Table I. Integrated Absolute En-or for a Second-Order Servomechanism Subject to a

Step Input of Magnitude Ρ

Ιωο Ρ

0 . 1 6 . 3 8 4 0 . 2 3 . 3 4 1 0 . 3 2 . 3 6 4 0 . 4 1.924 0 . 5 1.712 0 . 6 1 .618 0 . 6 5 1.606 0 . 7 1 .608 0 . 8 1.631 0 . 9 1.804 1.0 2 . 0 0 0

integrated absolute error of a second-order system subjected to a step input becomes

— σθι

(6)

This form of the equation is particularly

useful when ζ approaches unity, where the other equation leads to an indeterminate form. For f = l , equation 6 of the discus­sion gives directly

ωο (7)

which can also be obtained by direct in­tegration of the error for the critically damped case. For Γ > 1 , the integrated absolute error is the same as the integrated error and can be found ^ by adding the time constants of the closed-loop transfer func­tion.

For purposes of comparison and to save future workers some labor, values computed from equation 6 are given in Table I of the di.scussion.

In some experimental studies similar to those described in the paper, we have been using a slightly different method for deter­mining the integral of the error. In our case, a voltage proportional to the error was applied to an integrating amplifier in which the usual input resistor had been re­

placed by a diode rectifier. With this arrangement, only errors of one sign are integrated; the diode characteristics deter­mine whether the absolute or squared error, or some intermediate power of the error, is used. Part of the integral is found by application of a positive signal to the input of the simulated servomechanism, and the remainder is found by applying an equal negative input, which simply means re­moving the previous signal.

This technique, although less elegant than the procedure described in the paper, is useful when the time constants in the computer sinmlation are fairly large, and it eliminates the need for an absolute value unit.

R E F E R E N C E S 1. See reference 9 of t h e pape r .

R. R. Caldwell and V. C. Rideout: We wish to thank Mr. Stout for pointing out the simplifications possible in equation 2 of the paper.

Diesel-Electric Locomotive Ground

Relays

G . R. M C D o n a l d A S S O C I A T E M E M B E R AIEE

Synopsis: The following paper lists and describes the various protective functions of the ground relay as used on diesel-electric locomotives. The connections are de­scribed, and comments are made on the con­trol features initiated by the operation of the relay.

General Functions

TH E USE of ground relays s tar ted in the early 1920's in au tomat ic substa­

tions. These relays were primarily for the purpose of limiting the damage caused by armature breakdown. P rompt oi)era-tion of an instantaneous-type r d a y Hm-ited damage to conductors and ani ia ture stackings. The relay also furnished similar protection for commuta to r flash-overs.

The first use of a ground relay on a diesel-electric locomotive was about 1930. The first relays installed for this purpose were low-resistance high-current devices. In fact, some of these early relays did not have enough resistance to be self-protect­ing on the high currents imposed by flashovers, and as a consequence the re­lays were severely damaged by a flash-over. Higher resistance, higher voltage relays were, therefore, in order. Along

with the tendency in this direction, other miscellaneous functions were en­trusted to the ground relay until now the list of protective features which i t pro­vides is quite imposing. Any discussion of this relay, therefore, involves having a clear concept of these various functions and their interrelations.

The following list covers most of the protective functions now performed by the ground relay.

1. Armature grounds in either the main generator or in the traction motors. 2. Accidental power circuit grounds:

(a) On the positive power circuits. (b) On the power cables which are at one-half of the generator voltage during series-parallel operation. (c) On the negative power circuits.

3. High resistance creepage grounds, such as those caused by moisture and dirt:

(a) On the positive circuits. (b) On the half-voltage circuits.

P a p e r 53-77 , r e c o m m e n d e d b y t h e A I E E L a n d T r a n s p o r t a t i o n C o m m i t t e e a n d a p p r o v e d b y t h e A I E E C o m m i t t e e on Techn ica l O p e r a t i o n s for p r e s e n t a t i o n a t t h e A I E E W i n t e r Gene ra l M e e t i n g , N e w Y o r k , N . Y. , J a n u a r y 1 9 - 2 3 , 1953. M a n u ­scr ip t s u b m i t t e d O c t o b e r 20, 1952; m a d e ava i l ab l e for p r i n t i n g D e c e m b e r 3 , 1952.

