The magnet power servo system for the Princeton-Pennsylvania proton synchrotron

NUCLEAR

INSTRUMENTS

AND

METHODS

17 (1962)

87--93;

NORTH-HOLLAND

PUBLISHING

CO.

THE MAGNET POWER SERV0 SYSTEM FOR THE PRINCETON-PENNSYLVANIA PROTON SYNCHROTRON S. W A A B E N

Princeton-Pennsylvania Accelerator Princeton University, Princeton, New Jersey R e c e i v e d 6 M a y 1962 T h e m a g n e t p o w e r r e g u l a t i o n s y s t e m for t h e P r i n c e t o n P e n n s y l v a n i a 19 c/s r e p e t i t i o n r a t e 3 GeV P r o t o n S y n c h r o t r o n is d e s c r i b e d a n d a n a l y z e d . R o t a t i n g m a c h i n e r y is u s e d for s u p p l y i n g t h e losses of t h e c o m p l e x r e s o n a n c e s y s t e m p o w e r i n g the magnets.

T h e m a g n e t field v a r i a t i o n is a d c biased s i n e w a v e h a v i n g a m i n i m u m field of 250 g a u s s a n d a p e a k field of 13800 gauss. T h e f l u c t u a t i o n s a r e controlled so as to be less t h a n one g a u s s peak-peak.

1. Introduction

shows the magnets' bias current as a function of time. Standard industrial rotating machinery was chosen to be the power equipment supplying the a.c.

The Princeton Pennsylvania 3 GeV Proton Synchroton is designed to accelerate 19 times per second. The general features of this weak focusing machine are described in refs. 1) and 2). We are here primarily concerned with the servo system establishing and controlling the flow of information between the components of the magnet power equipment in such a w a y that a precision instrument is formed. The rapid cycling of the bending magnets' current is achieved b y a high Q hybrid L - C circuit tuned to a resonance of about 19 c/s. Fig. 1 shows the diagram of the resonance circuit. The principles of this distributed series parallel resonance network are presented in ref. 3). Fig. 2

19 c / s

13.8 k GAUSS

,

B dB/dt

T';M~ ~ 25 AMP

Fig. 2. T h e v a r i a t i o n of t h e m a g n e t bias. A. C. GENERATOR

circuit losses and for driving the d.c. bias current. Some of the reasons behind this choice are presented below. CHO

RATORS

2. Power Equipment

Disturbances originate either from within the resonant system or are introduced via the powering 1) M. G. W h i t e , F. C. S h o e m a k e r a n d G. K. O'Neill, C E R N S y m p o s i u m on H i g h E n e r g y Accelerators, P r o c e e d i n g s 1 (1956) 525 ( P P A D 119 E). 3) M. G. W h i t e , F. C. S h o e m a k e r , I n t e r n a t i o n a l Conference on H i g h E n e r g y Accelerators a n d I n s t r u m e n t a t i o n C E R N , P r o c e e d i n g s (1959) 362 ( P P A D 309 E). a) S. \Vaaben, Nucl. I n s t r . a n d M e t h . 9 (1960) 78 ( P P A D 300 E).

Fig. 1. T h r e e of t h e s e c t i o n s of t h e m a g n e t p o w e r r e s o n a n c e s y s t e m . F o r t h i s a c c e l e r a t o r t h e r e a r e 16 s u c h sections. All m a g n e t c u r r e n t s a r e identical. T h e s y s t e m is a d i s t r i b u t e d r e s o n a n c e s y s t e m , s t r o n g l y c o u p l e d v i a t h e p r i m a r y choke coils. T h e v a r i a t i o n of t h e m a g n e t bias is s h o w n in fig. 2. 87

88

S. W A A B E N

equipment. Thermal drift is the only disturbance generated within the resonant system. For any choice of powering equipment a slow drift type of disturbance is easily tamed by well known integral 715 A 5.6 k V ~

0001

2 kV

~

A.c.

GEN

A MEGAWATTS

X

KILOWATTS

~

- MAG.

