U.S. patent number 3,554,084 [Application Number 04/683,994] was granted by the patent office on 1971-01-12 for redundant force summing servo unit.
This patent grant is currently assigned to Honeywell Inc.. Invention is credited to Robert F. Rasmussen, John C. Taylor.
United States Patent |
3,554,084 |
Rasmussen , et al. |
January 12, 1971 |
REDUNDANT FORCE SUMMING SERVO UNIT
Abstract
The servosystem which will tolerate failure in two of its four
redundant channels and still be operational consists of four
identical redundant units. The force outputs of the four redundant
units are summed together, by means of a summing link or member,
with the resultant force on the member being applied to a control
valve of a fluid operated motor which is a low resistive load. The
input signal to each redundant unit is an electrical signal which
energizes a torque motor. The torque motor responds by developing a
force on its armature, which force is transmitted to a jet pipe
valve. The jet pipe is deflected thereby causing a fluid pressure
differential at the receiving holes coacting with the jet pipe
nozzle. This pressure differential is transmitted to both a mod
piston (which is the drive piston of the redundant unit) and the
pressure sensor piston in parallel with the mod piston. The motion
of both pistons is force fed back to the jet pipe by means of
yieldable, impositive members as flat springs, thus closing two
independent loops.
Inventors: |
Rasmussen; Robert F. (Brooklyn
Center, MN), Taylor; John C. (Golden Valley, MN) |
Assignee: |
Honeywell Inc. (Minneapolis,
MN)
|
Family
ID: |
24746290 |
Appl.
No.: |
04/683,994 |
Filed: |
November 17, 1967 |
Current U.S.
Class: |
91/3; 91/387;
91/433; 91/448; 91/509; 91/510 |
Current CPC
Class: |
F15B
18/00 (20130101); F15B 9/00 (20130101); G05D
1/0077 (20130101) |
Current International
Class: |
F15B
9/00 (20060101); G05D 1/00 (20060101); F15B
18/00 (20060101); F15b 013/02 (); F15b
013/16 () |
Field of
Search: |
;91/3,387(Cursory),363A,388,(Cursory),411,446,448,(Cursory),384,(Cursory) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Maslousky; Paul E.
Claims
We claim: In a fluid type actuator having a power output section
and a control section, receiving fluid under pressure, for porting
fluid through suitable passages to said power section, said control
section have a displaceable fluid conducting member and coacting
receiving ports to vary the flow rate to the power section and thus
the velocity of displacement thereof in combination:
means for variably displacing said member from a normal position
relative to said two coacting fluid receiving ports;
yieldable feedback means between the member and power output
section operated from said power section to modify the position of
or return said member and ports to the normal position; and
further fluid pressure monitor means including a controller having
subpassages, at least one having flow restrictive means, connected
to said passages to said power section thereby responsive by said
restrictive means to time duration of fluid pressures applied to
said power section, operating on the member to control the rate of
flow to the power section.
. The apparatus of claim 1 wherein operation of said controller in
said further means modifies the flow rate by modifying the position
of said member to provide long term equalization of fluid pressure
in the passages
to the power section. 3. In a fluid-type actuator having a power
output section and a control section for porting fluid from a
source of pressure fluid to said power section, said control
section having a displaceable member to vary the flow rate to the
power section and thus its displacement rate:
first means for variably displacing said member from a normal
position;
second or feedback means operated from said power section to return
said member to normal position;
a pressure fluid conduit means in said control section and having
an operable valve therein for transmitting fluid to said control
section; and
pressure fluid monitor means connected to said power section thus
responsive to fluid pressure therein and controlling the operation
of said
valve. 4. The apparatus of claim 3 characterized by the valve being
of the solenoid actuated type and the pressure fluid monitor means
also includes a switch means having contacts controlling a circuit
of said solenoid and responsive to pressure to said power means to
effect opening of said valve
to supply fluid to the conduit. 5. The apparatus of claim 4, in
including a pressure monitoring start switch for momentarily
energizing said solenoid to momentarily open the valve to passage
of fluid to apply
pressure to said power means and switch means. 6. The apparatus of
claim 3, pluralized to provide at least three redundant actuators,
and wherein each power section thereof is connected to a common
force summing member, all power sections normally applying effects
to an output of said summing member, whereby each feedback means is
operated in accordance with the average position of the output from
the summing member whereby failure to cause a first means in one
section to initially displace a displaceable member in the section
results in the feedback means repositioning said displaceable
member in the section causing a larger than normal operating
pressure thus a change in normal operating pressure to its power
