Aircraft and Method for Flight Control of an Aircraft During Flight

Abstract

A method for flight control of an aircraft with multiple actuators during flight is disclosed. For each actuator, a control command is computed according to at least one predetermined control law and based on pilot inputs and sensor measurements in relation to a physical state of the aircraft. The respective control commands are provided to the actuators. The control commands are independently monitored by estimating or measuring a current physical state of the aircraft and comparing it with the control commands. This comparison includes checking whether the control commands stabilize the aircraft in a stable state in the absence of both disturbances and pilot inputs according to at least one predefined criterion. If the monitoring indicates a lack of stability, transmission of the control commands is prevented and a backup control command is computed for each actuator.

Claims

1. A method for flight control of an aircraft during flight, the aircraft including a plurality of N, N? actuators for operating movable flight surfaces or propulsion of the aircraft units via pilot inputs (w), the method comprising: computing, for each actuator, a control command (u, u?R.sup.N) established according to at least one predetermined control law (K) for control of a movable flight surface or a propulsion unit operated by a respective actuator and based on the pilot inputs (w) and sensor measurements in relation to a physical state (x) of the aircraft and providing respective control commands (u) to the actuators; independently monitoring the control commands (u) by estimating or measuring a current physical state (x) of the aircraft and comparing the current physical state (x) to the control commands (u), the comparing including checking whether the control commands (u) stabilize the aircraft in a stable state x for time t.fwdarw.? in an absence of both disturbances (d) and pilot inputs (w) according to at least one predefined criterion; and in response to the monitoring indicating a lack of stability, preventing transmission of the control commands (u) to the actuators and independently computing, for each actuator, a backup control command (u.sub.dissimilar) established according to at least one predetermined backup control law (K.sub.dissimilar) for control of the flight surface or the propulsion unit operated by a given actuator and providing respective backup control commands (u.sub.dissimilar) to the actuators.

2. The method of claim 1, wherein the at least one predefined criterion is met if: $\frac{1}{2}.Math.\stackrel{.}{V}=x^{?}.Math.P.Math.\stackrel{.}{x}=x^{?}.Math.P.Math.f\left(x,-K\left(x\right),0\right)<0\left(\mathrm{for}.Math.x.Math.>?\right),$ wherein:
{dot over (x)}=?(x,u,d) denotes a temporal evolution or time derivative ({dot over (x)}) of the physical state (x) of the aircraft expressed as a mathematical function (?) dependent on the physical state (x) of the aircraft, the state (x) being defined by one or more of attitude angles, angular rates, position, or translational velocity, the mathematical function (?) being further dependent on the control commands (u) and further dependent on unknown disturbances (d), the disturbances including one or more of atmospheric disturbances, a system degradation, a mass distribution differing from a nominal configuration, or other disturbances that affect a motion of the aircraft; wherein V=x.sup.T.Math.P.Math.x denotes a quadratic Lyapunov function with P=P.sup.T>0, so that the relation: V>0: ?x?>0 holds and ? denotes a numerical parameter>0.

3. The method of claim 1, wherein the at least one predefined criterion is met if: $\frac{1}{2}.Math.\stackrel{.}{V}=x^{?}.Math.P.Math.\stackrel{.}{x}=x^{?}.Math.P.Math.f\left(x,-K\left(x\right),0\right)<\frac{?}{2}.Math.x^{?}.Math.P.Math.x\left(\mathrm{for}.Math.x.Math.>?\right),$ wherein:
{dot over (x)}=?(x,u,d) denotes a temporal evolution or time derivative ({dot over (x)}) of the physical state (x) of the aircraft expressed as a mathematical function (?) dependent on the physical state (x) of the aircraft, the state (x) being defined by one or more of attitude angles, angular rates, position, or translational velocity, the mathematical function (?) being further dependent on the control commands (u) and unknown disturbances (d), the disturbances comprising one or more of atmospheric disturbances, a system degradation, a mass distribution differing from a nominal configuration, or other disturbances that affect a motion of the aircraft; wherein V=x.sup.T.Math.P.Math.x denotes a quadratic Lyapunov function with P=P.sup.T>0, so that the relation: V>0: ?x?>0 holds and ? denotes a numerical parameter>0; wherein it is assumed that ?x(t)???.Math.e.sup.??t for d=w=0 with a decay rate ??0 and ?=const.

