Patent Description:
The current state of the art for aircraft brakes are centrally supplied hydraulic brakes, electrohydraulic actuator (EHA) brakes and, more recently, the emergence of electromechanical (EMA) brakes. For hydraulic brakes (either centrally supplied or EHA), the anti-skid function is achieved through metering of hydraulic fluid to and from the brake actuator to modify the braking pressure.

Anti-skid control is a highly dynamic application, which requires a fast response of the actuator. However, the responsiveness and resolution of conventional braking systems is limited, and dynamic operation of those systems leads to energy losses, resulting in a less effective and less efficient anti-lock braking system. Furthermore, using conventional hydraulic systems to provide anti-skid protection requires tightly toleranced manufacturing of servo-valves, and EMA systems often wear due to oscillating control.

UK patent application <CIT> describes an anti-locking brake arrangement in which a piezo-electric actuator is inserted between a brake piston and the back side of a brake pad and is included in the regular anti-locking feedback loop.

The present inventors have found that the high stiffness of piezoelectric actuators makes them more suitable for highly dynamic applications and fast response times, such as required in anti-lock braking systems. Piezoelectric actuators have high resolution (capable of very fine control) and, as there is no fluid returned to a tank, there is less energy lost. However, piezoelectric actuators cannot achieve the high displacement required to function as high-stroke actuators without substantial amplification, which results in a loss of force capability.

To mitigate this and other problems, the present inventors have separated the two conflicting demands of high displacement and high precision/response. The solution in essence uses a conventional braking system, such as centrally supplied hydraulics or EHA in series with a piezoelectric actuator.

The conventional, high-stroke actuator is responsible for the relatively large stroke of closing the initial clearance between the brake piston and stator of the brake, and applying the desired braking pressure. Once 'full' braking pressure is reached, anti-skid control can then be achieved through the use of the low-stroke, high-force piezoelectric actuator.

According to a first aspect of the invention, as defined in the appended claim <NUM>, there is provided an anti-lock braking system for an aircraft, comprising: a hydraulic braking actuator configured to apply a braking pressure to a brake stator; and a piezoelectric actuator connected in series with the braking actuator, wherein the piezoelectric actuator is operable to modify the braking pressure applied to the brake stator to provide anti-skid/slip protection, wherein the hydraulic braking actuator is configured to be locked in position during operation of the piezoelectric actuator by preventing entry and exit of hydraulic fluid to and from an active chamber of the braking actuator.

By connected in series, it is meant that the braking actuator and the piezoelectric actuator are mechanically connected, such that the respective displacements of the actuators combine. In other words, the extension axis (longitudinal axis defined by the extension of the actuator) of the piezoelectric actuator is aligned with and parallel to the extension axis of the braking actuator. A modification of the braking pressure can be a reduction in braking pressure, and/or a modulation (e.g. highfrequency alternation) of the braking pressure. The piezoelectric actuator can comprise a piezoelectric element, preferably a piezoelectric stack, and an electronic drive unit configured to control the piezoelectric element. The piezoelectric element with oscillatory high voltage/low current supply. Control can be purely supply ON/supply OFF.

With this arrangement, an improved anti-lock braking system can be provided. In particular, a more responsive anti-lock braking system can be achieved, capable of finer control and with reduced energy loss when operating dynamically. The system can also be less prone to wear and can be more reliable. Furthermore, the system can have reduced complexity, size and/or mass compared to the prior art arrangements.

The anti-lock braking system may be operable in a first phase and a second phase. The first phase may be referred to as a 'normal', or 'high-stroke', braking phase, which can be responsible for the relatively large stroke of closing the initial clearance between the brake piston and stator and applying the desired braking pressure. The braking pressure can be exerted by the braking actuator extending from a first extension state to a second (greater) extension state. The piezoelectric actuator is also operable between first and second extension states. In the first phase, the piezoelectric actuator is arranged to be in an extended state. The piezoelectric actuator can assume an extended state when energised by the electronic drive unit (i.e. when a voltage is applied to the piezoelectric element). In the second phase, the braking actuator is configured to remain in the second extension state and the piezoelectric actuator is configured to retract from the extended state to a retracted state. In other words, once 'full' braking pressure is achieved, the high-stroke braking actuator can be fixed at its current stroke and the anti-skid function can be provided by the piezoelectric actuator retracting to modify (in this instance by reducing) the braking pressure applied to the brake stator. The piezoelectric actuator can retract when it is de-energised (i.e. when the electronic drive unit stops applying a voltage to the piezoelectric element). Alternatively, the piezoelectric actuator could be configured to assume an extended state when de-energised by the electronic drive unit, and to retract when it is energised.

