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Feed-back in magnetic amplifiers
FEED-BACK
I N M A G N E T I C AMPLIFIERS. BY
ALAN S. FITZGERALDJ Part If.2
BINARY STABILITY.
T h e a r r a n g e m e n t of Fig. 9, w h e n the c i r c u i t c o n s t a n t s are a d j u s t e d so as t o give the c h a r a c t e r i s t i c s of Fig. 11, is c a p a b l e of m a n i f e s t i n g a m o s t interesting type of a c t i o n , s i m i l a r t o t h a t of the f a m i l i a r " l a t c h e d in" type of e l e c t r i c r e l a y . T h i s e f f e c t m a y , p e r h a p s , be m o r e e a s i l y d e s c r i b e d a n d e x p l a i n e d by r e f e r r i n g t o Fig. 17, w h i c h s h o w s a n e x a c t electro-mechanical a n a l o g u e Lamp
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PUSH BUTTONS
Polarized Relay
°-7_ i 1 : .......................
T :,,,,,
",00
FIG. 17. Electro-mechanical analogue of application of binary stability.
of the a c t i o n w h i c h is p o s s i b l e with a s a t u r a t i n g r e a c t o r c i r c u i t u n d e r c o n d i t i o n s of b i n a r y s t a b i l i t y . In Fig. 17 a load or i n d i c a t i n g d e v i c e s u c h a s a l a m p , is controlled by m e a n s of a p o l a r i z e d r e l a y w h i c h h a s a s n a p - a c t i o n s p r i n g c o n t r o l a r r a n g e m e n t so t h a t the a r m a t u r e c a n n o t r e m a i n in m i d - p o s i t i o n b u t s n a p s one w a y or the o t h e r a c c o r d i n g t o the p o l a r i t y of the d-c. c u r r e n t 1 Electrical Research Engineer, Los Angeles, Calif. 2 Part I appears in the JOURNAL OF THE FRANKLIN INSTITUTE, Vol. 247, No. 3, March, 1949, p. 223. 457
458
ALaN S. I?ITzGERALD.
[J. F. 1.
a p p l i e d t o the coil, a n d a l w a y s r e m a i n s in the p o s i t i o n of the last o p e r a t i o n w h e n the c o n t r o l c u r r e n t is w i t h d r a w n . T h e r e l a y is controlled over a t w o - w i r e c i r c u i t by m e a n s of two p u s h - b u t t o n s and a m i d t a p p e d b a t t e r y , so t h a t the coil of the polarized r e l a y m a y be e n e r g i z e d a t will with a s i g n a l of e i t h e r p o l a r i t y . O b v i o u s l y , p u s h i n g one b u t t o n down m o m e n t a r i l y will c a u s e the l a m p t o l i g h t a n d t o stay lit. O p e r a ting the o t h e r b u t t o n will put the l a m p out, w h i c h c o n d i t i o n will 1)¢' r e t a i n e d u n t i l the " O N " b u t t o n be a g a i n a c t u a t e d . A t f i r s t t h o u g h t it w o u l d a l m o s t seem t h a t a p h y s i c a l m o v e m e n t , or a c h a n g e of position of some m a t e r i a l m e m b e r , is a f u n c t i o n a l n e c e s s i t y i n h e r e n t in such a s y s t e m . H o w e v e r , e x a c t l y s i m i l a r a c t i o n is m a n i f e s t e d by the a r r a n g e m e n t s h o w n in Fig. 18, in w h i c h the l a m p is e n e r g i z e d from the c i r c u i t of Fig.
Lamp--(~) |
PUSH BUTTONS OFF ON
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B,as winding
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AC
~ J winding ~ % 1 Series ~ I/winding
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. . . . . . . . winding
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If0 VOLTS AC Fro. 18. Application of binary stability principle to control circuit. 9, the input coil of which is energized by the same battery and pushbutton arrangement as in Fig. 17. There are no moving parts in this system and all the circuits are permanently closed with the obvious exception of the signal-emitting device, duplicating that of Fig. 17. If the circuit is adjusted in accordance with Fig. Ii, when the " O N " button is depressed the ]amp lights. When the button is released the lamp remains illuminated. If the " O F F " button be now operated, despite the fact that the magnetic amplifier circuit remains connected to the power supply, the lamp will appear to go out; that is to say the current will be reduced to a low level insufficient to cause the lamp to emit any visible light. After the " O F F " button is released the ]amp remains out.
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It is believed that it will be apparent to the reader that this action logically results from the characteristic illustrated by the curve in Fig. 11 and that further explanation will be unnecessary. The analogy between this magnetic amplifier circuit and the electromechanical arrangement of Fig. 17 is maintained u n d e r another operating condition in an even more striking manner. If the a-c. s u p p l y to the magnetic amplifier circuit in Fig. 18 be interrupted, when the power is restored, neither push-button having been Operated in the meanwhile, the action existing at the m o m e n t of disconnection will be reproduced. If the lamp was not illuminated when the power was withdrawn, it will not l i g h t up when it is restored. But if the light be on when the power was disconnected, as soon as the circuit is a g a i n energized with a.c., the lamp lights up again.
i
E~EEE~
F:G. 19. Binary stability principle used for pendulum drive.
Furthermore, if the signal be applied at a time when the magnetic amplifier circuit is de-energized, whether or not the lamp will light when the a-c. power is restored, can be pre-determined. This "memory" action is due to a residual magnetism effect in the core. The application of an input signal of reversible polarity is not, however, the only effect by m e a n s of which the binary stability circuit may be caused to transfer from one r a n g e to the other, and therefore to undergo a c h a n g e in current v a l u e of substantial amplitude. A study of the curves of Figs. 5, 6, 7, and 11 will indicate that t h e r e are a n u m b e r of different changes in circuit constants that can cause such a transfer. For example, if we refer to Figs. 5, 6, and 7 it is apparent that a modification in the a m o u n t of feed-back such as m i g h t cause the circuit
460
ALAN S. FITZGERAI,D.
[J. F. I.
c h a r a c t e r i s t i c s t o c h a n g e f r o m , say, Fig. 5 t o Fig. 7 c a n c a u s e this t r a n s f e r . T h i s m a y be seen t o be the c a s e , for i n s t a n c e , a t the p o r t i o n of the c u r v e s c o r r e s p o n d i n g t o a n i n p u t e x c i t a t i o n in the n e i g h b o r h o o d of - 1 m a . S u c h a r e s u l t c o u l d o c c u r if the f e e d - b a c k t u r n s be c h a n g e d from 450 t o 350. H o w e v e r , it h a s been p o i n t e d out t h a t the f e e d - b a c k is a f u n c t i o n of the o u t p u t c i r c u i t r e s i s t a n c e a s well as the t u r n s . T h e s a m e r e s u l t m i g h t take p l a c e if i n s t e a d of c h a n g i n g the f e e d - b a c k t u r n s , the res i s t a n c e or i m p e d a n c e of the load c i r c u i t be m o d i f i e d . F i g u r e 19 s h o w s a n i n t r i g u i n g a p p l i c a t i o n of this principle. A p e n d u l u m is p r o v i d e d with a n a c t u a t i n g solenoid h a v i n g a s u i t a b l y s h a p e d p l u n g e r m o u n t e d u p o n the p e n d u l u m and e n t e r i n g and r e t r a c t i n g from the solenoid coil as the p e n d u l u m swings. T h e r e a c t a n c e of this s o l e n o i d w i n d i n g w i l l , of c o u r s e , v a r y a s the p l u n g e r e n t e r s t o a g r e a t e r or less e x t e n t i n t o the coil. W h e n the p l u n g e r s w i n g s f u l l y i n t o the coil the r e a c t a n c e will be r e l a t i v e l y h i g h ; w h e n it s w i n g s in the o t h e r d i r e c t i o n a l o w e r v a l u e of r e a c t a n c e will result. In Fig. 19 the s o l e n o i d w i n d i n g is e n e r g i z e d from a b i n a r y s t a b i l i t y type c i r c u i t such a s h a s b e e n d e s c r i b e d . It is c l e a r t h a t a n i n c r e a s e in the r e a c t a n c e will r e d u c e the effective f e e d - b a c k ; a n d t h a t , conversely, the f e e d - b a c k will be g r e a t e r w h e n the i m p e d a n c e is l e s s . I t f o l l o w s t h e r e f o r e t h a t the f e e d - b a c k will d e c r e a s e w h e n the p l u n g e r s w i n g s f u l l y i n t o the coil a n d will b e c o m e g r e a t e r w h e n it s w i n g s in the o u t w a r d direction. C l e a r l y , if this m e r e l y c a u s e d a m o r e or less p r o p o r t i o n a l v a r i a t i o n in the c u r r e n t in the coil, no effective d y n a m i c a c t i o n c o u l d result. H o w e v e r , in the a r r a n g e m e n t of Fig. 19 the c i r c u i t c o n s