US Navy Course NAVEDTRA 14029 - Aviation Electronics Technician-Intermediate

8/14/2019 US Navy Course NAVEDTRA 14029 - Aviation Electronics Technician-Intermediate

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NONRESIDENTTRAINING

COURSE

Aviation Electronics

Technician-

IntermediateNAVEDTRA 14029

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.


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PREFACE

By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy.

Remember, however, this self-study course is only one part of the total Navy training program. Practical

experience, schools, selected reading, and your desire to succeed are also necessary to successfully round

out a fully meaningful training program.

COURSE OVERVIEW: In completing this nonresident training course, you will demonstrate a

knowledge of the subject matter by correctly answering questions on the following: Servo systems, logic

devices, communications, navigation systems, optic and infrared systems, television, computers and

programming, waveform interpretation, and automatic test equipment.

THE COURSE: This self-study course is organized into subject matter areas, each containing learning

objectives to help you determine what you should learn along with text and illustrations to help you

understand the information. The subject matter reflects day-to-day requirements and experiences of

personnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers

(ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational or

naval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classifications

and Occupational Standards, NAVPERS 18068.

THE QUESTIONS: The questions that appear in this course are designed to help you understand the

material in the text.

VALUE: In completing this course, you will improve your military and professional knowledge.

Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you are

studying and discover a reference in the text to another publication for further information, look it up.

1992 Edition Prepared by

AVCM(NAC) Raymond A. Morin

and

ATC Richard M. Endres

Published by

NAVAL EDUCATION AND TRAINING

PROFESSIONAL DEVELOPMENT

AND TECHNOLOGY CENTER

NAVSUP Logistics Tracking Number

0504-LP-026-7060


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DISTRIBUTION STATEMENT C: Distribution authorized to U.S. Government agencies andtheir contractors because of proprietary information and classification of references asdetermined on 25 February 1992. Other requests for this document must be referred toCommanding Officer, Naval Education and Training Professional Development andTechnology Center, Code N315, 6490 Saufley Field Road, Pensacola, FL 32509-5237.

Although the words he, him, andhis are used sparingly in this course toenhance communication, they are notintended to be gender driven or to affront ordiscriminate against anyone.


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CONTENTS

CHAPTER Page

l. Servo-Systems . . . . . . . . . . . . . . . . . . . . .

2. Logic Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Navigation Systems . . . . . . . . . . . . . . . . . . . . . . . .

5. Anti-Submarine Warfare (ASW) . . . . . . . . . . . . . . . . . . . .

6. Radar Circuits . . . . . . . . . . . . . . . . . . . . . . .

7. Optic and Infrared Systems . . . . . . . . . . . . . . . . . . . . . . .

8. Television . . . . . . . . . . . . . . . . . . . . . . .

9. Computers and Programming . . . . . . . . . . . . . . . . . . . . . .

10 . Waveform Interpretation . . . . . . . . . . . . . . . . . . . . . . . . 10-1

11. Automatic Test Equipment . . . . . . . . . . . . . . . . . . . . . . . 11-1

APPENDIX

I. Glossary . . . . . . . . . . . . . . . . . . . . . . .AI-1

II . Symbols, Formulas, and Measurement . . . . . . . . . . . . . . . AII-1

III. References Used to Develop the Training Manual . . . . . . . . . . AIII-1

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . INDEX-1

1-1

2-1

3 - 1

4-1

5 -1

6 - 1

7-1

8-1

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SERVO SYSTEMS

Chapter Objective: Recall the purpose and functions of servo systems to

includ e oscillation, zeroing syn chro units, u se of the synchro alignm ent set,

antenna positioning servo systems, and hydraulic servo systems.

As an Aviation Electronics Technician (AT),

you will encounter various types of servo systems.

The particular type (electromechanical, electro-

hydraulic, hydraulic amplidyne, pneumatic, etc.)

will depend upon the type of load for which it wasdesigned. One of your primary jobs will be the

control of radar antennas from a remote controlsta tion. We will discuss some meth ods of ant enn a

control later in this chapter.This chapter will not provide a detailed

presentation of any one servo system. Instead, wewill discuss the basic systems, identify their

essential components, and explain the functionof each component. For details concerning the

theory and operation of a particular system, you

should refer to the applicable technical manualsfor that system. Before continuing, you should

review the basic theory of synchros and servo-mechanisms discussed in Module 15 of the NavyElect r ic i ty and E lect ronics Training Ser ies

(NEETS), NAVEDTRA 172-15-00-8.

BASIC SERVOMECHANISMS

Learning Objective: Identify the concepts

and components of a basic servomech-

anism to include a data transmissionsystem, servo control amplifier, and a

servom otor.

T h e e s s e n t i a l c o m p o n e n t s o f a s e r -vomechanism are a data transmission system, a

servo control amplifier, and a servomotor. These

components are shown in the block diagram of

figure 1-1, and are discussed in the following

paragraphs.The functions of the data transmission system

are as follows:

1. To measure the servo output2. To transmit or feedback the signal, which

is proportional to the output, to the error detector

(a differential device for comparing two signals)

Figu re 1-1.-Simplified b lock diagra m of a servomecha nism.

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3. To compare t he inpu t signal with the feed-

back signal4. To tra nsmit to the servo amplifier a signal

tha t is pr oportiona l to the difference between t he

input and output

The signal obtained by comparing the servo

input a nd outpu t is called the servo error, and isrepresented by the symbol E. Figure 1-1 shows

tha t the servo error (E) is t he difference between

the input (ei) and th e output (6.). This is statedmathemat ical ly as E =@i O..

In many servo systems, the physical location

of the servo input device and output device are

remotely located from each other, and may also

be remotely located from the servo amplifier. Thisrequires some means of transmitting the output

information back to the device receiving the inputcommand an d tra nsmitt ing the servo error to the

servo amplifier. This system of transmission, as

well as the comparing device (called an errordetector), is part of an overall data tra nsmission

system. We discuss data transmission later in thischapter. The function of the servo amplifier is to

receive the error signal from the error detector,

amplify it sufficiently to cause the output deviceto position t he ser vo load t o the comm an ded posi-

tion, and to transmit the amplified signal to the

servomotor.

The servomotor positions the servo load. The

motor must be capable of positioning the load

within a response t ime based on the requirements

of the system.

E R R O R D E T E C T OR S

The component of the data transmission

system that compares the input with the ser-vomechanism output is the error detector. An

error detector can be either a mechanical or an

electrical device. A simple form of a mechanical

error detector is the differential. However, in air-

craft weapons systems, most error detectors are

electrical devices because of their adaptability to

widely separated or remotely instal led com-

ponents. Most of the electrical devices used are

of either the potentiometer (resistive) or one ofseveral magnetic devices.

Electrical error detectors may be either ac or

dc devices, depending upon the requirements of

the servo system. An ac device used as an error

detector must compar e the two input signals and

produce an error signal. The phase and amplitude

of the error signal will indicate both the direc-

tion and the amount of control necessary to

accomplish correspondence. A dc device differs

because the polarity of the output error signal

de te rm ines the d i r ec t ion o f t he co r rec t ionnecessary.

Error detectors are also used extensively in

gyrostabilized platforms and rate gyros. In the

stabilized platform, synchros are a tta ched t o the

gimbals. Thus, any movement of the platform

ar ound the gyro axes is detected by th e synchro,

and the error voltage is sent to the appropriate

servo system.In rate gyros, an E-transformer (discussed

later) is commonly used to detect gyro precession.

It is extremely sensitive to very slight changes, but

its movement is limited to a very small amount.

Thus, it is extensively used with constrained gyros.

P O T E N T IO M E T E R

Potent iometer er ror detector sys tems are

generally used only where the input and output

of the servomechanism have limited motion. They

Figure 1-2.-B a la n c e d p o t e n t i om e t e r e r r o r d e t e c t or s y st e m .

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are characterized by high accuracy, small size, andthe fact that a dc or an ac voltage may be obtained

as the output. Their disadvantages include limitedmotion, a life problem resulting from the wear

of the brush on the potentiometer wire, and the

fact th at the voltage output of th e potent iometer

changes in discrete steps as the brush moves from

wire to wire. A further disadvantage of somepotentiometers is the high drive torque required

to rotate the wiper contact.An example of a balanced potentiometer

error detector system is shown in figure 1-2. Aswe have indicated, the purpose of the circuit is

to give an output error voltage that is proportionalto the difference between the input and output

signals.The command input shaft is mechanically

linked to R1, and the load is mechanically linked

to R2. An electrical source of 115 volts ac is

applied across both potentiometers.

