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The precision driving and absorption dynamometer-I. Digital speed control
The Precision Driving and Absorption Dynamometer-I. Digital Speed Control W. H. P. LESLIE Introduction
control voltage. This limits the applied accuracy of this method to between 0 ·1 and 0·25 per cent 3.4, and realistic accuracy claims have not altered in the last decade. In the alternative class the position of the shaft is controlled with respect to time; any error between the actual position and the desired position (linearly increasing in value with time) produces the controlling signal. Such a system may have transient speed errors during changing load conditions, but during steady conditions there can be no speed error. Figure 1 shows the simplest mechanical system in which the reference speed is supplied by a synchronous motor, a differential gear comparing shaft positions continuously. The output shaft of the differential indicates positional error. This shaft can be coupled to a field rheostat controlling the drive motor; such a system can be seen controlling old paper mills drives, giving exact control of speed. Figure 2 shows how the measuring apparatus can be removed from the main drive.
The Fluids Division of the Government's National Engineering Laboratory have a problem which requires the accurate control of shaft speed and torque measurement! with smaller error than is normally required. The tests involve efficiency measurements on model hydraulic turbines and pumps (and also on oil pumps and motors). Often the efficiency is over 90 per cent and modifications are made involving small changes in one of the several components of the 10 per cent loss; and alteration in torque of 0·1 per cent represents more than I per cent alteration in loss and may also represent an alteration in revenue of £1,000 a year from a large turbine whose model is being tested. The system described in a companion paper 2 measures torque with an error of 0·02 per cent of full-scale and in turn this requires speed control to better than 0·01 per cent (torque being proportional to square or cube of speed) . Although not limited in any way to motor or generator size the system has been applied to motors between 10 and 350 h.p. and speeds between 100 and 20,000 rev/min. Speed can be preset and obtained with an absolute error less than I part in 10 5 • Existing precision speed control methods are first dealt with, followed by the novel Digital Speed Control, which is all electronic.
Magslip
I Electrical
t transmission
Reference I speed
General Magslip
Speed control systems are of two main. ciasses. The more common type compares a signal (usually a voltage) whose magnitude is proportional to speed, with a fixed reference signal, and uses the error to provide the control of power to the prime mover. We shall consider an electric motor although most control systems can equally well be applied to any prime mover. With this type of system there must be an error in speed in order to produce the controlling action, although this error can be made small. The more serious difficulties lie in obtaining a voltage accurately related to speed and independent of environment and age, and in producing an equally accurate
clutch Stop Error Indicator and error signal generator
'-...
/
. Control panel remote from drive
Figure 2.
Addition of mags{ip-link to Figpre 1
A Practical System Standard frequency Reference speed synchronous motor Friction clutch Stop", Error indicator _ 0 and error signal generator
o,Sto p
Figure I. Basic speed COli/rot
If the control has to work over a range of speeds a gearbox can be introduced at A or B in Figure 2. The main drive motor then runs at exactly the speed of the reference motor multiplied by the gearbox ratio. A suitable gearbox which can be set to any ratio between 0 '100 and 0·999 has been described by Knapps and is shown diagrammatically in Figure 3. In the speed control system he describes, the gearbox is inserted at A in Figure 2. This has the disadvantage that all the gears have to be driven by the magslip-link and also that their backlash is in the control loop. Tn the system adopted in the author's laboratory the gearbox is inserted at B in Figure 2, where it runs 369
1470
W. H. P. LESLlE
9,fferentlal for adding tens to units
/ _Tumbler gear train select speed In un Its
-_ Jumbler gear train select speed In tens
Output Hundreds'" tens +-units
No drive through tumbler gear tra ins on zero stop
Select speed gate one for each train
Differential for add ing hundreds to tens plus units
Figure 3.
Multiratio gearbox
SynCh_ !motor I
Mains power supply
power ~~~========~~~·~=====3 Electronic amplif ier
11'Reference speed
~
Controlled_field
~ (
IndUtction mo or
supply
-
Electrical error signal
Con t rolled~a tu re SUPPlYllt Con trolled field suppy I 11 .....L.---JLL., Frequency represents
[fr( L:~~dr~ ~
Frequency represents
o
"~ D~;~e ~a~C~.U;~l~P~ Electro~~~ired ~
~enerator
r -'-----'----'----....L,
t
Boller and Chivens 3 decade gear box
motor
meucs~~~~cal
alternator
~d_ch ac o.
/
output
phase comparator
I
® © Reference speed
,_C
:m:ll x A_B alternator
l'
~Synch_ I '---1 i n d i ca tor I r.-;---o---...,....-,--, r Electronic l Hand control I
voltage reference _ _ Icomparator , I speed setting Voltage represen t s ' - - ' - - - - ' approx. speed Voltage represents approx_ speed desired Tuning fork _ Rotary st and a rd 1-----'''---1 power frequency amplifier Mains power supply
Figure 4.
Speed control for Hydraulic Machinery Laboratory
370
1471
--
t
THE PRECISION DRIVING AND ABSORPTION DYNAMOMETER-I. DIGITAL SPEED CONTROL
Mains power supply
I
--
Control signal
Thyratron
®
Electronic power amplifier
Electrical error signal Reference -Digital timing pulses
-
®
Supply tor motor
controlled
11 rectifier
d,c Drive motor
CD
CD
Actual frequency lOll range -
No
L Useful mechanical output
®
Fixed ratio differentiall==::::;;:;:;:::==~ electronic counter Frequency fs Hertz frequency divider L..----,.....------' Output pulses at fs 11 Hertz due to control action
Electronic frequency
N f Hertz divider lOll range 5 in steps ot 0'01%
Reference frequency
®®©@
Small alternator (batching counter) one pulse out for N pulses in
Tuning fork or crystal standard frequency
G)1Jl Figure 5.
