A METHOD

Abstract

A method for controlling an aircraft when taxiing comprising the steps of: measuring an angle of rotation of an active side stick about a first axis and a second axis; receiving an aircraft signal representative of an actual state of the aircraft; generating a control signal based on at least one of: the aircraft signal and the angle of rotation of the active side stick about a first axis and a second axis; transmitting the control signal to the aircraft, whereby the control signal causes an action affecting the actual state of the aircraft; determining a required state of the aircraft; generating a user feedback signal based on at least one difference between the actual state and the required state; and carrying out a user feedback action based on the user feedback signal.

Claims

1. A method for controlling an aircraft when taxiing comprising the steps of: measuring an angle of rotation of an active side stick about a first axis and a second axis; receiving an aircraft signal representative of an actual state of the aircraft; generating a control signal based on at least one of: the aircraft signal and the angle of rotation of the active side stick about a first axis and a second axis; transmitting the control signal to the aircraft, whereby the control signal causes an action affecting the actual state of the aircraft; determining a required state of the aircraft; generating a user feedback signal based on at least one difference between the actual state and the required state; and carrying out a user feedback action based on the user feedback signal.

2. The method according to claim 1 wherein the user feedback signal is generated such that the magnitude of the user feedback action is proportional to a difference between the actual state and the required state.

3. The method according to claim 1 wherein a difference between the actual state and the required state is a cross-track error representative of the shortest distance between an aircraft position on a taxiway and a centreline of the taxiway.

4. The method according to claim 1 further comprising the step of receiving a disabling signal and disabling the user feedback action from being carried out for a period of time.

5. The method according to claim 1 wherein the control signal comprises an impetus level and a brake level, each based on the angle of rotation of the active side stick about the first axis.

6. The method according to claim 5 comprising the further step of limiting the brake level to a maximum brake level such that a deceleration of the aircraft does not exceed a predetermined maximum deceleration value.

7. (canceled)

8. The method according to claim 5 wherein the aircraft signal further comprises an actual speed value and the method comprises the further steps of: limiting the impetus level to a maximum impetus level; and if the actual speed value is exceeding a maximum allowable speed, reducing the maximum impetus level.

9-11. (canceled)

12. The method according to a claim 1 wherein the control signal comprises a target steering angle based on the angle of rotation of the active side stick about the second axis.

13. The method according to claim 12 further comprises the step of determining an asymmetric thrust compensation factor, wherein the target steering angle is additionally based on the asymmetric thrust compensation factor.

14. The method according to claim 12 wherein if the target steering angle exceeds a predetermined steering value, the control signal is generated such that it further comprises a differential thrust/brake level.

15. The method according to claim 12 wherein the aircraft signal comprises an actual speed value and the method comprises the further steps of: limiting the target steering angle to a maximum target steering angle; and varying the maximum target steering angle based on the actual speed value.

16. The method according to claim 5 wherein the impetus level causes a throttle action affecting the actual state of the aircraft and the brake level causes a brake action affecting the actual state of the aircraft.

17. The method according to claim 16 wherein the control signal is generated such that if the angle of rotation of the active side stick about the first axis is less than or equal to 0° the throttle action caused is to set a throttle level to idle and if the angle of rotation of the active side stick about the first axis is a maximum positive angle the throttle action caused is to set the throttle level corresponding to the maximum impetus level.

18. The method according to claim 16 wherein the control signal is generated such that if the angle of rotation of the active side stick about the first axis is greater than or equal to 0° the brake action caused is to set a brake application level to none and if the angle of rotation of the active side stick about the first axis is a maximum negative angle the brake action caused is to set the brake application level corresponding to the maximum brake level.

19-21. (canceled)

22. The method according to claim 12 wherein the target steering angle causes a nose wheel action.

23-24. (canceled)

25. The method according to claim 1 comprising the further step of returning the active side stick to a neutral position such that the angles of rotation about the first axis and the second axis are 0 if there is no deflection pressure applied to the active side stick.

26. The method according to claim 1 comprising the further step of holding the active side stick in its current position such that the angles of rotation about the first axis and the second axis stay constant if there is no deflection pressure applied to the active side stick.

27. The method according to claim 1 wherein the step of generating a control signal comprises the step of using a control algorithm and optionally the control algorithm is a PID control algorithm or a fuzzy logic control algorithm.

28. The method according to claim 27 wherein the step of generating a control signal using a fuzzy logic control algorithm comprises the steps of: determining a fuzzified input based on one or both of the angle of rotation of the active side stick about the first axis and the second axis and the aircraft signal, determining a fuzzified output based on the fuzzified input and a set of fuzzy rules, determining a de-fuzzified output based on the fuzzified output wherein the control signal is representative of the de-fuzzified output.