When th e input a nd output shafts are in thesame angular position, they are in correspondence

and ther e is no out put error voltage. If the inpu t

shaft is rotated, moving the wiper contact of RI,

an error voltage is developed and applied to the

control amplifier. This error voltage is the

difference of the voltages at the wiper contacts

of R1 and R2. The output of the amplifier causesthe motor to rotate both the load and the wiper

cont act of R2 unt il both voltages a re equa l. When

equal, there is no output error voltage.

Figure 1-2 illustrates R1 and R2 grouped

together. In actual practice the potentiometers

ma y be positioned rem otely from ea ch oth er, withR2, the output potentiometer, being located at theoutput shaft or load. The remote location of one

of the components does not remove it from being

a part of the error detector.

E - T r a n s f o r m e r

The E-transformer is a type of magnetic device

used as an error detector. Its application is usefulin systems that do not require the error detector

to move through large angles. A simplified draw-ing, which is one of several possible devices in this

category, is shown in figure 1-3.The primary excitation voltage is applied to

coil A on the center leg of the laminated core.

The coupling between coil A and the secondarywindings, coils B and C, is controlled by the

armature, which is displaced linearly by the input

signal. When the armature is positioned so the

coupling between th e windings is balan ced (nu ll),

the output voltage is minimum because of the

Figure 1-3.-E - t r a n s f o r me r e r r o r d e t e c t o r .

series-opposing conn ections of th e seconda ry win -

dings. The ph ase of the outpu t volta ge on either

side of th e volta ge nu ll differs by 180 degrees. By

proper design of the transformer, the amplitudecan be ma de proport iona l to the displacement ofthe arm atu re from its n ull voltage position. This

type of error detector h as t he a dvanta ges of small

size and high accuracy. It has the disadvantage

of permitting only limited input motion.

C o n t r o l T r a n s f o r m e r

Synchros have been developed to a point of

re la t ively high accuracy, low noise level ,

reasonably small driving torques, and long life.

These qualities also apply to synchro controltransformers. A primary advantage of the synchro

control transformer over other types of errordetectors is its un limited rotat ion a ngle; tha t is,

both the input and the output to the synchro

control transformer may rotate through unlimitedangles. Among the disadvantages of synchros

(including the synchro control transformer) are

the large size necessary to maintain high accuracy,the power consumed, and the output supplied to

the servo control amplifier is always ac modulatedwith the servo error.

Alternating current may be used if the twofollowing conditions are met:

1. The frequency of the ac used must be

greater t han the ma ximum frequency response of

the measuring devices used.

2. If

allowed,

negative values of the variables are

the devices used can be phase-sensitive.

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Figure 1-4 shows a dc signal and the same

function represented by an ac voltage. The

instantaneous value of the ac signal does not

indicate the value of the function, but the averagevalue of the ac signal may be used to represent

the value of a function. If the ac signal is the in-

put to a servomotor, for example, the motor must

not attempt to follow every variation of the acsignal, but must follow the average value. The

second condition is essential because a negative

ac signal does not exist. However, negative values

can be indicated by a change in phase of the

signal. Note tha t in figure 1-4, during th e periodwhen the dc signal is positive, the positive peaks

of the ac signal correspond to the positive peaks

Figu re 1-4.-AC modulated w ith th e servo error .

of the ac reference. During the period when the

dc signal is negative, the positive peaks of the ac

signal correspond to the negative peaks of the

reference; i.e., the signal is 180 degrees out of

phase with the reference. Alternating-current ser-vomotors are available. These servomotors will

rotate in one direction when the input signal is

in phase with a reference voltage, and in the otherdirection when th e signa l is out of phase with t hereference voltage.

A synchro data transmission system is com-

prised of a synchro transmitter, a synchro con-

trol transformer, and, in some cases, a differentialt ransmit ter for addi t ional servo inputs . The

synchro transmitter transforms the motion of its

shaft into electrical signals suita ble for tra nsmis-

sion to the synchro control transformer, which

comprises the error detect or (fig. 1-5).

The stat or of the t ran smitter consists of thr ee

coils spaced 120 electrical degrees apart, Thevoltage induced into the stator windings is afunction of the tra nsmitt er r otor position. These

voltages are applied to the three similar statorwindings of the synchro control transformer. The

voltage induced in the rotor of the synchro control

transformer depends upon the relative position

of this rotor with respect to the direction of the

stator flux.

The variation of the synchro control trans-

former output voltage as a function of the rotor

position r elative to an a ssumed stat or flux direc-

tion is shown in figure 1-6. While there are two

positions of the rotor, 180 degrees apart, wherethe output voltage is zero, only one corresponds

to a stable operating position of the servo.

Figure 1-5.-T h e c o n t r o l t r a n s fo r m e r a s a n e r r o r d e t e c t o r .

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Figure 1-6.- Induced vol tage in synchro control t ransformer

ro to r .

When a synchro differential tra nsmitter is used

for additional inputs to the servo system, it is

connected between the synchro transmitter and

the synchro control transformer (fig. 1-7). When

the synchro differential rotor is in line with its

stat or windings, the different ial tran smitter a cts

as a one-to-one ratio transformer, and the voltages

applied to the synchro control transformer are the

same a s the voltages from t he synchro transm itter .If the synchro differential transmitter rotor is

displaced by a second input, the voltages from thesynchro transmitter to the control transformer aremodified by the synchro differential transmitter

by the amount and direction of its rotor displace-ment. Thu s, the t wo inputs are algebraically added

and fed to the synchro control transformer as a

single input .

F l u x G a t e

A flux gate element may be used to drive or

excite a control transformer and is usually usedin compass systems. The flux gate operates on theprin ciple of using th e ear th s ma gnetic field toproduce a second harmonic current flow in the

element. This, in tu rn, pr oduces a voltage in th e

stator windings of the control transformer that

is in direct proport ion to ear th s ma gnetic north .

Because it is desirable to use only the horizontal

componen t of the ea rt hs field, a gyro is used t o

hold the element level with t he ear th s sur face.

Another method is to suspend the element by a

spring and use the properties of a pendulum torigidly mount it to the aircraft so tha t it tu rns in

an azimuth as the aircraft turns.

Figure 1-7 .- S y n c h r o d i f f e r e n t i a l t r a n s mi t t e r u s e d f o r a d d i t i o n a l i n p u t .

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MULTIPLE-SPEED DATATRANSMISSION SYSTEMS

The static accuracy (how accurately the load

is controlled) of a servomechanism is frequently

limited only by the accuracy of the data transmis-sion system. The accuracy of the data transmis-

sion system may be increased considerably byemploying a multiple-speed data transmission

system along with a 1-speed system. The error-

detector elements of the multiple-speed transmis-

sion system rotate at some multiple of the shaft

being controlled. The elements of the 1-speed

transmission system operate one to one with

respect to the controlled shaft.The schematic diagram of a multiple- and a

1-speed system is shown in figure 1-8. If a systemcan transmit data at two different speeds, it is

refer red to as a dual -speed sys tem. In th is

example, if the input shaft tu rns thr ough 1 degree,

the 1-speed transmitter also is rotated 1 degreewhile the multiple-speed unit is rotated 10 degrees.The synchro control transformer associated with

each of these transmitters is geared in similar

ratios with respect to the servo output shaft. A

1-degree error bet ween th e position of th e input

and output shaf ts produces a re la t ive rotor

displacement of 1 degree in th e 1-speed synchr os,

and 10 degrees in th e mu ltiple-speed synchr os. If

the relation between the rotor displacement and

output voltage is linear , the err or signa l from th e

multiple-speed system is 10 times that from the

1-speed system. This amplification of the error

signal in the data transmission link reduces

the signal amplification required in the servo

controller. If the synchro has an inherent error

of 0.1 degree with respect to its own shaft,

the consequent servo error introduced by a

1-speed data transmission system will be ofcorresponding magnitude. The consequent servo

error intr oduced by a 10-speed data tra nsmission

system will be only one-tenth as great , or

0.01 degree.

A disadvantage of using a multiple-speed error

detector lies in the possibility of the system fallingout of step. If this happens, it will synchronize

in a position differing from the correct position

by an integral number of revolutions of themultiple-speed synchro. In the example shown in

figure 1-8, if th e outpu t sh aft were held fixed an d

the input shaft rotated 36 degrees, the 10-speed

synchro transmitter would turn one completerevolution. At this point, the error signal from

the multiple-speed error detector would be zero.

