Abstract:
A linear actuator comprises rotary motion producing means ( 1,2,3,4,5 ); linear motion producing means ( 6,7,10,11,12 ) coupled with the rotary motion producing means for converting rotary motion to linear motion; a driven member ( 18 ) movable linearly by the linear motion providing means from a first position to a second position; and backdrive means ( 22 ) for returning the driven member to the first position. The arrangement is such that the linear motion producing means includes torque reaction means ( 13,15,16,17 ) which, in normal operation, is in an activated condition and provides a torque reaction path to enable the driven member to be moved from the first to the second position but which, in the event of a fault, is in a de-activated condition so that it no longer provides the torque reaction path and the back-drive means can move the driven member to the first position, without disengaging the rotary motion producing means from the linear motion producing means.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to linear actuators.  
         BACKGROUND OF THE INVENTION  
         [0002]    Linear electric actuators, i.e. electric actuators with push-pull outputs, are becoming employed in fluid extraction installations as a replacement for the traditional hydraulic linear actuators, typically employed to operate valves. One of the features of such an actuator, particularly for a subsea installation, is that the device operated by the actuator should return to a required position in the event of a failure, such as a loss of electrical control or a mechanical failure. For example, if the actuator operates a valve, then the valve must revert to a closed position, or, more rarely, to an open position, in the event of a failure. There are many actuators available on the market most of which employ an electric motor which drives, via a gearbox, a rotary to linear mechanism such as a screw drive and a small percentage of them have a fail-safe mechanism built in. Those that are available as fail-safe often employ an integral mechanism that ‘rewinds’ the actuator back to its original position in the event of a failure of electric power. The actuator motor winds or compresses a spring when it is powered, so that on power failure the spring returns the actuator to its original position. Typically, the motor drives the linear mechanism to an electrically powered mechanical latch to fully operate a valve, and on failure of the power supply to the latch, the spring returns the linear mechanism to its original position.  
           [0003]    A hydraulic actuator normally comprises a simple piston and cylinder and has a fail-safe mechanism provided by the compression of a coil spring so that failure of the hydraulic power source results in the actuator reverting to its initial position by virtue of the potential energy in the spring returning the piston to its original position. Such a mechanism is very simple and reliable and is thus attractive to the fluid extraction contractor, which is one reason why hydraulic actuators have been popular.  
           [0004]    The disadvantage of an electric actuator as described above is that the fail-safe mechanism is not simple and has to reverse drive the actuator through its relatively complicated mechanism, which includes the motor, gearbox and rotary to linear mechanism. Furthermore, any failure of the relatively complicated drive mechanism involving seizing or jamming will also result in failure of the fail-safe feature. It is an additional problem that the provision of a fail-safe mechanism may prevent the actuator from being driven in both directions, ie extending and retracting. This is an important feature with several benefits, e.g. there are two methods of retracting the actuator as opposed to the fail-safe only, driving in reverse may allow “freeing-up” of sticky valves and driving in reverse could also give extra capability for wire-cutting operations.  
           [0005]    As prior art in the field of linear actuators, there may be mentioned: EP-A-1,024,422; U.S. Pat. Nos. 5,195,721; 5,497,672; WO 01/14775; GB-A-2,266,943; U.S. Pat. Nos. 5,983,743; 984,260; 6,041,857; 6,253,843; GB-A-2,216,625; GB-A-2,240,376; U.S. Pat. Nos. 4,920,811; 5,070,944; 6,257,549; GB-A-2,346,429; U.S. Pat. Nos. 6,152,167; WO 01/86370; WO 01/86371; U.S. Pat. Nos. 6,176,318; GB-A-2,116,790; GB-A-2,119,172; GB-A-2,120,349; GB-A-2,122,034; GB-A-2,196,414; GB-A-2,255,866; GB-A-2,283,061; GB-A-2,291,949; U.S. Pat. Nos. 5,865,272; and 4,584,902.  