4. The method of claim 2, wherein the parameter ? is used as a criterion to pause the monitoring if the physical state (x) is close to an equilibrium condition x=0, i.e., for ?x???.

5. The method of claim 2, wherein for ?x?|?? comparison is paused and the respective control commands (u, u?R.sup.N) are provided to the actuators.

6. The method of claim 5, wherein the method further comprises, in response to a disturbance (d?0) or pilot input (w?0), causing an excitement of the aircraft such that ?x?>? and the monitoring resumes.

7. The method of claim 1, wherein the control commands (u) are calculated based on a relation:
u=w?K(x), according to which the pilot inputs (w) are augmented with a control law (K(x)), the control law (K(x)) being non-linear and devised to asymptotically stabilize any undisturbed error dynamics in absence of pilot inputs: t.fwdarw.?: x.fwdarw.x.sub.0 for d=w=0, wherein x.sub.0 denotes an equilibrium condition.

8. An aircraft comprising: a plurality of N, N? actuators for operating movable flight surfaces or propulsion units of the aircraft via pilot inputs (w) and at least one pilot input device for providing the pilot inputs (w); a flight control system configured to control a flight of the aircraft using the actuators, the flight control system being in operative connection with the at least one pilot input device; at least one sensor configured to measure a current physical state (x) of the aircraft and provide corresponding measurement data (s) to the flight control system; the flight control system (2) being devised for augmenting a pilot input w for enhanced robustness to detect design and implementation errors, the flight control system having multiple independent channels including at least one primary control channel that generates control commands (u, u?R.sup.N) for the N actuators, the control commands (u) being a result of a control law (K=K(x)) implemented on a respective primary control channel that is configured to calculate the control commands (u) based on the pilot inputs (w) and the measurement data (s); wherein at least one independent monitoring channel is provided and configured to receive and to monitor an output (u) of the at least one primary control channel, the monitoring channel being configured to estimate or determine the current physical state (x) of the aircraft from the measurement data (s) and is further configured to check whether the received control commands (u) stabilize the aircraft in a stable state for time t.fwdarw.? in an absence of both disturbances (d) and pilot inputs (w) according to at least one predefined criterion; and the monitoring channel being further configured to isolate the primary control channel from the actuators based on the at least one predefined criterion.

9. The aircraft of claim 8, wherein the monitoring channel is further configured to verify a relation: $\frac{1}{2}.Math.\stackrel{.}{V}=x^{?}.Math.P.Math.\stackrel{.}{x}=x^{?}.Math.P.Math.f\left(x,-K\left(x\right),0\right)<0\left(\mathrm{for}.Math.x.Math.>?\right),$ wherein:
{dot over (x)}=?(x,u,d) denotes a temporal evolution or time derivative ({dot over (x)}) of the physical state (x) of the aircraft expressed as a mathematical function (?) dependent on the physical state (x), the physical state being defined by one or more of attitude angles, angular rates, position, or translational velocity, the mathematical function (?) being further dependent on the control commands (u) and unknown disturbances (d) that include one or more of atmospheric disturbances, a system degradation, a mass distribution differing from a nominal configuration, or other disturbances that affect a motion of the aircraft; wherein V=x.sup.T.Math.P.Math.x denotes a quadratic Lyapunov function with P=P.sup.T>0, so that the relation: V>0: ?x?>0 holds and ? denotes a numerical parameter>0.