29. The system for controlling an aircraft when taxiing configured to carry out a method according to claim 1.

Description

[0116] The invention will now be described by way of example only with reference to the accompanying drawings in which:

[0117] FIG. 1 is a schematic representation of a method according to an aspect of the invention;

[0118] FIG. 2 is a schematic representation of a system according to an aspect of the invention with which the method shown in FIG. 1 may be performed;

[0119] FIGS. 3 to 13 are each a schematic representation of control steps forming part of a method according to an embodiment of the invention being performed with the system shown in FIG. 2;

[0120] FIG. 14 is a schematic representation of control and feedback steps forming part of a method according to an embodiment of the invention using fuzzy logic control;

[0121] FIG. 15 is a graphical representation of a predetermined input membership function for acceleration;

[0122] FIG. 16 is a graphical representation of a predetermined input membership function for velocity error;

[0123] FIG. 17 is a graphical representation of a predetermined output membership function for throttle position;

[0124] FIG. 18 is a graphical representation of a predetermined output membership function for brake position;

[0125] FIG. 19 is a graphical representation of target speed and actual speed against time for an aircraft controlled with a method according to an embodiment of the invention;

[0126] FIG. 20 is a graphical representation of throttle and brake commands issued to achieve the actual speed shown in FIG. 19, based on the target speed shown in FIG. 19;

[0127] FIG. 21 is a schematic representation of user feedback steps forming part of a method according to an embodiment of the invention being performed with the control system shown in FIG. 2.

[0128] FIG. 22 is a graphical representation of a user feedback action carried out by an active side stick in response to a cross-track error according to an embodiment of the invention.

[0129] Referring initially to FIG. 1, a method according to the invention is generally defined by the reference numeral 100 and comprises the steps 101, 102, 103, 104, 105, 106 and 107. The method 100 is a method for controlling an aircraft when taxiing and may be carried out by a control system forming part of the aircraft. The control system may comprise a controller and an active side stick which a pilot of the aircraft may interact with in order to control the aircraft.

[0130] The active side stick may be adapted to be rotatable about the first and second axes, which may be equivalent to pitch and roll axes respectively. The pilot may rotate, or deflect, the active side stick about one of the first and second axes or a combination of both axes in order to issue a control command. For example, if the pilot wants the aircraft to accelerate forwards the pilot would deflect the active side stick forward, which may correspond to a positive angle of rotation about the first axis. Similarly, a command for the aircraft to steer right and left may be issued by deflecting the active side stick to the right and left respectively and a command for the aircraft to brake may be issued by deflecting the active side stick backwards.

[0131] Step 101 involves measuring an angle of rotation of the active side stick about the first axis and the second axis. Step 101 therefore translates the physical movement of the active side stick performed by the pilot into a data signal which may be received by the controller.

[0132] In order for accurate control of the aircraft to be determined, the controller is required to compare the command issued by the pilot with the current state of the aircraft. For example, if the pilot deflects the active side stick to issue a steering command, information relating to the current steering angle of the aircraft is crucial in order for the controller to determine the action required.

[0133] Further, the current state of the aircraft must be known in order to provide the pilot with feedback based on the current state of the aircraft.

[0134] Accordingly, step 102 involves receiving an aircraft signal comprising an actual state of the aircraft.

[0135] Step 103 involves generating a control signal based on at least one of: the aircraft signal and the angle of rotation of the active side stick about a first axis and a second axis. Step 103 may be carried out by the controller according to one or more algorithms and according to any suitable control technique such as PID control or fuzzy logic control.

[0136] The control signal may comprise a impetus level and a brake level, each based on the angle of rotation of the active side stick about the first axis.

[0137] Excessive braking of the aircraft can be uncomfortable for passengers of the aircraft. Therefore the brake level may be limited to a maximum brake level such that a deceleration of the aircraft does not exceed a predetermined maximum deceleration value. For example, the predetermined maximum deceleration value may be 1.5 m/s.sup.2.

[0138] However, in some instances higher brake levels may be required, in emergency situations for example. Accordingly, step 103 may comprise the further steps of measuring a deflection pressure applied to the active side stick and a pressure direction relative to the first axis in which the deflection pressure is applied; and, if the deflection pressure exceeds a predetermined pressure value and the pressure direction is negative, increasing the maximum brake level such that there is no limit to the deceleration of the aircraft.

[0139] Speed limits may be imposed on aircrafts when taxiing. In order to comply with a speed limit, the aircraft signal of step 102 may comprise an actual speed value and step 103 may comprise the further steps of limiting the impetus level to a maximum impetus level and, if the actual speed value is exceeding a maximum allowable speed, reducing the maximum impetus level. Further, the brake level may be increased in addition to reducing the maximum impetus level. The reduction to the maximum impetus level and the increase to the brake level may be calculated, according to an algorithm, in order to reduce the actual speed value slightly below the maximum allowable speed.

[0140] In order to protect the integrity of the nose wheel of the aircraft (which sets the steering angle), it may be advantageous to reduce the maximum allowable speed as the steering angle of the nose wheel increases. Therefore, the aircraft signal of step 102 may comprise an actual steering angle and step 103 may comprise the further step of varying the maximum allowable speed based on the actual steering angle.

[0141] The control signal may be generated to comprise a target steering angle based on the angle of rotation of the active side stick about the second axis. If the target steering angle exceeds a predetermined steering value, the control signal generated in step 103 may further comprise a differential thrust/brake level.

[0142] Similarly to reducing the maximum allowable speed in a turn to protect the integrity of the nose wheel, it may be advantageous to reduce the maximum amount that the nose wheel may be steered when the aircraft is travelling at a certain speed.

[0143] Accordingly, step 103 may comprise the further steps of limiting the target steering angle to a maximum target steering angle and varying the maximum target steering angle based on the actual speed value.

[0144] The control signal, which may comprise a impetus level, a brake level, a target steering angle or a combination of these elements, must be received by the aircraft in order for the commands to be actioned. Therefore, step 104 involves transmitting the control signal to the aircraft, whereby the control signal causes an action affecting the actual state of the aircraft.