In the second phase the braking actuator may be arranged to be locked in the second extension state. Where the braking actuator is a hydraulic actuator, the braking actuator can be locked in the second extension state by controlling entry and exit of hydraulic fluid to and from an active chamber of the braking actuator. With this arrangement, the stiffness of the system is increased when operating in the second phase, which increases responsiveness and reduces energy losses. When the braking actuator is an EMA, the braking actuator can be locked in position by using, for instance, a brake or a motor to hold the extension. This arrangement can avoid the need for fine manufacturing to deal with mechanical backlash in EMAs and can reduce wear due to the very small oscillations on mechanical transmissions.

The braking actuator and the piezoelectric actuator may be arranged such that the braking actuator is positioned adjacent to the brake stator. In other words, the braking actuator may be positioned between the piezoelectric actuator and the brake stator. Alternatively, the braking actuator and the piezoelectric actuator may be arranged such that the piezoelectric actuator is positioned adjacent to the brake stator, between the braking actuator and the brake stator. As will be appreciated, there may be a mechanical clearance between the braking actuator/piezoelectric actuator and the brake stator when the brakes are not being applied.

According to a further aspect of the invention, there is provided an aircraft landing gear comprising the anti-lock braking system according to any of the descriptions above. According to a further aspect of the invention, there is provided an aircraft comprising the aircraft landing gear.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:
<FIG> is a diagram of an anti-lock braking system according to an embodiment of the invention.

In the most general sense, the invention uses a conventional braking system, such as centrally supplied hydraulics or EHA, in series with a piezoelectric actuator. The conventional, high-stroke actuator is responsible for the relatively large stroke of closing the initial clearance between the brake piston and stator of the brake, and applying the desired braking pressure. Once 'full' braking pressure is reached, anti-skid control can then be achieved through the use of the low-stroke, high-force piezoelectric actuator.

<FIG> is a diagram of an anti-lock braking system <NUM> in accordance with an example of the invention. The braking system <NUM> comprises a high-stroke braking actuator <NUM> coupled in series with a piezoelectric actuator <NUM>.

The high-stroke braking actuator <NUM> is illustrated in <FIG> as a hydraulic actuator. The hydraulic braking actuator <NUM> comprises a cylinder <NUM> within which a piston and rod assembly <NUM>, <NUM>, is slidably housed so that the actuator <NUM> can extend and retract along a longitudinal axis A. The cylinder <NUM> includes a port P for coupling the actuator <NUM> to a hydraulic fluid supply circuit (not shown), such as a central hydraulic system. The high-stroke actuator <NUM> shown in <FIG> is single acting in that there is a single port P defining a single active chamber. The space within the cylinder <NUM> between the port P and piston <NUM> defines an active chamber AC (also referred to as an extension chamber) that hydraulic fluid such as oil can be supplied to in order to cause the actuator <NUM> to extend. As shown, the piston rod <NUM> of the braking actuator is coupled to one or more brake stators <NUM>. When the braking actuator <NUM> extends, the one or more brake stators <NUM> apply a braking pressure to one or more rotors fixed to a wheel (not shown). Friction between the stator(s) and rotor(s) applies a braking torque which acts to slow the rotation of the wheel (or prevent it from rotating in the case where the aircraft is stationary). Of course, in practice aircraft braking assemblies are more complicated and comprise many more elements besides. For instance, in practice a braking assembly for a single wheel may have multiple braking actuators. However, description of those elements is not necessary for the purposes of understanding the present invention.

