telemetry

telemetry, highly automated communications process by which measurements are made and other data collected at remote or inaccessible points and transmitted to receiving equipment for monitoring, display, and recording. Originally, the information was sent over wires, but modern telemetry more commonly uses radio transmission. Basically, the process is the same in either case. Among the major applications are monitoring electric-power plants, gathering meteorological data, and monitoring manned and unmanned space flights.

The original telemetry systems were termed supervisory because they were used to monitor electric power distribution. In the first such system, installed in Chicago in 1912, telephone lines were used for transmitting data on the operation of a number of electric-power plants to a central office. Such systems spread to other fields besides power networks and underwent extensive improvements, culminating in the introduction in 1960 of the so-called interrogation-reply principle, a highly automated arrangement in which the transmitter-receiver facility at the measuring point automatically transmits needed data only on being signalled to do so. The technique is applied extensively throughout the world in such fields as oil-pipeline monitor-control systems and oceanography, in which a network of buoys transmits information on demand to a master station.

Aerospace telemetry dates from the 1930s, with the development of the balloon-borne radiosonde, a device that automatically measures such meteorological data as temperature, barometric pressure, and humidity and that sends the information to an Earth station by radio. Aerospace telemetry for rockets and satellites was inaugurated with the Soviet satellite Sputnik, launched in 1957, and systems have grown in size and complexity since then. Observatory satellites have performed as many as 50 different experiments and observations, with all data telemetered back to a ground station. The techniques developed in aerospace have been successfully applied to many industrial operations, including the transmission of data from inside internal-combustion engines during tests, from steam turbines in operation, and from conveyor belts inside mass-production ovens.

Telemetering systems and components.

A typical telemetering system consists of an input device called a transducer, a medium of transmission (usually radio waves), equipment for receiving and processing the signal, and recording or display equipment.

The transducer.

The transducer converts the physical stimulus to be measured, such as temperature, vibration, or pressure, into an electrical signal and thus operates as the actual measuring instrument. Transducers can take many forms. They can be self-generating or externally energized. An example of the self-generating type is a vibration sensor based on the use of a piezoelectric material—i.e., one that produces an electrical signal when it is mechanically deformed. Many externally energized transducers operate by producing an electrical signal in response to mechanical deformation. Typical physical inputs producing such deformations are pressure, mechanical stress, and acceleration. A simple mechanical transducer-sensing device is a strain gauge based on the change in electrical resistance of a wire or a semiconductor material under strain. Another externally energized transducer, called the variable-reluctance type, is one in which the magnetic circuit is broken by an air gap. The mechanical movement to be measured is used to change this air gap, thus changing the reluctance, or opposition, to the production of a magnetic field in the circuit. The change in reluctance is then translated into an electrical signal.

Temperature sensors can be divided into two classifications: temperature-dependent resistance elements and self-generating thermocouples. Thermistors are of the first type; they have a high negative temperature coefficient—i.e., their resistance drops very rapidly as the temperature increases. The thermistor is small and provides rapid response to changes in temperature. Thermocouples are wire junctions of dissimilar metals that produce an electrical current when heated; they have a very low output, and each must be used with a second thermocouple held at a constant cold temperature for a reference point.

There are many types of specialized sensors and transducer systems. One is the previously mentioned radiosonde system, designed specifically to radio weather data from a balloon to a ground station. Most weather-sensing and transmitting elements measure temperature, pressure, and humidity. In manned space probes, sensors for measuring such factors as the astronaut’s blood pressure, heartbeat, and breathing rate are employed. Sensors have also been developed to indicate the rate of flow of a fluid through a pipe.

Communications links. Communications facilities for telemetry consist primarily of radio or wire links. Alternatives such as light beams or sonic signals are used in a few cases, but environmental factors (e.g., atmospheric obstructions) and local masking noises make them impractical for most applications.

Radio communication is used for aerospace work and for supervisory systems in which it is impractical to provide wire line links. For public utility installations in built-up areas, radio communication is usually ruled out by the difficulty of finding antenna sites and unobstructed line-of-sight radio paths. In such cases, cables and line links are used.

An important consideration in radio links is the choice of operating frequency, a choice limited to bands allocated by international agreement. Propagation varies enormously over the range of frequencies involved. For aerospace applications in which transmissions must penetrate the atmosphere, the frequency range is 100 megahertz (100,000,000 cycles per second) to 10,000 megahertz. Line links for supervisory applications usually employ a comparatively narrow band. They may utilize the whole or only a section of a conventional voice channel with a bandwidth of 3,000 hertz (cycles per second). The link may be either a direct wire circuit or one of the channels in a carrier communications system.