Abstract:
An actuator arrangement comprises a plurality of linearly extendable actuators arranged to be driven by a common electrically driven motor, each actuator, being provided with limit stops to limit extension and/or retraction thereof, wherein the limit stops of the actuators are positioned such that a first one of the actuators has a smaller range of permitted extension than at least a second one of the actuators.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an actuator arrangement and in particular to an actuator arrangement suitable for use in an electrically driven thrust reverser system for use in moving components of the thrust reverser system between stowed and deployed positions. 
     One form of thrust reverser system includes a pair of part generally cylindrical cowls each being arranged to be driven between stowed and deployed positions by a plurality of linearly extendable actuators. Commonly, the actuators used in this application have been hydraulically driven. However, the use of electrically powered thrust reverser systems is becoming more common. 
     One form of electrically powered thrust reverser system includes a cowl moveable by a plurality of ball or roller screw type actuators, for example three such actuators may be provided. A single electrically powered motor is used to drive all of the actuators associated with the cowl, an appropriate gear box and drive transmission system being provided to distribute and transmit power to the actuators and to ensure that the actuators operate in synchronism with one another at the same speed. 
     In normal use, the actuators are driven at high speed over the majority of their length, the motor being controlled to reduce the speed at which the actuators are driven as the actuators approach their stowed and fully deployed positions so that, when the stowed and fully deployed positions are reached, movement can be safely arrested by claw stops provided in the actuators. The stops are configured so that the stops of all of the actuators associated with the cowl engage to arrest further movement simultaneously. 
     There is a requirement for the stops provided in the actuators to be able to arrest movement even if the motor fails to slow the rate of operation of the actuators as they approach the stowed and fully deployed positions, for example as a result of a control failure in the thrust reverser control system resulting in the occurrence of a so called powered runaway condition in which the motor continues to be driven at full speed as these positions are approached. 
     One way in which the stops can be designed to be able to arrest such movement is to increase the radial height of the stops so as to enable them to withstand greater loadings as would occur in such circumstances. However, doing so would result in the diameter of the tail tube of each actuator being increased. As the actuators are approximately 1 m long, increasing the diameter of the tail tube even by a relatively small amount can result in a significant increase in the weight of the actuator arrangement, which is undesirable. 
     It is an object of the invention to provide an actuator arrangement in which the problem outlined hereinbefore is overcome or of reduced effect. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an actuator arrangement comprising a plurality of linearly extendable actuators arranged to be driven by a common electrically driven motor, each actuator being provided with limit stops to limit extension and/or retraction thereof, wherein the limit stops of the actuators are positioned such that a first one of the actuators has a smaller range of permitted extension than at least a second one of the actuators. 
     By arranging the limit stops in this manner, in normal use, only the limit stops of the said first one of the actuators will operate to arrest movement as the stowed and/or fully deployed positions are reached, the limit stops of the other actuators being redundant, in use, but useful during maintenance and fitting operations to avoid over extension of the actuator which may result in parts thereof becoming disengaged from one another. Consequently, only the limit stops of this one of the actuators need to be designed to withstand the increased loadings that may occur in the event of a control failure, and hence only the tail tube diameter of this one of the actuators needs to be increased. It will be appreciated, therefore, that a saving in the weight of the actuator arrangement can be achieved. 
     Conveniently, the limit stops of the first actuator engage to arrest movement both when the actuator reaches its fully deployed and stowed positions. 
     The first actuator is preferably provided with a position sensor, for example in the form of a linear variable differential transducer (LVDT), the output of which can be used by the control system of the thrust reverser actuator system to provide an indication of the position of the cowl thereof. 
     Preferably a series of drive shafts are provided to transmit drive to at least some of the actuators. The drive shafts may be capable of undergoing limited torsional movement. In such an arrangement, when the stops of the first actuator arrest movement thereof, limited continued movement of the remaining actuators may occur, such movement resulting in ‘winding up’ or torsional movement of the drive shafts. Such winding up of the drive shafts will apply a braking load to these actuators, preferably arresting movement thereof before the limit stops of these actuators engage. 
     The limit stops conveniently comprise claw stops. 
