Chapter 4
Telemetry Applications
1
4.1 Introduction:
• In modern measurement systems, the various components
comprising the Instrumentation system are usually located
at a distance from each other.
• It therefore becomes necessary to transmit data between
them through some form of communication channels.
• The term telemetry refers to the process by which the
information regarding the quantity being measured is
transmitted to a remote location
✓ for applications like data processing, recording or
displaying.
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Telemetry Systems Overview
• Telemetry is the process by which an object’s characteristics
are measured (such as velocity of an aircraft).
❖and the results transmitted to a distant station where they are
displayed, recorded, and analyzed.
• Telemetry is the process by which a process parameters(such
as temperature, pressure, flow rate, level etc) are sensed and
then transmitted that information to a central or host location.
❖it can be monitored and used to control a process at the
remote site
3
Cont…
• The transmission media may be air and space for satellite
applications.
• Or it can be a copper wire and fiber cable for static ground
environments like power generating plants and
Manufacturing industries.
• In today's telemetry applications, which support large
numbers of measurands,
➢it is too costy and impractical to use separate
transmission channels for each measured quantity.
4
Cont…
• The telemetry process involves grouping
measurements (such as pressure, speed, and
temperature) into a format that can be transmitted
as a single data stream.
• Once received, the data stream is separated into the
original measurement’s components for analysis.
❖Telemetry lets you to stay in a safe (or convenient)
location while monitoring what's taking place in an
unsafe (or inconvenient) location.
5
Cont…
Figure 4.1:
▪ Parts of telemetry system
1) Transmitter
2) Receiver
3) Channel
6
Cont…
• Parts of telemetry system
1) Transmitter
2) Receiver
3) Channel
Figure 4.2:
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4.2: Data Acquisition
• Data acquisition is the process by which:
➢physical phenomena (parameters)from the real
world are sensed and transformed into
electrical signals;
➢and converted into a digital format for
processing, analysis, and storage by a computer.
• The basic elements of a data acquisition system are
shown in figure below.
• Sensors or transducers: are devices which convert
the quantity which is to be measured(measurand)
in to a proportional electrical signal.
8
Cont…
Figure 4.3: A typical data acquisition system
9
Cont…
• Signal Conditioning: includes all the devices which
convert the signal from the sensor to the correct
level for the A/D(analogue to digital convertors)
➢It includes amplifiers, filters, ac to dc convertors
etc
• Anti-aliasing filters: used to remove high-frequency
noise signal.
10
Cont…
• Multiplexer(MUX): is selector switch connecting
one channel at a time.
➢The multiplexer may be electromechanical
(commutator) or;
➢completely electronic device.
• Analogue to digital convertor(A/D):
11
Cont…
• Random access memory(RAM): is used for the
temporary storage of data.
• A digital to analogue convertor(D/A): it is used if
analogue output from the computer is required.
• Interface: is a circuit used to interface analogue
signals with the digital computer.
• A bus consists of a number of wires carrying
signals and each connected to several devices.
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4.3: Multiplexer
• Multiplexing in Telemetering Systems is a means of
using the same transmission channel for
transmitting more than one quantity at a time.
• Multiplexing becomes necessary in measurement
systems when:
– the distance between transmitting and receiving point
is large.
– and many quantities are to be transmitted.
• If a separate channel is used for each quantity, the
cost of installation, maintenance and periodic
replacement becomes large. 13
Cont…
• Therefore a single channel is used which is
shared by the various quantities.
• There are two methods of multiplexing:
➢ Time Division Multiplexing (TDM)
➢ Frequency Division Multiplexing (FDM)
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Figure 4.4: Multiplexing versus no multiplexing 15
Classification of Multiplexing
Figure 4.5: Classification of Multiplexing
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Frequency Division Multiplexing (FDM)
• bandwidth of the transmission medium fm, is
split into a series of frequency bands, having a
bandwidth fch, each one of which is used to
transmit one signal.
• Between these channels there are frequency
bands, having bandwidth fg, called “guard bands,”
which are used to ensure that there is adequate
separation and minimum crosstalk between any
two channels.
17
Cont…
Figure 4.6: Frequency-division multiplexing:
18
Cont…
Figure 4.7: transmission of three signals using FDM.
19
Cont…
• The low-pass filters at the input to the modulators is used
to band limit the signals.
• Each of the signals then modulates a carrier.
• The individually modulated signals are then, summed and
transmitted.
• Band pass filters after reception are used to separate the
channels, by providing attenuation which starts in the
guard bands.
• The signals are then demodulated and smoothed.
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Time-division Multiplexing (TDM)
• Time-division multiplexing (TDM) is a digital process that can
be applied:
• when the data rate capacity of the transmission medium is
greater than the data rate required by the sending and
receiving devices.