If the output shaft were then released, the systemwould operate in a stable fashion with a 36-degreeerror between the input and output shafts. Thepurpose of using a 1-speed detector is to prevent

this ambiguous synchronization.

An error signal selector circuit is provided thatswitches control of the servo to the 1-speed data

transmission system. This occurs whenever the

Figure 1-8 . -Du a l - s p e e d d a t a t r a n s mi s s i o n s y s t e m.

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servo error becomes large enough to permit the

multiple-speed system to synchronize falsely.

The simplest device imaginable that could

control an error-selector circuit is shown in

figure 1-8. It is essentially a single-pole, double-

th row relay actuat ed by th e out put of th e 1-speed

error detector. The relay is shown in the de-

energ ized pos it ion . When the ou tpu t of the

1-speed synchro is high, the relay is energized andthe 1-speed circuit controls the servomotor. Whenthe output is low, the relay opens and the 10-speedsynchro controls the circuit. Keep in mind that

the synchro output is high only when there is a

large error.

The relationship of the coarse (1-speed)

synchro output and the fine (10-speed) synchro

output is shown in figur e 1-9, view A. The sh aded

portion represents the area where contr ol can be

switched from th e l-speed circuit to th e 10-speed

circuit. With the selector circuit shown, it is stillpossible to have a single ambiguous position of

the 1-speed (coarse) synchro. At this point the1-speed (coarse) and 10-speed (fine) shafts are

nulled (but are 180 degrees out of phase) and

control is switched to the 10-speed circuit.

One way of eliminating this false synchroniza-

tion position is to drive the multiple-speed syn-

chro at any odd multiple of the 1-speed synchro.Figure 1-9, view B, shows the phase relationship

of a 1-speed and 7-speed system. Although thereis still a null of both synchros at the 180-degree

position of th e 1-speed synchro, their out put s ar e

in phase. This position is an unstable one, and

the servo will not remain at this point.

The system illustrated in figure 1-8 is notfound in operating equipment due partly to theload the relay places on the 1-speed synchro. In

actual practice, the relay could be controlled by

an electronic circuit operated by the synchrovoltages. A met hod comm only used feeds th e out-

puts of the synchros to an electronic circuit biasedso tha t t he fine-synchro voltage is not used wh en

the coarse-synchro voltage is high. This method

does not require a relay.

The disadvantage of using multiple-speed error

detectors is the need for an additional synchrosystem and switching circuit. This additional

equipment is needed if increased servo accuracy

accounts for t he wide u se of th ese mu ltiple-speed

data tran smission systems. This results from the

am plificat ion of the er ror signa l an d th e effective

reduction of inherent synchro errors.

SERVO CONTROL AMPLIFIERS

Ear lier, we stat ed that the output of an err or

detector (error voltage) can be fed to a servo

control amplifier. This type of signal is small inamplitude and requires sufficient amplification toallow actuation of a prime mover. In addition to

amplification, the servo control amplifier must,

in some cases, transfer the error signal into

suitable form for controlling the servomotor or

output member. It may also include provisions

for special characteristics necessary to obtain

stable, fast, and accurate operation.

Servo amplif iers used in aircraf t weap-

ons sys tems a re l imi ted to e lec t ron ic and

magnetic types. The operation and explanation

of electronic amplifiers and their circuits are

discussed in Module 8 of the Navy Electricityand Electronics Training Series (NEETS), NAV-EDTRA 172-08-00-82.

Figure 1-9.-Phase relationship of f ine and coarse synchro voltages; (A) single-wed and 10-speed; (B) single-speed and 7-speed.

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In addi t ion to the requirements of bas ic

amplifiers, servo amplifiers must also meet

certain additional requirements as follows:

1. A flat gain versus frequency response for

a frequency well beyond the frequency range used.2. A minimum of phase shift with a change

in level of input signal. Zero phase shift is desired,but a small amount can be tolerated if constant.

3. A low output impedance.

4. A low noise level.

Servo amplifiers may use either ac or dc

am plifiers or a combina tion of both. The a pplica-

tion of dc amplifiers is limited by such problems

as drift and provisions for special bias voltages

needed in cascaded stages. Drift, a variation in

output voltage with no change in input voltage,

can be caused by a change in supply voltage ora change in value of a component. Consequently,

many servo amplifiers use ac amplifiers forvoltage amplification.

MODULATORS

As pointed out previously, ac amplifiers are

the best to use for amplifying an error signal. Theydo not need well-regulated power supplies and

costly precision components; however, some air-

craft weapons systems use a dc voltage for an

error signal. The dc error voltage maybe changed

to an ac signal by the use of a modulator

(sometimes called a chopper). Modulator circuits

used in servo control amplifiers must be phasesensitive and pr oduce an ac out put signa l, whose

amplitude is proportional to the dc input signal

and whose phase is indicative of the polarity.

Vi b r a t o r M o d u l a t o r s

A m odula to r m ay be e i the r an e l ec t ro -

mechanical vibrator or an electronic circuit. An

example of a vibrator modulator is shown infigure 1-10. An ac supply voltage is employed to

vibrate the contacts of the vibrator in synchronism

with the supply voltage. The dc error voltage is

applied to the center contact of the vibrator.Assume tha t the reference voltage will cause th e

cycle, and point B during the second half cycle.The output is represented by waveform B if the

error voltage is positive, and by waveform C if

it is negative.

Elec t ron ic Modu la to r

An example of an electr onic modulat or circuitis shown in figure 1-11. The circuit shown is a

diode ring modulator and works by causing achanging current to flow through one-half of the

primar y of tra nsformer T2, and th en th rough the

other half at a 400-hertz rate. Each half-cycle of

changing current produces a hal f -cycle of

sinusoidal output voltage. The phase of this out-

put voltage compared to the 400-hertz carrier

depends upon the direction of current through

each primary half.Diodes CR1 and CR4 are forward biased when

th e dc cont rol voltage is positive. Diodes CR2 an d

CR3 are forward biased when the dc controlvoltage is negative. When two of the diodes are

Figu re 1-10.-Vibrator modulator .

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Figu re 1-11.-An electron ic modulator .

forward biased by the dc control voltage, the othertwo are back biased and cut off. As long as the

instan tan eous a mplitude of th e carr ier voltage is

less th an th e dc contr ol volta ge, the cutoff diodes

remain back conducting diodes and through one

of the half windings.

When one of the back biased diodes becomes

forward biased (the amplitude of carrier voltage

exceeds the dc control voltage), the diode con-

ducts. This interrupts the current flowing throughthe half winding. The result is that the output

voltage amplitude is clipped at the value it had

when the current was interrupt ed.The capacitor connected across the primary

of T2 filters any high frequency components

associated with the clipped half-cycle of the sine

wave so th at a nea rly sinusoidal outpu t ha lf-cycle

occurs. The outpu ts am plitude is appr oximat ely

equal to the output voltage at the time of clipping.The capacitor operates by coupling the high

frequency components of the cl ipped voltage

through the nonconducting half windings. The

high frequency components are canceled because

they produce currents that flow in opposite direc-

tions in both ha lves of the cent er ta pped primar y

windings; that is, they produce magnetic fieldsthat cancel each other.

The amplitude of each half-cycle of the

400-hert z car rier voltage is modulated by the dc

control voltage. The polarity of the control

voltage determines the phase of the modulated

carrier voltage output relative to the unmodulatedcarrier voltage input. This is done as a result of

the direction of current flow through the half

winding. This direction depends upon which diode

is forward biased as a r esult of the polarity of the

dc control voltage.

P H AS E D E T E C T O R S

We have stated that an ac amplifier has

inherent advantages over a dc amplifier, that a

dc error voltage can be chan ged into an a c signal,

and the ac signal can be amplified and applied

to an ac servomotor. However, some systems use

dc servomotors, which necessitates converting theac signal to dc. To do this, use a phase detector,

sometimes called a demodulator.

B r i d g e P h a s e De t e c t o r s

Figure 1-12 displays a phase det ector using a

bridge circuit. With no error input signal and onlythe reference voltage applied, CR1 and CR2would conduct in series when point C is on its

positive half-cycle. When point C is on its negativeha lf-cycle, CR3 an d CR4 would condu ct in series.

Assuming the drops across the diodes and

resistances to be equal, points A and B would be

at ground potential on both half-cycles and the

output voltage would be zero.

