR ＝＝
,,,and let
and when the change in resistance is small in
comparison with the resistance itself,then the
output voltage is approximated as,
,
0
4321
4
1 e
R
R
R
R
R
R
R
Re
y
(5.11)
5.2.1 DC Bridges
The rule of the influence on the output due to the
resistance changes of arms:
1,The output voltage due to the resistance changes of
two adjacent arms is the difference between the
output voltages caused by the resistance change of
each adjacent arm independently;
2,The output voltage due to the resistance changes of
two opposite arms is the sum of the output voltages
caused by the resistance change of each opposite arm
independently,
5.2.1 DC Bridges
Fig,5.3 Cantilever strain
gage arrangement
The addition-subtraction property of bridge,
applied in many measuring circuits,also be used
for automatic compensation of errors due to
temperature variations.
5.2.1 DC Bridges
Sensitivity of a bridge,
R
eS y
,where ΔR/R is the input of the bridge.
Fig,5.4 Arrangements used
to balance dc resistance
bridges
5.2.1 DC Bridges
Disadvantages of a dc bridge:
Liable to be influenced by power line frequency
interference.
The output must be amplified with a dc amplifier,
which is relatively complex and is liable to be
influenced by zero drift and ground potential.
Advantages of a dc bridge:
Output of high accuracy.
No stray capacitance.
Balancing of a dc bridge is relatively simple,only
the arm resistances should be adjusted.
5.2.2 AC Bridges
An ac bridge has the structure similar to a dc
bridge,The only difference lies in that the ac
bridge uses ac power supply as its exciting
source,and the arms can be resistors,
capacitors or inductors.
Fig,5.5 Arrangement
of an ac bridge
5.2.2 AC Bridges
The balance equation for a dc bridge:
4231 zzzz?
(5.13)
where zi,complex reactance of each arm,ij
ii eZz
Zi,modulus of the complex reactance,
φi,phase angle of the complex reactance.
Substituting it in Eq,(5.13) yields
4231 4231 jj eZZeZZ (5.14)
4231
4231
ZZZZ (5.15) Equivalent:
5.2.2 AC Bridges
Since a reactance angle represents the phase-shift
between the current and the voltage of a bridge
arm,when the arm is a resistor,φ=0,that is,its
current and the voltage are in phase,For an
inductive arm,φ>0,while for a capacitive arm,
φ<0,Since balancing ac bridge must satisfy the
two conditions for both modulus and phase,
various combinations of bridge arms can be
employed.
5.2.2 AC Bridges
Fig,5.6 Commonly-used ac
bridge arrangements
(a) Capacitive (b) inductive
Fig,5.6(a) is a capacitive bridge,according to
Eq.(5.15),there is,
2
4
43
1
1
11 R
cjRRcjR
(5.16)
5.2.2 AC Bridges
Expanding Eq.(5.16),thus we have:
4
2
1
3
4231
c
R
c
R
RRRR (5.17)
As the balance of an ac bridge requires the
conditions for both modulus and argument,it is
much more complex than the balance of a dc
bridge,Even for a pure-resistance ac bridge,the
stray capacitances formed by lead wires also
influence the arm resistance values (Fig,5.7(a)).
5.2.2 AC Bridges
Fig,5.7 AC bridge balance (a) stray capacitances
(b) resistance ac bridge using both resistance and capacitance balances
5.2.3 Problems related to bridge applications
Bridge circuits have extensive applications in
strain-gage measuring circuits,A bridge
circuit will be also affected by the length
changes in lead wires of sensing elements,
Besides,the sensitivity must be adjusted in
different applications to suit to different
accuracies.
1.Compensation for leads
5.2.3 Problems related to bridge applications
Fig,5.9 (a) Simple bridge with remotely located sensor; (b) circuit similar to
that shown in (a),but with a compensating wire
5.2.3 Problems related to bridge applications
2,Bridge sensitivity adjustment
Adjustable bridge sensitivity may be desired for
several reasons,
① Attenuate inputs that are larger than desired.
② Provide a convenient relationship between
the system calibration and the scale of the
readout instrument,
③ Provide adjustment for adapting individual
transducer characteristics to recalibrated
systems,
5.2.3 Problems related to bridge applications
④ Provide means for controlling certain
extraneous inputs such as temperature
effects.
