Patent Publication Number: US-2004040797-A1

Title: Electromechanical aircraft brake system and method incorporating piezoelectric actuator subsystem

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to brake assemblies used on mobile platforms such as aircraft, and more particularly to a brake system incorporating an electromechanical subsystem for initially moving the braking elements of an aircraft brake assembly into contact with each other, in addition to the use of a piezoelectric subsystem for modulating the pressure applied to the braking elements to more closely control the braking action.  
       BACKGROUND OF THE INVENTION  
       [0002] The use of electrical brake actuation means as a replacement for existing hydraulic actuation technology commonly used with braking systems for mobile platforms, and more particularly for aircraft, has been pursued for many years. The dominant approach has been to use several electric motors on each brake housing to apply the force and motion required to bring the brake friction elements into contact with each other. The relatively large amount of electrical power consumed by the electric motors typically requires bulky wire bundles to be installed on the landing gear of an aircraft. This is undesirable from the perspective of the weight involved, as well as the cost involved for the large and complex wire bundles. The use of large and bulky wire bundles can also contribute to landing gear noise because the wire bundles are exposed to the airstream during takeoff and landing of the aircraft.  
       [0003] Another drawback with the use of conventional electric motors for aircraft braking systems is the high power consumption of such motors. The high power consumption requires that an electrical power controller being used to control the motors be constructed in a manner sufficient to reject a significant degree of heat caused by the high power consumption.  
       [0004] Electric motor driven actuators furthermore generally have an inherently low frequency response. A braking system which is capable of modulating the friction (i.e., braking) elements at a higher frequency would be highly desirable to better respond to anti-skid braking control signals produced by a braking system used on a commercial aircraft. A braking system which provides a higher frequency response would provide an advantage over a strictly electromechanical type of braking assembly because of its ability to even more effectively apply an anti-skid braking action to an aircraft wheel.  
       SUMMARY OF THE INVENTION  
       [0005] The above and other objects are provided by a braking system incorporating an electromechanical braking subsystem and a piezoelectric braking subsystem. The electromechanical braking subsystem is used to bring one or more braking elements into contact with one or more rotating elements of a brake assembly of a mobile platform, such as a brake rotor on an aircraft, during a braking sequence. Once the braking stationary elements are in reasonably close proximity to the rotating elements, the piezoelectric braking subsystem is modulated such that a piezoelectric element thereof controllably modulates the stationary braking elements contact with the rotating elements of the brake assembly of the mobile platform.  
       [0006] The use of an electromechanical braking subsystem and a piezoelectric braking subsystem provides several significant advantages over strictly electromechanical braking subsystems. For one, the electric motor used with the electromechanical braking subsystem can be significantly smaller in size and power rating since it is not required to produce high frequency response rates. Instead, it is required to only bring the stationary braking elements into close proximity to the rotating elements of the brake assembly. Thus, a much smaller and lightweight electric motor can be used than that required with previous braking systems that rely on the electric motor to modulate the braking element.  
       [0007] The use of a piezoelectric braking subsystem provides additional benefits over strictly electromechanical braking subsystems. The piezoelectric braking subsystem, with its piezoelectric element, provides an extremely fast-acting brake mechanism by which the brake elements can be modulated at an even higher frequency than what would be allowed by an electric motor. This allows even better modulation and control over the braking elements during anti-skid braking operation.  
       [0008] Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limited the scope of the invention.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
     [0010]FIG. 1 is a side view of a portion of a wheel/brake assembly of a commercial aircraft illustrating a quantity of braking systems in accordance with a preferred embodiment of the present invention being disposed circumferentially about the circumference of the wheel;  
     [0011]FIG. 2 is a schematic representation of one of the braking systems of the present invention shown in FIG. 1, with the system shown in a disengaged position relative to a brake rotor just prior to the beginning of a braking sequence; and  
     [0012]FIG. 3 is a simplified schematic representation of the braking system of FIG. 2 after the electromechanical braking subsystem has moved a pressure plate into close proximity to the brake rotor of the wheel; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0013] The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
     [0014] Referring to FIG. 1, there is shown a wheel assembly  12  and a brake frame  14  incorporating a plurality of braking systems  10  in accordance with a preferred embodiment of the present invention. The wheel  12  and brake frame  14  are illustrated as a single wheel/brake assembly of a commercial aircraft. However, it will be appreciated that the present invention can be used with virtually any form of mobile platform incorporating wheels that require a braking action in order to stop movement of the vehicle. FIG. 1 illustrates four working apparatuses  10  being disposed circumferentially about the brake frame  14 . Again, however, it will be appreciated that a greater or lesser plurality of braking apparatuses  10  could be employed depending upon the size of the mobile platform, the degree of braking action required in order to bring the mobile platform to a stop within a given distance, the speed at which braking may begin to take place, the weight of the vehicle, as well as various other considerations. Essentially, however, each of the braking apparatuses  10  operate independently, but in unison, to quickly and effectively arrest rotational movement of the wheel of the mobile platform with which the apparatuses  10  are employed.  
     [0015] Turning to FIG. 2, a more detailed illustration of one of the apparatuses  10  is provided. The apparatus  10  generally comprises an electromechanical actuator subsystem  16  and a piezoelectric actuator subsystem  18 . The electromechanical actuator subsystem  16  is formed by an electric motor, and in one preferred form a brushless DC electric motor  20 , having an output shaft  22 . The output shaft  22  is coupled to a gear reduction system  24  which is in turn engaged with a bevel gear  26  of a ball screw subassembly  28 .  