• TDM is a digital multiplexing technique to combine data.
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Time-division Multiplexing (TDM)
▪ Synchronous TDM
▪ Asynchronous TDM
Synchronous:
• The multiplexer allocates exactly the same time slot to each
device at all times, whether or not a device has anything to
transmit.
22
Cont…
• Time slots are grouped into frames.
• A frame consists of a one complete cycle of time slots
• A synchronization bits are added to the beginning of
each frame.
• These framing bits allows the demultiplexer to
synchronize with the incoming stream so that it can be
able to separate the time slots accurately.
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(a): Synchronous TDM
(b): Synchronous TDM 24
Asynchronous TDM
• Synchronous TDM does not guarantee that the full
capacity of a link is used.
• Because the time slots are pre-assigned and fixed,
whenever a connected device is not transmitting, the
corresponding slot is empty.
• Asynchronous TDM is designed to avoid this type of
channel waste.
25
Cont…
• The number of time slots in an asynchronous TDM frame (m) is
determined:
❖based on statistical analysis of the number of input lines that are
likely to be transmitting at any given time.
• With n input lines, the frame contains no more than m slots, with m
less than n.
• In asynchronous TDM each time slot must carry an address.
• This address tells the de-multiplexer from which input channel it is so
that it can easily direct the data to its correct output channel.
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Asynchronous TDM 27
4.4, Modulation
• Modulation involves the use of the information signal to
systematically vary either the amplitude, frequency or phase of
a sinusoidal carrier.
• This process involves two operations:
1, Frequency Translation : moving the signal to a high frequency
carrier
2, Vary some parameter of the high frequency carrier wave in
order to represent the message signal using the carrier signal
28
Cont…
Figure: Components of a continuous-
wave modulation system: (a)
Transmitter, and (b) receiver 29
Cont…
• Common form of carrier is sinusoidal wave
• The message(baseband)signal is referred to as modulating
wave.
• The result of the modulation process is referred to as the
modulated wave.
• In addition to the signal received from the transmitter, the
receiver input includes channel noise.
30
Cont…
Figure: Illustrating AM and FM signals (a) carrier wave. (b) sinusoidal
modulating signal (c) Amplitude-modulated signal. (d) Frequency-
modulated signal 31
Cont…
• In telemetry, the physical characteristics of a given application are
monitored by sensitive transducers,
• The sensors generate electric signals that vary in response to changes
in the status of the various physical characteristics.
• The sensor-generated information can be sent to a central location for
monitoring, or can be used as feedback in a closed-loop control
system.
• Most spacecraft and many chemical plants, e.g., use telemetry
systems to monitor characteristics, such as temperature, pressure,
speed, light level, flow rate, and liquid level.
32
Cont…
• Different sensors have different kinds of outputs.
• Many sensors have varying dc outputs, and others have ac
output.
• Each of these signals is typically amplified, filtered, and
otherwise conditioned before being used to modulate a
carrier.
• All the carriers are then added to form a single multiplexed
channel.
33
Cont…
• The conditioned transducer outputs are normally used to frequency-
modulate a subcarrier.
• The varying direct or alternating current voltage changes the
frequency of an oscillator operating at the carrier frequency.
• Such a circuit is generally referred to as voltage controlled oscillator
(VCO) or a subcarrier oscillator (SCO).
• To produce FDM, each VCO operates at a different center or carrier
frequency.
• The outputs of the subcarrier oscillators are added
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Types of telemetry system
• The function of the telemetery transmitter is to
convert output of a primary sensing element into an
electrical signal and to transmit it over a telemetering
channel.
• This signal is received by a receiver at remote
location and convert it to the usable form and is
recorded by an end device.
• Types of telemetry system
1.Land Line telemetry system
2.Radio Frequency telemetry system
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Landline Telemetry System
• Direct transmission of information through cables and
transmission lines.
• Direct transmission via cables employs current, voltage, position,
frequency, or pulses to convey information.
• Current, voltage and position type are used for short distance
whereas frequency, pulse systems are used for long distance.
• Information may be analog / digital.
• Pulse signals are used in digital telemetry .
• All the others are used for analog telemetry
Land line telemetry can be Classified as:
1.Voltage telemetry systems
2.Current telemetry systems
3.Position telemetry systems and so on.
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Voltage telemetery System
• Transmission signal is a direct voltage signal and
the signal transmission medium is essentially a
copper wire line.
37
Cont…
Sending-End Scheme:
• The transducer (sensor) converts the input physical
variable to an electrical signal.
• This output is processed by appropriate electronic
circuits (signal conditioner unit)
• Typically, the voltage is linearly proportional to the
value of the measurand.
• This voltage signal is then suitably amplified to a value
Vdc1 and given to the copper wire link.
