Patent Publication Number: US-2007120382-A1

Title: Coupling drive from an actuator to a mechanism

Description:
This apparatus relates to apparatus for coupling operational drive mechanically by way of a cable from an actuator, such as a handle or an electrical actuator mechanism to an actuable mechanism such as a door latch. It is particularly useful in automotive applications.  
      In modern vehicles such as passenger cars each of the side doors and the tailgate has an electrically-controlled latch, and there are usually systems for selective manual or electrical latch operation, to open the doors or the tailgate. Manual operation of the door latch is usually through the use of interior and exterior door handles connected by cables to the latch actuator. Such an arrangement is described for example in my publication WO 98/27301.  
      Safety standards such as UN Regulations 94 and 95 and EC Regulation No. 11, Amendment No. 2 require that car doors do not open accidentally upon impact of the vehicle, or for example if the vehicle rolls or spins following a side impact. At least one of the doors should however be capable of being opened manually after such an accident. When a vehicle crashes, spins or rolls, it has been found that accelerations of up to about 30 G may be experienced, this value being incorporated in the EC safety standard, and of course these accelerations may occur along any axis of the vehicle. Such accelerations can be sufficient to operate a door handle causing inadvertent opening of the door.  
      To prevent exterior door handles turning when a vehicle undergoes severe acceleration, the conventional approach has been to provide counterweights adjacent the door handle, as shown in  FIG. 1  of the accompanying drawings. (In this specification, references to acceleration are intended to include deceleration, i.e. a sudden shock along any direction). Typically, a counter-weight is rotationally coupled to the door handle using a spring arrangement, so that inertial movement of the door handle is countered by corresponding inertial movement of the counter-weight, in the event of abnormal accelerations. Due to the normal orientation of door handles on a vehicle, this would normally be relevant when the vehicle suffers a side impact or rolling about its main axis.  
      The problem with providing counterweights is that this adds to the weight of the vehicle and to the complexity and cost of manufacture of the door handle arrangement.  
      Most door handles have a return spring, and we have found that the maximum necessary force for lifting a typical handle is 10 N. In order to meet the safety standards described above, several vehicle manufacturers use harder springs, requiring say 35 N to open the handle—leading to unnecessary effort from the user. This has also led to the use of power-release mechanisms.  
      Alternative solutions have included providing the latch with internal inertia-responsive levers or other components, so that the latch is locked against opening movements, when the latch experiences undue acceleration in a specific predetermined axis. These arrangements introduce complexity and cost into the latches, and moreover, by their very nature, they cannot be incorporated retrospectively into latches of existing design.  
      Accordingly the purpose of the present invention is to overcome these disadvantages with prior arrangements, whilst at the same time reducing the cost of the system, preferably in a way which is compatible with existing systems.  
      The present invention provides apparatus for coupling operational drive mechanically by way of a cable from an actuator to a mechanism such as a door latch, comprising: a frame for mounting in a fixed position relative to the door latch and the actuator; an inertia lever pivotally mounted on a bracket constrained to slide along a predetermined path within the frame, the inertia lever having a centre of mass distant from its pivotal mounting on the bracket; a catch constrained to slide along a predetermined path within the frame, following the path of the inertia lever; means for connecting the bracket to the actuator; and means for connecting the catch to the said mechanism such as a door latch; the apparatus being configured such that when the inertia lever is at a position at which it locks against the catch to couple drive from the actuator to the cable, its centre of mass is shifted transversely from a line through its pivotal mounting on the bracket parallel at that point to the path of the inertia lever; such that when no driving force is applied from the actuator there is an axial gap between mutually-engaging surfaces of the catch and the inertia lever, but when the actuator applies normal driving force, the inertia lever slides to close that gap and then to lock against the catch; and such that axial acceleration of the inertia lever above a predetermined threshold, corresponding to an unsafe fault condition, causes the off-axis inertia lever to swing to move its centre of mass closer to axial alignment with its pivotal mounting point, sufficiently to bypass the catch by the time the gap has closed, whereby to decouple the operational drive.  
      The actuator may be a conventional door handle, or it may be an electrical actuator.  
      The apparatus may be provided entirely separately from conventional latches and conventional door handles, as a self-contained unit which may be connected in line to the drive cable. Alternatively, the apparatus may be formed adjacent, or integrated with, an electrical actuator. Either way, the apparatus embodying the invention is capable of ensuring that the door is not opened by erroneous operation of the latch from the actuator, in the event of excessive accelerations in the actuator, in any axis and in any direction.  
      It will be appreciated that the invention differs from inertia-responsive latch arrangements of prior publications, since the apparatus of the invention is responsive to the degree of acceleration applied from the door handle or other actuator. This allows the coupling apparatus and also the handle to be placed in any desirable location and at any desirable orientation, regardless of the axes of impacts or accelerations. This confers extra reliability on the invention, and greater freedom in vehicle design.  
      The invention avoids the need for hard return springs on the door handle, leading to increased user comfort.  
