Patent Description:
Aircraft actuators are widely used for driving spoilers, flaps, flight surfaces, and slats of an aircraft. The actuator drives the spoiler to a desired position to affect the airflow around the aircraft in a desired manner, e.g. to brake the aircraft or change the lift. Such spoilers can be subject to large aerodynamic loads from the air flowing around the spoiler and these large forces can feed back to the actuator and cause it to move in an undesired manner. It is desirable to provide an emergency brake to brake the actuator in such circumstances. <CIT> discloses an aircraft actuator.

According to a first aspect of the invention, there is provided an actuator comprising a motor shaft having an axis, an output arranged to rotate within an angular range of less than <NUM> degrees, wherein rotation of the motor shaft about the axis drives the output in rotation, a first ratchet comprising a first ratchet wheel and a first ratchet pawl, the first ratchet arranged such that, while the first ratchet pawl engages the first ratchet wheel, the first ratchet wheel is freely rotatable in a first direction and prevented from rotation in a second direction, opposite the first direction; a second ratchet comprising a second ratchet wheel and a second ratchet pawl, the second ratchet arranged such that, while the second ratchet pawl engages the second ratchet wheel, the second ratchet wheel is freely rotatable in the second direction and prevented from rotation in the first direction. The first and second ratchet wheels are mounted to a further shaft. A first cam surface is connected to rotate with the output, wherein the first cam surface is arranged to disengage the first ratchet pawl from the first ratchet wheel when the output is within a first predetermined angular range, and to allow the first ratchet pawl to engage with the first ratchet wheel when the output is outside the first predetermined angular range; a second cam surface connected to rotate with the output, wherein the second cam surface is arranged to disengage the second ratchet pawl from the second ratchet wheel when the output is within a second predetermined angular range, and to allow the second pawl to engage with the second ratchet wheel when the output is outside the second predetermined angular range; and an electrically-actuated clutch arranged to selectively connect the further shaft to the motor shaft.

The motor shaft may be connected to the output via a gearbox, e.g. an planetary or epicyclic gearbox.

The actuator may comprise a controller operable to order the output for rotation, and operable to control the electrically-actuated clutch, wherein the controller is configured to: detect a rotational speed of the output, and to engage the clutch to connect the further shaft to the motor shaft if the detected rotation speed of the output is greater than ar ordered rotational speed of the output by more than a predetermined amount.

The actuator may comprise an electromagnet, (e.g. a solenoid), wherein the controller is configured to control electrical power to the electromagnet in order to control the electrically-actuated clutch.

The further shaft and the motor shaft may be coaxial with one another.

The actuator may comprise an end stop, wherein the end stop defines a first end and a second end of the angular range of the output.

The first predetermined range may extend from a position where the output is in contact with the end stop at the first end of the angular range of the output, and extend to a first intermediate position of the output between the two ends of the angular range.

Put another way, the first predetermined range may extend from a first end of the range of motion allowed to the output by the endstop, to a position between the first and second ends of the allowed range of motion of the output. Once the output is beyond the first intermediate position, away from the endstop, the first cam surface no longer prevents the first ratchet pawl from engaging the first ratchet wheel.

Similarly, the second predetermined range may extend from a position where the output is in contact with the end stop at the second end of the angular range of the output and extend to a second intermediate position of the output between the two ends of the angular range.

Put another way, the second predetermined range may extend from a second end of the range of motion allowed to the output by the endstop, to a position between the two ends of the allowed range of motion of the output. Once the output is beyond the second intermediate position, away from the endstop, the second cam surface no longer prevents the second ratchet pawl from engaging the second ratchet wheel.

The cam surfaces may be arranged such that, when the output is at an angular position between the first and second intermediate positions, the first ratchet pawl engages the first ratchet wheel and second ratchet pawl engages the second ratchet wheel.

Put another way, in this embodiment of the invention, the first predetermined range does not overlap the second predetermined range. This means that, when the output is between the first and second intermediate positions, both ratchet pawls engage their respective ratchet wheels. If the clutch is engaged in this position to connect the shaft to the motor shaft, the ratchets act to prevent rotation of the motor shaft in either direction.