     [0016] The piezoelectric actuator subsystem  18  is comprised of a piezoelectric element  30  which is in contact with a piston head  32 . A piezoelectric control system  33  is used to modulate the piezoelectric element  30 . The piezoelectric element  30  is disposed within a ball nut  34  of the ball screw assembly. The ball nut  34  comprises part of the electromechanical actuator subsystem  16  and is able to move linearly within a ball nut housing  36  by movement of a plurality of balls  38 . The ball nut  34  is prevented from rotating by a spline  40 .  
     [0017] The entire electromechanical actuator subsystem  16  is mounted on a piston housing  42 . A seal  44 , such as an O-ring seal  44 , provides a seal between the piston housing  42  and the nut housing  36  of the ball screw subassembly  28 . A thrust bearing  45  receives the thrust experienced by the ball screw subassembly  28 . The piston head  32  is in contact with a pressure plate  46 . The pressure plate  46  essentially functions as a braking element to apply pressure against a brake rotor  48  and to thereby effectively squeeze the brake rotor  48  between the pressure plate  46  and a backing plate  50 . The pressure plate  46 , brake rotor  50  and backing plate  48  are all housed within a torque tube  52  which is part of the brake frame  14 . It will be appreciated that the torque tube  52 , pressure plate  46 , brake rotor  50  and backing plate  48  are all components of a conventional brake system presently employed on various commercial aircraft. Additional explanation of a braking system suitable for use with commercial aircraft can be found in U.S. Pat. Nos. 5,228,541 and 6,302,244, the disclosures of which are hereby incorporated by reference into the present application.  
     [0018] In operation, the electromechanical actuator subsystem  16  is used as a “long stroke” component to initially move the piston head  32  into close proximity to the pressure plate  46 , and to take up the running clearance between the pressure plate  46 , the rotor  48  and the backing plate  50 . Preferably, the pressure plate  46  is moved just into contact with the brake rotor  48 . This is accomplished by using DC motor  20  and gear reduction subsystem  24  to drive bevel gear  26 . Driving bevel gear  26  rotationally causes linear translating movement of the ball nut  34  in the direction of arrow  54  in FIG. 2, thus bringing pressure plate  46  into close proximity with the brake rotor  48 . It will be appreciated that brake rotor  48  will be rotating about an axle centerline  56 . It will also be appreciated that if a plurality of the brake apparatuses  10  are employed, that the braking action described in connection with FIG. 2 will preferably be performed simultaneously for all of the apparatuses  10  mounted on the brake frame  14 . A portion of a tire is denoted by reference numeral  57 .  
     [0019] The piezoelectric element  30  preferably comprises a multilayer piezoelectric component comprising a plurality of secured together layers of piezoelectric elements. It will be appreciated, however, that a single piezoelectric layer of suitable length and thickness might be employed to meet the needs of a specific application.  
     [0020] By using the electromechanical actuator subsystem  16  only to move the pressure plate  46  into close proximity to the brake rotor  48 , a much less complicated electromechanical actuator subsystem  16  can be employed. In practical terms, this results in wire bundles of significantly reduced size. A less complex electromechanical actuator subsystem, with a smaller motor, also reduces the cost associated with this portion of the braking apparatus  10 .  
     [0021] Referring to FIG. 3, after the pressure plate  46  has been moved into close proximity with the brake rotor  48 , the piezoelectric element  30  is activated via a suitable signal from the piezoelectric control system  33 . The electrical signal provided by the piezoelectric control system  33  causes the piezoelectric element  30  to move in accordance with the frequency of the electrical signal output from the system  33 . This causes the pressure plate  46  to be modulated into contact with the brake rotor  48  at a desired frequency as needed to implement anti-skid braking operation.  
     [0022] The piezoelectric element  30  and its associated control system  33  thus function as a “small stroke”, high frequency means of applying the needed pressure to the pressure plate  46  to effect a braking action on the brake rotor  48 . The electromechanical actuator subsystem  16  functions essentially as a means to take up the running clearance between the pressure plate  46  and the brake rotor  48 , and thus to account for brake frame  14  and torque tube  52  component deflections and wear of the friction material associated with the pressure plate  46 , brake rotor  48  and backing plate  50 .  
     [0023] Another benefit of the present invention is that the piezoelectric element  30  functions to provide improved brake whirl vibration suppression. The use of a piezoelectric actuator  30  and an associated piezoelectric control system  33  allows easier detection of the onset of brake whirl and a ready means to quickly adjust the pressure distribution of the piezoelectric elements  30  of a plurality of brake apparatuses  10  being used on a given wheel assembly to better actively suppress this brake vibration mode.  
     [0024] The present invention thus provides a means to even more effectively provide a braking action to a wheel of a mobile platform. By incorporating piezoelectric actuator subsystem  18 , much greater, high frequency control can be exerted over the mechanical elements of a braking system to even more effectively implement anti-skid braking operation. The use of piezoelectric technology also allows smaller, less complicated electromechanical actuator subassemblies to be employed, which thus in turn reduces the size and weight of the wire bundles used on wheel assemblies.  
     [0025] The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.