When an error signal is applied to the bridge

in phase with the referenced voltage and points

A and C are both on their positive half-cycle,

electron flow will be from point G on the referencetransformer T2 to point D, through CR2 to point

A, from point A to the center tap on T1, and to

E th rough to G. On th e next ha lf-cycle, bothpoints A and C will change polarity and the elec-

tr on flow will be from point G to point C, t hr oughCR3 to point B, through T1 to the center tap, to

Figure 1-12.-Br idge phase de tec tor .

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the right to point E, and through RL to G. Onboth half-cycles of reference and error voltage,

the electron flow was down through R~ to ground,deve lop ing a nega t ive dc ou tpu t vo l t age .

If the er ror signal is applied out of phase with

the reference voltage and positive at points A andD, electr on flow will be from point G up th rough

RL, left to the center tap of T1, down to pointB, through CR4, down to point D, and left to

point G. On the next half-cycle, both points A

and D will have G up through RL to the centertap of Tl, up to point A, through CR1 to point

C, and right t o the center tap to point G. On bothhalf-cycles of the error and reference voltages,

electron flow was up through RL, developing apositive voltage output at point E. The magnitudeof the dc produced at point E in both instances

was dependent on the amplitude of the ac error

signa l, and th e polar ity of the dc was dependent

on t he ph ase of the a c error signal. CL is used tof i l t e r t he pu l ses and p rov ide sm ooth dc .

T r i o d e P h a s e D e t e c t o r s

A phase detector that uses npn transistors andalso provides amplification of the error signal

in addition to phase detection is depicted in

figure 1-13. In this circuit, the collectors of the

transistors are supplied with the ac reference

voltage in such a manner that the collector

voltages are in phase. For the purpose of explana-

tion, assume that no error signal is present at T2.When the collectors of Q1 and Q2 are positive,

th e two tra nsist ors condu ct equa lly. The collector

current that flows sets up magnetic fields in the

dc motor exciter windings that are equal and

opposite; th erefore, th e fields cancel and produce

Figu re 1-13.-Triode pha se detector .

no output. When the collector voltages are on a

negat ive ha lf cycle, C1 and C2 discharge throughtheir respective exciter windings to maintain a

constant direct current through the windings.I f an er ror s ignal i s in t roduced in to the

primary of T2 with a phase relationship that

causes th e base of Q1 to be positive at t he sa me

instant that the collector of Q2 is positive, thefollowing conditions exist:

1. On this half cycle the conduction of Q1 is

increased above its no-error signal condition.

2. The heavier collector current causes a

stronger field to be created in the upper exciterwinding.

3. At t his same instan t, since the base of Q2

is on a negat ive half cycle, its aver age condu ction

is reduced to a level below that of its no-error

signal condition.

4. The lower level of collector current causes

a wea ker field to be produced in t he lower exciterwinding.

5. Since the magnetic fields produced in theexciter windings are no longer of equal amplitude,they no longer cancel each other.

6. The exciter produces an output voltage of

a polari ty controlled by the polari ty of the

resulta nt field a nd of an amplitude controlled by

the relative strength of this resultant field.

7. The exciter output causes the proper

mechanical actions necessary to reduce the

amplitude of the error to zero.

8. As the error signal is reduced to zero, the

current conduction through Q1 and Q2 is againbalanced. Also, the exciter fields are equal and

opposite, canceling each other, reducing the

e x c i t e r o u t p u t t o z e r o , a n d s t o p p i n g t h e

mechanical action. Resistors R1 and R2 prevent

excessive base current when the error angle is

large.

SPECIAL CIRCUITS

I t has been shown how a servo control

amplifier may have provisions for changing a dc

error signal to an ac signal, an d how an ac errorsignal may be detected to supply a dc voltage to

a servomotor or controller. In the following

paragraphs, other special amplifier circuits are

discussed.

T w o - S t a g e D C S e r v o C o n t r o l Am p l i fi e r

If somewhat more power is required by the

servomotor than can be supplied by the servo

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amplifier (fig. 1-13), a push-pull dc amplifier can

be inserted between the phase-sensitive transistorsand the servomotor. In the schematic diagram(fig. 1-14), the output of the phase detector

tran sistors is now tak en across the par allel RC net-works in the collector circuit.

The bias source, Ecc, for the dc amplifier is

connected with its positive term inal on t he ba seside. This positive voltage subtracts from the

highly negative voltage across the capacitor to givea resulting negative voltage, which allows the

transistor to operate on the linear portion of its

characteristic curve.When there is no signal input from the error

detector, the collector currents of the phase-

sensitive rectifiers are equal. The outputs of Q1

and Q2 are applied to the base of Q3 and Q4,

respectively. Equal output from Q1 and Q2 causesequal curren ts t o flow in Q3 an d Q4. With R5 a nd

R6 equal in resistance and current, the voltage

across th e motor is zero. Consequ ent ly, th e motordoes not turn.

For an analysis of a signal output from the

error detector, assume that the error signal makesthe base of Q1 positive and the base of Q2

negative. The collector current of Q1 increases and

the collector current of Q2 decreases. An in-

creasing collector current in Q1 increases thecharge on capacitor C1; conversely, a decreasing

collector current in Q2 decreases the charge on

capacitor C2. As a result of the change in error

signal, th e voltage on th e base of Q3 is now more

negative tha n t he volta ge on t he base of Q4. This

increased negative voltage on the base of Q3decreases its collector current and the voltage e3

decreases. The decreased negative voltage on thebase of Q4 increases its collector curr ent , and t he

vol tage e4 increases . As a resul t , a vol tage

difference appears across the motor armature andthe motor rotates.

When the output signal from the error detectorreverses in phase, the sequence of events that

follow causes the motor to reverse its direction

of rotation.

Magne t i c Ampl i f i e r s a s

Servo Con t ro l Ampl i f i e r s

The servomotor used in conjunction with the

magnetic amplifier shown in figure 1-15 is an ac

type. The uncontrolled phase may be connected

in par allel with tr ansformer T1 by using a ph ase

shifting capacitor, or it may be connected to a

different phase of a multiphase system. The

controlled phase is energized by the magnetic

amplifier, and its phase relationship is determinedby the polarity of the dc error voltage.

Figure 1-15.-M a g n e t i c a mp l i f i e r s e r v o c o n t r o l a mp l i f i e r .

Figure 1-14.-Two-stage dc servo contr ol amplif ier .

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T h e m a g n e t i c a m p l i f i e r c o n s i s t s o f a

tra nsformer (Tl) and two satu rable rea ctors, each

having three windings. Note that the dc bias

current flows through a winding of each reactor,

an d th e windings are conn ected in series-aiding.

This bias current is supplied by a dc bias power

source. A dc error current also flows through a

winding in each r eactor; however, these windin gs

are connected in series-opposing.

T he r eac to r s ZI and Z2 are equal ly andpartially saturated by the dc bias current when nodc error signa l is applied. The r eactan ce ofZ, andZz are now equal, resulting in points B and Dbeing at equal potential. There is no current flow

through the controlled phase winding.If an error signal is applied, causing the

current to further saturate Z2, the reactance ofits ac winding is decreased. This current thr oughZI ten ds t o can cel th e effect of the dc bias curr entand increase the reactance of its ac winding.

Within the operating limits of the circuit, thechange in r eac tance i s p ropor t iona l t o the

amplitude of the error signal. Hence, point D is

now effectively connected to point C, causing

motor rotation. Reversing the polarity of the errorsignal causes the direction of rotation to reverse,

since point D is effectively connected to point A.

The basic magnetic servo amplifier discussed

above has a response of approximately 6 to 20

Hz. In some applications, this delay would be

excessive, crea ting t oo mu ch err or. However, th isdelay can be reduced to about 1 Hz by using

special push-pull circuits.

Ampl i f i e r I n t eg ra to r

A servo system in a steady-state condition willha ve a const ant positiona l displacement between

input and output, which is called the error. The

only way to reduce this error is to increase

the drive torque. Thus, a new signal must be

introduced that is related to the error. The error

i s no t chang ing ; t he re fo re , i t canno t be a

derivative signal, nor can it be proportional to theerror, becau se it would th en decrease as t he err or

decreases and a new condition would be metwithout removing th e error. The only altern ative

is to produce a signal proportional to th e integral

of the error. Then, if a torque proportional to thetime integral of the er ror is added to the normal

torque th at is proportional to the err or, the error

will eventually be reduced to zero. A circuit that

is used for this purpose is called an amplifier

in tegrator .

