Anti-lock braking system

Abstract

The present application relates to an anti-lock braking system for an aircraft, including a brake actuator configured to apply brake pressure to a brake stator, and a piezoelectric brake actuator connected in series with the brake actuator. The piezoelectric actuator is operable to modify the brake pressure applied to the brake stator to provide anti-skid protection.

Description

Technical Field

[0001] This invention relates to aircraft braking systems, and more specifically, to aircraft anti-lock braking systems. Anti-lock or anti-skid systems are designed to correct braking force in the event of brake wheel slippage. Background Technology

[0002] Current technology for aircraft brakes includes centrally supplied hydraulic brakes, electro-hydraulic actuator (EHA) brakes, and the more recently developed electromechanical (EMA) brakes. For hydraulic brakes (centrally supplied or EHA), anti-slip functionality is achieved by metering the hydraulic fluid entering and leaving the brake actuator to correct the braking pressure.

[0003] Anti-slip control is a highly dynamic application that requires rapid actuator response. However, conventional braking systems have limited responsiveness and resolution, and their dynamic operation leads to energy losses, resulting in lower effectiveness and efficiency of anti-lock braking systems. Furthermore, using conventional hydraulic systems to provide anti-slip protection requires servo valves manufactured to tight tolerances, and EMA systems often suffer wear due to oscillating control. Summary of the Invention

[0004] The inventors have discovered that the high stiffness of piezoelectric actuators makes them well-suited for high-dynamic applications and fast response times, such as those required in anti-lock braking systems. Piezoelectric actuators offer high resolution (enabling very fine control) and minimal energy loss due to the absence of fluid return to the reservoir. However, without substantial scale-up, piezoelectric actuators cannot achieve the high displacements required for use as high-stroke actuators, resulting in a loss of force capability.

[0005] To mitigate this and other issues, the inventors have separated the conflicting requirements of high displacement and high precision / response. The solution essentially utilizes a conventional braking system combined with a piezoelectric brake, such as a centrally supplied hydraulic system, EHA, or EMA.

[0006] Conventional high-stroke actuators are responsible for closing the initial gap between the brake piston and stator with a relatively large stroke, and optionally also apply the desired braking pressure. Once the "full" braking pressure is reached, anti-slip control can be achieved by using a low-stroke, high-force piezoelectric actuator.

[0007] According to one aspect of the invention, an anti-lock braking system for an aircraft is provided, comprising: a brake actuator configured to apply braking pressure to a brake stator; and a piezoelectric actuator integral with the brake actuator, wherein the piezoelectric actuator is operable to modify the braking pressure applied to the brake stator to provide anti-slip protection.

[0008] Optionally, the piezoelectric actuator and the brake actuator are connected in series. This series connection means that the brake actuator and the piezoelectric actuator are mechanically connected, allowing the corresponding displacements of each actuator to be combined. In other words, the extended axis of the piezoelectric actuator (the longitudinal axis defined by the extension of the actuator) is aligned with and parallel to the extended axis of the brake actuator.

[0009] Correction of braking pressure can be achieved by reducing and / or modulating the braking pressure (e.g., high-frequency alternation). A piezoelectric actuator may include a piezoelectric element, preferably a piezoelectric stack, and an electronic drive unit configured to control the piezoelectric element. The piezoelectric element is coupled with an oscillating high-voltage / low-current power supply. Control can be simply turning the power on (ON) or off (OFF).

[0010] This arrangement provides an improved anti-lock braking system. Specifically, it enables a more sensitive anti-lock braking system with finer control and reduced energy loss during dynamic operation. The system is also less prone to wear and more reliable. Furthermore, compared to prior art arrangements, the system's complexity, size, and / or weight can be reduced.