Mains input
New precision speed control
at a constant speed driven by the synchronous motor. The differential is replaced by an electrical phase locking circuit, the complete system being shown in Figure 4. This system was designed and constructed by a prominent British manufacturer, only the gearbox being imported from America. The New Digital Speed Control This is explained in terms of the complete system shown in Figure 5, followed by details of the more important sub-units. The motor being controlled, 2, is fitted with a small alternator, 3, which usually gives out 60 pulses/rev so that the same number represents shaft speed in rev/min and the pulse repetition frequency in hertz. The repetition frequency bears an exact relation to shaft speed-no error is involved. The digital
differential counter, 5, accepts reference timing pulses from a frequency standard, 7 and 8, which can be shared by any number of drives 6 • It also receives pulses derived from shaft speed. Only when these pulse repetition frequencies are identical can the electrical error signal from 5 remain constant -any error in motor speed causes a change in error signal which passes via 6 and I to alter the power supplied to the motor so as to correct the error. The batching counter, 4, can be set to an integral number N , and will emit one pulse for every N pulses received from the alternator 7 • Thus the main drive motor must run at NIs rev/min in order that the output from the batching counter should be Is hertz to correspond exactly to the frequency Is from the standard. No error can exist without causing the speed to alter to eliminate it. A batching counter can be set to a new number NI whilst the system is Phonlc wheel (on shaft) /
Alternative coil shapes, \
\
Magnet Coils
LI•••blt Magnet Magnet
\
A
Plug,
Plugs ..---
Laminations
Figure 6.
Shaft speed transducer
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1472
Magnetic Inserts
W. H. P. LESLIE
operating, providing a new speed Nds rev/min; it has no teeth to suffer damage as in a gearbox. The digital differential counter has an output count which changes at a rate equal to the difference in its two input frequencies (with no error); it is analogous to the mechanical differential in Figure 2. The batching counter is analogous to a gearbox whose output shaft makes exactly one revolution for each N revolutions of the input shaft. Units 2, 3, 4 and 5 in Figure 5 are exactly analogous to Figure 2. Other speed control arrangements are possible using the differential counter but must be left out here. The units 3, 4 and 5 are described separately below. Shaft speed transducer
The small alternator, 3, referred to above can be a very simple device providing care is taken in its design. Figure 6 shows the type used in our laboratory; this has been designed to tolerate eccentricity and axial wobble or play, giving a constant output of over 1 V from 50 rev/min to the bursting speed of the disc (over 20,000 rev/min). This transducer is potted in Araldite to make it impervious to oil or water and finds wide use for shaft speed measurement. Digital d(fferential counter
A simple solution to the problem of simultaneous differential counting was found 6 in 1956. This relied on the Dekatron 8, a cold cathode decade counting tube in wide use. The circuits then described used vacuum therm ionic valves but they have been considerably simplified by the use of transistors. Figures 7 and 8 show one complete differential counter, and although other means of simultaneous differential counting can be devised by using complicated systems of storing pulses, no circuit has been devised with the simplicity of Figure 7. The operation can be described by first considering the basic
drive unit, based on a circuit published by Chaplin9 for unidirectional Dekatron operation. This circuit is shown inside dotted lines in Figure 7. The transistor Vj acts with the transformer T j and associated components as a blocking oscillator. On receipt of a negative triggering pulse via Cl> large enough to overcome the bias current flowing in Dl> the circuit produces a positive pulse at the transistor collector, immediately followed by a negative pulse of the same width and amplitude. The pulse width can be varied by adjusting R 2 • The third winding of the transformer is used for obtaining the output-each end of the winding in Chaplin's circuit was connected to one of the two sets of transfer electrodes on the Dekatron, and was also connected to earth through a diode D3 or D 4 • These diodes allow the end of the winding to go negative relative to earth, but not positive. Thus when the two pulses appear at the output, first D4 prevents its end being positive so that the other end goes negative and transmits a negative pulse to the Xl transfer electrodes. As the first pulse ends and the second commences with opposite polarity, owing to the action of D j a negative pulse appears at the other end of the winding and is transmitted to the X 2 transfer electrodes. The Dekatron makes a clockwise count when first Xl and then X z are made negative and returned to earth potential. In the case of the complete circuit of Figure 7, transistor VI and transformer TI are connected as described to drive the Dekatron clockwise. The negative pulses to Xl and X 2 are connected through isolating silicon diodes Ds and D6 which are there to allow Xl and X 2 to be driven negative from the V2 and T z circuit via the isolating diodes D7 and Ds. The circuit is simple to construct, contains few components and can be easily adjusted for optimum performance. The pulse width for clockwise rotation is adjusted by R2 to the nominal value recommended by the manufacturer (60 f1.sec for a 4 kHz Dekatron) .--------------~J
+
475
!R27
Transfer electrodes
Dekatron .--____Ox-r------------~-----------Anode
J X2 0
r
4
7
8
u:: ,,,' rN~1Io.19-L. NR' 2' 'O-L. ' 'R' '201',. . .L. .I\,RN2' '2w. .I\,RN2' '3w. .f'o.RN2' '4w. . ./'RoA2N5\. L. R,.,~N:"b~ j...AR
Cathodes
R:ont rol voltage output
--, I
t 475
9
, . . - - - - - 0 +475
I I I
- 30 o---+_--.
- r5
O--+---+--r":"'--"
- 30
I I I
- 15
Subtract )1--.---'-1 ~ c 4 input
( . . .) I BaSIC circuit
E
I
II
--------~
Figure 7.
