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CHAPTER 4
Computerized Data-Acquisition Systems
Since the late 1950s, computers have been used to monitor, and in
many cases to control, the performance of large process plants such
as refineries and chemical plants and to acquire data in major
testing programs such as the space program.
These systems were extremely expensive and required highly skilled
personnel to set them up. In the late 1960s, lower-cost computer
data-acquisition systems became available for smaller tests, but
since they were still difficult to set up and program, their use was
limited.
In the late 1970s, the simple and reliable Apple II personal
computer led to inexpensive and simple data-acquisition systems
that could readily be used for small experiments. Personal
computer systems (mostly using the Windows operating system)
are now so capable that they can be used for a significant fraction
of all engineering testing.
FIGURE 4.1 Computerized data-acquisition system.
Computer Systems for Data Acquisition
The computer systems most commonly used for data acquisition in
experiments are personal computers using the Windows operating
system. Sampling rates of over 10 million samples per second are
possible, and more than 3000 separate sensors can be
sampled (although not in the same system at the same time). Several
major companies supply software and hardware to make a personal
computer into a data-acquisition system, and some of the hardware
is available in low-cost generic form. Figure 4.2 shows a board
designed to plug into the interior bus of a personal computer and a
For monitoring and controlling many production systems,
specialized embedded computers are used. Probably the most
common embedded computers are those used for fuel control in
modern automobiles. However, embedded computer systems are
used in a wide variety of devices from medical imaging equipment
to assembly-line robots.
Figure 4.2
Data-acquisition board with
eight analog input
channels and two analog
output channels also showing
an analog signal connection
box. Manufactured by
National Instruments.
Components of Computer Systems
Although some computers used in data acquisition are highly
specialized (the engine control computer in an automobile, for
example), the computers normally used are quite standard and have
the following components:
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•
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Central processing unit (CPU)
Program (software)
Random access memory (RAM)
Mass storage systems
Display
User input device (keyboard, mouse, etc.)
Printers and plotters
Digital
input-output
(ports or
expansion bus)
FIGURE 4.3
Typical computer system.
Tne central processing unit (CPU) controls all aspects
of computer system operation and performs all of the
arithmetic operations (multiplication, addition, etc.).
The CPU operations follow instructions contained in
the user-provided program. The CPU also follows
instructions from the computer operating system
programs and from built in
programs.
Representing Numbers in Computer Systems
While numbers used in the everyday world are normally
represented in base 10 (decimal), it is far more practical in
computers to represent numbers in base 2 (binary).Information
in computers is stored in bi stable devices called flip-flops, which
can have two possible states. One state is defined as "on" and is
assigned a numerical value of 1, and the other state is defined as
"off" and is assigned a numerical value of 0. A series of flip-flops
are required to represent a number. For example, the binary
number 0101,, which corresponds to the decimal number 5, can
be represented in a computer Using four flip-flops. Each of these
flip-flops represents a "bit" of the number. The left most "1" in
the binary number 1001 is the most significant bit (MSB). The
rightmost "l" is the least significant bit (LSB). It is common in
computers to break long binary numbers up into segments of 8
bits, which are known as bytes.
= 9(101) + 2(100)
Find the 8-bit binary
number with the
same value as that
of the decimal
number 92.
Solution: This problem
can be solved by a series
of divisions by 2:
is zero for positive binary numbers
• These same 4 bits can alternatively be used to
represent numbers from -8 to +7. The positive
numbers from 0 to7 are represented by the three least
significant bits, ranging from 0000 to 0111. The
negative numbers from -8 to -1 are represented by the
binary numbers 1000 to 1111, respectively.
• For positive numbers, the most significant bit is always
0, while it is always 1 for negative numbers.
To convert a negative decimal integer to 2's-complement binary
the following procedure can be followed:
Example 4.3
Convert the decimal integer -92 to an 8-bit 2's-complement binary number.
+92 = 01011100
,invert all the bits, to obtain
10100011
1
then add 1 to obtain the final result,
10100100
Transistors amplify current, for example they can be used to amplify the
small output current from a logic IC so that it can operate a lamp, relay or
other high current device. In many circuits a resistor is used to convert the
changing current to a changing voltage, so the transistor is being used to
amplify voltage.
A transistor may be used as a switch (either fully on with maximum
current, or fully off with no current) and as an amplifier (always partly on).
Types of transistor: Watch @
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These logic circuits can be built very compact on a silicon chip
with 1,000,000 transistors per square centimeter. We can turn
them on and off very rapidly by switching every 0.000000001
seconds. Such logic chips are at the heart of your personal
computer and many other gadgets you use today.
Vacuum tubes were made containing several three
terminal devices called triodes.
•
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A p–n junction is formed at the boundary
between a P-type and Ntype semiconductorcreated in a single crystal of
semiconductor by doping, for example by ion
implantation,diffusion of dopants, or
by epitaxy (growing a layer of crystal doped
with one type of dopant on top of a layer of
crystal doped with another type of dopant). If
two separate pieces of material were used, this
would introduce a grain boundary between the
semiconductors which severely inhibits its utility
by scattering the electrons and holes.[citation
needed].
P–N junctions are elementary "building blocks"
of many semiconductor electronic devices such
as diodes, transistors, solar cells, LEDs,
andintegrated circuits; they are the active sites
where the electronic action of the device takes
place. For example, a common type
of transistor, the bipolar junction transistor,
consists of two p–n junctions in series, in the
form n–p–n or p–n–p.
•1946: ENIAC completed
•Electronic Numerical Integrator and Computer
•18,000 vacuum tubes.
•Occupied a 30 by 50 foot room
•Programming by plugging wires into a patch panel. Very difficult to do, because this style
programming requires intimate knowledge of the computer.
Four generations of computers
1.Vacuum tube (1939)
2.Transistor (invented in 1947, used in IBM 7090 in 1958)
3.Integrated circuit or chip (invented in 1959, used in IBM 360 in 1964)
1. A small wafer of silicon that has been photographically imprinted to contain
a large number of transistors together.
4.Large-scale integration: microprocessor (1975)
1. Entire processing unit on a single chip of silicon
Transistors
Old CPU's
Basics of Analog-to-Digital Converters
• To explain the function of the A"/D converter it is
necessary to describe two distinct methods by which
electronic systems process numerical information:
analog and digital.
• Many everyday electronic devices, such as television
sets and audio amplifiers, were basically analog devices
(although they may have some digital components).
• Modern computers, on the other hand, are digital
devices. If we are trying to represent a value of 5 V in
an analog device, we could, for example, charge a
capacitor to 5 V. In a digitaldevice,5 V will be
represented by a digital code (a digital binary
number such 0101), which is stored on bistable flipflops.
The two-light-bulb device is a 2-bit A/D
converter.
In general, the
output of an
analog-to-digital
converter has
2N possible
values,
A unipolar converter can only respond to analog inputs with the same sign. Examples of
the input range are 0 to 5 V or 0 to -10 V. Bipolar converters can convert both positive
and negative analog inputs, with +5 V or +10 V being typical input ranges.
Figure 4.6.