G. R . M C D O N A L D is w i th t h e G e n e r a l E l e c t r i c C o m p a n y , E r i e , P a .

(c) On the negative circuits. (d) On the armatures of generator or motors.

4. Flashover protection: (a) Generator armature. (b) Motor armature with motors oper­ating in parallel, (c) Motor armature on the positive side when operating in series parallel. (d) Motor armature on thenegative side when operating in series parallel. (e) Positive side contactor flashover. (f) Negative side contactor flashover. (g) Series contactor flashover (one-half generator voltage).

5. l^rotection during dynamic braking: (a) Motor flashover. (b) (jround in braking resistor. (c) Leakage or moisture ground. (d) Resistor blower motor groimd or flashover.

6. (jround protection during engine start­ing.

A short discussion of each of the pre­ceding j)oints will help to clarify the func­tions of this relay. This discussion is based on an Alco-GE road locomotive. The .ground relay considered has a pickup

GR RELAY

Figure 1 . Ground relay connected on nega-tive side of generator

1 7 0 McDonald—Diesel-Electric Locomotive Ground Relays J U L Y 1 9 5 3

h

)

•c PI •c PI IP

) )

^ 1 0 2 Ό 3 0 4 K ) 5 0 6 0 7 0 6 K } 9 O K D O I 10 IS

GENERATOR F R E O U E N C Y - ( C P S . )

Figure 2. Ground relay pickup with genera­tor armature grounded

of 38 volts with a coil current of 0.056 ampere. T h e coil resistance is 680 ohms. This relay is connected as shown in Figure 1.

ARMATURE GROUND

Consider a ground a t one point on a generator armatiu-e. As the a rma tu re rotates this ground will t ravel from one polarity to the other. A relay connected as shown by Figure 1 will have appHed to it an undulat ing voltage practically the equivalent of a completely displaced a-c wave. T h e relay pickup when operating on this voltage will be as shown by Figure 2. With this connection of the relay the drop through the generator commuta t ing field will modify the actual voltage re­quired by a small amoun t resulting from the drop through this field.

Op^eration in detecting an a n n a t u r e ground for a parallel connected motor is identical with t h a t described for a genera­tor, except for the slight modification caused by the commuta t ing field drop. If the a rma tu re ground were on a series-parallel connected motor operating on the positive side of the line, the voltage applied to the relay would be equal to one-half of the generator voltage plus the un­dulating voltage from this a rmature . The relay would pick up a t a slightly lower voltage t h a n indicated by Figure 2 be­cause of the larger d-c component . If the ground were in the a rmature of a series-parallel ctmnected motor operat ing on the negative side of the line, conditions would

0 A L T E R N A T E Λ G R R E L A Y V

C O N N E C T I O N v -C . C O M M . C F I E L D

be the same as for a generator a rmature except t h a t this relay would be subjected to the voltage of this negative armature . This would be one-half of the generator voltage, and therefore, the pickup of the relay with respect to generator voltage would be double t h a t shown by Figure 2 .

These comments on a rmature grounds presupposed a zero resistance ground in an a rma t tue . If the a n n a t u r e ground actu­ally has appreciable resistance as com­pared to the relay, then the voltage re­quired to operate the relay will be cor­respondingly increased.

ACCIDENTAL POWER CIRCUIT GROUNDS

(a) If a ground takes place on the positive side of the power system, genera­tor voltage would be applied directly to the ground relay. If this ground were of zero resistance, the relay would operate when the generator voltage reached 38 volts. If t he ground h a d appreciable re­sistance, the operat ing voltage would be correspondingly increased.

(b) If the ground took place in the connection between positive and negat ive motors when operating in the series-par­allel connection, the same comments would apply except t h a t the relay pickup in terms of generator voltage would be doubled.

(c) If a ground took place on the neg­at ive of a system i t would appear , a t first thought , t h a t the ground relay coil would be short-circuited. Figure 3 il­lustrates this circuit. If t he ground took place on the lead B, the coil of the ground relay would be short-circuited. If, how­ever, the ground took place on leads C or D, the series field would be intermediate between the ground location and the cir­cuit through the ground relay. Ordinary s teady-state voltages developed across the series field would no t be sufficient to operate the relay; however, any fluctua­tions in current would create enough in­ductive voltage across the series field to provide operation if the ground were on either C or D. An improvement on this circuit is to move the ground relay con­nection from A to E. In this case the

transient inductive voltages of both the generator commuta t ing field and the motor series field are effective for produc­ing relay operation with grounds on either C or D, and the generator commutat ing field inductive voltage is available for producing operation with a ground on B.

