Post on 13-Mar-2017
A Modulated-Frequency System of
Telemetering
HAROLD E. RENFRO ARTHUR PERRY PETERSON NONMEMBER AIEE ASSOCIATE AIEE
WITH today's centralized control and supervision of power systems, it is
essential that the man at the controls knows what is going on at all outlying stations and interconnection points. Telemetering equipment keeps him informed of load changes, transfer of power from one system to another, variations in voltage, and all other values that are necessary for efficient operation.
No one system of telemetering can meet all requirements. The speed of indication, accuracy, number of separate channels available, dependability, ease of totalizing, freedom from interference, the availability and need of repair parts, and the initial cost, as well as maintenance cost, are factors that determine the design of any system of remote metering. A resume of t i e telemetering practice up to 1941 will be found in the report of the joint committee of the AIEE committee on automatic stations and the AIEE committee on instruments and measurements, published in October 1941. That report also contains an excellent bibliography on telemetering systems and methods.
When the Department of Lighting of the City of Seattle wanted a telemetering system which would operate over a carrier-current channel they were unable to locate such a system that would- give them continuous and instantaneous readings. They felt that, for their purposes, an impulse system which provided readings at regular intervals was not the answer to their problem. Furthermore, they wanted a system that would permit several readings simultaneously over a single set of conductors. It was this need that led to the development of a modu-lated-frequency system of carrier telemetering.
The telemetering system herein described was designed to give continuous and almost instantaneous readings which are independent of supply voltage and are not affected by wide variations in remote-line characteristics. I t was designed to operate with a high degree of dependability during emergencies, to permit a large number of channels on one pair of conductors, to give very fast response,
Paper 45-19, recommended by the AIEE committee on instruments and measurements for presentation at the AIEE winter technical meeting, New York, N. Y., January 22-26, 1945. Manuscript submitted May 8, 1944; made available for printing December 1, 1944. HAROLD E. RENFRO'is an engineer in the Department of Lighting of the City of Seattle, Wash., and ARTHUR PERRY PETERSON is president of the Control Corporation, Minneapolis, Minn.
FEBRUARY 1945, VOLUME 64
and to incorporate certain other desirable features.
The transmitter generates its own carrier frequency making it unnecessary to provide a separate carrier-current system. Standard radio receiver tubes are used throughout and all parts subject to wear or breakdown are easily replaceable standard radio parts.
General
The block diagram, Figure 4, illustrates the principle of operation and this will be described briefly. In the telemeter transmitter is incorporated the primary instrument, such as a wattmeter, which determines the values to be transmitted. This primary instrument controls the frequencies of two high-frequency oscillators whose outputs are converted into a single beat frequency. The beat frequency, which varies with the magnitude of the quantity being measured, is amplified and transmitted over the line, whether it be a transmission line or a telephone line. Normally, capacitive coupling is used with a transmission line whereas inductive coupling, with a bandpass filter to separate the carrier frequencies from the voice frequencies, is used with a telephone line. Only telephone-line applications will be described here.
At the receiving point the incoming signal from the line is amplified and then mixed with the output of a local oscillator of constant frequency. This converter stage is followed by a low-pass filter which attenuates all but the beat frequency. The current at this beat frequency is then amplified and fed into a frequency meter, of which the final indicating instrument is a part.
Transmitter
The schematic diagram of the telemeter transmitter is shown in Figure 6. The primary instrument actuates two variable capacitors, C\ and C2, which, in turn, simultaneously control the frequencies of the two oscillators. The variable capacitors form part of the capacitance of the frequency-determining resonant circuit of each oscillator.
The greatest difficulty with instruments of this type that have been tried in the past has been that a variable capacitor on the shaft of the primary instrument develops a torque of its own that varies
Renfro, Peterson—Telemetering
with the potential applied to it. A variable inductance develops a somewhat similar torque due, to current through it. This destroys the calibration of the instrument and makes a reliable system impossible, unless counter measures are taken.
An analysis of the torque developed by a variable capacitor with a voltage across its plates shows the following to be true: two conductors separated by an insulator tend to move toward each other when a difference of potential exists between them. With parallel-plane conductors of infinite area, this force is perpendicular
Figure 1 . Front view of the assembled telemeter transmitter
The two wrousht-iron tanks at the bottom house the oscillatory circuits
Figure 2. Telemeter receiver
The indicating instrument is not shown
TRANSACTIONS 45
to the planes of the conductors. In a rotary variable air capacitor having two identical semicircular parallel fixed plates with a single semicircular rotor midway between them, a difference of potential causes several forces which are listed below, Figure 5 illustrates these forces and also shows the manner in which the two capacitors are formed by the stationary plates and damping disk of the primary instrument.