10. The aircraft of claim 8, wherein the monitoring channel is configured to verify a relation: $\frac{1}{2}.Math.\stackrel{.}{V}=x^{?}.Math.P.Math.\stackrel{.}{x}=x^{?}.Math.P.Math.f\left(x,-K\left(x\right),0\right)<\frac{?}{2}.Math.x^{?}.Math.P.Math.x\left(\mathrm{for}.Math.x.Math.>?\right),$ wherein:
{dot over (x)}=?(x,u,d) denotes a temporal evolution or time derivative ({dot over (x)}) of a physical state (x) of the aircraft expressed as a mathematical function (?) dependent on the physical state (x), the physical state being defined by one or more of attitude angles, angular rates, position, or translational velocity, the mathematical function (?) being further dependent on the control commands (u) and unknown disturbances that include one or more of atmospheric disturbances, system degradation, a mass distribution differing from a nominal configuration, or other disturbances that affect a motion of the aircraft; wherein V=x.sup.T.Math.P.Math.x denotes a quadratic Lyapunov function with P=P.sup.T>0, so that the relation: V>0: |?x?>0 holds, and E denotes a numerical parameter>0; wherein it is assumed that ?x(t)???.Math.e.sup.??t for d=w=0 with a decay rate ??0 and ?=const.

11. The aircraft of claim 9, wherein the monitoring channel is further configured to use the parameter ? as a criterion to pause its monitoring in response to the physical state (x) being close to an equilibrium condition x=0, i.e., for ?x???.

12. The aircraft of claim 9, wherein the monitoring channel is further configured to pause comparison for ?x??? and to allow provision of the control commands (u, u?R.sup.N) to the actuators.

13. The aircraft of claim 12, wherein in response to a disturbance (d?0) or a pilot input (w?0) leading to an excitement of the aircraft such that ?x?>?, the monitoring channel being further configured to resume the monitoring.

14. The aircraft of claim 8, wherein the primary control channel is configured to implement a relation:
u=w?K(x), according to which the pilot inputs (w) are augmented with a control law (K(x)) that is non-linear and devised to asymptotically stabilize any undisturbed error dynamics in absence of pilot inputs (t.fwdarw.?: x.fwdarw.x.sub.0 for d=w=0, wherein x.sub.0 denotes an equilibrium condition or state).

15. A non-transitory memory unit comprising instructions, that when executed, cause a processor of an aircraft during flight, the aircraft including a plurality of N, N? actuators for operating movable flight surfaces or propulsion units of the aircraft via pilot inputs (w), to: compute, for each actuator, a control command (u, u?R.sup.N) established according to at least one predetermined control law (K) for control of the movable flight surface or the propulsion unit operated by a respective actuator and based on the pilot inputs (w) and sensor measurements in relation to a physical state (x) of the aircraft and provide respective control commands (u) to the actuators; independently monitor the control commands (u) by estimating or measuring a current physical state (x) of the aircraft and comparing the current physical state (x) to the control commands (u), the comparing including checking whether the control commands (u) stabilize the aircraft in a stable state x for time t.fwdarw.? in an absence of both disturbances (d) and pilot inputs (w) according to at least one predefined criterion; and in response to the monitoring indicating a lack of stability, prevent transmission of the control commands (u) to the actuators and independently compute, for each actuator, a backup control command (u.sub.dissimilar) established according to at least one predetermined backup control law (K.sub.dissimilar)) for control of the flight surface or the propulsion unit operated by a given actuator and providing respective backup control commands (u.sub.dissimilar) to the actuators.

16. The non-transitory memory unit of claim 15, wherein the at least one predefined criterion is met if: $\frac{1}{2}.Math.\stackrel{.}{V}=x^{?}.Math.P.Math.\stackrel{.}{x}=x^{?}.Math.P.Math.f\left(x,-K\left(x\right),0\right)<0\left(\mathrm{for}.Math.x.Math.>?\right),$ wherein:
{dot over (x)}=?(x,u,d) denotes a temporal evolution or time derivative ({dot over (x)}) of the physical state (x) of the aircraft expressed as a mathematical function (?) dependent on the physical state (x) of the aircraft, the state (x) being defined by one or more of attitude angles, angular rates, position, or translational velocity, the mathematical function (?) being further dependent on the control commands (u) and further dependent on unknown disturbances (d), the disturbances including one or more of atmospheric disturbances, a system degradation, a mass distribution differing from a nominal configuration, or other disturbances that affect a motion of the aircraft; wherein V=x.sup.T.Math.P.Math.x denotes a quadratic Lyapunov function with P=P.sup.T>0, so that the relation: V>0: ?x?>0 holds and ? denotes a numerical parameter>0.