[0145] Step 105 involves determining a required state of the aircraft. The required state of the aircraft may be a target position of the aircraft relative to the taxiway, a target speed, a target acceleration or a target steering angle, for example.

[0146] Step 106 involves generating a user feedback signal based on a difference between the actual state and the required state.

[0147] Step 107 involves carrying out a user feedback action based on the user feedback signal. Step 107 may be performed by the active side stick.

[0148] The user feedback signal may be generated, in Step 106, such that the magnitude of the user feedback action of Step 107 is proportional to the difference between the actual state and the required state. For example, if the user feedback action of Step 107 is a vibration of the active side stick, the intensity of the vibration may vary proportionally to variation of the difference between the actual state and the required state.

[0149] The difference between the actual state and the required state may be a cross-track error representative of the shortest distance between an aircraft position on a taxiway and a centreline of the taxiway. Therefore, the user feedback action of Step 107 may indicate to the pilot that the aircraft is diverting from the centreline and the magnitude of the user feedback action (the intensity of vibration, for example) may indicate the degree to which the aircraft is diverting from the centreline.

[0150] Referring now to FIG. 2, an aircraft taxiing system is generally defined by the reference numeral 2 and comprises a pilot 10, a control system 12 and an aircraft 18.

[0151] The control system 12 may be configured to carry out the method 100 shown in FIG. 1 and comprises an active side stick 14 and a controller 16.

[0152] The pilot 10 may apply a deflection pressure 20 to the active side stick which may cause the active side stick to rotate about a first axis and/or a second axis. The angle of rotation of the active side stick 14 may then be measured, according to step 101 of the method 100, as an angle of rotation 22. The angle of rotation 22 may be transmitted from the active side stick 14 to the controller 16. Simultaneously, the controller may receive an aircraft signal 30 from the aircraft 18 in accordance with step 102. The controller 16 may then generate a control signal 24 based on at least one of: the aircraft signal 30 and the angle of rotation 22, in accordance with step 103.

[0153] The control signal 24 may then be transmitted to the aircraft 18 from the controller 16, in accordance with step 104.

[0154] The controller 16 may also generate a user feedback signal 32 based on the aircraft signal, in accordance with step 105, and transmit the user feedback signal 32 to the active side stick 14. The active side stick 14 may carry out a user feedback action 34 based on the user feedback signal 32.

[0155] Accordingly the pilot 10 may issue a control command to the aircraft 18 and receive feedback on a dynamic state of the aircraft 18 via the control system 12 wherein the control system 12 acts according to the method 100.

[0156] The control signal 24 may comprise an impetus level and a brake level, each based on the angle of rotation of the active side stick about the first axis. The impetus level may cause a throttle action affecting the actual state of the aircraft and the brake level may cause a brake action affecting the actual state of the aircraft.

[0157] Similarly, the control signal may comprise a target steering angle based on the angle of rotation of the active side stick about the second axis, wherein the target steering angle causes a nose wheel action.

[0158] However exact actions required of the pilot 10 in order to control the aircraft 18 as desired may depend on the configuration of the control system 12 according to a specific embodiment of the invention. Table 2 sets out five exemplary control strategies according to different embodiments of the invention.

TABLE-US-00002 TABLE 2 Axis: Deflection First axis (pitch) Second axis (roll) pressure: Forward Backward 0 Left/Right 0 Control 1 Adjust throttle Apply left and Neutral Steer nose Neutral strategy: position of right brakes. position. wheel position. both engines Emergency Idle thrust and left/right. 0° steering according to braking no brakes. Differential angle. deflection. beyond a (FIG. 6) thrust/brakes (FIG. 6) (FIG. 3) certain beyond a pressure. certain angle. (FIG. 4) (FIG. 5) 2 Adjust throttle Apply left and Side stick Steer nose Side stick position of right brakes. remains in wheel remains in both engines Emergency same position left/right. same position according to braking and Differential and deflection. beyond a commands are thrust/brakes commands (FIG. 3) certain unchanged. beyond a are pressure. (FIG. 7) certain angle. unchanged. (FIG. 4) (FIG. 5) (FIG. 7) 3 Adjust aircraft Emergency Neutral Steer nose Neutral speed braking position. wheel position. according to beyond a Reduce speed left/right. 0° steering deflection. certain to 0 (apply Differential angle. (FIG. 8) pressure. idle thrust and thrust/brakes (FIG. 9) brakes). beyond a (FIG. 9) certain angle. (FIG. 5) 4 Adjust aircraft Emergency Side stick Steer nose Side stick speed braking remains in wheel remains in according to beyond a same position left/right. same position deflection. certain and Differential and (FIG. 8) pressure. commands are thrust/brakes commands unchanged. beyond a are (FIG. 7) certain angle. unchanged. (FIG. 5) (FIG. 7) 5 Adjust aircraft Adjust aircraft Neutral Adjust rate of Neutral acceleration deceleration position. steering. position. according to according to Acceleration Differential Steering angle deflection. deflection. command is 0. thrust/brakes rate command (FIG. 10) (FIG. 11) Constant beyond a is 0. speed. certain angle. Constant (FIG. 13) (FIG. 12) steering angle. (FIG. 13)

[0159] Different aspects of the control strategies are represented in FIGS. 3 to 13 and further described below.