      The invention is also advantageous because it can be made with relatively few components, almost all of which can be made of plastics materials. This enables the invention to be made relatively inexpensively and of light weight. Vehicles fitted with the invention no longer require door handles to be counter-balanced, and this can significantly reduce vehicle weight.  
      The invention also provides a method of decoupling drive from an actuator to a mechanism such as a door latch in the event of abnormal acceleration such as upon impact, using mechanical coupling apparatus therebetween, in which the coupling apparatus couples the drive when operated normally but decouples the drive whenever the acceleration of the drive applied by the actuator exceeds a pre-determined threshold. 
    
    
      In order that the invention may be better understood, preferred embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1  shows a conventional door handle assembly including a counter-weight;  
       FIGS. 2   a  to  2   c  show coupling apparatus according to a first embodiment of the invention, with  FIG. 2   a  showing the apparatus at rest,  FIG. 2   b  showing it coupling drive under normal operation, and  FIG. 2   c  showing it decoupled under excessive acceleration;  
       FIGS. 3   a  to  3   d  show apparatus according to a second embodiment of the invention, with inertial components provided in tandem, in which  FIG. 3   a  shows the apparatus at rest,  FIG. 3   b  shows a first stage of normal operation,  FIG. 3   c  shows a second stage of normal operation, and  FIG. 3   d  shows decoupling under abnormal operation;  
       FIGS. 4   a  and  4   b  show apparatus according to a third embodiment of the invention, in which a ratchet is provided to lock movement of the door handle, in which  FIG. 4   a  shows the apparatus at rest, and  FIG. 4   b  shows the apparatus under abnormal operation, with drive decoupled and also with the door handle cable locked against the ratchet;  
       FIGS. 5   a  and  5   b  show apparatus according to a fourth embodiment of the invention, in which ratchets are provided on both sides, in which  FIGS. 5   a  shows the apparatus at rest, and  FIGS. 5   b  shows the effect of a transverse acceleration applied to the apparatus itself;  
       FIG. 6  is a perspective view of apparatus according to the first embodiment of the invention, but slightly modified with regard to the connection of the door handle cable, with the lid of the apparatus housing removed to show internal components;  
       FIG. 7  shows the underside of the lid of the apparatus of  FIG. 6 ;  
       FIGS. 8   a  and  8   c  show apparatus according to a fifth embodiment of the invention, in which  FIG. 12   a  illustrates normal safe door handle operation,  FIG. 12   b  illustrates door handle blocking due to excessive acceleration of the door handle cable, or lateral shock on the coupling apparatus in one direction, and  FIG. 12   c  illustrates door handle blocking on lateral shock applied to the coupling apparatus in the opposite direction;  
       FIGS. 9   a  and  9   b  show apparatus according to a sixth embodiment of the invention, with dual inertia levers in tandem, in which  FIG. 9   a  shows normal safe door handle operation and  FIG. 9   b  shows door handle blocking on lateral shock applied to the coupling apparatus;  
       FIG. 10  is a schematic view of a system embodying the invention for use between a door handle and a latch;  
       FIG. 11  is schematic view of a further system embodying the invention, for use with both an electrical actuator and a door handle; and  
       FIG. 12  is a schematic view of a further system embodying the invention, for use with an electrical actuator but without a manual door handle being connected. 
    
    
      As shown in  FIG. 1 , a conventional door handle  10  is mounted pivotally on a bar  11  running lengthwise of the vehicle on the vehicle door. Arms  101 ,  102  link the handle  10  to the bar  11 . A massive counter weight  12  is also mounted pivotally on the bar  11 . A coil spring  13  is mounted on the bar  11  to interconnect the counterweight and the handle rotationally so that excessive acceleration of the vehicle about the longitudinal axis of the vehicle does not have the effect of turning the handle  10 .  
      As explained above, one of the advantages of the present invention is to avoid the need for such a counterweight, by ensuring that drive from the door handle  10  to a door latch is decoupled in the event that there is such an excessive acceleration of the vehicle. This might occur for example upon side impact of the vehicle, or rolling of the vehicle about its longitudinal axis, or spinning about a vertical axis.  
      A first embodiment of the coupling apparatus according to the invention is shown in  FIGS. 2   a  to  2   c . This apparatus is shown also in  FIGS. 6 and 7 , which are described in greater detail below.  
      In this embodiment, a drive cable  21  is connected to a latch in a vehicle door, and a further drive cable  25  is connected to the door handle  10 , which could be similar to that of  FIG. 1  but without the counter-weight. An elongate box shaped housing  23  is in the form of a shell, with a lid, shown more clearly in  FIGS. 6 and 7 . Terminal sleeves  22  and  24  are fixed to this housing  23  at respective ends, for guiding the ends of the cables  21  and  25 . The cables may for example be Bowden cables with sheaths (not shown). Alternatively rods or strings or any other suitable couplings could be used.  
