Flight control surface actuation force fight mitigation system and method

A system and method of mitigating a force fight between hydraulically-operated actuators that are coupled to a single flight control surface is provided. The differential fluid pressure across each hydraulically-operated actuator is sensed. The position of a user interface is sensed using a plurality of user interface position sensors. Flight control surface position is sensed using one or more position sensors. The sensed differential pressures, the sensed user interface positions, and the sensed flight control surface position are used to generate a plurality of substantially equal actuator commands.

TECHNICAL FIELD

The present invention generally relates to aircraft flight control systems and, more particularly, an aircraft flight control system and method that mitigates any potential force fights between actuators coupled to the same flight control surface.

BACKGROUND

Aircraft typically include a plurality of flight control surfaces that, when controllably positioned, guide the movement of the aircraft from one destination to another. The number and type of flight control surfaces included in an aircraft may vary, but typically include both primary flight control surfaces and secondary flight control surfaces. The primary flight control surfaces are those that are used to control aircraft movement in the pitch, yaw, and roll axes, and the secondary flight control surfaces are those that are used to influence the lift or drag (or both) of the aircraft. Although some aircraft may include additional control surfaces, the primary flight control surfaces typically include a pair of elevators, a rudder, and a pair of ailerons, and the secondary flight control surfaces typically include a plurality of flaps, slats, and spoilers.

The positions of the aircraft flight control surfaces are typically controlled using a flight control surface actuation system. The flight control surface actuation system, in response to position commands that originate from either the flight crew or an aircraft autopilot, moves the aircraft flight control surfaces to the commanded positions. In most instances, this movement is effected via actuators that are coupled to the flight control surfaces. Though unlikely, it is postulated that a flight control surface actuator could become jammed, uncontrollably free, or otherwise inoperable. Thus, some flight control surface actuation systems are implemented with redundant (e.g., two or more) actuators coupled to a single flight control surface.

Flight control surface actuation systems that have two or more actuators coupled to a single flight control surface typically implement one of two operational configurations—an active-standby configuration or an active-active configuration. With the active-standby (or active-standby-standy) configuration, one actuator is actively powered while the other one (or two) are in a standby mode. With the active-active (or active-active-active) operational configuration, all of the actuators are simultaneously powered. This latter operational configuration provides certain advantages over the active-standby (or active-standby-standy) configuration. Specifically, it allows each individual actuator to be sized relatively smaller as compared to the actuators used to implement the active-standby (active-standby-standby) configuration. Additionally, there is no need for any redundancy management. It is noted, however, that the active-active (or active-active-active) operational mode does present the potential for a resultant force fight between the active actuators.

The force-fight results from the fact that the actuators, position sensors, control electronics, and mechanical components have independent, unique tolerances. Although installation and surface position rigging can reduce some of the differences between two channels, these differences can result in one channel attempting to position the flight control surface to a different position than the other channel(s). The resultant effect is torsion moment on the flight control surface as the two neighbouring channels compete with each other to move the flight control surface to different positions. This torsion moment introduces stress to the flight control surface and a resulting fatigue accumulation. Designing flight control surfaces to withstand the worst-case stress and fatigue that could occur in the active-active (or active-active-active) operational configuration would result in additional weight, and associated its costs.

Hence, there is a need for a system and method of preventing, or at least mitigating, the resultant force fights that can occur between actuators when flight control surface actuation system channels are configured in an active-active (or active-active-active) operational configuration without relying on undesirably heavy flight control surfaces. The present invention addresses at least this need.

BRIEF SUMMARY

In one embodiment, a flight control surface actuation system includes a plurality of differential pressure (DP) sensors, a plurality of user interface sensors, a position sensor, and a control. Each DP sensor is configured to sense a differential fluid pressure across a hydraulically-operated actuator and supply a differential pressure signal representative of the sensed differential fluid pressure. Each user interface sensor is configured to sense movement of a user interface and supply a position command signal representative of the sensed movement. The position sensor is configured to sense flight control surface position and supply a flight control surface position signal representative of the sensed flight control surface position. The control is coupled to receive the differential pressure signals, the position command signals, and the flight control surface position signal, the control is configured to process these signals and generate a plurality of substantially equal actuator commands.