t a n t s can be a d j u s t e d so t h a t the m o v e m e n t of the p e n d u l u m r e s u l t s in the d i s c o n t i n u i t y e f f e c t w h e r e b y the c i r c u i t t r a n s f e r s from one s t a b i l i t y r a n g e t o the o t h e r , and b a c k a g a i n ; this c a u s e s n o t a g r a d u a l , b u t an a b r u p t a n d s u b s t a n t i a l , c h a n g e in c u r r e n t due t o the p l u n g e r m o v e m e n t . T h a t is t o say, a t a c e r t a i n p o i n t on the o u t w a r d t r a v e l of the p l u n g e r the c u r r e n t s u d d e n l y i n c r e a s e s s e v e r a l h u n d r e d per c e n t ; in like m a n n e r a s the p e n d u l u m s w i n g s t o w a r d s the coil a s u d d e n c h a n g e in c u r r e n t in opposite sense takes place. I t is c l e a r t h a t this r e l a t i o n b e t w e e n the p l u n g e r position and the c u r r e n t in the s o l e n o i d coil, a n d a c c o r d i n g l y the pull e x e r t e d t h e r e b y u p o n the p l u n g e r , w h i c h p u l s a t e s in s y n c h r o n i s m with the s w i n g of the p e n d u l u m , has the characteristics n e c e s s a r y t o d r i v e the p e n d u l u m . T h u s , once the p e n d u l u m is s t a r t e d s w i n g i n g m o t i o n is c o n t i n u e d a s long a s the c i r c u i t is e n e r g i z e d with a l t e r n a t i n g c u r r e n t . W h i l e it is not o b v i o u s t h a t this d e v i c e h a s a n y g r e a t v a l u e in horology, n e v e r t h e l e s s it f o r m s a v e r y s i m p l e and i n e x p e n s i v e a c t i o n d e v i c e for d i s p l a y or o t h e r p u r p o s e s w h e r e a n oscillating m o v e m e n t
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m a y be of interest. T h e a b s e n c e of a n y c o n t a c t s s h o u l d be f a v o r a b l e t o reliability. I t is a l s o of p o s s i b l e i n t e r e s t a s a g e n e r a t o r of p u l s a t i n g electrical effects of more s u b s t a n t i a l m a g n i t u d e s i n c e , by f u r t h e r s t a g e s of m a g n e t i c amplification, t h e p u l s a t i n g c u r r e n t in the c i r c u i t of Fig. 19 m a y be s t e p p e d up t o s u b s t a n t i a l p o w e r l e v e l s . F o r e x a m p l e , i n s t e a d of the p e n d u l u m , a n oscillating s y s t e m b a s e d u p o n s p r i n g i n s t e a d of g r a v i t y control, m a y be u s e d t o p r o v i d e a n electric h a m m e r or r i v e t e r . I t is, h o w e v e r , by m e a n s of this b i n a r y s t a b i l i t y c i r c u i t principle, possible to produce a sustained pulsating effect w i t h o u t any mechanical m o v i n g e l e m e n t . If a pair of c i r c u i t s of the form s h o w n in Fig. 9 be
FIG. 20. Oscillating or pulsating magnetic amplifier circuit.
c o n n e c t e d t o g e t h e r so t h a t each one acts as a t e a s e r for the o t h e r , it is p o s s i b l e t o a r r a n g e for m u t u a l interaction s u c h t h a t the d u a l c i r c u i t c o n t i n u e s indefinitely in a c o n d i t i o n of oscillation or p u l s a t i o n . S u c h a n a r r a n g e m e n t is i l l u s t r a t e d in Fig. 20 (1). a In this d i a g r a m it will be n o t e d t h a t the o u t p u t c i r c u i t of each of the d u p l i c a t e m a g n e t i c a m p l i f i e r e l e m e n t s is c o n n e c t e d in s e r i e s with the i n p u t c i r c u i t of the o t h e r one. T h u s w h e n the r i g h t h a n d u n i t t r a n s f e r s from the l o w e r r a n g e of s t a b i l i t y t o the u p p e r r a n g e i t d e l i v e r s a s i g n a l t o the left h a n d u n i t s u c h a s t o c a u s e this l i k e w i s e t o c h a n g e from the low v a l u e t o t h e h i g h c u r r e n t v a l u e . T h i s in t u r n e n e r g i z e s the i n p u t of the r i g h t h a n d u n i t in the o p p o s i t e s e n s e t h r o w i n g the l a t t e r b a c k i n t o the l o w e r sta8 The boldface numbers in parentheses refer to the references appended to this part of the paper.
462
ALAN S. F[TZGEI~aLD.
[J. I;. Z.
b i l i t y r a n g e . T h i s closed-cycle c h a i n r e a c t i o n c o n t i n u e s a s long a s the c i r c u i t s are e n e r g i z e d from the a-c. s o u r c e . S p e c i a l f e a t u r e s m a y be i n c l u d e d in the b i a s c i r c u i t s for i n h i b i t i n g the p u l s a t i n g e f f e c t or for perm i t t i n g it t o o c c u r . O t h e r c i r c u i t a d j u s t m e n t s m a y be i n c l u d e d w h i c h
i
I
I
I
I
I
FIG. 2I. Lamp control application of pulsating circuit.
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FEED-BACK IN MAGNETIC AMPLIFIERS.
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have the effect of modifying in some degree the type of dynamic action which ensues. For further details of t h e s e arrangements reference is invited to the citation. An application of this latter principle is illustrated in Fig. 21 (1). In this arrangement one of the output currents of the system shown in F i g . 2 0 is connected in series with the input windings of a pair of single magnetic amplifier stages of the compensating winding type shown in a previous paper (2). Each of the output circuits of both of t h e s e magnetic amplifiers is connected to a signal lamp. The input windings of the pair of amplifiers therefore both receive identical signals consisting of a uni-directional current continuously pulsating between a high and a low value. Both of the lamp amplifiers are identical except that the compensating currents are not adjusted to like values. In one case, the compensating excitation is adjusted to be equal to the input signal when the current is at its minimum value; in the other case, the bucking ampere-turns are made equal to the m a x i m u m value. Thus one of the lamp amplifiers will deliver only a very low current when the signal has its minimum value and the lamp will be lighted when the input current has its high value. The other lamp, however, will be out with the high value of the pulsating current and on when it has the minimum value. Thus the two lamps will flash alternately in the m a n n e r of the familiar g r a d e crossing d a n g e r signal. This complete arrangement, it will again be noted, has the reliability feature of complete absence of contacts or moving parts. MODULATING CONDITION.
This p h a s e of the behavior of feed-back type magnetic amplifiers is probably of the greatest potential v a l u e in that it m a k e s possible amplifiers having very accurate linear response characteristics. We shall now consider more specially the action of the circuit of Fig. 9 when the circuit c o n s t a n t s are chosen so as to provide the response characteristics of the type shown in Fig. 12. Figure 22 shows the circuit of Fig. 9 with the addition of an output circuit which includes a milliammeter and an adjustable sei-ies resistance, together with a push-button arrangement similar to that of Fig. 18 for furnishing an input of reversible polarity. A second adjustable resistor is provided for determining the value of the bias current. Assuming that the type of rectifier and the n u m b e r of series or feedback t u r n s are suitably chosen, and that the resistance of the load or output circuit is somewhat less than the optimum value, the action of the circuit can be controlled by the two adjustable elements, the bias resistor and the series resistor. The latter serves as a trimmer; it may be of a low v a l u e as compared with that of the output circuit, and such
464
ALAN S. FITZGERAI.I).
IJ. b~. I.
a s e n a b l e s the sum of the t r i m m e r a n d the o u t p u t c i r c u i t r e s i s t a n c e v a l u e s t o be v a r i e d s o m e w h a t a b o v e a n d b e l o w the e s t i m a t e d o p t i m u m o u t p u t c i r c u i t resistance. In o r d e r t o s e c u r e the d e s i r e d characteristics t h e s e two c o n t r o l s are t o be a d j u s t e d with s o m e n i c e t y . T h e e x a c t v a l u e s w h i c h a r e r e q u i r e d a r e d e t e r m i n e d by o b s e r v i n g the b e h a v i o r of the p o i n t e r of the m i l l i a m m e t e r . A s d e s c r i b e d a b o v e w h e n the a d j u s t m e n t s h a v e been c o r r e c t l y m a d e this i n s t r u m e n t g i v e s an i n d i c a t i o n of i n d e t e r m i n a t e v a l u e ; t h a t is t o say, it b e h a v e s like a f l u x - m e t e r or o t h e r u n c o n t r o l l e d type of instrument.
no
O~fp~t ~[rcuit Resis'l'6v~ce
no
LIP
DOWN
FIG. 22. M o d u l a t i n g circuit.