         SUMMARY OF THE INVENTION  
         [0006]    According to the present invention, there is provided a linear actuator comprising:  
           [0007]    rotary motion producing means;  
           [0008]    linear motion producing means coupled with said rotary motion producing means for converting said rotary motion to linear motion, said linear motion producing means comprising a threaded shaft and nut arrangement;  
           [0009]    a driven member movable linearly by said linear motion producing means between a first position and a second position, said driven member being coupled to one of said shaft and nut arrangement; and back-drive means for returning said driven member to said first position in the event of a fault; characterised in that:  
           [0010]    said linear motion producing means includes torque reaction means which, in normal operation, is in an activated condition and provides a torque reaction path to enable the driven member to be reversibly moved between said first and said second positions, the driven member being restrained from rotating, but which, in the event of a fault, is in a de-activated condition so that it no longer provides said torque reaction path and said back-drive means can move the driven member to said first position, without disengaging the rotary motion producing means.  
           [0011]    The nut arrangement may be rotated by said rotary motion producing means to move said shaft linearly, said shaft being coupled to said driven member.  
           [0012]    Said torque reaction means is preferably such that it prevents rotation of said shaft during normal operation while said driven member is being moved from said first position to said second position but allows rotation of said shaft in the event of a fault so that said back-drive means can move the driven member to said first position.  
           [0013]    Said torque reaction means may includes a first gear wheel, on said shaft, coupled to a further gear wheel and means which prevents rotation of the further gear wheel during normal operation so that said gear wheels and said shaft cannot rotate but which allows said further gear wheel and thereby said first gear wheel and said shaft to rotate in the event of a fault.  
           [0014]    Said shaft and said driven member may be coupled so that said driven member can only move linearly when said shaft rotates.  
           [0015]    Alternatively, said shaft may be rotated by said rotary motion producing means, said shaft being coupled to said driven member. The shaft could be coupled to said rotary motion producing means via clutch means, said clutch means providing said torque reaction path.  
           [0016]    Said rotary motion producing means could comprise a worm and wheel arrangement.  
           [0017]    Said driven member could be moved from said first to said second position against the action of said back-drive means.  
           [0018]    Said back-drive means could comprise spring means. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a longitudinal section through an example of a linear actuator according to the invention;  
         [0020]    [0020]FIG. 2 is a longitudinal section through the linear actuator but at 90° relative to FIG. 1;  
         [0021]    [0021]FIG. 3 is a cut-away, perspective view of the actuator; and  
         [0022]    [0022]FIG. 4 is a longitudinal section through another example of a linear actuator according to the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]    Referring to FIGS.  1 - 3 , an electric motor unit  1  includes an electric motor  2  which drives via a shaft  3  a worm and wheel transmission in the unit comprising a worm  4  and a wheel  5 . The wheel  5  is fastened to a rotatable member  6  to give a rotary output about an axis A to the member  6 . The member  6  is attached to tubular roller screw mounting structure  7  such that structure  7  also rotates with member  6 , structure  7  being supported by tapered-roller bearings  8  and  9 . Reference numeral  10  designates an externally threaded shaft, there being an internally threaded tubular nut  11  coupled to shaft  10  as described below, tubular nut  10  being located between member  6  and the right-hand end in the figures of structure  7  and attached to the latter. Thus, tubular nut  11  rotates with member  6 .  
         [0024]    The thread on shaft  10  is coupled with the thread of the tubular nut  11 . The arrangement is not a simple nut on threaded shaft, but a low friction planetary roller screw arrangement, there being, between the inside of the nut  11  and around the shaft  10 , a plurality of longitudinal, externally threaded planetary rollers  12  whose axes are parallel to the axis A, these rollers having threads which engage with and match those of nut  11  and shaft  10 . Thus, when the nut  11  rotates, the shaft  10  moves axially because it is prevented from rotating by a prevention of rotation locking mechanism to be described below.  
         [0025]    The prevention of rotation locking mechanism provides a torque reaction path and comprises a gear wheel  13  (formed integrally as part of a member  14 ) with a mating pinion gear wheel  15 , a gearbox  16  and an electrically operated brake  17 . The gear wheel  15  is attached to the shaft  10  (which at this point has changed from having a thread to being a circular shaft) as a result of the shaft  10  engaging with member  14  as a result of a threaded or pinned connection or an interference fit, for example. The gear wheel  13  is permanently meshed with the pinion gear wheel  15  which in turn is attached to the output of the gearbox  16 . The electrically operated brake  17  locks the output of the gearbox  16  while the brake  17  is electrically energised. Thus, the energised brake  17  locks the shaft  10  from rotating.  
