chapter10.industrial electronic jm304

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TELEMETRY AND DATA ACQUISITION


OBJECTIVES

General objective : To understand the concept of telemetry and data acquisition.

Specific objectives : At the end of the unit you should be able to:

Identify the main concept of telemetry system and data acquisition.

Describe the structure of data collection system.

Define the specification of data acquisition system.

Identify the types of telemetry system.

Explain the function of multiplexing system.

UNIT 10

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10.1 INTRODUCTION OF TELEMETRY.

Telemetry means the transmission of data for monitoring and control over long

distances. Data can be sent directly as a DC voltage or current up to a few meters. At long

distances speed is severely limited, and noise becomes a serious problem. The original Morse

trans-Atlantic cables of 19th century used DC which transmitted at less than one word per

minute.

For longer distances, we convert DC voltage or current to audio tones and send them

over wire. This is called modulation, and the reverse (i.e. converting the varying signal to data)

is called demodulation. A device to perform it is called a modem.

An analog signal is a continuously varying wave. If we measure its height at specific

points in time, we obtain a series of voltages with numeric values. These values can be

represented in binary form and transmitted as a series of bits. A bit is a binary digit, either 0 or

1, whose combination in form of a code represents information in digital communication.

Figure 10.1(a) : Converting the analog to digital signal.

In other words, as indicated earlier, sensors in telemetry systems generate electrical

signals which change in some way in response to changes in physical characteristics. An

example of a sensor is a thermistor, a device used to measure temperature. A thermistor’s

resistance varies inversely with temperature: as the temperature increases, the resistance

INPUT

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decreases. The thermistor is usually connected into some kind of a resistive network, such as a

voltage divider or bridge, and also to a DC voltage source. The result is a DC output voltage,

which varies in accordance with temperature and which is transmitted to a remote receiver for

measurement, readout, and recording. The thermistor becomes one channel of an frequency

division multiplexing (FDM) system.

Other sensors have different kinds of outputs. Many simply have varying DC outputs,

while others are AC in nature. Each of these signals is typically amplified, filtered, and

otherwise conditioned before being used to modulate a carrier. All of the carriers are then

added together to form a single multiplexed channel.

10.2 STRUCTURE OF DATA ACQUISITION SYSTEMS.

Data acquisition system are used to measure and record signals obtained in basically

two ways:

a. signals originating from direct measurement of electrical quantities, these may include dc

and ac voltages, frequency or resistance and are typical found in such areas as electronic

component testing, environmental studies and quality analysis work.

b. Signals originating from transducers such as strain gage and thermocouple.

Data acquisition systems are used in a large and ever-increasing number of applications

in a variety of industrial and scientific areas, such as the biomedical, aerospace and telemetry

industries. The type of data acquisition system whether analog or digital, depends largely on

the intended use of the recorded input data. In general, analog data systems are used when wide

bandwidth is required or when lower accuracy can be tolerated. Digital systems are used when

the physical process being monitored is slowly varying (narrow bandwidth) and when high

accuracy and low per-channel cost is required. Digital systems range in complexity from

single-channel dc voltage measuring and recording systems to sophisticated automatic multi-

channel systems that measure a large number of input parameters, compare against preset

limits or conditions and perform computations and decisions on the input signal. Digital data

acquisition systems are general more complex than analog systems, both in terms of the

instrumentation involve and the volume and complexity of input data they can handle.

Data acquisition systems often use magnetic tape recorders. Digital system require

converts to change analog voltages into discrete digital quantities or numbers. Conversely,

digital information may have to be converted back into analog form such as a voltage or a

current which can then be used as a feedback quantity controlling an industrial process.

Instrumentation systems can be categorized into two major classes, analog systems and

digital system. Analog system deal with measurement information in analog form. An analog

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signal may be defined as a continuous function, such as a plot of voltage versus time, or

displacement versus pressure. Digital systems handle information in digital form. A digital

quantity may consist of a number of discrete and discontinuous pulse whose time relationship

contains information about the magnitude or the nature of the quantity.

Data acquisition is divided by two types, analog data acquisition and digital data

acquisition.

10.2.1 ANALOG DATA ACQUISITION.

An analog data acquisition system typically consists of some or all of the

following elements,

a. Transducers – translating physical parameters into electrical signals.

b. Signal conditioners – amplifying, modifying, or selecting certain portions of these

signals.

c. Visual display devices – continuous monitoring of the input signals. These

devices may include single-channel or multi-channel oscilloscope, storage

oscilloscope, panel meters, numerical display and others.

d. Graphic recording instruments – obtaining permanent records of the input data.

These instruments include stylus and ink recorders to provide continuous records

on paper chart, optical recording systems such as mirror galvanometer recorders

and ultraviolet recorders.

e. Magnetic tape instrumentation – acquiring input data, preserving their original

electrical form, and reproducing them at a later date for more detailed analysis.