V

A 0-200mA

ELIAS

BIAS

BIAS

V

Fig. 3. L a y o u t of t h e r o t a t i n g machinery. The a p p r o x i m a t e loads for t h e m a c h i n e r y are indicated. The m a i n machine d a t a are as follows: 3500 hp synchronous drive motor, two 750 A, 600 V d.c. generators, a n d a two pole 1650 kVA, 2000 V a.c. generator.

control techniques. Rapid disturbances are introduced via the powering equipment. Very fast fluctuations are attenuated by the resonant system itself. However, disturbances with a time of rise

Driving the shaft of rotating generators with a synchronous motor through a variable torque control device inherently results in a system with a strong attenuation of main power line fluctuations. This is so because the energy stored in the rotating mass is large compared to the energy delivered through the machinery per unit of time. In the megawatt size of power equipment the shaft speed of a flywheel is possibly the phenomena controllable to the highest degree of precision. Fig. 3 shows the mechanical layout of the machinery, also shown in the photograph, fig. 4. For sources of noise within the power equipment it holds that the solid mechanical coupling of the a.c. and d.c. generator shafts has the effect that the harmonics of the d.c. generator are phase coherent with the a.c. generator output voltage. Practical experience with the system has shown, as expected, that the dc generators' brush noise does not disturb to any appreciable amount the short term stability of the system. The generators' field current exciters are magnetic amplifiers driven by a well regulated 400 c/s generator. The three phase full wave magnetic amplifier system was picked because it is inherently

Fig. 4. The p h o t o g r a p h shows from left to right: 1200rpm synchronous motor, eddy current clutch, two d,c. generators, a n d t h e two pole, 19 c/s generator.

comparable to the three seconds time constant of the resonant system are difficult to control. This type of disturbance originates primarily from the main public power line.

short term stable. The 2400 c/s ripple'is reduced to a negligible level by the filtering action of the system. A brief study of fig. 1 shows how the load for the electrically cascaded d.c. generators is, approxi-

A MAGNET

POWER

mately, equal to a series connection of all the inductances in the resonant system. The inductance of this load is around 3 henry. The L/R time constant is about 3 seconds. The a.c. generator load is equivalent to a transformer where the secondary is tuned to a parallel resonance. At resonance the load impedance is, therefore, purely resistive. For other frequencies, the load has, in addition, a reactive component. The output voltage of the a.c. generator with fixed field excitation is dependent upon the phase angle of the load impedance. The reason for this is that the voltage drop, produced b y the reactive load current through the big internal inductive impedance of the a.c. generator, adds to the output voltage. Fig. 5 shows the measured generator output voltage for a fixed d.c. field bias current as a function of the phase angle of the load. It is of interest to note that the a.c. generator inherently tends to lock its shaft speed to the resonance frequency of the load. The resonance frequency drifts during a two to three hour warm-up from about 18.9 c/s to 19.3 c/s. The system's resonance frequency is tracked b y adjusting the slip of the eddy current clutch. After warm-up it is, however, perfectly feasible to operate GENERATOR OUTPUT VOLTAGE

INDUCTIVE ~

i 8

i 6

i 4

3. Tolerances

The magnet field variation is assumed to be represented b y the expression B = 7127.8-6872.2 cos o~t gauss. The stability of the critical injection field conditions is determined b y the difference between two large quantities Boo and Bac. Assume injection occurs at a prefixed magnetic field of 270 gauss. The stability of the rise of the magnetic field during injection time is important for a multiturn injection synchrotron. The quantity /~ at injection is inherently capable of being obtained in sampled form only. However, the time constants of the system are long and, therefore, the signal variation slow compared to the 19 c/s sampling rate so that the signal flow can be considered as if it were continuous. Corresponding tolerances can be calculated b y assuming a fixed B of injection as well as a fixed Bac. Shifting the Bd¢ level with the amount ABdc shifts the phase angle of injection and, therefore, the /~ at the time of injection. Equivalent tolerances are as follows

ABd~ (orBa¢) = 10 -4 Bde

= 3 x

10 - 3

ABi.j ~ 2 x ]0 -2 ¢"

i I0

be controlled, how they are measured, and how they are processed.

/~min

1050

i L 14 12

89

ABml, ...

II00

i 16

SERVO SYSTEM

i

. 2

.

4

.

6

.

i

9ooL

I

. . . . . 8 I0 12 14 16 " CAPACITIV7

&¢ = i',

A f = O,OOZc/t

Fig. 5. T h i s figure shows, for fixed a.c. g e n e r a t o r field bias c u r r e n t , t h e m e a s u r e d g e n e r a t o r v o l t a g e as a f u n c t i o n of t h e p h a s e angle b e t w e e n v o l t a g e a n d c u r r e n t of t h e g e n e r a t o r .

/~inj The corresponding generator shaft speed tolerance cart be calculated b y reference to fig. 5 showing the effect of the high Q resonant load on the ac generator output. A simple calculation shows that a tolerance of 10-* on the a.c. component is equivalent to a 10 -6 tolerance on the relative shaft speed. All other parameters are considered fixed in this calculation. 4. Measuring Schemes

at a fixed repetition frequency locked to a crystal oscillator. The performance achievable with this equipment depends upon which system signals are chosen to

No parameter can be controlled to a higher precision than the resolution of the equipment measuring the quantity. The various measuring methods used are described below.

