Stability and control augmentation system

Abstract

A Stability and Command Augmentation System (SCAS) for an aircraft, the SCAS including, a first input shaft providing a first rotational input; a second input shaft providing a second rotational input; and a device for summing the first and second rotational inputs to give a rotational output for controlling an actuator. A method of operating a SCAS on an aircraft, the method including providing a pilot input order in the form of a rotating first shaft, providing an augmentation input order in the form of a rotating second shaft, summing the rotation of the pilot input order and the augmentation input order to give an output order; and using the output order to control an actuator.

Claims

1. A Stability and Command Augmentation System (SCAS) for an aircraft, the SCAS comprising: a first input shaft providing a first rotational input; a second input shaft providing a second rotational input; a device for summing the first and second rotational inputs to give a rotational output for controlling an actuator, wherein the device comprises a planetary gear assembly comprising a ring gear, a sun gear and a plurality of planet gears, wherein each planet gear is driveably connected to both the ring gear and the sun gear; a housing surrounding at least a portion of the planetary gear assembly; and a centering mechanism for biasing the planetary gear assembly towards a predetermined position in relation to the housing, wherein the centering mechanism comprises a tension spring positioned between the ring gear and the housing.

2. The SCAS of claim 1, further comprising at least one of: a pilot input device arranged to control the rotation of the first input shaft; or a flight computer arranged to control the rotation of the second input shaft.

3. The SCAS of claim 1, further comprising an electric motor arranged to rotate the second input shaft.

4. The SCAS of claim 3, wherein an angular displacement of the second input shaft is at least one of: incrementally controlled by the electric motor; or determined by a position sensor.

5. The SCAS of claim 1, wherein the second input shaft is driveably connected to the ring gear.

6. The SCAS of claim 1, wherein the first input shaft is driveably connected to the sun gear.

7. The SCAS of claim 6, wherein the planet gears provide the rotational output for controlling an actuator.

8. The SCAS of claim 1, further comprising an actuator driveably connected to the rotational output of the summing device.

9. A method of controlling an actuator, the method comprising using a SCAS as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Some exemplary embodiments of the present disclosure will now be described by way of example only and with reference to FIGS. 1 to 4, of which:

(2) FIG. 1 shows an exploded three-dimensional view of a SCAS according to one embodiment of the present disclosure;

(3) FIG. 2 shows an assembled three-dimensional view of the SCAS of FIG. 1;

(4) FIG. 3 shows a cross-sectional view, taken in the axial direction, of the assembled SCAS of FIG. 2;

(5) FIG. 4 shows a schematic functional view of the SCAS of FIGS. 1 to 3.

DETAILED DESCRIPTION

(6) FIGS. 1 to 3 show parts of a SCAS 10. It should be understood, however, that a SCAS also contains other parts not shown in these figures, such as one or more actuators, the flight computer, electric and mechanical linkages and the pilot input device.

(7) The SCAS 10 comprises a planet gear assembly 20 having a first input shaft 22, a second input shaft 28, a ring gear 30 and three planet gears 36. Also provided is a gear 62 for controlling an actuator (not shown).

(8) The first input shaft 22 is driveably connected to a pilot input device (not shown) located in the cockpit of the aircraft. The first input shaft 22 can therefore be considered to be a pilot input shaft. The first input shaft 22 comprises a sun gear 24. The sun gear 24 may integrally formed on the first input shaft 22 or secured thereto in any suitable manner.

(9) The second input shaft 28 is connected to an electric motor 26. The motor 26 may be, for example, a stepper motor, so that the angular displacement of the shaft 28 can be incrementally controlled, or a torque motor used with a RVDT, so that the angular displacement of the second input shaft 28 can be closely monitored (by the flight control computer).

(10) The electric motor 26 is driveably connected to a flight control computer, for example an autopilot (not shown). The second input shaft 28 may therefore be considered to be a flight computer input order, an autopilot input order or an augmentation input order.

(11) The ring gear 30 comprises external teeth 32 arranged along its outer circumference and internal teeth 34 arranged along its inner circumference. The external teeth 32 are configured to engage the teeth of the second input shaft 28. The internal teeth 34 are configured to engage the teeth of the three planet gears 36, which are positioned between the ring gear 30 and the sun gear 24. The teeth of the sun gear 24 are thus configured to engage the teeth of the planet gears 36.

