Telemetry amplitude and wave. Multiplexing system; frequency
division multiplexing and time division multiplexing.
TELEMETRY AND DATA ACQUISITION
General objective : To understand the concept of telemetry and
data acquisition.
Specific objectives : At
the end of the unit you should be able to:
- Identify the main concept of telemetry system and data acquisition.
- Describe the structure of data collection system.
- Define the specification of data acquisition system.
- Identify the types of telemetry system.
- Explain the function of multiplexing system.
10.1 INTRODUCTION OF TELEMETRY.
For longer distances, we convert DC voltage or current to
audio tones and send them over wire. This is called modulation, and the reverse
(i.e. converting the varying signal to data) is called demodulation. A device
to perform it is called a modem.
An analog signal is a continuously varying wave. If we
measure its height at specific points in time, we obtain a series of voltages
with numeric values. These values can be represented in binary form and
transmitted as a series of bits. A bit is a binary digit, either 0 or 1, whose
combination in form of a code represents information in digital communication.
Other sensors have different kinds of outputs. Many simply have varying DC outputs, while others are AC in nature. Each of these signals is typically amplified, filtered, and otherwise conditioned before being used to modulate a carrier. All of the carriers are then added together to form a single multiplexed channel.
10.2
STRUCTURE OF
DATA ACQUISITION SYSTEMS.
a.
signals originating from direct measurement of electrical quantities, these may include dc
and
ac voltages, frequency or resistance and are typical found in such areas as electronic component
testing, environmental studies and quality analysis work.
ac voltages, frequency or resistance and are typical found in such areas as electronic component
testing, environmental studies and quality analysis work.
b.
Signals originating from transducers such as strain
gage and thermocouple.
.
Data acquisition systems are used in a large and
ever-increasing number of applications in a variety of industrial and
scientific areas, such as the biomedical, aerospace and telemetry industries.
The type of data acquisition system whether analog or digital, depends largely
on the intended use of the recorded input data. In general, analog data systems
are used when wide bandwidth is required or when lower accuracy can be
tolerated. Digital systems are used when the physical process being monitored
is slowly varying (narrow bandwidth) and when high accuracy and low per-channel
cost is required. Digital systems range in complexity from single-channel dc
voltage measuring and recording systems to sophisticated automatic multi-channel
systems that measure a large number of input parameters, compare against preset
limits or conditions and perform computations and decisions on the input
signal. Digital data acquisition systems are general more complex than analog
systems, both in terms of the instrumentation involve and the volume and
complexity of input data they can handle.Data acquisition systems often use magnetic tape recorders. Digital system require converts to change analog voltages into discrete digital quantities or numbers. Conversely, digital information may have to be converted back into analog form such as a voltage or a current which can then be used as a feedback quantity controlling an industrial process.
Instrumentation systems can be categorized into two major classes, analog systems and digital system. Analog system deal with measurement information in analog form. An analog signal may be defined as a continuous function, such as a plot of voltage versus time, or displacement versus pressure. Digital systems handle information in digital form. A digital quantity may consist of a number of discrete and discontinuous pulse whose time relationship contains information about the magnitude or the nature of the quantity.
Data acquisition is divided by two types, analog data acquisition and digital data acquisition.
10.2.1 ANALOG DATA ACQUISITION.
a.
Transducers – translating physical parameters into
electrical signals.
b.
Signal conditioners – amplifying, modifying, or
selecting certain portions of these signals.
c.
Visual display devices – continuous monitoring of the
input signals. These devices may include single-channel or multi-channel
oscilloscope, storage oscilloscope, panel meters, numerical display and others.
d.
Graphic recording instruments – obtaining permanent
records of the input data. These instruments include stylus and ink recorders
to provide continuous records on paper chart, optical recording systems such as
mirror galvanometer recorders and ultraviolet recorders.
e.
Magnetic tape instrumentation – acquiring input data,
preserving their original electrical form, and reproducing them at a later date
for more detailed analysis.
10.2.2 DIGITAL
DATA ACQUISITION.
A digital data acquisition included some or all of the elements shown in figure 10.2.2. The essential function operations within a digital system include handling analog signals, making the measurement, converting and handling digital data and internal programming and control. The function of each of the system elements of figure 10.2.2 is listed below.
a.
Transducer – translate physical parameters to
electrical signals acceptable by the acquisition system. Some typical
parameters include temperature, pressure, acceleration, weight displacement,
and velocity frequency, also may be measured directly.
b.
Signal conditioner – generally includes the supporting
circuitry for the transducer. This circuitry may provide excitation power,
balancing circuits, and calibration elements. An example of signal conditioner
is a strain- gage bridge balance and power supply unit.
c.