A very simple method of adjusting bridge
output is to insert a variable series resistor in
one or both of the input leads,the input to the
bridge will be reduced by the factor
RRRR
Rn
ss?
1 1
n,the bridge factor.
5.2.3 Problems related to bridge applications
The bridge output will be reduced by a
proportional amount,which makes this method
very useful for controlling bridge sensitivity.
Fig,5.10 Method for adjusting bridge sensitivity through use of
variable series resistance,Rs
Measurement Technology
(11)
Prof,Wang Boxiong
4.12 Infrared radiation detection
Inf rar ed rad iatio n,also kn ow n as inf rar ed ligh t,is
the inv isib le light ou ts id e the red l i gh t w ithin t he
sun ’ s light spect rum,A ny ob j ect,w hen its
tem perature is hig her th an the abso l ute zero gr a de
(i.e,
C 16.2 7 3
),is s aid to be i n a,ther m al
status,,T he m olecules a nd atom s of a sub stance in
therm al statu s are in a con tinuo us m ov e m ent and
rota tion,an d an elec t ron tr ansi tio n occu rs to
prod uce electrom agn etic wav es,
4.12 Infrared radiation detection
S inc e the w av elen g th o f the el ect ro m ag n etic w av e is
o u tsid e of tha t o f the v isi b le (red ) lig h t,so the lig h t is
call ed inf rar ed ray,W h en an o b ject los es it s the rm al
eq u il ibriu m w ith the am b ien t tem p er atu re,i t em its o r
ab so rbs inf r ared ray s,T h is p roc e ss is cal led th e rm al
radia tion o r inf rared rad i atio n,T h e i n f rared sp ec tru m
g en era lly is d ef ine d a s the rang e f rom
76.0
to
ab o u t
m?1000
,
Fig,4.106 Electromagnetic radiation spectrum
4.12 Infrared radiation detection
The relationship between an object’s temperature and
radiation power are described by Stefan-Boltzmann’s law:
4.12 Infrared radiation detection
4
TW ( 4,1 2 3 )
w h e r e
W n e t r a d ia n t h e a t tr a n sf e r o r h e a t f lux in
2
mW,
the Ste f a n - B o ltz ma n n c o n sta n t,
428
106 6 9 7.5
KmW,
T the r mo d y n a mic te m p e r a tur e in K,
the sp e c if ic r a d i a n c e o f o b je c t ( r a tio o f n o n - b la c k - b o d y
r a d ia n c e to b la c k - b o d y r a d ia n c e )
T h e r a d i a n t h e a t f lux o f a n o b je c t,
W
,is p r o p o r ti o n a l to the 4
th
p o w e r o f its a b so lute te mp e r a tur e
T
,
4.12 Infrared radiation detection
O bject,w hi ch abso rb s com pletely rad iatio n s of any
w aveleng ths f alling on i t in any tem peratu res,is called a
blackb od y,that is,its em is sivity 1,
C o m m on obje cts oth er th an a b lackb od y h ave the s pecif ic
radian ce
1
,in oth er w ords,they cann ot abs orb
tota lly th e radia n t p ow er on t heir su rfac es,T heir
em issivitie s are also s m aller than a blackb od y,and are
theref ore called,gray b od ies”,
The Planck’s Law describes the radiation intensity
distribution for an ideal radiator (blackbody) under
different temperatures:
4.12 Infrared radiation detection
1
2
5
1
T
C
e
C
W
（ 4.131 ）
in which
W — the e ner g y e m it te d by wave le ngth
,in
12
mWm? ；
1
C
— the 1
st
r adiation coe f f ic ie nt,
24
1
/15.37 4 mmMWC ；
2
C
— the 2
nd
r adi at ion coe f f ic ie nt,
mKC?14388
2；
T — the a bsol ute t em per at ur e,in K ；
— the wave le ngth,in
m?
。
If we should heat an ideal radiator to various
temperatures and determine the relative intensities
at each wavelength,we would obtain characteristic
energy-distribution curves such as those shown in
Fig,4.107,Not only is the radiation intensity of
the higher-temperature body increased,but the
wavelength of maximum emission is also shifted
toward shorter waves (from red toward blue).