         [0026]    When the motor  2  of electric motor drive unit  1  rotates and thereby causes the shaft  10  to move axially, a housing  18  of the gear wheels  13  and  15 , gearbox  16  and brake  17  assembly is also moved axially in the same direction, the housing  18  sliding along a key or spline connection at  19 , reference numeral  34  designating a bracket fastening gearbox  16  to housing  18  to close the torque reaction path. At the same time, an annular member  20  attached to the housing  18  presses against an actuator return spring  21  and compresses it. The actuator is held in position by the worm  4  and wheel  5  arrangement in the electric motor drive unit  1 , i.e. the worm and wheel arrangement cannot be backdriven and acts as a brake. In this connection the worm  4  and wheel  5  arrangement is an inefficient (high friction) arrangement for converting rotary motion from motor  2  to rotary motion about axis A, the rotary motion to linear motion converting arrangement of nut  11 , shaft  10  and rollers  12  being a relatively efficient (low friction) motion converting arrangement.  
         [0027]    In use, the axial movement of the shaft  10  is transmitted to a valve stem  22  of a valve controlled by the actuator, the valve stem being locked to housing  18  via a lock nut  23 . Reference numeral  24  designates a valve bonnet of the valve and reference numeral  25  designates a gate of the valve, which (when the valve is open as a result of axial movement rightwards in the figures of stem  22 ) allows fluid flow through an opening  26 . The valve bonnet  24  is attached to an annular housing  27  of the actuator, reference numeral  28  designating a sealing arrangement through which valve stem passes. The valve could, for example, be for controlling the operation of an underwater hydrocarbon production system.  
         [0028]    The coupling between the shaft  10  and valve stem  22  is such that shaft  10  and member  14  are able to rotate but stem  22  is not. Such a coupling is, by way of example, via a spherical roller thrust bearing  29  and a flange  30  which is part of member  14 , which flange can rotate against a ring  31  bolted to housing  18 , there being a thrust bearing between ring  31  and flange  30 .  
         [0029]    As long as power is applied to the electric brake  17 , the actuator may be controllably driven in both directions, ie extending and retracting.  
         [0030]    If power is removed from the electric brake  17 , fail-safe operation occurs as a result of de-activation of the prevention of rotation locking mechanism, allowing the pinion gear wheel  15  to rotate. This in turn permits the gear wheel  13  and the shaft  10  to rotate. The return spring  21  provides sufficient force to overcome friction in the planetary roller screw arrangement comprising nut  11 , rollers  12  and shaft  10  and the prevention of rotation locking mechanism comprising gear wheels  13  and  15 , gearbox  16  and brake  17  such that the whole mechanism back-drives via rotation of the shaft  10  inside the nut  11 . Thus, during this fail-safe axial return action, the shaft  10  is both rotating and moving axially. This is permitted as a result of the relative efficiency of the planetary roller screw arrangement comprising nut  11 , rollers  12  and shaft  10  and the relative inefficiency of the worm  4  wheel  5  arrangement, the latter arrangement acting as a brake and permitting such back-driving without the need to disengage the rotary motion producing means ( 2 ,  3 ,  4 ,  5 ) from the linear motion producing means ( 6 ,  7 ,  10 ,  11 ,  12 ) or otherwise prevent motor  2  being back-driven. Reference numeral  35  designates a ball roller thrust bearing to take up torque from spring  22 .  
         [0031]    Instead of brake  17 , gearbox  16  and gear wheels  13  and  15  alternative means could be used-for example a single component such as a toothed brake.  
         [0032]    In order to re-activate the drive, the electrical supply to the brake  17  is restored, but the electric drive motor  2  needs to know that it must drive again in the same direction as it operated the actuator in the first place. To achieve this a position sensor  32  (see FIG. 1) is fitted to determine the axial position of the shaft  10  and thus feed position information to an electronic control of the electric motor of unit  1 , the position sensor  32  being for example an inductive sensor.  
         [0033]    If desired, means may be provided at  33  (see FIG. 1) for shaft  10  to be rotated manually or by an underwater remotely operated vehicle (ROV) for example, as an override operation.  