Transistorized Dekatroll dijJerelllial counter (circuit)
372
1473
THE PRECISION DRIVING AND ABSORPTION DYNAMOMETER-1. DIGITAL SPEED CONTROL
The pulse width for anti-clockwise rotation is adjusted by R7 until the differential counter will count in either direction for applied signals of approximately 4 kHz applied to each input, and capable of being easily adjusted from a difference of + 1 Hz to - 1 Hz. Wrong adjustment of R7 results in the counter jumping in a preferred direction instead of operating smoothly through zero frequency difference. Figllre 8 shows the complete differential counter and its output potential divider. This output is obtained from resistors RIO to R 26 in Figure 7. When the Dekatron conducts on cathode 1, current flows through RIO to - 30 V and produces no output. At cathode 2 the current path is through RI I and R 19 ; the volt drop in the latter appears at the output. At cathode 3 the volt drop is now due to the same value of current flowing through RI9 and R 2() and thus is larger. This process is similar as the Dekatron conducts to each higher numcered cathode so that eventually at cathode 9 the volt drop is due to the current flowing in RI 9 to R 26 and is arranged to make the output voltage approximately zero; the output with the count at cathode I is - 30 V. The resistors Rq to RI 8 are proportioned so that each cathode is at the same potential when conducting.
over a wide speed range the alternators used for generating the pulse frequencies may be made with the appropriate ratio 111 their numbers of poles. Beaching coullter Batching counters can take two forms. The first type is reset to the desired number of counts below zero (953 if a batch of 47 is required on a 3 decade counter) and the batch is known to have been reached when the counter reaches zero, an easily recognized state. At this point a pulse is emitted and the counter reset to 953 (or whatever is required). The use of a batching counter in this form of speed control sets an extra requirement not often met with on batching counters: it must be reset and be ready to count the next pulse at speeds up to 10 kHz. A cold cathode counter has been described 7 which meets this requirement, and this counter has been employed successfully for the purpose.
Figllre 9.
Figure 8.
Complete specd cOlltrol (Follt)
The second type of batching counter is reset to zero and the desired batch is known to be reached when the counter reaches the number required. This is normally recognized by a diode matrix which only allows the reset pulse to occur when the counter reaches the required number. The advent of commercial transistor decades for counters encouraged the construction of a transistor batching counter which can batch any number from I to 99,999 at input frequencies up to 20 kHz. This uses a very simple but effective form of reset gating, again using the Chaplin double-pulse circuit, and will be described elsewhere. Figure 9 shows the batching counter built into a complete experimental speed control unit.
Trallsistorized Dekatroll di/krelllial COllllta (photograph)
At tlrst sight only 9 discrete voltages are available, but in practice the count oscillates between two cathodes, jumping from say 4 to 5 for every clockwise input pulse, and from 5 to 4 for every anti-clockwise pulse. The output then depends on the relative phase of the two signals and its mean value varies smoothly through all intermediate values as the phase alters. The cathode 10 can be connected, in the' Test' position of the switch, so that the count can progress continuously round the tube if there are different frequencies applied (corresponding to the output shaft of a mechanical differential) or 10 can be disconnected so that the glow cannot pass 9 or 0 (corresponding to the differential with a slipping clutch and stops as in Figure 1). This is the way it is used in speed control since the end to which the count is driven indicates whether the motor speed is higher or lower than the desired value. This information is necessary during automatic starting or in the event of a suddenly applied load momentarily overloading the system. This digital differential counter can be applied to many other tasks 6 such as the generation of very low frequencies, and the control of slave shafts to follow a master shaft in a multi-drive machine. Where a fixed ratio of speeds has to be preserved
Experimental Results The system described is still under development but has been successfully applied to the control of a fractional horse-power motor over the speed range 100-4,500 rev/min. With this motor it has been possible to set up any desired speed, switch on, and to obtain this speed with an accuracy of ± 1 in 10 5 without any manual intervention. The speed can be changed during running simply by turning the knobs on the batching counter to set a new number. With a reference frequency of 100 Hz the speed can be set in steps of 100 revlmin over the range, and with a 10 Hz reference frequency the speed can be set in steps of 10 rev Imin.
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1474
W. H. P. LESLIE
pulses and the standard pulses every 6'0 of a revolution of the motor shaft (assuming a 60 pulse/rev/alternator). In the case of method (b), if the number N set on the batching counter is 660, say for 6,600 rev/min, then the differential counter only checks the relative phase every 660/60 = 11 revolutions. This is quite satisfactory with a relatively slowly changing load, as in a large hydraulic installation, but would not do if there was an appreciable change in load during the eleven revolutions. Method (c) is not so suitable if an exact number of rev/min is required since it is necessary to set the oscillator by successive approximations, checking each setting by a counting type frequency meter. Once the oscillator is set, the frequency meter can be used to check the actual shaft speed.
The system was then applied experimentally to a 100 h.p. and a 20 h.p. driving dynamometer (in each case a compound wound d.c. motor) via a thyratron-controlled Ward-Leonard generator system and was used successfully to control the speed over a ± 400 rev /min range at any base speed manually selected from 500 to 4,000 rev/min. In these experiments the prototype transistor drive circuits for the differential counter were used. Future Development
As a result of the satisfactory results obtained, the control system is now being applied to the drive pumps for the NEL Water Tunnel, 350 h.p., and to a new dynamo meter of 100 h.p. for 6,000 to 20,000 rev/min. The control unit shown in Figure 9 has been constructed for this purpose; in the interest of reliability and size no therm ionic valves have been employed and nearly every transistor is used as a controlled switch, either • on' or • off'. So far it has not been found possible to devise a simple transistor circuit to perform the actual differential counting so the cold cathode gas-filled • Dekatron' has been retained for this purpose. Further development will aim at eliminating this tube, which is now the least reliable part although it is very dependable. This particular speed control has been designed with three types of control in one unit so that the water tunnel operators can decide which method is most suitable for their purpose. Emphasis has been laid on ease of switching from one system to another without surges in speed. The three systems are: (a) d.c. tachogenerator and hand-set potentiometer, (b) the digital system described above, and (c) a digital system without the batching counter. The third system uses an R-C oscillator as the source of reference frequency to the digital differential, and the pulses direct from the motor as the other source. A commercial R-C oscillator can be purchased with a half-hour frequency stability of 0·05 per cent. Thus once adjusted for a desired condition the speed can be held at that value for long enough to perform a test. Absolute accuracy can be obtained by monitoring the frequency of the oscillator with an electronic counter frequency meter and this can also check the drive frequency. The components of the batching counter are switched to form a frequency meter when system (c) is in use. There is an advantage attached to method Cc) which will sometimes make it attractive, and this is quick response. The differential counter is checking the relative phase of the motor
The author acknowledges the co-operation of Mr. A. Russell in the development of the original speed control and of Mr. J. J. Hunter in the development of the transistorized speed control. The speed control system is the subject of patent applications ill several countries. The work described has been carried out in the Fluid Mechanics Division of the National Engineering Laboratory* (D.S.I.R.). This paper is published with the permission of the Director, Dr. Sop with.