MOISTURE OR CREEPAGE GROUNDS

One place where the ground relay does, if anything, too good a job is in con­nection with moisttire or creepage grounds. These are always a mat te r of degree and most types of ground relays overprotect, causing unnecessarv^ inter­ference with locomotive operation. How­ever, this ma t t e r is, somewhat involved, particularly from the standpoint t h a t all of the leakage current does not necessarily pass through the relay coil.

(a) If leakage is taking place from the positive cable terminations and connec­tions of the system, then conditions are identical with item (a) in the preceding section. Tak ing into account the equiva­lent resistance of the leakage path , the generator voltage with respect to relay pickup may readily be calculated.

(b) T h e same comments apply for leakage taking place on the cable ter­minat ions and connections between two or more series connected motors .

(c) vSimilar leakage from the negative does not tend to cause relay operation for the same reason as discussed under item (c) in the preceding section. The change in relay connections already mentioned would improve this situation, b u t the leakage resistance from the negative alone would have to be of a low value before relay operation could be expected.

(d) Leakage m a y be experienced from armatures because of the collection of dir t and moisture. This would usually be expected to occur a t the commutator string bands or a t the ends of the arma­ture coils. If distr ibuted leakage of this

BRUSHI

! S E R I E S i F I E L D

" ^ R E V E R S E R

• G R R E L A Y

Figure 3 (left). Schematic dia­gram of negative

power circuits

Figure 4 (right). Paths of distribu­ted leakage over commutator string

band

- BRUSHES

^ GR LEAKAGE ^ RELAY

lURFACE (string bond)

J U L Y 1 9 5 3 McDonald—Diesel-Electric Locomotive Ground Relays 1 7 1

1000 ,

h

§ (Τ

Ζ3

5

8 0 0

6 0 0

4 0 0

ω 200^

>

Fisure 5 (left). Ground relay pickup with distributed armature leakage

ARMATURE RESISTANCE TO G R O U N D - O H M S

Fisure 6 (lower leit). Equivalent circuits ior distributed cable leakase, parallel

connected motors

< 6R ^ R E L A Y ,

PCS. LEAKAGE RES.

Ϊ Ϊ r NE6. LEAKAGE RES. Fisure 7 (risht). Total leakase

current at s^ound relay operation

SOOr

40d

o) 300f

a:

I I 200 UJ

100

α I 0.2 0.3 0.4 0.5 0.6 0.7 TOTAL GENERATOR LEAKAGE CURRENT-AMPERES

type takes place, t he voltage apphed to the leakage pa ths varies between brushes or polarities around the a rmature . T h e leakage is in all directions even from one segment to the next, as shown by Figure 4 . T h e relay sees only a small pa r t of this total leakage current. Tests indicate tha t with uniform leakage around an armature , relay operation may be ex­pected as shown by the curve of Figure 5. T h e armature insulation resistances used in plotting this curve are, of course, resist­ances taken a t standstill . T h e a rmature insulation resistance values shown are surprisingly low, ye t tests did not indicate any tracking or damage to the anna tu r e s as a result of these creepage grounds. This point is mentioned because ordi­narily a rmature resistance to ground meas-lu-ements might be expected to be on the order of megohms. Obviously an arma­ture having low leakage resistance is badly in need of maintenance. T h e im-phcationis t h a t in emergency cases motive power having low a n n a t u r e resistance to ground could be dispatched wi thout ex­pecting damage to t he a rmature .

(e) Generalized moisture leakage con­dit ions: I n t he preceding text, moisture leakage from various par t s of the locomo­tive circuits ha s been considered on an in­dividual basis. Usually such extreme leakage conditions would be expected to occur uniformly throughout a locomotive. On this basis the exposure on the negative side, with parallel connected motors, might be three tunes t h a t of the positive side of the system, as indicated by Figure 3. Equivalent drcui t s , neglecting the possibiHty of a rmature leakage and with motors operating in the parallel connec­

tion, would be as indicated by Figure 6. To ta l leakage current supphed by the generator, as plot ted against generator voltage a t t h e point where ground relay operation is produced would be as shown in Figure 7 . Similar plots could be m a d e for the series-parallel connection. T h e point i l lustrated is t h a t the to ta l leakage current which is causing damage to the equipment m a y be very m a n y times the actual current t h a t goes through the ground relay.

