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Патент USA US3070343

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Dec. 25, 1962
H. HEcH-r ETA».
A
3,070,333
AIRCRAFT ALTITUDE CONTROL SYSTEM
Filed Jan. A6, .1961
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Patented Dec. 25, 1962
2
tially instantaneous absolute vertical displacement of the
3,070,333
AIRCRAFT ALTIÈFUDE CÜNTRÜL SYSTEM
Herbert Hecht, Scottsdale, and Lawrence Kaufman,
Phoenix, Ariz., assigner-s to Sperry Rand Corporation,
Great Neck, NX., a corporation of Delaware
Il
craft above the terrain while the barometric altimeter 2
provides a signal representative of the pressure altitude of
the craft. Preferably, the best characteristics of each of
the altimeters 1 and 2 are combined in order that the
Filed `1an. 6, 1961, Ser. No. 81,170
4 Claims. (Cl. 244-77)
radar altimeter signal is used for long term stabilization
while the barometric altimeter signal is used for short
This invention relates to an aircraft control system for
controlling the altitude of an aircraft and particularly to
is not subject to calibration for airport elevation above
sea level and errors due to variations of atmospheric
means for eliminating the engage, disengage and switching
transients in aircraft altitude control apparatus.
Conventional altitude controllers, particularly of the
clutched type, introduce engage and disengage transients
into the elevator channel of the automatic pilot ‘control
system when the altitude control system is engaged, dis
pressure. However, the radar altimeter signal is fre~
quently noisy due to the nature of the signal system and
also because step discontinuities may exist in the terrain
such as tall buildings, trees, cliffs, etc. and it is undesirable
to follow these in the course of normal aircraft flight.
Therefore, smoothing of the radar altimeter signal is de
engaged or switched from one mode of operation to
sired. To overcome the deficiencies of the radar altimeter
term control. The radar altimeter 1 is more accurate and
signal, a rate of change of altitude signal is obtained from
the barometric altimeter 2. In this respect the present
it is possible for a large error to exist at the output of 20 invention combines the advantages of two independent
another. In conventional altitude control systems utiliz
ing a combination of radar and barometric altimeters,
the barometric sensor at the time of disengagement, par
measuring devices in a manner disclosed in U.S. Patent
ticularly when operating in a radar mode. This happens,
2,841,345, issued July 1, 1958, to Halpert et al. entitled
for example, when a change in elevation of the terrain
“Glide Path System With Flare-Out.”
The altitude control system of the present invention
has taken place since engagement and, under the influ
ence of an integrated radar altimeter signal, the aircraft 25 may be operated in any one of three modes of operation,
has changed its altitude in order to maintain its original
i.e., (l) follow-up, (2) radar, or (3) barometric.
In the follow-up mode of operation, the engage switch
terrain clearance. The barometric altimeter will then
3 is in the off position as shown in solid lines thereby de
provide a signal which if suddenly removed from the input
energizing the relay 4 in order that its ganged contact
of the automatic pilot system would lead to an undesirable
30 arms da, 4b and 4c are in their lowermost positions as
disengage transient.
It is a primary object of the present invention to pro
shown in solid lines. The radar altimeter 1 is connected
to one input terminal of an algebraic summation device
vide altitude control apparatus which is not subject to
5 while the output terminal thereof is connected through
engage, disengage or mode switching transients.
It is a further object of the present invention to provide
the contact arm 4a to an input terminal of a summing
altitude control apparatus wherein the control signals 35 amplifier 6 of an electromechanical integrating device 7
which functions as a synchronizer. The integrator 7 com
emanating therefrom are smoothly introduced into and
disengaged from the autopilot control system without any
prises the amplifier 6 which is connected in controlling
noticeable undesirable effect on the flight path.
relation to a servomotor 1t) which in turn drives a tach
The above objects are accomplished in an altitude con
ometer generator 11 and a potentiometer 12, the latter
trol system which utilizes combined radar and barometric
through reduction gearing 13. The tachometer generator
altimeters to provide signals representative of the sub
11 provides a rate stabilization feedback signal to another
stantially instantaneous absolute vertical displacement of
input terminal of the summing amplifier 6. The output
the craft above the terrain and a signal representative
terminal of the potentiometer 12 is connected to another
of the rate of change of altitude of the craft respectively.