According to another aspect of the invention, there is provided an aircraft comprising: an airframe; a spoiler movably mounted to the airframe; and the actuator of the first aspect mounted to the airframe, wherein the output is connected to the spoiler to control a position of the spoiler relative to the airframe.

According to a further aspect of the invention, there is provided a method of controlling the rotary actuator the first aspect, the method comprising detecting a speed of the output; determining whether the speed of the output is greater than an ordered speed by more than a predetermined amount; and, if so, actuating the electrically-actuated clutch to connect the shaft to the motor shaft.

Certain embodiments of the present invention will now be described in greater detail by way of example only and with reference to the accompanying drawings in which:.

<FIG> shows an actuator <NUM> having a motor shaft <NUM> connected to a gearbox <NUM>. The actuator <NUM> may be a rotary actuator. The motor shaft defines an axis X and the motor shaft <NUM> is driven by a motor (not shown). Rotation of the motor shaft <NUM> turns the gears of the gearbox <NUM>. The actuator <NUM> has connection points 16a,b for connecting the actuator to an aircraft frame <NUM> of an aircraft <NUM> (see <FIG>). Rotation of the motor shaft <NUM>, via the gearbox <NUM>, causes an output <NUM> to rotate around the axis X. In use, the output <NUM> connects to a spoiler <NUM> of the aircraft <NUM>, such that movement of the output <NUM> causes movement of the spoiler. The actuator <NUM> has an end stop <NUM> that limits the range of motion of the output <NUM> to less than <NUM> degrees around the axis X.

The motor shaft <NUM> connects to one side of an electrically-actuated clutch <NUM> that is controlled by an electromagnet <NUM>, for example a solenoid.

A controller <NUM> is provided to control actuation of the electromagnet <NUM> and thereby control whether the clutch <NUM> is engaged or disengaged. The same controller <NUM> may also be used to control the actuator <NUM>, shown schematically by dashed- line <NUM>, e.g. to control a motor of the actuator <NUM> as well as to detect a position of the output <NUM>. The angular position and angular speed of the output <NUM> may be monitored by the controller <NUM> or by another controller in a variety of ways, e.g. by a sensor detecting the output <NUM> or a sensor monitoring the motor shaft <NUM> or by a sensor monitoring the motor controlling the motor shaft <NUM>.

A first ratchet <NUM> and a second ratchet <NUM> are provided on the other side of the electrically-actuated clutch <NUM>, and are connected to the clutch via a shaft <NUM> having an axis Y. The shaft axis Y may be coaxial with the motor shaft axis X or may be non-coaxial therewith. As described in detail below, the ratchets <NUM>,<NUM> provide an anti-extension and anti-retraction function for the actuator <NUM> when they are connected, via the clutch <NUM> to the motor shaft <NUM>.

The first ratchet <NUM> comprises a first ratchet wheel <NUM> mounted for rotation with the shaft <NUM>. The first ratchet wheel <NUM> has one or more teeth <NUM> on its outer circumference. A first ratchet pawl <NUM> is mounted adjacent the teeth <NUM> and is biased by a spring to bear against the outer circumference of the ratchet wheel <NUM>. When the ratchet wheel <NUM> rotates in a first direction (which is anticlockwise in the direction shown in <FIG>), the teeth <NUM> slide freely under the first ratchet pawl <NUM>. When the first ratchet wheel <NUM> rotates in a second direction, opposite the first direction (i.e. clockwise in the orientation shown in <FIG>), one of the teeth will come to bear against the pawl <NUM> and thereafter further rotation of the ratchet wheel <NUM> in that direction will be prevented.

The second ratchet <NUM> comprises a second ratchet wheel <NUM> mounted for rotation with the shaft <NUM>. The second ratchet wheel <NUM> has one or more teeth <NUM> on its outer circumference. A second ratchet pawl <NUM> is mounted adjacent the teeth <NUM> and is biased by a spring to bear against the outer circumference of the second ratchet wheel <NUM>. The second ratchet <NUM> is oriented in the opposite direction from the first ratchet wheel. That is, when the second ratchet wheel <NUM> rotates in the second direction (which is clockwise in the direction shown in <FIG>), the teeth <NUM> slide freely under the second ratchet pawl <NUM>. When the second ratchet wheel rotates in the first direction (i.e. anticlockwise in the orientation shown in <FIG>), one of the teeth <NUM> will come to bear against the pawl <NUM> and further rotation of the second ratchet wheel <NUM> in that direction will be prevented.