         [0034]    In the case of the actuation of devices such as large valves, typically fitted to  2  inch and 5 inch bore pipes, the linear actuator described above is particularly practical. However, in the case of smaller bore systems, typically ½ and ¾ inch, such as those involved in the chemical injection processes employed in fluid extraction from wells, the large size of the actuator might not be suitable. The following example overcomes this problem by providing a compact fail-safe linear actuator using an electric clutch which can be powered by low power available, whilst transmitting the lower torque required to operate the actuator to open and close a valve in such a smaller bore system. However, it does need to back-drive part of the actuator drive mechanism to achieve fail-safe operation.  
         [0035]    Referring to FIG. 4, an electric motor  41  (typically  3 , phase 300 Watt) in a housing  42  drives a shaft  43  via an electric clutch assembly  44 / 45  and a worm and wheel gearbox  46 . The two sections  44  and  45  of the clutch assembly are locked together when electric power is fed to it. The clutch section  44  is attached to the drive output of the worm and wheel gearbox  46  and the other section  45  of the clutch assembly is attached to the shaft  43 . Thus, when the clutch assembly  44 / 45  (providing a torque reaction path) is electrically energised, the shaft  43  is rotated by the electric motor  41 , and when the clutch assembly  44 / 45  is electrically de-energised, the shaft  43  is detached from the drive and is free to rotate independently from it.  
         [0036]    The shaft  43  extends from the housing  42 , to provide the facility of enabling an ROV to rotate the shaft  43  in the event of emergency or during commissioning. The other end of the shaft  43  is threaded as shown at  47  as part of a planetary roller screw drive mechanism having a nut  48 . The shaft  43  terminates at the position  49 . The nut  48  of the planetary roller screw drive mechanism keyed to a carrier  50  in such a way that the nut cannot rotate. The carrier  50  is also keyed to a ring  51  which, to facilitate assembly, is screwed into the housing  42  and then locked such that the carrier  50  and nut  48  can only move axially when the shaft  43  is rotated. Thus, the planetary roller screw drive mechanism translates the rotation of the shaft  43  about axis B-B to an axial linear motion of the carrier  50 .  
         [0037]    By selecting the appropriate direction of rotation of the electric motor  41 , the carrier  50  will move to the left or right in FIG. 4. The carrier  50  is attached to an axial actuating shaft  52  of a valve  53 . A circlip  54  in the carrier  50  retains a plate  55  which mates with an assembly  56  of Belleville disc springs. Thus, when the carrier  50  moves to the right, the shaft  52  also moves to the right to operate the valve  53 , typically to open it, whilst at the same time the plate  55  compresses the spring assembly  56 .  
         [0038]    Compression of the spring assembly  56  stores the energy required for fail-safe operation. When the actuator has fully operated the valve  53 , the electric motor  41  is de-energised, but the electric clutch assembly  44 / 45  is kept energised. The relatively inefficient (high friction) worm and wheel gearbox  46  does not permit back-driving of the shaft  43  and thus the valve  53  remains operated. In the event of failure, or deliberate removal in an emergency of the electric supply to the clutch assembly, the clutch sections  44  and  45  separate, i.e. they are no longer locked together, and thus shaft  43  is free to rotate. Under these conditions, the stored energy in the spring assembly  56  will push the carrier  50  towards the left through the high efficiency (low friction) drive of the planetary roller screw drive mechanism  47 / 48  and the now free to rotate shaft  43 , thus returning the valve  53  to its original, typically closed position. Thus this process provides a fail-safe operation of the actuator, without the need to disengage the rotary motion producing means ( 41 ,  46 ) from the linear motion producing means ( 43 ,  48 ,  50 ,  52 ).  
         [0039]    In the linear actuator of FIG. 4 the actuator housing and the valve housing are separate and mated, for example by bolting, together but they could instead be integral with each other.  
         [0040]    The fail-safe linear actuator of FIG. 4 can be used for the linear actuation of any suitable device, e.g. for the operation of sub-sea devices, such as valves, for small pipe bore chemical injection for sub-sea, production fluid, extraction wells. The actuator is kept compact by the use of a clutch, a high efficiency planetary roller screw drive mechanism providing conversion of rotary to linear motion and the use of a Belleville disc spring assembly to store energy.