Download Basic Input Circuits

Basic Input Circuits
Basic Input Circuits
Most common input circuits used in transducer work:
Prof. Dr. M. Zahurul Haq
[email protected]
http://teacher.buet.ac.bd/zahurul/
Department of Mechanical Engineering
Bangladesh University of Engineering & Technology
1
Current-sensitive Circuit
2
Voltage-sensitive (ballast) Circuit
3
Voltage-divider Circuit
4
Voltage-balancing Potentiometer Circuit
5
Bridge Circuit
6
Amplifier Circuit
ME 361: Instrumentation & Measurement
c Prof. Dr. M. Zahurul Haq (BUET)
Basic Input Circuits
ME 361
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Current-sensitive Input Circuit
c Prof. Dr. M. Zahurul Haq (BUET)
Basic Input Circuits
ME 361
Voltage-sensitive (ballast) Circuit
! i = R +EiR
i
! i = R +EiR
i
)
i
Ei =Ri
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)
= 1 + (R=Rm1)(Rm =Ri )
Eo
Ei
= 1 +(R(=RR=mR)(R)(mR=R=iR) )
m
m
i
f002.eps
f001.eps
Current flow through the circuit indicates sensor’s resistance R.
Current flow varies linearly with R for some ranges of operation.
Voltmeter impedance is very high & draws negligible current.
Change in R is indicated by voltage change.
Ei Ri
dEo
Sensitivity, S
dR
(R+Ri )2
=
=
For maximum sensitivity,
dS
dRi
= 0 = E(R(R+RR) ) :=)
i
i
i
3
Ri
=R
Homework: Example 4.1 (Holman)
c Prof. Dr. M. Zahurul Haq (BUET)
Basic Input Circuits
ME 361
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c Prof. Dr. M. Zahurul Haq (BUET)
Basic Input Circuits
ME 361
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Voltage-divider Circuit
Voltage-balancing Potentiometer Circuit
h
Loading Error =
Eo
Ei
h
i
htrue i
Eo
Ei
Eo
Ei
i
ind
true
f003.eps
f004.eps
Source voltage Ei is impressed across the total transducer
resistance Rm , while the variable contact is connected to a
voltmeter with internal resistance Ri .
For Ri
Rm : =)
For Ri =Rm :
=)
h
h
Used for precise measurements of small electrical potentials by
comparison, particularly those generated by thermocouples.
A known portion of source voltage Ei is balanced against the
unknown voltage Eo through the use of a variable resistor Rm .
At balance point, current through the galvanometer is zero.
i
=
=
ind 1+(R =R
Eo
R
Ei true Rm
Eo
Ei
i
m
R =Rm
R =R m R m =R i
)(1
)(
=) EE = RR
)
o
i
Homework: Example 4.2 (Holman)
c Prof. Dr. M. Zahurul Haq (BUET)
Basic Input Circuits
ME 361
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Wheatstone Bridge Circuit
m
Galvanometer resistance Ri does not effect the reading.
Source voltage Ei must be accurately known to determine Eo .
c Prof. Dr. M. Zahurul Haq (BUET)
Basic Input Circuits
ME 361
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Null Method:
Vout
= 0 7 ! ig = 0 =) RR = RR
2
1
3
4
If the resistor R2 varies with changes in the measured physical
variable, one of the other arms of the bridge can be adjusted to
null the circuit and determine the resistance. The balancing
mechanism may be accomplished manually or automatically
through a closed loop controller circuit. The values of the other
resistors must be precisely known.
Deflection Method:
Used for dynamic signals where time is insufficient for balancing.
Galvanometer/detector deflection indicates the deviation at one of
the arms (transducer arm) from some balanced condition.
Bridge output may be measured by:
1
e671.eps
c Prof. Dr. M. Zahurul Haq (BUET)
2
Basic Input Circuits
ME 361
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Low-impedance device
High-impedance device
c Prof. Dr. M. Zahurul Haq (BUET)
Current-sensitive circuit.
Voltage-sensitive circuit.
Basic Input Circuits
ME 361
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WB Circuit: Deflection Method
Strain Gauge using Wheatstone Bridge
Voltage-sensitive Circuit: Vout is measured by high impedance
R2 .
device. At balanced condition, Vout 0. If R2 ! R2
R2 =R R1 R
R2
R3
) VVoutin R1R+2R+
R
4
R 3 +R 4
4+2(R2 =R )
2 +R2
=
=
=
=
+
=
=
Current-sensitive Circuit: ig is measured by galvanometer with Rg .
) ig RTHVout+ Rg VCout R2R4 R1R3
C R1 R2 R3 R4
R3 R4 R1 R2
Rg R1 R2 R3 R4
At null position ) R2 R4 R1 R3
ig 0
If transducer resistance, R2 ! R2
R2
) ig ! ig .
) ig VCout R2 R2 R4 R1 R3 VCout R4 R2
=
=
(
)
=
( + )+
( + )+ ( + + + )
=
=
: =
+ =
= = [( + )
]=
Homework: Example 4.4, 10.6 & 10.7 (Holman)
Homework: Example 6.4 & 6.5 (Figliola)
c Prof. Dr. M. Zahurul Haq (BUET)
e677.eps
Basic Input Circuits
ME 361
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c Prof. Dr. M. Zahurul Haq (BUET)
Basic Input Circuits
ME 361
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