1. Forces between the rotor and upper stator which are balanced by forces between the rotor and lower stator. These forces, which are not shown on Figure 5, are parallel to the shaft and do not tend to rotate it. 2. Forces between the stator plates and the extended parts of the rotor, shaft, and so forth, which are symmetrical about the axis. These forces, shown as Au A2, Az, and AA, are radial and do not tend to produce rotation. 3. Forces between the diametrical edge of the rotor and the stators when the rotor is not fully meshed. These forces, shown as JBi, Bz, Bs, and B*, are tangential and tend to turn the rotor in a direction that will mesh it deeper between the plates of the stator.
Thus, the only rotary electrostatic force is due to edge effect so that, with plates so shaped that the edge exposure is constant with any angle of rotation, the torque produced is dependent only on difference of potential and is entirely independent of angular position except near zero mesh and full mesh. Throughout practically the whole angle of rotation, the torque may be kept constant with constant applied potential. Currents flow radially and so do not produce any torque.
Two variable capacitors having a common rotor are incorporated in the transmitting instrument in such a manner that increasing the mesh of one capacitor de-
damping disk passes between them. The four plates are identical and have the same voltage applied between each plate and the rotor, thus neutralizing any torque which is produced.
Figure 4. Block diagram of the modu-lated-frequency system of telemetering
maximum mesh and 60,500 cycles per second at minimum mesh. At the time one capacitor is at maximum mesh the other is at minimum mesh so that the two oscillator frequencies approach or re-
TRANSMITTER
PRIMARY INSTRU
MENT
OSCILLATOR 49,500 TO 50,00p_CPS
r'ELECTROSTATIC \ff COUPLING VERTER
CON- u p-™ 14
OSCILLATOR 60,000 TO 60,500 CPS 1
FILTER
\
AMPLIFIER
COUP-HLING
UNIT LINE
10,000 TO 11,000 CPS OUTPUT
RECEIVER
LINE COUP LING H UNIT
[AMPLIFIER
Since the two capacitors formed by the plates and damping disk are of small capacitance, the oscillator frequencies must be high in order to obtain the desired frequency change with a given angle of rotation of the instrument shaft. A fast response to changes in measured values is obtained since no mechanical connection is made to the shaft and all torques due to electrostatic forces are eliminated through cancellation.
Various oscillator frequencies can be used and, in the case of several metering sets operating over a common line, the frequencies must be separated sufficiently in order to provide the required number of channels. A remote-metering arrangement using typical oscillator frequencies will be described.
One of the variable capacitors de
creases the mesh of the other. At the time that one capacitor is at maximum mesh the other is at minimum mesh so that the two oscillator frequencies approach or recede from each other with rotation of the common rotor. The two capacitors have a common rotor which is formed by the damping, disk of the wattmeter. Each of the two stators consists of two parallel aluminum plates spaced 0.083 inch apart and mounted so the
Figure 3. Indicating instruments at the receiving end of six telemetering systems
scribed herein is in an oscillator circuit arranged to generate 50,000 cycles per second at minimum mesh of the capacitor plates and 49,500 cycles per second at maximum mesh. The other variable capacitor is in an oscillator circuit which generates 60,000 cycles per second at
LOW HCONVERTER W PASS
FILTER AMPLI
FIER FREQUENCY
METER
100 TO 1100 CPS OUTPUT
OSCILLATOR 9900 CPS
VOLTAGE [REGULATOR I
STATOR PLATES
Figure 5. Electrostatic forces in two rotary variable air capacitors, each having two iden
tical semicircular parallel fixed plates
A single semicircular rotor midway between the fixed plates is common to both capacitors
cede from each other with rotation of the transmitting instrument shaft. The oscillator tubes are A and B in Figure 6.
In order to insure an accuracy of two per cent, the maximum frequency variation of each of the three oscillators used in the system cannot vary more than two thirds of one per cent, assuming zero error in other components. Investigation showed that electron-coupled oscillators, tuned by a combination of positive- and negative-temperature-coefficient ceramic fixed capacitors plus a very small variable capacitor, have excellent stability over long periods of time. All oscillator circuits used in this system of telemetering are stabilized in this manner. The amount of negative-temperature-coefficient capacitor capacitance necessary to compensate for the coil and positive-temperature-coefficient capacitors in the set as described here was 10.71 per cent of the total capacity. Coils and mountings were developed that repeated their temperature-frequency characteristics faithfully.
The two voltages which are generated by the oscillators are fed into a converter
46 TRANSACTIONS Renfro, Peterson—Telemetering ELECTRICAL ENGINEERING
tube (tube C in Figure 6) where the difference or beat frequency, which varies between 10,000 and 11,000 cycles per second, is produced.