[0160] In control strategy (CS) 1 the control signal is generated such that if the angle of rotation of the active side stick about the first axis is less than or equal to 0° the throttle action caused is to set a throttle level to zero (resulting in idle thrust) and if the angle of rotation of the active side stick about the first axis is a maximum positive angle the throttle action caused is to set the throttle level corresponding to the maximum impetus level. Further, the control signal is generated such that if the angle of rotation of the active side stick about the first axis is greater than or equal to 0° the brake action caused is to set a brake application level to none and if the angle of rotation of the active side stick about the first axis is a maximum negative angle the brake action caused is to set the brake application level corresponding to the maximum brake level. The maximum brake level may be limited such that the deceleration does not exceed a predetermined value (unless emergency braking is applied).

[0161] FIGS. 3 and 4 show the controls that may therefore be implemented according to CS1. In particular, in FIG. 3 the pilot 10 applies a deflection pressure 320 to the active side stick 14 (which started in a neutral position) resulting in a full forward deflection. The angle of rotation 322 that is measured therefore changes from 0° about the first axis to a maximum positive angle (+max°). The controller 16 then generates a control signal 324 comprising an impetus level and transmits the control signal 324 to the aircraft 18. The impetus level causes a throttle action involving the throttle level changing from idle to a maximum throttle level (40% in this case) in accordance with the measured angle of rotation 322. The throttle action may be carried out with respect to the actual throttle levers of the aircraft 18.

[0162] The maximum throttle level may not be a maximum possible throttle level and may instead correspond to a maximum impetus level. The maximum impetus level may be limited in order to avoid exceeding a maximum allowable speed and/or limit the thrust level to prevent damage due to jet blast or ingestion of foreign objects into the engine. Further, the thrust level may be varied based on environmental conditions or steering angle. Therefore the maximum throttle level corresponding to the maximum impetus level may be a fraction of the maximum possible throttle level of the aircraft 18. Hence, in this embodiment of the invention the maximum throttle level is 40% of the possible throttle level range of the aircraft 18. However, in other embodiments of the invention the maximum throttle level may be a different percentage of the possible throttle level of the aircraft.

[0163] In FIG. 4, the pilot 10 applies a deflection pressure 420 to the active side stick 14 (which started in a neutral position) resulting in a full backward deflection. The angle of rotation 422 that is measured therefore changes from 0° about the first axis to a maximum negative angle (−max°). The controller 16 then generates a control signal 424 comprising a brake level and transmits the control signal 424 to the aircraft 18. The brake level causes a brake action involving the brake application level changing from no braking to a maximum brake application level in accordance with the measured angle of rotation 422. The brake action may be carried out with respect to the actual brake pedals of the aircraft 18.

[0164] The maximum brake application level may not be a maximum possible brake application level and may instead correspond to a maximum brake level forming part of the control signal 424. The maximum brake level may be limited such that a deceleration of the aircraft 18 does not exceed a predetermined maximum deceleration value. Therefore, the maximum brake application level corresponding to the maximum brake level may be a fraction of the maximum possible brake application level of the aircraft 18. However, if the deflection pressure 420 exceeds a predetermined pressure value and the pressure direction is negative, the controller may increase the maximum brake level such that there is no limit to the deceleration of the aircraft 18.

[0165] Also in CS1, the target steering angle is representative of a nose wheel angle.

[0166] FIG. 5 demonstrates how steering control may be performed by the pilot 10 according to CS1. In this example, the pilot 10 applies a deflection pressure 520 to the active side stick 14 (which started in a neutral position) resulting in a full right deflection. The angle of rotation 522 that is measured therefore changes from 0° to a maximum positive angle (+max°) about the second axis. The controller 16 then generates a control signal 524 comprising a target steering angle and transmits the control signal 524 to the aircraft 18. The target steering angle causes a nose wheel action involving the nose wheel angle changing from 0° to a maximum steering angle in the right (positive) direction in accordance with the measured angle of rotation 522. In this embodiment the maximum steering angle is 75°, but the maximum steering angle may be any suitable angle. Additionally, the control signal may be generated such that it further comprises a differential thrust/brake level once the target steering angle exceeds a predetermined steering value (70° for example).

[0167] Another aspect of CS1 is that, if there is no deflection pressure applied to the active side stick, the active side stick returns to a neutral position such that the angles of rotation about the first axis and the second axis are 0.

[0168] Referring now to FIG. 6, the pilot 10 releases the active side stick such that the deflection pressure 620 reduces to 0 in all directions. In accordance with CS1 the active side stick returns to the neutral position. Therefore the angle of rotation 622 that is measured changes from the last angle of rotation measured when a deflection pressure was being applied to the active side stick (X°, Y°) to 0° about both the first and second axes (0°, 0°). The controller 16 then generates a control signal 624 comprising an impetus level, a brake level and a target steering angle and transmits the control signal 624 to the aircraft 18. The impetus level, brake level and target steering angle cause the throttle level, brake application level and the nose wheel angle respectively to each go to 0 in accordance with the measured angle of rotation 622 (wherein a throttle level of 0 corresponds to idle thrust). The aircraft 18 therefore enters an idle condition in which the aircraft 18 may coast to a gradual stop while travelling in a straight line (if the aircraft is not already stationary).

[0169] CS2 is similar to CS1 except that, if there is no deflection pressure applied to the active side stick, the active side stick maintains its current position rather than returning to a neutral position. This means that the angles of rotation about the first axis and the second axis stay constant with respect to the last angles of rotation measured when a deflection pressure was being applied to the active side stick.