means with the accompanying operation of the pressure monitor means
to terminate
operation of said valve. 7. The apparatus of claim 6 wherein the
first means displacing said member and the second feedback means
thereof are
structurally of the impositive type. 8. The apparatus of claim 7
wherein the power section includes a power piston and wherein the
displaceable member controls the position of a nozzle or jet pipe
which normally applies equal pressures to a pair of receiving ports
or openings directly connected to opposite ends of the power piston
of the power means to apply
normally equal pressures thereto. 9. The apparatus of claim 7,
including a pressure equalizer monitor responsive to time duration
of fluid pressure applied to said power section and operatively
connected to said
displaceable member. 10. The apparatus of claim 3, characterized by
the valve being of the solenoid actuated type and the pressure
fluid monitor means also includes a switch means controlling a
circuit of said solenoid and responsive to continued large
displacements of said member from normal
position opening said circuit to return the valve to closed
position. 11. In control apparatus, means for operating a low
resistive load output comprising:
a plurality of redundant channels each having a piston type
proportional operated fluid servomotor, all servomotors through
their pistons operating a force summing common member connectable
for operation to said output;
first means displaceable for applying differential fluid pressure
to said redundant servo motor pistons to effect operation
thereof;
second means responsive to excessive differential pressures in each
servomotor operating a failure switch connected to the first means
for terminating application of fluid pressure to said servomotor
thus terminating operation of said servomotor; and
a differential pressure responsive device in each channel connected
through fluid flow restrictive conducting means to opposite sides
of its associated piston to respond only to time duration
differential pressures across said piston and connected to the
first means to modify the displacement thereof.
Description
The purpose of the pressure sensor piston is to provide long-term
hydraulic pressure equalization on all of the jet pipe receiving
holes. For example, ideally if each of the four redundant units was
identical and received the same input signal, the differential
force on each mod piston because of the feedback, in the steady
state, would be very low. However, this ideal condition can never
be exactly attained because of differences during manufacturing
resulting from tolerances and driving signal mismatches so it is
possible for steady state extraneous forces to build up on the mod
pistons and summing link. The forces are in balance and do no
useful work. Some forces would be directed to the right, and others
to the left, with a net force on the summing link being zero.
Since a pressure sensor piston receives the same pressure
differential as its respective mod piston, it is possible to
effectively measure the force on each mod piston by noting the
displacement of its pressure sensor piston over a long period. The
possibility obviously exists then of feeding back negative pressure
sensor displacement to the jet pipe. By so doing, the differential
pressure forces on the mod pistons could be relieved by
repositioning the jet pipes.
Each redundant channel of the servo in one embodiment additionally
includes a third piston arrangement which acts as a failure
monitor. If a hardover failure (or a loss of input or feedback)
occurred in any individual redundant channel, the pressure
differential on both its mod and sensor pistons would build up to a
large value. Both pistons would move in response to the pressure.
However, the mod piston is constrained by the summing link, which
is being held in place (within a small error) by the other
redundant units; whereas the monitor pressure sensor piston is free
to move, and does so, until it trips a failure switch which
disengages the particular redundant channel or unit as by removing
hydraulic pressure.
This invention relates to a novel "fly-by-wire control apparatus"
incorporating duplicate or redundant units. The term "fly-by-wire"
may be visualized as a structural departure with respect to a
conventional aircraft having force transmitting control cables
extending between the control stick operated by the pilot and the
control surface or control surface actuator. In the "fly-by-wire
apparatus," such cables are omitted and signals are taken directly
from the pilot's stick or other signal sources for operating a
motor to position the actuator.
CONVENTIONAL CONTROL SYSTEMS
Existing conventional mechanical primary control systems, wherein
the pilot through his control wheel and cables or linkage operates
a control surface of an aircraft or controls the main actuator for
the control surface, have reached a limit of performance due to the
basic operation limitation of mechanical elements in such system.
For example, such primary control system has mechanical elements
wherein there is lost motion between the moving parts all whereby
there is lack of concurrence in time or similarity between the
movement of the control wheel and the operation of the surface.
While the above lost motion effect has been referred to
specifically there are other limitations on such a mechanical
element system.
