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Chapter 4 : Signal conditioning

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Title: Welcome! Author: NEC Computers International Last modified by: naila Created Date: 7/1/2005 8:05:27 AM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Chapter 4 : Signal conditioning


1
Chapter 4 Signal conditioning
  • 4.1 Introduction to signal conditioning
  • 4.2 Bridge circuits
  • 4.3 Amplifiers
  • 4.4 Protection
  • 4.5 Filters

2
Introduction
3
ELECTRICAL MEASUREMENT SYSTEM
WHY?
  • Easy to transmit signal from measurement site
  • the data collection site
  • Easy to amplify, filter and modify
  • Easy to record the signal

4
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.

5
Signal conditioning
6
Bridge circuits
7
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

8
Wheatstone bridge
  • D voltage detector

9
Exercise 1
  • Determine
  • R4 if a Wheatstone bridge nulls with
    R1 1000 O, R2 842 O, and R3 500 O.
  • The voltage offset if the supply voltage is 10.0
    V. The resistors in a bridge are given by R1 R2
    R3 120 O and R4 121 O.

10
Galvanometer detector
11
Exercise 2
  • A bridge circuit has a resistance of
    R1 R2 R3 2.00 kO and R4 2.05 kO
    and a 5.00 V supply. If a galvanometer with a
    50.0 O internal resistance is used for a
    detector, calculate the offset current.

12
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

13
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

14
Current balance bridge
15
Current balance bridge
  • Used current to null bridge

16
Exercise 3
  • A current balance bridge has a 10 V supply
    voltage and resistors R1 R2
    10 kO, R3 1 kO, R4 950 O, R5
    50 O and a high impedance null detector.
    Determine the current required to null the bridge
    if R3 increased by 1 O.

17
Potential measurements using bridges
18
Potential measurements using bridges
19
Exercise 4
  • A bridge for potential measurement nulls when
    R1 R2 1 kO, R3 605 O, and R4 500 O with a
    10.0 v supply. Determine the unknown potential.

20
Exercise 5
  • A current balance bridge is used for potential
    measurement. The fixed resistors are R1 R2 5
    kO, R3 1 kO, R4 990 O, and R5
    10 O with a 10 V supply. Calculate the current
    necessary to null the bridge if the potential is
    12 mV.

21
Amplifiers
22
Op amp characteristic
23
Summing amplifier
24
Noninverting amplifier
25
Exercise 7
  • Design a high impedance amplifier with a voltage
    gain of 42 if R1 1 kO is chosen.

26
Differential amplifier
  • The transfer function
  • Common mode rejection

27
Voltage-to-Current converter
28
Current-to-Voltage converter
29
Exercise 8
  • For a voltage-to-current converter using an
    op-amp, show that the relationship between
    current and voltage is given by
  • .

30
Integrator
31
Exercise 9
  • Use an integrator to produce a linear ramp
    voltage rising at 10 V per ms. Determine the R
    and C.

32
Differentiator
33
Linearization
34
Linearization
35
Filters
36
Filters
  • Filter a circuit that is designed to pass
    signals with desired frequencies and reject or
    attenuate others
  • 4 types of filters
  • Low-pass filter passes low frequencies and stops
    high frequencies
  • High-pass filter passes high frequencies and
    rejects low frequencies
  • Band-pass filter passes frequencies within a
    frequency band and blocks or attenuates
    frequencies outside the band
  • Band-reject filter passes frequencies outside a
    frequency band and blocks or attenuates
    frequencies within the band

37
Low-pass RC filter
38
Low-pass RC filter
  • Critical frequency
  • Output-to-input voltage ratio

39
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?

40
High-pass RC filter
41
High-pass RC filter
  • Critical frequency
  • Output-to-input voltage ratio

42
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.

43
Design Methods
  1. Determine critical frequency, fc
  2. Select standard capacitor (µF pF)
  3. Calculate required resistance (1 k? - 1 M?)
  4. Use nearest resistance standard value to
    calculated value
  5. Consider tolerance in resistors and capacitors

44
Practical considerations
  1. Very small resistance -gt lead to large currents
    and loading effects -gt avoid large capacitance
    (R kO -MO, 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
    follower
  4. Cascade RC filters to improved fc sharpness -gt
    consider loading

45
Band-pass RC filter
46
Band-pass RC filter
  • Critical frequency
  • Output-to-input voltage ratio

47
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.

48
Band-pass RC filter
49
Band-reject RC filter
50
Twin-T notch filter
51
Twin-T notch filter
  • Critical frequency
  • Grounding resistor and capacitor

52
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.
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