In another embodiment, a flight control surface actuation system includes a plurality of hydraulically-operated actuators, a plurality of differential pressure (DP) sensors, an inceptor, a plurality of user interface sensors, and a control. Each actuator is coupled to receive an actuator command and is adapted to receive a flow of hydraulic fluid. Each actuator is configured, upon receipt of the actuator command, to move a flight control surface to a position. Each DP sensor is configured to sense a differential fluid pressure across one of the hydraulically-operated actuator and supply a differential pressure signal representative of the sensed differential fluid pressure. The inceptor is configured to receive user input and is configured, upon receipt of the user input, to move to a control position. Each user interface sensor is configured to sense movement of the inceptor and supply a position command signal representative of the control position. The position sensor is configured to sense the position of the flight control surface and supply a flight control surface position signal representative thereof. The control is coupled to receive the differential pressure signals, the position command signals, and the flight control surface position signal. The control is configured to process these signals and generate a plurality of substantially equal actuator commands for supply to the actuators.

In yet a further embodiment, a method of mitigating a force fight between hydraulically-operated actuators that are coupled to a single flight control surface includes sensing differential fluid pressure across each hydraulically-operated actuator, sensing a position of a user interface using a plurality of user interface position sensors, and sensing flight control surface position using one or more position sensors. A plurality of substantially equal actuator commands is generated from the sensed differential pressures, the sensed user interface positions, and the sensed flight control surface position.

Furthermore, other desirable features and characteristics of the flight control surface actuation system and method will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

DETAILED DESCRIPTION

Turning first toFIG. 1, a perspective view of an exemplary aircraft is shown. In the illustrated embodiment, the aircraft100includes first and second horizontal stabilizers101-1and101-2, respectively, a vertical stabilizer103, and first and second wings105-1and105-2, respectively. An elevator102is disposed on each horizontal stabilizer101-1,101-2, a rudder104is disposed on the vertical stabilizer103, and an aileron106is disposed on each wing105-1,105-2. In addition, a plurality of flaps108, slats112, and spoilers114are disposed on each wing105-1,105-2. The elevators102, the rudder104, and the ailerons106are typically referred to as the primary flight control surfaces, and the flaps108, the slats112, and the spoilers114are typically referred to as the secondary flight control surfaces.

The primary flight control surfaces102-106control aircraft movements about the aircraft pitch, yaw, and roll axes. Specifically, the elevators102are used to control aircraft movement about the pitch axis, the rudder104is used to control aircraft movement about the yaw axis, and the ailerons106control aircraft movement about the roll axis. It is noted, however, that aircraft movement about the yaw axis can also be achieved by varying the thrust levels from the engines on opposing sides of the aircraft100.

The secondary control surfaces108-114influence the lift and drag of the aircraft100. For example, during aircraft take-off and landing operations, when increased lift is desirable, the flaps108and slats112may be moved from retracted positions to extended positions. In the extended position, the flaps108increase both lift and drag, and enable the aircraft100to descend more steeply for a given airspeed, and also enable the aircraft100get airborne over a shorter distance. The slats112, in the extended position, increase lift, and are typically used in conjunction with the flaps108. The spoilers114, on the other hand, reduce lift and when moved from retracted positions to extended positions, which is typically done during aircraft landing operations, may be used as air brakes to assist in slowing the aircraft100.

The flight control surfaces102-114are moved to commanded positions via a flight control surface actuation system200, an exemplary embodiment of which is shown inFIG. 2. In the depicted embodiment, the flight control surface actuation system200includes a plurality of primary flight control surface actuator assemblies, which include elevator actuator assemblies202, rudder actuator assemblies204, and aileron actuator assemblies204, and a plurality of controls208.