I t will be r e c a l l e d t h a t the floating c o n d i t i o n w h i c h is d e s i r e d , a n d w h i c h is i l l u s t r a t e d by the c u r v e in Fig. 12, lies b e t w e e n the s i n g u l a r s t a b i l i t y c h a r a c t e r i s t i c of Fig. l 0 a n d the b i n a r y t y p e of. r e s p o n s e of Fig. 11. C l e a r l y , if the a d j u s t m e n t d e p a r t s from the o p t i m u m in one d i r e c t i o n the a c t i o n will tend t o w a r d s s i n g u l a r s t a b i l i t y , a n d if the a d j u s t m e n t d e v i a t e s in the opposite s e n s e , t h e r e will be a n a p p r o a c h t o the b i n a r y c o n d i t i o n of s t a b i l i t y . In each case the b e h a v i o r of the i n s t r u m e n t p o i n t e r will be c h a r a c t e r i s t i c of the s e n s e of d e v i a t i o n . I t is b e l i e v e d t h a t a d e s c r i p t i o n of the r e l a t i o n b e t w e e n the c i r c u i t a d j u s t m e n t and the a c t i o n of the i n s t r u m e n t p o i n t e r m a y p o s s i b l y be
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s o m e w h a t simplified by c o n s i d e r a t i o n of the v i s u a l aid or a n a l o g u e s h o w n in Fig. 23. T h e a c t i o n of the m e t e r pointer, in r e s p o n s e t o a s l o w l y v a r y i n g s i g n a l , m a y be c o m p a r e d with t h a t of a c i r c u l a r o b j e c t rolling on an a p p r o x i m a t e l y h o r i z o n t a l c u r v e d s u r f a c e . A s s u g g e s t e d by the d i a g r a m the r o l l e r r e s t s on the u p p e r s u r f a c e of a c o n t a i n i n g m e m b e r c a p a b l e of b e i n g t i l t e d in e i t h e r d i r e c t i o n . N i n e c o n c e p t s of this s t r u c t u r e are i l l u s t r a t e d . I t is s h o w n a s h a v i n g a c o n c a v e , flat, or c o n v e x s u r f a c e . I t is also s h o w n in t h r e e d i f f e r e n t positions, t i l t e d t o the left, horizontal, a n d t i l t e d t o the r i g h t . T h e r e is a c l o s e a n a l o g y b e t w e e n the m a n n e r in w h i c h the p o i n t e r of the m i l l i a m m e t e r r e s p o n d s t o a s i g n a l a p p l i e d t o the i n p u t of the amplifier, and the w a y in w h i c h the r o l l e r will b e h a v e if a f o r c e be app l i e d t o it laterally, t e n d i n g t o d i s p l a c e it t o the r i g h t o r t o the left. TOO MUCH BIAS
TOO MUCH RESISTANCE
CORRECT RESISTANCE
TOO L I T T L E RESISTANCE
CORRECT BIAS
TOO L I T T L E BIAS
% % %
SINGULAR STABILITY
MODULATING CONDITION
BINARY STABILITY
FIG. 23. Mechanical analogue of action of modulating circuit.
S i m i l a r l y , a d j u s t m e n t of the b i a s c u r r e n t in the electrical c i r c u i t corr e s p o n d s c l o s e l y with the e f f e c t of a tilt in the m e c h a n i c a l c o u n t e r p a r t s h o w n in Fig. 23. F o r e x a m p l e , s u p p o s e we f i r s t c o n s i d e r the case of a c o n c a v e s u r f a c e . If t h e r e be no tilt a n d in the a b s e n c e of a n y controlling f o r c e b r o u g h t t o b e a r u p o n the roller, the l a t t e r will take up a position a t the l o w e s t p o i n t of the c u r v e , a p p r o x i m a t e l y a t the c e n t e r . If, by the a p p l i c a t i o n of an e x t e r n a l force, it be d i s p l a c e d from this p o s i t i o n it will r e t u r n t h e r e w h e n the f o r c e is w i t h d r a w n . This is c l e a r l y a case of s i n g u l a r s t a b i l i t y a n d c o r r e s p o n d s with the a c t i o n of the electrical c i r c u i t w h e n this is a d j u s t e d in like m a n n e r , t h a t is t o say, w h e n the c i r c u i t c o n s t a n t s are s u c h a s t o p r o d u c e a t e n d e n c y t o w a r d s the c h a r a c t e r i s t i c s of Fig. 10. In Fig. 22 the p u s h - b u t t o n w h i c h e m i t s the p o s i t i v e s i g n a l is m a r k e d " U P " a n d the o t h e r one " D O W N . " T h e e x p l a n a t i o n of the a c t i o n w h i c h f o l l o w s r e f e r s specifically t o very w e a k s i g n a l s , u n d e r w h i c h c o n d i tion the p h e n o m e n a t o be d e s c r i b e d are m a n i f e s t e d . A c c o r d i n g l y , a low
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resistance is connected in parallel with the i n p u t winding for the dual purpose of precluding opening of the i n p u t circuit by the push-buttons, and to limit the current in the i n p u t winding to a very low o r d e r of magnitude. A variable series resistance of relatively high value permits the signal level to be controlled with convenience and precision at the desired low values. In the absence of any signal the bias is adjusted so that the m e t e r r e a d s at mid-scale. If now the " U P " button be held down so that a positive signal of a low value be delivered to the i n p u t of the amplifier, the m e t e r pointer will be deflected to the right. When the button is released the pointer will r e t u r n to its previous position at mid-scale. Exactly similar action in the opposite sense will result from the operation of the " D O W N " button. If the bias current be increased the m e t e r pointer will have a normal position somewhat to the left of or below mid-scale. Similarly if in the mechanical analogue the surface be tilted to the left the n o r m a l position of the roller will likewise be more to the left. Decreasing the bias current gives an increased reading on the m e t e r in the same way that tilting the surface to the r i g h t displaces the roller to the right. Let us now consider the action when the surface is convex. In this case it will not be possible to r e t a i n the roller in the centre of the surface without the application of an external force. It will tend to remain either at one or the other end of the convex surface. If by applying an external force it be removed from either extremity, when the force is withdrawn it will, if the surface is not tilted, roll towards whichever of the two ends it is nearer. In the same manner, if the electric circuit be adjusted such that the b i n a r y stability condition obtains, the pointer of the instrument will not remain in mid-scale. It will take up a position towards the high or low reading end of the scale, and it will only be possible to transfer it from one position to the other by m e a n s of an i n p u t signal of sufficient magnitude of either positive or negative polarity as may be required. If the bias current be increased it will require a stronger positive signal to move the pointer from the low to the high reading position; but a signal of reduced strength will be sufficient to transfer it in the downward direction. This corresponds with a tilt to the left. Reducing the bias has the opposite effect corresponding to a tilt to the right; that is to say, it will be easier to move the pointer in the u p w a r d direction than downward. If now the circuit constants be adjusted to an intermediate condition so that the desired "floating" action is approached, the instrument pointer will behave in the following manner. If we operate the " U P " button and a p p l y a fairly strong positive signal the instrument pointer will move towards the right, indicating an output current value in the u p p e r portion of the scale. If we then release the push-button the
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pointer will be seen to remain in the vicinity of the l a t t e r position for a substantial period of time. Ultimately it will tend very slowly to drift away from this position. If on the o t h e r hand we a p p l y a negative signal by operating the " D O W N " push-button the pointer will be deflected towards the left and will take up a position indicating a low current value. And again, when the push-button is released the pointer will remain for a substantial time a t or near that position. In like m a n n e r if the pointer be positioned in mid-scale as may be done by suitably operating the push-buttons, it will retain this approximate position when the positioning signals are withdrawn. Suppose now we a p p l y a very weak signal. If this signal be of positive polarity the milliammeter pointer will be observed slowly to move towards the right, that is to say, in the direction of increasing output current. If the push-button be operated intermittently the pointer may be "inched" or "jogged" upwards. So long as the pushb u t t o n is depressed the pointer will move to the right. When the pushbutton is released the m o v e m e n t will stop. Exactly the same action will be noted in the opposite direction if the o t h e r push-button be operated so as to a p p l y a weak negative signal. U n d e r these conditions the action of the circuit will correspond to the condition suggested in the mechanical analogy when the surface is approximately both flat and horizontal. If the roller be displaced in either direction t h e r e is no strong restoring force tending to r e t u r n it to the central position, as when the surface is concave; or tending to cause it to move either to the right or left hand extremity, as is the case when the surface is convex. Continuing to consider the mechanical analogy, it will be evident that the force necessary to displace or position t h e roller when the surface is flat and horizontal, will be very much less than that which is required when the surface is curved or inclined. Correspondingly, therefore, when the electrical circuit is adjusted so as to give the "floating" action, and has exactly the proper a m o u n t of bias, the pointer may be displaced or positioned by m e a n s of a signal of much w e a k e r value than is necessary with either a singular or b i n a r y condition of stability. That is to say, the v a l u e of the output current can be controlled or modulated with transient i n p u t excitation of a very low order of magnitude. As has been mentioned above, the o p t i m u m adjustment is arrived at by observation of the behavior of the instrument pointer. This procedure may be described with some gain in conciseness if we may r e f e r to the action of the electrical circuit in terms based upon the mechanical analogue. Thus the correct adjustment of the bias current determines the degree of "levelness." This is indicated by the rate of m o v e m e n t of the instrument pointer up or down when signals of the
468
ALAN S. FITzGERALI).