17. The non-transitory memory unit of claim 15, wherein the at least one predefined criterion is met if: $\frac{1}{2}.Math.\stackrel{.}{V}=x^{?}.Math.P.Math.\stackrel{.}{x}=x^{?}.Math.P.Math.f\left(x,-K\left(x\right),0\right)<\frac{?}{2}.Math.x^{?}.Math.P.Math.x\left(\mathrm{for}.Math.x.Math.>?\right),$ wherein:
{dot over (x)}=?(x,u,d) denotes a temporal evolution or time derivative ({dot over (x)}) of the physical state (x) of the aircraft expressed as a mathematical function (?) dependent on the physical state (x) of the aircraft, the state (x) being defined by one or more of attitude angles, angular rates, position, or translational velocity, the mathematical function (?) being further dependent on the control commands (u) and unknown disturbances (d), the disturbances comprising one or more of atmospheric disturbances, a system degradation, a mass distribution differing from a nominal configuration, or other disturbances that affect a motion of the aircraft; wherein V=x.sup.T.Math.P.Math.x denotes a quadratic Lyapunov function with P=P.sup.T>0, so that the relation: V>0: ?x?>0 holds and ? denotes a numerical parameter>0; wherein it is assumed that ?x(t)???.Math.e.sup.??t for d=w=0 with a decay rate ??0 and ?=const.

18. The non-transitory memory unit of claim 16, wherein the parameter ? is used as a criterion to pause the monitoring if the physical state (x) is close to an equilibrium condition x=0, i.e., for ?x???.

19. The non-transitory memory unit of claim 16, wherein for ?x??? comparison is paused and the respective control commands (u, u?R.sup.N) are provided to the actuators.

20. The non-transitory memory unit of claim 19, wherein the memory unit comprises further instructions, that when executed, cause the processor to: cause an excitement of the aircraft such that ?x?>? and the monitoring resumes, in response to a disturbance (d?0) or pilot input (w?0).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Characteristics and advantages of example embodiments are described by way of example with reference to the drawings.

[0059] FIG. 1 shows an aircraft having a flight control system as known from the prior art for comparison;

[0060] FIG. 2 shows an aircraft according to an example embodiment having an improved flight control system; and

[0061] FIG. 3 shows further features of an aircraft according to an example embodiment.

DETAILED DESCRIPTION

[0062] As shown in FIG. 1, which represents the prior art, an aircraft 1, which comprises a plurality of actuators (not shown) for operating movable flight surfaces and/or propulsion units (not shown), is controlled during flight with respect to said actuators, i.e., said flight surfaces and propulsion units, by means of a flight control system 2, which flight control system 2 is actually, i.e., physically, comprised within aircraft 1.

[0063] Flight control system 2 presents a COM/MON architecture comprising a primary control or COM channel (COM) 2.1 and an independent monitoring or MON channel (MON) 2.2. The COM channel 2.1 performs the actual control task and provides corresponding control commands u=w?K(x) to the actuators, while the MON channel 2.2 monitors the validity of the actions of the COM channel, i.e., the validity of the control commands generated by the COM channel 2.1 and, if necessary, passivates (or isolates) the COM channel via a switch 2.3 located at the output of the COM channel 2.1, so that the COM channel cannot provide control commands to the aircraft (or actuators). This is shown in FIG. 1 by annotation (Isolate COM (y/n)). To ensure continued operation in this case, flight control system 2 further comprises a dissimilar backup controller (BACKUP) 2.4 that takes over aircraft control and provides backup control commands u.sub.dissimilar to aircraft 1, i.e., said actuators.

[0064] All of said channels or controllers 2.1, 2.2, 2.4 are devised as programmable computing devices including the required hardware (processors, memory units, etc.) and software/firmware, as known to any skilled person, although this is not shown in any detail for reason of clarity.