10.2.2 DIGITAL DATA ACQUISITION.

A digital data acquisition included some or all of the elements shown in figure

10.2.2. The essential function operations within a digital system include handling

analog signals, making the measurement, converting and handling digital data and

internal programming and control. The function of each of the system elements of

figure 10.2.2 is listed below.

a. Transducer – translate physical parameters to electrical signals acceptable by the

acquisition system. Some typical parameters include temperature, pressure,

acceleration, weight displacement, and velocity frequency, also may be measured

directly.

b. Signal conditioner – generally includes the supporting circuitry for the transducer.

This circuitry may provide excitation power, balancing circuits, and calibration

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elements. An example of signal conditioner is a strain- gage bridge balance and

power supply unit.

c. Scanner or multiplexer – accept multiple analog inputs and sequentially connects

them to one measuring instrument.

d. Signal converter – translates the analog signal to a form acceptable by the analog-

to-digital converter. An example of signal converter is an amplifier for amplifying

low-level voltages generated by thermocouples or strain gages.

e. Analog –to-digital (A/D) converter - Converts the analog voltage to its equivalent

digital form. The output of the A/D converter may be displayed visually and also

available as voltage outputs in discrete steps for further processing or recording

on a digital recorder.

f. Auxiliary equipment – This section contains instruments for system programming

functions and digital data processing. Typical auxiliary functions include

linearizing and limit operation. These functions may be performed by individual

instruments or by a digital computer.

g. Digital recorder – Records digital information on punched cards, perforated paper

tape, magnetic tape, typewritten pages, or a combination of systems. The digital

recorder may be preceded by a coupling unit that translates the digital information

to the proper form for entry into the particular digital recorder selected.

Fig. 10.2.2: Elements of digital data-acquisition system.

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Activity 10A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT

INPUT…!

10.1 Name THREE places where telemetry is used.

10.2 What is the basic principles of telemetry ?

10.3 Describe that TWO categorized system of instrumentation.

10.4 What the differential between analog and digital data acquisitions ?

Hii !!!!!…..Good Luck and

Try your best ….

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Feedback To Activity 10A

10.1 Three places used a telemetry : telephone system, modern radio and TV broadcasting,

semiconductor industries, control system and others.

10.2 Telemetry means the transmission of data for monitoring and control over long

distances. For longer distances, telemetry convert DC voltage or current to audio tones

and send them over wire. This is called modulation, and the reverse (i.e. converting the

varying signal to data) is called demodulation. A device to perform it is called a modem.

10.3 Instrumentation systems can be categorized into two major classes, analog systems and

digital system. Analog system deal with measurement information in analog form. An

analog signal may be defined as a continuous function, such as a plot of voltage versus

time, or displacement versus pressure. Digital systems handle information in digital

form. A digital quantity may consist of a number of discrete and discontinuous pulse

whose time relationship contains information about the magnitude or the nature of the

quantity.

10.4 The type of data acquisition system whether analog or digital, depends largely on the

intended use of the recorded input data. In general, analog data systems are used when

wide bandwidth is required or when lower accuracy can be tolerated. Digital systems are

used when the physical process being monitored is slowly varying (narrow bandwidth)

and when high accuracy and low per-channel cost is required. Digital systems range in

complexity from single-channel dc voltage measuring and recording systems to

sophisticated automatic multi-channel systems that measure a large number of input

parameters, compare against preset limits or conditions and perform computations and

decisions on the input signal. Digital data acquisition systems are general more complex

than analog systems, both in terms of the instrumentation involve and the volume and

complexity of input data they can handle.

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10.3 FREQUENCY OF TELEMETRY.

In the frequency of telemetry process, the carrier frequency is varied above and below

its center value (modulated) in accordance with the amplitude of the data signal. The rate at

which the carrier frequency deviates from its center value is a function of the frequency signal.

The amplitude and frequency characteristics that define the data signal are therefore contained

in the frequency variations of the frequency telemetry carrier around its center value. When

this modulated frequency demodulator by detecting the number and rate of zero crossings.

It is clear that frequency telemetry recording is extremely sensitive to variations in tape

speed (flutter) because tape speed variations introduce apparent modulation of the carrier and

are interpreted by system as unwanted signal (noise). Instability in tape speed therefore reduces

the dynamic range of the system.

Since the data signal is contained entirely in the frequency characteristics of the

frequency carrier, the system is not sensitive to amplitude instability. Two important factors in

telemetry recording are deviation ratio and percentage deviation. Deviation ratio is defined as

the ratio of deviation of the carrier from the center frequency to the signal frequency, or

=

m

where = deviation ratio

= carrier deviation from center frequency

m = data signal frequency

INPUT

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10.4 MULTIPLEXING SYSTEM.