A simple and commonly used integrator

consists of two circuit elements: a resistor and

capacitor. (See fig. 1-16. ) The voltage across the

capacitor is proportional to the integral of the

charging current. It can be explained by consider-ing that the voltage across a capacitor is

For any given capacitor (C), the voltage depends

directly on t he cha rge (Q), which is t he imba lan ce

of electrons on the two capacitor plates. The

amount of this charge depends on the current flowand the time that this flow exists.

Because the voltage is proportional to the

integral of the charging current, it allows the

RC circuit to be used as an integrator output.

Provision must be made to supply a charging

curr ent t hat is proportional to the input informa -tion. The purpose of the resistor is to produce thisproportional current from a n inpu t signal voltage(ei). At the instant this voltage is applied, thecharging current becomes

U nfor tuna tely, th i s p ropor t iona l i ty does no t

continue to exist . As the capacitor becomescharged, the capacitor voltage opposes the

charging cur ren t , and the cha rg ing cur ren t

becomes less proportional to the input signal. Thisresults in an error in the output. The ideal out-

put for a constant input signal is a steadily

increasing output. This steady increase is attained

only when t he signal voltage is first applied an d

the capacitor has not become appreciably charged.

Figure 1-16.-S imple in t egra tor .

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A remedy to this error in the RC integrator

is to use a circuit with a long time const an t. Such

a circuit delays th e char ging of the capacitor. The

result is a more accurate integration of an input

signal . The ideal output would be a perfect

triangular wave. Although a long time constant

produces more accurate results, it also provides

a much lower output for the same input signal,

Better int egrat ion is possible by the u se of a high

gain, feedback amplifier.

An amplifier integrator is i l lustrated in

figure 1-17. The c i rcui t ar rangement uses a

high gain amplifier and is known as the Miller

integrator. The amplif ier produces an output thatis not limited by the input signal as it is in the

simple RC int egrat or. The a mplifier also supplies

any energy that is required in the output. The

function of the input signal is to control the

charging current.

The operation can be explained by assuminga consta nt inpu t, as shown in figur e 1-17, view A.

At t he st art , assum e th e initial condition is zero,

that is,

Also assume t hat the capacitor is dischar ged. The

posit ive voltage to be integrated, ei , is thenapplied. The capacitor charges with a polarity as

shown, since electrons are attracted from the left

plate. The cha rging path is shown in figure 1-17,

view B.

A voltage measu red at the a mplifier input, eg ,tends to rise in the positive direction since this

point is dir ectly coupled to e i. However, this riseten ds to be opposed by the degener at ive feedback

voltage from the output. The output will be

Aeg(eO). The letter A stands for the amplifiergain. The minus sign indicates that the output

polarity or phase is opposite to the input. The out-put changes A t imes faster or steeper than eg. Th eoutput voltage is negative and aids the charging

of the capacitor.

For a certain input voltage, the charging

curr ent is limited to a particular value tha t tends

to keep eg practically zero. If the current shouldexceed this value, eg would decrease a smallamount due to th e increased voltage drop across

R. The eO would decrease, and the chargingcurrent would decrease to the original value. If

the initial charging current should decrease, the

opposite action would occur. The value of the

charging curr ent is th erefore s tabilized t o a specificvalue proportional to the input voltage. This

eliminates the error caused by ei and the chargingcur ren t no t r em ain ing p ropor t iona l i n the

fundamental RC integrator.

This constant charging current must beproduced by eO despite the fact that the steadilyincreasin g capa citor volta ge opposes the char ging

current. To do this, eO must also steadily increase.This steady increase in eO is exactly the integratoroutput voltage desired for a constant signal input.

Similar action would be produced for a

condition in which the input signal suddenly

became negative. Polarities would then be inreverse to those shown in the example given.

Remember that simple examples are used forexplanation on the assumption that the desired

result will also be produced for a more com-

plicated signal input. Removal of e i w ouldproduce little effect u pon t he outpu t t hat existed

at that instant, since the amplifier output would

oppose the t endency fo r t he capac i to r t o

discharge.

The limits for eO are determined by theamplifier and not by ei or the range of eg . Th eout put ra nge would be designed to produce an in-

creasing output for any probable input amplitudeand period of application. The exception to this

would be an integrator that was designed to

function also as a limiter.

OUTPUT DEVICES

Figu re 1-17.-Amplif ier integr ator .

The output of the servo control amplifier is

fed to an output device. The functions of this

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device, usually a servomotor, are to supply torque,power, and dynamic characteristics required to

position t he s ervo load. Id eally, the p ower d evice

should require small power from the control

am plifier, accelerat e ra pidly, be of sma ll size an dweight, be of lasting endurance, have small time

lags, and have an adequate speed range. In air-

craft weapons syst ems, th e electr ic motor is mostfrequently used as an output device. However,

electromagnetic clutches, hydraulic devices, and

pneumatic devices are also used.

Elec t r i c Moto r s

In aircraft weapons systems, electric motors

are primarily used to drive the servo load. The

type of electric motor used within a particular

equipment is determined by power factors such

as type of power available, output power, speed

range, inertia, and electrical noise.

ALTERNATING-CURRENT MOTORS.

Alternating-current motors are frequently used inlow power servo applications because of theirsparking and rapid response. However, they have

a disadvantage of having a narrow speed rangecharacteristic. The theory of operation of ac

motors is discussed in Navy Electrici ty and

Electronics Training Series (NEETS), module 5.

We briefly discuss the types of motors used with

servo systems in this chapter.

The two-phase induction motor is the most

widely used ac servomotor. The stator of the

motor consists of two similar windings that arepositioned at right angles to each other. The rotormay be wound with short-circuited turns of wire

or it may be a squirrel cage rotor. The squirrel

cage rotor is the type most frequently en-

countered. It is made up of heavy conductingbars, which are set into armature slots, the bars

being shorted by conducting rings at the ends.

Two ac voltages 90 degrees out of pha se mu st

bc applied to the stator windings for the motor

to turn. These out-of-phase voltages generate a

rotating magnetic field, which induces a voltage

in the rotor. This induced voltage generates a

magnetic field in the rotor that is displaced 90

degrees from the stator magnetic f ield. Theinteraction ofthese two magnetic fields causes thearmature to rotate.

As stated previously, the voltage to the two

stat or windings must be 90 degrees out of phase

to cause the rotor to turn. The direction of rota-tion is determined by the phase relationship of thestat or windings, which, in t urn , is determined by

the servo error detector. One phase is connected

directly to one of the stator windings while the

other pha se is used to energize an err or det ector.The resulting error voltage is either in phase or

180 degrees out of pha se with t he signal a pplied

to the error detector. This will cause the controlledphase to either lead or lag the uncontrolled phase

by 90 degrees.Most induction motors have low start ing

torque and high torque at high speed. For servo

applications, it is desirable to have high st art ing

torque so that the system may have a low timelag. This may be accomplished by increasing the

arm atu re resistance with t he use of mat erials suchas zinc for the conducting bars. This increasedtorque at low speed results in decreased torque

at high speed. However, increased st ability of th eservo system is a desirable result of this change.

Split-phase ac motors are similar t o the two-

phase induction motor. It differs only in that a

phase sh ifting network is used to shift t he phaseof th e volta ge supp lied to one of the wind ings by90 degrees. This is usually accomplished b yconnecting a capacitor in series with the un-

controlled winding of the stator. Direction of rota-

tion and reversal is accomplished in the samemanner as in the two-phase motor discussed

above.

Other t ypes of motors tha t ma y be used withan ac power supply are shaded pole, universal,

and repulsion motors. They use var ious methods

of obtaining rotation reversal. However, they are

seldom found in aircraft weapons systems.

DIRECT-CURRENT MOTORS. Direct-

current motors have an advantage of havinghigher sta rting t orque, reversing torque, and less

weight for equal power than ac motors.

Series motors ar e chara cterized by their highstar ting torque an d poor speed regulat ion with a

chan ge in torque. Higher torque can be obtained

on reversal of direction with a series motor than

any other type. However, it is a unidirectionalmotor and requires special switching circuits to

obtain bidirectional characteristics. This is nor-

mally done by switching either the armature orfield connections, but not both.

A var ia t ion of the ser ies motor that has

bidirectional characteristics is the split-series

motor. The motor has two field windings on its

frame, only one of which is used for each direc-

tion of rotation. This reduces the number of

relay contacts required for reversing by one-half.

This double winding also reduces the torque

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capabil i t ies of the motor as compared to a

straight-series motor wound on the same frame.