[0065] As already explained, aircraft pilot provides pilot input w to all three channels 2.1, 2.2 and 2.4 through a pilot input device (not shown), which channels implement a (flight) control law K (x), wherein x denotes a physical state of aircraft 1, which state can be either directly measured (via sensors, not shown) or estimated (e.g., from sensor measurements). Typically, said control law is used by COM channel 2.1 to augment the pilot input w according to the equation:

u=w?K(x).

[0066] For the MON channel 2.2 and the backup controller 2.4 this equation reads u.sub.MON=W?K(x) and u.sub.dissimilar=K.sub.dissimilar(x, w), respectively. In this way, a more generic control law structure is considered for backup. If K.sub.dissimilar=w?K(x) is chosen, the primary control structure is retained as a special case for backup, too. The aircraft's physical state x changes with time according to the equation

{dot over (x)}=?(x,u,d)

wherein ? is a mathematical function with variables x, u as previously defined and d, wherein d denotes an (external) disturbance of the aircraft or the aircraft's state, e.g., caused by a gust of wind, which disturbance is typically unknown.

[0067] There can be more than one COM channel 2.1, and backup controller 2.4 is only used if all existing COM channels have been passivated previously, as explained above.

[0068] In general, the MON channel 2.2 is an exact functional copy of the COM channel 2.1. Thus, if there is a random fault in execution or a systematic implementation error on either the COM channel 2.1 or the MON channel 2.2, this is identified by a mismatch between an output of the COM channel 2.1 (i.e., control command u) and an output of the MON channel 2.2 (i.e., control command u.sub.MON), cf. annotation Compare u and u.sub.MON in FIG. 1. In this context, a dissimilar software implementation and dissimilar hardware/part numbers (particularly for any processors and memory modules) is used for COM channel 2.1 and MON channel 2.2, respectively, to ensure an absence of common or shared errors.

[0069] FIG. 2, in contrast, shows the solution according to an example embodiment. In the following, only the differences with respect to the prior art will be discussed. Identical reference numerals or annotations denote the same elements as in FIG. 1.

[0070] As can be gathered from FIG. 2, the MON channel 2.2 is devised differently than in the prior art:

[0071] Instead of simply comparing u (from COM channel 2.1) with alternative control commands u.sub.MON (from MON channel 2.2, cf. FIG. 1), MON channel 2.2 implements and continuously verifies the relation/criterion:

[00009]$x^{?}.Math.P.Math.f\left(x,-K\left(x\right),0\right)<0\left(.Math.x.Math.>?\right),$

based on Lyapunov's method as explained further up. Matrix P is related to a so-called quadratic Lyapunov function V=x.sup.T.Math.P.Math.x with P=P.sup.T>0, so that the relation: V>0: ?x?>0 holds, and ? denotes a numerical parameter>0.

[0072] If said relation is true, i.e., if said criterion is met, then the aircraft's state is assumed to be asymptotically stable (for t.fwdarw.0), which assumption holds even in case of non-negligible disturbance (d?0) and/or pilot input (w?0). In this case, control command u is applied to the aircraft 1, i.e., its actuators.

[0073] Instead of the above criterion, another relation/criterion (not shown in FIG. 2) can be used, which is stricter in terms of required stability:

[00010]$x^{?}.Math.P.Math.f\left(x,-K\left(x\right),0\right)<\frac{?}{2}.Math.x^{?}.Math.P.Math.x\left(.Math.x.Math.>?\right),$

[0074] wherein ? (?>0) denotes an exponential decay rate.

[0075] If said other criterion is met, i.e., if the corresponding relation is true, then the aircraft's state is assumed to be exponentially stable (for t.fwdarw.?), which assumption holds even in case of non-negligible disturbance (d?0) and or pilot input (w?0). In this case, too, control command u from COM channel 2.1 is applied to the aircraft 1, i.e., its actuators.

[0076] If the implemented criterion is not met, then the COM channel 2.1 is isolated/passivated via switch 2.3 and backup controller 2.4 takes over, as described before.