[0011] The anti-lock braking system can operate in a first stage and a second stage. The first stage, which may be referred to as the "normal" or "high-stroke" braking stage, is responsible for closing the initial gap between the brake piston and the stator with a relatively large stroke and applying the required braking pressure. The braking pressure can be applied by a brake actuator extending from a first extended state to a second (greater) extended state. The piezoelectric actuator is also operable between the first and second extended states. In the first stage, the piezoelectric actuator is arranged in the extended state. When energized by the electronic drive unit (i.e., when voltage is applied to the piezoelectric element), the piezoelectric actuator can present the extended state. In the second stage, the brake actuator is configured to remain in the second extended state, and the piezoelectric actuator is configured to retract from the extended state to the retracted state. In other words, once the "full" braking pressure is reached, the high-stroke brake actuator can be fixed at its current stroke, and the retraction of the piezoelectric actuator can provide an anti-slip function to correct (in this case, reduce) the braking pressure applied to the brake stator. When the piezoelectric actuator is de-energized (i.e., when the electronic drive unit stops applying voltage to the piezoelectric element), the piezoelectric actuator can retract. Alternatively, the piezoelectric actuator can be configured to be in an extended state when the electronic drive unit is de-energized and retract when energized.

[0012] In the second stage, the brake actuator can be arranged to lock in the second extended state. When the brake actuator is a hydraulic actuator, it can be locked in the second extended state by controlling the flow of hydraulic fluid into and out of the actuator's active chamber. With this arrangement, the system stiffness increases during second-stage operation, which increases responsiveness and reduces energy loss. When the brake actuator is an EMA (Electrical Expansion Part), the extension can be maintained by using, for example, a brake or motor, thereby locking the brake actuator in place. This arrangement avoids the need for precision manufacturing to address the mechanical clearances in the EMA and reduces wear caused by minute vibrations in the mechanical transmission.

[0013] The brake actuator can be a hydraulic actuator, such as a centrally supplied hydraulic actuator or an EHA. Alternatively, the brake actuator can be an EMA.

[0014] The brake actuator and piezoelectric actuator can be arranged such that the brake actuator is positioned adjacent to the brake stator. In other words, the brake actuator can be positioned between the piezoelectric actuator and the brake stator. Alternatively, the brake actuator and piezoelectric actuator can be arranged such that the piezoelectric actuator is positioned adjacent to the brake stator, between the brake actuator and the brake stator. As will be understood, when the brake is not applied, there may be mechanical clearance between the brake actuator / piezoelectric actuator and the brake stator.

[0015] Preferably, the brake actuator is a hydraulic actuator, and the piezoelectric actuator is further configured to lock the hydraulic brake actuator when the braking pressure is adjusted to provide anti-slip protection. In this arrangement, the piezoelectric actuator can simultaneously provide locking and anti-slip protection, thereby reducing system complexity and / or the number of individual components. This improves reliability and increases system stiffness when locked, enhancing the performance of the piezoelectric actuator.

[0016] An operable piezoelectric actuator can lock a hydraulic brake actuator by controlling the entry and exit of hydraulic fluid into and out of the active chamber of the brake actuator. For example, an operable piezoelectric actuator can lock the brake actuator by physically blocking the entry and exit of hydraulic fluid into and out of the active chamber.

[0017] An operable piezoelectric actuator is used to lock a hydraulic brake actuator and to correct the braking pressure applied to the brake stator by moving within the active chamber of the hydraulic brake actuator. The piezoelectric actuator is operable to move between at least a first position, a second position, and a third position. In the first position, the piezoelectric actuator allows hydraulic fluid to enter and exit the active chamber. In the second and third positions, the piezoelectric actuator can block the entry and exit of hydraulic fluid into the active chamber. The operable piezoelectric actuator, by moving between the second and third positions, corrects the braking pressure applied to the brake stator to provide anti-slip protection. Thus, the brake actuator can be hydraulically locked by the piezoelectric actuator during anti-slip operation.

[0018] The second position of the piezoelectric actuator can be a larger extended state than the first position, and the third position of the piezoelectric actuator can also be a larger extended state than the second position. For example, the first position can be a retracted (e.g., de-energized) state, in which the piezoelectric actuator occupies minimal space within the active chamber of the hydraulic brake actuator. By applying an increased voltage to the piezoelectric stack to cause the piezoelectric actuator to occupy more space in the active chamber, thereby increasing the hydraulic pressure in the active chamber, the piezoelectric actuator can extend to the second position, and subsequently to the third position.