The most frequently used dc servomotor is theshunt motor. Its direction of motion is controlled

by varying the direction of flow of either the

armature or f ield current. The uncontrolled

current is usually maintained constant to preserve

a linear relationship between the motor outputtorque an d th e voltage or curr ent input. The fieldwindings are usually two differentially wound

coils to aid in direction control of the field currentby the servo control amplifier. The field current

is usually controlled with receiving type vacuum

tubes . The larger armature currents require

thyratrons or generators as current regulators, but

are not normally found in aircraft weapons

systems.

Magne t i c C lu tches

Any device using an electr ical signal th at ma y

be used to control the coupling of torque from

an input shaft to an output shaft is a magnetic

clut ch. This coupling m ay be a ccomplished by th e

cont act between friction sur faces or by the a ction

of one or more ma gnetic fields. A magnet ic clutchis used only to couple the input torque to th e out -

put shaft. Thus, it is capable of controlling large

amounts of power and torque for its size andweight. The magnetic clutch may be used with a

large flywheel driven at high speed by a small

motor. This allows the flywheel to impart very

large acceleration t o the load when the magnet icclutch is energized.

There are two distinct types of magnetic

clutches. Some transmit torque by physical con-

tact of frictional surfaces. Others use the action

of magnetic flux produced by two sets of coils,

or one set of coils and induced eddy currents

resulting from rotating the one set of coils near

a conducting surface. The eddy current type of

clutch offers smoother operation and has no

problem of wear because of friction. Both types

have suitable control characteristics and are foundin servomechanisms.

HYDRAULIC DEVICES

Hydrau lic components used in servomechan-

isms are frequently found in aircraft weapons

systems. Hydraulic power devices, such as motorsand associated control valves, have the advantageof a response much faster than the best electric

motors and equal to that of a magnetic clutch

system. They also require a minimum of main-

tenance, have very high accuracy, and are adaptedto heavy loads.

The essential components of a hydraulic

system are as follows:

1. A source of high-pressure oil and sump to

receive discharge oil2. A control valve and means of employingan actuating signal

3. An actuator (motor or cylinder)

The theory of operation of a hydraulic system is

discussed in Fluid Power, NAVEDTRA 16193

(series).

The source of high-pressure oil serves as a

source of power to operate the actuator. However,this source of power is controlled by the controlvalve. This valve is actuated by the output from

the servo control amplifier. This control is nor-ma lly accomplished by feeding th e err or signal to

a solenoid-controlled valve. However, the error

signal could be used to drive an electric motor,

which, in turn, would actuate the control valve.

The actuator is usually in the form of an axial

motor, which must be a reversible and variable

speed type. Some applications may employ a

cylinder where linear motion is required forpositioning.

SERVOMECHANISM OSCILLATION

Learning Object ive: Iden t i f y fac tors

affecting servomechanism oscillations to

include damping, integral control, and the

relationship of gain, phase, and balance.

In aircraft weapons systems, servomechanismsare used for various functions and must meet

certain performance requirements. These require-ments not only concern such things as speed of

response and accura cy, but the m ann er in which

the system r esponds in carr ying out its comma nd

function. All systems contain certain errors; the

problem is keeping them within allowable limits.

As discussed pr eviously, th e servomotor mus t

be capable of developing sufficient torque and

power to position th e load in a minim um of time.The servomotor and its connected load have

sufficient iner tia t o drive the load past the point

of command position. This overshooting results

in an opposite error voltage, reversing the

direction of rotation of the servomotor and the

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21/537

load. The servomotor again attempts to correct

the error, and again overshoots the point of

correspondence, with each reversal requiring less

correction until the system is in correspondence.The time required for the oscillations to die out

determines the transient response of the system

and can be greatly reduced by the use of damping.

DAMPING

The function of damping is to reduce the

amplitude and duration of the oscillations that

may exist in the system. The simplest form of

dam ping is viscous dam ping. Viscous dam ping is

the application of friction to the output load or

shaft t ha t is pr oportional to th e output velocity.

The amount of friction applied to the system is

critical and will materially affect the results of thesystem. When just enough friction to prevent

overshoot is applied, the system is said to be

critically damped. When the friction is greaterthan that needed for cr i t ica l damping, thesystem is over-damped. However, when damping

is slightly less than critical, the system is said to

be slightly underdamped, which is usually the

desired condition. The application of friction

absorbs power from the motor and is dissipated

in the form of heat.

A pure viscous damper would absorb an

excessive amount of power from the system.However, a system having some of the char-

acteristics of a viscous damper with somewhat lesspower loss is used in actual practice. The first of

this type of damper to be discussed uses a dryfriction clutch to couple a weighted flywheel to

the output dr ive shaf t . A f lywheel has the

property of inertia, which maybe defined a s t hat

property of matt er by which it will remain at rest

or in uniform motion in the sa me str aight line ordirection unless acted upon by some external

force.

Since the flywheel is coupled to the output

shaft with a friction clutch, any rapid change in

velocity of the output member causes the clutch

to slip. This slipping effectively disconnects the

flywheel, instantaneously, but allows sufficient

power to be coupled to the flywheel to overcome

its inertia. As the inertia is gradually overcome,

the flywheel gains speed and approaches thevelocity of the output member. As the point of

correspondence is neared a nd t he err or signal is

reduced, the inertia of the flywheel gives up powerto the system, causing the load to increase its over-shoot. When the system attempts to correct for

this overshoot, the inertia of the flywheel adds

to the output load, reducing the effect of the

corr ecting signal. The effect dam pens th e oscilla-

tions in the system, reducing its transit time.

Another type of damper used is the eddy

curren t dam per. This damper uses th e interactionof induced eddy currents and a permanent magnetfield to couple the output shaft to a weighted

flywheel.The effect of dam ping is s hown in figure 1-18.

The solid line shows the action of the load without

damping. The time required to reach a steady-

state condition without damping should be noted.This time is greatly reduced alth ough t he initial

overshoot is increased.

As shown in figure 1-18, a viscous damper

effectively reduces transient oscillations, but it

also produces an undesired steady-state error.

How well the load is controlled is a measure

of the steady-state performance of a servo system.If the load is moved to an exact given position,

then the servo system is said to have perfectsteady-state performance. If the load is not moved

to an exact position, then the system is not perfectand the difference in error is expressed as thesteady-state error. Steady-state error may be eithervelocity la g or position err or. Velocity er ror is th e

steady-state error due to viscous drag during

velocity opera tion. Position err or is th e differen ce

in position between the load and the position

order given to th e servo system. Since the friction

damper absorbs power from the system, its use

is normally limited to small servomechanisms.To overcome t he disa dvan ta ges of th e viscous

dampers and still provide damping, error-ratedamping is used. This type of damping consists

of introducing a voltage that is proportional to

the rate of change of the error signal. This voltage

is fed to the ser vo contr ol am plifier a nd combined

with the error signal. Figure 1-19 shows the

Figu re 1-18.-Effect of friction damper.

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Figure 1-19.-To r q u e v a r i a t i o n s u s i n g e r r o r - r a t e d a mp i n g .

effect of error-rate damping on the torque out-

put of the servomotor. Curve A shows the torqueresulting from the error voltage, curve B shows

the t orque resulting from the err or-rate dam per,an d curve C depicts t he resu ltan t of curves A an d

B. It should be noted that torque resulting from

the damper increases the total torque as long asth e error component is increas ing. Once the err orcomponent starts to decrease, the error-rate

dam per p roduces a to rque in an oppos i t e

direction, reducing the transit time of the system.

There are two methods of generating an error-rate voltage normally found in aircraft weapons

systemstachometer and electrical networks. The

tachometer error-rat e damper u ses a device tha t

is essentially a generator having an output voltageproportional to its shaft speed. The tachometer

is connected to the shaft of the output member,giving a voltage proportional to its speed. The out-put voltage is fed to a network that modifies thisvoltage so that it is proportional to a change in

input voltage. This voltage is fed back to the servo

control amplifier and added to the error signal,

as shown in figure 1-19.

Electrical networks used for error-rate damping

consis t of a combinat ion of res is tors and

capacitors used to form an RC differentiatingnetwork. For a detailed explanation of RC

circuits, r efer to Na vy Electr icity a nd Electr onicsTraining Series (NEETS), module 2, NAVED-

TRA 172-02-00-85. These networks, sometimes

referred to as phase advance or lead networks,vary in design, depending on the type of error

signal. H owever , in pr a ct ice , n et wor k s a r enorma lly limited to the dc type (fig. 1-20) because

Figure 1-20.-E r r o r - r a t e s t a b i l i z a t i o n n e t wo r k .

of the unstable results that would be caused bya s mall chan ge in frequency of the power source.