[0077] As before, x denotes a physical state of aircraft 1, which state can be either directly measured (via sensors, not shown) or estimated (e.g., from sensor measurements). The output of aircraft block 1 is x, which actually results from an integration of {dot over (x)}=?(x, u, d), i.e., x=?.sub.0.sup.t dx dt, within block 1, as known to the skilled person.

[0078] In the special case of the aircraft state x being too close to an equilibrium condition x=0, i.e., for ?x???, the flight control system can be numerically unstable, which shall be avoided. Furthermore, the state x has proven to be convergent towards said equilibrium condition, which is the required result.

[0079] Therefore, in such a situation the monitoring (MON) channel 2.2 is configured to pause its monitoring based on said parameter ? if the aircraft state x is close to an equilibrium condition x=0, i.e., for ?x???. In this case, too, control command u from COM channel 2.1 is applied to the aircraft 1, i.e., its actuators.

[0080] If ?x? becomes greater than E, e.g., in case of a disturbance, then the monitoring action of MON channel 2.2 is resumed.

[0081] FIG. 3 illustrates an aircraft 1 according to an example embodiment in some more detail.

[0082] Aircraft 1 is devised in the form of an eVTOL multicopter, i.e., an aircraft with a plurality of actuators in the form of motor controllers for operating a plurality of electrically driven propulsion units 3a, only one of which motor controllers is denoted by reference numeral 3 for reason of clarity, which aircraft 1 has vertical take-off and landing capability and can be manned. The motor controllers 3, which are operatively connected with respective rotors and electric motors comprised in said propulsion units 3a, are controlled by providing them with corresponding control commands u by means of flight control system 2, as explained earlier. In this way, a flight behaviour of the entire aircraft 1 (in terms of attitude, altitude, velocity, etc.) can be controlled.

[0083] Aircraft 1 may have further actuators (not shown), e.g., in connection with movable surfaces (flaps, wings, etc.) or a payload winch, that also influence flight behaviour of the entire aircraft 1 (in terms of attitude, altitude, velocity, etc.) and can be controlled by means of flight control system 2 and appropriate control commands u.

[0084] Aircraft 1 further has a number of sensors for providing sensor data (measurement data) in connection with said physical state of aircraft 1, only one of which is denoted by reference numeral 4. Said sensors can comprise, e.g., temperature sensors, magnetometers, gyro sensors, accelerometers, atmospheric pressure sensors, etc., without limitation. Said sensors 4 provide corresponding data to flight control system 2, which data can be used to directly measure and/or estimate a current physical state x or part of such a state of aircraft 1. Said data is therefore said to be in relation with a physical state x or part of such a state of aircraft 1 and is denoted s in FIG. 3. The actual state x of aircraft 1 is established by flight control system 2, preferably by using data s from a variety of different sensors 4.

[0085] Reference numeral 5 denotes the pilot input device that was mentioned earlier, which can take the form of a control stick, a touch screen, a pushbutton, a lever, a speech input device, etc., or any combination or number of the aforementioned elements. Said pilot input device 5 is used to provide said pilot input denoted w in FIGS. 1 through 3 to flight control system 2.

[0086] Although pilot input device 5 has been described in terms of a device that can be operated by a human pilot, the embodiments are not limited in this respect. Pilot input could also be received from a robot, an auto-pilot and/or via remote control.

[0087] Flight control system 2 is devised as explained earlier with reference to FIG. 2. It thus comprises at least one COM channel, at least one MON channel and at least one backup channel or backup controller. The MON channel is preferably devised to apply a Lyapunov method online, i.e., during flight of aircraft 1, in order to monitor functioning of said COM channel in terms of (aircraft) stability and integrity.

[0088] Although embodiments have been described with reference to an aircraft and its behavior during flight as an exemplary embodiment, the skilled person will appreciate that the concept underlying the example embodiments can be easily extended to any system or plant the overall behavior of which is governed or controlled by a plurality of actuators, which actuators are themselves controlled by means of a control system that comprises at least one primary control channel and at least one monitoring channel, which monitoring channel is devised to monitor/verify operation of said primary control channel in accordance with the described techniques and systems.