[0019] A hydraulic brake actuator may include one or more ports for connecting the active chamber of the hydraulic brake actuator to a hydraulic fluid supply (e.g., a supply line or return line). An operable piezoelectric actuator can lock the hydraulic brake actuator by blocking (i.e., covering or blocking in a fluid-tight manner) one or more ports. The number of ports can be selected taking into account the required flow rate and the strength of the hydraulic brake actuator. As will be understood, a greater number of ports can increase the flow rate into and out of the active chamber, while fewer ports can increase the strength of the cylinder.

[0020] A piezoelectric actuator may include a piezoelectric stack and a piston assembly (e.g., a metal disc coupled to the piezoelectric stack). The piston assembly can move under the influence of the piezoelectric stack to lock a hydraulic brake actuator and correct braking pressure. The piezoelectric actuator may be arranged such that the piston assembly provides an interface between the piezoelectric stack and hydraulic fluid in the active chamber.

[0021] The anti-lock braking system may also include a dual-fluid control valve that can operate to selectively connect the hydraulic brake actuator between the hydraulic fluid source line and the hydraulic fluid return line.

[0022] According to another aspect of the invention, an aircraft landing gear is provided, which includes an anti-lock braking system according to any of the above descriptions. According to another aspect of the invention, an aircraft is provided that includes the aircraft landing gear. Attached Figure Description

[0023] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

[0024] Figure 1 This is a schematic diagram of an anti-lock braking system according to an embodiment of the present invention.

[0025] Figure 2 This is a schematic diagram of an anti-lock braking system according to another embodiment of the present invention. Detailed Implementation

[0026] In its most general sense, this invention utilizes a conventional braking system in conjunction with a piezoelectric actuator, such as a centrally supplied hydraulic system, EHA, or EMA. A conventional high-stroke actuator is responsible for closing the initial gap between the brake piston and stator with a relatively large stroke, and optionally also applies the desired braking pressure. Once the "full" braking pressure is reached, anti-slip control can be achieved by using a low-stroke, high-force piezoelectric actuator.

[0027] Figure 1 This is a schematic diagram of an anti-lock braking system 10 according to an example of the present invention. The braking system 10 includes a high-stroke brake actuator 20 connected in series with a piezoelectric actuator 30.

[0028] High-stroke brake actuator 20 Figure 1 The actuator is shown as a hydraulic actuator, but it can be any suitable brake actuator. The hydraulic actuator 20 includes a cylinder 22 in which piston and rod assemblies 24, 26 are slidably housed, allowing the actuator 20 to extend and retract along the longitudinal axis A. The cylinder 22 includes a port P for connecting the actuator 20 to a hydraulic fluid supply circuit (not shown), such as a central hydraulic system. Figure 1 The high-stroke actuator 20 shown is single-acting, characterized by a single port P defining a single active chamber. The space between port P and piston 24 within cylinder 22 defines an active chamber AC (also called an extension chamber), to which hydraulic fluid, such as oil, is supplied to cause actuator 20 to extend. As shown, piston rod 26 of the brake actuator is coupled to one or more brake stators 40. When brake actuator 20 extends, the one or more brake stators 40 apply braking pressure to one or more rotors fixed to a wheel (not shown). Friction between the one or more stators and one or more rotors applies a braking torque, which serves to slow the rotation of the wheel (or prevent the wheel from rotating when the aircraft is stationary). Of course, in practice, aircraft braking assemblies are more complex and include many more elements. For example, in practice, a braking assembly for a single wheel may have multiple brake actuators. However, a description of these elements is not necessary for understanding the purposes of this invention.