38
Cont…
Receiving-End Scheme:
• The end device at the receiving end is a permanent-
magnet moving-coil (PMMC) voltmeter( high
sensitivity and scale linearity).
• The meter measures the voltage at the receiving end of
the line, Vdc2.
• Its scale is calibrated in terms of measurand (M), so that
the user can read the value of M(variable to measured)
directly.
39
Cont…
Transmission Error:
• The voltage at the receiving end is given by
Vdc2 = Vdc1 – I R.
• where I is the line current and R is the resistance of the line
Demerits
❖It can be used only for short distances.
❖Because both the error and the cost of line increase with the
length of the wire line.
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Current Telemetry System
• Transmission signal is a direct current signal and the signal
transmission medium is essentially a copper wire line.
• The most commonly used current signal is 4-20mA
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Cont…
Sending-End Scheme:
• Similar the direct voltage telemetry scheme.
• The difference in the sending-end schemes is that the
direct current system employs a voltage to current
converter
Receiving-End Scheme:
• The end device is a PMMC milli-ammeter as it has to
read the value of the line current at the receiving end,
Idc2, which is in milli-ampere range.
• Its scale is calibrated in terms of the measurand (M)
42
Cont…
Transmission Error:
• The line current at the receiving end is given by
Idc2 = Idc1 – Ileakage
• where Idc1 is the line current at the sending end.
• Ileakage is the small current leaking from one wire of the line to
the other wire or to the ground due to a finite value of insulation
resistance
Disadvantages:
• The only demerit or limitation of this telemetry system is that:
➢Used only for short distances.
➢Because the error due to leakage can become substantial if
the length of the wire line is large.
➢Cost of the line increases directly with its length
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Balanced Bridge Position Telemetry
• In this system, the transmitter adjusts the relation between
the signal in correspondence to the measurement.
• The receiver converts the signals into displacement to
represent the measurement.
• The system requires at least 3 conductors between the two
ends.
• Out of three, one conductor is taken common. The value
of one quantity is adjusted with the help of the
potentiometer.
44
Cont…
• There are two potentiometers.
• One at transmitting end and other at receiving end.
• Both are energized by a common supply.
45
Cont…
• The slide contact at the transmitting end is positioned by
Bourdon tube, when pressure is applied on the tube.
• If sliding contact at the receiving end is positioned, till the
galvanometer indicates zero;
• the position of the contact will assume the same position as
the contact at the transmitter end.
• The receiving end sliding contact moves the pointer, which
indicates the pressure to be measured and the scale is
calibrated in Nm/m2.
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Radio Frequency(RF) Telemetry System
• The signal transmission medium is a radio link.
• It consists of:
➢a transmitting antenna ,
➢receiving antenna and the space.
• The transmitter consists of a RF modulator, and amplifier
• The receiver comprises an amplifier and a demodulator.
Mainly divided into
• Short-Range Radio Telemetry System
• Satellite Radio Telemetry System
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Cont…
48
Satellite Radio Telemetry System
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Short-Range Radio Telemetry System
• Transmission signal is a frequency-modulated AC signal.
• Generally a RF sinusoidal signal is used as the carrier and
a radio link as the transmission medium.
• FM telemetry has been largely used for short range radio
telemetry
Sending-End Scheme:
• A transducer converts the given physical variable into an
electrical output.
• which is conditioned/ processed by an appropriate signal
conditioner to yield a dc voltage proportional to the value
of the measurand, M.
50
Cont…
51
Cont…
• This voltage signal is used for the frequency modulation of
a radio-frequency (RF) carrier.
• The frequency-modulated radio-frequency (FM-RF) signal
is applied to a transmitting antenna
Receiving-End Scheme
• The receiver selects the desired signal by employing a band-
pass filter.
• This signal is demodulated using a frequency demodulator
thereby recovering the information signal.
• Signal conditioner processes the information signal to make
it compatible to the given end device
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Pulse Telemetry System
• Commonly used in long distance telemetry
• Carrier is a train of Pulses
• Classified into
1.Analog(PAM,PWM) telemetry
2.Digital( PCM) Telemetry
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PAM Telemetry
Pulse-Amplitude Modulation (PAM)
Flat-Top Sampling
• Natural sampling is not usually employed.
• Flat-top sampling is the basis for pulse amplitude
modulation.
• In natural sampling the pulse amplitude takes the shape
of the analogue waveform for the period of the
sampling pulse.
54
Cont…
Ideal Sampling
Natural Sampling
Flat Top Sampling
55
Cont…
• In flat top sampling the continuous analogue waveform is
converted into a series of pulses;
❖whose amplitude is equal to the amplitude of the
analogue signal at the start of the sampling process.