[J. f. I.
same magnitude but of positive and negative polarity are alternately applied. If the reading of the milliammeter goes up, in response to a positive signal more rapidly than it is reduced by a negative signal the bias should be increased. Conversely, if tile pointer moves with greater speed towards the left with a negative signal but more slowly towards the right of the scale with a positive signal of tile same magnitude, then the bias should be decreased. The "flatness" of the characteristic is indicated in the first place, in making the preliminary adjustment, by whether tile pointer tends to drift towards mid-scale, indicating singular stability, or whether it tends to drift to either the left extremity or to the right extremity of the scale, which signifies that the circuit adjustments are such as to cause b i n a r y stability, in the absence of any signal. When action indicative of strongly marked "concave" or "convex" characteristic has been eliminated by adjustment of tile trimmer or series resistance, indication of the degree of flatness may be deduced by noting the speed of m o v e m e n t of the pointer tit different portions of tile scale, in response to weak signals. Thus, if the pointer is indicating a low current value towards the left hand end of tile scale, and a positive signal be applied the pointer will commence to move towards the right. If with no change in the signal the rate of m o v e m e n t of the pointer increases, this will be an indication that there is still some "convexity" present. If on the o t h e r hand, tile rate of m o v e m e n t of the pointer decreases, and if, perhaps, the pointer comes to rest without reaching the u p p e r portion of the scale, it may be inferred that some degree of "concavity" is still present. In other words, tile objective which is aimed at in making this adjustment is that the application of weak signals of the same wflue of both positive anti negative polarity should, in the first place, approach a speed of response which is equal in both directions; and in tile second place, this speed of response should be substantially the same at all points on the scale of the instrument. An electro-mechanical counterpart of the above circuit would be represented by a variable-output device such as a rheostat, driven by a reversing m o t o r controlled by the two " U P " and " D O W N " pushbuttons. With such an arrangement the output current will be determined by the position of the rheostat arm, which l a t t e r will move in one direction or the other as the m o t o r is operated. Thus the output current will increase steadily as long as the " U P " push-button is held down; or will continue to be reduced as long as tile " D O W N " button is operated: and will remain at a constant value when neither of the pushbuttons is depressed. The practical utility of this modulating circuit is not predicated upon the achievement of absolute perfection in the reproduction of the action of the above electro-mechanical counterpart. That is to say, it is
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MAGNETIC AMPLIFIERS.
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n o t e s s e n t i a l t h a t the v a l u e of the o u t p u t c u r r e n t r e m a i n a t a c o n s t a n t v a l u e , w h e n t h e r e is n o s i g n a l , for a n indefinitely p r o l o n g e d p e r i o d of t i m e . In s e l f - b a l a n c i n g r e g u l a t i n g or c o n t r o l s y s t e m s of the type t o w h i c h r e f e r e n c e h a s b e e n m a d e a b o v e , it is of e q u a l significance if the C u r r e n t v a l u e be held for m o d e r a t e p e r i o d s of time of the o r d e r of a m i n u t e or two. In s u c h s y s t e m s , w h e n e v e r t h e r e is a d e v i a t i o n from the p r o p e r v a l u e t o w h i c h the c u r r e n t s h o u l d be m o d u l a t e d , a c o r r e c t i v e s i g n a l is a u t o m a t i c a l l y set up so t h a t the r e q u i r e d v a l u e is c o n t i n u o u s l y maintained. I n d e e d , in m a n y s u c h c o n t r o l a r r a n g e m e n t s the o u t p u t c u r r e n t is n o t r e q u i r e d t o be r e g u l a t e d t o a c o n s t a n t v a l u e b u t r a t h e r one s u b j e c t t o c o n t i n u o u s or f r e q u e n t v a r i a t i o n . In s u c h c i r c u m s t a n c e s a p e r f e c t type of a c t i o n c a p a b l e of h o l d i n g a c u r r e n t a t a fixed v a l u e for long p e r i o d s of time w o u l d have no pertinence. LINEAR RESPONSE AMPLIFIER.
T h i s m o d u l a t i n g p r i n c i p l e m a y be u t i l i z e d t o p r o v i d e a self-regulating t y p e of m a g n e t i c a m p l i f i e r c a p a b l e of g i v i n g a n a c c u r a t e l i n e a r r e s p o n s e t o an i n p u t s i g n a l . S u c h a n a r r a n g e m e n t is s h o w n in Fig. 24, in w h i c h the o u t p u t c u r r e n t is c o n t i n u o u s l y a u t o m a t i c a l l y r e g u l a t e d So t h a t it b e a r s a cons t a n t r e l a t i o n t o the i n p u t s i g n a l . This r a t i o is m a i n t a i n e d by m e a n s of a t a p p e d r e s i s t o r or p o t e n t i a l d i v i d e r w h i c h is e n e r g i z e d from t h e o u t p u t of the amplifier. A g i v e n f r a c t i o n of the o u t p u t v o l t a g e , obt a i n e d from the tap on the p o t e n t i a l d i v i d e r , is c o m p a r e d with the s i g n a l , a n d a n y difference b e t w e e n t h e s e two v o l t a g e s is a m p l i f i e d by a p o l a r i z e d type a m p l i f i e r a n d a p p l i e d as a c o n t r o l s i g n a l t o a m o d u l a t o r type c i r c u i t s u c h a s t h a t in Fig. 22. T h u s , the o u t p u t of the m o d u l a t o r is i n c r e a s e d if the v o l t a g e o b t a i n e d from the p o t e n t i a l d i v i d e r t a p is less t h a n the s i g n a l v o l t a g e , or is d e c r e a s e d if the o u t p u t is g r e a t e r t h a n the proper value. T h e o u t p u t of the m o d u l a t o r is fed t o a t h i r d s t a g e w h i c h is of the n e u t r a l type and w h i c h , b e c a u s e it is r e q u i r e d t h a t the o u t p u t be r e d u c e d t o zero w h e n t h e r e is no s i g n a l , is of the counter-poise type w h i c h h a s a l r e a d y b e e n d e s c r i b e d by the a u t h o r e l s e w h e r e (3). I t h a s a l r e a d y b e e n m e n t i o n e d t h a t , a s is seen Fig. 12, the r a n g e of the m o d u l a t o r type c i r c u i t does not go c o m p l e t e l y down t o z e r o . F o r this r e a s o n the t h i r d s t a g e is p r o v i d e d with a c o m p e n s a t i n g w i n d i n g , the excitation of w h i c h is a d j u s t e d so t h a t w h e n the o u t p u t of the s e c o n d o r m o d u l a t o r s t a g e is a t its m i n i m u m v a l u e the n e t a m p e r e t u r n s a p p l i e d t o the core of the t h i r d s t a g e s a t u r a t i n g r e a c t o r is z e r o , u n d e r w h i c h c o n d i t i o n the o u t p u t of the t h i r d s t a g e , a n d therefore the c u r r e n t in the p o t e n t i a l d i v i d e r , is l i k e w i s e zero. T h e p o l a r i z e d a m p l i f i e r u s e d in the f i r s t s t a g e is s h o w n only in b l o c k d i a g r a m form in o r d e r t o s i m p l i f y the i l l u s t r a t i o n of the p r i n c i p l e of the l i n e a r a m p l i f i e r circuit, a n d is of the type s h o w n in Fig. 14.
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ALAN S. FITZGERALD.
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T h e b l o c k i n g l i m i t e r of Fig. 16 is also i n c l u d e d . T h i s m a k e s ~ i t possible for the i n p u t s i g n a l t o be a p p l i e d i n s t a n t a n e o u s l y at its m a x i mum v a l u e , or t o be s u d d e n l y r e d u c e d t o zero, w i t h o u t c a u s i n g a n y hysteresis zero-shift in the polarized amplifier.
/
COUNTERPOISE
/
SAT REACTOR
MODULATOR
BLOCKING
LIMITER AMPLIFIER
¢J
......................
o
OUTPUT
INPUT FIG. 24. Linear r e s p o n s e m a g n e t i c amplifier.
I n l i n e a r amplifiers of t h i s type t h e r e is no difficulty in obtaining an accuracy, over t h e full operating r a n g e , of within o n e microampere. T h a t is to say, t h e difference between t h e i n p u t signal and t h e fraction
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of the o u t p u t which should have the same value, will not, except when the signal is-rapidly varying, exceed one microampere. In order to simplify the description and explanation only a single stage of amplification is shown in Fig. 24. However, any required number of additional stages of amplification may be incorporated in the linear amplifier so as to step up the output power to any desired level. Amplifiers of this type can be b u i l t with outputs in the kilowatt range. It will be noted that the action of the linear response magnetic amplifier is very similar to that of the rotary device known as the amplidyne. While it is possible that the magnetic amplifier may not at present compare favorably with the amplidyne in speed of response, it embraces a range of i n p u t and output power level which would require a combination of both amplidyne and electronic apparatus. It may also present some advantage in applications where, for any reason, it may be desired to avoid rotating parts, commutators, and brushes. REFERENCES.
(1) FITZGERALD, U. S. Patent N o . 2,168,402. (2) ALAN S. FITZGERALD, "Magnetic Amplifier Circuits Neutral T y p e , " JOURNAL OF THE FRANKLIN INSTITUTE, Vol. 244, N o . 4 (1947). See Fig. 11. (3) Ibid. See Fig. 10.
