Digital Data-Acquisition SystemsSince the late 1950s, computers have been used to monitor, and inmany 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 AcquisitionThe 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 besampled (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.

Data-acquisition board with eight analog inputchannels and two analog output channels also showing an analog signal connection box. Manufactured by National Instruments.

Figure 4.2

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:

• Central processing unit (CPU)• Program (software)• Random access memory (RAM)• Mass storage systems• Display• User input device (keyboard, mouse, etc.)• Printers and plotters

Components of Computer Systems

FIGURE 4.3Typical computer system.

Digitalinput-output(ports or expansion bus)

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 inprograms.

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.

Representing Numbers in Computer Systems

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

= 9(101) + 2(100)

• 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:

Convert the decimal integer -92 to an 8-bit 2's-complement binary number.Example 4.3

+92 = 01011100 ,invert all the bits, to obtain 10100011

then add 1 to obtain the final result,

1

10100100

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

Four generations of computers

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 flip-flops.

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.