The piezoelectric actuator <NUM> can be any suitable piezoelectric actuator. For example, the piezoelectric actuator <NUM> can comprise a piezoelectric element, preferably a piezoelectric stack, driven by an electronic drive unit. The piezoelectric actuator <NUM> is operable to extend and retract along the longitudinal axis A. The piezoelectric stack may be configured to assume a retracted position when de-energised and to respond to an applied voltage by extending. Alternatively, the piezoelectric stack may be configured to assume an extended position when de-energised and to respond to an applied voltage by retracting. As will be appreciated, either configuration can be achieved through appropriate selection of the piezoelectric material (e.g. the polarisation), or by means of amplification.

The piezoelectric actuator <NUM> is coupled in series with the high-stroke braking actuator <NUM>, for example as shown in <FIG>. In other words, the piezoelectric actuator <NUM> is mechanically connected to the high-stroke braking actuator <NUM> such that the extension axis of each actuator is aligned with and parallel to the axis A, as shown. As an example, the piezoelectric actuator <NUM> can be placed in mechanical communication with (e.g. affixed to) an end of the cylinder casing <NUM> of the high-stroke actuator, on the opposite side to the piston rod/stator. Alternatively, the piezoelectric actuator <NUM> and high-stroke actuator <NUM> could be arranged such that the piezoelectric actuator <NUM> is adjacent to (and acts on) the brake stator <NUM>. In either case, the total displacement delivered to the brake stator <NUM> is a combination of the respective displacements of the piezoelectric actuator <NUM> and the high-stroke actuator <NUM>.

As described above, the high-stroke actuator <NUM> is responsible for the relatively large stroke of closing the initial clearance between the brake piston and stator of the brake, and applying the desired braking pressure. This is referred to as a first phase. Sticking to the hydraulic braking actuator example, in the first phase a hydraulic fluid supply circuit supplies hydraulic fluid under pressure to the port P of the braking actuator <NUM> (e.g. from a reservoir of a central hydraulic system). Hydraulic fluid entering the port P into the active chamber AC forces the piston <NUM> towards the side of the casing <NUM> from which the rod <NUM> extends. This causes the actuator <NUM> to change during the first phase between first and second extension states, which in turn applies a braking force to the brake rotor(s) via one or more brake stators <NUM>, as discussed above. The piezoelectric actuator <NUM> can be extended throughout the first phase, by energising the piezoelectric stack (applying a voltage using the electronic drive unit). Equally, the piezoelectric actuator could be configured such that it is extended when de-energised (no voltage is applied).

Once 'full' braking pressure is reached (i.e. once the desired braking force is applied, but the wheels have started to or are close to slipping), anti-skid control can then be achieved in a second phase through the use of the low-stroke, high-force piezoelectric actuator <NUM>. Any suitable anti-skid sensing and control can be used.

In the second phase, the high-stroke braking actuator <NUM> is locked, or fixed at its current stoke position (or otherwise remains in the second extension state). In the hydraulic braking actuator example, this can be achieved by controlling (i.e. preventing) entry and exit of hydraulic fluid into the active chamber AC. The anti-skid function is then achieved by retracting (i.e. de-energising) the piezoelectric actuator <NUM> to modify the braking pressure applied to the brake stator <NUM>. As will be appreciated, a retraction of the piezoelectric actuator <NUM> whilst the high-stroke actuator <NUM> is locked in position results in a relatively small reduction in braking pressure applied to the brake stator <NUM>, which in turn relieves the braking torque applied to the wheel(s). The braking pressure can be continuously modulated for a period of time, as required to prevent skidding. For instance, a closed loop control system may be used to automatically regulate the braking pressure as required. In some instances, the piezoelectric actuator may be driven to alternate between extended and retracted states, but it will be appreciated that the form of brake pressure modulation will depend on the circumstances (as determined, for example, by closed loop control). The piezoelectric stack can be driven with high voltage/low current supply.

Claim 1:
An anti-lock braking system (<NUM>) for an aircraft, comprising:
a hydraulic braking actuator (<NUM>) configured to apply a braking pressure to a brake stator (<NUM>); and
a piezoelectric actuator (<NUM>) connected in series with the braking actuator, wherein the piezoelectric actuator is operable to modify the braking pressure applied to the brake stator to provide anti-skid protection, characterised in that the hydraulic braking actuator (<NUM>) is configured to be locked in position during operation of the piezoelectric actuator (<NUM>) by preventing entry and exit of hydraulic fluid to and from an active chamber of the braking actuator.