input terminal of the summation device 5. Thus in the
The displacement and rate signals are algebraically com 45 off or follow-up mode of the system, the signal from the
bined and the resultant signal is integrated in an integrat
radar altimeter 1 is applied to the amplifier 6 to drive
ing device before being applied as a control signal to the
the motor 10 which produces a signal on the potentiom
elevator channel of the automatic pilot. When engaging
eter 12 representative of the altitude sensed by the radar
the altitude control system, the starting of the integrator
altimeter 1. The potentiometer signal is applied in op
is delayed until the rate signal has decayed to zero there 50 position to the radar altimeter signal thus maintaining that
by permitting the integrated control signal to be intro
portion of the altitude control system synchronized when
duced slowly into the elevator channel to avoid engage
the switch 3 is inthe off position at which time the altitude
transients which would otherwise occur if the control sig
control system does not control the altitude of the craft.
nal was introduced directly into the elevator channel.
Inorder to prevent the output signal from the sum
55
Similarly when switching from one mode of altitude con~
mation device 5 from being effective in the autopilot
trol to another, the integrator is temporarily stopped to
elevator channel 14, the output trerninal of a summing
permit the new control signal to be introduced slowly into
amplifier 15 is short-circuited to ground potential 16
the elevator channel to avoid switching transients. To
through the contact arm 17 of a relay 18. In this mode
avoid disengage transients, the integrator is immediately
of operation, the contact arm 17 is in its lowermost po
stopped when the altitude control system is disengaged.
sition as viewed in solid lines.
Referring now to the drawing, it discloses a schematic
In the radar mode of operation, a mode `switch 20 has
diagram of a preferred embodiment of the present inven
its ganged contact arms 20a, 20h and 20c disposed to
tion.
the right as viewed in solid lines. When the engage
The altitude control system of the present invention
switch 3 is placed in its uppermost position as shown in
includes a radar altimeter 1 and a barometric altimeter
dotted lines, the relay 4 is energized which attracts its
2. The radar altimeter may be of the type disclosed in
contact arms da, 4b and 4c to their uppermost positions
the M.I.T. Radiation Laboratory Series, volume l, on
as shown in dotted lines. The relay 4 is energized by
pages 143 to 147 while the barometric altimeter 2 may be
means of a D_C. power source 21 through the contact
of the type disclosed in U.S. Patent No. 2,446,546 issued
August l0, 1948, in the name of A. W. Meston entitled 70 arm 26a, a contact 22 and the engage switch 3. Energiz
ing the relay 4 results in the following: (a) with the
“Pressure Responsive Controller Device.” The radar
mode switch 2l) in the radar position described, the out
altimeter 1 provides a signal representative of the substan
4
3
put from the summing device 5 is disconnected from the
summing amplifier 6, thus the signal representative of the
prevailing radar-detected altitude on the potentiometer
12 becomes the reference signal for the ensuing radar
mode control phase, (b) the output terminal of the
barometric altimeter 2 is connected through a rate cir
cuit 27 comprising a series-connected capacitor 23 and
a parallel resistor 24 and through the contact arm 4b to
an input terminal of an algebraic summation device 25.
the craft had when the integration began. This results
in the engage transient error being dependent upon the
start of an integrator'such as 31 which can be much more
closely controlled than the exact time of mechanical
engagement of a clutched-type altitude controller.
When it is desired to disengage the altitude control
system of the present invention, the engage switch 3 is
placed in its lower position as indicated in solid lines
thereby deenergizing the relay 4 and causing its Contact
The other input terminal of the summation device 25 in 10 arms 4a, 4b and 4c toreturn to their lowermost posi
tions as shown in solid lines. This causes the relay 18
this mode of operation is connected to the output ter
minal of the summation device 5, and (c) the D_C. power
to become dcenergized since its coil power is derived
source 21 is connected through the contact arm 4c to en
through the contact arm 4c of the relay 4. With the
ergize a time delay relay 26 in a manner to be explained.
output terminal of the amplifier 15 again connected to
The output terminal of the summation device 25 is
the ground potential 16 through the contact arm 17, the
connected to a modulator 30 for modulating the corn
integrator 31 is immediately stopped thereby preventing
disengage transients. Disengage transients would other
bined signal lfrom the summation device 25. The modu
lated signal from the modulator 30 is then integrated by
wise be prevalent because, particularly when operating in
means of an electromechanical integrator 31 which in
the radar mode of altitude control, it is possible for a
cludes the summing amplifier 15 provided the output ter 20 large error to exist at the output terminal of the baro
metric altimeter 2 at the time of disengagement. This
minal of the amplifier 15 is not grounded. The integra
can happen, for example, when a change in elevation of
tor 31 comprises the amplifier 1S which is connected to
the terrain has taken place since engagement and under
control a servomotor 32 that in turn drives a tachom
the infiuence of the integrated signal from the radar
eter generator 33 and the rotor of a synchro control trans
altimeter 1, the aircraft has changed its altitude to main
former 34, the latter through a reduction gearing 35.