As the primary control system complexity multiplies to meet the
increasingly stringent requirements imposed by high performance and
variable geometry aircraft, sensitivity of the system to
maintenance errors is magnified in such system, and the safety and
reliability of the system decreases.
Additionally, as the mechanical linkage system is basically a
single channel system with considerable cross-sectional area from
the arrangement of its operable elements, a high vulnerability to
combat damage exists.
REDUNDANT FLIGHT CONTROL LINKAGE APPARATUS
The above disadvantages of conventional mechanical control linkages
in an aircraft control surface positioning apparatus can be
eliminated by use of electrical and electronic techniques. Thus, an
electrohydraulic mechanization of the primary flight control system
will provide an improved level of performance through elimination
of the mechanical linkage nonlinearities. However, as an electronic
or hydraulic element is more prone to failure than a comparable
mechanical device multiple channels also known as redundant
channels of the electrohydraulic system must preferrably be
provided. Use of properly isolated redundant channels magnifies
many times the probability that satisfactory system operation will
remain after failures in one or more channels have occurred;
therefore, a redundant automatic flight control system can provide
higher reliability and safety as well as improved performance over
the above type of primary control system.
One object of this invention therefore is to provide an improved
apparatus with channel redundancy and with a minimum requirement
for cross channel monitoring.
Another object of this invention is to provide for channel
redundancy monitoring with a simple "in-line" monitor configuration
eliminating requirements for cross channel comparisons and voting
logic.
A further object of this invention is to provide redundant channels
each having a separate force summed servo which servos coact on a
common output member to minimize movement of the output member upon
occurrence of a failure.
A further object of the invention is to provide in each of multiple
redundant channels a hydraulic or fluid pressure equalization
monitor arrangement to minimize channel mistracking.
A further object of the invention is to provide for each of the
redundant channels an individual improved malfunction or failure
monitor with provisions for disengaging a malfunctioning
channel.
The above and other objects and advantages of the invention will
appear upon consideration of the accompanying specification along
with the subjoined drawings showing various embodiments of the
invention.
In the drawings:
FIG. 1 is a block diagram of the novel four channel redundancy
control apparatus;
FIG. 2 is a schematic of one modification of a pressure
equalization disengage monitor arrangement;
FIG. 3 is a block diagram of a redundant channel along with the
summing link or summing member for the four channels;
FIG. 4 is a block diagram of a second form of jet pipe equalization
pressure arrangement and monitor failure control;
FIG. 5 is a schematic diagram of the arrangement of FIG. 4; and
FIG. 6 is a schematic of a modified form of equalizer
piston-cylinder configuration.
In the apparatus, four identical channels, which receive like
control signals, each include a force summed fluid servo. The
servos may have separate fluid sources or there may be a lesser
number of fluid sources than servos with suitable pressure operated
switching means for each source for switching out a failed source
from a servo and switching in an unfailed source to the servo, for
example. The four servos are utilized to drive a common output
member with each servo driven directly from operation of a member
of one of the redundant channels. Multiple force summing minimizes
the output link response to a failure of one channel thus allowing
a simple failure monitoring arrangement to disengage a failed
channel.
Also a simple pressure equalization arrangement for a fluid servo
is used in each channel to minimize the mistracking due to
mismatching of inputs or tolerances within the servo.
Response requirements of the servo failure monitor are not as
critical with force summed servos (relative to displacement summed
servos) as slowness or even failure of the monitor to disengage a
hardover servo, for example results in only a small movement of the
output link.
In each channel, a jet pipe valve utilizing an electrically driven
moveable orifice is used to provide proportional control and also
to minimize hydraulic contamination effects which sometimes occur
in other arrangements.
A simple hydraulic pressure sensor piston monitors the differential
pressure across each individual servo power piston and feeds back a
time integrated and limited signal to the torque motor output which
reduces the servo differential pressure and thereby channel
mistracking. The primary positional feedback to the torque motor
linkage that positions the jet pipe is obtained from the common
multiple force summing member that is positioned by the four
redundant servos.