Before proceeding further, it is noted that the flight control surface actuation system200additionally includes a plurality of secondary control surface actuator assemblies, such as flap actuator assemblies, slat actuator assemblies, and spoiler actuator assemblies. However, the operation of the secondary flight control surfaces108-114and the associated actuator assemblies is not needed to fully describe and enable the present invention. Thus, for added clarity, ease of description, and ease of illustration, the secondary flight control surfaces and actuator assemblies are not depicted inFIG. 2, nor are these devices further described.

Returning now to the description, it will be appreciated that the flight control surface actuation system200may be implemented using various numbers and types of primary flight control surface actuator assemblies202-206. In addition, the number and type of primary flight control surface actuator assemblies202-206per primary flight control surface102-106may be varied. In the depicted embodiment, however, the flight control surface actuation system200is implemented such that two primary flight control surface actuator assemblies202are coupled to each elevator102, three primary flight control surface actuator assemblies204are coupled to the rudder104, and two primary flight control surface actuator assemblies206are coupled to each the aileron106. Moreover, each primary flight control surface actuator is implemented using a hydraulic actuator assembly202-206. It will be appreciated that this number of primary flight control surface actuator assemblies202-206is merely exemplary of a particular embodiment, and that other numbers of actuator assemblies202-206could also be used.

The flight control surface actuation system200may also be implemented with various numbers of controls208. However, the flight control surface actuation system200is preferably implemented such that the primary flight control surface actuator assemblies202-206that are coupled to a common flight control surface102-106are controlled by at least one control208. Thus, at least in the depicted embodiment, the flight control surface actuation system200includes five controls208. It will be appreciated that, although the controls208are depicted as being disposed remote from the associated actuator assemblies202-206, some or all of the actuator controls208could be collocated or integral with the associated actuator assemblies202-206.

No matter its specific implementation, each control208is preferably configured, during normal system operation, to operate in an active-active (or active-active-active) mode. Thus, each control208is configured to receive flight control surface position commands from one or more non-illustrated external systems, such as one or more pilot controls. In response to the flight control surface position commands, each actuator control208appropriately controls its associated plurality of flight control surface actuator assemblies202-206to supply a force to its associated flight control surface102-106that moves the associated flight control surface102-106to the commanded position. The controls208are additionally each configured to eliminate, or at least mitigate, any potential force fight that might otherwise occur between its associated flight control surface actuator assemblies202-206. It will be appreciated that the controls208may be variously configured to implement this functionality. However, particular preferred configurations are depicted inFIGS. 3 and 6, and will each be described.

Referring first toFIG. 3, a functional block diagram of a portion of the flight control actuation system associated with a single primary flight control surface is depicted and includes an inceptor system302, a plurality of actuator assemblies304(e.g.,304-1,304-2), and a control306. The inceptor system302includes user interface308and a plurality of user interface sensors312(e.g.,312-1,312-2). The user interface308is configured to move in response to an input force supplied from, for example, a pilot. The user interface position sensors312are configured to sense the position of the user interface308, and supply position command signals314representative of the sensed user interface position. It will be appreciated that the user interface308may be implemented using any one of numerous user interface configurations including, for example, a side stick, a yoke, or a rudder pedal, just to name a few, and may be implemented as an active device or a passive device. It will additionally be appreciated that the user interface sensors312may be variously configured. For example, the user interface sensors312may be implemented using any one of numerous types of force sensors, or position sensor, just to name a few. No matter its specific implementation, the inceptor system302supplies the position command signals314to the control306. As will be described further below, the control306is configured to generate and supply actuator commands to each of the actuators304.

The actuator assemblies304, as described above, are each implemented as hydraulic actuator assemblies and are coupled to the same flight control surface310. The hydraulic actuator assemblies304may be implemented using any one of numerous types of hydraulic actuator assemblies. In the depicted embodiment, the hydraulic actuator assemblies304each include a servo control valve316and an actuator318. The servo control valve316, which is implemented as an electro-hydraulic servo valve (EHSV) in the depicted embodiment, is configured, when appropriately energized, to control the supply of pressurized hydraulic fluid to and from the actuator318. The actuator318, depending on the position of the EHSV316, and in response to the pressurized hydraulic fluid, moves in either an extend direction322or a retract direction324, and thereby supplies a force to the flight control surface310. It will be appreciated that the hydraulic actuator assemblies304could additionally be implemented as electrohydraulic actuator assemblies (EHAs), which do not include servo valves.