tain its original terrain clearance. The barometric altim
The tachometer generator 33 provides a rate stabilization
eter 2 would therefore necessarily provide an error sig
feedback signal to the other input terminal of the sum
nal which if suddenly removed from the input of the
ming amplifier 15.
.
autopilot elevator channel 14 would lead to undesirable
A vertical gyro 40 is connected to provide a signal rep
disengage transients. In the present invention, this is
resentative of the pitch attitude of the aircraft to the
avoided as explained above, by having the relay 18 ef
stator of the control transformer 34. The output ter
fectively clamp the output of the integrator 31 at the
minals of the control transformer 34 are connected to
same time as the barometric signal is removed from its
the autopilot elevator channel 14 for maneuvering the
input.
v
craft in pitch to maintain a desired altitude.
The altitude control system of the present invention
In order to avoid engage transients, the integration 35
is also operable in a barometric mode of operation, in
function performed by the integrator 31 is prevented from'
which event, the mode switch 20 has its contact arms
starting until the relay 18 is energized Ythereby attracting
29a, 2Gb and 20c in the leftward position as shown in
the contact arm 17 to the upward position as shown in
dotted lines. In this mode of operation, only barometric
dotted lines which disconnects the output terminal of the
altitude information -is utilized to control the autopilot
summing amplifier 15 from the ground potential 16.
elevator channel 14. The radar altimeter 1 is in a fol
The operation of the relay 18 is controlled by the time
low-up mode of operation with its output connected
delay relay 26 and the time delay of the latter is selected
through the summation device 5 and contact arm 20c
in such a manner that the transient signal appearing at
to the input of the `summing amplifier 6 and the follow
the output of the rate network 27 must have decayed
Iup operation is performed in a manner previously de
to substantially zero prior to energizing the relay 18.
scribed above with respect to the follow-up mode of op
This is accomplished by having the time delay of the time
eration When the engage switch '3 is in the off position.
delay relay 26 substantially exceed the time constant of
In the barometric mode of operation, a Signal repre
the rate circuit 27. For example, a system which oper
sentative of the displacement of the craft above the
ated satisfactorily utilized a 1.25 second time delay for
the time delay relay 26 and a 0.1 second time constant 50 terrain or from a predetermined altitude is obtained from
the barometric altimeter 2 and it is applied through a
for the rate circuit 27.
resistor 42 and the contact arm 2Gb to an input terminal
In this mode of operation for the reasons explained
of the summation device 25. The signal representative
above, the radar altimeter 1 provides a long term sta
of the rate of change of altitude of the craft is obtained
bilization signal representative of the instantaneous dis
from the rate circuit 27 and it is applied to the other
placement of the craft above the terrain which is com
input terminal of the summation device 25. The corn
pared in the summation device 5 4with the reference
bined displacement and trate signal from the summation
altitude signal from the potentiometer 12. The differ
device 25 is applied to the modulator 3€) and then in
ence therebetween, i.e., the error signal is applied to the
tegrated by the integrator 31 in a manner substantially
summation device 25 Where it is compared with the short
term' stabilization signal which is representative of the 60 as described above with respect to the radar mode of
operation. To avoid engage and disengage transients in
rate of change of the altitude of the craft from the rate
the barometric mode of operation, the integrator 31 is
circuit 27 as sensed by the barometric altimeter 2.
effectively held clamped for a predetermined time in
This arrangement is very desirable for maintaining the
terval after the initiation of operation of the altitude
altitude of the craft during power changes, lowering of the
control system and is immediately clamped after dis
flaps and other aerodynamic changes of this type because
the use of displacement and rate terms to drive the in
tegrator 31 then yields an altitude control which provides
integral and displacement terms for controlling the craft
altitude. This results in Smooth altitude control in spite
continuing this mode of operation in a manner similar
to that described above with respect to the radar mode
of operation.