An ac system m ay use a dc network by first usinga demodulator (detector) prior to the network.However, the output of the network must be

modulated for use in the remainder of the ac

system. Like the tachometer, the output of the

network is fed to the servo control amplifier.

INTEGRAL CONTROL

Servomechanisms used in aircraft weapons

systems are sometimes required to follow an in-

put function, the magnitude of which changes at

a constant rate with time, such as an antenna

system tracking a tar get. Thus, if the input is the

angle of a shaft, the velocity of the shaft may beconstant for a substantial percentage of time. The

servomechanism may be required to respond tothis type of input with substantially zero error.The error that characterizes the servo response to

a constan t velocity input is known a s th e velocityerror.

To correct for velocity error or an inaccuracy

due to a steady-state error, an integral control maybe used. This control modifies the error voltage

in such a mann er tha t th e signal fed to the servo

control amplifier is a function of both theamplitude an d time dur ation of the error signal.This is accomplished by the use of a variablevoltage divider, whose output is increased with

time for a constant input. As in al l voltage

dividers, the output is only a portion of the in-

put that effectively reduces the amplitude of theerror signal. To compensate for the loss of

amplitude, additional amplification must be usedeither in the form of a preamplifier or a highergain servo control amplifier. With the overall gainof th e system n ow increased t o give a norma l out -put for transient error signals, small velocity or

steady-state error signals of long duration will

resul t in somewhat increased output to the

servomotor due to the action of the integral

control.

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The integral control (fig. 1-21) consists of a

combination of resistors and capacitors connected

to make an integrator circuit for a dc error signal.The value of the components are such that the

capacitor does not have sufficient time to changewith fluctuations in error voltage. Only that

portion of the transient error signal developed

across R1 is impressed on the amplifier. However,with a velocity error or steady-state error of longerduration, the capacitor (C1) charges, increasing

the amplitude of the amplifier input.

Networks shown in figure 1-21 are not limited

to dc systems, as a demodulator maybe used priorto the integrator and its output modulated for

easier amplification.

GAIN, PHASE, AND BALANCE

The overall system gain h as a most importa nt

e f f e c t o n t h e s e r v o m e c h a n i s m r e s p o n s e

characteristics and is one of the more easily

ad jus t ab le pa ram ete r s in e l ec t ron ics se rvocontrollers. Increasing the system gain reduces the

Figure 1-21 .- I n t e g r a l s t a b i l i z a t i o n n e t wo r k .

system velocity errors and those steady-state

errors resulting from restraining torques on the

servo load or misalignment in the system. An

increase in system gain a lso increases t he speed

of response to transient inputs. Excessive gain

always decreases the rate at which oscillatory

transients disappear. Continued increase in the

system gain eventually produces instability.Servo systems using push-pull amplifiers must

be balanced to ensure equal torque in both

directions of the servomotor. This adjustment

should be checked periodically as a change in

value of a component may cause an unbalanced

output. Balancing is accomplished by adjusting

the system for zero output with no signal applied.A phase control is included in some servo

systems u sing ac motors. The t wo windings of th e

ac servomotor must be energized by ac signals thatare 90 degrees apart. A phasing adjustment is

normally included in the system to compensate

for any phase shift in the amplifier circuit ,resulting in unstable operation of the system. Thisad jus tm en t m ay be loca ted in the con t ro l

am plifier, or in t he case of a sp lit-phase m otor, it

may be in the uncontrolled winding.

ZEROING SYNCHRO UNITS

Learning Objective: Recognize the impor-

tan ce of zeroing tran sm ittin g and receiving

synchro units.

In this chapter, we have stressed the impor-

tance of accuracy with servomechanisms. In any

Figure 1-22.-S y n c h r o e l e c t r i c a l z e r o p o s i t i o n s .

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servomechanism using synchro units, it is also veryimportant that the units be zeroed electrically

(fig. 1-22, view A).

For a synchro tr ansm itter or r eceiver t o be in

a position of electrical zero, the rotor must be

aligned with S2, the voltage between S1 a nd S3

must be zero, and t he ph ase of the volta ge at S2

must be the same a s the pha se of the voltage at R1.The most common m eth ods of zeroing synchro

transmitters and receivers are the ac voltmetermethod and the electrical lock method. The

meth od used to zero a synchro depends upon h ow

the synchro is used. Where the rotor is free to

turn, the electrical lock method can be used. This

is accomplished by connecting S1 and S3 to R2

using a jumper wire and connecting S2 to R1(fig. 1-23). When power is applied, the rotor will

position itself in the zero position. After the

synchro is zeroed, the pointer is adjusted to

indicate zero.

T he g rea t m a jo r i ty o f synchros used inaviation systems have t heir rotors gear dr iven or

mechanically coupled to a driving member. In

these cases it is n ecessary t o use th e ac voltmetermethod, zeroing the synchro by rotating the statoror housing until its electrical zero is reached.

Before you zero the synchro, the mechanical unit

that positions the synchro must be set to its

indexing or ZERO position. This is done by

aligning the unit to its index and installing its

indexing pins in the holes provided for this

purpose. The pins hold the unit to its index and

keep it from moving.

The ac voltmeter method is done by connectingthe meter and jumper wires, as shown in fig-ure 1-24, view A. Rotate the energized synchrount il a zero reading is obta ined on t he voltmeter .

Since rotor positions of 0 degree a nd 180 degrees

produce this zero reading, it is necessary to

Figure 1-23.-E l e c t r i c a l lo c k me t h o d o f ze r o i n g a s y n c h r o .

Figure 1-24.-Ac v o l t me t e r me t h o d o f e l e c t r i c a l l y z e r o i n gsynchro r ece ive r o r t r ansm i t t e r .

determine if the phase of S2 is the same as thatof R1. Make the connections as shown in figure

1-24, view B. If the proper polarity relationshipexists, the voltmeter indicates less than the

excitation voltage being applied to the rotor. If

the indication is greater tha n t he rotor excitation

voltage, the rotor (or stator) must be rotated

180 degrees and

performed again.

D IF F E R E N T IA L

the previous s tep must be

TRANSMITTER

zero position of a synchroThe electrical

differential transmitter or receiver is when the

thr ee windings of the r otor are in correspondencewith their respective stator windings and their

respective voltages are in phase (fig. 1-22, view B).Because the differential transmitter synchro

is normally used to insert a correction into a

synchro system, it is usually driven either directlyor through a gear train. Before you zero the

differential transmitter synchro, the unit whose

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position the differential synchro transmits shouldfirst be zeroed. After th is has been a ccomplished,

connect the differential synchro as shown infigure 1-25, view A. Turn the synchro in its

mounting until the voltmeter shows a minimum

indication. After you complete this step, make the

conn ections shown in figure 1-25, view B. Again,

turn the synchro slightly in its mounting until aminimum voltage is indicated by the voltmeter.

DIFFERENTIAL RECEIVER

Electrical zero for a differential receiver is

illustrated in figure 1-22, view B. To zero a

differential receiver synchro, make the connec-

tions shown in figure 1-26. As soon as the power

is applied to the synchro, the rotor assumes a

position of electrical zero. The dial can then be

set at zero, and the unit reconnected to its circuit.

CONTROL TRANSFORMER

The synchro control transformer is normally

zeroed by using the ac voltmeter method. You

Figure 1-25.-E lect r ical ly zeroing a di f ferent ia l t ransmit ter .

Figure 1-26.-Electr ically zeroingreceiver .

a d i f f e r e n t i a l s y n c h r o

should remember tha t the electrical zero position

of th e contr ol tr an sform er is 90 degrees from th at

of a receiver, since the rotor winding must be

perpendicular to the stat ors resu lting magnetic

field to have a zer o out put (fig. 1-22, view C). The

coarse adjustment is made by connecting the

meter and u nit as shown in figure 1-27, view A.

The rotor is rotated to give a minimum or nullreading on the voltmeter. The final adjustment

is made by conn ecting th e unit as sh own in figure

1-27, view B, and displacing the rotor a few

degrees in both directions to determine the null

or electrical zero position. On ce th e zero positionha s been determined, the u nit mu st be locked, as

discussed previously.

SYNCHRO ALIGNMENT SET

TS-714/U

Learning Objective: Recall the purpose anduse of the synchro alignment set.

