Basics of Telemetry Study Notes

Data Transmission and Telemetry

Introduction

In modern measurement systems, the various components comprising the system are usually located at a distance from each other. It, therefore, becomes necessary to transmit data between them through some form of communication channels.

The terms data transmission and telemetry refer to the process by which the information regarding the quantity being measured is transmitted to a remote location for applications like data processing, recording or displaying. This may involve transduction of the quantity with the help of transducers and in addition, signal conditioning also.

Methods of Data Transmission

The transmission of a measured variable from the point of measurement to a remote point is an important function in instrumentation systems because of the size and complexity of modem industrial plants. The most common variable encountered in industrial plants are: temperature, pressure and flow. The measuring elements can be pressure springs, thermocouples, bellows, floats etc. Most measuring devices such as mercury thermometers, pressure gauges. or flow-rate meters would require fluid line connections of great length. This cannot be done because it would result in excessive measuring lags. Hence some form of transmission must be employed.

The method employed for data transmission depends upon the variable and also the distance involved. In case the data is to be transmitted over a short distance, the following methods may be used:

(i) Hydraulic transmission,

(ii) Pneumatic transmission, and

(iii) Electrical or Electronic transmission.

The hydraulic and pneumatic methods are useful for transmitting data over small distances and hence are limited in scope.

The electrical and electronic methods of data transmission are extensively used in industrial measurement and instrumentation systems. In this text data transmission by electrical and electronic means is only discussed.

Telemetry

Telemetering may be defined as measuring at a distance. In fact, according to ASA "telemetering is the indicating, recording or integrating of a quantity at a distance by electrical means".

Several methods of classifying telemetering systems are used. IEEE bases its classification on the characteristics of the electric signal.

This may be : (i) voltage, (ii) current, (iii) position, (iv) frequency, and (v) pulse

Telemetering Systems may be classified as: (i) analog, and (ii) digital.

In addition, telemetering systems have been classified as:

(i) short distance type, and (ii) long distance type.

Another classification may be based upon whether the user has control over transmission channel or not.

Table 2.1 shows a comparison between the above mentioned ways of classification. All of the IEEE's classification can be used for short distance telemetering, but only the frequency and the pulse types are suited to long distance telemetering. The voltage, current, position, frequency and pulse can be used for analog telemetry while only pulse type can be used for digital telemetering.

Voltage, current and position telemetering require a physical connection between the transmitter and the receiver. This physical connection is normally called a channel, which consists of one, two or more wires depending upon the system. In case of radio frequency (R. F.) telemetery, the channel is not a physical link.

The frequency and pulse systems can operate on a physical wire channel and also can utilize other channels such as telegraph, teletype, telephone, radio or microwave. Hence for long distance telemetering either frequency or pulse techniques are used.

General Telemetering System

A general telemetering system is shown in Fig. 2.1. The primary detector and end device of the telemetering system have the same functional positions as in any general measurement system.

However, there are three system elements in the intermediate state which are peculiar to a telemetering system. They are:

Fig. 2.1 Functional representation of a general telemetering system.

(i) telemeter transmitter, (ii) telemeter channel, and (iii) telemeter receiver.

The function of the telemeter transmitter is to convert the output of a primary detector into a related quantity (translating means) which can be transmitted over the telemeter channel. The function of the telemeter receiver at the remote location is to convert the transmitted signal (translating means) into a related suitable quantity.

Electrical Telemetering Systems

The electrical telemetering system consist of a transmitter which converts the measurand into an electrical signal that is transmitted through a telemetering channel and is received by a receiver located at a remote location. This signal is converted into usable form by the receiver and is indicated or recorded by an end device which is graduated in terins of the measurand.

The electrical telemetering systems may be broadly classified as:

(i) d.c. systems and (ii) a.c. systems.

D.C. Telemetry Systems The d.c. systems may be classified as:

(i) voltage telemetering systems,

(ii) current telemetering system, and

(iii) position telemetering systems.

In d.c telemetering systems, the signal is transmitted through a telemetering or communication channel which utilizes direct transmission via cables in order to convey the information. This is called Land Line Telemetry.

a. Voltage Telemetering Systems

A voltage telemetering system transmits the measured variable as a function of an a.c. or d.c. voltage. A simple voltage system is shown in Fig 2.2. A slide wire potentiometer is connected in series with a battery. The sliding contact is positioned by a pressure sensitive bourdon tube. The telemetering channel consists of a pair of wires connected to a voltage measuring device such as a null balance d.c. potentiometer indicator or recorder. As the measured pressure changes, the bourdon tube actuates the sliding contact thereby changing the voltage. The d.c. null balance potentiometer measures the voltage and positions pointer on a scale calibrates in terms of the pressure being measured.