Referring to FIG. 1, a redundant control system 10 includes a
plurality of redundant channels A, B, C, and D. Since the channels
are similar, a description of one will suffice for a description of
all. Thus referring to channel A, an electrical variable signal
source such as control transducer 17 operated by movement of a
member such as the control stick of an aircraft supplies the
control signal in each case to a demodulator amplifier 19 which in
turn through a shaping network 20 supplies a control signal to
summing device 21. The summing device also receives control signals
from stabilization and control augmentation systems (not shown)
through conductor 22. The signal from summing device 21 is
transmitted by conductor 23 to a servoamplifier 25. The output of
the amplifier 25 reversibly controls the operation of a
conventional torque motor 26 which through motion transmission
means 27 controls the operation of a moveable servo control member
that differentially ports fluid to the ends of a piston of a servo
38. The piston under control of the fluid applied thereto operates
through piston rod 61 a common member 40 of the four channels, and
the common member 40 has an output 41 that operates for example the
control valve of a main actuator for a control surface.
The movement of the servopiston and its rod 61 to force summing
member 40 is also supplied in a feedback arrangement of the force
transmission type to a force summing arrangement within servo 38.
The output from the force summing arrangement within servo 38 is
transmitted to the moveable servocontrol member.
Additionally, the force summing arrangement within servo 38
receives an input over transmission means 48, of the force type,
from a pressure equalization arrangement 49.
The equalization arrangement 49 and a monitor 51 receives as inputs
thereto the pressure across the mod piston in servo arrangement 38
to be described.
FIG. 2 shows a force summed fluid-type servomotor 38 for one
channel. Servo 38 includes a torque motor 26 (that receives the
variable transducer signal) having an output arm 54 which through a
link 59 operates to variably displace a jet pipe or moveable
servocontrol member 52. The jet pipe 52 receives pressure fluid
through a solenoid operated engage valve 50 and conduit 51 from a
fluid pressure source P. A return 53 is provided from the discharge
of jet pipe 52. The jet pipe 52 coacts with two holes or ports 57,
58 which normally receive equal fluid pressure discharge from the
jet pipe 52. The holes 57, 58 communicate by suitable conduits to
opposed sides of a mod piston 60. Operation of the torque motor arm
54 and its connected link 59 in either direction according to the
transducer signal displaces the jet pipe 52 to cause reversible
movement of the mod piston 60. An output rod 61 of the mod piston
60 connects to a mod piston summing link or summing member 63 which
may be used to position a control valve of a main actuator, now
shown. The jet pipe 52 is repositioned toward its normal position
by a feedback arrangement comprising summing member 63 on common
member 40, a feedback spring 64 and link 65 connected to torque
motor arm 54. The redundant servos operated member 63 thus exerts
supervision of the feedback to jet pipe 52.
A hydraulic fluid cylinder-piston type monitor having a piston 67
has applied to opposite ends thereof pressure. The fluid is
conducted to the ends through suitable orifices 62 or restrictors
in subconduits connected with main conduits extending to opposite
sides of the mod piston 60. The orifices provide a time delay to
operation of the piston 67 following initial displacement of mod
piston 60. Movement of the monitor piston 67 is transmitted through
a feedback spring 69 (thus through a force or impositive feedback
member) to the torque motor arm 54 to reposition jet pipe 52 to
thereby provide pressure equalization from jet pipe 52 on the holes
or ports 57, 58.
In addition, although a separate monitor may be provided, the
hydraulic fluid clinder-piston monitor having piston 67 operates
through a suitable connection a disengage limit switch 72 for
opening the electrical circuit through a solenoid operated engage
valve 50 permitting the valve to close by suitable means such as a
spring to interrupt the flow of fluid to the adjustable jet pipe
52, thus indicating a failure in the particular channel
involved.
FIG. 3 is an analysis block diagram of the force summed servo 38 of
FIG. 2 wherein the electrical torque motor 26 receives a variable
voltage signal over a conductor from amplifier 25 FIG. 1. The
torque motor as a transducer converts the electrical input to a
mechanical torque output proportional to its electrical input.
Through its arm 54 and connecting link 59, it initially positions
the jet pipe 52 a distance proportional to the difference between
the torque motor output and the two feedback spring (64 and 69)
forces.
Due to its displacement, the jet pipe 52 varies the fluid pressure
in receiving holes 57 and 58, proportional to its displacement,
whereby the mod piston 60 is positioned in inches per second in
accordance with the flow resulting from that pressure differential
which is applied to its opposite sides. The piston 60 moves in its
cylinder in accordance with the integral of the time period of the
differential pressure and resulting rate of flow, and through its
rod 61 positions the mod piston summing link 63 and member 40 which
operates the feedback spring 64 to provide a force feedback
proportional to the displacement of the summing link 63 from the
neutral position. The force on spring 64 varies the normal
dimension or shortest distance between its output-input points.