4.12 Infrared radiation detection
Fig,4.107 Graphical representation illustrating the basis for Wien’s
displacement law
4.12 Infrared radiation detection
4.12 Infrared radiation detection
For a nonideal body,the inte nsity dist ri bution
m ust be m ulti plie d by the em issi vity,?,The
w avele ngth of peak int ensit y is given by the
W ien’ s displa cem ent l aw,
mT /2 89 8
m a x
（ 4.132 ）
Devices converting infrared radiations into electric
quantities are called infrared detectors.
Two types,
Thermo-sensitive detectors
Photosensitive detectors
4.11.2 Infrared detectors
Nonselective infrared detectors,
Thermo-sensitive detectors take advantage of the
heat effect produced by semiconductor films when
exposed to infrared radiation.
The thermo-sensitive device has a relatively long
response time of the order of 10-3 s.
Basically,thermo-sensitive detectors have the same
response to radiations with different wavelengths,
They have a flat optical spectrum,and a nearly
constant sensitivity over the whole wavelength
range of measurement.
4.11.2 Infrared detectors
4.11.2 Infrared detectors
A ph oto electr ical detector i s a sem icond uctive o ne,
W hen ph oto ns are striking o n th e ph oto sensitive
com po nen t,elect ron - ho le pairs ar e deco m po sed to
gen erat e electr ical signals,A s the p ho toelectr ical
ef f ect appears v ery f ast,respon se of a ph oto electr ical
detector to inf rar ed radiation is m uch f aster t han that
of a therm o - sensitive d etector,usu ally in the o rder of
ms
or even
s?
,
T h e d etecto r ha s an u n ch an g ed w av elen g th rang e,an d a
p eak v alu e at th e w av elen g th,
p
,T h e resp o n se c o m es
rapid ly to sto p w h en th e w av elen g th ex ceed s
p
(see
cu rve 2 in F ig,4,1 0 8 ),T h e reaso n is th at th e am o u n t o f
p h o ton s is ins u f f icien t to activ ate th e releas e o f electro n s
w ithin a ran g e o f w av elen g ths lar g er tha n a ce rtain
w av elen g th,
4.11.2 Infrared detectors
Photoelectrical detectors operate only in low
temperatures
4.11.2 Infrared detectors
Fig,4.108 Optical spectrum response of infrared detector
1,Radiation pyrometers
4.11.3 Applications
Fig,4.109 Operating principle for radiation pyrometer
The Stefan-Boltzmann’s law can be used in radiation
pyrometers.
4.11.3 Applications
T he m easured ob ject i s usu ally a gr ay bo dy w ith its
1,U sing the blackb od y radia tion as a c riter ion
f or calib ration,it is po ssible to evalu ate the
tem perature of the m easured ob j ect by m eans of E q,
(4.13 0) and the def init ion of?,if the? of the
ob ject i s kn ow n,
Assuming that all energy radiated by the gray body is
absorbed by the blackbody,their total energies must
be identical,that is,
4.11.3 Applications
4
00
4
TT
w h ere
= sp ecif ic rad ian ce o f th e m easu red o b ject
0
= sp ecif ic rad ian ce o f b lack b o d y,1
0
T
= ab so lute tem p eratu re o f o b ject
= th e S tef an - B o ltzm an n co n stan t
T h u s w e h av e
4
o
T
T? （ 4,1 2 6 ）
2,Infrared thermometry
4.11.3 Applications
Radiati on py rom ete rs ar e usuall y used in m easur em ent
of te m pera ture s higher than C?800,Ho w ever,the
inf ra r ed ther m om etr y w e ar e now deal ing w it h
concer ns w it h m easur em ents i n low te m pera ture and
inf ra re d light r ange,
Fig,4.110 Block diagram of an infrared thermometering device
4.11.3 Applications
3,Infrared thermal imaging
As the infrared light is invisible to human eyes,it is
impossible to use common cameras to take infrared
pictures,Infrared thermal imaging technique can be
used to convert infrared radiation into visible light
for display.
4.11.3 Applications
Fig,4.111 Active infrared thermal imaging
1-infrared light source
2-infrared camera
3-monitor
There are two types of thermal imaging:
Active thermal imaging,utilizes an infrared
radiation source to irradiate the measured object,
The reflected radiation from the measured object is
received by an infrared camera,
Passive thermal imaging,acquires the thermal
images directly from the infrared radiation of the
object itself.
