unit - 1 telemetry principles€¦ · unit – 1 telemetry principles 6 jnkothari,ic department...

UNIT – 1 TELEMETRY PRINCIPLES

1 JNKOTHARI,IC DEPARTMENT

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.



Telemetry system classification:

1) Basis of domain

Hydraulic

Pneumatic

Electric & electronic

2) Basis of characteristic of electrical signal

Current

Pulse

Voltage

Frequency

Position

3) On the basis of transmission

Analog

Digital

4) On the basis of distance

Short distance

Long distance



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.



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



ELECTRICAL TELEMETRY:

.

CURRENT TELEMETRY SYSTEM:



� 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.



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)



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.



•



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



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

unit - 1 telemetry principles€¦ · unit – 1 telemetry principles 6 jnkothari,ic department...

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