TELEMETRY SYSTEM

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INTRODUCTION TO TELEMETRY:
• Telemetry may be defined as measurement at a distance.
• Telemetry is the process by which the measured quantities such as temperature,
level, pressure, flow, displacement, velocity, acceleration etc. are transmitted to a
convenient remote location, in a form, suitable for displaying, recording, actuating a
process etc
BLOCK DIAGRAM OF A TYPICAL TELEMETRY SYSTEM:
• A general telemetering system is shown in fig.

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Telemetry system classification:
1) Basis of the signal transmission medium used
Wire-Link Telemetry or Wire Telemetry
It uses a pair of copper wires (or conductors) as the signal transmission medium or link
between the sending and receiving ends.
Radio Telemetry or Wireless Telemetry
It uses a radio link between the transmitting and receiving ends. Very often, references are
Made to the following two special types of radio telemetry:
(i) Short-Range Radio Telemetry: When radio link is used for the reason that the sensor
Output cannot be taken through wires whereas the distance involved is so short that
even conventional methods of measurement could have been used, then this type of
radio telemetry is called short range radio telemetry.
(ii) Satellite-Radio Telemetry: When satellite radio communication is used for linking
Widely spaced transmitter and receiver of a radio telemetry system, it is referred to as
satellite-radio telemetry.
Optical-Fibre Telemetry or Fibre-Optic Telemetry
It uses optical fibre as the signal transmission medium or link between the transmitting and
receiving ends.
2) Telemetry Classification Based on Modulation Method
In regard to the modulation, there are three possibilities and accordingly there are three
categories of telemetry systems as follows:
DC Telemetry Systems
These telemetry systems use no modulation. The information signal which varies very slowly
and is considered as a DC signal is transmitted as such.
There are two telemetry systems in this category:
1. Direct voltage telemetry system
2. Direct current telemetry system
AC Telemetry Systems
These telemetry systems use an AC carrier, which is modulated using one of the AC
modulation techniques.
There are two telemetry systems in this category:
1. Amplitude modulation (AM) telemetry system
2. Frequency modulation (FM) telemetry system
Pulse Telemetry System
These telemetry systems use a pulse carrier, which is modulated using one of the pulse
modulation techniques.
There are five telemetry systems in this category:
1. Pulse amplitude modulation (PAM) telemetry system

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2. Pulse width modulation (PWM) telemetry system
3. Pulse phase modulation (PPM) telemetry system
4. Pulse frequency modulation (PFM) telemetry system
5. Pulse code modulation (PCM) telemetry system
3) Telemetry Classification Based on Type of Information Signal
Telemetry methods and systems are very often also classified on the basis of the type of the
Information signal, which can be either analog or digital. Accordingly the telemetry system is
called analog or digital telemetry system. It may be noted that in all the telemetry systems listed
in the last section, the input to the transmitter is an analog signal, with only one exception, viz.
PCM telemetry system. Accordingly, the above telemetry systems can be re-classified under
two categories as follows:
3.1 Analog Telemetry Systems
1. Direct voltage telemetry system
2. Direct current telemetry system
3. Amplitude modulation (AM) telemetry system
4. Frequency modulation (FM) telemetry system
5. Pulse amplitude modulation (PAM) telemetry system
6. Pulse width modulation (PWM) telemetry system
7. Pulse phase modulation (PPM) telemetry system
8. Pulse frequency modulation (PFM) telemetry system
3.2 Digital Telemetry System
Or
Pulse code modulation (PCM) telemetry system
4.Single- and Multi-Channel Telemetry Systems
A telemetry system may be a single-channel or a multi-channel telemetry system depending
on the number of physical variables (measurands) telemetered by it. One of the following
methods of multiplexing is used in a telemetry system of multi-channel type in order to send
signals of all the channels on a single transmission link:
(a) Time Division Muliplexing (TDM): It can be used with all types of signal transmission
links.
(b) Frequency Division Multiplexing (FDM): It can be used with copper wire links and radio
links only.

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PNEUMATIC TELEMETRY:
• In a pneumatic telemetry system, compressed air is used to communicate the values
of measured quantity from one location to the other location.
• A block diagram of a pneumatic telemetry system is shown in fig.
• There are four bellows elements A and B transmitting with stroke lever & the
interface disc d.C & D form the receiving & display block along link.
• The two blocks are connected by pneumatic lines. with the float rising or falling ,the
push rod moves up pressing bellows element B or expanding it so pressure increase
in line 1 or 2 expanding element D or C at the receiving end
ADVANTAGES OF PNEUMATIC TELEMETRY SYSTEM:
• They are safe and explosion proof.
• They are unaffected by electric power failures.
• Pneumatic actuators and control valves are directly operated through pneumatic
signals without requiring any conversion.
• Improved dynamic response and facilities for calibration and checking.
DISADVANTAGES OF PNEUMATIC TELEMETRY SYSTEM:
• They are slow to respond.
• System is not suitable for larger distances.

