Lecture 4A SignalConditioning DCBRIDGE SV
-
Upload
fazdrul-akiff -
Category
Documents
-
view
248 -
download
1
Transcript of Lecture 4A SignalConditioning DCBRIDGE SV
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
1/48
Signal Conditioning for
Electronic Instrumentation
1
MCT 3332 : Instrumentation and Measurements
Dr. Hazlina Md Yusof
Department of Mechatronics Engineering
International Islamic University Malaysia
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
2/48
2
Content:
Introduction
Bridge Circuits
Op-Amp circuits for Inverting
Non-inverting amplifier High impedance buffer
Differential amplifier
Instrumentation amplifier Active filters
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
3/48
Analog Signal Conditioning
1. Introduction
Signal conditioning is used to process an output
signal from a sensor to make it suitable for the nextstage of operation.
3
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
4/48
Analog Signal Conditioning
Signal conditioning is applied when signal:-
is too small and needs to be amplified
contains interference which need to be attenuated
is non linear and needs to be linearized
Need to be differentiated/integrated
in analogue form and digital form is needed
in the form of resistance change needs to be converted into
current change
in the form of voltage change needs to be converted intocurrent change
4
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
5/48
Analog Signal Conditioning
Advantages of signal conditioning
to prepare the signal to be in a suitable form for interfacing with
other elements in the process-control loop.
In analog conversions, the conditioned output signal is still in
an analog representation of the variable.
Even in applications involving digital processing, some type of
analog conditioning is usually required before analog-to-
digital conversion is made.
5
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
6/48
Analog Signal Conditioning
Principles of analog signal conditioning
sensor measures a variable/measurand by convertingthe information about that variable into a dependentsignal of either electrical or mechanical in nature.
analog signal conditioning provides the necessaryoperations to transform the sensor output into a formnecessary to interface with other elements.
the effect of the signal conditioning is defined by atransfer function
6
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
7/48
7Measurement Sub-system
A/D board
Analog Signal Conditioning
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
8/48
Analog Signal Conditioning
Signal Conditioning is categorized into several types:
(a) Linearization
(b) Conversion(c) Filtering and Impedance Matching
(d) Concept of Loading
8
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
9/48
Analog Signal Conditioning
(a) Linearization
a sensor output varies nonlinearlywith a process variable.
a linearization circuit will condition
the sensor output to be linear withthe process variable.
generally, these types of circuits aredifficult to design and operate onlywithin narrow limits.
9
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
10/48
Analog Signal Conditioning
Modern approach :
i. provide the nonlinear signal as input to a computer and
perform the linearization using software.
ii. virtually any nonlinearity can be handled in this manner
and, with the speed of modern computers, can be done inreal time.
10
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
11/48
Analog Signal Conditioning
(b) Conversion
Circuits can be used to convert the resistance change to a :
(i) voltage signal
(ii) current signal (i.e. bridge)
11
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
12/48
Analog Signal Conditioning
(c) Filtering and Impedance Matching
(i) Filtering
high-pass, low-pass, or notch filtersare used to attenuate
unwanted signals from the process control loop.
filtering can be accomplished by, passivefilters; resistors, capacitors, and inductors activefilters, using amplifier and feedback.
(ii) Impedance matching
active and passive circuit networks are employed to provide
impedance matching to maximum power transfer from source to
load
12
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
13/48
Analog Signal Conditioning
(d) Concept of Loading
the loading of one circuit byanother is important.
loading occurs when something is
connected to an open circuit
(across the output), and the output
voltage of the element drops to
some value, Vy < Vx.
Different loads result in different
drops.
13
RLis connected across the output of
the element. When a current flow,and voltage will be dropped across
The loaded output voltage will be
The voltage that appears across theload, instead of Vx is reduced by the
voltage dropped across the internal
resistance.
xL
xxy
RR
R1VV
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
14/48
Analog Signal Conditioning
Problem 1
An amplifier outputs a voltage that is ten times the
voltage on its input terminals. It has an input
resistance of 10k. A sensor outputs a voltage
proportional to temperature with a transfer function
of 20mV/C. the sensor has an output resistance of
5.0k . If the temperature is 50 C, find the
amplifier output.
14
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
15/48
Analog Signal Conditioning
3. Passive circuits
bridge and divider circuits are two passive
techniques that is used for signal conditioning bridge circuits are used for measuring changes in
impedance.
a passive circuit consists of a resistor and capacitor
can be used to eliminate high- and low-frequencynoise without changing the desired signal
information.
15
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
16/48
Analog Signal Conditioning
(a) Divider Circuits
The voltage of such a divider isgiven by the well-known
relationship
(2.2)
where
VS, = supply voltage
R1, R2=divider resistors16
21
2
RR
VRV
S
D
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
17/48
Analog Signal Conditioning
Problem 2
The divider of figure has R1=10.0kandVs=5V. Suppose R2is a sensor whoseresistance varies from 4.00 to 12.0k as some dynamic variable varies over arange. Then find
(a) The maximum and minimum of VD(b) The range of output impedance
(c) The range of power dissipated by R2
17
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
18/48
Analog Signal Conditioning
Issues to be considered for converting resistance tovoltage :
1. the variation of VDwith either R1or R2is nonlinear,
even if the resistance varies linearly with themeasured variable.