(12) The teeth of the planet gears 36 are also configured to engage the teeth of the gear 62. The gear 62 is connected to (or formed on) an input shaft 62 for a control valve 64. The control valve 64 rotates, together with the gear 62 and the input shaft 62, to control hydraulic flow within actuator chambers (not shown), which, in turn, controls the actuation of an actuator (not shown).

(13) As can be seen in FIG. 3, the planet gears 36 each have a first end 36a and a second end 36b. The first end 36a engages the sun gear 24 while the second end 36b engages the gear 62. There is a gap, along the length of the planet gear 36, between the end of the sun gear 24 and the opposing end of the gear 62.

(14) A housing 50 is provided into which the planet gear assembly 20 is positioned. The housing 50 comprises a first face 50a and a second face 50b. The second face 50b is generally planar and has an aperture through which the first input shaft 22 passes. The first face 50a has a first aperture 52 that receives the ring gear 20 and a second, smaller aperture 54 (connected to the first aperture 52) that receives the electric motor 26. As shown in FIG. 3, a cover plate 56 can be secured to the first face 50a once the planet gear assembly 20 is in place. The cover plate 56 has an aperture through which the control valve input shaft 62 passes.

(15) Secured to the ring gear 30 is a centering mechanism in the form of a tension spring 40. The tension spring 40 sits in a concave portion 38, i.e. an indentation, on the ring gear 30, and extends into the second aperture 54 in the housing first face 50a. The tension spring 40 may be secured to a portion of the second input shaft 28.

(16) FIG. 4 shows a schematic functional plan of the planet gear assembly 20. Electric motor 26, ring gear 30, first input shaft 22 and control valve input shaft 60 are each shown and are each secured to a ground 70 in such a way that they can rotate relative thereto. A single planet gear 36 is shown in engagement with internal teeth 34 of ring gear 30 and gear 24 of first input shaft 22. As can be seen, the first input shaft 22 and the control valve input shaft 60 are aligned along planet gear assembly axial centerline 70, about which these shafts 22, 60 as well as ring gear 30 and planet gears 36 rotate.

(17) In use, a pilot will operate the pilot input device (not shown), which will cause the first input shaft 22 and sun gear 24 to rotate.

(18) This rotation will be imparted to the three planet gears 36 which engage the sun gear 24. Due to the weight of the ring gear 30 and the kinematics of the system, the three planet gears 36 will move with the sun gear 24 around the internal circumference of the ring gear 30 as they are engaged with internal teeth 34. As they move around the ring gear 30, the planet gears 36 will rotate about their own axes. The movement of the planet gears 36 around the ring gear 30 and around their own axes causes the input gear 62 of the control valve 64 to rotate. This will, in turn, actuate the actuator (not shown) to, for example, vary a flight control surface, such as a yaw rudder. The operation of the pilot input device thus operates the actuator.

(19) If the flight control computer determines that the command from the pilot input device needs to be augmented, for example, by increasing the extent to which the actuator varies the flight control surface, the computer sends a command to the electric motor 26 to rotate the second input shaft 28 one direction or another by a certain angular displacement.

(20) Rotating the second input shaft 28 causes the ring gear 30 to rotate, which, in turn, causes the three planet gears 36 to move, with the ring gear 30, around the sun gear 24. Again, each planet gear 36 will also rotate about their own axes as it travels around the sun gear 24. This will cause the input gear 62 to rotate.

(21) When both the first input shaft 22 and the second input shaft 28 are rotated, their rotation will be summed algebraically, i.e. if both rotations are positive, the rotations will be added together, but if one rotation is negative, then that rotation will be subtracted from the other positive rotation.

(22) It should be understood, however, that in order to increase the rotation of the input gear 62, (to more than that cause by the first input shaft 22), the electric motor 26 must rotate the second input shaft 28 in the opposite direction to the first input shaft 22. This is because rotating the first input shaft 22 and second input shaft 28 in opposite directions causes the three planet gears 36 to rotate in the same direction. As such, if the flight control computer determines that too much rotation has been imparted to the input gear 62 (and thus the actuator) by the pilot input device, this rotation could be reduced by rotating the second input shaft 28 in the same direction as the first input shaft 22 is being rotated. Varying the rotation of the first input shaft 22 and the second input shaft 28 changes the amount the three planet gears 36 rotate the input gear 62.

(23) The tension spring 40 biases the ring gear 30 (relative to the housing 50) to the normal position shown in FIGS. 1 to 3, where the spring 40 is vertical. This prevents the ring gear 30 rotating freely should the motor 26 fail. The spring 40 thus acts as a safety mechanism.