Hoeppner, C.H. “Telemetry”
The Electrical Engineering Handbook
Ed. Richard C. Dorf
Boca Raton: CRC Press LLC, 2000
77
Telemetry
77.1 Introduction to Telemetry
77.2 Measuring and Transmitting
77.3 Applications of Telemetry
Power Sources ? Power Plants
77.4 Limitations of Telemetry
77.5 Transmitters and Batteries
77.6 Receivers and Discriminators
77.7 Antennas and Total System Operation
77.8 Calibration
77.9 Telemetry Frequency Allocations
77.10 Telemetry Antennas
77.11 Measuring and Transmitting
77.12 Modulating and Multiplexing
77.13 Passive Telemeters
77.14 The Receiving Station
77.1 Introduction to Telemetry
Telemetry, or measurement at a distance, takes many and varied forms. It may use the principles of radio,
electricity, optics, mechanics, or hydraulics to convey measurements made at one place to indicators, actuators,
recorders, or computers at another. By far the most popular telemetry systems are electrical and use radio or
wire links to convey information. In this respect, all of the considerations in the foregoing chapters on com-
munications apply, as well as considerations of antennas, power supplies and convertors, heat removal, and
radio frequency interference. Additional considerations that are unique to telemetry are treated here.
The deeper an instrumented vehicle probes into the remote reaches of outer space, the more technologically
spectacular seem the achievements of telemetry. There is still something exciting and uncanny about performing
measurements of a physical quantity at a distant location and precisely reproducing them at a more convenient
place for reading or recording them.
Yet the vast distances spanned by telemetry signals are less challenging technically than the stubborn problems
of almost sheer inaccessibility in some industrial applications to the quantities being measured. Signals from
a missile-launched space probe soaring toward the sun are often easier to obtain than measurements from
inside a stolid, earthbound motor only a foot or two away. To find the temperature of the spinning rotor,
housed in a steel casing and surrounded by a strong alternating magnetic field, may require more ingenuity to
transcend the operating environment than taking measurements from the most distant instrument payload
speeding through the unaccommodating environment of space.
The technology that has produced missile and space telemetry is also spawning new forms of industrial radio
telemetry, capitalizing on the development of new transducers, powerful miniature radio transmitters, improved
self-contained power sources, and better techniques of environmental protection.
Simply enough, to telemeter is to measure at a distance. First, at the remote point, is needed a transducer,
a device that converts the physical quantity being measured into a signal, usually an electrical one, so that it
Conrad H. Hoeppner
The Johns Hopkins University
? 2000 by CRC Press LLC
can be more conveniently transmitted. Then a connecting link between the location where the measurement
is being made and the point where one can read or record the signal being sent is required. This link can be
either an electrical circuit—there have been wired telemetry systems since long before the turn of the cen-
tury—or pneumatic or hydraulic lines, a beam of light, or now, more practically, a radio carrier.
A radio telemetry system comprises (1) transducers that convert measurements into electrical systems, (2) a
subcarrier oscillator modulated by the transducers, (3) a radio transmitter modulated by the subcarrier, (4) a
transmitting antenna, (5) a receiving antenna, (6) a radio receiver, and (7) a subcarrier discriminator. The radio
link can transmit an analog of the continuous variable being measured, or, with pulse-code methods, it sends
the measurement data digitally as a finite number of symbols representing a finite number of possible values
of the measurement signal at the time it is sampled. The range of a radio link is limited by the power radiated
toward the receiver from the transmitter and by the sensitivity of the receiver. The wider the bandwidth, the
more the effect from noise, and therefore the more transmitted power required for a detectable signal.
Optical, mechanical, and hydraulic telemeters represent a smaller segment of the telemetry field than do
electrical and radio telemeters; they will be given only brief treatment here.
Optical telemeters use light transmitted through space or through optical fibers, the light being modulated
by the measurement signal. The modulation may be produced either electrically or mechanically. Electrically,
light-emitting diodes, lasers, or electroluminescent material are used to convert the electricity to light. The light
may then be modulated with the measurement information by modulating the electricity that produces the
light or it may be polarized and rotated by Kerr or Pockels cells, absorbed by electrically activated chromofors,
or converted to another wavelength by electrically controlled nonlinear elements, all of which are activated by
the modulating signal.
Hydraulic telemeters are generally used in conjunction with hydraulic sensors and hydraulic displays, such
as pressure gauges. They are immune to all electrical and optical interference and hence find application in
unfavorable electrical and optical environments. A typical hydraulic telemeter is used to measure load lifted
and boom angle on a crane. Here a hydraulic piston is activated by the tension of a lifting rope pushed sidewise
by the piston roller. An increase in tension produced by the load tends to straighten the deflected rope and
press the piston into its cylinder. The change in pressure is communicated through fluid in a tube to a remote
indicator. The hydraulic telemeter measures the boom angle by simply placing a fluid reservoir on the boom,
which when it is raised provides increased pressure through its tube to a second remote indicator. In this way,
with remote indicators, the operator monitors the crane to prevent overloading and/or overturning.
Electrical telemeters proliferate through (1) space, (2) battlefields, and (3) industrial sites, varying in size,
configuration, and information-carrying capacity with their various applications. Space research and missile
development used the first significant multichannel telemeters. Telemeters developed at the Naval Research
Laboratories and built by the Raytheon Company were first used to explore outside the earth’s atmosphere in
German V-2 rockets launched at the White Sands Proving Ground. These telemeters used 1000-MHz pulse
position modulated signals at ranges greater than 100 miles. Conrad H. Hoeppner designed the equipment and
managed the installations and operation. From 1945 onward, telemetry developed rapidly and found its way
to the various missile ranges also being developed.