The actuator assemblies304additionally include a plurality of sensors. In the depicted embodiment, these sensors include a differential pressure (DP) sensor326and a position sensor328. It will be appreciated, however, that the actuator assemblies304could include additional sensors if needed or desired. Each DP sensor326is configured to sense the differential fluid pressure across its associated actuator318(e.g., between to hydraulic fluid actuator chambers), and supply a differential pressure signal332representative of the sensed differential fluid pressure to the control306. The DP sensors326, as may be appreciated, may be implemented using any one of numerous types of suitable pressures or DP sensors now known or developed in the future. In those embodiments in which pressure sensors are used, it will be appreciated that each pressure sensor senses the fluid pressure in one hydraulic fluid actuator chamber, and the DP sensors326preferably include suitable processing circuitry to calculate the differential fluid pressure from the individual sensed fluid pressures.

The position sensors328are each configured to sense the position of the associated actuator318and supply an actuator position signal334representative of the sensed position to the control306. It will be appreciated that the actuator position signal334is also representative of, and may indeed be scaled to, the position of the flight control surface310. Thus, it may additionally be appreciated that in some embodiments the actuator position sensors328may be replaced with one or more sensors that directly sense the position of the flight control surface310. In any case, it will be appreciated that the one or more position sensors328, whether configured to sense actuator position or flight control surface position directly, may be implemented using any one of numerous position sensors now known or developed in the future.

The control306is coupled to receive the position command signals314from the inceptor system302, the differential pressure signals332from the actuator assemblies304, and the flight control surface position signals334from either the actuator assemblies304or the flight control surface310. The control306is configured to process these signals314,332,334and generate a plurality of substantially equal actuator commands336. The actuator commands336are supplied to the actuator assemblies304, and more specifically, at least in the depicted embodiment, to the servo control valves316, to control the supply of pressurized hydraulic fluid to and from the actuator318. Because the actuator commands336supplied to the actuator assemblies304are at least substantially equal, any potential force fight between the actuator assemblies304, both during movement of the flight control surface310or when the flight control surface310is static, is mitigated or even eliminated. AsFIG. 3depicts, the control306implements what is referred to herein as a force fight mitigation function350. It is the force fight mitigation function350that provides for the generation of the substantially equal actuator commands336. A particular preferred implementation of the force fight mitigation function350will be described further below. Before doing so, however, a particular configuration of the control306will be described.

With continued reference toFIG. 3, it is seen that each of the controls306, at least in the depicted embodiment, includes a plurality of actuator control modules338(e.g.,338-1,338-2) and a flight control module342. This is, of course, merely one example as to how each control306could be configured, and that various other configurations could be implemented. Nonetheless, in the depicted embodiment each of the actuator control modules338, which may also be referred to as actuator control electronics (ACE) modules, is coupled to receive one of the position command signals314from the inceptor system302, the differential pressure signals332from one of the actuator assemblies304, and the flight control surface position signals334either from one of the actuator assemblies304or from the flight control surface310. The actuator control modules338in turn supply these signals314,332,334to the flight control module342.

The flight control module342, and more specifically to the force fight mitigation function350, processes the position command signals314, the differential pressure signals332, and the flight control surface position signals334, and generates and supplies force fight command signals352(352-1,352-2) to the actuator control modules338. Each of actuator control modules338combines the force fight command signals352it receives from the flight control module342with the position command signals314it receives from the inceptor system302to generate and supply actuator commands336to the appropriate actuator assembly304. As noted above, the actuator command336supplied to one actuator304-1will be equal, or at least substantially equal, to the actuator command336supplied to the other actuator304-1. As was noted above, the force fight mitigation function350provides for the generation of the substantially equal actuator commands336. A particular preferred implementation of the force fight mitigation function350is depicted inFIG. 4, and with reference thereto will now be described

