A D-C Telemeter or D-C Selsyn for

Aircraft

R. G . J E W E L L A S S O C I A T E A I E E

Synopsis: This paper describes a d-c tele-meter which is particularly adapted for transmitting to the instrument board indi-cations of pressure, temperature, liquid level and of the position of the various controlling members of the airplane. The description comprises the principle, variations, char-acteristics, and application of this telemeter-ing system. Although the applications de-scribed are primarily in connection with aircraft, the versatile nature of this device makes it suitable for many other applica-tions.

The large number of indications which must be transmitted accurately to the instrument board of an airplane has created a demand for a telemetering system of small size and light weight. For many years the a-c Selsyn telemetering system has given excellent service in marine applications. Variations of this system are used success-fully on aircraft. A search for a simpler and lighter system which will operate on

P a p e r 4 2 - 7 , r e c o m m e n d e d b y the A I E E c o m m i t t e e o n air t r a n s p o r t a t i o n , fo r p r e s e n t a t i o n a t t h e A I E E w i n t e r c o n v e n t i o n , N e w Y o r k , Ν . Y . , J a n u a r y 2 6 - 3 0 , 1 9 4 2 . M a n u s c r i p t s u b m i t t e d O c t o b e r 1 0 , 1 9 4 1 ; m a d e a v a i l a b l e fo r p r i n t i ng N o v e m b e r 3 , 1 9 4 1 .

R . G . JEWELL a n d H . T . FAUS are d e v e l o p m e n t e n g i n e e r s in t h e W e s t L y n n W o r k s o f G e n e r a l E l e c t r i c C o m p a n y , L y n n , M a s s .

H . T. F A U S M E M B E R A I E E

direct current has led to the development of (the d-c Selsyn telemetering system.

Principle

ASIMPLIFIED concept of the opera-tion of the d-c Selsyn may be ob-

tained by referring to Figure 1 which shows a coil wound on a toroidal iron core. This is a continuous single layer winding with two brushes supplying direct current at diametrically opposite points. It is evi-dent that a magnetic pole will be set up in the core under each of the brushes, and that the magnetic field inside the core will be as shown. This field will follow the brushes as they are rotated. If a polar-ized permanent magnet rotor is placed in-side the core, it will revolve so as to keep its direction of magnetization in line with the brushes.

The next step in the development is shown in Figure 2. The circular rheostat transmitter is tapped at intervals of 120 degrees and these taps are connected to similarly spaced taps on the receiver. This gives a result similar to that obtained

in Figure 1 so that, within the accuracy limits to be defined later, the permanent magnet rotor will turn to a position having the same relation to the taps on the re-ceiver winding as the brushes have to the corresponding taps on the transmitter winding. The windings shown are similar to delta connected three-phase windings. Connections similar to other polyphase windings may also be used and the wind-ings may be concentrated in coils rather than being uniformly distributed on the core.

The use of a permanent magnet of high coercive force in combination with a cop-per damping shell fixed in the air gap be-tween the rotor and the stator gives effec-tive damping combined with high torque. Damping by a permanent magnet used in this manner involves no power loss as would be the case if an alternating field were used. The torque is a function of the product of the rotor and stator fluxes. Since the rotor field is of much higher magnetomotive force than the stator field, and since its excitation involves no elec-trical losses, it is evident that the inherent torque producing effectiveness is high. The high ratio of torque to weight obtain-able with this system is one of its out-standing advantages. This makes it pos-sible to use a comparatively light moving element which requires neither ball bear-ings nor jewel bearings, since the fric-tional errors are inappreciable with steel and bronze bearings. Since these bear-

For another type of controller response

AP D

R Rt(TiP+l) (8)

the equivalent correction is directly propor-tional to the tie-line power change, but it is delayed by the time lag 7Y

The effects of an intermittent controller are introduced as .follows. The tie-line power change, A P , is read at regular time intervals. Within the period Δ/ following each reading a corrective indication is ap-plied at a constant rate ki and for a length of time proportional to A P . This rate of cor-rection is again expressed as an equivalent prime-mover torque change per unit of time. If the correction is applied through the syn-chronizing motor then k \ is also a measure of the synchronizing-motor speed.