Thus, in both the radar and barometric
modes of operation engage transients are minimized
of aircraft aerodynamic changes which would otherwise 70 since the integrator 3-1 slowly introduces a change in
the control signal from the control transformer 34 into
introduce altitude flight path errors.
the autopilot elevator channel 14 and since the integrator
Another important feature of the altitude control sys
3ft is immediately clamped upon disengage there are no
disengage transients in either mode of operation.
altitude signal which is integrated to yield a steady state
Another «feature of the present invention is that the
term proportional to the deviation from the altitude that 75
tem disclosed immediately above is the use of a rate of
3,070,333
5
6
system may be switched from a radar to a barometric
2. In an aircraft altitude control system, radar alti
meter means for providing a signal representative of the
vertical deviation of the craft from a Vreference altitude,
mode of operation or vice versa without introducing any
switching transients. Assuming that the system is in
the radar mode, the contact arm 20a of the mode switch
20 is touching the contact 22. When the switch 20 is
thrown to the left for the barometric mode, the contact
arm 20a then touches a contact 41. During the short
time interval when the contact arm 20a is going from
the contact 22 to the contact 41 and is not touching
barometric altimeter means, rate circuit means connected
to said barometric altimeter means for providing a sig
nal representative of the rate of change of the altitude of
said craft, said rate circuit means having a predetermined
time constant, algebraic summation means responsive
to said deviation and rate signals for providing a com
either of them, the relay 4 is deenergized thereby deen 10 bined signal representative of the algebraic summation
ergizing the relay 18 and causing the output of the
thereof, electromechanical integrating means responsive
amplifier 15 to be connected to the ground potential 16
through the contact arm 17.
to said combined signal for providing an output repre
When the contact arm 20a
sentative of the integral thereof, first switching means
touches the contact 41, relay 4 is again energized there
for engaging and disengaging said altitude control sys
by initiating the time delay cycle of the time delay relay 15 tem, and time delay means responsive to the operation
26. After this predetermined time delay, the relay 18 is
of said first switching means including second switching
energized thereby unclamp-ing the integrator 31 and per
means associated with said integrating means for effec
mitting it to slowly introduce the error signal into the
tively maintaining said integrating means short-circuited
autopilot elevator channel 14 by means of the control
when said system is disengaged and for a predetermined
transformer 34. A similar sequence of operations takes 20 time delay substantially greater than that of said time
place when the mode switch 20 is switched from a baro
constant when said first switch-ing means is initially en
metric to a radar mode of operation thereby preventing
gaged, said second switching means effectively short
switching transients lfrom occurring.
circuiting said integrating -means immediately upon dis
While the invention has been described in its preferred
engaging said first switching means whereby engage and
embodiments, it is to be understood that the words which 25 disengage transients are minimized.
have been used are words of description rather than of
3. A -system of the character described with respect
limitation and that changes within the purview of the
to claim 2 further including third switching means hav
appended claims may be made without departing from
ing at least a portion thereof associated with said first
the true scope and spirit ot the invention in its broader
switching means for momentarily disengaging said sys
aspects.
30 tem when switching from one mode of operation to an
What is claimed is:
other thereby causing said time delay cycle to be initiated.
l. In an aircraft altitude control system, means for
4. A system of the character described with respect
to claim 3 further including pitch attitude reference means
providing a signal representative of the vertical displace
ment of the craft above the terrain, means including
means hav-ing a predetermined time constant for provid
for providing a signal representative of the pitch attitude
35 of the aircraft, differential signal generating means hav
ing a signal representative of the rate of change of the
ing one portion thereof connected to the integrating
altitude of -said craft, algebraic summation means re
means and the other portion thereof connected to the
sponsive to said displacement and rate signals for pro
pitch attitude reference means for providing a control
viding a combined signal representative of the algebraic
signal representative of the algebraic summation thereof,
summation thereof, integrating means selectively respon 40 and autopilot elevator channel means responsive to said
sive to said combined signal for providing an output
control signal for controlling the pitch attitude of the
representative of the integral thereof, time delay means
craft in accordance therewith.
including switching means associated with said integrat
ing means for effectively rendering said integrating means
References Cited in the file of this patent
responsive to said combined signal after a predetermined 45
UNITED STATES PATENTS
time delay following engagement of said system and
2,809,340
Bernhart ____________ __ Oct. 8, 1957
unresponsive to said combined signal immediately upon
disengagement of said system and engage switch means
for engaging and disengaging said system having at least
a portion thereof connected to said time delay means.
al.
2,841,345
2,889,509
Halpert et al. ________ __ July 1, 1958
Gorzelany et al ________ __ June 2, 1959
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