Transcript Unit 12 PowerPoint Slides

EET 2259 Unit 12
Data Acquisition
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Read Bishop, Chapter 8.
Lab #12 and Homework #12 due next
week.
Floyd, Digital Fundamentals, 10th ed
Data Acquisition
The text’s Chapter 8 covers several kinds
of material:
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Data acquisition hardware (Sections 8.1 to
8.5): Not much on LabVIEW in these sections.
LabVIEW data acquisition without the DAQ
Assistant (Sections 8.6 to 8.8): This is how you
had to do it before LabVIEW version 7.0.
LabVIEW data acquisition using the DAQ
Assistant (Sections 8.9 to 8.12): You’re already
familiar with much of this.
Floyd, Digital Fundamentals, 10th ed
Data Acquisition Hardware
Outline:
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Components of a DAQ System (§8.1)
Types of Signals (§8.2)
Transducers and Signal Conditioning (§8.3)
Signal Grounding and Measurements (§8.4)
Analog-to-Digital Conversion (§8.5)
Each of these topics could occupy us for a
week, so we’ll just hit the high points.
For lots more detail, see NI’s website:
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LabVIEW Measurements Manual
Floyd, Digital Fundamentals, 10 ed
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Measurement Fundamentals Main Page
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Components of a DAQ System (§8.1)
Floyd, Digital Fundamentals, 10th ed
Image from NI’s website
Types of Signals (§8.2)
Floyd, Digital Fundamentals, 10th ed
Image from LabVIEW Measurements Manual
Transducers and Signal Conditioning
(§8.3)
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A transducer (or sensor) is a
device that converts a physical
quantity (such as temperature,
pressure, light intensity, …) to an
electrical signal (such as voltage or
current).
Floyd, Digital Fundamentals, 10th ed
Some Common Transducers
Floyd, Digital Fundamentals, 10th ed
Image from LabVIEW Measurements Manual
Signal Conditioning
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In many cases the electrical signal
coming from the transducer must
be conditioned in some way before
it is converted into digital format for
use by the computer.
Examples of signal conditioning
include:
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Amplifying a weak signal
Filtering out noise from the signal
Floyd, Digital Fundamentals, 10th ed
Signal Conditioning For Some
Transducers
Floyd, Digital Fundamentals, 10th ed
Image from LabVIEW Measurements Manual
Signal Grounding and Measurements
(§8.4)
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Signal sources can be either:
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Grounded (also called referenced)
Floating (also called non-referenced)
Floyd, Digital Fundamentals, 10th ed
Grounded Signal Source
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Examples:
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Function generator or power supply
whose negative terminal is tied to earth
ground through the power cord.
Floyd, Digital Fundamentals, 10th ed
Image from LabVIEW Measurements Manual
Floating Signal Source
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Examples:
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Our red trainer’s function generator or
power supply, whose negative terminal
is not tied to earth ground through the
power cord.
Battery-powered devices
Transformers
Thermocouples
Floyd, Digital Fundamentals, 10th ed
Image from LabVIEW Measurements Manual
Measurement System Configuration
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Measurement systems can be
configured as either:
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Differential
Referenced Single-Ended (RSE)
Non-Referenced Single-Ended (NRSE)
The PCI-6221 cards inside our
computers can be configured in any
of these modes.
 The myDAQ can only be configured
in differential mode.
Floyd, Digital Fundamentals, 10 ed
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Differential Measurement System
Floyd, Digital Fundamentals, 10th ed
Image from LabVIEW Measurements Manual
Referenced Single-Ended (RSE)
Measurement System
Floyd, Digital Fundamentals, 10th ed
Image from LabVIEW Measurements Manual
Non-Referenced Single-Ended
(NRSE) Measurement System
Floyd, Digital Fundamentals, 10th ed
Image from LabVIEW Measurements Manual
Matching Signal Grounding to
Measurement Configuration
Image from
LabVIEW Measurements Manual.
See also pages 11-14 of NI
myDAQ User Guide and
Specifications.
See also NI tutorial on
Ground Loops and Returns.
Floyd, Digital Fundamentals, 10th ed
Analog-to-Digital Conversion (§8.5)
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Before a computer can process
analog information, we must first use
an analog-to-digital converter
(ADC) to transform the analog values
into digital binary values.
Floyd, Digital Fundamentals, 10th ed
Number of Bits and Resolution
•The ADC periodically samples the analog
signal, and converts each sampled value of the
analog signal into a binary code.
•The more bits that are used in this code, the
better the ADC’s resolution is. Better
resolution results in a more accurate
representation of the original signal.
•The following slides (from Floyd’s Digital
Electronics) show an example of how using 4
bits results in much better resolution than using
2 bits.
Figure 12.8 Light gray = original waveform. Blue = Reconstructed waveform using four quantization levels (2 bits).
Digital Fundamentals, Tenth Edition
Thomas L. Floyd
Copyright ©2009 by Pearson Higher Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Figure 12.10 Light gray = original waveform. Blue = Reconstructed waveform using sixteen quantization levels (4 bits).
Digital Fundamentals, Tenth Edition
Thomas L. Floyd
Copyright ©2009 by Pearson Higher Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Resolution
•There are several common ways of specifying
an ADC’s resolution:
•Number of bits, n
•Number of output codes, = 2
n
•Step size (which Bishop calls code width),
n
= Vref / 2
Resolution: Examples
Number of bits
Number of output
codes
Step size (assuming
5 V reference
voltage)
Formula
4-bit ADC
n
2n
4
16
Vref / 2n
312.5 mV
10-bit ADC
Sampling Rate
•Sampling rate is another factor determining
how well the digitized data match the original
analog input.
•It’s expressed either as number of samples
per second or as a frequency.
•Examples: 1000 samples/sec or 1 kHz.
Specs for Our Equipment
•Let’s find the resolution and sampling rate for
the ADCs inside our equipment.
•NI myDAQ User Guide and Specifications
•NI PCI-6221 Specifications
LabVIEW Data Acquisition without
the DAQ Assistant (§§8.6 – 8.8)
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Before the DAQ Assistant was
introduced, LabVIEW programmers
had to use the functions on the
Measurement I/O > NI-DAQmx
palette to perform data acquisition.
Express VIs
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Express VIs (the DAQ Assistant is
one example) were introduced in
LabVIEW 7.0.
They provide user-friendly ways to
do things that you could also do
using other LabVIEW functions and
subVIs.
Seeing the Code Hidden Inside an
Express VI
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You can convert an Express VI to
standard LabVIEW code by rightclicking and selecting “Open Front
Panel.”
In the case of a DAQ Assistant, you
can also right-click and select
“Generate NI-DAQmx Code.”
LabVIEW Data Acquisition with the
DAQ Assistant (§§8.9 – 8.12)
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You already know much of what is
discussed in these sections, since
you’ve been using the DAQ Assistant
since the start of this class.
Read these sections to review and
strengthen your knowledge.