     The limit stop of the first actuator operable as the actuator reaches its fully deployed position is conveniently compliant. The limit stop operable as the first actuator reaches its stowed position may also be compliant, but this need not always be the case as the provision of larger limit stops capable of withstanding the loadings applied in the event of control failure leading to a powered runaway condition as the actuator reaches its stowed position need not involve the use of an increased diameter tail tube. 
     In one arrangement, the actuator includes a screw shaft of hollow form, and the compliant stop is provided by locating a torsion bar within the screw shaft, a stop member being associated with the torsion bar, the stop member projecting through a slot formed in the shaft and engageable by a stop associated with a nut translatable along the screw shaft. Stop members may be provided at or adjacent both ends of the torsion bar. Alternatively, a stop member may be provided at or adjacent just one end thereof. In either case, winding up of the torsion bar occurs, in use, when the nut reaches its stowed and/or fully deployed position, thus slowing and arresting further movement. 
     In another arrangement, at least one of the compliant stops may comprise a stop member spring biased towards a stop position such that, in use, once the stop position is reached, the spring biasing applies a braking load prior to arresting actuator movement. 
     The stop member may be mounted on the actuator shaft by means of an acme screw thread. The spring could comprise, for example, a coiled spring or, alternatively, may comprise at least one disc spring. 
     If desired, a spring biased stop may be provided to arrest actuator movement in one direction, a torsion bar arrangement being used to arrest movement in the opposite direction. 
     The invention also relates to an electrical thrust reverser system comprising an extendable actuator driven by an electric motor, the actuator including a rotatable screw shaft and a translatable nut, the actuator being provided with limit stops to limit the permitted range of extension of the actuator, wherein at least one of the limit stops is compliant. 
     The compliant limit stop may comprise a stop member biased by a spring towards a stop position. The stop member may be mounted upon the screw shaft of the actuator by means of an acme screw thread. The spring may comprise a coil spring, or alternatively may comprise at least one disc spring. 
     Alternatively, the compliant limit stop may be provided by locating a torsion bar within the screw shaft, a stop member being associated with the torsion bar, the stop member projecting through a slot formed in the shaft and engageable by a stop associated with the nut translatable along the screw shaft. Stop members may be provided at or adjacent both ends of the torsion bar. Alternatively, a stop member may be provided at or adjacent just one end thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will further be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic representation of an actuator arrangement in accordance with one embodiment of the invention; 
         FIGS. 2 and 3  illustrate one form of compliant stop in accordance with an embodiment of the invention and suitable for use in the arrangement of  FIG. 1 ; and 
         FIG. 4  illustrates another form of compliant stop. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1  there is illustrated an actuator arrangement for use in a thrust reverser system. The actuator arrangement comprises an electrically driven motor  10  arranged to drive a first actuator  12  through a gearbox  14 . The first actuator  12  comprises a screw shaft  16  mounted to be rotatable by the operation of the motor  10 , but held against axial movement by bearings (not shown). A translating nut  18  cooperates with thread formations formed on the screw shaft  16 , the nut  18  being held against rotation but being free to undergo translational motion. It will be appreciated that, in use, operation of the motor  10  to drive the screw shaft  16  results in translation of the nut  18 , the direction of movement of the nut  18  being dependent upon the direction of rotary motion of the screw shaft  16 . In order to reduce frictional forces resisting movement of the nut  18 , the cooperation between the nut  18  and thread formations is conveniently in the form of a ball-screw or roller-screw type cooperation. 
     The translating nut  18  is connected to or forms part of a tail tube  20  which is formed with a mounting  22  whereby it is connected to a generally part cylindrical thrust reverser cowl  24 . 
     The motor  10  is controlled, in use, by an electrical trust reverser actuator system controller (not shown) which receives information representative of the position of first actuator  12  and the cowl  24  from a LVDT  26  driven from the gearbox  14 . 
     The actuator arrangement further comprises a pair of second actuators  28  which are identical to one another, the first actuator  12  being located between the second actuators  28 . The second actuators  28  are driven from the gearbox  14  through drive shafts  30  and gear arrangements  32 . Each second actuator  28 , like the first actuator  12 , comprises a rotatable screw shaft  34  which cooperates through a ball-screw or roller-screw type arrangement with a translatable nut  36 . The nut  36  is connected to or forms part of a tail tube  38  which, like the tail tube  20  of the first actuator  12 , is connected to the cowl  24 . 