To permit tests to be made interchangeably at all ranges, it was necessary to standardize types of telemeters
at the ranges. To this end the Department of Defense Research and Development Board formed the Guided
Missiles Committee, which in turn formed the Working Group on Telemetry. This later became the Inter-Range
Instrumentation Group (IRIG), which has published telemetry standards that are widely accepted.
Meanwhile, industrial telemetry has developed along different lines, producing miniaturized complete cap-
sules for applications to process control, detection of defects, and machine design. Medical science is currently
using telemetry in experimental, clinical, and diagnostic applications. Some of the particular body characteristics
telemetered include heartbeat, brain waves, blood pressure, temperature, voice patterns, heart sounds, respira-
tion sounds, and muscle tensions. Similar studies are being pursued in the biological and psychological fields,
where more experimental latitude permits embedding of transmitters within living animals.
The basic telemetry system consists of three building blocks. These are (1) input transducer, (2) the trans-
mitter, and (3) the receiving station. Transducers convert the measured physical quantity into a usable form
for transmission. The conversion of the desired information into a form capable of being transmitted to the
receiver is a function of the type of transducer employed. Transducers convert the physical quantities to be
? 2000 by CRC Press LLC
measured into electrical, light, pneumatic, or hydraulic energy. The type of energy conversion is determined
by the type of transmission desired. In a radio telemetry system, the transmitter and receiver have much in
common with communications equipment. The transducers, however, are unique to telemetry and will be
described in some detail here.
One of the most common types of transducers generates electrical signals as a function of the changing
physical quantity, and one of the most common varieties of this type is the resistance wire strain gauge. In this
transducer, the ability of the wire to change its dimension as it is stressed causes a corresponding change in its
electrical resistance. A decrease in wire diameter generally results in greater resistance to the flow of electricity.
Similarly, temperature-sensitive materials that have electrical characteristics changing with temperature make
temperature detection possible.
In most transducers, the electrical output is varied as a function of changes in the physical parameter. These
electrical changes can be transmitted by wire direct to a control center, data display area, or to a data analysis
section for evaluation. The difficulties with the use of wire in many applications have given rise to wireless
telemetry. In order to transmit the transducer information through the air, it is necessary to apply this
information to a high-frequency electrical carrier, as is commonly done in radio. Application of the transducer
information to a high-frequency carrier is commonly called modulation. High-frequency or rapidly changing
electricity has the capability of being propagated through space, whereas low-frequency or battery, nonchanging
voltage does not possess this ability.
The technique used for applying or modulating the high-frequency carrier by the transducer output involves
any one of three different methods. It is possible to modulate a carrier by a change in amplitude, a change in
frequency, or a change in the carrier phase. The last technique is similar to the modulation used in transmitting
color by television. In color TV the brightness signal is transmitted as amplitude modulation (AM), the sound
as frequency modulation (FM), and the color as phase modulation (PM), or pulse coding. Pulse coding is used
to modulate the radio frequency carrier in either AM, FM, or PM.
A common and extremely useful technique for increasing the information-carrying capability of a single
transmitting telemetry line is called multiplexing. When it is desirable to monitor different physical parameters,
such as temperature and pressure, it may be wasteful to have duplicating telemetry transmission lines. Multi-
plexing techniques can usually be considered to be of two types: frequency division multiplexing and time
division multiplexing. In the frequency division multiplexing system, different subcarrier frequencies are mod-
ulated by their respective changing physical parameter; these subcarrier frequencies are then used to modulate
the carrier frequency, enabling the transmission of all desired channels of information, simultaneously by one
carrier. At the receiver, these subcarrier frequencies must be individually removed. This is accomplished by
filters that allow any one of the respective subcarrier frequencies to pass. Each subcarrier frequency is then
converted back to a voltage by the discriminator. The discriminator voltages can be used to actuate recorders
and/or similar devices. Time division telemetry systems may use pulse modulation or pulse code modulation.
In these systems the information signal is applied, in time sequence, to modulate the radio carrier. The
characteristics of a pulse signal can be affected by modulating its amplitude, frequency, or phase.
Telemetry began as a wire communication technique between two remotely located stations. As science
extends its domains into the realm of space, telemetry will be the essential communicating link among satellites,
spaceships, robots, and other scientific devices yet to be designed.
The range of a radio link is limited by the strength of the signal radiated by the transmitter toward the
receiver and by the sensitivity of that receiver. A 10-microwatt (mW) output will transmit data easily 100 feet
with a bandwidth of 100 kHz. The wider the bandwidth, the more the effect from noise, and therefore the more
transmitting power required for an acceptable signal.
At the receiving station, there are usually no space restrictions in accommodating large antennas, sensitive
radio tuners and recorders, and an ample power supply, but the transmitting station often must be small,
possibly doughnut-size, but sometimes no bigger than a pea, and must be self-sufficient, carrying its own power
or perhaps receiving it by radio.
On the surface, industrial radio telemetry seems to be simply a matter of hardware. It almost is, except that
the functional requirements are a lot different from those in missile and space telemetry. Distances are much
shorter, a matter of a few feet to a few hundred yards; signal power can be radiated directly from the transmitter
circuitry or from an antenna as simple as an inch or two of wire. Most tests are repeatable—no missile blowing
? 2000 by CRC Press LLC
up on the pad here, taking with it valuable instruments and invaluable records of the events leading up to that
failure.