This force is transmitted through the connecting link 65 to the
force summing point 66 which may be considered the remote end of
arm 54.
While the servoloop has been described, there is also included a
second loop for repositioning the jet pipe 52 through the pressure
sensor monitor piston 67. The differential pressure on ends of the
mod piston 60 resulting from any load on said mod piston because of
disagreements with other mod pistons connected to summing link 40
is also applied through the suitable filters or orifices to the
pressure sensor piston 67 which has a total displacement or
movement dependent upon the time integral of the differential
pressure supplied to the opposite sides of piston 67. This
displacement is converted through the spring member 69 to pounds
per inch of displacement of piston 67 and is also applied to the
force summing point 66.
Additionally, in the embodiment of FIG. 2 the displacement of
pressure sensor monitor piston 67 is supplied to the disengage
on-off limit switch 72 which operates as will be described
hereinafter to break the circuit through the operating solenoid of
engage valve 50 thereby terminating the flow of fluid from the
pressure source P to the jet pipe 52. The switch 72 also controls
the energization of an indicator for notification to the pilot of
such failure in the channel.
As indicated in FIG. 3, X2, X3, X4, the mod pistons of the three
other redundant, force servos are connected to the summing member
40. It will be understood that each channel torque has similar
feedback provisions as that in channel A.
While mechanical feedback has been described specifically to apply
impositive torques to the jet pipe 52, it is contemplated that the
feedbacks from summing member 40 and pressure sensor piston 67
could be electrical signals that would, instead of being applied to
torque arm 54 as the mechanical feedback are, be applied to
amplifier 25.
Concerning a second embodiment, and advancing from FIG. 3 the
arrangement in FIG. 4 resembles FIG. 3 except for the fact that the
differential pressure applied to the mod piston 60 is not only
applied to the mod piston pressure equalizer 67 but is separately
applied to the engage valve control circuit, switch 72 having a
separate pressure responsive piston operating means. FIG. 4 thus
has reference characters corresponding to those in FIG. 3.
In addition to the mod piston pressure sensing or responsive
switches 72 controlling the solenoid engage valve 50, the valve may
additionally be controlled from engage and monitor circuits to be
described. FIG. 4 consequently includes logic statements which will
hereinafter be reviewed.
FIG. 5 shows the force summed servo with the mechanical feedback of
FIG. 4 shown in detail. The features common with FIG. 2 have the
same reference characters.
FIG. 5, since it presents merely novel electrical circuits over
material previously considered, will be described concurrently with
its operation, thus momentary closing of a manually operable start
monitor switch 80 upper left closes the circuit from a DC voltage
source (not shown), conductor 81, switch 80, conductor 82,
conductor 83, presently closed solenoid operated switch 84,
conductor 85, through a pull-in electrical winding 86 of a
push-type solenoid, conductor 87 to DC return. Previous to
operation of switch 80, the engage valve 50 as shown is in closed
position whereby pressure from the source P cannot be transmitted
to the jet pipe 52. Following closing of switch 80 and energization
of winding 86, the valve moves rightward in the FIG. thus
permitting pressure fluid to be supplied to the jet pipe 52 and
concurrently opening the circuit through switch 84 by an extension
of the valve and valve 50 remains open by the following
arrangement.
Pressure monitor disengage switch 72 as shown consists of four
relatively moveable end mounted members two of which, 90, 91 are
elongated to engage with shoulders on moveable plungers 93, 94, of
the mod piston pressure monitor. Each of the separate plungers 93,
94 have engaged therewith suitable outwardly biasing springs 95,
97. It is evident that when pressure fluid through suitable
conduits shown is admitted to one end of each of the plungers 93
and 94 that the normal pressure in ports 57, 58 as transmitted by
the conduits compress the biasing springs 95, 97 causing the
operable inner switch members 90, 91 to contact each other. The
outer switch contacts 98, 99 which normally engage their respective
inner contacts 90, 91 remain so engaged.
Thus, as involving switch 72 and its closed contacts, upon opening
of engage valve 50 by operation of switch 80, a holding circuit for
the engage valve solenoid is established from an electrical supply
as a DC source (not shown), conductor 88 closed engage switch 104,
outer contact 99, contacts 91, 90, 98, conductor 100, conductor
101, hold winding 102, pull-in winding 86, conductor 87 to DC
return.