90

S. WAABEN

4.1. b SIGNAL The dB/dt at injection time is sampled in the following way: the /3 signal comes from a pick up coil located in a magnet. At the instant of injection, a two transistor switch samples the B signal;

with the modification that shortly before the peak occurs the memory capacitor is moderately discharged. A data clamp device is achieved, having a resolution to < 0.01% of the peak. Fig. 7 shows the basic schematic of this dynamic peak detector. 4.3. S P E E D DETECTOR

IN~

oOUT

o

-F

I

o

Fig. 6. Sample and hold circuit. The switch is a two transistor switch.

fig. 6 shows the basic schematic of this sampler. At the present, the sampling pulse, corresponding to a fixed field injection of 270 gauss, comes when a supermalloy core biased b y the magnet current and a fixed bias current flips its saturation state as the magnet current is increasing. The resolution of this sampler is far better than 2 % of /} at injection. This excellent transistor switch was brought to attention b y M. Isaila4). Eventually it m a y prove to be more satisfactory to generatore the injection timing pulse when the Van de Graaff injector energy matches the magnetic field. The trick of a selftriggered injection is used at the 3 GeV "Saturne" Saclay, France. The magnet power supply m a y thus tend to track Van de Graaff energy fluctuations. 4.2. B P E A K SIGNAL

An analog of the magnets' B field is obtained b y integrating the B signal. The peak is sampled b y a conventional diode-capacitor peak voltage detector

INo 0

i•[

RESET

1 OUTPUT ]/'~\\

OUTo / 0

/ /

/

\ \ INPUT

\

\

Fig. 7, The principle of the dynamic peak detector performing the operation of sampling and holding the peak value.

The generator shaft speed, relative to the resonance frequency of the system, is measured b y determining the phase angle difference between the a.c. generator voltage and current. The phase error signal is obtained directly b y sampling the voltage when the current passes zero. The speed detector, encountering the steep dependence of phase angle on speed, has a relative frequency resolution of detection Af/f easily better than 10-6. 4.4. DC CURRENT MEASUREMENTS

The d.c. generator output current is measured in a conventional w a y with a shunt and a low noise amplifier. The generator field currents are measured ill the same way. Since the highest frequency of interest in these servo-loops is in the 20-40 c/s range, signal resolutions of better than the necessary 0.01% can be achieved b y filtering noise.

5. Signal Flows The complete signal flow diagram for the total system is exceedingly complex because of interactions via the common generator shaft. However, practical experience indicates that the following framework is reasonably representative. The system is analyzed, in a first order approximation, b y considering it as consisting of three main subsystems operating independently: a. The generator shaft speed clutch loop. b. Tile d.c. generator current loop. c. The a.c. generator loop. The layout of these three main servoloops is as shown in figs. 8, 9, 10. For all 3 subsystems it holds that the time constant of the regulated load block is some seconds. The time constants of the generator fields are some seconds also. Conventional field current subloops are, therefore, introduced to reduce these field time constants. Fig. 11 shows the block diagram of an ideal 4) M. Isaila, PPAD 245 C (November 1958).

A MAGNET

POWER

subloop with gain/~ around a simple time constant z. The transfer function is determined by the following well known analytical expression 0°(s)-1 [(1+2- ) ] 0~(s) 1+ # s+ 1 .

SERVO SYSTEM

91

damperwinding. The loop does, however, reduce disturbances introduced via the field exciter. The clutch transfer function details are complex and highly nonlinear due to hysteresis effects and eddy current build up phenomena. The field current subloop improves this somewhat. However, these nonlinear phenomena are not too bothersome since

ERRORSIGNAL I ~ ERROR SIGNAL

4 0 0 c/s

BIAS

mASi Ac

~ ' ' ~

[M~

4 5 ~ , ~~< ;US

CHOKE

__~,j,~~

CLUTCH GENERATOR

~

i~.~

]i~ ~

t~.

I I i "~

[

~

AC GENERATORCHOKE

o ]

,~ CLUTCH

I so,,.ccP

KINTEL

"MEASURING ,TRANSFORMERS

114A --

T

F L/"Li

REFERENCE E GEN .m

ERROR

~

I

j

i lGEN

_ _ J

Fig. 8. C l u t c h loop l a y o u t . COIL ERROR SIGNAL

BIAS I i

r'-. . . .

t

c-~

I~ ~>

400 C/S ~ ! GENERATOR I. ~ FIELD ~ -- - -

:

~ -

Fig. 10. T h e B.c. g e n e r a t o r loop l a y o u t . I

! ~ l L.