Multiplexing is the process of simultaneously transmitting two or more individual

signals over a single communications channel. Multiplexing has the effect of increasing the

number of communication channels so that more information can be transmitted.

There are many instances in communication where it is necessary or desirable to

transmit more than one voice or data signal. The application itself may require multiple signals

and money can be saved by using a single communications channel to send multiple

information signals. Telemetry and telephone applications are good examples. In satellite

communications, multiplexing is essential to making the system practical and for justifying the

expense.

The concept of a simple multiplexer is illustrated in figure 10.7(a). Multiple input

signals are combined by the multiplexer into a single composite signal that is transmitted over

the communications medium. Alternatively , the multiplexed signals may modulate a carrier

before transmission. At the other end of the communications link, a demultiplexer is used to

sort out the signal into their original form.

Figure 10.4(a) : Concept of multiplexing.

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There are two basic types of multiplexing, frequency division multiplexing (FDM) and

time division multiplexing (TDM). Generally speaking , FDM systems are used to deal with

analog information and TDM systems are used for digital information.

10.4.1 FREQUENCY DIVISION MULTIPLEXING.

Frequency division multiplexing is based on the idea that a number of signal can share

the bandwidth of a common communications channel. The multiple signal to be transmitted

over this channel are each used to modulate a separate carrier. Each carrier is on a different

frequency. The modulated carriers are then added together to form a signal complex signal that

is transmitted over the single channel.

Figure 10.4.1(a) shows a general block diagram of FDM system. Each signal to be

transmitted feeds a modulator circuit. The carriers for each modulation fc is on a different

frequency. The carrier frequencies are usually equally spaced from one another over a specific

frequency range. Each input signal is given a portion of bandwidth . the result is illustrated in

figure 10.4.1(b). As for the type of modulation any of the standard kinds can be used including

AM, SSB, FM or PM.

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Figure 10.7.1(a) : The transmitting end of an FDM system.

Figure 10.7.1(b) : Spectrum of an FDM signal.

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The modulator output containing the sideband information are added together in a

linear mixer. In a linear mixer, modulation and the generation of sidebands do not take place.

Instead , all the signals are simply added together algebraically. The resulting output signal is a

composite of all carriers containing their modulation. This signal is then used to modulate a

radio transmitter. Alternatively, the composite signal itself may be transmitted over the single

communication channel. Another option is that the composite signal may become one input to

another multiplexer system.

10.4.2 TIME DIVISION MULTIPLEXING.

In FDM, multiple signals are transmitted over a single channel by sharing the channel

bandwidth. This is done by allocating each signal a portion of the spectrum within that

bandwidth. In TDM , each signal can occupy the entire bandwidth of the channel. However ,

each signal is transmitted for only a brief period of time. In other words, the multiple signals

take turns transmitting over the single channel. This concept is illustrated graphically in figure

10.4.2(a).

Figure 10.4.2(a) : The basic TDM concept

Here, four signals are transmitted over a single channel each signal is allowed to use the

channel for a fixed period of time, one after another. Once all the signals have been

transmitted, the cycle repeats again and again.

Time division multiplexing may be used with both digital and analog signals. To

transmit multiple digital signals, the data to be transmitted is formatted into serial data words.

For example, the data may consist of sequential bytes. One byte of data may be transmitted

during the time interval assigned to a particular channel. For example , in figure 10.4.2(a), each

time slot might contain 1 byte from each channel. One channel transmits 8 bits. The third

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channel then transmits its data word and so on. One transmission of each channel completes

one cycle of operation called a frame. The cycle repeats itself at high rate of speed. In this way,

the data bytes of the individual channel are simply interleaved. The resulting single –channel

signal is a digital bit stream that must somehow be deciphered and reassembled at the receiving

end.

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Activity 10B

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT

INPUT…!

10.5 Describe the concept of multiplexing process.

10.6 Describe briefly the differential between , frequency division multiplexing

(FDM) and time division multiplexing (TDM).

10.7 Referring to equation 10(a), what is the meaning of , and m .

Equation 10(a):

Are you ready to check

your answer ??

=

m

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Feedback To Activity 10B

10.5 Multiplexing is the process of simultaneously transmitting two or more individual

signals over a single communications channel. Multiplexing has the effect of increasing

the number of communication channels so that more information can be transmitted.

There are many instances in communication where it is necessary or desirable to transmit

more than one voice or data signal. The application itself may require multiple signals

and money can be saved by using a single communications channel to send multiple

information signals. Telemetry and telephone applications are good examples. In satellite

communications, multiplexing is essential to making the system practical and for

justifying the expense.

10.6 Please refer to Input 10.4.1 and Input 10.4.2 for the answer.

10.7 The answer for this question, is deviation ratio, = carrier deviation from center

frequency and m = data signal frequency

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SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment section and

check your answers with those given in the Feedback on Self-Assessment given on the

next page. If you face any problems, discuss it with your lecturer. Good luck.