[0029] The piezoelectric actuator 30 can be any suitable piezoelectric actuator. For example, the piezoelectric actuator 30 may include a piezoelectric element driven by an electronic drive unit, preferably a piezoelectric stack. The piezoelectric actuator 30 is operable to extend and retract along the longitudinal axis A. The piezoelectric stack may be configured to present a retracted position when de-energized and to respond to an applied voltage by extending. Alternatively, the piezoelectric stack may be configured to present an extended position when de-energized and to respond to an applied voltage by retracting. As will be understood, either configuration can be achieved by appropriately selecting the piezoelectric material (e.g., polarization) or by means of amplification.

[0030] The piezoelectric actuator 30 is connected in series with the high-stroke brake actuator 20, for example, as... Figure 1 As shown. In other words, the piezoelectric actuator 30 is mechanically connected to the high-stroke brake actuator 20 such that the extension axis of each actuator is aligned with and parallel to axis A, as shown. For example, the piezoelectric actuator 30 can be positioned on the opposite side of the piston rod / stator and mechanically communicates (e.g., is attached) with the end of the cylinder housing 22 of the high-stroke actuator. Alternatively, the piezoelectric actuator 30 and the high-stroke actuator 20 can be arranged such that the piezoelectric actuator 30 is adjacent to (and acts on) the brake stator 40. In either case, the total displacement transmitted to the brake stator 40 is a combination of the respective displacements of the piezoelectric actuator 30 and the high-stroke actuator 20.

[0031] As described above, the high-stroke actuator 20 is responsible for closing the initial gap between the brake piston and stator of the brake with a relatively large stroke and applying the desired braking pressure. This is referred to as the first stage. Following the example of a hydraulic brake actuator, in the first stage, the hydraulic fluid supply circuit supplies hydraulic fluid under pressure (e.g., from a reservoir in the central hydraulic system) to port P of the brake actuator 20. The hydraulic fluid entering the active chamber AC from port P forces the piston 24 toward the rod 26 of the housing 22 from its extended side. This causes the actuator 20 to change between a first extended state and a second extended state during the first stage, which in turn applies braking force to one or more brake rotors via one or more brake stators 40, as described above. It should be understood that the same function can be achieved with different types of actuators, such as EMA. By energizing the piezoelectric stack (applying voltage using an electronic drive unit), the piezoelectric actuator 30 can extend throughout the first stage. Similarly, the piezoelectric actuator can be configured such that it extends when de-energized (no voltage applied). Alternatively, the piezoelectric stack can be initially retracted (or at least not fully extended) during brake actuator movement. In this case, the brake actuator can be arranged to close the initial gap between the brake piston and one or more stators without generating braking torque, and the (extended) piezoelectric actuator can be used to apply braking pressure.

[0032] Once “full” braking pressure is reached (i.e., once the required braking force has been applied, but the wheels have already begun to slip or are close to slipping), anti-slip control can be achieved in the second stage using a low-stroke, high-force piezoelectric actuator 30. Any suitable anti-slip sensing and control can be used.

[0033] In the second stage, the high-stroke brake actuator 20 is locked or fixed in its current stroke position (or otherwise held in a second extended state). In the example of a hydraulic brake actuator, this can be achieved by controlling (i.e., preventing) the entry and exit of hydraulic fluid into the active chamber AC. The anti-slip function is then achieved by retracting (i.e., de-energizing) the piezoelectric actuator 30 to correct the braking pressure applied to the brake stator 40. As will be understood, the retraction of the piezoelectric actuator 30, while the high-stroke actuator 20 is locked in place, results in a relatively small reduction in the braking pressure applied to the brake stator 40, which in turn reduces the braking torque applied to one or more wheels. The braking pressure can be continuously modulated for a period of time as needed to prevent slippage. For example, a closed-loop control system can be used to automatically adjust the braking pressure as needed. In some cases, the piezoelectric actuator can be driven to alternate between the extended and retracted states; however, it should be understood that the form of brake pressure modulation will depend on the environment (e.g., determined by closed-loop control). The piezoelectric stack can be driven with a high-voltage / low-current power supply.