' I

~.i

dz >H;';,;H / [DCGENERATORS i ~ L::':] [' / - - . / 2

r

i i

"~

-

d ~

SUB X,O

~

I LOOP / ~ / -~

IKINT E L ~

KINTEL 114 A ERROR

---~.(LM+Lch) 16

] i

'~-- TOO AMP

SHUNT

~

!I L

~

•

~

[ 12

[

,

0 i

(s)

f• ~ {~:~ - - ,

i

lINGl _ REFERENCE ORKI ~ V O L T A G E FROM CONTROL ROOM

J

Fig. 9. T h e d.c. g e n e r a t o r loop l a y o u t .

The d.c. generators' field current subloop increases the speed of response of the generator field in this conventional way. The a.c. generator field current subloop is not completely effective in reducing the a.c. generator field time constant since this loop does not affect the one second time constant of this generator's

Fig. 11. Block d i a g r a m r e d u c t i o n of a n ideal subloop w i t h a gain of F a r o u n d a single t i m e c o n s t a n t T.

the dynamic operating range is very small indeed. The transfer function of a sample and hold operation followed by a plant transfer function G(jco) can be approximated by the well known analytical expression

HG* (jco)

~ e -~:j°'T G(jog) .

92

S. WAABEN

The factor e -H°'r indicates how the data hold operation introduces a time delay of half a sampling period. The high frequency end of the sampled error signal loops is inherently limited b y tiffs time delay. It results in a violent phase pick up for frequencies above 5 c/s in the equivalent continuous data approximation of the transfer function representing the sampled data system properties. The high frequency end of the transfer function of the dc generators load goes through a resonance at about 3 c/s. These phenomena limiting the high end of the transfer functions are of such a nature that little can be achieved b y introduction of first derivative filters. This is so despite the fact that the sampled controlled parameters can readily be measured with higher precision than they can be controlled. The/~ error signal can, in principle, correct the system via any of the 3 generator channels. For optimum performance a semiempirical amount of B error signal is injected into the commanding summing points of the d.c. and a.c. channels. All other loops .may thus be considered subloops helping the B loops. The system can be said to be equivalent to a two parallel channel single loop system controlling /~ at the moment of injection. 6. Electronic Components The low power level electronics system is, for the most part, an interconnection of the following standard module boards: 1) The transistor operational amplifier shown in fig. 12 consists of two long tail pairs in cascade and an emitter follower output stage. 2) The Schmitt trigger and a blocking oscillator. The blocking oscillator sends out a 3 #sec pulse when the Schmitt trigger input voltage goes from a negative to a positive voltage. 3) The switching circuit performing the sampling operation consists of a two transistor switch which is closed b y a 3/~sec pulse from a blocking oscillator. The sampled voltage on the memory capacitor is read out continuously b y a high impedance emitter follower. All these circuits are of a conventional type. Silicon transistors have been generously used

throughout. The reliability of this electronic system of several hundred transistors has so far been very good compared to even a highly reliable tube system. PIN 20

-24V

I 0 0 PF ~ -

62 K

I000 PF"

:5,K !5,K

33oa

L~

PIN 13 INPUT 2 PNP

NPN 2N336A '\

2N652A

2K /

~

HEAT

PIN 8

~PUT,

L~ 5~ K + 24v

Fig. 12. Diagram of the operational amplifier used as building block. This amplifier is described in ref.5).

The + 24-volt power supplies, as well as the reference sources, are of T. Coor's design. Multiple signal ground loops can present serious problems in an extensive system. The floating input floating output dc amplifier 114A, b y Kintel, has eased this problem substantially. 7. Performance The performance of the system is more or less as anticipated from a conventional continuous and sampleddata servo system analysis. The measured fluctuations of /~ at the moment of injection are somewhat better than 1%. A second harmonic null detector, an extremely useful device designed by M. Awschalom, is used as a valuable independent check of the minimum current stability. The fluctuations observed with this instrument are less than 100 mA corresponding to 1 gauss of magnetic field. Acknowledgements The author wishes to White, Project Director, fessor F. C. Shoemaker discussions. The rotating power

thank Professor M. G. and, particularly, Profor many constructive equipment

s) S. Waaben, P P A D 311 C (June 6, 1960).

came

from

A MAGNET

POWER

General Electric, Schenectady, where the insight and experience of M. Horton was instrumental in this choice. P. R. Cheeseman has designed many of the protection interlock circuits making the operation almost fool proof. E. de Haas designed many of the power engineer-

SERVO

SYSTEM

93

ing features of the installation, such as the 16 capacitor banks and the complex power cable system. S. Steinitz designed the reliable magnetic amtiers for the generator field exciters. The skills of P. McCann and D. Baker have been relied upon in the construction of the transistor electronics.