4.11.3 Applications
The image thus formed is called the thermal
image,and the device for acquiring thermal
images is called the thermal imaging system,
A thermal imaging system doesn’t need any
extra infrared light source,is easy to manipulate,
and can acquire accurately the thermal images
representing object’s temperature field,
4.11.3 Applications
Fig,4.112 Arrangement of optical system of
an infrared thermal imaging system
4.11.3 Applications
Infrared thermal imaging systems have been
widely used in industry,in measurement of
temperatures for different environmental
conditions,such as thermal deformation of
machine parts due to temperature rising.
4.11.3 Applications
Fig,4.113 Isothermal curve plot of the section of a lathe spindle bearing
4.11.3 Applications
Fig,4.114 Thermal imaging detection for supersonic cavitation
4.11.3 Applications
1-nozzle
2-model
3-thermal camera
4-observation window
Thermal imaging systems are used to detect
temperature rising of electrical equipment,especially
switches and power lines,to prevent possible faults.
In non-destructive detection of fissures,air pores,
foreign substances,and sectional area abberances
of sectional areas of different materials such as
metal,ceramics,plastics,multi-layer fiber plates,
etc
In power industry
Thermal imaging systems are used for safety control,
in petrochemical and metallurgical industries
4.11.3 Applications
In public security and fire control,thermal imaging
systems are used to detect in-site the damages by fire
and the situations of the persons remaining in the
building in fire,
Another very useful application of thermal imaging is
in the medical diagnosis.
4.11.3 Applications
Fig,4.115(a) Thermal image (breast cancer) acquired by thermal
imaging system; Female; sustained high-temperature in left breast;
4.11.3 Applications
N i p p l e s t e m p e r a t u r e d i f f e r e n c e,1,3 C? ; D i a g n o s i s,l e f t b r e a s t c a n c e r
4.11.3 Applications
Fig,4.115(b) Periarthritis of shoulder (scapulohumeral periarthritis);
Male; Symptom,sustained high-temperature in right shoulder
Solid-state image sensor is a solid-state integrated
element,whose kernel part is a charge-coupled
device (CCD),
A CCD device is composed of a number of metal-
oxide-semiconductor (MOS) capacitors arranged
on a substrate,which has the functions of
generating photo-electric charges,storing and
transferring the charges,
4.13 Solid-state image sensors
The basic principle for CCD camera:
Usually a silicon wafer has hundreds or thousands of
mutually-independent,regularly-lined MOS
photosensitive elements known as the photosensitive
element array,If a positive bias is applied to metal
electrodes,hundreds or thousands of mutually-
independent potential traps will be generated in silicon
wafer,If an image with dark-bright variations is
exposed to the photosensitive elements,they will
produce a photoelectrical image corresponding to the
light intensities,This is the basic principle for CCD
camera,
4.13 Solid-state image sensors
Line-array and area-array cameras based on CCD
devices can transmit image information,and have
been widely used in areas like TV,fax,
photography,image transmission and processing,
Especially for such advantages as small size,high
speed,high sensitivity,high stability,and non-
contact measurement,CCD devices are also used
in size and position measurements,event counting,
image recognition,and automatic monitoring and
control,etc,
4.13 Solid-state image sensors
Fig,4.116 Self-scanning diode arrays and cameras.
(a) MOS photo-sensitive unit
4.13 Solid-state image sensors
Fig,4.116 Self-scanning diode arrays and cameras,
(b) 1024 element array
4.13 Solid-state image sensors
Fig,4.116 Self-scanning diode arrays and cameras.
(c) line-scan camera
4.13 Solid-state image sensors
Fig,4.117 Application example,two-dimensional measurand acquisition
with CCD line-array camera
4.13 Solid-state image sensors
Fig,4.118 shows an example of object position
detection by use of a laser-optical triangulation sensor.
