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Chapter 4 : Signal conditioning. 4.1 Introduction to signal conditioning 4.2 Bridge circuits 4.3 Amplifiers 4.4 Protection 4.5 Filters. Introduction. ELECTRICAL MEASUREMENT SYSTEM. WHY?. Easy to transmit signal from measurement site the data collection site - PowerPoint PPT Presentation
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1BMCC 3743Signal ConditioningNH
Chapter 4 : Signal conditioning
4.1 Introduction to signal conditioning4.2 Bridge circuits4.3 Amplifiers4.4 Protection4.5 Filters
2BMCC 4743 Signal ConditioningNH
Introduction
3BMCC 4743 Signal ConditioningNH
ELECTRICAL MEASUREMENT SYSTEM
WHY?
1. Easy to transmit signal from measurement site the data collection site2. Easy to amplify, filter and modify3. Easy to record the signal
4BMCC 4743 Signal ConditioningNH
Signal conditioning• Used in factory or machine automation : to convert
sensor or transducer measurement signal levels to industry standard control signals
• Provide computer and control system manufacturers a common communication method to effectively receive and transmit measurement and control data
• Examples of measurement data : temperature or AC/DC voltage/current signals from various transducers
• Examples of control data : on/off signals for a heating element or proportional signals for a valve actuator.
5BMCC 4743 Signal ConditioningNH
Signal conditioning
6BMCC 4743 Signal ConditioningNH
Bridge circuits
7BMCC 4743 Signal ConditioningNH
Bridge circuits
• Used to convert impedance variations into voltage variations
• Can be design so the voltage produced varies around zero
• Amplification can be used to increase voltage level for increased sensitivity to variation of impedance
8BMCC 4743 Signal ConditioningNH
Wheatstone bridge• D : voltage detector
4123
4231
4123
42
4
31
3
RRRR
VRRRR
RRRRV
VRR
RV
VRR
RV
VVV
b
a
ba
9BMCC 4743 Signal ConditioningNH
Exercise 1
Determine;1. R4 if a Wheatstone bridge nulls with
R1 = 1000 Ω, R2 = 842 Ω, and R3 = 500 Ω.
2. The voltage offset if the supply voltage is 10.0 V. The resistors in a bridge are
given by R1 = R2 = R3 = 120 Ω and R4 = 121 Ω.
10BMCC 4743 Signal ConditioningNH
Galvanometer detector
GTh
ThG
Th
Th
RRVI
RRRR
RRRRR
VRRRR
RRRRV
42
42
31
31
4231
4123
11BMCC 4743 Signal ConditioningNH
Exercise 2
A bridge circuit has a resistance of R1 = R2 = R3 = 2.00 kΩ and R4 = 2.05 kΩ and a 5.00 V supply. If a galvanometer with a 50.0 Ω internal resistance is used for a detector, calculate the offset current.
12BMCC 4743 Signal ConditioningNH
Bridge resolution
• Resolution function of detector : to determine the bridge offset
• Resistance resolution : resistance change in 1 arm bridge that causes an offset voltage equal to detector resolution
• Detector can measure change of 100 µV
13BMCC 4743 Signal ConditioningNH
Resolution
• The smallest discernible change in input; the smallest change in input that manifests itself as perceptible change in output that can be measured (example : 0.000 1 mm)
• Primary factor in deciding precision• Good resolution does not imply in good
precision
14BMCC 4743 Signal ConditioningNH
Current balance bridge
15BMCC 4743 Signal ConditioningNH
Current balance bridge• Used current to null bridge
5542
54
31
3
5542
54
542
54
IRVRRR
RRVRR
RV
IRVRRR
RRV
RRRRR
b
16BMCC 4743 Signal ConditioningNH
Exercise 3
A current balance bridge has a 10 V supply voltage and resistors R1 = R2 = 10 kΩ, R3 = 1 kΩ, R4 = 950 Ω, R5 = 50 Ω and a high impedance null detector. Determine the current required to null the bridge if R3 increased by 1 Ω.
17BMCC 4743 Signal ConditioningNH
Potential measurements using bridges
18BMCC 4743 Signal ConditioningNH
Potential measurements using bridges
0
0
0
5
5542
54
31
3
42
4
31
3
IRV
IRVRRR
RRVRR
RV
VRR
RVRR
RV
VVVVVV
x
x
x
bc
axc
19BMCC 4743 Signal ConditioningNH
Exercise 4
A bridge for potential measurement nulls when R1 = R2 = 1 kΩ, R3 = 605 Ω, and R4 = 500 Ω with a 10.0 v supply. Determine the unknown potential.
20BMCC 4743 Signal ConditioningNH
Exercise 5
A current balance bridge is used for potential measurement. The fixed resistors are R1 = R2 = 5 kΩ, R3 = 1 kΩ, R4 = 990 Ω, and R5 = 10 Ω with a 10 V supply. Calculate the current necessary to null the bridge if the potential is 12 mV.
21BMCC 4743 Signal ConditioningNH
Amplifiers
22BMCC 4743 Signal ConditioningNH
Op amp characteristic
23BMCC 4743 Signal ConditioningNH
Summing amplifier
2
3
21
1
2 VRRV
RRVout
24BMCC 4743 Signal ConditioningNH
Noninverting amplifier
inout
outinin
VRRV
RVV
RV
II
1
2
21
21
1
0
0
25BMCC 4743 Signal ConditioningNH
Exercise 7
Design a high impedance amplifier with a voltage gain of 42 if R1 = 1 kΩ is chosen.