FLASHOVER PROTECTION

One of the main duties of the ground relay is to detect generator or motor arma­ture flashovers. A point which can be made in connection with the equipment under discussion is t h a t an a rmature flashover always involves ground in the circuit.

(a) Consider a generator a n n a t u r e flashover involving ground, as indicated by Figure 8. F rom the positive of the a rma tu re there is an anode drop, an arc s t ream drop, and a cathode drop where the connection is m a d e to ground. From ground back to the negative of the arma­ture there are corresponding anode, arc s tream, and cathode drops. I t m a y be presumed, therefore, t h a t ground poten­tial assumes a position approximately midway between the positive and nega­tive of the a rmature . Oscillographic tests show t h a t this asstunption is sub­stantially correct. I t mus t be appreci­a ted t h a t dmring a flashover, arcing con­ditions va ry continuously and ground po­tential may , therefore, fluctuate consider­ably wi th respect to potential a t one side of the a rmature . T h e ground relay in

this case received half of the armature voltage. Experience has shown tha t with the type of relay here discussed, and the connections shown in Figure 1 or modified as suggested in Figure 3, the ground relay always received sufficient voltage to re-hably detect a generator flashover.

(b) T h e operation of the ground relay in detecting a parallel connected motor flashover is identical with t ha t of de­tecting a generator flashover. The cur­rent in a flashover of this type will be part ly generator current—which ser\'es as exciting current for the motor by re­turning to the generator via the motor field—and part ly current generated by the motor a rmature because the motor is being driven by train movement . A motor flashover wih very frequently re­sult in a generator flashover.

(c) If the motors are operating in the series-parallel connections and a positive

Fisure 8. Generator flashover involvins a Sround

1 7 2 McDonald—DieseUElectrk Locomotive Ground Relays J U L Y 1 9 5 3

G R O U N D

Figure 9 (left). Flashover of positive connected motor in

series-parallel connection

ψ R E L A Y Figure 10 (right). Flashover of negative connected motor in

series-parallel connection

G R O U N D

connected motor flashes over the voltage across this motor will t end to collapse. The voltage across the negat ive motor connected in series will be increased as a result of the increase in generator cur­rent. T h e voltage available for oper­ating the ground relay will be the sum of the voltage across the negat ive connected motor and half the voltage across the positive connected motor , see Figure 9.

(d) If in the series-parallel connection a negative connected moto r flashes over, the relay will be operated b y one-half of the voltage across this a rma tu re as indi­cated by Figure 10. In this case the inductance of the moto r field m a y also be considered as contr ibuting toward operat­ing the relay and the ne t result is t h a t rehable indication of negative connected motors flashing while operating in the series-parallel connections is obtained.

(e) Positive side contactors flashover: A commonly used locomotive power cir­cuit for a typical 4-motor locomotive us­ing series-paraUel and parallel connections is shown in Fig 11 (A).

T h e restricted space available on a lo­comotive makes i t impossible to supply completely adequate arc distance for taking care of circuit interruptions of un­usual severity on the main line contac­tors. Contactor flashover to ground al­though rare is always a possibility. A well-designed contactor tends to build up a fixed voltage across its contacts during circuit interruption and main ta ins this voltage unti l the arc is extinguished. This type of operation is indicated by Figure 11(B). Normally a contactor will mainta in about 150 per cent of the chrcuit voltage across its contacts during circuit opening. If the contactor arc goes to ground i t would be expected t h a t the ground voltage would be midway be­tween the voltages across the contactor tips. For a flashover of contactor PI operation of the ground relay m a y be es­t imated from the assumed vector relation­ship of Figure 11(C).

(f) Negative side contactor flash-over: An analysis of the ability of the

ground relay to detect a flashover of the P2 contactor , which is on the negat ive side of t he system, m a y also be m a d e from Figure 11(B). Because of the induct ive voltages involved, i t will be observed t h a t no difficulty should be experienced with the operation of the ground relay on th is t ype of flashover.