The use of a null balance d.c. potentiometer reduces the current carried by the telemetering (or communication) channel to minimum where the resistance is negligible.

Most of the systems use primary elements which produce a voltage signal. These elements include thermocouples, tachometers and differential transformers. The application of voltage systems in industrial plants is limited to distances up to about 300 metres. Self balancing potentiometers are the usual receivers for such system. The deflection type of indicators may also be used if they are calibrated for the line resistance involved.

Voltage telemetering system requires high quality circuits than current systems. The signal to noise ratio (S/N) must be comparatively high. Since, the power level is small in voltage telemetering systems, the transmission channel must be protected from sources of interference which are of the same order as that of the signal.

A voltage telemetering system is suitable for adding several output voltages in series. This is subject to the condition that the measurement is linear. However, the voltage system requires a relatively more expensive receiving end equipment. The system is generally not adaptable to the use of the many receivers at the same time.

b. Current Telemetering Systems

The earlier version of a current telemetering system is shown in .Fig ..2.3. It has a slide wire potentiometer connected in series with a battery. The sliding contact of the potentiometer is positioned by a pressure sensitive bourdon tube. The telemetering channel consists of a pair of wires connected to a current measuring device.

As the pressure, which is the measurand, changes the bourdon tube moves and changes the position of the sliding contact on the slide wire, thereby changing the current in the circuit. This current is measured with the help of a milliammeter whose scale is graduated terms of pressure.

The commonly used current telemetering system are motion and force balance types which are improved forms of the basic current telemetering system described above.

In a Motion Balance System the slide wire is replaced by a position detector like an LVDT as shown in Fig. 2.4. A capacitive transducer may also be used.

The pressure acting on the bourdon tube causes a displacement which moves the core of the LVDT, thereby producing a voltage output which is amplified and rectified. This voltage produces a d.c. current of the order of 4 to 20 mA in the telemetering channel and is measured by a d.c. milli-ammeter. The scale of the d.c. milli-ammeter is directly calibrated in terms of pressure being measured.

A Force Balance System is shown in Fig. 2.5. In this system, a part of the current output is fed back to oppose the motion of the input variable. The system is operated by the bourdon tube which rotates the feedback force coil which in turn changes the flux linkages between the primary and the secondary coils. This change in flux linkages varies, the amplitude of the amplifier. The output signal is connected to the feedback force coil which produces a force opposing the bourdon tube input.

Fig. 2.5 Force balance current telemetering system.

A force balance system increases the accuracy as smaller motions are required which result in better linearity.

Position Telemetering System

A position telemetering system transmits and reproduces the measured variable by positioning variable resistors or other electrical components in a bridge circuit form so as to produce proportional changes at both the transmitter and the receiver ends. This is known as Bridge type System.

Fig. 2.6 shows two potentiometers, one at transmitting end and the other at the receiving end. The two potentiometers are energized by a common power supply. The sliding contact at the transmitting end is positioned by the bourdon tube as pressure is applied to the latter. If the sliding contact at the receiving end is positioned until the centre zero galvanometer indicates zero, the position of the contact will assume the same position as the contact at the transmitter. The receiving contact moves the pointer which indicates on the scale the pressure which is being measured (the scale is directly calibrated in terms of pressure). The principle involved is the same as that of a Wheatstone bridge.

Another most commonly used position telemetry system utilizes a Synchro Transmitter and Receiver. (This is purely an a.c. telemetry system. This has described here because it uses land line telemetry. Fig; 2.7 shows a pair of synchro transmitter and receiver as used in position type telemetering. The rotors and stators of the synchro transmitter-receiver pair are connected in torque transmission configuration. The input to this type of system is angular position of the synchro-transmitter. When the rotor of the torque transmitter is in the same position as that of the receiver, the emfs induced in the stator windings of both of them are equal and there is no current in the telemetering (or the transmission) channel.

If the rotor of the transmitting synchro is rotated, there is an emf unbalance in the stator windings of synchro transmitter and the receiver, resulting in a current flow in the telemetering channel. This current flowing in the stator of the receiver produces a torque on the receiver rotor till it occupies the same position as the rotor of the synchro transmitter.

The indicator in such a system may be a simple pointer attached to rotor shaft of the receiver synchro for the end devices. The telemetering channel includes a connecting line and its associated terminal equipment.

6.2 A.C. Telemetry Systems

The methods described above are used for d.c. telemetering purposes, the exception being the telemetering through synchro transmitter-receiver pair. Alternating quantities can also be transmitted using telemetering circuits such as telephone cables using transformers and amplifiers. A.C. telemetry is used both for land line and ratio frequency airborne telemetry techniques. Electronic means are used for a.c. telemetry.

The a.c. telemetry is used for sensors that provide an a.c. output or voltage to frequency converters. The data is available in the form of current or voltage and is modulated by an a.c. carrier produced by an oscillator.