• In pulse-amplitude modulation (PAM) the amplitudes of
regularly spaced pulses are varied in proportion to the
corresponding sample values of a continuous message
signal
56
Cont…
• There are two operations involved in the generation of
PAM signal:
1, Instantaneous sampling of the message signal m(t)
every Ts seconds
– where the sampling rate fs = 1⁄Ts is chosen according to
the sampling theorem
2, Lengthening the duration of each sample to some
constant value τ.
• In digital communication these two operations are jointly
referred to as “sample and hold”.
57
Cont…
• Figure : Pulse Amplitude modulation
58
Cont…
59
Cont…
• Transmission signal takes the form of amplitude-
modulated pulses.
• A multi-channel operation is achieved through time-
division multiplexing (TDM).
Sending-End Scheme
• A 4-channel PAM telemetry system is shown in figure
above
• Measurands, M1 to M4, are applied to appropriate
transducers, T1 to T4, respectively.
• The transducer outputs are processed in suitable signal
conditioners, SC-1 to SC-4
• and produces dc voltages V1 to V4, proportional to M1
to M4, respectively.
60
Cont…
• These voltage signals are applied to a 4-channel
multiplexing switch at its input terminals, IN1 to IN4.
• The multiplexing switch functions under the control of
a clock in a cyclic order.
• The output is thus a time-multiplexed PAM signal.
61
Cont…
Receiving-End Scheme
• The receiver gets the sync pulse as well as PAM signals.
• The sync pulse is identified by a sync pulse detector (on the
basis of amplitude)
• and delivered to the synchronization circuit.
• The time-multiplexed PAM signal is applied to the de-
multiplexing switch, which outputs the pulses P1 to P4 at its
output terminals, OUT1 to OUT4, respectively.
62
Cont…
• These pulsed signals are interpolated by the signal
interpolators(estimation of the unknown samples) followed
by reconstruction filters SI-1 to SI-4
• The reconstruction filters produce continuous voltage
signals, V1 to V4, proportional to m1 to m4, respectively.
• Finally, these voltages are read on respective PMMC
voltmeters.
• The PMMC are calibrated in terms of the values of the
measurand M1 to M4.
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Pulse duration modulation(Pulse Width
modulation) Telemetry System
• The sampled message signal is used to vary the
duration of the individual pulses in the carrier
Pulse-Position modulation(PPM):
➢ the position of a pulse relative to its un-modulated time
of occurrence is varied in accordance with the message
signal.
64
Figure (a), Modulating wave. (b), Pulse carrier.
(c) PDM(PWM) wave. (d) PPM wave 65
Pulse-Code Modulation(PCM)
Telemetry System
• The combined operations of sampling and quantizing generate a
quantized PAM signal, i.e. a train of pulses whose amplitudes are
restricted to a number of discrete magnitudes.
• In Pulse-Code Modulation (PCM), a message signal is represented by
a sequence of code pulses.
• This is accomplished by representing the signal in discrete form in
both time and amplitude.
66
Cont…
• The basic operations performed at a PCM transmitter
are sampling, quantizing and encoding
67
Cont…
Regenera
tive Ckt
Figure : A PCM system (Transmitter& Receiver)
Encoder
• In combining the process of sampling and quantization, the
baseband signal(signal that is proportional to the
measurand) becomes limited to a discrete set of values:
• This discrete set of values are not suitable for digital
transmission.
68
Cont…
• The quantized samples are applied to the encoder.
• The encoder responds to each quantized sample by the
generation of a unique and identifiable code in a form of
binary pulse (binary pattern).
• A binary code uses two digits, 0 and 1(called bits or
symbols), to represent the quantized sample values.
• If we have M quantization levels, then in order to represent
these levels in a binary code we require N bits, where N is
given by:
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PCM stream Reconstruction(Regeneration)
• The most important feature of PCM system is :
➢the ability to control the effects of distortion and noise;
❖produced by transmitting the PCM signal
through a transmission channel.
• This is accomplished by reconstructing the signal using a
chain of regenerative repeaters along the transmission
route(channel).
70
Cont…
Figure: Block diagram of Regenerative repeater
71
Cont…
• The equalizer shapes the received pulses so as to
compensate the effects of amplitude and phase distortions
produced by the transmission characteristics of the channel
• The timing circuit provides a periodic pulse train, derived
from the receive pulses.
• This train of pulses are used for sampling the equalized
pulses at the instants of time where the signal-to –noise ratio
is maximum.
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Decoder
• The first operation in the receiver is to regenerate the
received pulses.
• these pulses are then regrouped into code words and
decoded(i.e., mapped back) into a quantized PAM
signal.
• The pulse amplitude of the PAM signal is the linear
sum of all the pulses in the code word.
• Each pulse of the code word is weighted by its place
value(20, 21, . . ., 2N-1) where N is the number of bits
per sample.
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