[0170] FIG. 7 shows the same control action being applied by the pilot 10 as in FIG. 6. However, in this example CS2 is applied rather than CS1. Therefore, when the pilot 10 releases the active side stick, the angle of rotation 722 that is measured remains constant at the angle of rotation measured when a deflection pressure was last being applied by the pilot 10 (X°, Y°). This means that the controller 16 generates a control signal 724 wherein the impetus level, brake level and target steering angle each remain constant. The aircraft 18 receives the control signal 724 and the throttle level, brake application level and nose wheel angle are each caused to remain constant. This means that the pilot 10 may momentarily release the active side stick 14 to perform another action and aircraft 18 will continue as if there has been no change to the controls applied by the pilot 10.

[0171] In CS3 the impetus and brake levels are representative of a target speed for the aircraft to reach. The control signal may be generated such that if the angle of rotation of the active side stick about the first axis is 0° the throttle action and brake action caused are to set the throttle level and the brake application level respectively to achieve an actual speed value of 0 km/h and if the angle of rotation of the active side stick about the first axis is a maximum angle the throttle action and brake action caused are to set the throttle level and the brake application level respectively to achieve an actual speed value equal to a maximum allowable speed.

[0172] FIG. 8 shows an example of a control that may be implemented according to CS3. The pilot 10 applies a deflection pressure 820 to the active side stick 14 (which started in a neutral position) resulting in a full forward deflection. The angle of rotation 822 that is measured therefore changes from 0° about the first axis to a maximum positive angle (+max°). The controller 16 then generates a control signal 824 comprising a impetus/brake level representative of a target speed and transmits the control signal 524 to the aircraft 18. The impetus/brake level causes throttle/brake actions wherein the throttle level and brake level are set in order to achieve the speed changing from 0 km/h to a maximum allowable speed (max km/h) in accordance with the measured angle of rotation 822. The throttle and brake actions may be carried out with respect to the actual throttle levers and brake pedals of the aircraft 18. For example, the braking applied may be reduced so that no braking occurs, and the throttle levers may be increased to a maximum amount until the target speed is reached. The throttle levers may then be adjusted to maintain the speed, rather than increase it further, beyond the maximum allowable speed. The throttle level and brake application level required may be calculated according to any suitable control technique, such as PID control or fuzzy logic.

[0173] An advantage of CS3 is that the pilot is simply required to set the speed of the aircraft by deflecting the active side stick and may allow the controller to generate the required throttle and braking levels required to achieve the speed. This reduces the requirement of the pilot to adjust controls based on external disturbances (such as gradient and wind speed) and therefore simplifies operation of the aircraft when taxiing.

[0174] According to CS3, the target steering angle is representative of a target angle for a nose wheel, similarly to CS1 and CS2. Therefore the pilot may implement steering control similarly to the example shown in FIG. 5, described above.

[0175] CS3 is also similar to CS1 in that if there is no deflection pressure applied to the active side stick, the active side stick returns to a neutral position such that the angles of rotation about the first axis and the second axis are 0. However, as the impetus/brake levels are representative of a target speed, the effect of the pilot releasing the active side stick is different for CS3 when compared to CS1. FIG. 9 shows that when the pilot 10 releases the active side stick and the measured angle of rotation 922 changes to (0°, 0°) the target speed changes to 0 km/h (provided it was not already 0 km/h), as well as the target steering angle changing to 0° in accordance with CS1 and FIG. 6. This means that, rather than the aircraft 18 entering an idle condition, the aircraft 18 is caused to apply the brakes (via a brake application level) in order to reduce the speed to 0 km/h (although the braking may be limited to a maximum brake level in order to limit deceleration). This control strategy may reduce the risk of the aircraft being involved in an accident if the pilot is distracted or otherwise unable to control the aircraft.

[0176] CS4 is similar to CS3 except that, if there is no deflection pressure applied to the active side stick, the active side stick maintains its current position rather than returning to a neutral position. This means that the angles of rotation about the first axis and the second axis stay constant with respect to the last angles of rotation measured when a deflection pressure was being applied to the active side stick. In this aspect of the control strategy, CS4 is similar to CS2 and is therefore similarly represented by FIG. 7 in which the control signal 724 remains constant when the active side stick 14 is released by the pilot 10.

[0177] However, in this case rather than specific throttle/brake levels remaining constant, it is the target speed of the aircraft that remains constant. Therefore, when the pilot releases the active side stick and a constant target speed is set, the controller 16 may adjust impetus and brake levels in order to ensure that the speed of the aircraft is maintained at the desired target speed. Factors such as taxiway gradient and wind speed may affect the actual speed of the aircraft and this may be fed back to the controller as part of an aircraft signal, hence allowing the controller to adjust the generated control signal accordingly.

[0178] In CS5 the impetus level is representative of a target acceleration such that if the angle of rotation of the active side stick about the first axis is 0 the throttle action caused is to set the throttle level to achieve an acceleration of 0 m/s.sup.2 and if the angle of rotation of the active side stick about the first axis is a maximum positive angle the throttle action caused is to set the throttle level to achieve a maximum acceleration. Similarly, the brake level is representative of a target deceleration such that if the angle of rotation of the active side stick about the first axis is 0 the brake action caused is to set the brake application level to achieve a deceleration of 0 m/s.sup.2 and if the angle of rotation of the active side stick about the first axis is a maximum negative angle the brake action caused is to set the brake application level to achieve a maximum deceleration, which may be a predetermined value such as 1.5 m/s.sup.2.