4.13 Solid-state image sensors
Fig,4.118 Displacement and profile measurement using
triangulation method
An example of triangulation measurement of
workpiece profile,
4.13 Solid-state image sensors
Fig,4.119 3-D profile measurement by use of triangulation method
An example of the measurement of object’s 3-D
profile using line-projection method
4.13 Solid-state image sensors
Fig,4.120 Measuring principle of line-projection method
The moiré fringes of a measured object
4.13 Solid-state image sensors
Fig,4.121 Measuring principle of moiré method
Fig,4.122 Example of a moiré structure
4.13 Solid-state image sensors
Chapter 5 Signal conversion and
conditioning
Prof,Wang Boxiong
Chapter 5 Signal conversion and
conditioning
5.1 Introduction
5.2 Bridges
5.3 Modulation and demodulation
5.4 Filters
5.1 Introduction
A mechanical quantity,after it has been detected and
transduced into an electrical form,needs to be further
converted,amplified,filtered and analyzed to remove
unwanted noise and to improve signal-to-noise ratio
before it is used in a suitable form for driving an
indicator or recorder as well as for further possible
processing with a computer,
The signal conversion and conditioning covers a very
wide area,our study will focus on some techniques in
common use such as bridges,signal modulation and
demodulation (MODEM),signal filtering,analog-to-
digital and digital-to-analog conversions,
5.2 Bridges
A bridge is a measuring circuit,which converts
a variation in resistance,capacitance and
inductance into an equivalent voltage or
current output.
According to the types of excitation power
supply,bridges are usually divided into dc
bridges and ac bridges; according to their
operating principles,they are divided into the
null and the deflection types.
5.2.1 DC Bridges
Of all the possible configurations,the
Wheatstone resistance bridge is undoubtedly
used to the greatest extent,
In applications,one or more of the arms is a
resistance transducer whose resistance is to be
determined,Typical resistances used with a
circuit of this kind include resistance
thermometers,thermistors,or resistance-type
strain gages,
5.2.1 DC Bridges
Fig,5.1 DC bridge circuit
Requirements for bridge
balance,When the output
terminals are connected with
meter or amplifying circuit
having large input impedance,
the bridge circuit can be
considered as an open circuit,
and the output current is zero,
hence:
5.2.1 DC Bridges
43
0
2
21
0
1
KR
e
I
KR
e
I
0
21
1
41 eRR
RRIU
ab
0
43
4
42 eRR
RRIU
ad
(5.1)
(5.2)
(5.3)
The output voltage:
5.2.1 DC Bridges
04321
4231
0
43
4
0
41
1
e
RRRR
RRRR
e
RR
R
e
RR
R
UUe adaby
(5.4)
In balance,the voltage across the meter is zero,
then,
4231 RRRR?
is the condition for the balance of a dc bridge,
Any change in one or more of the four
resistances will cause the bridge unbalanced,
resulting in a voltage output,
(5.5)
5.2.1 DC Bridges
Commonly-used bridges are usually connected
into forms of single-arm-half-bridges,double-
arm-half-bridges,and full-bridges.
Single-arm-half-bridge circuit,Letting the
resistance be R1 and the variation ΔR,
0
43
4
21
1 e
RR
R
RRR
RRe
y
(5.6)
When R1=R2=R0,R3=R4=R0’,Further,we
assume R0=R0’,Eq.(5.6) becomes
5.2.1 DC Bridges
0
0 24
eRR Re y
(5.7)
Usually ΔR<<R0,so
0
04
eRRe y
(5.8)
The output voltage ey is proportional to the
exciting voltage e0,and has pure linear
relationship with the resistance change of the
varied arm,if e0 and R0 are fixed.
5.2.1 DC Bridges
Fig,5.2 Configurations of DC bridges
(a) single-arm-half-bridge circuit;
(b) double-arm-half-bridge circuit;
(c) full-bridge circuit
5.2.1 DC BridgesDouble-arm-half-bridge connection,
two bridge arms (usually two adjacent arms)
change with the measurand:
11 RR 22 RR
,
When:
021 RRR,'43 RRR?＝,RRR 21
and '
00 RR?
Then,the output voltage
0
02
eRRe y
(5.9)
5.2.1 DC BridgesFull-bridge connection,
four bridge arm resistances vary with the
measurand:
11 RR 22 RR
,
When:
021 RRR,'43 RRR?＝,RRR 21
and '
00 RR?
Then,the output voltage
(5.10)
,33 RR and 44 RR
0
0
eR Re y
5.2.1 DC Bridges
When the resistance changes of the four arms are
of the same sign,that is:
11 RR 22 RR 33 RR 44 RR
04321 RRRRR ＝＝
,,,and let
and when the change in resistance is small in
comparison with the resistance itself,then the
output voltage is approximated as,
,
0
4321
4
1 e
R
R
R
R
R
R
R
Re
y
(5.11)
5.2.1 DC Bridges
The rule of the influence on the output due to the
resistance changes of arms:
1,The output voltage due to the resistance changes of
two adjacent arms is the difference between the
output voltages caused by the resistance change of
each adjacent arm independently;
2,The output voltage due to the resistance changes of
two opposite arms is the sum of the output voltages
caused by the resistance change of each opposite arm
independently,
5.2.1 DC Bridges
Fig,5.3 Cantilever strain
gage arrangement
The addition-subtraction property of bridge,
applied in many measuring circuits,also be used
for automatic compensation of errors due to
temperature variations.