Concerning the operation of the pressure monitor disengage switch
72, in the event that there has been a failed operation malfunction
in signal operated amplifier 25 of channel A for example whereas
the remaining channels have operated, motion is applied in channel
A through the force summing link 40 and rod 61 to piston 60 moving
the cylinder feedback flat spring 64 and displacing the jet pipe
52, which has no torque applied thereto from torque 26, in
accordance with the displacement of the output member 40 or summing
link 63. This unrestricted displacement of pipe 52 results in a
large pressure in one or the other of holes or ports 57, 58 which
can approximate 80 percent of the total pressure P.sub.s resulting
in a large displacement say of plunger 94 relative to plunger 93 or
vice versa and as to plunger 94 engaging switch arm 90 thereby
opening the electrical circuit between outer contact 98 and inner
contact 90 or as to plunger 93 being operated disengaging outer
contact 99 and inner contact 91 thereby opening the electrical
circuit between conductors 88 and 100 thus deenergizing the
windings 102, 86 of the solenoid causing the engage valve 50 to
move to its closed position as shown in FIG. 5. The closing
movement of the valve 50 may be obtained by a suitable auxiliary
spring means 89 connected thereto aided by the pressure from the
hydraulic power source.
As in FIG. 2, in FIG. 5 the spring biased, mod piston pressure
equalizer piston 67 is supplied with fluid through a restrictor 62
to limit the flow rate thereto so that it operates on a long term
basis. In one mode of operation of the pressure equalizer piston
67, in the event that the A channel mod piston 60 is jammed and
will not move, a differential pressure on the opposite ends of the
mod piston 60 for an extended time period occurs. The equalizer
piston 67, because of the presence over the time period of the
differential pressure, is displaced and through its spring
connection 69 will return the jet pipe 52 to normal position where
it applies equal pressure to the two openings 57, 58 and thus to
both ends of the mod piston.
The circuit between conductor 88 and outer contact 99 of limit
switch 72 as stated includes a manually operable engage-disengage
switch 104 so that the channel may be also manually disengaged. It
is moved to closed position following momentary operation of switch
80 to hold the solenoid valve 50 in the open or engaged
position.
With respect to the logic statements of FIG. 4, note that in FIG. 5
the pressure line, to one end of monitor pressure switch plunger 93
remote from its biasing means, has been labeled c.sub.2 and the
line conveying pressure to one end of plunger 94 remote from its
biasing spring is labeled c.sub.1. Additionally the line conveying
the pressure from the source to the jet pipe 50 is labeled P.sub.s.
In FIG. 4, the disengage logic is tabulated. In other words, the
electrical engage circuit for valve 50 is opened between contacts
90, 91, if c.sub.1 + c.sub.2 be less than .5 P.sub.s. Also the
circuit is opened between contacts 90, 98 of the limit switch 72 if
c.sub.1 - c.sub.2 be greater than .8 P.sub.s. Finally, if c.sub.2 -
c.sub.1 be greater than .8 P.sub.s the engage circuit is opened at
contacts 91, 99. In other words, for the first condition, the inner
contacts 90, 91, will become disengaged or are not closed. For the
second condition, outer contact 98 and inner contact 90 will be
disengaged. For the third condition or third logic statement, outer
contact 99 and contact 91 will be disengaged. In other words, the
solenoid winding for the engage valve 50 will be deenergized if
there is very low pressure supplied to the mod piston 60 by the jet
pipe 52. Also, if the pressure be excessive on one side of the mod
piston relative to the other, either switch contacts 98, 90 or 99,
91 will be disengaged.
FIG. 6 shows a modified form of cylinder-piston equalizer 110,
where the cylinder has a small diameter portion 111 housing the
piston 67 and two enlarged cylindrical end sections 112, 113. The
end sections of chambers 112, 113 are connected to the lines
c.sub.1, c.sub.2 FIG. 5, with the chamber 113 including in the
supply line thereto a fluid flow restrictor 62. End chamber 112
includes a washer 117 which abuts one end of chamber 112 while
subject to the force of a prestressed coil spring 119. Similarly,
the chamber 113 includes a washer 118 biased by a preloaded coil
spring 120. The arrangement is such that the washers 117 and 118
with no pressure applied to conduits c.sub.1 and c.sub.2 apply no
force to the piston 67. However, when a differential pressure is
applied to the conduits c.sub.1, c.sub.2 the pressure must overcome
either one of the preloaded springs 119, 120 before any movement is
applied to the piston 67 for moving the jet pipe 52.