Quantities can be measured one or two at a time, rather than requiring an enormous amount of information
to be transmitted at once. This results in relatively inefficient use of the radio link but enables simpler circuitry
at both the transmitting and receiving ends.
Surprisingly, environment plays the most critical role in industrial telemetry. It makes by far the largest
difference between telemetry operations from missiles and spacecraft and those used in industrial remote
measurement. While missile telemetry equipment is expected to withstand accelerations of 10 to 20 g, the
rotating applications of telemetry in industry, such as the embedding of a transducer in a spinning shaft, require
immunity to 10,000 or 20,000 g centrifugal accelerations.
The environmental extremes under which industrial telemeters must work are considered normal operating
conditions by their users. Unlike missile telemetry equipment, which is shielded and insulated against extremes
of temperature, shock, and vibrations and which is carefully calibrated for weeks before it is used only once in
an actual shot, industrial telemeters must operate repeatedly without adjustment and calibration. Used out-
doors, they are often subjected to a temperature range of –40 to +140°F. They must operate when immersed
in hot or cold fluids, and thus it is almost mandatory that they be completely encapsulated to be impervious
not only to humidity and water but to many other chemical fluids and fumes. Many lubricating oils operate
at temperatures of 300 or 350°F.
We know that missile telemetry components must be small and light, yet an order of magnitude reduction
in size and weight has been necessary to make telemetry suitable for high-speed rotating shafts or for biological
implants. They must be so reliable that no maintenance is required, for there are no service centers set up to
handle this kind of equipment, and it must work without failure to continue to gain industrial acceptance.
Information theory has been used extensively to develop space telemetry for the most efficient data trans-
mission over a maximum distance with a minimum of transmitted power. Inefficiencies, being of no real
consequence in industrial telemetry, make for less elaborate, less costly equipment. Radio channels are used in
a relatively inefficient manner, and the distances between transmitter and receiver are usually so short that there
are few problems of weak signals. In many cases, measurement and testing via telemetry links takes place in
completely shielded buildings or in metal housings.
Although telemetry is usually defined as measurement at a distance, it has also gradually begun to embody
the concept of control from a distance. In telemetry—the transmission of the value of a quantity from a remote
point—it may serve merely to communicate the reading on an instrument at a distance. The output of the
instrument can also be fed into a control mechanism, however, such as a relay or an alarm, so that the telemetered
signal can activate, stop, or otherwise regulate a process. Measurement may be taken at one location, indication
provided at a second location, and the remote control function initiated at one of those two locations or at a
third point.
For example, a motor might be pumping oil from one location while oil pressure is being measured at
another. When the pressure reading is telemetered to a control station, a decision can be made there to reduce
pump motor speed when the pressure is too high, or a valve can be opened at still another location to direct
the oil to flow in another path. The decision-making controller may be an experienced pipeline dispatcher or
an automatic device.
77.2 Measuring and Transmitting
Telemetry, then, really begins with measurement. A physical quantity is converted to a signal for transmission
to another point. The transducer that converts this physical quantity into an electrical signal may be a piezo-
electric crystal, a variable resistance, or perhaps an accelerometer. Telemetered information need be no less
accurate than that obtained directly under laboratory conditions. For instance, in telemetering strain
measurements, it is possible to achieve accuracies of a few microinches per inch or greater. The only limitation
is usually the degree of stability in the bond of the strain gauge to the specimen, and not the strain gauge itself.
If great accuracy in temperature measurement is desired, it can be attained by choosing a transducer that
provides a large variation of output signal over a small range of process property variation. The resolution
which this provides may be translated into true accuracy by careful transducer calibration. Accuracy is reduced,
? 2000 by CRC Press LLC
of course, if a wider range of temperature needs to be detected. Typical single-channel analog telemetry links
maintain a measurement accuracy of 1–5%. This is not a limitation of the total system, however, since 1% of
a 100° temperature change would only be 1°, so several telemetry channels can easily share the total temperature
range to be measured, say a 100°F range divided into four 25°F ranges, to produce an accuracy of one-fourth
of a degree.
Special temperature probes have been produced for the range of 70 to 400°F and higher to maximize the
stability and accuracy of temperature telemetry. These probes, when used with the proper choice of transmitters
and receivers, can provide temperature measurements to closer than 0.05°F.
One of the limitations to accuracy and to repeatability in telemetry is the output level of the transducer. The
low electrical levels produced by thermocouples and strain gauges (millivolts) are much more difficult to
telemeter than a higher-voltage level of, say, 5 V. At low signal levels, extraneous electrical noises produce great
degradation. This noise may be thermally generated, caused by atmospheric effects, or generated by nearby
electrical equipment. When low-level transducers are used, stable amplifiers are required to raise the signal
voltage to useful modulation levels.
There may be great variations in the strength of the radio signal received because of variations in distance
between transmitter and receiver or because of the interposition of metallic objects. In industrial radio telemetry
transmission, these effects can be prevented from disturbing the data by resorting to FM of both the subcarrier
and the carrier so that the telemetered signal is unchanged by undesirable amplitude variations. This method
is called FM/FM telemetry.