I N M A G N E T I C AMPLIFIERS. BY
ALAN S. FITZGERALDJ Part If.2
BINARY STABILITY.
T h e a r r a n g e m e n t of Fig. 9, w h e n the c i r c u i t c o n s t a n t s are a d j u s t e d so as t o give the c h a r a c t e r i s t i c s of Fig. 11, is c a p a b l e of m a n i f e s t i n g a m o s t interesting type of a c t i o n , s i m i l a r t o t h a t of the f a m i l i a r " l a t c h e d in" type of e l e c t r i c r e l a y . T h i s e f f e c t m a y , p e r h a p s , be m o r e e a s i l y d e s c r i b e d a n d e x p l a i n e d by r e f e r r i n g t o Fig. 17, w h i c h s h o w s a n e x a c t electro-mechanical a n a l o g u e Lamp
©
PUSH BUTTONS
Polarized Relay
°-7_ i 1 : .......................
T :,,,,,
",00
FIG. 17. Electro-mechanical analogue of application of binary stability.
of the a c t i o n w h i c h is p o s s i b l e with a s a t u r a t i n g r e a c t o r c i r c u i t u n d e r c o n d i t i o n s of b i n a r y s t a b i l i t y . In Fig. 17 a load or i n d i c a t i n g d e v i c e s u c h a s a l a m p , is controlled by m e a n s of a p o l a r i z e d r e l a y w h i c h h a s a s n a p - a c t i o n s p r i n g c o n t r o l a r r a n g e m e n t so t h a t the a r m a t u r e c a n n o t r e m a i n in m i d - p o s i t i o n b u t s n a p s one w a y or the o t h e r a c c o r d i n g t o the p o l a r i t y of the d-c. c u r r e n t 1 Electrical Research Engineer, Los Angeles, Calif. 2 Part I appears in the JOURNAL OF THE FRANKLIN INSTITUTE, Vol. 247, No. 3, March, 1949, p. 223. 457
458
ALaN S. I?ITzGERALD.
[J. F. 1.
a p p l i e d t o the coil, a n d a l w a y s r e m a i n s in the p o s i t i o n of the last o p e r a t i o n w h e n the c o n t r o l c u r r e n t is w i t h d r a w n . T h e r e l a y is controlled over a t w o - w i r e c i r c u i t by m e a n s of two p u s h - b u t t o n s and a m i d t a p p e d b a t t e r y , so t h a t the coil of the polarized r e l a y m a y be e n e r g i z e d a t will with a s i g n a l of e i t h e r p o l a r i t y . O b v i o u s l y , p u s h i n g one b u t t o n down m o m e n t a r i l y will c a u s e the l a m p t o l i g h t a n d t o stay lit. O p e r a ting the o t h e r b u t t o n will put the l a m p out, w h i c h c o n d i t i o n will 1)¢' r e t a i n e d u n t i l the " O N " b u t t o n be a g a i n a c t u a t e d . A t f i r s t t h o u g h t it w o u l d a l m o s t seem t h a t a p h y s i c a l m o v e m e n t , or a c h a n g e of position of some m a t e r i a l m e m b e r , is a f u n c t i o n a l n e c e s s i t y i n h e r e n t in such a s y s t e m . H o w e v e r , e x a c t l y s i m i l a r a c t i o n is m a n i f e s t e d by the a r r a n g e m e n t s h o w n in Fig. 18, in w h i c h the l a m p is e n e r g i z e d from the c i r c u i t of Fig.
Lamp--(~) |
PUSH BUTTONS OFF ON
~ ~l'lq'lq'l
,lll,l~ltlll--~+
B,as winding
-------_-_---~ --_-_--i~
~ ~
~
L
t
AC
~ J winding ~ % 1 Series ~ I/winding
]
. . . . . . . . winding
~,j
If0 VOLTS AC Fro. 18. Application of binary stability principle to control circuit. 9, the input coil of which is energized by the same battery and pushbutton arrangement as in Fig. 17. There are no moving parts in this system and all the circuits are permanently closed with the obvious exception of the signal-emitting device, duplicating that of Fig. 17. If the circuit is adjusted in accordance with Fig. Ii, when the " O N " button is depressed the ]amp lights. When the button is released the lamp remains illuminated. If the " O F F " button be now operated, despite the fact that the magnetic amplifier circuit remains connected to the power supply, the lamp will appear to go out; that is to say the current will be reduced to a low level insufficient to cause the lamp to emit any visible light. After the " O F F " button is released the ]amp remains out.
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It is believed that it will be apparent to the reader that this action logically results from the characteristic illustrated by the curve in Fig. 11 and that further explanation will be unnecessary. The analogy between this magnetic amplifier circuit and the electromechanical arrangement of Fig. 17 is maintained u n d e r another operating condition in an even more striking manner. If the a-c. s u p p l y to the magnetic amplifier circuit in Fig. 18 be interrupted, when the power is restored, neither push-button having been Operated in the meanwhile, the action existing at the m o m e n t of disconnection will be reproduced. If the lamp was not illuminated when the power was withdrawn, it will not l i g h t up when it is restored. But if the light be on when the power was disconnected, as soon as the circuit is a g a i n energized with a.c., the lamp lights up again.
i
E~EEE~
F:G. 19. Binary stability principle used for pendulum drive.
Furthermore, if the signal be applied at a time when the magnetic amplifier circuit is de-energized, whether or not the lamp will light when the a-c. power is restored, can be pre-determined. This "memory" action is due to a residual magnetism effect in the core. The application of an input signal of reversible polarity is not, however, the only effect by m e a n s of which the binary stability circuit may be caused to transfer from one r a n g e to the other, and therefore to undergo a c h a n g e in current v a l u e of substantial amplitude. A study of the curves of Figs. 5, 6, 7, and 11 will indicate that t h e r e are a n u m b e r of different changes in circuit constants that can cause such a transfer. For example, if we refer to Figs. 5, 6, and 7 it is apparent that a modification in the a m o u n t of feed-back such as m i g h t cause the circuit
460
ALAN S. FITZGERAI,D.
[J. F. I.
c h a r a c t e r i s t i c s t o c h a n g e f r o m , say, Fig. 5 t o Fig. 7 c a n c a u s e this t r a n s f e r . T h i s m a y be seen t o be the c a s e , for i n s t a n c e , a t the p o r t i o n of the c u r v e s c o r r e s p o n d i n g t o a n i n p u t e x c i t a t i o n in the n e i g h b o r h o o d of - 1 m a . S u c h a r e s u l t c o u l d o c c u r if the f e e d - b a c k t u r n s be c h a n g e d from 450 t o 350. H o w e v e r , it h a s been p o i n t e d out t h a t the f e e d - b a c k is a f u n c t i o n of the o u t p u t c i r c u i t r e s i s t a n c e a s well as the t u r n s . T h e s a m e r e s u l t m i g h t take p l a c e if i n s t e a d of c h a n g i n g the f e e d - b a c k t u r n s , the res i s t a n c e or i m p e d a n c e of the load c i r c u i t be m o d i f i e d . F i g u r e 19 s h o w s a n i n t r i g u i n g a p p l i c a t i o n of this principle. A p e n d u l u m is p r o v i d e d with a n a c t u a t i n g solenoid h a v i n g a s u i t a b l y s h a p e d p l u n g e r m o u n t e d u p o n the p e n d u l u m and e n t e r i n g and r e t r a c t i n g from the solenoid coil as the p e n d u l u m swings. T h e r e a c t a n c e of this s o l e n o i d w i n d i n g w i l l , of c o u r s e , v a r y a s the p l u n g e r e n t e r s t o a g r e a t e r or less e x t e n t i n t o the coil. W h e n the p l u n g e r s w i n g s f u l l y i n t o the coil the r e a c t a n c e will be r e l a t i v e l y h i g h ; w h e n it s w i n g s in the o t h e r d i r e c t i o n a l o w e r v a l u e of r e a c t a n c e will result. In Fig. 19 the s o l e n o i d w i n d i n g is e n e r g i z e d from a b i n a r y s t a b i l i t y type c i r c u i t such a s h a s b e e n d e s c r i b e d . It is c l e a r t h a t a n i n c r e a s e in the r e a c t a n c e will r e d u c e the effective f e e d - b a c k ; a n d t h a t , conversely, the f e e d - b a c k will be g r e a t e r w h e n the i m p e d a n c e is l e s s . I t f o l l o w s t h e r e f o r e t h a t the f e e d - b a c k will d e c r e a s e w h e n the p l u n g e r s w i n g s f u l l y i n t o the coil a n d will b e c o m e g r e a t e r w h e n it s w i n g s in the o u t w a r d direction. C l e a r l y , if this m e r e l y c a u s e d a m o r e or less p r o p o r t i o n a l v a r i a t i o n in the c u r r e n t in the coil, no effective d y n a m i c a c t i o n c o u l d result. H o w e v e r , in the a r r a n g e m e n t of Fig. 19 the c i r c u i t c o n s t a n t s can be a d j u s t e d so t h a t the m o v e m e n t of the p e n d u l u m r e s u l t s in the d i s c o n t i