The force fight mitigation function350includes a DP averaging function402, a position command averaging function404, first, second, third, and fourth subtraction functions406-1,406-2,406-3, and406-4, first and second proportional-plus-integral (PI) control paths408-1,408-2, first and second addition functions412-1,412-2, and first and second gain functions414-1,414-2. The DP averaging function402is coupled to receive the differential pressure signals332and is configured to supply a value416representative of the average of the sensed differential fluid pressures. It will be appreciated that the DP averaging function402may generate the value416representative of the average of the sensed differential fluid pressures using various techniques. For example, the DP averaging function402may determine the mathematical difference between the sensed differential pressures and divide the determined mathematical difference by two. Alternatively, it may determine the mathematical sum of the sensed differential pressures and divide the determined mathematical sum by two.

The position command averaging function404is coupled to receive the position command signals314from the user interface system202, and is configured to supply a value418representative of the average of the position command signals. It will be appreciated that the position command averaging function404, like the DP averaging function402, may generate the value418representative of the average of the position command signals using various techniques. For example, the position command averaging function404may determine the mathematical difference between the position command signals and divide the determined mathematical difference by two. Alternatively, it may determine the mathematical sum of the position command signals and divide the determined mathematical sum by two.

The first and second subtraction functions406-1,406-2are each coupled to receive one of the differential fluid pressure signals332and the value416representative of the average of the sensed differential fluid pressures. The first and second subtraction functions406-1,406-2are each configured to determine the mathematical difference between the sensed differential pressure and the value representative of the average of the sensed differential fluid pressures and supply a value representative of this determined mathematical difference. These determined mathematical differences, which are referred to herein as differential fluid pressure offsets422-1,422-2, are supplied one each to the PI control paths408-1,408-2.

The PI control paths408-1,408-2are each coupled to receive one of the differential fluid pressure offsets422-1,422-2, and are each configured to supply differential pressure equalization output signals424-1,424-2. The PI control paths408each include an integral path and a proportional path. The integrals paths provide long term control of the differences in fluid differential pressure between the actuator assemblies304, driving the DP to the average value. Each integral path also preferably eliminates any tolerances in the control electronics and any mechanical offsets in the actuator assemblies304. Although the integral paths are able to effectively mitigate steady state force fights, these paths are unable to keep up with dynamic flight control surface movements. The proportional paths, however, compensate for high rate pressure changes during such movements.

The differential pressure equalization output signal424-1from one of the PI control paths408-1is supplied to the third subtraction function406-3, and the differential pressure equalization output signal424-2from the other PI control path408-2is supplied to the first addition function412-1. The value418representative of the average of the position command signals is supplied to both the third subtraction function406-3and the first addition function412-1. Thus, the value418representative of the average of the position command signals is mathematically subtracted from one of the differential pressure equalization output signals424-1to generate a first equalization output signal426-1, and is mathematically added to the other differential pressure equalization output signal424-2to generate a second equalization output signal426-2.

The first and second equalization output signals426-1,426-2are supplied to the second addition function412-2and the fourth subtraction function406-4, respectively. The second addition function412-2and the fourth subtraction function406-4also each receive an actuator/flight control surface position equalization command signal428(e.g.,428-1,428-2). The outputs of the second addition function412-2and the fourth subtraction function406-4are the above-described force fight command signals352-1,352-2that are supplied to the actuator control modules338-1,338-2. The actuator surface position equalization command signals428-1,428-2are supplied from the first and second gains414-1,414-2. The first and second gains414-1,414-2are responsive to the one or more flight control surface position signals334in a manner, and for reasons, that will now be described.