Question 10-1

a. Based on figure 10(b), what is an analog signal ?

b. Referring to figure 10(b), describe that how to convert the analog signal to digital

signal ?

c. Describe the function of thermistor according to telemetry concept.

Figure 10(b) : Converting the analog to digital signal.

Question 10-2

a. Data acquisition system is used to measure and record signals obtained in basically

two ways. Describe that the two ways of data acquisition system.

b. List FIVE elements in analog data acquisition and describe each item.

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Question 10-3

a. Referring to Figure 10(c), describe the relationship of each element according to

digital data acquisition.

b. Referring to multiplexer diagram, describe briefly the concept of multiplexer and

de-multiplexer. List TWO basic types of multiplexer.

Figure 10(c): Elements of digital data-acquisition system.

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Feedback To Self-Assessment

Answer 10-1

a. An analog signal is a continuously varying wave.

b. If we measure its height at specific points in time, we obtain a series of voltages with

numeric values. These values can be represented in binary form and transmitted as a

series of bits. A bit is a binary digit, either 0 or 1, whose combination in form of a

code represents information in digital communication.

c. A thermistor is a device used to measure temperature. A thermistor’s resistance varies

inversely with temperature: as the temperature increases, the resistance decreases.

The thermistor is usually connected into some kind of a resistive network, such as a

voltage divider or bridge, and also to a DC voltage source. The result is a DC output

voltage, which varies in accordance with temperature and which is transmitted to a

remote receiver for measurement, readout, and recording. The thermistor becomes

one channel of an frequency division multiplexing (FDM) system.

Answer 10-2

a. Two way of data acquisition system are used to measure and record signals is,

i. signals originating from direct measurement of electrical quantities, these may

include dc and ac voltages, frequency or resistance and are typical found in such

areas as electronic component testing, environmental studies and quality

analysis work.

ii. Signals originating from transducers such as strain gage and thermocouple.

b. An analog data acquisition system typically consists of some or all of the following

elements,

i. Transducers – translating physical parameters into electrical signals.

ii. Signal conditioners – amplifying, modifying, or selecting certain portions of

these signals.

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iii. Visual display devices – continuous monitoring of the input signals. These

devices may include single-channel or multi-channel oscilloscope, storage

oscilloscope, panel meters, numerical display and others.

iv. Graphic recording instruments – obtaining permanent records of the input

data. These instruments include stylus and ink recorders to provide continuous

records on paper chart, optical recording systems such as mirror galvanometer

recorders and ultraviolet recorders.

v. Magnetic tape instrumentation – acquiring input data, preserving their original

electrical form, and reproducing them at a later date for more detailed analysis

Answer 10-3

a. Refer to figure 10(c), the function of transducer is translate physical parameters to

electrical signals acceptable by the acquisition system. Some typical parameters

include temperature, pressure, acceleration, weight displacement, and velocity

frequency, also may be measured directly. Signal conditioner ; generally includes the

supporting circuitry for the transducer. This circuitry may provide excitation power,

balancing circuits, and calibration elements. An example of signal conditioner is a

strain- gage bridge balance and power supply unit. Scanner or multiplexer; accept

multiple analog inputs and sequentially connects them to one measuring instrument.

Signal converter; translates the analog signal to a form acceptable by the analog-to-

digital converter. An example of signal converter is an amplifier for amplifying low-

level voltages generated by thermocouples or strain gages. Analog to digital (A/D)

converter ; converts the analog voltage to its equivalent digital form. The output of

the A/D converter may be displayed visually and also available as voltage outputs in

discrete steps for further processing or recording on a digital recorder. Auxiliary

equipment; This section contains instruments for system programming functions and

digital data processing. Typical auxiliary functions include linearizing and limit

operation. These functions may be performed by individual instruments or by a

digital computer. Digital recorder; Records digital information on punched cards,

perforated paper tape, magnetic tape, typewritten pages, or a combination of systems.

The digital recorder may be preceded by a coupling unit that translates the digital

information to the proper form for entry into the particular digital recorder selected.

b. Multiple input signals are combined by the multiplexer into a single composite signal

that is transmitted over the communications medium. Alternatively , the multiplexed

signals may modulate a carrier before transmission. At the other end of the

communications link, a de-multiplexer is used to sort out the signal into their original

form.

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Figure 10.4(a) : Concept of multiplexing.

There are two basic types of multiplexing, frequency division multiplexing

(FDM) and time division multiplexing (TDM). Generally speaking , FDM systems

are used to deal with analog information and TDM systems are used for digital

information.

HAVE A FUN AND NICE DAY.

CONGRATULATIONS

!!!!…..May success be

with you always….

chapter10.industrial electronic jm304

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