n u i t y e f f e c t w h e r e b y the c i r c u i t t r a n s f e r s from one s t a b i l i t y r a n g e t o the o t h e r , and b a c k a g a i n ; this c a u s e s n o t a g r a d u a l , b u t an a b r u p t a n d s u b s t a n t i a l , c h a n g e in c u r r e n t due t o the p l u n g e r m o v e m e n t . T h a t is t o say, a t a c e r t a i n p o i n t on the o u t w a r d t r a v e l of the p l u n g e r the c u r r e n t s u d d e n l y i n c r e a s e s s e v e r a l h u n d r e d per c e n t ; in like m a n n e r a s the p e n d u l u m s w i n g s t o w a r d s the coil a s u d d e n c h a n g e in c u r r e n t in opposite sense takes place. I t is c l e a r t h a t this r e l a t i o n b e t w e e n the p l u n g e r position and the c u r r e n t in the s o l e n o i d coil, a n d a c c o r d i n g l y the pull e x e r t e d t h e r e b y u p o n the p l u n g e r , w h i c h p u l s a t e s in s y n c h r o n i s m with the s w i n g of the p e n d u l u m , has the characteristics n e c e s s a r y t o d r i v e the p e n d u l u m . T h u s , once the p e n d u l u m is s t a r t e d s w i n g i n g m o t i o n is c o n t i n u e d a s long a s the c i r c u i t is e n e r g i z e d with a l t e r n a t i n g c u r r e n t . W h i l e it is not o b v i o u s t h a t this d e v i c e h a s a n y g r e a t v a l u e in horology, n e v e r t h e l e s s it f o r m s a v e r y s i m p l e and i n e x p e n s i v e a c t i o n d e v i c e for d i s p l a y or o t h e r p u r p o s e s w h e r e a n oscillating m o v e m e n t
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m a y be of interest. T h e a b s e n c e of a n y c o n t a c t s s h o u l d be f a v o r a b l e t o reliability. I t is a l s o of p o s s i b l e i n t e r e s t a s a g e n e r a t o r of p u l s a t i n g electrical effects of more s u b s t a n t i a l m a g n i t u d e s i n c e , by f u r t h e r s t a g e s of m a g n e t i c amplification, t h e p u l s a t i n g c u r r e n t in the c i r c u i t of Fig. 19 m a y be s t e p p e d up t o s u b s t a n t i a l p o w e r l e v e l s . F o r e x a m p l e , i n s t e a d of the p e n d u l u m , a n oscillating s y s t e m b a s e d u p o n s p r i n g i n s t e a d of g r a v i t y control, m a y be u s e d t o p r o v i d e a n electric h a m m e r or r i v e t e r . I t is, h o w e v e r , by m e a n s of this b i n a r y s t a b i l i t y c i r c u i t principle, possible to produce a sustained pulsating effect w i t h o u t any mechanical m o v i n g e l e m e n t . If a pair of c i r c u i t s of the form s h o w n in Fig. 9 be
FIG. 20. Oscillating or pulsating magnetic amplifier circuit.
c o n n e c t e d t o g e t h e r so t h a t each one acts as a t e a s e r for the o t h e r , it is p o s s i b l e t o a r r a n g e for m u t u a l interaction s u c h t h a t the d u a l c i r c u i t c o n t i n u e s indefinitely in a c o n d i t i o n of oscillation or p u l s a t i o n . S u c h a n a r r a n g e m e n t is i l l u s t r a t e d in Fig. 20 (1). a In this d i a g r a m it will be n o t e d t h a t the o u t p u t c i r c u i t of each of the d u p l i c a t e m a g n e t i c a m p l i f i e r e l e m e n t s is c o n n e c t e d in s e r i e s with the i n p u t c i r c u i t of the o t h e r one. T h u s w h e n the r i g h t h a n d u n i t t r a n s f e r s from the l o w e r r a n g e of s t a b i l i t y t o the u p p e r r a n g e i t d e l i v e r s a s i g n a l t o the left h a n d u n i t s u c h a s t o c a u s e this l i k e w i s e t o c h a n g e from the low v a l u e t o t h e h i g h c u r r e n t v a l u e . T h i s in t u r n e n e r g i z e s the i n p u t of the r i g h t h a n d u n i t in the o p p o s i t e s e n s e t h r o w i n g the l a t t e r b a c k i n t o the l o w e r sta8 The boldface numbers in parentheses refer to the references appended to this part of the paper.
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ALAN S. F[TZGEI~aLD.
[J. I;. Z.
b i l i t y r a n g e . T h i s closed-cycle c h a i n r e a c t i o n c o n t i n u e s a s long a s the c i r c u i t s are e n e r g i z e d from the a-c. s o u r c e . S p e c i a l f e a t u r e s m a y be i n c l u d e d in the b i a s c i r c u i t s for i n h i b i t i n g the p u l s a t i n g e f f e c t or for perm i t t i n g it t o o c c u r . O t h e r c i r c u i t a d j u s t m e n t s m a y be i n c l u d e d w h i c h
i
I
I
I
I
I
FIG. 2I. Lamp control application of pulsating circuit.
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have the effect of modifying in some degree the type of dynamic action which ensues. For further details of t h e s e arrangements reference is invited to the citation. An application of this latter principle is illustrated in Fig. 21 (1). In this arrangement one of the output currents of the system shown in F i g . 2 0 is connected in series with the input windings of a pair of single magnetic amplifier stages of the compensating winding type shown in a previous paper (2). Each of the output circuits of both of t h e s e magnetic amplifiers is connected to a signal lamp. The input windings of the pair of amplifiers therefore both receive identical signals consisting of a uni-directional current continuously pulsating between a high and a low value. Both of the lamp amplifiers are identical except that the compensating currents are not adjusted to like values. In one case, the compensating excitation is adjusted to be equal to the input signal when the current is at its minimum value; in the other case, the bucking ampere-turns are made equal to the m a x i m u m value. Thus one of the lamp amplifiers will deliver only a very low current when the signal has its minimum value and the lamp will be lighted when the input current has its high value. The other lamp, however, will be out with the high value of the pulsating current and on when it has the minimum value. Thus the two lamps will flash alternately in the m a n n e r of the familiar g r a d e crossing d a n g e r signal. This complete arrangement, it will again be noted, has the reliability feature of complete absence of contacts or moving parts. MODULATING CONDITION.
This p h a s e of the behavior of feed-back type magnetic amplifiers is probably of the greatest potential v a l u e in that it m a k e s possible amplifiers having very accurate linear response characteristics. We shall now consider more specially the action of the circuit of Fig. 9 when the circuit c o n s t a n t s are chosen so as to provide the response characteristics of the type shown in Fig. 12. Figure 22 shows the circuit of Fig. 9 with the addition of an output circuit which includes a milliammeter and an adjustable sei-ies resistance, together with a push-button arrangement similar to that of Fig. 18 for furnishing an input of reversible polarity. A second adjustable resistor is provided for determining the value of the bias current. Assuming that the type of rectifier and the n u m b e r of series or feedback t u r n s are suitably chosen, and that the resistance of the load or output circuit is somewhat less than the optimum value, the action of the circuit can be controlled by the two adjustable elements, the bias resistor and the series resistor. The latter serves as a trimmer; it may be of a low v a l u e as compared with that of the output circuit, and such
464
ALAN S. FITZGERAI.I).
IJ. b~. I.
a s e n a b l e s the sum of the t r i m m e r a n d the o u t p u t c i r c u i t r e s i s t a n c e v a l u e s t o be v a r i e d s o m e w h a t a b o v e a n d b e l o w the e s t i m a t e d o p t i m u m o u t p u t c i r c u i t resistance. In o r d e r t o s e c u r e the d e s i r e d characteristics t h e s e two c o n t r o l s are t o be a d j u s t e d with s o m e n i c e t y . T h e e x a c t v a l u e s w h i c h a r e r e q u i r e d a r e d e t e r m i n e d by o b s e r v i n g the b e h a v i o r of the p o i n t e r of the m i l l i a m m e t e r . A s d e s c r i b e d a b o v e w h e n the a d j u s t m e n t s h a v e been c o r r e c t l y m a d e this i n s t r u m e n t g i v e s an i n d i c a t i o n of i n d e t e r m i n a t e v a l u e ; t h a t is t o say, it b e h a v e s like a f l u x - m e t e r or o t h e r u n c o n t r o l l e d type of instrument.
no
O~fp~t ~[rcuit Resis'l'6v~ce
no
LIP
DOWN
FIG. 22. M o d u l a t i n g circuit.