It is assumed that the impulse starts after an elapsed time h dependent upon the value of A P at the beginning of the interval, and ends after a fixed time te. If the reading of tie-line power deviation is equal to or greater than A P m , the full correction is applied, that is, h is equal to some fixed time tm. Then for any smaller power deviation AP the impulse is applied at the time t\ given by

t 9 - t x _ AP

te tm APm

The correcting adjustment applied at the governor head is of the form

D kt

R (TAP+1) (10)

where (with /o^time at beginning of inter-val At, and te<t0-\-At)

k = 0 for / i>/>/o £ = £i for te>t>h

Ti = time lag of the controller

At a fixed power deviation equal to A P m , and neglecting the controller time lag (TA = 0), there is an average correcting rate over the entire interval equal to

JD\ kxjte-U) P\R)=^T ( I D

This average rate can be defined by an equivalent R/ similar to that of equation 7:

'(eq.)

or from the above two equations :

APm At (9) 2V(eq.) =

ki(te — tm)

(12)

(13)

The effects of variations of the constants ki and APm were obtained from the dif-ferential analyzer solutions. The following time intervals were used :

Δ/ = 2.12 seconds / m = 0.265 second / e

8 1.325 seconds

All time constants and the factors Ri and l/&i are given in units of electrical radians in the above equations but in other parts of the paper they are given in seconds for a 60-cycle system. Otherwise the nota-tion is consistent throughout.

R e f erences 1. PRIME-MOVER SPEED GOVERNORS FOR INTER-CONNECTED SYSTEMS, R . J . C a u g h e y a n d J . B . M c C l u r e . A I E E TRANSACTIONS, v o l u m e 6 0 , 1 9 4 1 , A p r i l s e c t i o n , p a g e s 1 4 7 - 5 1 ; d i s c u s s i o n s : p a g e s 7 2 8 - 3 1 .

2 . EFFECT OF PRIME-MOVER SPEED-GOVERNOR CHARACTERISTICS ON POWER-SYSTEM FREQUENCY VARIATIONS AND TIE-LINE POWER SWINGS, C . C o n c o r d i a , S . B . C r a r y , a n d Ε . E . P a r k e r . A I E E TRANSACTIONS, v o l u m e 6 0 , 1 9 4 1 , p a g e s 5 5 9 - 6 7 .

3 . SUPPLEMENTARY CONTROL OF PRIME-MOVER SPEED GOVERNORS, S . B . C r a r y a n d J . B . M c C l u r e . A I E E TRANSACTIONS, v o l u m e 6 1 , 1 9 4 2 , Apr i l s e c t i o n , p a g e s 2 0 9 - 1 4 .

3 1 4 T R A N S A C T I O N S Jewell, Faus—A D-C Telemeter ELECTRICAL ENGINEERING

Figure 1 . S imp l i f i ed d -c Selsyn

M A G N E T I Z E D R O T O R

P E R M A L L O Y L A M I N A T I O N S

T R A N S M I T T E R I N D I C A T O R

«« C U R R E N T \ F O R P O S I T I O N O F T R A N S M I T T E R

· « — F L U X / A R M A S S H O W N

27-3 T O R E C E I V E R

Figure 3 ( l e f t ) . D - c Selsyn

transmitter

Indicator makes three revo-lutions per revolution of

transmitter

Figure 5 ( l e f t ) .