Scanner or multiplexer – accept multiple analog inputs
and sequentially connects them to one measuring instrument.
d.
Signal converter – translates the analog signal to a
form acceptable by the analog-to-digital converter. An example of signal
converter is an amplifier for amplifying low-level voltages generated by
thermocouples or strain gages.
e.
Analog –to-digital (A/D) converter - Converts the analog voltage to its equivalent
digital form. The output of the A/D converter may be displayed visually and
also available as voltage outputs in discrete steps for further processing or
recording on a digital recorder.
f.
Auxiliary equipment – This section contains instruments
for system programming functions and digital data processing. Typical auxiliary
functions include linearizing and limit
operation. These functions may be performed by individual instruments or by a
digital computer.
g. Digital recorder – Records digital information
on punched cards, perforated paper tape, magnetic tape, typewritten pages, or a
combination of systems. The digital recorder may be preceded by a coupling unit
that translates the digital information to the proper form for entry into the
particular digital recorder selected.
10.2
FREQUENCY OF
TELEMETRY.
In the frequency
of telemetry process, the carrier frequency is varied above and below its
center value (modulated) in accordance with the amplitude of the data signal.
The rate at which the carrier frequency deviates from its center value is a function of the frequency signal.
The amplitude and frequency characteristics that define the data signal are
therefore contained in the frequency variations of the frequency telemetry
carrier around its center value. When this modulated frequency demodulator by
detecting the number and rate of zero crossings.
It
is clear that frequency telemetry recording is extremely sensitive to
variations in tape speed (flutter) because tape speed variations introduce
apparent modulation of the carrier and are interpreted by system as unwanted
signal (noise). Instability in tape speed therefore reduces the dynamic range
of the system.
Since
the data signal is contained entirely in the frequency characteristics of the
frequency carrier, the system is not sensitive to amplitude instability. Two
important factors in telemetry recording are deviation ratio and percentage
deviation. Deviation ratio is defined as the ratio of deviation of the carrier
from the center frequency to the signal frequency, or
10.2
MULTIPLEXING
SYSTEM.
Multiplexing is
the process of simultaneously transmitting two or more individual signals over
a single communications channel. Multiplexing has the effect of increasing the
number of communication channels so that more information can be transmitted.
The concept of a simple multiplexer is illustrated in figure 10.7(a). Multiple input signals are combined by the multiplexer into a single composite signal that is transmitted over the communications medium. Alternatively , the multiplexed signals may modulate a carrier before transmission. At the other end of the communications link, a demultiplexer is used to sort out the signal into their original form.
There are two basic types of multiplexing, frequency division multiplexing (FDM) and time division multiplexing (TDM). Generally speaking , FDM systems are used to deal with analog information and TDM systems are used for digital information.
10.2.1 FREQUENCY DIVISION MULTIPLEXING.
Frequency division
multiplexing is based on the idea that a number of signal can share the
bandwidth of a common communications channel. The multiple signal to be
transmitted over this channel are each used to modulate a separate carrier.
Each carrier is on a different frequency. The modulated carriers are then added
together to form a signal complex signal that is transmitted over the single
channel.
Figure 10.4.1(a)
shows a general block diagram of FDM system. Each signal to be transmitted
feeds a modulator circuit. The carriers for each modulation fc is on a different frequency. The
carrier frequencies are usually equally spaced from one another over a specific
frequency range. Each input signal is given a portion of bandwidth . the result
is illustrated in figure 10.4.1(b). As for the type of modulation any of the
standard kinds can be used including AM, SSB, FM or PM.
The modulator output containing the sideband
information are added together in a linear mixer. In a linear mixer, modulation
and the generation of sidebands do not take place. Instead , all the signals
are simply added together algebraically. The resulting output signal is a
composite of all carriers containing their modulation. This signal is then used
to modulate a radio transmitter. Alternatively, the composite signal itself may
be transmitted over the single communication channel. Another option is that
the composite signal may become one input to another multiplexer system.
10.2.1 TIME DIVISION MULTIPLEXING.
In FDM, multiple
signals are transmitted over a single channel by sharing the channel bandwidth.
This is done by allocating each signal a portion of the spectrum within that
bandwidth. In TDM , each signal can occupy the entire bandwidth of the channel.
However , each signal is transmitted for only a brief period of time. In other
words, the multiple signals take turns transmitting over the single channel.
This concept is illustrated graphically in figure 10.4.2(a).
Here, four signals
are transmitted over a single channel each signal is allowed to use the channel
for a fixed period of time, one after another. Once all the signals have been
transmitted, the cycle repeats again and again.