If FM is used in the subcarrier of the transmitter, the transducer signal modulates the frequency of the
subcarrier oscillator. This can be done by a simple resonant circuit that produces a given frequency in the audio
range, say 1,000 Hz, which is varied above or below by the signal from the transducer as it responds to the
variable it is measuring. If the signal were fed to a loudspeaker, a rising or falling tone would be heard. The
subcarrier oscillator then modulates a radio frequency carrier, varying its frequency (FM) or its amplitude
(AM) in accordance with the subcarrier signal. The radio frequency in FM industrial radio telemetry links is
usually in the 88- to 108-MHz band. At the receiving end of the link, the radio receiver demodulates the signal,
removing the carrier and feeding the subcarrier to a special discriminator circuit that removes the modulation
and precisely reproduces the original measurement signal for calibrated indication or recording.
Multiple measurements can also be transmitted over the carrier by sampling the output of several transducers
in rapid sequence, a technique called time-division multiplexing. This technique has been employed to handle
as many as a million samples per second. It provides for simple data displays and easier separation of channels
for recording or analysis, and it is free of cross talk. If possible, it is advantageous to use no multiplexing at all
for concurrent data taking, but to use separate radio carriers for each measurement being transmitted.
Many and varied kinds of modulation have been used in telemetry systems. All have general usefulness, with
cost and application being the drivers. Synchronized modulation is generally used with other systems, being
synchronized to them to give additional information such as range. Typically, a command control uplink to
an aircraft is used to synchronize a telemetry downlink with the delay being proportional to distance or the
length of the link. Signal-to-noise advantages are also achieved. Another example is one in which telemetry is
tacked on to a radar transponder to add additional pulses to indicate altitude, heading identification, or other
conditions of the vehicle carrying the transponder. This is usually accomplished with pulse position modulation.
In many instances, a reconnaissance vehicle will carry a television camera. Its signals may be recorded on
board but are often telemetered for real-time observations. Data may be placed on the same carrier using a few
lines of the TV picture or an additional subcarrier. The much greater bandwidth of the TV signal seriously
compromises the combination of range, transmitter power, and antenna directivity, and typically signal-to-
noise ratio is reduced as much as 30 dB.
77.3 Applications of Telemetry
High-voltage transmission lines are an excellent example of how inaccessible an object of measurement may
be. These lines vibrate in the wind, and the stresses and strains require measurement under the dynamic
conditions that contribute to fatigue failure. Strain tests to determine fatigue will show quickly whether the
endurance limit of the line has been exceeded, and only if it is exceeded need we be concerned about fatigue
? 2000 by CRC Press LLC
failure. Therefore, it is necessary to measure the number and magnitude of the strain reversals in order to
predict the time of failure. Telemetry techniques permit dynamic testing under actual service conditions rather
than by simulated laboratory conditions or static tests.
While the transducer that produces an electrical signal proportional to strain may have an output of 0.01 V,
the live transmission line to which it is attached may be at a potential of several hundred thousand volts. The
problem is to detect this hundredth of a volt in the presence of a very large signal. In the language of the
telemetry engineer, this is rejection of a common mode voltage on the order of 10
8
to 1. Then why not deenergize
the line? It’s a simple matter of economics—an idle line transmits no power, and the wind forces that cause
the line to vibrate are neither predictable nor constant. So, weeks or months may be spent in gathering
measurements for a particular set of spans. However, a radio telemetry link makes it possible to transmit the
strain signal even while power is being carried.
A self-contained FM radio transmitter is attached to the transmission line at a point adjacent to a strain gauge.
All remain at the same electrical potential as the line, much like a bird sitting safely on the wire, transmitting
the strain gauge output to a radio receiver and recorder located at some convenient point on the ground, where
vibration analysis can be made. As a result, armor rods may be placed around the line at the vulnerable points,
or vibration absorbers of the correct resonant frequency can be installed at the proper points on the line.
More down to earth, but equally inaccessible to measurement, is strain on the chain belt of an earth mover.
Too light a chain will quickly fail from fatigue caused by the alternating stresses imposed by the full and empty
buckets it transports. Measurements made under actual operating conditions of the earth hoist mean attaching
strain gauges to a chain traveling at 500 ft per minute, subjecting them to violent shock and vibration. On this
kind of moving equipment, slip rings and wire-link remote measurements will not work. Here again, radio
telemetry is now providing the dynamic measurements needed to test the earth-moving equipment at work.
A transducer and a small, rugged transmitter are attached to points along the chain—strain varies from link
to link, depending on the proximity to the bucket—until the most vulnerable part of the chain is found. It is
preferable to use several transducers and multiple-channel telemetry equipment for such measurements to
simplify correlation between load and the resulting strain at various links.
Telemetry can also determine water levels and flow rates of rivers to provide vital data for flood control or
for efficient hydroelectric power generation. Data on the potential watershed into rivers can be obtained by
analyzing the water content of snow that would eventually melt and feed them. One requirement is to measure
the depth and water content of snow in the mountains, then transmit these data from remote points to a central
receiving station. The snow-measuring transducer may consist of a radioactive source atop a tall pole and a
radiation intensity meter on the ground beneath the snow. The gamma-ray intensity reaching the meter is a
function of the height and water content of the intervening snow. Both the meter and the transmitting
equipment can be powered by a storage battery and controlled by a clock timer that sets the time of transmission
to a few seconds per day.