2. the effective output impedance of the divider is theparallel combination ofR2and R1.
3. in a divider circuit, current flows through bothresistors; power will be dissipated by both, includingthe sensor.
18
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
19/48
Analog Signal Conditioning
(b) Bridge Circuits
Bridge Circuit is a null method, operates on the principleof comparison. That is a known (standard) value isadjusted until it is equal to the unknown value.
used to convert impedance variations into voltagevariations.
advantages : it can be designed so the voltage produced varies around
zero. amplification can be used to increase the voltage level for
increased sensitivity to variation of impedance.
19
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
20/48
Analog Signal Conditioning
20
Bridge Circuit
DC Bridge(Resistance)
AC Bridge
Inductance Capacitance Frequency
Schering Bridge Wien BridgeMaxwell Bridge
Hay BridgeOwen BridgeEtc.
Wheatstone Bridge
Kelvin BridgeMegaohm Bridge
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
21/48
Analog Signal ConditioningWheatstone bridge
A Wheatstone bridge is an electrical circuit used to
measure an unknown electrical resistance by balancing
two legs of a bridge circuit, one leg of which includes the
unknown component.
Its operation is similar to the original potentiometer. It was invented by Samuel Hunter Christie in 1833 and
improved and popularized by Sir Charles Wheatstone in
1843.
One of the Wheatstone bridge's initial uses was for thepurpose of soils analysis and comparison
21
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
22/48
Analog Signal ConditioningWheatstone bridge
Wheatstone Bridge simplest and common bridge circuit .
used in signal-conditioning applicationswhere a sensor changes resistance withprocess variable changes.
Dis a high-input impedance differential
amplifiervoltage detector used tocompare the potentials of the points aand bof the network.
22
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
23/48
Analog Signal ConditioningWheatstone bridge
Instrument for making
comparison in measurements
Operates on null-indication
principle
Indication is independent of the
calibration of the indicating device
Able to achieve higher degree of
accuracy
Used in control circuits
One arm contains a resistive
element that is sensitive to thephysical parameterbeing
controlled
23
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
24/48
Analog Signal ConditioningWheatstone bridge
Accurate and reliable
Heavily used in industry
Accuracy of 0.1% compared to ordinary ohmmeter for resistance
measurement
Bridge used to determine an unknownresistor. Vary one of the remaining resistors until the current through the null
detector (usually a galvanometer other sensitive current meters ) is
zero.
This indicates that the bridge is in balanced condition.24
GC D
A
B
R1
R3
R2
R4
I4
I1
I3
I2Ratio arms
Standard arm
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
25/48
Analog Signal ConditioningWheatstone bridge
25
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
26/48
Analog Signal ConditioningWheatstone Bridge
26
Analog Signal Conditioning
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
27/48
Analog Signal ConditioningWheatstone bridgeMeasurement Errors
27
1. Limiting error of the known resistorsUsing 1st order approximation:
2. Insufficient sensitivity of Detector3. Changes in resistance of the bridgearms due to the heating effect (I2R) or
temperatures4. Thermal emf or contact potential in
the bridge circuit5. Error due to the lead connection3, 4 and 5 play the important role in the
measurement of low value resistance
Analog Signal Conditioning
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
28/48
Example
In the Wheatstone bridge circuit, R3is a decade
resistance with a specified in accuracy 0.2% and R1 and
R2= 500 0.1%. If the value of R3at the null position is
520.4 , determine the possible minimum and maximumvalue of RX
28
Analog Signal ConditioningWheatstone bridgeMeasurement Errors
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
29/48
Example
A Wheatstone bridge has a ratio arm of 1/100 (R2/R1). At
first balance, R3 is adjusted to 1000.3 . The value of Rx
is then changed by the temperature change, the new value
of R3 to achieve the balance condition again is 1002.1 .Find the change of Rx due to the temperature change.
29
Analog Signal ConditioningWheatstone bridgeMeasurement Errors
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
30/48
Analog Signal ConditioningWheatstone bridge
Sensitivity of the Wheatstone Bridge
At an unbalanced condition, current that passes through the
galvanometer will cause a deflection of the pointer.
The amount of deflection is a function of the sensitivity of thegalvanometer.
Deflection, D per unit current.
S = millimeters or degrees or radians
A A A
The total deflection D is then becomes,
D = S x I (I in A) 30
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
31/48
Analog Signal ConditioningWheatstone bridge
Sensitivity of the Wheatstone Bridge A galvanometer is use to detect an unbalance condition in Wheatstone bridge. Its
sensitivity is governed by: Current sensitivity (currents per unit defection) and internal
resistance.
consider a bridge circuit under a small unbalance condition, and apply circuit analysis
to solve the current through galvanometer
To calculate the current that passes the galvanometer Use TheveninsTheorem to find the Thevenin equivalent voltage seen by the
galvanometer.
