Chapter 4 : Signal conditioning

transcript

1BMCC 3743Signal ConditioningNH

4.1 Introduction to signal conditioning4.2 Bridge circuits4.3 Amplifiers4.4 Protection4.5 Filters

2BMCC 4743 Signal ConditioningNH

Introduction

ELECTRICAL MEASUREMENT SYSTEM

1. Easy to transmit signal from measurement site the data collection site2. Easy to amplify, filter and modify3. Easy to record the signal

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.

Signal conditioning

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

Wheatstone bridge• D : voltage detector

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.

Galvanometer detector

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.

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

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

Current balance bridge

Current balance bridge• Used current to null bridge

IRVRRR

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.

Potential measurements using bridges

IRVRRR

VVVVVV

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.

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.

Amplifiers

Op amp characteristic

Summing amplifier

2 VRRV

RRVout

Noninverting amplifier

outinin

Exercise 7

Design a high impedance amplifier with a voltage gain of 42 if R1 = 1 kΩ is chosen.

Differential amplifier

CMRRCMRAACMRR

10log20

baout VVAV • The transfer function;

• Common mode rejection;

2 VVRRVout

Voltage-to-Current converter

Current-to-Voltage converter

IRVout

Exercise 8For a voltage-to-current converter using an op-amp, show that the relationship between current and voltage is given by

2 VRRR

I R1 R2

Integrator

Exercise 9

Use an integrator to produce a linear ramp voltage rising at 10 V per ms. Determine the R and C.

Differentiator

dtdVRCV

Linearization

outin 0

Linearization

eincout

outout

log1log1exp

Filters

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

Low-pass RC filter

• Critical frequency:

• Output-to-input voltage ratio:RC

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?

High-pass RC filter

• Critical frequency:

• Output-to-input voltage ratio:RC

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.

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

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

Band-pass RC filter

Band-pass RC filter• Critical frequency:

• Output-to-input voltage ratio:HH

ffrffff

2222 1

LLH CRf

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.

Band-pass RC filter

Band-reject RC filter

Twin-T notch filter

Twin-T notch filter• Critical frequency:

• Grounding resistor and capacitor:

cn ff 785.0 RCfC 2

cH ff 57.4cL ff 187.0

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.

Chapter 4 : Signal conditioning

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