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Telemetry is the process of transfer of signals from one place to another without the help of any intermediate medium. It has become a necessary part of our lives nowadays. There isn’t any part where telemetry hasn’t been used by us. Whether its starting and controlling your TV using a remote or talking to a friend far away, say, in Gujarat, it is telemetry which has always come to our rescue.
Telemetry has its inception in the wireless technology, which was pioneered by Alexander Graham bell. Through the years, it has developed and spread into various branches, which will be discussed here.
“Higher-quality telemetry coupled with high bandwidth communications means we can collect information in real time, as it is happening,”
True to every word, this quote was coined by Kenneth Graham. It puts the main aim of telemetry in a nutshell, i.e, collecting data and transferring them in the most efficient of ways possible. With the digital age finally coming to its foremost times, telemetry has been needed to mould itself accordingly. And for this reason,pathbreaking technologies,like cluetooth,WiFi came into existence.
Telemetry sure would have come in handy for the Wright brothers at Kitty Hawk, North Carolina back in 1903.
But engineers from different disciplines and industries didn’t pioneer it until some years later. As fate would have it, each deployed their own unique set of telemetry nomenclature. Even today, definitions used in airborne, ground, space, flight test, or industrial applications may differ.
TELEMETRY……. GETTING STARTED
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. The transmission media may be air and space for satellite applications, or copper wire and fiber cable for static ground environments like power generating plants.
TELEMETRY……. AN OVERVIEW
A telemetry system is often viewed as two components, the Airborne System and the Ground System. In actuality, either or both may be in the air or on the ground.
TDM (TIME DIVISION MULTIPLEXING)
Data acquisition begins when sensors & transducers measure the amount of a physical attribute and transform the measurement to an engineering unit value. Some sensors produce a voltage directly while others require excitation. Sensors attached to signal conditioners provide power for the sensors to operate or modify signals for compatibility with the next stage of acquisition. Since maintaining a separate path for each source is cumbersome and costly, a multiplexer is employed. It serially measures each of the analog voltages and outputs a single stream of pulses, each with a voltage relative to the respective measured channel. The rigorous merging of data into a single stream is called Time Division Multiplexing or TDM.
PAM (PULSE AMPLITUDE MODULATION)
The scheme where the pulse height of the TDM stream is proportional to the measured value is called Pulse Amplitude Modulation (PAM). A unique set of synchronization pulses is added to identify the original measurands and their value. PAM has many limitations, including accuracy, constraints on the number of measurands supported, and the poor ability to integrate digital data.
PCM (PULSE CODE MADOLATION)
Pulse Code Modulation (PCM) is today’s preferred telemetry format for the same reasons that PAM is inadequate.
Accuracy is high, with resolution limited only by the analog to digital converter (ADC), and thousands of measurands can be acquired along with digital data from multiple sources, including the contents of the computer’s memory and data buses.
In a PCM-based system, the original PAM multiplexer’s analog output is digitized to a parallel format. The Output Formatter along with synchronization data for measurand identification merges this, plus other sources of digital data. The Output Formatter serializes the composite parallel data stream to a binary string of pulses (1’s and 0’s) for transmission on copper wire, fiber cable. All components from after the sensor to the formatter comprise the encoder. Other, often remote encoders are used to multiplex additional sensor data into the main encoder’s output. Not only does this expand the number of measurands to thousands per stream, but it also eliminates the weight of cables required for each sensor.
The output of the main encoder is filtered and transmitted via radio transmitter and antenna, coax cable, telephone line, tape recorder, etc. Filtering rounds or smoothes the square data pulses to reduce frequency content and thus the required transmitter bandwidth. At the Ground Station, the received data stream is amplified. Since the transmission path often distorts the already rounded signal, a bit synchronizer reconstructs it to the original serial square wave train. Then, a decommutator (decom) recognizes the synchronization pattern and returns the serial digital stream to parallel data. The decom also separates the PCM stream into its original measurands (also known as prime parameters) and data.