(g) Series contactor flashover: A similar diagram m a y be drawn for a flash-over of the series contactor SI.

GROUND PROTECTION DURING DYNAMIC BREAKING

A dynamic braking circuit is shown in Figure 12. T h e motor a rmatures a n d braking resistors are located to i l lustrate the direction of their voltage depar ture from the positive of the generator. The actual voltage developed by the generator during dynamic braking is qui te small as compared to motor a rma tu re voltage. I t will be noted t h a t during dynamic brak­ing all of the features previously dis­cussed—armature grounds, creepage grounds, and flashovers—produce action of the ground relay. Hence i t affords full protection during this type of locomotiA^e operation.

ENGINE STARTING

While the generator is being used as a motor to s ta r t the diesel engine, the power

Figure 1 1 . Dia­grams relating to power contactor

flashovers

A . Simplified schematic of cir­

cuits B. Vol tage and current during contactor open­

ing C. Vector dia­gram for flashover

of P1 D. Vector d ia­gram for flashover

of P2

circuits of the locomotive are connected to the control ba t te ry . T h e ground relay then responds to any ground tha t might exist on the control ba t te ry . If the ground relay is connected to the point Ε of Figure 3, i t will be noted t h a t when voltage is first applied to the generator, the arma­ture drop is qui te small consequently the ground relay will, in effect, be connected to the positive side of the ba t te ry . Mos t of the voltage will be across the generator start ing field. As the generator comes u p to speed the a rmature develops back electromotive force, the current decreases and the ground relay, in effect, becomes connected to t he negative side of the ba t ­tery. T h e relay is thus responsive to a ground a t any location along the length of the control ba t t e ry and circuits.

Relay Connections

A ground relay connected as in Figure 1 has provided ver\ ' good protection on a large number of equipments . As dis­cussed in connection with Figure 3, i t is considered bet ter to change the connec­tions from point Λ to point Ε of Figure 3 in order to minimize the possibihty of mak­ing the relay inoperative by having a ground on a negative lead.

A resistance bridge connection has sometimes been used in order to provide

GEN. NEG.

' GEIt VOLTS M2

VOLTS M2

VOLTS P2 CONT

VOLTS Hi-

GEN. COMM. FLO VOLTS

GR RELAY

J U L Y 1 9 5 3 McDonald—Diesel-Electric Locomotive Ground Relays 1 7 3

Figure 12 (leit). Sche­matic diagram of dy­

namic braking circuit

mid-point grounding of tbe generator voltage as indicated by Figtire 13. Referring to the preceding headings, i t would appear t h a t this type of connection will detect armatvue grounds of ei ther generators or motors . A relay so con­nected would recognize power circuit grounds except those during series-paral­lel operation which migh t occur on the cables operat ing a t potent ial between the positive and negat ive motors . A bridge-connected relay should provide successful protection in the case of a creepage ground on either the positive or negative of the power curcuits. I t would no t how­ever respond to this type of ground on the cables connected between the positive and negative motors during series-parallel operation. Nei ther would i t t end to de­tect generalized a rma tu re leakage since such leakage would produce a ground potential midway between the terminals of the generator or motor on which i t ex­isted. For generalized leakage conditions from the power cables the bridge-con­nected relay could give good operation on the basis t h a t there would be more leakage from the negative of the system than from . the positive. Theoretically this type of relay would no t be able to detect a generator or moto r flashover if the motors are operating in t he parallel

Figure 13 (right). Re­sistance bridge connec­

tion for ground relay

connection since ground voltage should be midway of the system. Actual ly there are fluctuations in this voltage and these migh t produce operat ion of such a relay. T h e bridge-type relay should provide pro­tection for a motor flashover if t he motors are being operated in the series-parallel connection and only one armatiu-e flashes over. For contactor flashovers i t migh t be expected t h a t this t ype relay would supply adequa te protection.

There has been some though t t h a t the bridge-connected relay relieves voltage stress on t he equipment b y grounding the mid-point of the system. Actually m a ­chine designers are most ly concerned with providing sufficient insulation and creep-age distances in series windings such as coimnutat ing fields and series fields. A negative connected ground relay will nonnal ly keep all of these field windings a t or near ground potent ial except in series parallel operation when some of the motor fields are raised to 50 per cent of system voltage above ground.