[0179] FIGS. 10 and 11 show the controls that may be implemented according to CS5. In particular, in FIG. 10 the pilot 10 applies a deflection pressure 1020 to the active side stick 14 (which started in a neutral position) resulting in a full forward deflection. The angle of rotation 1022 that is measured therefore changes from 0° about the first axis to a maximum positive angle (+max°). The controller 16 then generates a control signal 1024 comprising an impetus level representative of target acceleration and transmits the control signal 1024 to the aircraft 18. The impetus level causes a throttle action wherein the throttle level is set in order that the acceleration of the aircraft 18 changes from 0 m/s.sup.2 to a maximum acceleration (max m/s.sup.2) in accordance with the measured angle of rotation 1022. The throttle action may be carried out with respect to the actual throttle levers of the aircraft in order to achieve the target acceleration forming part of the control signal 1024.

[0180] Similarly, in FIG. 11, the pilot 10 applies a deflection pressure 1120 to the active side stick 14 (which started in a neutral position) resulting in a full backward deflection. The angle of rotation 1122 that is measured therefore changes from 0° about the first axis to a maximum negative angle (−max°). The controller 16 then generates a control signal 1124 comprising a brake level representative of a target deceleration and transmits the control signal 1124 to the aircraft 18. The brake level causes a brake action wherein the brake application level is set in order that the deceleration of the aircraft changes from 0 m/s.sup.2 to a maximum deceleration (max m/s.sup.2) in accordance with the measured angle of rotation 1122. The brake action may be carried out with respect to the actual brake pedals of the aircraft in order to achieve the target deceleration forming part of the control signal 1124.

[0181] Further, the maximum deceleration may be limited such that the deceleration of the aircraft 18 does not exceed a predetermined maximum deceleration value. Therefore, the maximum deceleration may not be the maximum possible braking achievable by the aircraft 18. However, if the deflection pressure 1120 exceeds a predetermined pressure value, the controller may increase the maximum deceleration such that there is no limit to the deceleration of the aircraft 18.

[0182] Also in CS5, the target steering angle is representative of a steering angle rate, that is the rate at which the angle of the aircraft's nose wheel is changed.

[0183] FIG. 12 demonstrates how steering control may be performed by the pilot 10 according to CS5. In this example, the pilot 10 applies a deflection pressure 1220 to the active side stick 14 (which started in a neutral position) resulting in a full right deflection. The angle of rotation 1222 that is measured therefore changes from 0° to a maximum positive angle (+max°) about the second axis. The controller 16 then generates a control signal 1224 comprising a target steering angle representative of a steering angle rate and transmits the control signal 1224 to the aircraft 18. The target steering angle causes a nose wheel action wherein the steering angle rate is set such that it changes from 0°/s to a maximum steering angle rate (+max °/s) in the right (positive) direction in accordance with the measured angle of rotation 1222. Additionally, if the steering angle exceeds a predetermined steering value (±70° for example), the control signal may be generated such that it further comprises a differential thrust/brake level. The differential thrust/brake level may be increased from zero to a maximum level as the steering angle increases from the predetermined steering value to a maximum possible angle (e.g. 700 to 750).

[0184] Another aspect of CS5 is that, similarly to CS1 and CS3, the active side stick returns to a neutral position if there is no deflection pressure applied to the active side stick.

[0185] However, as the impetus/brake levels are representative of a target acceleration or deceleration, the effect of the pilot releasing the active side stick is different for CS5 when compared to CS1 or CS3. FIG. 13 shows that when the pilot 10 releases the active side stick and the measured angle of rotation 1322 changes to (0°, 0°) the target acceleration/deceleration changes to 0 m/s.sup.2 (provided it was not already 0 m/s.sup.2). In other words, the aircraft maintains a constant speed if the active side stick is released.

[0186] A further difference to CS1 and CS3 is that the target steering angle is representative of a steering angle rate. Therefore when the pilot 10 releases the active side stick and the measured angle of rotation 1322 changes to (0°, 0°) the steering angle rate changes to 0°/s (provided it was not already 0°/s). In other words, the aircraft maintains the steering angle that was set before the pilot released the active side stick.

[0187] Control of an aircraft while taxiing may substantially involve maintaining a constant speed in a straight line or while performing a turn with a constant steering angle. CS5 may therefore be advantageous as the pilot is only required to deflect the active side stick in order to change the speed and/or steering angle of the aircraft. When the pilot wants the aircraft to maintain its current course and speed there is no requirement to deflect the active side stick from its neutral position.

[0188] Control strategies 1 to 5 represent examples of how different features of the invention may be combined to define particular embodiments of the invention. Other embodiments of the invention may comprise any suitable combination of the features described above. For example, some embodiments of the invention may combine the throttle/brake level features of CS1 with the steering angle rate features of CS5.