In the design of machinery, one of the most difficult factors to cope with is alternating fatigue-producing
stresses that occur at some parts of the machine. It has long been the custom to measure stress in equipment
with bonded strain gauges to predict the failure limits before actual failure occurs. This had only been possible
on those portions of the equipment that could be connected by wires. With radio telemetry, it is not possible
on all members. Costly fatigue failures are now avoidable through installation of miniature telemetry compo-
nents that are reliable, rugged, and accurate in heretofore inaccessible locations and environments. Industrial
uses are virtually limitless; systems can be built to specifications and encapsulated to withstand the most adverse
conditions. Low-cost measurement and telemetry systems have been applied to read internal vibrations and
strain in rotating equipment, chains, vehicles, and projectiles—eliminating slip rings and wires. Measurement
can be made under operating conditions of vibration, acceleration, strain, temperature, pressure, magnetic
fields, electrical current, and voltage, under such adverse conditions as in a field of high electrical potential, in
fluids, in steam, or in high-velocity gases.
Power Sources
Power sources for the transmitter in industrial telemetry applications are seldom a problem. Batteries can be
used for temporary applications and at temperatures below 200°F. Small and light, rechargeable and expendable
? 2000 by CRC Press LLC
batteries are available solidly encapsulated in epoxy resin to withstand almost as rugged environments as the
telemeter itself.
In a moving or rotating application, stationary magnets can be placed so that they generate electricity in a
moving coil and are used to provide automatic power generation. If this method is not feasible, a stationary
coil can be placed in the vicinity of the transmitter and fed electrical energy at a high frequency, so that its
field can easily couple into a moving coil in almost any environment. The stationary coil ring may be large,
even encompassing a whole room; usually only one turn of wire is necessary. The stationary coil may also be
made extremely small, 1?4 to 1?2 in. in diameter, and coupled to the end of a rotating shaft. These power supplies
and coil configurations are standard available units.
Power Plants
In power plants, coal is fed in turn to a number of hoppers by conveyor belt. A tripper on the conveyor belt diverts
the coal into a particular hopper until it is full. Either an operator or a mechanical sensing device determines
when the hopper is full, and a signal is transmitted to the conveyor to move onto the next hopper. Before
telemetering equipment was in use, costly accidents could occur if the operator should be away momentarily or
if the sensor failed to function. As much as six tons of coal a minute could overflow onto the power station floor.
To prevent this, pressure switches are installed in the tripper chute to activate a radio transmitter if coal
backs up into the tripper. The transmitter sends its signal to a receiver located at the conveyor belt and sounds
an alarm. This type of control is difficult if not impossible to achieve by wired power connections because the
tripper is moving and because the corrosive coal dust atmosphere attacks the wires. For this reason, a radio
transmitter equipped with long-life batteries is mounted on the tripper. The receiver at the control end is
powered by ac. Subcarrier tone (frequency) coding is used to eliminate the effects of interference and noise,
giving positive protection at all times.
77.4 Limitations of Telemetry
The preceding paragraphs describe a number of the requirements placed upon telemetry systems by the
transducers and quantities being measured. Unfortunately, the development of telemetry has not been such as
to satisfy all requirements, and in many cases the telemetry system seriously limits the measurement. A
compromise is therefore required between telemetry capabilities and the requirements of measurement. The
shortcomings and limitations of the telemetry system place restrictions upon measurements above and beyond
those encountered in the laboratory when the telemeter is not used. In the first place, an electrical output from
the measuring device is required in order that the measurement may be placed on a radio link. Consequently,
transducers that produce an electrical output on one form or another are necessary. Also, the telemetry system
may not be perfectly stable down to zero frequency (dc), and transducers and methods of measurement must
be chosen to minimize the effects of drift. Overmodulating the subcarrier, or the time-division multiplexer,
may also affect adjacent channels, as well as produce erroneous data in its own channel. If various measuring
devices are switched, the switching transients must be minimized, or the accuracy of the telemetry system may
be impaired. When mechanical commutators or time multiplexers are used, the measurement of the time
occurrence of the event, such as the impact of cosmic particles or the receipt of a guidance pulse, is made more
difficult and the time ambiguity of the multiplexed system is a serious limitation.
The measurement of a large number of parameters requires extensive and bulky equipment, unless the
parameters can be combined in groups of similar inputs to minimize the signal conditioning required. This
fact generally dictates a relatively standard transducer rather than an optimum one for each particular mea-
surement.
The bandwidth of the measurement, or the frequency with which the measured quantity changes, is also
seriously limited by the telemeter. In the FM/FM telemeter, the permissible bandwidth varies from a relatively
low value on the lower-frequency subcarriers to a reasonably high value on the high-frequency subcarriers. The
bandwidth of the measurement must not exceed the subcarrier bandwidth limitations, or sidebands will be
generated in adjacent channels, thereby reducing the accuracy of other measurements (if multiplexed), or
interference with adjacent RF signals will be caused.
? 2000 by CRC Press LLC
In a time-multiplexed system, the problem of “folded data” is present whenever the rate of data change is
faster than one-half the sampling rate. When this occurs, it is not known whether the measured quantity has
reversed itself several times between samples or if there has been no reversal at all. It is considered desirable to
limit the bandwidth of the data so that this ambiguity is not present; however, with refined techniques of
analysis, this is not a rigid requirement. The form in which the data is displayed or recorded is also a limitation
on measurement. In general, time-history plots of the measured quantity are desired. In this case, the speed at
which the recording medium moves is often a severe limitation. If sampling is not regular, demultiplexing
difficulties are magnified.
77.5 Transmitters and Batteries
The transmitter is made up of two components: the subcarrier oscillator and the radio frequency oscillator.
The subcarrier can be bridge controlled (BCO) or voltage controlled (VCO). The subcarrier center frequency
is 4,000 Hz, which can be modulated ±400 Hz by the strain or voltage being measured. Using BCOs, a strain
as large as 2,500 microinches per inch (min./in.) and as small as 2 min./in. can be measured and transmitted.