The output of the converter is fed into a low-pass filter which attenuates the high frequencies and all harmonics and
amplified by the audio amplifier (tube C) which feeds the frequency meter. Incorporated as a part of the frequency meter is the final indicating instrument, Its pointer is at the left end of the scale with 100-cycle input and at the right end of the scale with 1,100-cycle input.
ci^S^Tn
Figure 6. Schematic diagram of the telemeter transmitter
passes only the beat frequency to an amplifier which is arranged in push-pull. In Figure 6 the amplifier tubes are D and E.
The amplifier feeds the line through an inductive coupling unit (7\) which has laminated phenolic-sheet insulation between the primary and secondary windings and is capable of withstanding the highest voltage exposure of the line. A single-stage low-pass filter with a cutoff frequency.of 4,500 cycles per second prevents the carrier power from entering the telephone instrument circuit where it would be wasted. This filter is not shown in Figure 6.
Receiver
The schematic diagram of the telemeter receiver is shown in Figure 7. At the receiving point the telephone line connects to a coupling unit similar to that at the transmitter. Voice frequencies are passed on to the telephone unit and the carrier frequency goes to the metering receiver.
The receiver consists of one 10,000 to 11,000-cycle amplifier stage, local oscillator, detector, low-pass filter, audio amplifier, frequency meter with indicating instrument, and a voltage regulator. The high-frequency amplifier stage (tube A and associated circuits) is fixed in frequency and is adjusted to 10,490 cycles per second. This frequency is approximately the geometric mean between 10,000 and 11,000 cycles per second. The local oscillator (tube F) has a constant output frequency of 9,900 cycles per second. The oscillator output and the high-frequency output are fed into the converter stage (tube B) where a beat frequency is produced. This beat frequency varies between 100 and 1,100 cycles per second. The high frequencies and all harmonics are eliminated by the low-pass filter which follows the converter stage. The output of the low-pass filter (100 to 1,100 cycles per second) is then
The frequency meter consists of two triode tubes, D and E, connected in series; that is, the plate of E is connected to the positive line, its cathode is connected to the plate of the other triode, D, and the other cathode is connected to the negative line. A fixed capacitor is shunted across the plate and cathode of each tube. These capacitors are shown as Cz and C4. The grids in normal operation are biased beyond cutoff so that no plate current flows except that due to a-c grid excitation. At the time one grid is made more positive, the other is made more negative so that both tubes cannot conduct at the same time. Both tubes may be nonconducting at once but conduction is alternate. When either tube becomes conducting, it discharges the capacitor across its plate-cathode circuit and charges the capacitor which is connected across the nonconducting tube. When the grid polarities change, the op-
voltage and wave form within wide limits* The frequency meter and oscillator in
the receiver are supplied from an electronic voltage regulator which is of a type frequently used and will not be described here. The neon-glow tube, / , and vacuum tubes G and H with their associated circuits comprise the voltage regulator. The actual output potential remains constant at 80 volts with the line voltage between 100 and 130.
Zero current in the indicating instrument puts the pointer off scale to the left. Such a condition indicates that the d-c power supply is off at the receiver or that the frequency-meter tubes have failed. If the transmitter power is off, or if the line is in trouble so that no carrier current is received, the bias is removed from the frequency-meter tubes and the indicating instrument goes off scale on the upper end. In this case the current is limited by degenerative resistors in the cathodes of the frequency-meter tubes. If the indicating instrument pointer is on its scale, the observer- knows that none of the troubles just mentioned are being experienced.
Totalizing at the transmitter may be accomplished by a multiple-wound current transformer in the a-c circuits to the wattmeter. Totalizing at the receivers may be done in the frequency-meter output circuits by adding the direct current in a lead common to several instruments. Strip chart recording d-c milliammeters are available for continuous records at the receiving point.
Six complete sets of this system of telemetering are now in continuous service and have been for the past few years. The experience thus gained definitely
posite capacitors charge and discharge. A d-c milliammeter in series with the d-c supply shows the average current used to charge the capacitors. With a constant direct voltage each charge requires a definite current for a definite time. The time of discharge and charge is shorter than the period of one input alternation at the highest frequency used and therefore the d-c milliammeter reading is a definite indication of the number of charges per second. The number of charges is controlled by the input frequency and is independent of the input
Figure 7. Schematic diagram of the telemeter
indicates that the equipment has met the requirements', expected of it. It provides almost instantaneous indications with standard accuracy. It permits a large number of readings to be transmitted over a single circuit and it is a simple matter to totalize readings from several transmitters. It is dependable and free from interference. It has proved easy to maintain at a minimum of expense.
FEBRUARY 1945, VOLUME 64 Renfro, Peterson—Telemetering TRANSACTIONS 47