[0189] Referring now to FIG. 14, an aircraft taxiing system is generally defined by the reference numeral 1402 and comprises a pilot (not shown), an active side stick 1414, a controller 1416 and an aircraft 1418. The active side stick 1414 and the controller 1416 form part of a control system which may carry out a method according to an embodiment of the invention configured according to CS3 and wherein the control signal is generated according to fuzzy logic. (FIG. 14 refers to speed control, but it is to be understood that this is for the purpose of demonstration and that embodiments of the invention are not limited to speed control only.) In this example, the active side stick 1414 is deflected by the pilot and an angle of rotation 1422 about the first axis is measured. The angle of rotation 1422 is transmitted to the controller 1416. The controller 1416 then translates the angle of rotation 1422 to the target speed that it is representative of in accordance with CS3. Simultaneously the controller 1416 receives a first aircraft signal 1430a which is representative of the actual acceleration of the aircraft 1418 and a second aircraft signal 1430b which is representative of the actual speed of the aircraft 1418. The controller 1416 may then generate a control signal based on the aircraft signals 1430a, 1430b and the angle of rotation 1422 of the active side stick 1414.

[0190] In this particular example, the controller 1416 calculates a velocity error 1440 based on the difference between the target speed set by the angle of rotation 1422 and the actual speed represented in the second aircraft signal 1430b. The controller 1416 then determines inputs 1442 for a fuzzy logic control algorithm based on the first aircraft signal 1430a and the velocity error 1440.

[0191] Table 3 comprises examples of fuzzy sets corresponding to acceleration and FIG. 15 shows a graphical representation of the membership function corresponding to each fuzzy set.

TABLE-US-00003 TABLE 3 Fuzzy set Acceleration (A) NA Negative acceleration   −3 m/s.sup.2 ≤ A ≤ −1.5 m/s.sup.2 ZA Zero acceleration −2 m/s.sup.2 ≤ A ≤ 2 m/s.sup.2   PA Positive acceleration 1.5 m/s.sup.2 ≤ A ≤ 3 m/s.sup.2  

[0192] For example, if the first aircraft signal 1430a is representative of an actual acceleration of 3 m/s.sup.2 the controller would determine that the acceleration value belongs to fuzzy set PA (positive acceleration). It is also possible for an acceleration value to belong to multiple fuzzy sets. For instance, if the first aircraft signal 1430a is representative of an actual acceleration of 1.8 m/s.sup.2 the controller would determine that the acceleration value belongs to fuzzy sets PA (positive acceleration) and ZA (zero acceleration).

[0193] Table 4 comprises examples of fuzzy sets corresponding to velocity error and FIG. 16 shows a graphical representation of the membership function corresponding to each fuzzy set.

TABLE-US-00004 TABLE 4 Fuzzy set Velocity error (Ve) NE Negative error −50 kts ≤ Ve ≤ −9 kts  (−92.6 km/h ≤ Ve ≤ −16.7 km/h) NZE Negative-Zero error −10 kts ≤ Ve ≤ 0 kts    (−18.5 km/h ≤ Ve ≤ 0 km/h)     ZE Zero error −0.3 kts ≤ Ve ≤ 3 kts     (−0.556 km/h ≤ Ve ≤ 5.56 km/h)    PZE Positive-Zero error  2 kts ≤ Ve ≤ 10 kts (3.70 km/h ≤ Ve ≤ 18.5 km/h) PE Positive error  9 kts ≤ Ve ≤ 50 kts (16.7 km/h ≤ Ve ≤ 92.6 km/h)

[0194] For example, if the velocity error 140 is −20 kts (−37.0 km/h) the controller would determine that the velocity error belongs to fuzzy set NE (negative error). It is also possible for a velocity error value to belong to multiple fuzzy sets. For instance, if the velocity error 1440 is 2.5 kts (4.63 km/h) the controller would determine a fuzzified input of ZE (zero error) and PZE (positive zero-error).

[0195] The controller comprises a fuzzy logic controller 1417 which is configured to determine an output 144 based on an input 1442. The behaviour of the fuzzy logic controller 1417 may be established through a set of fuzzy rules which are based on ‘if then’ conditions. Table 5 provides an example for a set of rules for the fuzzy logic controller 1417 to follow based on inputs 1442 determined in relation to velocity error and acceleration.

TABLE-US-00005 TABLE 5 Velocity Throttle Brakes Rule # Error Acceleration Command Command 1 PE PA Medium Zero 2 PE ZA High Zero 3 PE NA High Zero 4 PZE PA Low Zero 5 PZE ZA Medium Zero 6 PZE NA Medium Zero 7 ZE PA Zero Low 8 ZE ZA Zero Zero 9 ZE NA Low Zero 10 NZE PA Zero Medium 11 NZE ZA Zero Low 12 NZE NA Zero Low 13 NE PA Zero High 14 NE ZA Zero Medium 15 NE NA Zero Medium

[0196] In this example the outputs 1444 are determined in relation to throttle and brake commands.

[0197] The basic logic behind the rules defined in Table 5 is the following: [0198] If the actual speed is lower than the target speed, then the velocity error is positive (PE/PZE) and the aircraft needs to speed up. This is achieved by increasing the throttle and/or reducing brake pressure, especially if the velocity error is large (PE) and the aircraft is slowing down (NA). [0199] If the actual speed is greater than the target speed, then the velocity error is negative (NE/NZE) and the aircraft needs to slow down. This is achieved by reducing the throttle and/or increasing brake pressure, especially if the velocity error is large (NE) and the aircraft is speeding up (PA). [0200] If the actual speed is close to the target speed, then the velocity error is small (ZE) and the aircraft either needs to speed up or slow down a small amount (relative to the two cases described above). This is achieved by applying very small throttle and/or brake commands. [0201] The brake command is zero whenever the throttle command is non-zero (and vice-versa).