A first cam surface <NUM> is connected for rotation with the output <NUM>. The first cam surface <NUM> is adjacent the first ratchet <NUM> and is shaped such that, within a first predetermined range of angular positions of the output <NUM>, the first cam surface <NUM> pushes the first ratchet pawl <NUM> away from engagement with the first ratchet wheel <NUM>. The first cam surface <NUM> is further shaped such that at other positions, i.e. at angular positions of the output <NUM> outside the first predetermined range, the first ratchet pawl <NUM> is left free to abut against the first ratchet wheel <NUM>. Put another way, the first cam surface disengages the first ratchet <NUM> when the output <NUM> (which is connected to the first and second cam surfaces) is within the first predetermined angular range.

A second cam surface <NUM> is also connected for rotation with the output <NUM>. The second cam surface <NUM> is adjacent the second ratchet <NUM> and is shaped such that, within a second predetermined range of angular positions of the output <NUM>, the second cam surface <NUM> pushes the second ratchet pawl <NUM> away from engagement with the second ratchet wheel <NUM>. The second cam surface <NUM> is further shaped such that at other positions, at positions of the output <NUM> outside the second predetermined range, the second ratchet pawl <NUM> is left free to abut against the second ratchet wheel <NUM>. Put another way, the second cam surface <NUM> disengages the second ratchet <NUM> when the output (which is connected to the first and second cam surfaces) is within the second predetermined angular range.

Typically, the first predetermined range will be different from the second predetermined range, e.g. as shown below in relation to <FIG>.

During normal operation of the actuator <NUM>, the clutch <NUM> is disengaged and the two ratchets <NUM>,<NUM> are not connected for rotation with the motor shaft <NUM>. As such, the actuator <NUM> may drive the output <NUM> in either direction of rotation about the axis X.

When the clutch <NUM> is engaged, the motor shaft <NUM> is connected for rotation with the shaft <NUM> that connects to both ratchet wheels <NUM>,<NUM>. As such, the ratchet wheels <NUM>,<NUM> will attempt to rotate with the motor shaft <NUM>. Depending on the current position of the output <NUM> (and therefore the current positions of the first and second cam surfaces <NUM>,<NUM>) when the clutch <NUM> is engaged, rotation of the motor shaft <NUM> may or may not be arrested by the ratchets <NUM>,<NUM>.

In the orientation shown in <FIG>, the shaft <NUM> is free to rotate clockwise while the first cam surface <NUM> is within the first predetermined range. This is because the second ratchet wheel <NUM> is always free to rotate in the clockwise direction (due to the orientation of its one or more teeth <NUM> relative to the its pawl <NUM>), and the first ratchet wheel <NUM> is free to rotate because the first ratchet pawl <NUM> is currently disengaged from the first ratchet wheel <NUM> by the first cam surface <NUM>. When the output <NUM> moves out of the first predetermined range, however, the first ratchet pawl <NUM> is allowed to re-engage with the first ratchet wheel <NUM>, under the bias from the spring, and will quickly arrest further rotation of the first ratchet wheel <NUM> by engaging with one of the teeth <NUM>.

Similarly, in the orientation shown in <FIG>, the shaft <NUM> is free to rotate anticlockwise while the second cam surface <NUM> is within the second predetermined range. This is because the first ratchet wheel <NUM> is always free to rotate in the anticlockwise direction (due to the orientation of its teeth <NUM> relative to its pawl <NUM>) and the second ratchet wheel <NUM> is free to rotate because the second ratchet pawl <NUM> is currently disengaged from the second ratchet wheel <NUM> by the second cam surface <NUM>. When the output <NUM> moves out of the second predetermined range, however, the second ratchet pawl <NUM> is allowed to re-engage with the second ratchet wheel <NUM>, under bias from the spring, and will quickly arrest further rotation of the second ratchet wheel <NUM> by engaging with one of the teeth <NUM>.

