Telemetry and Data Acquisition Systems

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TELEMETRY AND DATA ACQUISITION

UNIT 10


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.

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INPUT

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

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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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INPUT

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

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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):  = 
m

Are you ready to check
your answer ??

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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.

CONGRATULATIONS
!!!!…..May success be
with you always….

HAVE A FUN AND NICE DAY.

Telemetry and Data Acquisition Systems

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