* East Kilbride, Glasgow, Scotland. References 1
HUTTON, S. P. Techniques for hydraulic machinery research
2
LESLIE, W. H. P. The precision driving and absorption dynamometer-II. Torque measurement. This book, p. 376 JAESCHKE, R. L. Electronic speed regulation of magnetic clutches. Electronics Aug. (1945) 102 DRAKE, L. S., Fox, J. A. and GUNNELL, G. H. A. Speed control oflarge wind tunnels. Proc. Instn. elect. Engrs. 105A, Dec. (1957)
Trans. Instn. Engrs. Shipb. Scot. (1957)
3 4
204 5
6 7
8 9
KNAPP, R. T. Hydraulic Machinery Laboratory at California Institute of Technology. Trans. Amer. Soc. mech. Engrs. (1936) 663 LESLIE, W. H. P. A digital differential. Electron. Engng. May (1956) 190 GRIMMOND, W. and LESLIE, W. H. P. Batching and counting using fast gas-filled decade tubes. Electron. Engng. April (1956) 138 Cold Cathode Handbook. 1954/55. Ericsson Ltd. CHAPLIN, G. B. B. and WILLIAMSON, R. Dekatrons and electromechanical registers operated by transistors. Instn. elect. Engrs. Paper M2440, Nov. (1957)
Summary
The precise measurement of steady torque in rotating shafts, with an error smaller than 2 parts in 104 , primarily depends on the control of the shaft speed to a smaller error because many of the engines, pumps, turbines, etc., tested have a relation T = RNx where T is torque, N is speed, and x may be as large as 3. A digital speed control is described which can be applied to any prime mover. It has been used to control the speed of electric motors of ,'0 to 350 h.p. to any desired value in their operating range with an absolute error of 1 part in 105 • The speed to be measured is converted into a train of pulses whose
repetition frequency in hertz is numerically equal to shaft speed in revolutions per minute. This frequency is divided by an integer N, set at will on a batching counter, and then compared in a digital differential counter with a standard frequency, Is hertz, derived from a quartz crystal oscillator. The differential counter provides an electrical output which is used to control the power to the electrical motor until it runs exactly at NIs rev/min. By making Is equal to 100, 10 or 1 hertz, a direct decimal setting of shaft speed on the batching counter is made possible. Apart from one cold cathode gas-filled counting tube, simple transistor circuits are used throughout.
Sommaire
La mesure precise du couple it l'arret dans les arbres tournants, avec une erreur inferieure it 2 . 10-" depend essentiellement du contr6le de la vitesse de l'arbre avec une erreur plus faible parce que beaucoup des moteurs, pompes, turbines, etc... essayes ont une relation T = RN', ou T est le couple, N la vitesse et x peut atteindre 3. On decrit une commande de vitesse numerique qui peut et re appliquee it
n'importe quel moteur primaire. Elle a ete utilisee pour regler la vitesse de moteurs electriques de it 350 CV it la valeur desiree dans leur gamme de fonctionnement avec une erreur absolue de 1 x 10- 5 . La vitesse it mesurer est convertie en un train d'impulsions dont la frequence de repetition en Hertz est numeriquement egale it la vitesse de rotation de l'arbre en tours par minute. Cette frequence est l.!.O
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THE PRECISION DRIVING AND ABSORPTION DYNAMOMETER-I. DIGITAL SPEED CONTROL
divisee par un nombre entier N, regie a volonte sur un diviseur de frequence et comparee ensuite dans un compteur differentiel numerique, a une frequence etalon f, (Hz) provenant d'un oscillateur a quartz. Le compteur differentiel fournit une sortie electrique, qui est utilisee pour regler la puissance du moteur eIectrique de fa90n qu' il
tourne exactement a Nfs tours/minute. En faisantfs egal a 100, 10 ou 1 Hz on peut realiser un reglage decimal direct de la vitesse de I'arbre par le diviseur de frequence. En dehors d'un tube de comptage a gaz, a cathode froide, on n'utilise que de simples circuits a transistors.
Zusammenfassung Die Genauigkeit der Messung eines stationaren Drehmomentes an einer sich drehenden Welle mit einem Fehler, der kleiner als 2 x 10- ' ist, hangt hauptsachlich von der Stabilisierung der Drehzahl ab; denn fUr vicle der untersuchten Antriebsmaschinen, Pumpen, Turbinen usw . gilt die Beziehung T = RN', wobei T das Drehmoment , N die Umfangsgeschwindigkeit und x gleich 3 ist. Im ersten Teil des Vortrages wird ein digitales System zur Regelung der Umfangsgeschwindigkeit beschrieben, das fUr beliebige Motoren verwendet werden kann . Es wurde zur Regelung von Elektromotoren mit einer Leistung zwischen 0,1 und 350 PS verwendet, wobei die Geschwindigkeit im Arbeitsbereich auf 10- 5 genau gehalten wurde.