While the output <NUM> is in a position that is outside both the first and second predetermined ranges, the ratchets <NUM>,<NUM> will together prevent rotation of the shaft <NUM> in either direction. That is to say, in the orientation shown, the first ratchet <NUM> will prevent clockwise rotation of the shaft <NUM> and the second ratchet <NUM> will prevent anticlockwise rotation of the shaft <NUM>.

<FIG> respectively depict the first and second predetermined angular ranges. The skilled reader will appreciate that, in some implementations the particular arrangement of e.g. the gearbox <NUM> and/or the clutch <NUM> might mean that a clockwise rotation of the motor shaft <NUM> actually leads to anticlockwise rotation of either or both of the shaft <NUM> and/or output <NUM>. However, for the sake of clarity, it is assumed in the following discussion that clockwise rotation of the motor shaft <NUM> results in clockwise rotation of the shaft <NUM> (when connected via the clutch <NUM>) and results in clockwise rotation of the output <NUM>, where all such directions are in the orientation shown in <FIG>.

<FIG> shows the entire mechanical stroke of the output <NUM>, between <NUM>° and <NUM>°, with zero degrees indicating a center point of the end stop <NUM>. As the end stop <NUM> has some angular width, the total operational stroke available to the output <NUM> is less than the full <NUM> degrees and this range is depicted by lines A and B which define the first and second ends of the operational stroke. Linking this diagram to <FIG>, movement to the right (e.g. moving from a position at <NUM> degrees from the end stop to <NUM> degrees from the end stop) in <FIG> corresponds to clockwise rotation of the shaft <NUM> in the orientation shown in <FIG> (and consequently, as discussed in the preceding paragraph, clockwise rotation of the output <NUM>). <FIG> shows only the clockwise movement of the output <NUM> - anticlockwise motion of the output <NUM> is shown in <FIG> and discussed below.

The shaded box <NUM> indicates the first predetermined range defined by the first cam surface <NUM>, which is connected for rotation with the output <NUM>, and thus the angular position of the output <NUM> may be considered to be identical to the angular position of the first cam surface <NUM> and, indeed, with the second cam surface <NUM>. While the first cam surface <NUM> is anywhere within the range depicted by box <NUM>, the first cam surface <NUM> is holding the first ratchet pawl <NUM> away from the first ratchet wheel <NUM>, which means the shaft <NUM> is free to rotate clockwise and anticlockwise. The end 40a of the box <NUM> that is between points A and B is the first intermediate position.

When the first cam surface <NUM> and output <NUM> are at an angular position outside the box <NUM> of <FIG>, the shaft <NUM> is no longer free to rotate clockwise because the first ratchet pawl <NUM> now engages the teeth <NUM> of the first ratchet wheel <NUM>. In this region, if the shaft <NUM> is connected to the motor shaft <NUM> via the clutch <NUM>, then the motor shaft <NUM> is also prevented from clockwise rotation. That is, the motor shaft <NUM> is braked against clockwise rotation. This means that when the clutch <NUM> is engaged, the output may be moved clockwise only within the region defined by box <NUM>.

<FIG>, similar to <FIG>, shows the entire mechanical stroke of the output <NUM>, between <NUM>° and <NUM>°, with zero degrees indicating a center point of the end stop <NUM>. <FIG> depicts only the anticlockwise movement of the output <NUM>. <FIG> also shows a box <NUM> depicting the angular range of the second cam surface <NUM>, within which range the second cam surface <NUM> holds the second ratchet pawl <NUM> away from the second ratchet wheel <NUM>. The end 50a of box <NUM> between points A and B is the second intermediate point. As such, when the output <NUM> is at an angular position within box <NUM>, the shaft <NUM> may be rotated anticlockwise, which is movement to the left in <FIG>. When the output <NUM> and, correspondingly, the second cam surface <NUM> reaches the edge of the box <NUM>, further anticlockwise rotation of the shaft <NUM> is prevented by the second ratchet <NUM>. In this region, if the shaft <NUM> is connected to the motor shaft <NUM> via the clutch <NUM>, then the motor shaft <NUM> is prevented from anticlockwise rotation when the output <NUM> is outside box <NUM>. That is, the motor shaft <NUM> is braked against anticlockwise rotation.