T h e S y n c h r o A l i g n m e n t S e t T S - 7 1 4 / U

(fig. 1-28) is a porta ble, general-pur pose test set

used to check the al ignment of synchros orresolvers. It can be used to align any 400 Hz

synchro or resolver. In addition to its higher

sensitivity, the test set has an additional advantage

over the methods previously discussed because thetest set can also supply excitation voltage for the

synchro or resolver being aligned.

The test set (fig. 1-28) basically consists of abandpa ss am plifier an d power su pply, a synchro

Figure 1-27.-Electr ically zeroing a control tr ansformersynchro .

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222.15

Figur e 1-28.-Synchro Alignment Set TS-714/U front panel.

or reso lver exc i ta t ion supply wi th ou tpu ts

from 3 to 115 volts rms (1) and switching

circuits. The output voltages from the synchros

or resolvers are applied to the amplifier, the

output of which is fed to a phase sensi t ive

detector circuit. The detectors outpu t is met eredby the microammeter (2). A meter switch (3)

selects the meter sensitivity from 300 volts full

scale to 0.1 volt full scale,

The meter has a ZERO center scale and

indicates 0 when the synchro or resolver is

adjusted to either of its two nulls. The synchro

or resolver is adjusted to a null position with

the function switch (4) in the ZERO position.When the null is reached, the function switch

is switched to the POL position and a reading

is taken from the meter . Then the funct ion

switch is returned to the ZERO position and

t h e s y n c h r o i s r o t a t e d 1 8 0 d e g r e e s t o i t s

opposite nu ll. When t he opposite nu ll is reached,

the function switch is again switched to the POL

position and a note made of the reading. The

corr ect nu ll will be the one indicat ing th e lowestreading with the function switch in the POL

position. When the synchro is adjusted to this

null, it is electrically zeroed with the correct

polarity.

For detailed instructions on the use of the

TS-714/U test set, consult Operation and Service

Instruction Manual, NA 11-70-FAG-510.

ANTENNA POSITIONING

SERVO SYSTEM

Learn ing Objec t ive : Explain the pro-

cedures for the application of servo-

m echanism s to include positioning a radar

antenna and supplying information to the

weapons system.

In this section, the application of a servo-mechanism t o posi t ion a radar antenna and supplytarget information to the weapons system is

discussed. However, before discussing the servo

system, consider the scan pattern of a typical

aviation fire control radar.

The antenna radiator and reflector form a

conical pattern of circular symmetry with beam

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dimensions, as shown in figure 1-29. The antennaassembly contains a spinner motor that rotatesthe beam about the antenna axis to produce

a 7-degree conical scan. While the radar isin the search mode of operation, the rotating

cone scans both horizontally and vert ically,

covering an area of 10 degrees vertically by

90 degrees horizontally (fig. 1-30). The searchpattern may be posit ioned vert ically from a

positive 30 degrees to a negative 30 degrees by theantenna positioning level.

The operator normally observes the targets,

identifies each as friend or foe, and determineswhich t arget, if any, to pursue. Since the a nten na

uses its 7-degree conical pattern only during trackoperation, some means must be provided for

positioning the a nten na on th e selected ta rget to

begin the track operation. This is accomplished

by bracketing the selected target with strobe lines.

When the target has been selected and bracketed,a lock-on switch is depressed, positioning theantenna on the predetermined target, and placing

the equipment in the automatic track mode of

operat ion. The a nten na is now positioned by th e

radar receiver output, keeping the target centeredin the 7-degree beam.

A block diagram of a typical fire control

antenna servo system is shown in figure 1-31. It

should be noted that the azimuth channel of the

antenna control system has been omitted, as its

operation is similar to the elevation channel.

Since the antenna servo system uses differentcomponents during search and track operation,

the system used in each mode of operation is

discussed separa tely.

Figure 1-30.-T y p i c a l a n t e n n a s c a n p a t t e r n .

SEARCH OPERATION

The main components of the antenna servo

system used during a search operation are as

follows:

1. Error detector and its ac voltage source

2. Servo amplifier

3. Servomotor

4. Data transmission system

The ac generator supplies voltage to the in-

put an d feedback potentiometers of the balan ced

potentiometer error detector. However, thevoltage fed to the input potentiometer is fed

through a gyro space stabilizer and scan generator.The function of the gyro space stabil izer

is to cause the antenna to scan a selected area90 degrees horizontally and 10 degrees vertically,

222.16

Figu re 1-29.-Anten na bea m with con ical scan.

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Fi gur e 1 -31 .-An t e n n a s e l e v a t i o n s e r v o s y s t e m f u n c t i on a l b l o c k d i a g r a m .

regardless of any roll or pitch of the aircraft. As

in all fire control equipment of this type, the

amount of correction that can be made by the

gyro space stabilizer is limited by the limits of theradar scanner. The output of the gyro space

sta bilizer is an ac voltage, th e am plitude of which

is a function of the roll and pitch of the aircraft.The pr inciples of opera tion of gyros a re discussedlater in this manual.

The function of the vertical scan generator isto automatically position the antenna in thevertical geometric plane. Refer to figure 1-30.

Note that the antenna scans horizontally and

vertically. The scan generator provides the

necessary voltage change to cause t he a ntenn a t o

change its angle of elevation by 3 degrees when

the antenna reaches its azimuth limits.The error detector has three inputs that are

summed an d compar ed against th e ant ennasposit ion. The gyro space stabil izer and scan

genera tor const itut e two inpu ts by controlling th e

amplitude of the voltage supplied to the inputpotentiometer. The third input is the control

handle, which positions the wiper contact of theinput potentiometer. The output of the error

detector is an ac voltage, whose amplitude andphase is determined by the voltages on the wipers

of the potentiometers.

The error signal is fed to the servo amplifier,where it is amplified and compared with the phase

of the reference voltage. The phase of the outputvoltage causes the servomotor to rotate in a

direction reducing the error voltage.T h e d a t a t r a n s m i s s i o n s y s t e m i s t h e

mechanical linkage necessary to drive the wiper

of the feedback potentiometer, indicating the

actual position of the antenna in the vertical planeat all times.

TRACK OPERATION

The main components of the servo systememployed during track operation are as follows:

1, Radar receiver and 50-Hz amplifier

2. Servo amplifier

3. Servomotor

4. 50-Hz spin generator

The radar receiver functions as the error

detector, sup plying a 50-Hz err or volta ge. Before

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Figu re 1-32.-Derivation of e levation err or s ignal .

222.20

Figu r e 1-33.-(A) S e r v o s y s t e m s c h e m a t i c d i a g r a m .

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discussing the other components of the system,

first we must determine how the receiver providesthe error signal. As stated previously, the antennaaxis is centered approximately on a target prior

to going into track operation. The antenna isrotating at 50 revolutions per second while the

radar transmitter is transmitting a pulse of energy

450 times per second. When the antenna axis ispointing directly at the target, the target return

and receiver video output remain at a constant

level. However, if the target were above the

antenna axis , as shown in figure 1-32, t h e

amplitude of the video would vary as the a nt ennarotated about its axis. You should note that the

video amplitude is ma ximum when th e beam axis

is at its highest elevation and minimu m when th e

beam axis is at its lowest elevation. The video out-

put from the receiver is filtered, leaving only the

50-Hz envelope to be employed as an errorvoltage.

The function of the servo amplifier is toamplify the 50-Hz error voltage and compare its

phase with the phase of the 50-Hz reference

voltage originating in the 50-Hz spin generator.

The phase of the output voltage to the servomotorcauses the motor to rotate in the direction that

reduces the amplitude of the error signal.

THEORY OF SEARCH OPERATION

The schematic diagram of the antenna servosystem described above is shown in figure 1-33.

As in th e case of th e block diagra m, th e system s

search mode of operation is discussed first.

S c a n G e n e r a t o r

The elevation scan generator is used during

au tomat ic sear ch only. It consist s of two resistors

an d one double-pole relay. Since only one r esistor

is in the circuit at a time, they serve alternately

to unbalance the voltage applied to the error-

detector potentiometer R3. The inpu t t o the scan

generator is an ac voltage with its center pointgrounded by a resistor network. With both RI andR2 shorted, the center of R3 would also be at

222.21

F i g u r e 1 - 3 3 . - ( B) S e r v o s y s t e m s c h e ma t i c d i a g r a m Co n t i n u e d .

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ground potential. Inserting R1 in the circuit wouldcause the center of R3 to be at some potential just

as t hough th e wiper of R3 ha s been moved to the

right. Shorting R1 and insert ing R2 should ha ve

the same effect as moving the wiper of

Documents

US Navy Course NAVEDTRA 14029 - Aviation Electronics Technician-Intermediate