L inear transmitter for

d-c Selsyn

Indicator makes three r e v o l u t i o n s as brushes move from end to end of trans-

mitter

Figure 2 ( l e f t ) . D - c

Selsyn t e l e m e t e r i n g

system

T O R E C E I V E R

Figure 6 ( a b o v e ) . D - c Selsyn transmitter

Indicator makes one revolution as brushes traverse entire length of respective resistors

8 0 I 6 0 2 4 0 3 2 0

T R A N S M I T T E R S E T T I N G - D E G R E E S

Figure 8 . Curves showing vo l tage variat ions

at taps o n the transmitter w i th various loads

c o n n e c t e d to the transmitter a n d 1 2 volts

a p p l i e d

A—Taps 1 - 2 , no load Β—Taps 2 - 3 , no load C—Taps 3 - 1 , no load 0—Taps 1 - 2 , one indicator Ε—Taps 2 - 3 , one indicator F—Taps 3 - 1 , one indicator

G—Taps 1 - 2 , two indicators H—Taps 2 - 3 , two indicators

/—Taps 3 - 1 , two indicators

Δ 2 ϋ Ι û O û l - 2 0 . ZÛCD

g i i « - i o

8 0 I 6 0 2 4 0 3 2 0

T R A N S M I T T E R S E T T I N G — D E G R E E S

Figure 9 . D e v i a t i o n of d - c Selsyn from a

l inear f o l l o w - u p characteristic

A—Instrument without power-failure indicator

Β—Instrument with power-failure indicator

Either instrument wi l l repeat its calibration curve within plus or minus 0.1 degree

Figure 4 ( b e l o w ) . L inear transmitter for d -c

Selsyn

Indicator makes one revolution as brushes move from end to end of transmitter

2 0 4 0 6 0

P O W E R D E F L E C T I O N F R O M

F R E E P O S I T I O N - D E G R E E S

Figure 7 ( a b o v e ) . T o r q u e characteristics of d -c Selsyn

A—Maximum-torque condition with transmitter set with one brush on a tap

Β—Minimum-torque condition with both brushes set 3 0 degrees from a tap Figure 1 0 . D - c Selsyn pos i t ion transmitter

JUNE 1 9 4 2 , V O L . 6 1 Jewell, Faus—A D-C Telemeter T R A N S A C T I O N S 3 1 5

ings are lightly loaded they withstand severe vibration very well.

The use of sliding contacts has been cited as an objection to this type of telemeter. By using brushes of precious metal alloys operating at carefully deter-mined pressures, a reliable contact is ob-tained with very little friction. Life tests have shown these brushes to be capable of operating for over 40,000,000 complete revolutions of the transmitter.

Variations

In the transmitter shown in Figure 2, the output voltage passes through 360 electrical degrees for each revolution so

that the ratio of angular motions of the transmitter and indicator is unity. By making the number of evenly spaced taps equal to 3w, where η is the number of indi-cator revolutions per transmitter revolu-tion, it is possible to obtain a ratio equal to any whole number. Brushes of oppo-site polarity must be 180 electrical de-grees (360/(2«) angular degrees) apart. In order to utilize the full capacity of the winding, 2n brushes must be used, but the transmitter is operable with only two brushes. Figure 3 shows a transmitter de-signed for a ratio of 3.

The transmitter shown in Figure 4 is designed to transmit linear motion with-out converting it to circular motion by mechanical means. Motion of the brushes for the complete length of this transmitter gives one revolution of the indicator. A multi-revolution modification of this cir-cuit is shown in Figure 5 which gives three revolutions of the indicator. This scheme can obviously be modified to give any number of revolutions of the indicator for a given linear motion of the brushes. Figure 6 is a modification of this circuit which will give fractional ratios with a circular transmitter. Obviously with this circuit, the motion is limited to less than one revolution of the transmitter.

Two or more indicators may be oper-ated in parallel in connection with any of these transmitters. The only effect on their operation is to lower the torque in proportion to the reduction in current through each indicator.

Since there is no torque acting on the indicator when the power is disconnected, the pointer will remain unchanged in position. For applications where an indi-cation of power failure is desired, the indi-cator is provided with a small fixed per-manent magnet which attracts the rotor magnet to a position at which the pointer is off the calibrated part of the scale. Due to the high torque of the instrument, it is possible to use a fixed magnet strong enough to provide a reliable indication of power failure without having any signifi-cant effect on the accuracy.