<FIG> shows <FIG> put together. Thus, when considering clockwise rotation of the output <NUM>, one must refer to the upper half of the diagram (i.e. corresponding to <FIG>) and when considering anticlockwise rotation of the output <NUM>, one must refer to the lower half of the diagram (i.e. corresponding to <FIG>).

If the output <NUM> is at angular position X when the clutch <NUM> is engaged, then due to the position of the first cam surface <NUM>, the output <NUM> may move clockwise (i.e. to the right) up to the end of box <NUM>, but no further. The output <NUM> is also prevented from moving anticlockwise (i.e. to the left) by the second ratchet <NUM>.

The first and second predetermined ranges (shown as boxes <NUM> and <NUM>) in <FIG> are non-overlapping. This means that, if the output is at position Y, between the first and second intermediate points, when the clutch <NUM> is engaged, then neither cam surface <NUM>,<NUM> is holding a respective pawl <NUM>,<NUM> away from a respective ratchet wheel <NUM>,<NUM>, and therefore movement of the output <NUM> in either direction is prevented. That is, in this region between boxes <NUM> and <NUM>, the first ratchet <NUM> prevents clockwise rotation of the shaft <NUM> and the second ratchet <NUM> prevents anticlockwise rotation of the shaft <NUM>. As such, the output is held in place at position Y unable to move while the clutch <NUM> is engaged.

If the output is at position Z when the clutch <NUM> is engaged, the second cam surface <NUM> is holding the second ratchet pawl <NUM> away from the second ratchet wheel <NUM> and therefore the output may be moved anticlockwise (i.e. to the left) up to the end of box <NUM>, but no further. The output is also prevented from moving clockwise (i.e. to the right) by the first ratchet <NUM>.

Thus, depending on the angular position of the output <NUM> when the clutch <NUM> is engaged, the output <NUM> may be free to move towards a more-central position within the diagram, or, if it is already near a central position (i.e. between the two boxes <NUM>,<NUM>), it may be locked in place.

When the actuator <NUM> is installed for use in an aircraft, the output <NUM> is connected to a spoiler of the aircraft (in other uses, the actuator may be connected to a flap, slat or flight surface etc.). The actuator will typically be arranged such that a given angular position of the output <NUM> that is somewhere between the ends of the operational stroke corresponds to a neutral position of the spoiler. This means the actuator <NUM> can move the spoiler in either direction away from the neutral position. For the sake of simplicity, the following discussion will assume that this neutral position is point Y in <FIG>, but it could be other places between the ends A,B of the operational stroke too. In such a setup of the aircraft, clockwise movement of the output <NUM> may correspond to an upward movement of the spoiler while anticlockwise movement corresponds to a downward movement of the spoiler. Taking point Y as the neutral position, point X, being to the left of point Y, corresponds to a spoiler-up position, and point Z corresponds to a spoiler-down position.

When the clutch <NUM> is engaged, the above-discussed arrangement of ratchets <NUM>,<NUM> and respective cam surfaces <NUM>,<NUM>, means that if the output <NUM> is at point X and the spoiler is up, the spoiler can be returned at least partially towards the neutral position. That is the output <NUM> may be rotated clockwise, which shows as rightward movement, up to the end of box <NUM>. However, while the clutch <NUM> is engaged, the spoiler cannot be moved further up from its current position. The ratchets <NUM>,<NUM> therefore act here as an anti-extension device, i.e. disallowing further upward movement of the spoiler, by preventing anticlockwise movement of the output <NUM>.

Similarly, when the clutch <NUM> is engaged, the above-discussed arrangement means that if the output is at point Z in Fig 3C, and the spoiler is therefore down, the spoiler can be returned at least partially up towards the neutral position. That is, the output <NUM> may be rotated anticlockwise up to the end of the box <NUM>. The ratchets <NUM>,<NUM> therefore act here as an anti-retraction device by disallowing further downward movement of the spoiler, by preventing clockwise movement of the output <NUM>.

If the clutch is engaged while the output <NUM> is at point Y or at any point within the angular region between the ends of boxes <NUM> and <NUM>, then the ratchets <NUM>,<NUM> act to prevent rotation of the output in either direction, and therefore act to fully lock the output <NUM> in its current position.