[0034] The above concept attempts to improve the dynamic response of anti-skid braking systems by combining the highly dynamic characteristics of piezoelectric actuators with "normal" hydraulic or EMA / EHA systems to handle the relatively high displacement and velocity required to close mechanical backlash in braking systems. Once backlash is overcome, the dynamic response required for anti-skid braking can practically only be achieved through a rigid system, which includes not only the piezoelectric actuator but also the transmission of force from the piezoelectric actuator to the brake piston. In the example above using a standard aircraft hydraulic system, this can be achieved by locking the hydraulic fluid at maximum pressure to overcome compressibility in the fluid lines and piston, thus forming a rigid fluid column. The hydraulic actuator can be locked, for example, via an external fluid control valve used to selectively connect port P to the supply or return line of the hydraulic circuit. Once locked, the piezoelectric element with low displacement but high force capability enables the required high precision and responsive braking control.

[0035] The discussions so far have envisioned a piezoelectric actuator mechanically connected to a separate high-stroke brake actuator, such that the extension axis of each actuator is parallel to axis A, and the displacement of the actuators along axis A is combined to apply braking pressure. However, in another arrangement, the piezoelectric actuator can be integrated with a hydraulic brake actuator and arranged to correct the hydraulic pressure within the active chamber AC. An example of this arrangement is shown below. Figure 2 As shown, and described below.

[0036] The inventors have discovered that, with this arrangement, the system can be designed such that the piezoelectric actuator used to provide anti-slip protection can also be used as a locking brake actuator. Specifically, the piezoelectric actuator can be arranged to selectively block the entry and exit of hydraulic fluid during anti-slip operation. This can reduce the complexity and / or number of components in the system, while increasing the system's stiffness and improving reliability.

[0037] Figure 2 An example of a system 100 that can achieve this is shown. The braking system 100 includes a high-stroke hydraulic brake actuator 200 with an integrated piezoelectric actuator 300. (Compared to...) Figure 1 Similar to the brake actuator 20, Figure 2 The illustrated hydraulic brake actuator 200 includes a cylinder 220, within which piston and rod assemblies 240, 260 are slidably housed, allowing the actuator 200 to extend and retract along the longitudinal axis A. However, those skilled in the art will understand other suitable arrangements. The cylinder 220 includes one or more ports P for connecting the actuator 200 to a hydraulic fluid supply circuit, such as a central hydraulic system, via a binary fluid control valve (not shown). When unobstructed, port P allows hydraulic fluid to flow into or out of the active chamber AC, depending on the position of the binary fluid control valve. Figure 2 Two ports P are visible in the cross-section shown, but additional ports can be distributed around the periphery of cylinder 220 as needed to achieve the desired flow rate. The space within cylinder 220 (defined in some examples by piston 240 at one end and piezoelectric actuator 300 at the other) defines an active chamber AC (also called an extension chamber) to which hydraulic fluid, such as oil, can be supplied via ports P to extend actuator 200. Figure 1 As shown, the piston rod 260 of the brake actuator is connected to one or more brake stators 400, which applies braking pressure to one or more rotors fixed to the wheel as described above.

[0038] The piezoelectric actuator 300 can be any suitable piezoelectric actuator, as described above. Figure 1 The piezoelectric actuator 300 is arranged to move within the active chamber AC of the hydraulic brake actuator 200 to correct the braking pressure and control the entry and exit of hydraulic fluid. For example, extending the piezoelectric actuator 300 into the active chamber AC (by reducing the volume of the active chamber AC) increases the hydraulic pressure within the active chamber AC while also hindering the exit of hydraulic fluid via port P.

[0039] The piezoelectric actuator 300 is operable to move between at least three positions, such as Figure 2Positions 1, 2, and 3 are shown. Some or all of these positions can be discrete extensions of the piezoelectric actuator 300, which are pre-programmed (e.g., by applying a predetermined voltage to the piezoelectric stack) and / or consecutive points (e.g., upper and lower limits). For example, the piezoelectric actuator 300 can be operated to move discretely between positions 1 and 2, but continuously between positions 3 and 4.