The temperature measurement range of the VCO is from –200 to 4,000°F. With a copper-constantan thermo-
couple, a temperature change as small as 2°F can be sensed and transmitted.
The single-resistance strain gauge transmitter does not have a subcarrier oscillator and can be used from
–40 to 212°F. It has only a radio frequency oscillator, which is modulated by the sensor signal. For this reason,
it is not suitable for static strain measurements and must be used for dynamic strain measurements only. It
has a frequency response to 25,000 Hz or greater. A static strain signal transmitted by this device will drift. It
is provided with self-contained rechargeable nickel-cadmium batteries. Pins protruding through the epoxy case
are used for all electrical connections. Only one screw adjustment is provided, and this is used to set the radio
frequency.
Rechargeable nickel-cadmium batteries are used with the BCO and the VCO. The BCO batteries have useful
lives of 4 and 9 hr. A VCO battery has 40 hr useful life. The single-resistance strain gauge transmitter has a
built-in nickel-cadmium battery with a life of 4 hr.
77.6 Receivers and Discriminators
A typical industrial receiver has a tuning range of 88 to 108 MHz. When the transmitter is used in its greatest
sensitivity mode, the output of the discriminator is approximately 1 V for a 25-min./in. strain with a single
active gauge in the bridge. At the most insensitive mode 1 V is obtained for an approximately 500-min./in.
strain. The discriminator can withstand a 500% overload, which means that a 5-V signal will be obtained from
a strain of 125-min./in. at the maximum sensitivity and from 2,500-min./in. at the minimum sensitivity.
77.7 Antennas and Total System Operation
A nickel-cadmium battery supplies the power to the transmitter. For the BCO, the resistance change of the
strain gauge changes the frequency of the subcarrier. In the case of the VCO, the millivolt output of the
thermocouple changes the frequency of the subcarrier. This change modulates the radio frequency transmitted
by the antenna. The receiving antenna picks up the signal and conducts it by wire link to the radio receiver,
which is tuned to the transmitting frequency. The radio receiver demodulates the FM carrier to reproduce the
subcarrier signal. The subcarrier signal is then fed to the discriminator, which demodulates this signal to obtain
a dc voltage, which is then amplified by the dc amplifier and recorded on the oscillograph. The oscillograph
record, properly calibrated, is then a display of the strain in microinches per inch for the BCOs, or the
temperature in degrees for the VCO. At the same time the dc signal can be read on a VTVM and can be used
as a check on the oscillograph.
The transmitter subcarrier oscillators are factory set to operate at a center frequency of about 4,000 Hz. They
have a frequency range of ±400 Hz about the 4,000-Hz center frequency. The center frequency is set with a
? 2000 by CRC Press LLC
counter at the time of testing. The change of ±400 Hz is the information frequency change brought about by
the change in strain or temperature measured by the sensor. It is this information frequency change that the
discriminators isolate as a dc voltage change, which is proportional to the measured strain and is recorded on
the oscillograph.
77.8 Calibration
Batteries are calibrated under simulated service conditions for voltage drop versus time. Bridge-controlled
transmitters are calibrated for strain subcarrier frequency change using a cantilever beam instrumented with
resistance strain gauges. The beam is calibrated for load versus strain using a strain indicator. It is then used
to calibrate the bridge-controlled transmitters statically, by measuring the subcarrier frequency change as a
function of strain. A dynamic calibration can also be made by using a second cantilever beam driven by a
vibration generator. Two resistance strain gauges are mounted back-to-back on the second beam and calibrated.
One of the gauges is monitored through the telemetry system and the other by wire link to the oscillograph,
and the two signals are then compared. The single-resistance strain gauge transmitter is similarly calibrated,
but in this case, the beam is fixed in a fatigue machine operating at 30 Hz. Calibrations are performed at various
strain levels. Again, two calibrated gauges are monitored and compared, one using the telemetry systems and
the other using wire link.
The effect of temperature on a transmitter and battery is measured at temperatures from 65 to 135°F by
placing both in an air-circulating oven, with the receiving equipment and the calibration beams to room
temperature outside the oven.
The voltage-controlled transmitter is calibrated for temperature subcarrier frequency change from 78 to
640°F. Two calibrated thermocouples, welded next to one another on a piece of stainless steel, are heated
simultaneously. After determining by wire link instrumentation that both thermocouples are indicating the
same temperature, the millivolt output of one is fed into the transmitter, and the output of the other is fed by
wire into a precision potentiometer. The subcarrier frequency change is determined as a function of temperature,
and the radio signal is recorded on the calibrated oscillograph, with a galvanometer determining its deflection
as a function of temperature. The data obtained by wire link and radio are then compared to establish the
calibration. The effect of thermocouple lengths can also be investigated in the same test setup. The receivers
and discriminators are calibrated before these tests.
Cold junction compensation may be investigated from –40 to 258°F by cooling the transmitter and battery,
with leads shorted and with a 20-mV input, in a cold chamber below room temperature and by heating in an
air-circulating oven to above room temperature. The 20-mV input is imposed with a dc power supply kept
outside the temperature chamber.
The discriminators are calibrated with the transmitters. The subcarrier frequency, which is the input to the
discriminator, is monitored with a digital counter as the calibration beam is loaded. The voltage output
corresponding to the frequency change can be monitored with a vacuum tube voltmeter. The strain, subcarrier
frequency change, and the voltage output of the discriminator are then correlated. A digital frequency counter
is used to set the transmitter center frequency.