T h e functions performed by the ground relay should no t interfere with emergency plugging of a locomotive.

T h e ground relay is normally arranged to control a generator field contactor. Part icular ly in case of a flashover i t is necessar}^ t h a t the ground relay be fast in operation and t h a t it control circuits which will promptly remove the excitation from the generator. T h e equipment here

RES.

- Α Λ Α - | | | ι .

6R RELAY

RES,

discussed has a generator provided with a single shun t field winding. T h e ground relay operates a field contactor which in­serts a field killing resistance in the field circuit having approximately 16 times the resistance of the field. T h e voltage of the generator is thus quickly reduced below the point which will sustain a flashover.

Wi th all ground relay systems it is cus tomary to provide a cu tout switch in series with the relay coil. This m a y be opened to permit emergency operation of the locomotive wi th a ground on the power circuit. Making the ground relay inoperat ive will, of course, eliminate its protect ive functions and such things as a flashover or a double ground on the sys­tem migh t cause considerable damage. T h e ground relay function should be re­tained during any load test since there is ordinarily no other way provided to clear a flashover.

T h e foregoing discussion illustrates the point t h a t the ground relay has a large number of functions to perform. In most cases the exact operat ing current or voltage of the relay is no t too critical; however, any changes made in the relay operat ing characteristics should be care­fully considered with respect to all of the functions.

Considerable work is being done in con­nection with moisture groimds which, it is believed, will result in further improve­ments in locomotive operation.

Di iscussion

W. M. Hutchison (Westinghouse Electric Corporation, East Pittsburgh, Pa . ) : The importance of a ground relay to the pro­tection of the electric equipment of a diesel-electric locomotive is well illustrated in Mr. McDonald's thorough paper on this inter­esting subject.

Our experience with several types of ground relay and with various ways of connecting them leads us to the following comments.

Mr. McDonald suggests that a ground on the negative system, by short-circuiting the relay coil, does not necessarily nullify the relay. He suggests tha t if the acci­dental ground occurs between the motor and the series field, fluctuations in current would create enough inductive voltage to operate the relay. Actually there is much more of the circuit at a potential which, if grounded, would short-circuit the relay coil (potential 5 , Figure 3 ) than there is between the motor armature and field (potential C or Z>, Figure 3) . Therefore, the probability of a ground which would nullify the relay is

correspondingly great. Even if the ground occius between the armature and field, only a violent change in generator or motor arma­ture ciurent, such as a motor flashover to ground, will cause it to operate. These are the greatest objections to this type of ground protection.

Tests with a certain relay with one side grounded showed no response to a single armature ground and erratic response to flashover.

Several years ago ground relays connected equivalent to one side grounded were used. So many accidental grounds to the con-

1 7 4 McDonald—Diesel-Electric Locomotive Ground Relays J U L Y 1 9 5 3

ductor directly connected to the line side of the relay resulted in making the relay in­operative that it was necessary to reconnect the relay to another point which was not used in the locomotive wiring and which was at suiiicient difference in potential to all the locomotive circuits tha t any ground would be depended upon to trip the relay.

Since the protection offered by the relay depends on its operation on an induced

voltage supplied by either the generator commutating field or a motor series field, both of which have only a few turns, it appears tha t more data should be offered on the current surge required through these fields to develop the 38 volts required to operate the relay. I t appears unlikely tha t such voltages can be developed in normal operation.

The resistance bridge connection has

been found to be much more reliable. It is our practice, in our permanent series-paral­lel connection, to put the main fields all on the negative side and to use the resistance bridge ground relay.

Biasing the bridge-connected ground relay from a separate power supply with a suitable relay should give the best over-all response to grounds anywhere on the system from all causes.

Hydraulic Servos Incorporatins

a High-Speed Hydraulic-Amplifier

Actuated Valve

ROBERT L. SCRAFFORD A S S O C I A T E MEMBER A I E E

EL E C T R O H Y D R A U L I C systems have certain advantages over pure

electromechanical systems, which make them part icular ly suitable as an adjunct to electronic control for high-power level actuation. T h e main factor which has hmited the application of hydraul ic actuators unt i l recently has been the awkardness of the control systems re­quired. However, developments in the technique of controlling hydraul ic flow electrically now permi t the flexibility of electronic control to be applied to the linear operat ion of hydraul ic valves a t high speeds.