Die zu messende Geschwindigkeit wird in eine Impulsfolge umgeformt, deren Frequenz in Hertz gleich der Umdrehungszahl pro Minute ist. Diese Frequenz wird durch eine ganze Zahl N, die in einem Zahler eingestellt ist, dividiert und danach in einem reversiblen Zahler mit einer quarzstabilisierten Normalfrequenz verglichen. Vom reversiblen Zahler wird eine Ausgangsgrof3e zur Regelung der Motorleistung entnommen, die genau die Umdrehungszahl Nf, einstellt. Wenn!, gleich 100, 10 oder 1 gesetzt wird, ist eine direkte dezimale Eingabe der Drehzahl moglich . Auf3er einer gasgefUllten Kaltkatodenrohre zum Zahlen werden nur einfache Transistorschaltungen verwendet.
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1476
control voltage. This limits the applied accuracy of this method to between 0 ·1 and 0·25 per cent 3.4, and realistic accuracy claims have not altered in the last decade. In the alternative class the position of the shaft is controlled with respect to time; any error between the actual position and the desired position (linearly increasing in value with time) produces the controlling signal. Such a system may have transient speed errors during changing load conditions, but during steady conditions there can be no speed error. Figure 1 shows the simplest mechanical system in which the reference speed is supplied by a synchronous motor, a differential gear comparing shaft positions continuously. The output shaft of the differential indicates positional error. This shaft can be coupled to a field rheostat controlling the drive motor; such a system can be seen controlling old paper mills drives, giving exact control of speed. Figure 2 shows how the measuring apparatus can be removed from the main drive.
The Fluids Division of the Government's National Engineering Laboratory have a problem which requires the accurate control of shaft speed and torque measurement! with smaller error than is normally required. The tests involve efficiency measurements on model hydraulic turbines and pumps (and also on oil pumps and motors). Often the efficiency is over 90 per cent and modifications are made involving small changes in one of the several components of the 10 per cent loss; and alteration in torque of 0·1 per cent represents more than I per cent alteration in loss and may also represent an alteration in revenue of £1,000 a year from a large turbine whose model is being tested. The system described in a companion paper 2 measures torque with an error of 0·02 per cent of full-scale and in turn this requires speed control to better than 0·01 per cent (torque being proportional to square or cube of speed) . Although not limited in any way to motor or generator size the system has been applied to motors between 10 and 350 h.p. and speeds between 100 and 20,000 rev/min. Speed can be preset and obtained with an absolute error less than I part in 10 5 • Existing precision speed control methods are first dealt with, followed by the novel Digital Speed Control, which is all electronic.
Magslip
I Electrical
t transmission
Reference I speed
General Magslip
Speed control systems are of two main. ciasses. The more common type compares a signal (usually a voltage) whose magnitude is proportional to speed, with a fixed reference signal, and uses the error to provide the control of power to the prime mover. We shall consider an electric motor although most control systems can equally well be applied to any prime mover. With this type of system there must be an error in speed in order to produce the controlling action, although this error can be made small. The more serious difficulties lie in obtaining a voltage accurately related to speed and independent of environment and age, and in producing an equally accurate
clutch Stop Error Indicator and error signal generator
'-...
/
. Control panel remote from drive
Figure 2.
Addition of mags{ip-link to Figpre 1
A Practical System Standard frequency Reference speed synchronous motor Friction clutch Stop", Error indicator _ 0 and error signal generator
o,Sto p
Figure I. Basic speed COli/rot
If the control has to work over a range of speeds a gearbox can be introduced at A or B in Figure 2. The main drive motor then runs at exactly the speed of the reference motor multiplied by the gearbox ratio. A suitable gearbox which can be set to any ratio between 0 '100 and 0·999 has been described by Knapps and is shown diagrammatically in Figure 3. In the speed control system he describes, the gearbox is inserted at A in Figure 2. This has the disadvantage that all the gears have to be driven by the magslip-link and also that their backlash is in the control loop. Tn the system adopted in the author's laboratory the gearbox is inserted at B in Figure 2, where it runs 369
1470
W. H. P. LESLlE
9,fferentlal for adding tens to units
/ _Tumbler gear train select speed In un Its
-_ Jumbler gear train select speed In tens
Output Hundreds'" tens +-units
No drive through tumbler gear tra ins on zero stop
Select speed gate one for each train
Differential for add ing hundreds to tens plus units
Figure 3.
Multiratio gearbox
SynCh_ !motor I
Mains power supply
power ~~~========~~~·~=====3 Electronic amplif ier
11'Reference speed
~
Controlled_field
~ (
IndUtction mo or
supply
-
Electrical error signal
Con t rolled~a tu re SUPPlYllt Con trolled field suppy I 11 .....L.---JLL., Frequency represents
[fr( L:~~dr~ ~
Frequency represents
o
"~ D~;~e ~a~C~.U;~l~P~ Electro~~~ired ~
~enerator
r -'-----'----'----....L,
t
Boller and Chivens 3 decade gear box
motor
meucs~~~~cal
alternator
~d_ch ac o.
/
output
phase comparator
I
® © Reference speed
,_C
:m:ll x A_B alternator
l'
~Synch_ I '---1 i n d i ca tor I r.-;---o---...,....-,--, r Electronic l Hand control I
voltage reference _ _ Icomparator , I speed setting Voltage represen t s ' - - ' - - - - ' approx. speed Voltage represents approx_ speed desired Tuning fork _ Rotary st and a rd 1-----'''---1 power frequency amplifier Mains power supply
Figure 4.