77.9 Telemetry Frequency Allocations
Frequency bands for telemetry have been allocated as follows:
88–108 MHz Low power, noninterference
216–260 MHz General telemetry
400–475 MHz Command destruct
1435–1540 MHz General telemetry
1710–1850 MHz Video telemetry
2.2–2.3 GHz General telemetry
? 2000 by CRC Press LLC
The low-power, 88 to 108 MHz, band is shared with FM broadcast stations. Telemetry is allowed to be used,
but it must not interfere with broadcast reception. Transmitter power and antennas are limited to provide a
signal strength no greater than 50 mV/m at 50 ft from the antenna and/or transmitter. In use, the telemetry
transmitters are generally tuned to operate at frequencies between local FM broadcast stations.
The remaining frequency bands are used mainly with aircraft, unmanned vehicles, space vehicles, and for
military applications. Equipment for these applications is rigidly constrained for stability, low spurious emis-
sions, low cross talk, good linearity, etc.
77.10 Telemetry Antennas
When transmitter and receiver are stationery, antenna considerations for telemetry are no different than for
communications. The usual case, however, is that the transmitter is moving, both rotating and translating and
often obscuring the transmission with reflective material. This poses problems in both receiving the signal and
tracking the moving transmitter with a directive receiving antenna. In many cases it is necessary to have two
or more receiving antennas to receive from a single transmitter.
Transmitting antennas may be either conformal or protruding. The protruding antennas are usually cheaper
and simpler. The radiation pattern must include downward directivity if an aircraft or space vehicle will fly
directly overhead. A vertical whip antenna does not provide this coverage. A simple choice to provide smooth
coverage from the nadir to the horizon is circular polarization at the nadir and elliptic polarization in between.
A circular polarized receiving antenna is used to receive the signals over the complete pattern. It must be
polarized in the same circular direction as the transmitting antenna. A spiral or helix antenna is usually used
on the ground. Two complete radio frequency systems are generally used to receive signals of any polarization.
One system uses a circular polarized right-hand antenna and the other uses a circular polarized left-hand
antenna. In this manner, as the vehicle tilts or spins, signals are received continuously unless the transmitting
antenna is occluded. To receive occluded signals, diversity reception is required. Each receiving station is located
such that one fills in the occluded pattern of the other.
In short distance telemetry the same problems are encountered but from a different cause. If the transmitter
is occluded at one position by the shaft of a rotating machine, it would be expected that reflections from nearby
objects or walls would fill the gap. In practice it is the usual occurrence that a gap is generated once per
revolution. This is caused by multiple signal cancellation. There are two methods of overcoming this signal
drop-out: (1) with diverse polarization and (2) by locating the receiving antenna close to the transmitting
antenna and effectively surrounding the shaft with it.
While information theory has been used extensively to develop space telemetry for the most efficient data
transmission over a maximum distance with a minimum of transmitted power, the very inefficiencies permitted
in industrial telemetry make for less elaborate, less costly equipment.
Radio channels are used in a relatively inefficient manner, and the distances between transmitter and receiver
are usually so short that there are few problems of weak signals. In many cases, measurement and testing via
telemetry links take place in completely shielded buildings or in metal housings.
Although telemetry is usually defined as measurement at a distance, it has also gradually begun to embody
the concept of control from a distance. In a telemeter—the transmission of the value of a quantity from a
remote point—it may only be necessary to observe the reading of an instrument to determine the temperature,
pressure, or vibration of a distant or inaccessible object. One can also feed the output of the instrument into
a control mechanism, however, such as a relay or an alarm device, so that the telemetered signal may activate
or stop a controllable process. Measurement may be performed at one location, indication provided at a second
location, and the remote control function initiated at one of the first two locations or even at a third point.
Take, for example, an oil pipeline in which a motor is pumping oil from one location and oil pressure is
being measured at a second location. The pressure reading is telemetered to a station where a decision can be
made to reduce the speed of the pump motor when the pressure is too high, or a valve may be opened at still
another location to cause the oil flow in another path. The decision-making element may be human, an
experienced pipeline dispatcher, or an automatic controller. Human or automatic device—either one telemeters
a command to the control points.
? 2000 by CRC Press LLC
77.11 Measuring and Transmitting
Telemetry, then, really begins with measurement. A physical quantity is converted to a signal for transmission
to another point. The transducer that converts the physical quantity into an electric signal typically may be a
piezoelectric crystal, a variable resistance, or perhaps an accelerometer.
Telemetering the measurement signal of the best transducers in no way degrades the measurement below
accuracies attainable under laboratory conditions. For instance, in strain measurement it is possible to achieve
accuracies of a few microinches per inch or greater, but the limitation is usually the degree of stability in the
bond of the strain gauge to the specimen.
If one wants accuracy in temperature measurement, it can be attained by choosing a transducer that provides
a large variation in output signal over a small range of temperature. The resolution that this provides may be
translated to true accuracy by careful transducer calibration. Typical analog telemetry links maintain a measure-
ment accuracy on a single channel to 1%. This is not a limitation of the total system, since 1% of a 100-degree
temperature change would only be 1 degree, so several telemetry channels can easily share the total temperature
range to be measured, say, a 100 range divided into four 25 ranges to produce an accuracy of 1?4 degree.