26BMCC 4743 Signal ConditioningNH
Differential amplifier
CMRRCMRAACMRR
VVV
cm
bacm
10log20
2
baout VVAV • The transfer function;
• Common mode rejection;
121
2 VVRRVout
27BMCC 4743 Signal ConditioningNH
Voltage-to-Current converter
543
354
42531
31
2
RRR
RIVRR
R
RRRRR
VRRRI
m
sat
ml
in
28BMCC 4743 Signal ConditioningNH
Current-to-Voltage converter
IRVout
29BMCC 4743 Signal ConditioningNH
Exercise 8For a voltage-to-current converter using an op-amp, show that the relationship between current and voltage is given by
.in
31
2 VRRR
I R1 R2
R3
R4R5
RL
I
-+
Vin
30BMCC 4743 Signal ConditioningNH
Integrator
tRCKV
dtVRC
V
dtdVC
RV
out
inout
outin
1
0
31BMCC 4743 Signal ConditioningNH
Exercise 9
Use an integrator to produce a linear ramp voltage rising at 10 V per ms. Determine the R and C.
32BMCC 4743 Signal ConditioningNH
Differentiator
dtdVRCV
RV
dtdVC
inout
outin
0
33BMCC 4743 Signal ConditioningNH
Linearization
RVGV
VIRV
inout
outin 0
34BMCC 4743 Signal ConditioningNH
Linearization
RIVV
VIVI
eincout
outout
0
0
log1log1exp
35BMCC 4743 Signal ConditioningNH
Filters
36BMCC 4743 Signal ConditioningNH
Filters• Filter : a circuit that is designed to pass signals with
desired frequencies and reject or attenuate others• 4 types of filters:
1. Low-pass filter: passes low frequencies and stops high frequencies
2. High-pass filter: passes high frequencies and rejects low frequencies
3. Band-pass filter: passes frequencies within a frequency band and blocks or attenuates frequencies outside the band
4. Band-reject filter: passes frequencies outside a frequency band and blocks or attenuates frequencies within the band
37BMCC 4743 Signal ConditioningNH
Low-pass RC filter
38BMCC 4743 Signal ConditioningNH
Low-pass RC filter
• Critical frequency:
• Output-to-input voltage ratio:RC
fc 21
2/1
1
cin
out
ffVV
39BMCC 4743 Signal ConditioningNH
Exercise 10
A measurement signal has a frequency less than 1 kHz, but there is unwanted noise at about 1 MHz. Design a lowpass filter that attenuates the noise to 1% if a capacitor 0.01 µF has been used. What is the effect on the measurement signal at its maximum of 1 kHz?
40BMCC 4743 Signal ConditioningNH
High-pass RC filter
41BMCC 4743 Signal ConditioningNH
High-pass RC filter
• Critical frequency:
• Output-to-input voltage ratio:RC
fc 21
2/1
/
c
c
in
out
ff
ffVV
42BMCC 4743 Signal ConditioningNH
Exercise 11
Pulses for a stepping motor are being transmitted at 2000 Hz. Design a highpass filter to reduce 60 Hz noise and reduce the pulses by no more than 3 dB.
43BMCC 4743 Signal ConditioningNH
Design Methods
1. Determine critical frequency, fc
2. Select standard capacitor (µF – pF)3. Calculate required resistance (1 kΩ - 1 MΩ)4. Use nearest resistance standard value to
calculated value 5. Consider tolerance in resistors and capacitors
44BMCC 4743 Signal ConditioningNH
Practical considerations
1. Very small resistance -> lead to large currents and loading effects -> avoid large capacitance (R= kΩ -MΩ, C= µF – pF)
2. The exact fc is not important, choose R and C of approximately to the fc
3. Isolation filter input/output with voltage follower4. Cascade RC filters to improved fc sharpness -> consider
loading
45BMCC 4743 Signal ConditioningNH
Band-pass RC filter
46BMCC 4743 Signal ConditioningNH
Band-pass RC filter• Critical frequency:
• Output-to-input voltage ratio:HH
L CRf
21
L
H
HLLH
H
in
out
RRr
ffrffff
ffVV
2222 1
LLH CRf
21
47BMCC 4743 Signal ConditioningNH
Exercise 12
A signal conditioning system uses a frequency variation from 6 kHz to 60 kHz to carry measurement information. There is considerable noise at 120 Hz and at 1 MHz. Design a bandpass filter to reduce the noise by 90%. What is the effect on the desired passband frequencies if r = 0.01? Determine all the resistors and capacitors.
48BMCC 4743 Signal ConditioningNH
Band-pass RC filter
49BMCC 4743 Signal ConditioningNH
Band-reject RC filter
50BMCC 4743 Signal ConditioningNH
Twin-T notch filter
51BMCC 4743 Signal ConditioningNH
Twin-T notch filter• Critical frequency:
• Grounding resistor and capacitor:
cn ff 785.0 RCfC 2
1
cH ff 57.4cL ff 187.0
101RR
CC 10
1
52BMCC 4743 Signal ConditioningNH
Exercise 13
A frequency of 400 Hz prevails aboard an aircraft. Design a twin-T notch filter to reduce the 400 Hz signal if 0.01 µF has been used and calculate the grounding resistor and capacitor. What effect would this have on voice signals at 10 to 300 Hz? Determine the higher frequency when the output is down by 3 dB.