     The first actuator  12  is provided with limit stops operable to determine the range of extension thereof. The limit stops comprise first claw stop members  40   a ,  40   b  associated with the shaft  16  and nut  18 , respectively, and engageable with one another to limit movement of the nut  18  in the stow direction, and second claw stop members  42   a ,  42   b  associated with the shaft  16  and nut  18 , respectively, and engageable with one another to limit movement of the nut  18  in the deploy direction. Similarly, the second actuators  28  are each provided with corresponding first and second claw stop members  44   a ,  44   b ,  46   a ,  46   b . However, the positioning of the stop members  44   a ,  44   b ,  46   a ,  46   b  of the second actuators  28  is chosen such that the permitted range of extension of the first actuator  12  as determined by the first and second stop members  40   a ,  40   b ,  42   a ,  42   b  is smaller than the permitted range of movement of the second actuators  28  as determined by the stop members  44   a ,  44   b ,  46   a ,  46   b . In  FIG. 1 , the permitted range of extension of the first actuator  12  is denoted by ‘A’, and the range of extension of the second actuators  28  is denoted by ‘a’, these dimensions being exaggerated in  FIG. 1  for clarity. It will be apparent that range ‘a’ extends beyond range ‘A’ at both limits of movement. Thus, as the actuator arrangement is driven towards and reaches both its stowed limit position and its deployed limit position, the claw stop members  40   a ,  40   b ,  42   a ,  42   b  of the first actuator  12  engage to arrest movement, the stop members  44   a ,  44   b ,  46   a ,  46   b  of the second actuators remaining out of engagement with one another. As only the stop members of the first actuator  12  serve to arrest movement, in use, only these stops need to be of sufficiently large dimensions to be able to withstand the loadings applied in a powered runaway event. The stop members of the second actuators serve primarily to prevent overextension of the second actuators, and associated disengagement of components thereof, during servicing, fitting and maintenance operations, and so can be of relatively small dimensions as the loadings applied thereto are relatively small. 
     In normal use, when the cowl  24  is to be moved to its deployed position, the motor  10  is operated to drive the first and second actuators  12 ,  28  towards their deployed positions. As the deployed position is approached, as sensed by the LVDT  26 , the controller reduces the operating speed of the motor  10  such that the rate of extension of the actuators  12 ,  28  is reduced, extension being arrested when the fully deployed position is reached by engagement of the second stop members  42   a ,  42   b  of the first actuator  12  with one another. The stop members of the second actuators  28  remain spaced apart from one another. 
     To return the cowl  24  to its stowed position, the motor  10  is driven in the reverse direction. As the stowed position is approached, the motor operation is controlled to slow the actuator speed, and movement is arrested once the stowed position is reached by the cooperation of the first stop members  40   a ,  40   b  of the first actuator  12 . The stop members of the second actuators again remain spaced apart from one another. 
     In the event of a control failure leading to a powered runaway condition during movement in either the stow direction or the deploy direction, the motor  10  will drive the actuators  12 ,  28  at high speed, and the appropriate ones of the stop members  40   a ,  40   b ,  42   a ,  42   b  of the first actuator  12  will engage one another (depending upon the operating direction of the actuator when the powered runaway condition arises) to arrest the movement of the first actuator  12  when its limit position is reached. Continued movement of the second actuators  28  will result in torsional movement or winding up of the drive shafts  30 . Such winding up of the drive shafts  30  will apply a braking force to the second actuators  28  which will result in movement of these actuators being slowed and arrested before the stop members thereof engage, and subsequently the drive shafts  30  will return to their unwound condition. 
     It will be appreciated that as only the stop members  40   a ,  40   b ,  42   a ,  42   b  of the first actuator  12  serve to arrest actuator movement at the stowed and deployed positions, only these stop members need to be sufficiently large to bear the loading applied in a powered runaway event, thus only the first actuator  12  needs to have a tail tube of increased diameter. Consequently, significant weight savings can be made. 
     In the arrangement described hereinbefore, the claw stop members are all fixed stops. Rather than provide fixed stops, it is thought that the use of compliant stops to reduce the actuator operating speed before it is finally arrested will allow a reduction in the radial dimensions of the limit stops provided on the first actuator  12 . Consequently, a small diameter tail tube may be used which may result in further weight savings being possible. 