[0202] For instance, Rule 2 states that, if the velocity error is positive (PE) and the acceleration is zero (ZA), then the throttle command should be high while the brake command should be zero.

[0203] Since both the velocity error and the acceleration can belong to multiple fuzzy sets, multiple fuzzy rules may be activated at the same time (to different degrees). For instance, if the velocity error is 20 knots (and therefore belongs to fuzzy set PE) and the acceleration is 1.7 m/s2 (and therefore belongs to fuzzy sets ZA and PA), then fuzzy rules 1 and 2 are activated simultaneously.

[0204] In order for the controller 1416 to generate a control signal with exact numerical values for, in this example, the throttle level and brake application level the throttle and brake commands must undergo defuzzification.

[0205] Table 6 comprises examples of fuzzy sets corresponding to throttle command and FIG. 17 shows a graphical representation of the membership function corresponding to each fuzzy set.

TABLE-US-00006 TABLE 6 Fuzzy set Throttle command (T.sub.cmd) Zero Tcmd = 0 Low   0 ≤ Tcmd ≤ 0.17 Medium 0.15 ≤ Tcmd ≤ 0.25 High 0.23 ≤ Tcmd ≤ 0.40

[0206] For example, if the velocity error and acceleration are determined to belong to fuzzy sets PE and ZA respectively then, according to Rule 2, the fuzzified output relating to throttle command would be “High”. Therefore the fuzzy logic controller 1417 would determine a throttle level in the range of 0.23 to 0.40.

[0207] The exact throttle level is determined by applying a defuzzification method which combines the fuzzy outputs of all of the rules that are activated simultaneously in order to produce a single crisp value.

[0208] Table 7 comprises examples of fuzzy sets corresponding to brake command and FIG. 18 shows a graphical representation of the membership function corresponding to each fuzzy set.

TABLE-US-00007 TABLE 7 Fuzzy set Brake command (B.sub.cmd) Zero Bcmd = 0 Low  0. ≤ Bcmd ≤ 0.4 Medium 0.35 ≤ Bcmd ≤ 0.75 High 0.70 ≤ Bcmd ≤ 1.0 

[0209] For example, if the velocity error and acceleration are determined to belong to fuzzy sets NZE and PA respectively then, according to Rule 10, the fuzzified output relating to brake command would be “Medium”. Therefore, the fuzzy logic controller 1417 would determine a brake level in the range 0.35 and 0.75.

[0210] The crisp throttle and brake command values are generated, as set out above, as outputs 1444 from the fuzzy logic controller 1417. The controller 1416 then generates a first control signal 1424a based on the throttle command and a second control signal 1424b based on the brake command. Each of the control signals 1424a, 1424b are transmitted to the aircraft 1418 in order to cause a throttle action and a brake action respectively.

[0211] To demonstrate the result of an aircraft's speed being controlled according to the embodiment of the invention described above, FIG. 19 shows a graphical representation of the actual speed of the aircraft varying based on the target speed set by the pilot by deflecting the active side stick. FIG. 20, then shows the associated throttle actions and brake actions caused by the controller over the same period of time shown in FIG. 19.

[0212] Referring now to FIG. 21, steps forming part of a method according to an embodiment of the invention are represented. In this embodiment of the invention the aircraft signal 2130 comprises a position of the aircraft on the taxiway. The controller 16 may determine a required state of the aircraft, i.e. the required position of the aircraft on the taxiway—the centreline. The controller 16 may then calculate a cross-track error, which is representative of the shortest distance between the position of the aircraft 18 on a taxiway and a centreline of the taxiway.

[0213] It is beneficial for an aircraft to remain as close as possible to the centreline of the taxiway it is travelling along as this reduces the risk of the aircraft leaving the taxiway which can result in damage to the aircraft. However, it can be difficult for a pilot to gauge how accurately the aircraft is following the centreline, particularly during a turn.

[0214] To assist with reducing the cross-track error, this embodiment of the invention further involves generating a user feedback signal 2132 such that the magnitude of a user feedback action 2134 to be carried out by the active side stick 14 is proportional to the cross-track error. The active side stick 14 receives the user feedback signal 2132 and carries out the user feedback action 2134 accordingly, thereby providing feedback to the pilot 10.

[0215] As the user feedback action 2134 is proportional to the cross-track error, the user feedback action 2134 indicates the degree of cross-track error to the pilot 10 to allow the pilot to make the necessary corrections to the aircraft's steering. For example, the user feedback action 2134 may be a vibration that becomes more intense (for example by increasing the amplitude and/or the frequency of the vibration) as the cross-track error increases. Alternatively, the user feedback action 2134 may be a pressure or force bias applied to the pilot's hand via the active side stick 14 in the direction opposite to the direction of the cross-track error, therefore prompting the pilot to correct the active side stick's positioning.

[0216] FIG. 22 shows a graphical representation of a user feedback action (in the form of a force bias applied to the pilot's hand through the active side stick) being carried out in response to varying magnitude and direction of a cross track error.

[0217] The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

[0218] Preferences and options for a given aspect, feature or parameter of the invention should, unless the context indicates otherwise, be regarded as having been disclosed in combination with any and all preferences and options for all other aspects, features and parameters of the invention. For example, some embodiments of the invention may combine disclosed features of the invention in different configurations to those which are given as examples in Table 2.