Chapter 9Computer-Based

Instrumentation Systems

1. Operation of Computer-based Instrumentation system.

2. Possible Errors encountered in instrumentation systems.

3. Avoid common pitfalls such as ground loops, noise coupling, andloading when using sensors.

4. Determine specifications for the elements of computer-based instrumentation systems such as data-acquisition boards.

Goal

Overview of Computer-Based Instrumentation

Analog Analog-to-Digital DigitalPhysical-to-Analog

. . . when we need to measure the internal voltage of the sensor, we should specify a signal-conditioning amplifier having an input impedance that is much larger in magnitude than the Thévenin impedance of the sensor.

insensor

insensorinin ZZ

ZVZIV+

==

Model for Voltage Sensing

sensorinsensorin at ZZVV >>=

. . . when we want to sense the current produced by a sensor, we need a current-to-voltage converter having a very small (ideally zero) input impedance magnitude.

Model for Current Sensing

Variable Resistance Sensor

Generally Sensors have varying Impedance. To Get Reliable Data from Sensor, Very High Impedance Amplifier is Necessary

Errors in Measurement Systems

trueError xxm −= %100Error %full

true ×−

=x

xxm

1. Accuracy: Maximum expected difference in magnitude between measured and true values

Usually defined as a percentage of full-scale value

2. Precision: The ability to Repeat the Measurement of a constant measurand.

More precise measurements have less random error.

3. Resolution:The Smallest Increment discernible between measured values.

Higher resolution means smaller increments

Additional Terms for Sensing

SIGNAL CONDITIONING

1. Amplification of the sensor signals

2. Conversion of Currents to Voltages

3. Supply of (ac or dc) Excitations to the sensors so changes in resistance, inductance, or capacitance are converted to changes in voltage

4. Filtering to eliminate noise or other unwanted signal components

Typical functions of Signal conditioners are:

Single-Ended vs Differential Amplifiers

Signals from Sensor are very small Amplification is Necessary- Single Ended Amplifier : Grounded Signal : Two Wires- Differential Amplifier : Floating Signal : 3 Wires

21 vvvd −=

( )21cm 21 vvv +=

Differential Signal

Differential Signal

Common Mode Signal

CMRR : Common Mode Rejection Ratio

cm

log 20CMRRAAd=

Instrumentation Amplifier has Large CMRR

Ground Loops

+ + + + +

1. Many Electrical System Connected to Ground with Different Wires2. Ground Noise (such as 60Hz) swings

Ground Loops

. . . in connecting a sensor to an amplifier with a single-ended input, we should select a floating sensor

Cableg RIVV1Sensorin −=

Various Connections

Noise

Electric field coupling of noise can be reduced by using shielded cables.Magnetically coupled noise is reduced by using coaxial or twisted-pair cables.

Noise due to Electric or Magnetic Coupling from Nearby Circuits

DIGITAL SIGNAL PROCESSING

Analog-DigitalConverter

Digital-AnalogConverter

DSP

If a signal contains no components with frequencies higher than fH, the signal can be exactly reconstructed from its samples, provided that the sampling rate fs is selected to be more than twice fH.

ADC : Conversion of Signals from Analog to Digital Form

Hs ff 2> : Sampling Rate (frequency)

DAC : Conversion Digital form into Analog Signal

DigitalDigital--toto--Analog Converter (DAC) Analog Converter (DAC) ]222[ 2

21

1n

nRout bbbVV −−− +++= L

output voltageanalog=outV

voltagereference=RVdbinary worbit -...21 nbbb n =

Rnout VNV2

= input DAC of equivalentnumber whole10 base −=N

nRout VV −=∆ 2 changeoutput smallest =∆ outV

voltagereference=RV wordin the bits ofnumber =n

RR VVV 9375.0]2222[ 4321max =+++= −−−−

RR VVV 9961.0]22222222[ 87654321max =+++++++= −−−−−−−−

4bit

8bit

Generic DAC diagram Typical DAC Circuit

DAC CircuitDAC Circuit

AnalogAnalog--toto--Digital Converters(ADCs)Digital Converters(ADCs)

R

innn V

Vbbb ≤+++ −−− 222 22

11 L

nRVV −=∆ 2

ageinput volt analog=inV voltagereference analog=RV

output digitalbit -...21 nbbb n =

= n

R

in

VVN 2INTRnin VNV

2=

Conversion Resolution

kN 2=32rms

∆=qN

The effect of finite word length can be modeled asadding quantization noise to the reconstructed signal.

Generic ADC diagram

Data-acquisition timing diagram Read Operation at any time after the end-of-convert

Analog-to-digital conversion is a two-step process. First, the signal is sampled at uniformly spaced points in time. Second, the sample values are quantized so they can be represented by words of finite length.

Analog-to-Digital Conversion

ADC CharacteristicsADC Characteristics

1. Analog Voltage Input : Signal Input to be Converted2. Power Supply3. Reference Voltage4. Digital Output

N output Lines Connected to Memory Bus5. Control Lines

SC : Start of ConversionEOC : End of ConversionRD : Read

6. Conversion Time

Aliasing

Aliasing : Sampling Rate too Low, High frequency Componentappears to be those of Low frequency

Anti-aliasing filter removes High frequency component (Low Pass Filter or High Order Butterworth Low Pass filter)

Successive approximation using Internal DAC

Parallel Feedback ADCParallel Feedback ADC

DualDual--Slope ADCSlope ADC

Op amp integrator, comparator, counter. Commonly used in digital voltmeters.Typical timing diagram

∫= dtVRC

V x1

1 xVTRC

V 111

=

∫−= dtVRC

VV R1

12 Rx tVRC

VTRC

V 1112 −=

Rx

x VTtV

1

=

Basic Concept of a sample-and-hold circuit for use with the ADC.

Sampled signal is literally “held”during the ADC conversion

Sample and HoldSample and Hold

ADC interfaced to Computer bus with Tri-state bufferAddress decoding by computer software.

MicroMicro--Processor Compatible ADCProcessor Compatible ADC

FrequencyFrequency--based Convertersbased Converters

General diagram of a frequency-based analog-to digital converter.

Common V-to-F Converteruseful in frequency-based ADCs.

Useful for generation of frequencyBy resistance or capacity.

LM331LM331 555 timer555 timer

DAS HardwareDAS Hardware

Typical layout of a data-acquisition board for use in a personal computer expansion slot.

DATADATA--ACQUISITION SYSTEMSACQUISITION SYSTEMS

Multi-position switch for selecting particular inputs to ADC

Analog MultiplexerAnalog Multiplexer

DAS SoftwareDAS Software

Start ADC-Test EOC-Input

Temperaturehistory

No loading

Loading

No loading Loading

DAS for TRIPDAS for TRIP

TRIP(TRIP(변태유기소성) : TRansformation Induced Plasticity

t=0

t=t

L01

L02

dL

dl

∆L1 ∆L2

0102

12

LLLL

−∆−∆

=ε

● Power : 50 kW●Maximum temp. : 1500 oC●Maximum load : 90 kgf●Measure of disp. : LVDT &

Laser disp. Sensor● Cooling Method : Gas cooling (Ar)

변태소성측정장치