Modulation

It is the modulation of a carrier waveform, which is usually sinusoidal, in response to the information to be carried.

A sinusoidal carrier can be described by e_c = A_c sin (2π f₀ t + θ)

Where A_c = amplitude of carrier, f₀ = frequency of carrier; Hz,

θ = relative phase shift of carrier; Hz,

There are many ways of modulation of a signal. They are described below :

Amplitude Modulation (A.M.)

In amplitude modulation the value of A_c changes. The carrier amplitude level swings about its unmodulated value. For a sinusoidal information signal, the expression for A_o is :

A_o= A_o(1 + m cos 2π f₀t) ...(2.2)

where A_o = amplitude of the carrier at frequency f_o when the signal or modulation at frequency f₀ is zero, and f_s = signal or modulating frequency ; Hz.

Actual amplitude of the modulation = A_om …(2.3)

where m = modulation index

From. Eqns. 2.1 and.2.2, we have the modulated output:

e_fm = A_o (1 + m cos 2π f_s t) sin (2π f₀ t)

Assuming θ = 0, e_ℓm = A₀[sin 2π f₀t + ½ m sin 2π (f₀ + f_s)t + ½ m sin 2π (f₀ – f₈) t] …(2.5)

The output voltage waveform has three frequencies f_c, f_o+ f_s and f_e – f_s.

The two side frequencies (f_o + f_s) and (f_c –f_s) have an amplitude proportional to the message signal A_o m and are called side frequencies.

Bandwidth BW = (f_o + f_s) – (f_o – f_s) = 2f_s …(2.6)

∴ The bandwidth is twice the frequency of the message signal.

A multiplier circuit can act as an AM modulator while an AM demodulator consists of a band pass filter, rectifier and output low-pass filter. Fig. 2.8 shows an amplitude modulated signal.

The amplitude of the a.c. voltage, which carries the data to be measured, is affected by the impedance of the cable as well as the burden. Stray fields also effect the magnitude of this voltage. Hence other methods of modulation are preferred for long distance transmission.

Fig. 2.8, Amplitude modulation.

Frequency Modulation (FM). In frequency modulation the instantaneous frequency of the carrier is varied in response to the amplitude of the modulating signal. The amplitude A_c and the relative phase, θ, of the carrier do not change. A schematic diagram of a frequency modulation system is shown in Fig. 2.9 (a). The frequency, f_o, of the oscillator is dependent upon the LC

resonant circuit. The capacitance, C, is varied periodically with the signal or modulation frequency f_o· Therefore, the instantaneous value of the frequency modulated signal is:

e_fm= A_o sin (w_ct + m_f sin w_st) …(2.7)

where m_f = modulation index.

The modulation index, m_f, is analogous to the modulation index min amplitude modulation systems. The frequency modulated waveform is shown in Fig. 2.9 (b).

This waveform represents a spectrum of the carrier wave and Symmetrically grouped sidebands. The number and the relative amplitude of the sidebands depend not only on the signal frequency but also on the modulation index m_f. An example of a simple spectrum is shown in Fig. 2.9 (c).

The frequency components of an FM signal extend to infinity on either side of the carrier frequency. However, their amplitudes fall off rapidly outside a certain range as shown in Fig. 2.9 (b).

According to Carson's rule, the approximate value of bandwidth for an FM signal is:

Bandwidth BW = 2(D + f_h) …(2.8)

where and D = maximum deviation in the carrier from its unmodulated frequency,

and f_h = highest frequency in the message signal.

Frequency modulation requires a greater bandwidth than amplitude modulation. It is used for land line and radio frequency telemetering.

The peak value of the carrier voltage must be much greater than the noise and stray emfs. With frequency modulation, the signal to be measured is much susceptible to disturbances than amplitude modulation. Frequency modulated signal can therefore be transmitted error-free over long distances.

Phase Modulation: The phase modulation techniques (PM) employ the use of varying the phase angle θ for telemetering purposes. The angle θ is varied about its unmodulated value and can be analyzed in a similar fashion as the frequency modulation (FM) techniques.

Three cables are used for phase modulated signals. The phase of the a.c. voltage in one conductor is changed with respect to that in the other conductor depending on the magnitude of the measured quantity. The third conductor provides the return path. The transmission frequency ranges from power frequency to about 400 Hz. Rotating field systems like synchros serve as transmitters and receivers. Angular positions are transmitted up to distances of a few kilometres using this method.

2.7 Pulse Telemetering Systems

When pulse telemetering is used, the measurement is transmitted in terms of time rather the magnitude of an electrical quantity. The information may be conveyed through land-line connections or through radio frequency links.

Pulse telemetry may be classified as:

(i) Analog pulse telemetry, and

(ii) Digital telemetry.

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