Speed control for Hydraulic Machinery Laboratory
370
1471
--
t
THE PRECISION DRIVING AND ABSORPTION DYNAMOMETER-I. DIGITAL SPEED CONTROL
Mains power supply
I
--
Control signal
Thyratron
®
Electronic power amplifier
Electrical error signal Reference -Digital timing pulses
-
®
Supply tor motor
controlled
11 rectifier
d,c Drive motor
CD
CD
Actual frequency lOll range -
No
L Useful mechanical output
®
Fixed ratio differentiall==::::;;:;:;:::==~ electronic counter Frequency fs Hertz frequency divider L..----,.....------' Output pulses at fs 11 Hertz due to control action
Electronic frequency
N f Hertz divider lOll range 5 in steps ot 0'01%
Reference frequency
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Small alternator (batching counter) one pulse out for N pulses in
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New precision speed control
at a constant speed driven by the synchronous motor. The differential is replaced by an electrical phase locking circuit, the complete system being shown in Figure 4. This system was designed and constructed by a prominent British manufacturer, only the gearbox being imported from America. The New Digital Speed Control This is explained in terms of the complete system shown in Figure 5, followed by details of the more important sub-units. The motor being controlled, 2, is fitted with a small alternator, 3, which usually gives out 60 pulses/rev so that the same number represents shaft speed in rev/min and the pulse repetition frequency in hertz. The repetition frequency bears an exact relation to shaft speed-no error is involved. The digital
differential counter, 5, accepts reference timing pulses from a frequency standard, 7 and 8, which can be shared by any number of drives 6 • It also receives pulses derived from shaft speed. Only when these pulse repetition frequencies are identical can the electrical error signal from 5 remain constant -any error in motor speed causes a change in error signal which passes via 6 and I to alter the power supplied to the motor so as to correct the error. The batching counter, 4, can be set to an integral number N , and will emit one pulse for every N pulses received from the alternator 7 • Thus the main drive motor must run at NIs rev/min in order that the output from the batching counter should be Is hertz to correspond exactly to the frequency Is from the standard. No error can exist without causing the speed to alter to eliminate it. A batching counter can be set to a new number NI whilst the system is Phonlc wheel (on shaft) /
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Shaft speed transducer
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W. H. P. LESLIE
operating, providing a new speed Nds rev/min; it has no teeth to suffer damage as in a gearbox. The digital differential counter has an output count which changes at a rate equal to the difference in its two input frequencies (with no error); it is analogous to the mechanical differential in Figure 2. The batching counter is analogous to a gearbox whose output shaft makes exactly one revolution for each N revolutions of the input shaft. Units 2, 3, 4 and 5 in Figure 5 are exactly analogous to Figure 2. Other speed control arrangements are possible using the differential counter but must be left out here. The units 3, 4 and 5 are described separately below. Shaft speed transducer
The small alternator, 3, referred to above can be a very simple device providing care is taken in its design. Figure 6 shows the type used in our laboratory; this has been designed to tolerate eccentricity and axial wobble or play, giving a constant output of over 1 V from 50 rev/min to the bursting speed of the disc (over 20,000 rev/min). This transducer is potted in Araldite to make it impervious to oil or water and finds wide use for shaft speed measurement. Digital d(fferential counter
A simple solution to the problem of simultaneous differential counting was found 6 in 1956. This relied on the Dekatron 8, a cold cathode decade counting tube in wide use. The circuits then described used vacuum therm ionic valves but they have been considerably simplified by the use of transistors. Figures 7 and 8 show one complete differential counter, and although other means of simultaneous differential counting can be devised by using complicated systems of storing pulses, no circuit has been devised with the simplicity of Figure 7. The operation can be described by first considering the basic
drive unit, based on a circuit published by Chaplin9 for unidirectional Dekatron operation. This circuit is shown inside dotted lines in Figure 7. The transistor Vj acts with the transformer T j and associated components as a blocking oscillator. On receipt of a negative triggering pulse via Cl> large enough to overcome the bias current flowing in Dl> the circuit produces a positive pulse at the transistor collector, immediately followed by a negative pulse of the same width and amplitude. The pulse width can be varied by adjusting R 2 • The third winding of the transformer is used for obtaining the output-each end of the winding in Chaplin's circuit was connected to one of the two sets of transfer electrodes on the Dekatron, and was also connected to earth through a diode D3 or D 4 • These diodes allow the end of the winding to go negative relative to earth, but not positive. Thus when the two pulses appear at the output, first D4 prevents its end being positive so that the other end goes negative and transmits a negative pulse to the Xl transfer electrodes. As the first pulse ends and the second commences with opposite polarity, owing to the action of D j a negative pulse appears at the other end of the winding and is transmitted to the X 2 transfer electrodes. The Dekatron makes a clockwise count when first Xl and then X z are made negative and returned to earth potential. In the case of the complete circuit of Figure 7, transistor VI and transformer TI are connected as described to drive the Dekatron clockwise. The negative pulses to Xl and X 2 are connected through isolating silicon diodes Ds and D6 which are there to allow Xl and X 2 to be driven negative from the V2 and T z circuit via the isolating diodes D7 and Ds. The circuit is simple to construct, contains few components and can be easily adjusted for optimum performance. The pulse width for clockwise rotation is adjusted by R2 to the nominal value recommended by the manufacturer (60 f1.sec for a 4 kHz Dekatron) .--------------~J
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Transistorized Dekatroll dijJerelllial counter (circuit)
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THE PRECISION DRIVING AND ABSORPTION DYNAMOMETER-1. DIGITAL SPEED CONTROL
The pulse width for anti-clockwise rotation is adjusted by R7 until the differential counter will count in either direction for applied signals of approximately 4 kHz applied to each input, and capable of being easily adjusted from a difference of + 1 Hz to - 1 Hz. Wrong adjustment of R7 results in the counter jumping in a preferred direction instead of operating smoothly through zero frequency difference. Figllre 8 shows the complete differential counter and its output potential divider. This output is obtained from resistors RIO to R 26 in Figure 7. When the Dekatron conducts on cathode 1, current flows through RIO to - 30 V and produces no output. At cathode 2 the current path is through RI I and R 19 ; the volt drop in the latter appears at the output. At cathode 3 the volt drop is now due to the same value of current flowing through RI9 and R 2() and thus is larger. This process is similar as the Dekatron conducts to each higher numcered cathode so that eventually at cathode 9 the volt drop is due to the current flowing in RI 9 to R 26 and is arranged to make the output voltage approximately zero; the output with the count at cathode I is - 30 V. The resistors Rq to RI 8 are proportioned so that each cathode is at the same potential when conducting.