The computer (in the Visual Test System) or the telemetry front end system selects prime parameters for real-time processing; archiving to disk or tape; display; output to strip chart recorders or distribution to other computing resources according to the test plan.
1. DATA ACQUISITION:
In the airborne system, variety of sensors & transducers are used for to measure and acquire a physical property’s value. Signal conditioners serve as the interface of the data acquisition system from the transducers. Many transducers require ac or dc power (e.g., thermistors, strain gages, and LVDTs), while others generate signals (tachometers, thermocouples, and piezoelectric strain gages) as they provide excitation (power), network calibration, signal amplification, and filtering.
In airborne data acquisition, sensor output characteristics must be transformed, filtered, or modified for compatibility with the next stage of the system. The absolute relationship between the output and the actual property value of the measurand may vary with time, altitude, pressure, temperature, etc. Therefore, signal conditioners also incorporate calibration features to assist in defining the relationships.
Whatever the quantities monitored at the data source (whether electrical or physical), the cost to transmit each quantity through a separate channel would be prohibitive. One way to conserve resources is to share time or frequency spectrum with techniques such as Time-Division Multiplexing (TDM) &
Frequency Multiplexing (FM), respectively.
Think of the equipment and cables or frequency
spectrum required to monitor and transmit several hundred or thousands of measurands!
Today, the most popular form of telemetry multiplexing (originally called commutation) is TDM. Here, each channel is serially sampled for an instant by the multiplexer.
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10-08-2011, 03:18 PM
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By Definition Telemetry is the science and technology of automatic measurement and transmission of data by wire, radio, or other means from remote sources, as from aircraft, to receiving stations or a drive shaft to a stationary location on the vehicle . 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. The transmission media may be air and space for satellite applications, or copper wire and fiber cable for static ground environments like power generating plants. In today's telemetry applications, which support large numbers of measurands, it is too costly and impractical to use separate transmission channels for each measured quantity. 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 .
1.2 The purpose of telemetry
Telemetry lets stay in a safe (or convenient) location while monitoring what's taking place in an unsafe (or inconvenient) location. Aircraft development, for example, is a major application for telemetry systems. During initial flight testing, an aircraft performs a variety of test maneuvers. The critical flight data from a maneuver is transmitted to flight test engineers at a ground station where results are viewed in real time or analyzed within seconds of the maneuver. Real-time monitoring allows the "safety officer" to make instant decisions on whether to proceed with or terminate a test. With real-time analysis, the flight test engineer can request a maneuver be repeated, the next maneuver be performed, or test plan alternatives be substituted. Real-time data is also captured to storage media, such as disk and tape, for later analysis and archiving.
1.3 Reason for Telemetry
Telemetry(or Remote Monitoring, or M2M, or Telematics, or Machine2Man, Man2Machine, Machine2Machine... ) can be used for too many applications to mention here, but vehicle and mobile asset tracking, storage tank level monitoring, remote meter reading, pump condition monitoring, vending machine monitoring all have one thing in common and that is reducing costs. The technology is now sufficiently proven, the mobile network operators have now embraced the solution and the cost of the solutions have fallen dramatically over the years opening most if not all of these applications. Using telemetry can seriously help business - know where things are, how things are, be the first to react to a potential problem, know what has caused the problem before set off on a long journey, get more from operatives and assets.
1.4 Types of Telemetry transmission
• Cell phone – It can use GSM and GPRS. CDMA technology is also used in this. Data phones and modems are very cheap. It is limited to areas having cell phone coverage.
• VHF or UHF Radio – It requires radio system usually with repeaters in required locations.
• Satellite receiving systems – Transmission is one way (data acquisition) only. It is very expensive and slow.
• Satellite phone (satphone) – The operation is similar to cell phone telemetry, but more expensive (both hardware and calls). It will work almost anywhere in the world. Costs are coming down gradually.
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16-02-2012, 01:31 PM
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