One of the limitations to accuracy and repeatability in telemetry is the output level of the transducer. The
low electrical levels produced by thermocouples and strain gauges (0.010 V) are more difficult to telemeter
than high-voltage levels of 5 V. At low signal levels, extraneous electrical noises produce greater degradation.
These may be thermally generated or caused by atmospheric effects or generated by nearby electrical equipment.
When low-level transducers are used, stable amplifiers are required to raise their signal voltage to useful
modulation levels.
77.12 Modulating and Multiplexing
The transducer signal modulates the frequency of the subcarrier oscillator. This is simply a resonant circuit
that produces a given frequency in the audio range, say 100 Hz, and is varied plus or minus this center frequency
by the signal from the transducer as it responds to the variable that it is measuring. When the signal is fed to
a loudspeaker, one actually hears a rising or falling tone. The subcarrier oscillator modulates a radio frequency
carrier, varying its frequency in accordance with the subcarrier voltage signal. The radio frequency in FM
industrial radio telemetry links is usually in the 88- to 108-MHz band, permitting the use of high-grade radio
tuners already mass produced for the high-fidelity market. The radio receiver demodulates the signal, removing
the carrier and feeding it to a special discriminator circuit that removes the double modulation and reproduces
an analog of the original measurement signal for calibrated indication or recording.
There can be great variations in the strength of the radio signal received because of variations in distance
between transmitter and receiver or because of the interposition of metallic objects. In industrial radio telemetry
transmission, these effects are prevented from disturbing the data by resorting to FM of both the subcarrier
and the carrier so that the telemetered signal is unchanged by undesirable amplitude variations. This method
is called FM/FM telemetry. There are other methods of carrier modulation, such as pulse amplitude modulation,
phase modulation, and pulse duration modulation. Each has its proper place in missile and space telemetry,
where great distances must be spanned with a maximum of data over crowded and often noisy communication
channels. Pulse code modulation of an FM link, however, may be expected to become more widespread in
industrial telemetry.
Particularly in missile telemetry, it is important that multiple measurements be transmitted over a single
carrier to save power and minimize electronic equipment and antennas. Such simultaneous transmission of
signals over a common path, called multiplexing, is sometimes used in industrial telemetry. When concurrent
data about several simultaneous events are transmitted by several subcarriers, each subcarrier oscillator has a
distinctive reference frequency and swings from this center frequency toward arbitrary maximum and minimum
frequencies in response to signals from a corresponding transducer. Thus a number of separate audio frequency
bands are sent over the radio frequency carrier. This is called frequency division multiplexing. The frequency
division multiplex requires careful adjustment of subcarrier frequencies and the corresponding filters at the
receiver and strong suppression of harmonics to avoid cross talk or interaction between channels.
? 2000 by CRC Press LLC
Multiple measurements may also be transmitted over the carrier by sampling the output of each transducer
in rapid sequence, a technique called time-division multiplexing. The technique has been used to handle as
many as a million samples per second. It provides for very simple data displays, easier separation of channels
for recording or analysis, and is free of cross talk. If possible, though, it is advantageous to use no multiplexing
at all for concurrent data talking but to use separate radio carriers for each measurement being transmitted.
The multiplex telemeter requires careful adjustment of subcarrier frequencies and precisely tuned filters to
separate them at the receiver. This adds to the cost of the equipment and requires considerable experience of
an operator.
The telemetry data received may be recorded in a number of ways, but such records must preserve the
accuracy of the entire system. For example, if one is monitoring a 1% system and can distinguish 1/64th in.
on a paper graph, the minimum graph size for full scale should be approximately 2 in. Similarly, numeric data
should be printed to enough decimal places to preserve the accuracy of the system.
A single channel of industrial FM/FM telemetry equipment may cost between $1000 and $2000, depending
on the flexibility required and the measurements being made. It buys everything needed for a given remote
measurement—transducer, radio link, power supply, and simple indicator.
77.13 Passive Telemeters
Passively powered telemeters offer some interesting advantages. When a number of telemeters are used, they
can be powered in sequence or only when measurements are required, thus preventing radio frequency con-
gestion. In medical applications of telemetry, passive devices eliminate the danger involved in swallowing or
implanting batteries.
Most passive telemeters are essentially an inductance and a capacitance coupled as a resonant circuit. Either
of these components may be pressure sensitive or temperature sensitive. A nearby magnetic coil coupled to this
circuit can, by means of a varying frequency, determine the resonant point of the telemeter, which can then be
a function of the temperature or pressure being measured.
77.14 The Receiving Station
The industrial telemetry receiving station differs vastly in purpose and principle from the transmitting station.
Its usual environment is no more difficult to cope with, in terms of ambient temperature, shock, and vibrations,
than an automobile radio. It receives signals over relatively short distances in which the subcarrier frequencies
are so widely spaced that harmonics and drift are no problem.
In the FM broadcast band, available professional-grade high-fidelity tuners have 1 or 2 mV sensitivity for 30
dB of quieting of extraneous noises and automatic frequency control circuits, which compensate for both
transmitter and receiver drift. They feed telemetry phase-lock discriminators, which lock the receiver into the
frequency and phase of the incoming signal.
The transmitters usually radiate from the resonant elements themselves, avoiding elaborate antennas that
might be required for longer distance transmission; the receivers use simple dipole or commercial TV antennas.
Thus, industrial radio telemetry has become a carefully engineered blend of the borrowed and the new.
Related Topics
38.1 Wire ? 69.1 Modulation and Demodulation ? 69.2 Radio
? 2000 by CRC Press LLC