The advantages of hydraul ic systems stem mainly from their abil i ty to provide large amoun t s of power from a "hydraul ic ba t t e ry" (accumulator) wi th minimtun of appa ra tus and storage volume, and with a small t ime cons tant . One-shot hydraulic systems have m a n y applica­tions in guided missiles, where high-power output to opera te control surfaces is re­quired for shor t period of flight. Another advantage of hydraul ic systems is the ease of obtaining either l inear or rota t ional motion.

Applications requiring cont inuous op-

P a p e r 53 -103 , r e c o m m e n d e d b y t h e A I E E F e e d ­back C o n t r o l S y s t e m s C o m m i t t e e a n d a p p r o v e d by t h e A I E E C o m m i t t e e on Techn i ca l O p e r a t i o n s for p r e s e n t a t i o n a t t h e A I E E W i n t e r Genera l Mee t ing , N e w Y o r k . N . Y. , J a n u a r y 1 9 - 2 3 , 1953. M a n u s c r i p t s u b m i t t e d S e p t e m b e r 29 , 1952; m a d e avai lab le for p r i n t i n g D e c e m b e r 19, 1952.

R O B E R T L . S C R A F P O R D is w i t h t h e Corne l l Aero­nau t i ca l L a b o r a t o r y , I n c . , Buffalo, N . Y.

T h e d-c t r ans fe r v a l v e desc r ibed in th i s p a p e r was deve loped in t h e Corne l l Ae ronau t i ca l L a b o r a ­t o r y H y d r a u l i c L a b o r a t o r y u n d e r t h e d i rec t ion of W. C. M o o g .

eration m a y also benefit from the mini­m u m apparatus- to-horsepower rat io by providing a recharging system in a less restr icted location t h a n the ac tuator , and conveniently connected to i t by copper tubing.

I t is the purpose of this paper to de­scribe a d-c transfer valve developed a t Cornell Aeronautical Laborator>% Inc. , and to call to the a t ten t ion of servo en­gineers, especially in industrial positions, the progress t h a t has been made in the field of hydraulic control , which m a y be familiar to engineers associated with the aircraft indus t ry .

This valve, operat ing in conjunction with a hydraul ic s t ru t or piston, has a combinat ion of gain, power ou tpu t , and torque-to-inert ia rat io which is difficult to obta in in an electrical ac tua tor . How­ever, ra t ings are part icular ly difficult to apply to hydraul ic systems because of the inherent nonlinear relationships. I t is desirable, for the sake of simplicity, to m a k e a comparison on the basis of closed servo loops. These analyses indicate

superiori ty over o ther servo systems operat ing a t power levels of 1 to 10 horse­power, even when used in a control loop which does no t fully reahze the potentiali­ties of the device.

Typical Hydraulic Control Systems

A typical control system is shown in block diagram form in Figure 1. In the case of a hydraulic system, the power source consists of a source of hydraulic fluid under pressure, and the power ampli­fier is a transfer or control valve. The ac tua to r m a y be ei ther of the linear or rotat ional-motion variety.

The General Servo Problem

We m a y divide all servomechanisms into two p a r t s :

1. Intelligence section, which includes voltage-amplifying and phase-compensating functions;

2. Power output and driver stages

The first pa r t contains devices which perform the various signaling and com­put ing functions. These are shown as dot ted lines in Figure 1. They provide no power to the load and m a y accomplish intelhgence transmission by means of any. of the systems used by communica­tions and telemetering engineers. The second pa r t contains the power-converting devices, in which power from some energy source is transferred to the load, in re­sponse to a signal from the intelhgence por t ion of the loop.

P O W E R

S U M P

/SFDL J S I G 1 J . 1 S I G N A L |

MfU.' I G A I N J I_ J ] _ A M P U F I E R

P O W E R A M P L I F I E R

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I F E E D B A C K ! G A I N j

Figure 1 . Servo block diasram

S I G N A L P O W E R

S E C T I O N S S E C T I O N S

J U L Y 1 9 5 3 Scrafford—Hydraulic Servos Incorporalifig Actuated Valve 1 7 5