I t will be r e c a l l e d t h a t the floating c o n d i t i o n w h i c h is d e s i r e d , a n d w h i c h is i l l u s t r a t e d by the c u r v e in Fig. 12, lies b e t w e e n the s i n g u l a r s t a b i l i t y c h a r a c t e r i s t i c of Fig. l 0 a n d the b i n a r y t y p e of. r e s p o n s e of Fig. 11. C l e a r l y , if the a d j u s t m e n t d e p a r t s from the o p t i m u m in one d i r e c t i o n the a c t i o n will tend t o w a r d s s i n g u l a r s t a b i l i t y , a n d if the a d j u s t m e n t d e v i a t e s in the opposite s e n s e , t h e r e will be a n a p p r o a c h t o the b i n a r y c o n d i t i o n of s t a b i l i t y . In each case the b e h a v i o r of the i n s t r u m e n t p o i n t e r will be c h a r a c t e r i s t i c of the s e n s e of d e v i a t i o n . I t is b e l i e v e d t h a t a d e s c r i p t i o n of the r e l a t i o n b e t w e e n the c i r c u i t a d j u s t m e n t and the a c t i o n of the i n s t r u m e n t p o i n t e r m a y p o s s i b l y be
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s o m e w h a t simplified by c o n s i d e r a t i o n of the v i s u a l aid or a n a l o g u e s h o w n in Fig. 23. T h e a c t i o n of the m e t e r pointer, in r e s p o n s e t o a s l o w l y v a r y i n g s i g n a l , m a y be c o m p a r e d with t h a t of a c i r c u l a r o b j e c t rolling on an a p p r o x i m a t e l y h o r i z o n t a l c u r v e d s u r f a c e . A s s u g g e s t e d by the d i a g r a m the r o l l e r r e s t s on the u p p e r s u r f a c e of a c o n t a i n i n g m e m b e r c a p a b l e of b e i n g t i l t e d in e i t h e r d i r e c t i o n . N i n e c o n c e p t s of this s t r u c t u r e are i l l u s t r a t e d . I t is s h o w n a s h a v i n g a c o n c a v e , flat, or c o n v e x s u r f a c e . I t is also s h o w n in t h r e e d i f f e r e n t positions, t i l t e d t o the left, horizontal, a n d t i l t e d t o the r i g h t . T h e r e is a c l o s e a n a l o g y b e t w e e n the m a n n e r in w h i c h the p o i n t e r of the m i l l i a m m e t e r r e s p o n d s t o a s i g n a l a p p l i e d t o the i n p u t of the amplifier, and the w a y in w h i c h the r o l l e r will b e h a v e if a f o r c e be app l i e d t o it laterally, t e n d i n g t o d i s p l a c e it t o the r i g h t o r t o the left. TOO MUCH BIAS
TOO MUCH RESISTANCE
CORRECT RESISTANCE
TOO L I T T L E RESISTANCE
CORRECT BIAS
TOO L I T T L E BIAS
% % %
SINGULAR STABILITY
MODULATING CONDITION
BINARY STABILITY
FIG. 23. Mechanical analogue of action of modulating circuit.
S i m i l a r l y , a d j u s t m e n t of the b i a s c u r r e n t in the electrical c i r c u i t corr e s p o n d s c l o s e l y with the e f f e c t of a tilt in the m e c h a n i c a l c o u n t e r p a r t s h o w n in Fig. 23. F o r e x a m p l e , s u p p o s e we f i r s t c o n s i d e r the case of a c o n c a v e s u r f a c e . If t h e r e be no tilt a n d in the a b s e n c e of a n y controlling f o r c e b r o u g h t t o b e a r u p o n the roller, the l a t t e r will take up a position a t the l o w e s t p o i n t of the c u r v e , a p p r o x i m a t e l y a t the c e n t e r . If, by the a p p l i c a t i o n of an e x t e r n a l force, it be d i s p l a c e d from this p o s i t i o n it will r e t u r n t h e r e w h e n the f o r c e is w i t h d r a w n . This is c l e a r l y a case of s i n g u l a r s t a b i l i t y a n d c o r r e s p o n d s with the a c t i o n of the electrical c i r c u i t w h e n this is a d j u s t e d in like m a n n e r , t h a t is t o say, w h e n the c i r c u i t c o n s t a n t s are s u c h a s t o p r o d u c e a t e n d e n c y t o w a r d s the c h a r a c t e r i s t i c s of Fig. 10. In Fig. 22 the p u s h - b u t t o n w h i c h e m i t s the p o s i t i v e s i g n a l is m a r k e d " U P " a n d the o t h e r one " D O W N . " T h e e x p l a n a t i o n of the a c t i o n w h i c h f o l l o w s r e f e r s specifically t o very w e a k s i g n a l s , u n d e r w h i c h c o n d i tion the p h e n o m e n a t o be d e s c r i b e d are m a n i f e s t e d . A c c o r d i n g l y , a low
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resistance is connected in parallel with the i n p u t winding for the dual purpose of precluding opening of the i n p u t circuit by the push-buttons, and to limit the current in the i n p u t winding to a very low o r d e r of magnitude. A variable series resistance of relatively high value permits the signal level to be controlled with convenience and precision at the desired low values. In the absence of any signal the bias is adjusted so that the m e t e r r e a d s at mid-scale. If now the " U P " button be held down so that a positive signal of a low value be delivered to the i n p u t of the amplifier, the m e t e r pointer will be deflected to the right. When the button is released the pointer will r e t u r n to its previous position at mid-scale. Exactly similar action in the opposite sense will result from the operation of the " D O W N " button. If the bias current be increased the m e t e r pointer will have a normal position somewhat to the left of or below mid-scale. Similarly if in the mechanical analogue the surface be tilted to the left the n o r m a l position of the roller will likewise be more to the left. Decreasing the bias current gives an increased reading on the m e t e r in the same way that tilting the surface to the r i g h t displaces the roller to the right. Let us now consider the action when the surface is convex. In this case it will not be possible to r e t a i n the roller in the centre of the surface without the application of an external force. It will tend to remain either at one or the other end of the convex surface. If by applying an external force it be removed from either extremity, when the force is withdrawn it will, if the surface is not tilted, roll towards whichever of the two ends it is nearer. In the same manner, if the electric circuit be adjusted such that the b i n a r y stability condition obtains, the pointer of the instrument will not remain in mid-scale. It will take up a position towards the high or low reading end of the scale, and it will only be possible to transfer it from one position to the other by m e a n s of an i n p u t signal of sufficient magnitude of either positive or negative polarity as may be required. If the bias current be increased it will require a stronger positive signal to move the pointer from the low to the high reading position; but a signal of reduced strength will be sufficient to transfer it in the downward direction. This corresponds with a tilt to the left. Reducing the bias has the opposite effect corresponding to a tilt to the right; that is to say, it will be easier to move the pointer in the u p w a r d direction than downward. If now the circuit constants be adjusted to an intermediate condition so that the desired "floating" action is approached, the instrument pointer will behave in the following manner. If we operate the " U P " button and a p p l y a fairly strong positive signal the instrument pointer will move towards the right, indicating an output current value in the u p p e r portion of the scale. If we then release the push-button the
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pointer will be seen to remain in the vicinity of the l a t t e r position for a substantial period of time. Ultimately it will tend very slowly to drift away from this position. If on the o t h e r hand we a p p l y a negative signal by operating the " D O W N " push-button the pointer will be deflected towards the left and will take up a position indicating a low current value. And again, when the push-button is released the pointer will remain for a substantial time a t or near that position. In like m a n n e r if the pointer be positioned in mid-scale as may be done by suitably operating the push-buttons, it will retain this approximate position when the positioning signals are withdrawn. Suppose now we a p p l y a very weak signal. If this signal be of positive polarity the milliammeter pointer will be observed slowly to move towards the right, that is to say, in the direction of increasing output current. If the push-button be operated intermittently the pointer may be "inched" or "jogged" upwards. So long as the pushb u t t o n is depressed the pointer will move to the right. When the pushbutton is released the m o v e m e n t will stop. Exactly the same action will be noted in the opposite direction if the o t h e r push-button be operated so as to a p p l y a weak negative signal. U n d e r these conditions the action of the circuit will correspond to the condition suggested in the mechanical analogy when the surface is approximately both flat and horizontal. If the roller be displaced in either direction t h e r e is no strong restoring force tending to r e t u r n it to the central position, as when the surface is concave; or tending to cause it to move either to the right or left hand extremity, as is the case when the surface is convex. Continuing to consider the mechanical analogy, it will be evident that the force necessary to displace or position t h e roller when the surface is flat and horizontal, will be very much less than that which is required when the surface is curved or inclined. Correspondingly, therefore, when the electrical circuit is adjusted so as to give the "floating" action, and has exactly the proper a m o u n t of bias, the pointer may be displaced or positioned by m e a n s of a signal of much w e a k e r value than is necessary with either a singular or b i n a r y condition of stability. That is to say, the v a l u e of the output current can be controlled or modulated with transient i n p u t excitation of a very low order of magnitude. As has been mentioned above, the o p t i m u m adjustment is arrived at by observation of the behavior of the instrument pointer. This procedure may be described with some gain in conciseness if we may r e f e r to the action of the electrical circuit in terms based upon the mechanical analogue. Thus the correct adjustment of the bias current determines the degree of "levelness." This is indicated by the rate of m o v e m e n t of the instrument pointer up or down when signals of the
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ALAN S. FITzGERALI).