Various system components and associated tolerances are relatively significant contributors to the force fight between actuator assemblies304coupled to the same flight control surface310. A reduction in component tolerances would, more than likely, lead to higher component price. As an alternate approach, the effect of the system rigging was investigated. That is, the static offsets between the actuator assemblies304associated with a common flight control surface310were measured at the zero deflection (or null) position, and at the two extreme flight control surfaced actuation positions. These measured offsets are recorded and used to generate, via interpolation, additional actuator surface position equalization command signals428-1,428-2. Hence, the first and second gains414-1,414-2, which may be implemented using a look-up table, for example, supplies the actuator surface position equalization command signals428-1,428-2based on the one or more flight control surface position signals334. The actuator surface position equalization command signals428-1,428-1are supplied, in a feed forward manner, to the second addition function412-2and the fourth subtraction function406-4to generate the force fight command signals352-1,352-2. As may be readily apparent to the skilled artisan, the force fight command signals352-1,352-2that are generated will be equal in magnitude and opposite in sign.

Before proceeding further, it is noted thatFIG. 4depicts two gains412-1,412-2that each receive a position feedback signal334from separate position sensors328. This, however, is merely exemplary of one embodiment. In other embodiments, as depicted inFIG. 4, the force fight mitigation function350could be implemented using a single gain412having a single position signal334as an input. This is possible because of the relatively high accuracy associated with the position sensors328that are used to sense actuator or flight control surface position, as the case may be.

It was previously noted that for some aircraft, such as the one depicted inFIG. 1, three primary flight control surface actuator assemblies may be coupled to a single flight control surface, such as the rudder104. A functional block diagram of a portion of the flight control actuation system associated with a primary flight control surface to which three actuator assemblies are coupled is depicted inFIG. 6. This system portion includes also an inceptor system302, a plurality of actuator assemblies304(e.g.,304-1,304-2,304-3), and a control306. It will be appreciated that this portion of the primary flight control system operates substantially identical to the one depicted inFIG. 3, and thus includes like reference numerals to refer to like parts ofFIG. 3. The major difference, of course, is that the system portion depicted inFIG. 6includes an additional processing channel associated with the additional actuator assembly304-3. Because the overall operation of this system portion is substantially identical to that of the one depicted inFIG. 3, the skilled artisan will readily appreciate that a detailed description of this system portion need not be repeated.

The additional actuator assembly304-3, and its associated processing channel, does result in the force fight mitigation function350being slightly modified from the one depicted inFIG. 4and described above. In particular, asFIG. 7depicts, the force fight mitigation function350for this system portion includes the DP averaging function402, the position command averaging function404, first, second, third, fourth, fifth, sixth, and seventh subtraction functions406-1,406-2,406-3,406-4,406-5,406-6,406-7, first, second, and third proportional-plus-integral (PI) control paths408-1,408-2, and408-3, first, second, third, fourth, and fifth addition functions412-1,412-2,412-3,412-4, and412-5, and first, second, and third gain functions414-1,414-2, and414-3. The overall operation of the three-channel force fight mitigation function350depicted inFIG. 7is substantially identical to that of the one depicted inFIG. 4. As such, the skilled artisan will readily appreciate that a detailed description of this embodiment of the force fight mitigation function350need not be repeated. As may be readily apparent to the skilled artisan, the force fight command signals352-1,352-2,352-3that are generated in this embodiment will be such that the sum of two of the force fight command signals will be equal in magnitude and opposite in sign to that of the remaining force fight command signal. In other words, the force fight command signals352-1,352-2,352-3will mathematically sum to zero.

Just as the two-channel force fight mitigation function350depicted inFIG. 4included two gains that each receive a position feedback signal334from separate position sensors328, the three-channel force fight mitigation function350depicted inFIG. 5includes three gains414-1,414-2,414-3that each receive a position feedback signal334from separate position sensors328. This, however, is merely exemplary of one embodiment. In other embodiments, as depicted inFIG. 8, the three-channel force fight mitigation function350could be implemented using two gains414-1,414-2that each have the same, single position signal334as an input. Again, this is possible because of the relatively high accuracy associated with the position sensors328that are used to sense actuator or flight control surface position, as the case may be.

The system and method described herein prevent, or at least mitigate, the resultant force fights that can occur between actuators when flight control surface actuation system channels are configured in an active-active (or active-active-active) operational configuration