       FIG. 2  illustrates one way of providing the first actuator  12  with compliant stops. As illustrated in  FIG. 2 , the screw shaft  16  is of hollow form. Within the screw shaft  16  is located a multi-part torsion bar  50  to which are secured pins  52 , the pins  52  each passing through slots  54  formed in the screw shaft  16 , the projecting parts of the pins  52  defining or having the stop members  40   a ,  42   a  secured thereto. As best seen in  FIGS. 3   a  and  3   b , the slots  54  are each shaped so as to allow the pins  52  to undergo angular movement about the axis of the screw shaft  16 . Although illustrated as lying in the same plane as one another, the pins  52  may be angularly displaced relative to one another. The torsion bar  50  is prestressed in that, during assembly, the torsion bar  50  is introduced into the screw shaft from the free end thereof and one of the pins  52  furthest from the free end of the shaft  16  is introduced through the corresponding slot  54  and through an opening in the bar  50 . A torque is then applied to the bar  50  to prestress the bar  50  by a predetermined amount, and the other pin  52  is then introduced axially into its respective slot  54  and coupled to the bar. The torque may be applied by using an appropriate tool to engage, for example, flats formed at an end of the bar  50 . 
     In use, when the actuator is being driven towards and approaches its deployed position, the stop member  42   b  moves into engagement with the stop member  42   a , urging the corresponding pin  52  to undergo angular movement as permitted by the slot  54 . The arrangement is such that, in order for such angular movement to occur, further stressing of the torsion bar  50  must occur, the other pin  52  already being located hard against the corresponding end of its slot  54  and so being unable to undergo further angular movement. The further stressing of the torsion bar  50  serves to apply a braking load, slowing and subsequently arresting movement of the actuator  12 . Similarly, movement of the actuator  12  towards its stowed position is slowed and arrested by stressing of the torsion bar  50 , in this case by engagement between the stop members  40   a ,  40   b.    
     The arrangement illustrated in  FIGS. 2 and 3  uses the torsion bar  50  to apply a braking load to slow and arrest movement in both directions. However, if desired, one end of the torsion bar  50  could be anchored against movement relative to the screw shaft  12 , for example by a pin received in a correspondingly sized opening formed in the screw shaft  16 , so that the torsion bar  50  serves to slow and arrest movement in only one direction. Such an arrangement may be advantageous where there are concerns that the formation of a slot  54  may unduly weaken the screw shaft  16 . 
     Although the arrangement illustrated in  FIG. 2  has two separate pins  52 , an arrangement is envisaged in which one of the pins  52  (the one adjacent the free end of the screw shaft  16 ) is integral with the torsion bar  50 , the prestressing of the torsion bar being achieved by applying a torque to the other end thereof prior to introduction of the corresponding pin  52 . 
       FIG. 4  shows an arrangement in which the compliant stop is formed by a stop member  60  mounted upon the screw shaft  16  by an acme screw thread  62 . The stop member  40   a  is formed on the stop member  60 . A spring  64  is engaged between the stop member  60  and a shoulder  66  formed on the screw shaft  16 . 
     In use, as the actuator  12  is moved towards its stowed position, the stop member  40   b  engages the stop member  40   a . The acme screw thread  62  allows the stop member  60  to move relatively freely on the screw shaft  16 , but movement of the stop member  60  is resisted by the spring  64 . It will be appreciated, therefore, that upon engagement of the stop members  40   a ,  40   b , movement of the actuator is slowed and subsequently arrested. The spring  64  is illustrated as comprising a series of disc springs, but could alternatively comprise a coiled spring, for example. 
     The arrangement shown in  FIG. 4  is intended for use in slowing and arresting movement to the stowed position, but could alternatively or additionally be used to slow and arrest movement to the deployed position. Further, the  FIG. 4  arrangement could be used in combination with, for example, a torsion bar arrangement. For example, the torsion bar arrangement could be used to slow and arrest movement to the deployed position whilst the spring biased arrangement is used to slow and arrest movement to the stowed position. 
     The position of arrangements using compliant stops is thought to allow a reduction in the radial size of the stops, thus permitting a smaller size of tail tube to be used. 
     It will be appreciated that a wide range of modifications and alterations may be made to the arrangements described hereinbefore without departing from the scope of the invention.