31
A l Si l C diti i
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
32/48
Analog Signal ConditioningWheatstone bridge
Galvanometer Detector Usage of a galvanometer as a
null detector in the bridgecircuit introduces difference incalculation because :
the detector resistance may below but it is still there
the bridge offset as current offsetmust be determined first.
32
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
33/48
33
Analog Signal ConditioningWheatstone bridge
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
34/48
Example
Figure below show the schematic diagram of a Wheatstone bridge with
values of the bridge elements. The battery voltage is 5 V and its internal
resistance negligible. The galvanometer has a current sensitivity of 10
mm/Aand an internal resistance of 100 . Calculate the deflection of
the galvanometer caused by the 5- unbalance in arm BC
34
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
35/48
Example
The galvanometer in the previous example is replaced by one
with an internal resistance of 500 and a current sensitivity of
1mm/A. Assuming that a deflection of 1 mm can be observedon the galvanometer scale, determine if this new galvanometer is
capable of detecting the 5- unbalance in arm BC
35
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
36/48
Analog Signal ConditioningWheatstone bridge
Example 3
Calculate the current, IG
through the galvanometer in the
circuit. Given
E=6V, R1=1k,R2=1.6k,R3=3.5k,
R4=7.5k , RG=200,
36
Ca
A
B
D
E
R1
R3
R2
R4
b
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
37/48
Analog Signal ConditioningWheatstone bridge
Bridge Resolution
is used to determine the bridge offset.
viewed as an overall accuracy of the instrument.
R represents the uncertainty in the resistor used in the givenbridge and error signal detector (i.e. across terminal a and b).
37
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
38/48
Analog Signal ConditioningWheatstone bridge
Slightly Unbalanced Wheatstone Bridge
38
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
39/48
Example
Circuit in Figure (a) below consists of a resistor Rv which is sensitive to
the temperature change. The plot of R vs Temp. is also shown in Figure
(b). Find (a) the temperature at which the bridge is balance and (b) The
output signal at Temperature of 60C.
39
Analog Signal Conditioning
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
40/48
Analog Signal Conditioning
Kelvin Bridge
40
RY
=1
2
3
Low resistance Bridge: R
x
< 1
Analog Signal Conditioning
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
41/48
Analog Signal Conditioning
Kelvin Double Bridge: 1 to 0.00001
41
Analog Signal Conditioning
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
42/48
42
Analog Signal Conditioning
Kelvin Double Bridge: 1 to 0.00001
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
43/48
Kelvin Bridge
Example 4
If in the figure given, the ratio of Rato Rbis 1000, R1=5
and R1=0.5R2,what is the value of Rx?
43
A l Si l C diti i
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
44/48
Analog Signal ConditioningLead Compensation
used where changes in lead resistance
are introduced equally to two (both)
arms of the bridge circuit without
causing effective changes in bridge
offset.
assume R4is the sensor
wire 3 is the power leadhas no
influence on bridge balance condition changes in wire 2 will give some
changes to R4.
wire 1 is exposed to the same
environment (i.e. lead) and changed by
the same amount
R3and R4are changed identically, no
change in the bridge; null occurs.
this type of compensation is used with
long leads to the active element of the
bridge.
44
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
45/48
Analog Signal ConditioningCurrent Balanced Bridge
a technique that provides an electronicnulling of the bridge (i.e. zeroing network)
and uses only fixed resistors by using
current
this method is used to overcome the
disadvantage of a Wheatstone Bridge
where a null by variation of resistors is
needed.
the Wheatstone bridge is modified by
splitting one arm resistor into two, R4and
R5.
current, I is fed into the bridge throughjunction R4 &R5.
R4R5 (1)
if a high impedance null detector (high
Rm) is used, Eq. 1 will be
(R2+R4)R5 (2)
45
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
46/48
Analog Signal ConditioningCurrent Balanced Bridge
5
542
54
b IRRRR
)RV(RV
46
Vbis the sum of the divided supply voltage + the voltage dropped across R5from the current, I
(3)
the voltage at point a is
the bridge offset voltage is V = Va-Vb
(4)
31
3
aRR
VRV
5
542
54
31
3IR
RRR
)RV(R
RR
VRV
Analog Signal Conditioning
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
47/48
Analog Signal ConditioningCurrent Balanced Bridge
Problem 4
A current balance bridge as
shown has resistors R1=R2=10k
R4= 950 R3=1.0kR5=50
and a high impedance nulldetector. Find the current
required to null the bridge if R3
changes by 1 . The supply
voltage is 10V.
47
A l Si l C diti i
-
8/10/2019 Lecture 4A SignalConditioning DCBRIDGE SV
48/48
Analog Signal Conditioning
Bridge Controlled Circuits
If the ratio of R2to Rvequals the ratio ofR1and R3then the error signal is zero
If the physical parameter changes, Rvwillchange. the bridge becomes unbalanced
in most control applications, the measuredand control parameter is corrected, restoringRvto the value that creates a null conditionat the output of the bridge.
the Rvvalue can be calculated using thebalanced bridge equation.
due to small value of Rv, the error signal isquite lowneeds to be amplified beforebeing used for control purposes
48