[0040] Initially, the piezoelectric actuator 300 can be in position 1. In position 1, port P is unobstructed, so when braking is requested, the binary control valve can direct hydraulic fluid toward the brake piston. The inflow of hydraulic fluid causes the brake piston to extend over the relatively low resistance provided by the sealing friction of the brake piston, thereby closing the gap between the brake piston and the stator. Since the brake piston is in contact with the stator, and the stator is in contact with the rotor of the wheel, braking pressure can be achieved. Similarly, the full pressure (i.e., the "fill" pressure) achievable by filling the active chamber AC from the hydraulic supply may only close the mechanical gap in the braking system without applying torque to the wheel. As the full charge pressure of the hydraulic supply approaches, the piezoelectric actuator 300 extends from position 1 to position 2 (e.g., ...). Figure 2 (As shown). In position 2, the port of cylinder 220 is shielded by piezoelectric actuator 300 (in this simplified example, by the piezoelectric stack itself), which closes the active chamber AC and locks the hydraulic fluid inside. A seal around piezoelectric actuator 300 ensures fluid tightness. When the fluid is locked in place at the fill pressure, piezoelectric actuator 300 can extend to position 3 to increase the pressure to the maximum braking pressure, thereby increasing the maximum braking torque on the wheel. Piezoelectric actuator 300 can be controlled (e.g., in a continuous manner using a closed-loop controller) between positions 2 and 3 to control the anti-slip braking pressure. For example, piezoelectric actuator 300 can be controlled to move continuously between positions 2 and 3 as needed to maintain maximum braking torque without slippage.

[0041] When brake actuator 200 is locked by piezoelectric actuator 300, the binary fluid control valve can switch to the return position to backfill the fluid in the line between piezoelectric actuator 300 and the binary control valve back to the reservoir. Once the desired braking is achieved, piezoelectric actuator 300 can move rearward toward position 1, which unlocks brake actuator 200 by reopening the fluid connection between the active chamber AC and the binary control valve. Depending on the volume between the binary control valve and piezoelectric actuator 300, this position can be used to moderate the slow release of the brake. If the volume is large, the brake will simply open and return the fluid to the reservoir.

[0042] Alternatively, for safety reasons, the binary control valve can remain in place (i.e., supply fluid from the source) until, for example, the aircraft comes to a complete stop. In this way, if the piezoelectric actuator 300 is released, braking pressure can be maintained and / or the brakes can be reapplied more quickly (without having to reclose the mechanical clearance).

[0043] As can be seen from the above operation, the piezoelectric actuator 300 mates with port P to provide an internal / integrated fluid control valve that controls the entry and exit of hydraulic fluid into the active chamber AC. In position 1, the integrated fluid control valve can be considered open (or partially open), while in positions 2 and 3, it can be considered closed. By locking the hydraulic fluid at the maximum fill pressure, the hydraulic fluid in the active chamber AC is compressed to create a rigid fluid column. Therefore, once locked, the low displacement but high force capability of the piezoelectric actuator 300 can be used to achieve highly accurate and responsive braking control. However, depending on the desired application (i.e., the required force, the displacement of the piezoelectric actuator, etc.), the piezoelectric actuator 300 can be operated to lock the brake actuator 200 at high, medium, or low pressure. This will be further affected by the specific method of closing port P to the active chamber AC.

[0044] Figure 2 The example shown is a simplified illustration. More complex modifications might involve enlarging the piezoelectric actuator or directing the piston to another inlet to reduce the displacement required for the piezoelectric stack.

[0045] In the example shown, the piezoelectric stack is positioned to extend into the active chamber AC. However, in some cases, it may be desirable to avoid direct contact between the piezoelectric stack and the hydraulic fluid, as they are generally incompatible. Therefore, the piezoelectric actuator 300 may advantageously include a piston assembly (e.g., a metal disc, etc.) coupled to the piezoelectric stack, such that the piston assembly provides an interface to the hydraulic fluid and extends into the active chamber AC under the influence of the piezoelectric stack.