The arrangement in FIG. 6 has its advantages over that in FIG. 5
during the application of small static loads or small steady state
loads on the summing member 40. For example, the apparatus
including the summing member 40 may be associated with the primary
controls such as the control stick and feel system in an aircraft
whereby static loads from the feel system may be applied to the
member 40 to apply a steady static load thereto. By the arrangement
of FIG. 6, there will be no action or movement of the equalizer
piston 67 until the static load exceeds the preloading of either
one of the springs 119, 120, in other words by use of the springs
119, 120 and the arrangement of FIG. 6 a "dead spot" is provided so
that operation of the piston 67 does not occur until the
differential pressure exceeds the preloading of either spring 119
or 120 thus static loads are tolerated.
OPERATION
Each of the servo amplifiers, such as amplifier 25 in channel A,
for the force summed fluid servos receive a similar electrical
control signal. The multiple mod pistons are normally linked
together to a common output member 63. In accordance with the
common control signals, the mod pistons normally or ideally have
the same operation. In actual practice, when such a mechanical
arrangement is made, the null of each of the servounits will vary
in accordance with manufacturing tolerances, material variations,
etc. To compensate for these effects and to equalize the
load-carrying capacity of the individual mod pistons, a
differential pressure sensor equalizer is connected to the
corresponding output cylinder to have applied thereto the
differential pressure on a mod piston. To permit only long term
pressure variations to effect operation of this mod piston pressure
equalizer, flow to its opposite ends is limited by orifices. This
limitation--plus the effect of the pressure sensor displacement
characteristics --permits the input signal to the jet pipe to be
modified such that the loads imposed on all the mod pistons are
equalized, or nearly so.
Should a failure occur in but one of the redundant units --for
example a hardover failure due to loss of servo feedback --the
pressure sensor (failure switch) of that unit because of high
pressure due to displacement of pipe 52 from center will move off
towards one extreme. The opposing load on the remaining redundant
mod pistons will be a fraction of this pressure value since they
oppose the failed channel collectively. With the failure switch 72
set to 80 percent of the available pressure, P.sub.s, from the
pressure source, only the failed mod piston or servo unit achieves
this pressure level and, thereby, trips its failure switch 72
indicating failure and disabling the fluid supply to its jet pipe.
The remaining channels continue engaged or operative.
This type of operation requires that the mod pistons handle little
or no load as stated. This is accomplished by having the mod
pistons drive a standard control valve for a main actuator. While
there are some dynamic loads reflected to the mod pistons, the
effect of the pressure sensor (equalizer) line orifices minimize
these effects.
Either soft or hardover failures are detected directly by the
failure switch 72. Dead failures are detected due to the relatively
high-pressure gain utilized, which, when a command appears to the
good servos makes the dead servo appear to be hardover in the
opposite direction.
The four channels may have individual fluid sources and their jet
pipes may be engaged and will remain engaged with at least one
fluid source through pressure responsive switching in the event its
original pressure source fails.
It will be understood that the gains of the redundant channels are
so designed that when the overall system calls for the operation of
a pressure equalization monitor, such as 67 FIG. 5, there is little
probability of operation of a disengage switch 72, in response to
such call.
FAILURE MONITORING
The force summed servos with hydraulic mod piston pressure
equalization provide unique, straight-forward monitoring of channel
performance. When channel mistracking reaches a predetermined
level, it is, by definition, a failure of that channel and a switch
is actuated. Essentially, this method of monitoring compares each
servo against the average output for all servos in that axis. Thus,
no complicated logic arrangement is required to sort out the failed
channel. In the four channel embodiment herein, even after two
channels fail and are disengaged, the system is fail safe since if
a malfunction occurs in one remaining channel, any tendency for
irregular operation of the output member 40 is compensated by the
remaining properly operating channel.
It will now be evident that there has been provided a novel control
system such as a "fly-by-wire system" consisting of redundant
channels, having fluid operated servos. Such systems, because of
the redundancy, provide a fail operational arrangement more
reliable than the conventional primary control system. Further, the
arrangement includes a self-equalization arrangement for equalizing
the fluid pressure on the holes cooperating with a jet pipe in each
redundant channel. Additionally a disengage monitoring arrangement
has been included responsive to fluid pressure on a servo to
disable a failed servounit.
* * * * *