over a wide speed range the alternators used for generating the pulse frequencies may be made with the appropriate ratio 111 their numbers of poles. Beaching coullter Batching counters can take two forms. The first type is reset to the desired number of counts below zero (953 if a batch of 47 is required on a 3 decade counter) and the batch is known to have been reached when the counter reaches zero, an easily recognized state. At this point a pulse is emitted and the counter reset to 953 (or whatever is required). The use of a batching counter in this form of speed control sets an extra requirement not often met with on batching counters: it must be reset and be ready to count the next pulse at speeds up to 10 kHz. A cold cathode counter has been described 7 which meets this requirement, and this counter has been employed successfully for the purpose.
Figllre 9.
Figure 8.
Complete specd cOlltrol (Follt)
The second type of batching counter is reset to zero and the desired batch is known to be reached when the counter reaches the number required. This is normally recognized by a diode matrix which only allows the reset pulse to occur when the counter reaches the required number. The advent of commercial transistor decades for counters encouraged the construction of a transistor batching counter which can batch any number from I to 99,999 at input frequencies up to 20 kHz. This uses a very simple but effective form of reset gating, again using the Chaplin double-pulse circuit, and will be described elsewhere. Figure 9 shows the batching counter built into a complete experimental speed control unit.
Trallsistorized Dekatroll di/krelllial COllllta (photograph)
At tlrst sight only 9 discrete voltages are available, but in practice the count oscillates between two cathodes, jumping from say 4 to 5 for every clockwise input pulse, and from 5 to 4 for every anti-clockwise pulse. The output then depends on the relative phase of the two signals and its mean value varies smoothly through all intermediate values as the phase alters. The cathode 10 can be connected, in the' Test' position of the switch, so that the count can progress continuously round the tube if there are different frequencies applied (corresponding to the output shaft of a mechanical differential) or 10 can be disconnected so that the glow cannot pass 9 or 0 (corresponding to the differential with a slipping clutch and stops as in Figure 1). This is the way it is used in speed control since the end to which the count is driven indicates whether the motor speed is higher or lower than the desired value. This information is necessary during automatic starting or in the event of a suddenly applied load momentarily overloading the system. This digital differential counter can be applied to many other tasks 6 such as the generation of very low frequencies, and the control of slave shafts to follow a master shaft in a multi-drive machine. Where a fixed ratio of speeds has to be preserved
Experimental Results The system described is still under development but has been successfully applied to the control of a fractional horse-power motor over the speed range 100-4,500 rev/min. With this motor it has been possible to set up any desired speed, switch on, and to obtain this speed with an accuracy of ± 1 in 10 5 without any manual intervention. The speed can be changed during running simply by turning the knobs on the batching counter to set a new number. With a reference frequency of 100 Hz the speed can be set in steps of 100 revlmin over the range, and with a 10 Hz reference frequency the speed can be set in steps of 10 rev Imin.
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pulses and the standard pulses every 6'0 of a revolution of the motor shaft (assuming a 60 pulse/rev/alternator). In the case of method (b), if the number N set on the batching counter is 660, say for 6,600 rev/min, then the differential counter only checks the relative phase every 660/60 = 11 revolutions. This is quite satisfactory with a relatively slowly changing load, as in a large hydraulic installation, but would not do if there was an appreciable change in load during the eleven revolutions. Method (c) is not so suitable if an exact number of rev/min is required since it is necessary to set the oscillator by successive approximations, checking each setting by a counting type frequency meter. Once the oscillator is set, the frequency meter can be used to check the actual shaft speed.
The system was then applied experimentally to a 100 h.p. and a 20 h.p. driving dynamometer (in each case a compound wound d.c. motor) via a thyratron-controlled Ward-Leonard generator system and was used successfully to control the speed over a ± 400 rev /min range at any base speed manually selected from 500 to 4,000 rev/min. In these experiments the prototype transistor drive circuits for the differential counter were used. Future Development
As a result of the satisfactory results obtained, the control system is now being applied to the drive pumps for the NEL Water Tunnel, 350 h.p., and to a new dynamo meter of 100 h.p. for 6,000 to 20,000 rev/min. The control unit shown in Figure 9 has been constructed for this purpose; in the interest of reliability and size no therm ionic valves have been employed and nearly every transistor is used as a controlled switch, either • on' or • off'. So far it has not been found possible to devise a simple transistor circuit to perform the actual differential counting so the cold cathode gas-filled • Dekatron' has been retained for this purpose. Further development will aim at eliminating this tube, which is now the least reliable part although it is very dependable. This particular speed control has been designed with three types of control in one unit so that the water tunnel operators can decide which method is most suitable for their purpose. Emphasis has been laid on ease of switching from one system to another without surges in speed. The three systems are: (a) d.c. tachogenerator and hand-set potentiometer, (b) the digital system described above, and (c) a digital system without the batching counter. The third system uses an R-C oscillator as the source of reference frequency to the digital differential, and the pulses direct from the motor as the other source. A commercial R-C oscillator can be purchased with a half-hour frequency stability of 0·05 per cent. Thus once adjusted for a desired condition the speed can be held at that value for long enough to perform a test. Absolute accuracy can be obtained by monitoring the frequency of the oscillator with an electronic counter frequency meter and this can also check the drive frequency. The components of the batching counter are switched to form a frequency meter when system (c) is in use. There is an advantage attached to method Cc) which will sometimes make it attractive, and this is quick response. The differential counter is checking the relative phase of the motor
The author acknowledges the co-operation of Mr. A. Russell in the development of the original speed control and of Mr. J. J. Hunter in the development of the transistorized speed control. The speed control system is the subject of patent applications ill several countries. The work described has been carried out in the Fluid Mechanics Division of the National Engineering Laboratory* (D.S.I.R.). This paper is published with the permission of the Director, Dr. Sop with.