The reader will therefore appreciate that if the output starts at point X when the clutch is engaged, and is returned partially towards the neutral position, i.e. to the end of the box <NUM>, the spoiler is then locked in place by the ratchets <NUM>,<NUM> at that point. Similarly, if the output <NUM> starts at point Z when the clutch is engaged and is returned partially towards the neutral position, i.e. to the end of box <NUM>, the spoiler is then locked in place by the ratchets <NUM>,<NUM> at that point.

It is common to have large gear ratios in the gearbox <NUM>, such that many turns of the motor shaft <NUM> correspond to only a partial rotation of the output <NUM>. Common gear ratios may be <NUM>:<NUM> or <NUM>:<NUM>, for example.

The skilled reader will appreciate that when a ratchet pawl is allowed to engage its ratchet wheel, it will only arrest rotation of the ratchet wheel once it comes into abutment with a tooth on the wheel. Thus, there is typically some small angular range of rotation of the ratchet wheel that is allowed before further rotation is arrested by the pawl - the small angular range being defined by the number and spacing of the teeth on the ratchet wheel and where these are in relation to the pawl when it first engages the wheel. However, with the aforesaid gear ratios, a full turn of the motor shaft may correspond to only a very small angular change in the position of the output <NUM>. Thus, when the clutch <NUM> engages, the rotation of the output <NUM> may be arrested by the ratchets essentially instantly, i.e. within a very small angular range of movement of the output <NUM>.

Providing the ratchets <NUM>,<NUM> to brake the motor shaft <NUM> allows for the use of a smaller end stop <NUM>. It is much easier to prevent rotation of the motor shaft <NUM>, using the ratchets <NUM>,<NUM> (i.e. lower forces involved), compared to relying on the end stop <NUM> to prevent movement of the output <NUM>. For an electromechanical actuator, the inertia of the actuator is mainly determined by the inertia of the motor. The inertia at the output <NUM> is equal to motor inertia x gear ratio<NUM>. Therefore, with a large gear ratio, a small motor inertia gives high inertia at gearbox output. Designing an end stop <NUM> able to stop such high inertia may require heavy parts. By contrast, providing the present ratchets <NUM>,<NUM> to stop motion of the motor shaft <NUM> may allow the use of lighter and/or smaller components to brake the actuator in the event of runaway, and reduce the overall actuator weight.

The angular position and angular speed of the output <NUM> may be monitored in a variety of ways, e.g. by a sensor detecting the output <NUM> or a sensor monitoring the motor shaft <NUM> or monitored by the motor controlling the motor shaft <NUM>. Runaway may be defined as any situation where movement of the output <NUM> is not fully controlled by the motor or is inaccurately controlled by the motor e.g. due to inaccurate or delayed information from the position sensor. When runaway is detected, the clutch <NUM> may be engaged to connect the ratchets <NUM>,<NUM> to the motor shaft <NUM>. This engages the above-discussed anti-extension/anti-retraction functionality of the actuator <NUM>. As such, if the output <NUM> is currently an angular position in either box <NUM> or box <NUM> of Fig 3C, further movement towards the end of the operational stroke is arrested. While the clutch <NUM> remains engaged, movement of the output <NUM>/spoiler partially back towards the neutral position is allowed, as described above.

<FIG> shows a diagram of an example movement of the output <NUM> in a clockwise direction (upper half of the diagram) and an example movement of the output <NUM> in an anticlockwise direction (lower half of the diagram).

Considering first the upper half, the dashed line depicts a speed order, wherein the controller <NUM> orders the actuator to move the output <NUM> clockwise at <NUM> degrees per second, and to slow the output <NUM> down as it nears the end of the operational stroke at point B, which is the same as point B in <FIG>. The solid line depicts the actual speed of the output <NUM>, i.e. as measured by the controller <NUM> from a sensor. In this example, the output <NUM> is being helped to move clockwise by aerodynamic forces on the spoiler that is controlled by the actuator <NUM>. Thus, the actual speed of the output <NUM> is, in this example, <NUM> degrees per second. This difference in speeds may be acceptable when the output <NUM> is moving faster than the ordered speed by only a predetermined amount. However, at point D, the gap between the ordered speed and the actual speed increases beyond the predetermined amount. The controller <NUM> determines from this that runaway is occurring, i.e. movement of the output is no longer under its control. In response, the controller <NUM> may energize the electromagnet <NUM> so as to engage the clutch <NUM>. There may be a brief delay for the electronic signaling (which corresponds to some further angular movement of the output <NUM> from D to D'), and then the ratchets <NUM>,<NUM> take effect to brake further movement of the output <NUM>. The ratchets then have the space between D' and B to fully brake the output <NUM> without the output <NUM> contacting the end stop <NUM>.