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HYDRAULIC TELEMETRY:
• The open center system may employ any number of subsystems, with a selector valve
for each subsystem. Unlike the closed center system, the selector valves of the open
center system are always connected in series with each other. In this arrangement, the
system pressure line goes through each selector valve. Fluid is always allowed free
passage through each selector valve and back to the reservoir until one of the selector
valves is positioned to operate a mechanism. When one of the selector valves is
positioned to operate an actuating device, fluid is directed from the pump through one
of the working lines to the actuator. [Figure 12-3B] With the selector valve in this
position, the flow of fluid through the valve to the reservoir is blocked. The pressure
builds up in the system to overcome the resistance and moves the piston of the
actuating cylinder; fluid from the opposite end of the actuator returns to the selector
valve and flows back to the reservoir. Operation of the system following actuation of the
component depends on the type of selector valve being used. Several types of selector
valves are used in conjunction with the open center system. One type is both manually
engaged and manually disengaged. First, the valve is manually moved to an operating
position. Then, the actuating mechanism reaches the end of its operating cycle, and the
pump output continues until the system relief valve relieves the pressure. The relief
valve unseats and allows the fluid to flow back to the reservoir. The system pressure
remains at the relief valve set pressure until the selector valve is manually returned to
the neutral position. This action reopens the open center flow and allows the system
pressure to drop to line resistance pressure. The manually engaged and pressure
disengaged type of selector valve is similar to the valve previously discussed.
• When the actuating mechanism reaches the end of its cycle, the pressure continues to rise to a
predetermined pressure. The valve automatically returns to the neutral position and to open center flow

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ELECTRICAL TELEMETRY:
.
CURRENT TELEMETRY SYSTEM:

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ADVANTAGES OF CURRENT TELEMETRY SYSTEM:
• The current system can develop higher voltages than most voltage systems.
• Simple D.C. milliammeters can be used.
• Several receivers can be operated simultaneously.
• The energy level is high.
DISADVANTAGE OF CURRENT TELEMETRY SYSTEM:
• This system is not suitable for larger distance.

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PULSE TELEMETRY:
• In this type of telemetry, the measurand is transmitted in terms of time rather than
magnitude of an electrical quantity.
• The information may be conveyed through radio frequency links to the remote
control room.
• Pulse telemetry is classified into two categories:
1) Analog pulse telemetry
2) Digital pulse telemetry
1. ANALOG PULSE TELEMETRY:
• In an analog pulse telemetry, the signal which is transmitted to a remote location is
converted into the number of pulses which are d.c. or a.c. voltages of constant
amplitude and small width.
• Analog pulse telemetry is classified into three categories:
1) Pulse amplitude modulation (PAM)
2) Pulse width modulation (PWM)
3) Pulse position modulation (PPM)

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2. DIGITAL PULSE TELEMETRY:
• In digital pulse telemetry system, the data to be measured is quantized and
transmitted serially as data words in pulse code.
• Due to this reason, the process is also called pulse code modulation (PCM).
• The analog signal is sampled at regular intervals and then each sample value is
converted into coded form.

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FREQUENCY SPECTRUM FOR TELEMETRY APPLICATIONS:
NAME FREQUENCIES APPLICATIONS
Very low frequency (VLF) Below 30 kHz Radio location equipment
Low frequency (LF) 30 kHz to 300 kHz Wartime radio navigation
Medium frequency (MF) 300 kHz to 3 MHz Includes AM radio
broadcast band
High frequency (HF) 3 MHz to 30 MHz Radio
Very high frequency (VHF) 30 MHz to 300 MHz Includes FM broadcast
band and television VHF
channels
Ultra high frequency
(UHF)
300 MHz to 3 GHz Includes television UHF
channels
Super high frequency
(SHF)
3 GHz to 30 GHz Satellite communications
Extremely high frequency
(EHF)
30 GHz to 300 GHz Satellite communications

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Pneumatic telemetry system:3 -15psi
Electrical telemetry system:4 -20 mA or 0-24 v
Hydraulic telemetry system:0.2 to 1 kg/sem^2