* East Kilbride, Glasgow, Scotland. References 1
HUTTON, S. P. Techniques for hydraulic machinery research
2
LESLIE, W. H. P. The precision driving and absorption dynamometer-II. Torque measurement. This book, p. 376 JAESCHKE, R. L. Electronic speed regulation of magnetic clutches. Electronics Aug. (1945) 102 DRAKE, L. S., Fox, J. A. and GUNNELL, G. H. A. Speed control oflarge wind tunnels. Proc. Instn. elect. Engrs. 105A, Dec. (1957)
Trans. Instn. Engrs. Shipb. Scot. (1957)
3 4
204 5
6 7
8 9
KNAPP, R. T. Hydraulic Machinery Laboratory at California Institute of Technology. Trans. Amer. Soc. mech. Engrs. (1936) 663 LESLIE, W. H. P. A digital differential. Electron. Engng. May (1956) 190 GRIMMOND, W. and LESLIE, W. H. P. Batching and counting using fast gas-filled decade tubes. Electron. Engng. April (1956) 138 Cold Cathode Handbook. 1954/55. Ericsson Ltd. CHAPLIN, G. B. B. and WILLIAMSON, R. Dekatrons and electromechanical registers operated by transistors. Instn. elect. Engrs. Paper M2440, Nov. (1957)
Summary
The precise measurement of steady torque in rotating shafts, with an error smaller than 2 parts in 104 , primarily depends on the control of the shaft speed to a smaller error because many of the engines, pumps, turbines, etc., tested have a relation T = RNx where T is torque, N is speed, and x may be as large as 3. A digital speed control is described which can be applied to any prime mover. It has been used to control the speed of electric motors of ,'0 to 350 h.p. to any desired value in their operating range with an absolute error of 1 part in 105 • The speed to be measured is converted into a train of pulses whose
repetition frequency in hertz is numerically equal to shaft speed in revolutions per minute. This frequency is divided by an integer N, set at will on a batching counter, and then compared in a digital differential counter with a standard frequency, Is hertz, derived from a quartz crystal oscillator. The differential counter provides an electrical output which is used to control the power to the electrical motor until it runs exactly at NIs rev/min. By making Is equal to 100, 10 or 1 hertz, a direct decimal setting of shaft speed on the batching counter is made possible. Apart from one cold cathode gas-filled counting tube, simple transistor circuits are used throughout.
Sommaire
La mesure precise du couple it l'arret dans les arbres tournants, avec une erreur inferieure it 2 . 10-" depend essentiellement du contr6le de la vitesse de l'arbre avec une erreur plus faible parce que beaucoup des moteurs, pompes, turbines, etc... essayes ont une relation T = RN', ou T est le couple, N la vitesse et x peut atteindre 3. On decrit une commande de vitesse numerique qui peut et re appliquee it
n'importe quel moteur primaire. Elle a ete utilisee pour regler la vitesse de moteurs electriques de it 350 CV it la valeur desiree dans leur gamme de fonctionnement avec une erreur absolue de 1 x 10- 5 . La vitesse it mesurer est convertie en un train d'impulsions dont la frequence de repetition en Hertz est numeriquement egale it la vitesse de rotation de l'arbre en tours par minute. Cette frequence est l.!.O
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divisee par un nombre entier N, regie a volonte sur un diviseur de frequence et comparee ensuite dans un compteur differentiel numerique, a une frequence etalon f, (Hz) provenant d'un oscillateur a quartz. Le compteur differentiel fournit une sortie electrique, qui est utilisee pour regler la puissance du moteur eIectrique de fa90n qu' il
tourne exactement a Nfs tours/minute. En faisantfs egal a 100, 10 ou 1 Hz on peut realiser un reglage decimal direct de la vitesse de I'arbre par le diviseur de frequence. En dehors d'un tube de comptage a gaz, a cathode froide, on n'utilise que de simples circuits a transistors.
Zusammenfassung Die Genauigkeit der Messung eines stationaren Drehmomentes an einer sich drehenden Welle mit einem Fehler, der kleiner als 2 x 10- ' ist, hangt hauptsachlich von der Stabilisierung der Drehzahl ab; denn fUr vicle der untersuchten Antriebsmaschinen, Pumpen, Turbinen usw . gilt die Beziehung T = RN', wobei T das Drehmoment , N die Umfangsgeschwindigkeit und x gleich 3 ist. Im ersten Teil des Vortrages wird ein digitales System zur Regelung der Umfangsgeschwindigkeit beschrieben, das fUr beliebige Motoren verwendet werden kann . Es wurde zur Regelung von Elektromotoren mit einer Leistung zwischen 0,1 und 350 PS verwendet, wobei die Geschwindigkeit im Arbeitsbereich auf 10- 5 genau gehalten wurde.
Die zu messende Geschwindigkeit wird in eine Impulsfolge umgeformt, deren Frequenz in Hertz gleich der Umdrehungszahl pro Minute ist. Diese Frequenz wird durch eine ganze Zahl N, die in einem Zahler eingestellt ist, dividiert und danach in einem reversiblen Zahler mit einer quarzstabilisierten Normalfrequenz verglichen. Vom reversiblen Zahler wird eine Ausgangsgrof3e zur Regelung der Motorleistung entnommen, die genau die Umdrehungszahl Nf, einstellt. Wenn!, gleich 100, 10 oder 1 gesetzt wird, ist eine direkte dezimale Eingabe der Drehzahl moglich . Auf3er einer gasgefUllten Kaltkatodenrohre zum Zahlen werden nur einfache Transistorschaltungen verwendet.
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