[J. f. I.
same magnitude but of positive and negative polarity are alternately applied. If the reading of the milliammeter goes up, in response to a positive signal more rapidly than it is reduced by a negative signal the bias should be increased. Conversely, if tile pointer moves with greater speed towards the left with a negative signal but more slowly towards the right of the scale with a positive signal of tile same magnitude, then the bias should be decreased. The "flatness" of the characteristic is indicated in the first place, in making the preliminary adjustment, by whether tile pointer tends to drift towards mid-scale, indicating singular stability, or whether it tends to drift to either the left extremity or to the right extremity of the scale, which signifies that the circuit adjustments are such as to cause b i n a r y stability, in the absence of any signal. When action indicative of strongly marked "concave" or "convex" characteristic has been eliminated by adjustment of tile trimmer or series resistance, indication of the degree of flatness may be deduced by noting the speed of m o v e m e n t of the pointer tit different portions of tile scale, in response to weak signals. Thus, if the pointer is indicating a low current value towards the left hand end of tile scale, and a positive signal be applied the pointer will commence to move towards the right. If with no change in the signal the rate of m o v e m e n t of the pointer increases, this will be an indication that there is still some "convexity" present. If on the o t h e r hand, tile rate of m o v e m e n t of the pointer decreases, and if, perhaps, the pointer comes to rest without reaching the u p p e r portion of the scale, it may be inferred that some degree of "concavity" is still present. In other words, tile objective which is aimed at in making this adjustment is that the application of weak signals of the same wflue of both positive anti negative polarity should, in the first place, approach a speed of response which is equal in both directions; and in tile second place, this speed of response should be substantially the same at all points on the scale of the instrument. An electro-mechanical counterpart of the above circuit would be represented by a variable-output device such as a rheostat, driven by a reversing m o t o r controlled by the two " U P " and " D O W N " pushbuttons. With such an arrangement the output current will be determined by the position of the rheostat arm, which l a t t e r will move in one direction or the other as the m o t o r is operated. Thus the output current will increase steadily as long as the " U P " push-button is held down; or will continue to be reduced as long as tile " D O W N " button is operated: and will remain at a constant value when neither of the pushbuttons is depressed. The practical utility of this modulating circuit is not predicated upon the achievement of absolute perfection in the reproduction of the action of the above electro-mechanical counterpart. That is to say, it is
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n o t e s s e n t i a l t h a t the v a l u e of the o u t p u t c u r r e n t r e m a i n a t a c o n s t a n t v a l u e , w h e n t h e r e is n o s i g n a l , for a n indefinitely p r o l o n g e d p e r i o d of t i m e . In s e l f - b a l a n c i n g r e g u l a t i n g or c o n t r o l s y s t e m s of the type t o w h i c h r e f e r e n c e h a s b e e n m a d e a b o v e , it is of e q u a l significance if the C u r r e n t v a l u e be held for m o d e r a t e p e r i o d s of time of the o r d e r of a m i n u t e or two. In s u c h s y s t e m s , w h e n e v e r t h e r e is a d e v i a t i o n from the p r o p e r v a l u e t o w h i c h the c u r r e n t s h o u l d be m o d u l a t e d , a c o r r e c t i v e s i g n a l is a u t o m a t i c a l l y set up so t h a t the r e q u i r e d v a l u e is c o n t i n u o u s l y maintained. I n d e e d , in m a n y s u c h c o n t r o l a r r a n g e m e n t s the o u t p u t c u r r e n t is n o t r e q u i r e d t o be r e g u l a t e d t o a c o n s t a n t v a l u e b u t r a t h e r one s u b j e c t t o c o n t i n u o u s or f r e q u e n t v a r i a t i o n . In s u c h c i r c u m s t a n c e s a p e r f e c t type of a c t i o n c a p a b l e of h o l d i n g a c u r r e n t a t a fixed v a l u e for long p e r i o d s of time w o u l d have no pertinence. LINEAR RESPONSE AMPLIFIER.
T h i s m o d u l a t i n g p r i n c i p l e m a y be u t i l i z e d t o p r o v i d e a self-regulating t y p e of m a g n e t i c a m p l i f i e r c a p a b l e of g i v i n g a n a c c u r a t e l i n e a r r e s p o n s e t o an i n p u t s i g n a l . S u c h a n a r r a n g e m e n t is s h o w n in Fig. 24, in w h i c h the o u t p u t c u r r e n t is c o n t i n u o u s l y a u t o m a t i c a l l y r e g u l a t e d So t h a t it b e a r s a cons t a n t r e l a t i o n t o the i n p u t s i g n a l . This r a t i o is m a i n t a i n e d by m e a n s of a t a p p e d r e s i s t o r or p o t e n t i a l d i v i d e r w h i c h is e n e r g i z e d from t h e o u t p u t of the amplifier. A g i v e n f r a c t i o n of the o u t p u t v o l t a g e , obt a i n e d from the tap on the p o t e n t i a l d i v i d e r , is c o m p a r e d with the s i g n a l , a n d a n y difference b e t w e e n t h e s e two v o l t a g e s is a m p l i f i e d by a p o l a r i z e d type a m p l i f i e r a n d a p p l i e d as a c o n t r o l s i g n a l t o a m o d u l a t o r type c i r c u i t s u c h a s t h a t in Fig. 22. T h u s , the o u t p u t of the m o d u l a t o r is i n c r e a s e d if the v o l t a g e o b t a i n e d from the p o t e n t i a l d i v i d e r t a p is less t h a n the s i g n a l v o l t a g e , or is d e c r e a s e d if the o u t p u t is g r e a t e r t h a n the proper value. T h e o u t p u t of the m o d u l a t o r is fed t o a t h i r d s t a g e w h i c h is of the n e u t r a l type and w h i c h , b e c a u s e it is r e q u i r e d t h a t the o u t p u t be r e d u c e d t o zero w h e n t h e r e is no s i g n a l , is of the counter-poise type w h i c h h a s a l r e a d y b e e n d e s c r i b e d by the a u t h o r e l s e w h e r e (3). I t h a s a l r e a d y b e e n m e n t i o n e d t h a t , a s is seen Fig. 12, the r a n g e of the m o d u l a t o r type c i r c u i t does not go c o m p l e t e l y down t o z e r o . F o r this r e a s o n the t h i r d s t a g e is p r o v i d e d with a c o m p e n s a t i n g w i n d i n g , the excitation of w h i c h is a d j u s t e d so t h a t w h e n the o u t p u t of the s e c o n d o r m o d u l a t o r s t a g e is a t its m i n i m u m v a l u e the n e t a m p e r e t u r n s a p p l i e d t o the core of the t h i r d s t a g e s a t u r a t i n g r e a c t o r is z e r o , u n d e r w h i c h c o n d i t i o n the o u t p u t of the t h i r d s t a g e , a n d therefore the c u r r e n t in the p o t e n t i a l d i v i d e r , is l i k e w i s e zero. T h e p o l a r i z e d a m p l i f i e r u s e d in the f i r s t s t a g e is s h o w n only in b l o c k d i a g r a m form in o r d e r t o s i m p l i f y the i l l u s t r a t i o n of the p r i n c i p l e of the l i n e a r a m p l i f i e r circuit, a n d is of the type s h o w n in Fig. 14.
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T h e b l o c k i n g l i m i t e r of Fig. 16 is also i n c l u d e d . T h i s m a k e s ~ i t possible for the i n p u t s i g n a l t o be a p p l i e d i n s t a n t a n e o u s l y at its m a x i mum v a l u e , or t o be s u d d e n l y r e d u c e d t o zero, w i t h o u t c a u s i n g a n y hysteresis zero-shift in the polarized amplifier.
/
COUNTERPOISE
/
SAT REACTOR
MODULATOR
BLOCKING
LIMITER AMPLIFIER
¢J
......................
o
OUTPUT
INPUT FIG. 24. Linear r e s p o n s e m a g n e t i c amplifier.
I n l i n e a r amplifiers of t h i s type t h e r e is no difficulty in obtaining an accuracy, over t h e full operating r a n g e , of within o n e microampere. T h a t is to say, t h e difference between t h e i n p u t signal and t h e fraction
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of the o u t p u t which should have the same value, will not, except when the signal is-rapidly varying, exceed one microampere. In order to simplify the description and explanation only a single stage of amplification is shown in Fig. 24. However, any required number of additional stages of amplification may be incorporated in the linear amplifier so as to step up the output power to any desired level. Amplifiers of this type can be b u i l t with outputs in the kilowatt range. It will be noted that the action of the linear response magnetic amplifier is very similar to that of the rotary device known as the amplidyne. While it is possible that the magnetic amplifier may not at present compare favorably with the amplidyne in speed of response, it embraces a range of i n p u t and output power level which would require a combination of both amplidyne and electronic apparatus. It may also present some advantage in applications where, for any reason, it may be desired to avoid rotating parts, commutators, and brushes. REFERENCES.
(1) FITZGERALD, U. S. Patent N o . 2,168,402. (2) ALAN S. FITZGERALD, "Magnetic Amplifier Circuits Neutral T y p e , " JOURNAL OF THE FRANKLIN INSTITUTE, Vol. 244, N o . 4 (1947). See Fig. 11. (3) Ibid. See Fig. 10.