Characteristics

Each indicator element weighs 25 grams and can be mounted in a îy^inch-diame-ter circle.

The power required by the transmitter and indicator combination is 1.8 watts. With two indicators connected to one transmitter, the power required is 2.2 watts.

Figure 1 1 . D - c Selsyn pos i t ion indicators, showing three posi t ions of flaps a n d l e n d i n g Ρ ' 9 " ' « 1 2 . T y p i c a l l i q u i d - l e v e l transmitter F igure 1 3 . T y p i c a l t h r e e - e l e m e n t l i q u i d - l e v e l

gear ind ica tor

3 1 6 T R A N S A C T I O N S Jewell, Faus—A D-C Telemeter ELECTRICAL ENGINEERING

Varying the voltage of the supply does not affect the indication provided no power failure indicator is used. With the power failure indicator a variation of ten per cent in the voltage will cause a maxi-mum error of one degree.

The torque obtained is shown by the curves in Figure 7. The slope of the torque curve at zero is important since a steep slope at this point gives less fric-tional error. The torque curves show that the steepest part is at zero.

Temperature effect on indications is negligible because of the symmetry of the circuit.

The wave form of the voltage between taps in the transmitter is shown in Figure 8. At no load the form is triangular with the tops cut off flat. With load, the sides and tops of the form bend inward. The change in the form is of such nature that at any given point the ratio of voltage be-tween one pair of taps to that between another pair of taps does not change with load so that an indicator connected to the transmitter will not change its calibration when another indicator is placed in par-allel with it.

The peculiar shape of the voltage wave does prevent the indicator from following the transmitter linearly. This inherent deviation has been determined to have a maximum peak of 1.3 degrees. The

peaks occur at six points above the axis, and six points below the axis. Figure 9 shows a typical distribution curve A which consists of the 1.3 degree deviation and additional deviation due to manufac-turing tolerances. Curve Β of the same figure shows how the distribution curve is changed by the power failure indicator.

Applications

One of the first applications of the d-c Selsyn was for the remote indication of the positions of the landing gear and wing flaps on an airplane. For this purpose four elements were mounted in a single case, one element being used for each of the three landing wheels and one for the flaps.

The remote indication of oil and fuel pressure is accomplished by connecting a bellows to the oil or fuel pressure line and utilizing the motion of the bellows to ro-tate the transmitter brushes.

The temperature transmitter has a bi-metal helix which rotates the transmitter brushes as the temperature changes.

The manifold pressure transmitter has a partially evacuated sealed bellows in a pressure tight compartment and the mani-fold pressure is applied to the outside of the bellows. Motion of the bellows is transmitted mechanically through a flex-

ible wall to the d-c Selsyn transmitter. Temperature compensation is obtained by having the right amount of air in the sealed bellows.

The differential fuel pressure transmit-ter is similar to the manifold pressure ex-cept the bellows is not evacuated. The fuel pressure is connected to the inside of the bellows while the surrounding cham-ber is connected to the manifold. The bellows deflection is then proportional to the differential pressure.

The liquid level measurement utilizes the motion of a float to actuate the trans-mitter brushes. Magnetic coupling is used between the transmitter outside the tank and the float mechanism inside the tank so as to provide a positive gasoline seal.

Conclusions

The d-c Selsyn has been shown to be particularly suited to applications re-quiring high accuracy, light weight, and rapid and stable indications. These quali-ties make it particularly suitable for air-craft use. The fact that it can be oper-ated on any source of low-voltage direct current without auxiliary equipment is an advantage, not only for aircraft use but for other applications where special power sources are not available.

JUNE 1 9 4 2 , V O L . 6 1 Jewell, Faus—A D-C Telemeter T R A N S A C T I O N S 317