5.2.1 DC Bridges
Sensitivity of a bridge,
R
eS y
,where ΔR/R is the input of the bridge.
Fig,5.4 Arrangements used
to balance dc resistance
bridges
5.2.1 DC Bridges
Disadvantages of a dc bridge:
Liable to be influenced by power line frequency
interference.
The output must be amplified with a dc amplifier,
which is relatively complex and is liable to be
influenced by zero drift and ground potential.
Advantages of a dc bridge:
Output of high accuracy.
No stray capacitance.
Balancing of a dc bridge is relatively simple,only
the arm resistances should be adjusted.
5.2.2 AC Bridges
An ac bridge has the structure similar to a dc
bridge,The only difference lies in that the ac
bridge uses ac power supply as its exciting
source,and the arms can be resistors,
capacitors or inductors.
Fig,5.5 Arrangement
of an ac bridge
5.2.2 AC Bridges
The balance equation for a dc bridge:
4231 zzzz?
(5.13)
where zi,complex reactance of each arm,ij
ii eZz
Zi,modulus of the complex reactance,
φi,phase angle of the complex reactance.
Substituting it in Eq,(5.13) yields
4231 4231 jj eZZeZZ (5.14)
4231
4231
ZZZZ (5.15) Equivalent:
5.2.2 AC Bridges
Since a reactance angle represents the phase-shift
between the current and the voltage of a bridge
arm,when the arm is a resistor,φ=0,that is,its
current and the voltage are in phase,For an
inductive arm,φ>0,while for a capacitive arm,
φ<0,Since balancing ac bridge must satisfy the
two conditions for both modulus and phase,
various combinations of bridge arms can be
employed.
5.2.2 AC Bridges
Fig,5.6 Commonly-used ac
bridge arrangements
(a) Capacitive (b) inductive
Fig,5.6(a) is a capacitive bridge,according to
Eq.(5.15),there is,
2
4
43
1
1
11 R
cjRRcjR
(5.16)
5.2.2 AC Bridges
Expanding Eq.(5.16),thus we have:
4
2
1
3
4231
c
R
c
R
RRRR (5.17)
As the balance of an ac bridge requires the
conditions for both modulus and argument,it is
much more complex than the balance of a dc
bridge,Even for a pure-resistance ac bridge,the
stray capacitances formed by lead wires also
influence the arm resistance values (Fig,5.7(a)).
5.2.2 AC Bridges
Fig,5.7 AC bridge balance (a) stray capacitances
(b) resistance ac bridge using both resistance and capacitance balances
5.2.3 Problems related to bridge applications
Bridge circuits have extensive applications in
strain-gage measuring circuits,A bridge
circuit will be also affected by the length
changes in lead wires of sensing elements,
Besides,the sensitivity must be adjusted in
different applications to suit to different
accuracies.
1.Compensation for leads
5.2.3 Problems related to bridge applications
Fig,5.9 (a) Simple bridge with remotely located sensor; (b) circuit similar to
that shown in (a),but with a compensating wire
5.2.3 Problems related to bridge applications
2,Bridge sensitivity adjustment
Adjustable bridge sensitivity may be desired for
several reasons,
① Attenuate inputs that are larger than desired.
② Provide a convenient relationship between
the system calibration and the scale of the
readout instrument,
③ Provide adjustment for adapting individual
transducer characteristics to recalibrated
systems,
5.2.3 Problems related to bridge applications
④ Provide means for controlling certain
extraneous inputs such as temperature
effects.
A very simple method of adjusting bridge
output is to insert a variable series resistor in
one or both of the input leads,the input to the
bridge will be reduced by the factor
RRRR
Rn
ss?
1 1
n,the bridge factor.
5.2.3 Problems related to bridge applications
The bridge output will be reduced by a
proportional amount,which makes this method
very useful for controlling bridge sensitivity.
Fig,5.10 Method for adjusting bridge sensitivity through use of
variable series resistance,Rs
