Telemetry

Telemetry is a branch of engineering that deals with the detection and measurement of physical quantities at one location, the transmission of the measurement data to a distant receiving station, and the recording, display, or interpretation there of the data. Physical quantities, such as temperature, pressure, and radiation.

The distance between the point of measurement and the receiving station may be only a few inches, as in ground tests of engines, or it may be many millions of miles, as in the transmission of data from deep-space probes. Sometimes the transmission link is used exclusively for telemetry and sometimes for telemetry and other purposes such as human communications or flight commands to a spacecraft.

Such systems monitor the performance of space vehicles, check the physical condition of astronauts, and determine environmental conditions. Telemetry is also used in some remote-control systems in industry.

Telemetering systems consist of three essential parts: a device for picking up information, a means of transmitting the data, and a receiver for indicating or recording the information at a distant point.

The information is picked up by transducers, which measure the physical quantities in the form of electrical signals suitable for transmission. A simple transducer for measuring temperature employs a heat-sensitive metal, such as platinum or tungsten. Because the electrical resistance of the metal varies with changes in temperature, a signal corresponding to temperature can be generated. Signals corresponding to strains measured with a wire strain gage can be produced because the electrical resistance of the wire varies as it is stretched.

The data is transmitted as a signal by wire or radio.

Complex multiplexing systems are used to send several signals simultaneously over a single pair of wires or over a single radio-frequency band. The combined signals are later separated by the receiving equipment, which also converts them into meaningful forms for observing or back-up storage.

Historical Background

Long-distance telemetry was first used in 1874, when electrical signals representing meteorological data were transmitted by wire from the slopes of Mont Blanc to Paris, a distance of about 300 miles (485 km). Wire-link telemetry also was used in 1913-1914 to report the water level in the locks of the Panama Canal and the position and movement of the lock gates, and to display to measured quantities at a central control board. In the 1920's telemetry became more widely used, including the reporting of load conditions in electrical power transmission systems and of pressure conditions in long petroleum pipelines.

An early use of radio telemetry was in France in 1931, when small unmanned balloons sent weather data, such as temperature, pressure, and humidity measurements, to ground stations. Soon after 1940, the first radio telemetry systems for flight tests of aircraft were designed and used for measuring structural strains and flight characteristics. This occurred in response to the need for more complete and reliable measurements than could be provided by on-board recorders or pilots' observations during experimental flights of military aircraft. By 1945 radio telemetry was used for flight measurements taken during tests of small rockets. About the same time rockets were provided with telemetry equipment for exploration of the upper atmosphere at altitudes far above those reached by balloons.

By 1955 large and complex rocket-propelled vehicles were being designed and built in the United States, spurring the development of radio telemetry systems that could transmit hundreds of measurements for subsequent data processing. By 1970 the development of unmanned satellites for space research, communications, meteorology, and earth resources evaluations, of unmanned space probes for investigating the solar system, and of programs for manned space flight had led to even more advanced telemetry systems. These made use of computer control and data processing before transmission. Telemetry also became increasingly useful in biomedical research.

How a Telemetry System Works

In the operation of a telemetry system the major steps are those that: (1) make a measurement; (2) originate a signal that represents a measured quantity needed for eventual interpretation or decision; (3) encode, or modulate, this signal to give it greater immunity to errors that may arise during its passage over the transmission link; (4) transmit the encoded signal over the link, generally by wire or radio; (5) receive the encoded signal; (6) decode, or demodulate, the received signal to obtain the wanted measurement data; and (7) record, display, store, or interpret the data that have been obtained.

In most uses of telemetry systems, it is necessary to send signals representing many measurements over the same channel. This is accomplished by the use of some form of multiplexing. In one technique, called frequency-division multiplexing, each measurement signal is assigned to a frequency band that does not overlap any of the other frequency bands to which signals are assigned. In another technique, called time-division multiplexing, each measurement signal is assigned to a time interval that does not overlap any of the other time intervals to which signals are assigned.

The coding techniques that are used include pulse-amplitude modulation (PAM), pulse-duration modulation (PDM), and pulse-code modulation (PCM). In the 1960's, PCM was used increasingly in all areas of telemetry as a result of the development of complex microelectronic circuits, digital computers for data processing, and improved coding methods.

Applications

Telemetry has two major applications: as a permanent part of a fully developed operational system and as a temporary part of a system being tested. When part of an operational system, as for industrial control or for space probes, the telemeter must be compatible with the rest of the system with respect to cost, reliability, maintainability, and other factors. When part of a system being tested, as in rocket vehicle tests or in biomedical research experiments, the telemeter need not be fully compatible with the rest of the system.

Industrial Control. A telemetry system for supervisory control of a gas pipeline may extend over 2,000 miles (3,220 km), often using wire circuits for much of the basic transmission link. At a typical compressor station on the pipeline route, the telemetry system makes measurements to determine station status, pressure set points, actual pressures, and gas flow, and converts the data to digital form by using pulse-code modulation. On demand, the system transmits the data to a central office, where they are evaluated by a computer. Based on this evaluation, appropriate control signals are sent to various compressor stations along the route.

Rocket Vehicle Tests. A radio telemetry system for flight tests of the Saturn rocket launch vehicle made as many as 1,000 measurements and transmitted all the data by the time-division multiplex method. The rocket was so large that the time-division multiplexer was divided into subunits distributed throughout the rocket in order to reduce the excessive amount of signal circuit wiring that would otherwise be needed.

Dental Research. A telemeter placed inside the human mouth can transmit as many as 10 measurements, including some on the forces and motions of the teeth. The entire telemeter, including a battery and a radio transmitter, fits in the space of the crown of a molar tooth. In this application time-division multiplexing and pulse-amplitude modulation are used in transmitting the data to a radio receiver.

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