Considering the lower half of the figure, the dashed line depicts a speed order, wherein the controller <NUM> orders the actuator to move the output anticlockwise at <NUM> degrees per second, and to slow the output down as it nears the end of the operational stroke. The solid line depicts the actual speed of the output <NUM>, i.e. as measured by the controller from a sensor. In this example, the output <NUM> is being helped to move anticlockwise by aerodynamic forces on the spoiler controlled by the actuator <NUM>. Thus, the actual speed of the output <NUM> is, in this example, <NUM> degrees per second. As before, this difference in speeds may be acceptable when the output <NUM> is moving faster than the ordered speed by only a predetermined amount. However, at point E, the gap between the ordered speed and the actual speed increases beyond the predetermined amount. The controller <NUM> determines from this that runaway is occurring, i.e. movement of the output <NUM> is no longer under its control. In response, the controller <NUM> may energize the electromagnet <NUM> so as to engage the clutch <NUM>. There may be a brief delay for the electronic signaling (which corresponds to some further angular movement of the output <NUM> between points E and E'), and then the ratchets <NUM>,<NUM> take effect to brake further movement of the output <NUM>. The ratchets then have the space between E' and A to fully brake the output without the output <NUM> contacting the end stop <NUM>.

<FIG> shows a flowchart of a method <NUM> of using the actuator <NUM>. The method starts at a step <NUM> of detecting a speed of the output <NUM>. At step <NUM>, a determination is made (e.g. by the controller <NUM> or by another aircraft controller) whether the speed of the output is greater than an ordered speed (e.g. as ordered by the controller or other aircraft controller) by more than a predetermined amount. If the determination is "yes", the method proceeds to step <NUM> where the electrically-actuated clutch <NUM> is engaged, so as to connect the shaft <NUM> to the motor shaft <NUM>.

Claim 1:
An actuator (<NUM>) comprising:
a motor shaft (<NUM>) having an axis (X),
an output (<NUM>), wherein rotation of the motor shaft about the axis drives the output in rotation,
a first ratchet (<NUM>) comprising a first ratchet wheel (<NUM>) and a first ratchet pawl (<NUM>), the first ratchet arranged such that, while the first ratchet pawl engages the first ratchet wheel, the first ratchet wheel is freely rotatable in a first direction and prevented from rotation in a second direction, opposite the first direction;
a second ratchet (<NUM>) comprising a second ratchet wheel (<NUM>) and a second ratchet pawl (<NUM>), the second ratchet arranged such that, while the second ratchet pawl engages the second ratchet wheel, the second ratchet wheel is freely rotatable in the second direction and prevented from rotation in the first direction;
characterised in that the output is arranged to rotate within an angle range of less than <NUM> degrees, in that the first and second ratchet wheels are mounted to a further shaft (<NUM>), and in that the actuator further comprises:
a first cam surface (<NUM>) connected to rotate with the output (<NUM>), wherein the first cam surface is arranged to disengage the first ratchet pawl from the first ratchet wheel when the output is within a first predetermined angular range, and to allow the first ratchet pawl to engage with the first ratchet wheel when the output is outside the first predetermined angular range;
a second cam surface (<NUM>) connected to rotate with the output (<NUM>), wherein the second cam surface is arranged to disengage the second ratchet pawl from the second ratchet wheel when the output is within a second predetermined angular range, and to allow the second pawl to engage with the second ratchet wheel when the output is outside the second predetermined angular range; and
an electrically-actuated clutch (<NUM>) arranged to selectively connect the further shaft to the motor shaft.