[0046] Furthermore, although the piezoelectric actuator 300 is shown extending and retracting along the longitudinal axis A, it should be understood that any orientation and / or position of the piezoelectric actuator can be used to achieve the same function (i.e., changing the pressure within the active chamber while blocking one or more ports). Alternative arrangements for achieving the function described herein will be apparent to those skilled in the art.

[0047] It should be noted that the above embodiments are illustrative rather than limiting of the invention, and those skilled in the art will be able to devise many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference numerals placed in parentheses should not be construed as limiting the claims. The word "comprising" does not exclude the presence of elements or steps other than those listed as a whole in any claim or specification. A singular reference to an element does not exclude a plural reference to that element, and vice versa. The various components of the invention can be implemented by means of hardware comprising several different elements. In device claims enumerating several components, several of these components may be implemented by one or the same piece of hardware. The fact that specific measures are described in mutually different dependent claims does not mean that a combination of these measures cannot be used to produce good results.

Claims

1. An anti-lock braking system for an aircraft, comprising: A hydraulic brake actuator configured to apply braking pressure to a brake stator; as well as A piezoelectric actuator, integral with the brake actuator, wherein the piezoelectric actuator is operable to correct the braking pressure applied to the brake stator to provide anti-slip protection, and wherein the hydraulic brake actuator is configured to be locked in place during operation of the piezoelectric actuator by preventing hydraulic fluid from entering and leaving the active chamber of the brake actuator.

2. The anti-lock braking system according to claim 1, characterized in that, The brake actuator is a hydraulic brake actuator.

3. The anti-lock braking system according to claim 1, characterized in that, The piezoelectric actuator can be operated to lock the hydraulic brake actuator, and the brake pressure applied to the brake stator can be corrected by moving within the active chamber of the hydraulic brake actuator.

4. The anti-lock braking system according to claim 2, characterized in that, The piezoelectric actuator can be operated to move between at least a first position, a second position, and a third position, wherein: In the first position, the piezoelectric actuator allows hydraulic fluid to enter and exit the brake actuator; In the second and third positions, the piezoelectric actuator prevents hydraulic fluid from entering and leaving the brake actuator; The piezoelectric actuator can be operated to correct the braking pressure applied to the brake stator by moving between the second position and the third position, so as to provide anti-slip protection.

5. The anti-lock braking system according to claim 2, characterized in that, The hydraulic brake actuator includes one or more ports for connecting the active chamber of the hydraulic brake actuator to a supply of pressurized hydraulic fluid, wherein the piezoelectric actuator can be operated to lock the hydraulic brake actuator by blocking the one or more ports.

6. The anti-lock braking system according to claim 2, characterized in that, The piezoelectric actuator includes a piezoelectric stack coupled to the piston assembly.

7. The anti-lock braking system according to claim 2, characterized in that, It also includes a binary fluid control valve, which can be operated to selectively connect the hydraulic brake actuator between the hydraulic fluid source line and the hydraulic fluid return line.

8. The anti-lock braking system according to claim 1 or 2, characterized in that, The piezoelectric actuator is connected in series with the brake actuator.

9. The anti-lock braking system according to claim 8, characterized in that, The anti-lock braking system can operate in both the first and second phases, wherein: In the first stage, the piezoelectric actuator is arranged in an extended state, and the braking pressure is applied by the braking actuator extending from the first extended state to the second extended state. In the second stage, the brake actuator is configured to remain in the second extended state, and the piezoelectric actuator is configured to retract from the extended state to the retracted state.

10. The anti-lock braking system according to claim 9, characterized in that, In the second phase, the brake actuator is arranged to be locked in the second extended state.

11. The anti-lock braking system according to claim 10, characterized in that: The brake actuator is a hydraulic actuator; and The brake actuator is locked in the second extended state by controlling the entry and exit of hydraulic fluid into and out of the active chamber of the brake actuator.

12. An aircraft landing gear comprising an anti-lock braking system according to any one of claims 1 to 11.

13. An aircraft comprising the aircraft landing gear according to claim 12.

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