      A coupling catch  27 , generally L-shaped, is mounted pivotally at one end to the end nipple  26  of the cable  21 . A boss  261  projecting from the coupling catch  27  rides along an elongate groove  231  formed in the base of the housing  23 , so that the pivotal point of the coupling catch slides axially along the housing. An elongate rectangular boss  28  projects from another limb of the coupling catch  27 , and is guided along an elongate groove  29  in the lid  60  of the housing. In this way, the coupling catch  27  is constrained to move lengthwise with a sliding motion. The coupling catch  27  has an operative surface  281  for engagement with a corresponding operative surface  311  of an inertia lever  31 .  
      The inertia lever  31  is in the shape of a comma, and is pivotally mounted at one end to the nipple  30  at the end of the cable  25  which connects to the door handle  10 . A round boss  301  projecting from the inertia lever  31  slides along the elongate groove  231 , to guide it longitudinally. Also, a circular boss  34  projecting upwardly from the inertia lever  31  is guided along an elongate track  341  in the housing lid  60 . Together, these bosses  34 ,  301  constrain the inertia lever to longitudinal movement.  
      A metallic, massive cylinder  33  typically weighing about 3 g is held within the inertia lever  31 , in a complementary recess, remote from its pivoted end, so that the overall centre of mass of the inertia lever  31  is remote from its pivot point. In one example, it is 15 mm from the pivot point. The cylinder  33  could of course be of any material, preferably substantially denser than the material from which the other components are made, apart from the spring  32  described below.  
      A torsion coil spring  32  disposed around the boss  301  biases the inertia lever  31  clockwise in  FIG. 2   a , such that its centre of mass is disposed transversely off the longitudinal axis through the pivot point  30 . At the position shown in  FIG. 2   a , a flat surface of the inertia lever  31  abuts against a flat surface of the coupling catch  27 , to prevent its continued clockwise rotational movement.  
      A longitudinal gap exists between the operative surfaces  311  and  281  of the inertia lever and the coupling catch respectively, in the rest position shown in  FIG. 2   a . This allows for inertial decoupling under fault conditions, as described below.  
      A finger  35  formed as a projection in the housing lid  60 , and shown in  FIGS. 2   a  and  7 , guides the boss  34  of the inertia lever into one or other of two parallel channels  36  and  37 , defined in the lid  60 . Once the boss  34  has moved past the tip of the finger  35 , it cannot change channels between channels  36  and  37 : in this way, the apparatus stays either coupled or decoupled until the cable  25  is released.  
      Under normal operation, where the acceleration applied to cable  25  is below a predetermined threshold which may for example be 2 G, corresponding to a vehicle impact at about 5 km per hour, but could be in a range of 2 G to 3 G or 1.5 G to 4 G, tension on cable  25  pulls the inertia lever  31  towards the right, to move it from the position shown in  FIG. 2   a  to that shown in  FIG. 2   b . The value of 2 G is equivalent to a spring force of 0.2N acting on the inertia lever at a radius of 15 mm from the pivot point. Once the gap has closed between the operative surfaces  311  and  281 , the inertia lever is locked against the coupling catch and drags it lengthwise up the housing to the final position shown in  FIG. 2   b . This causes operation of the latch, since cable  21  is pulled. Release of the door handle  10  causes the apparatus to return to its rest position shown in  FIG. 2   a , due for example to a return spring in the latch pulling back the cable  21 .  
      Under conditions of abnormal acceleration, above the predetermined threshold on cable  25 , the inertia lever  31  swings counter clockwise, so that the centre of mass tends to move towards and usually past the longitudinal axis passing through its pivot point. As the inertia lever swings counter clockwise, so it is moved slidingly along the housing, closing the gap between operative services  311  and  281 . If the acceleration on cable  25  is exactly at the predetermined threshold, the inertia lever  31  would have swung counter clockwise just sufficiently for surface  311  to clear surface  281  as it passes it, so that the components do not lock together. At accelerations above the threshold, the inertia lever will have swung even further than this. Accordingly, under fault conditions, the inertia lever continues its longitudinal sliding movement, to the position shown in  FIG. 2   c , at which the apparatus is decoupled. It will be seen that boss  34  slides along the lower channel  37  of the two possible channels  36 ,  37 , under this fault condition. Once the tension on cable  25  is released, the coupling apparatus resets itself to the position shown in  FIG. 2   a.    
      A coupling apparatus very similar to that of  FIGS. 2   a  to  2   c  is shown in perspective view in  FIGS. 6 and 7 , where like reference numerals are used for like components. The main difference is that in  FIG. 6  the nipple  30  at the end of cable  25  is held in a bracket with a separate boss for mounting the torsion spring  32 . Although this introduces complexity into the inertia lever  31 , it can facilitate assembly of the components.  
      A second embodiment of the invention is shown in  FIGS. 3   a  to  3   d . This operates in a similar fashion to the first embodiment, except that there are two inertia levers  312 ,  313  which operate in opposite rotational directions. Correspondingly, there are two coupling catches  271 ,  272  facing each other; with operative engaging surfaces  282  and  283 . In this example, a single torsion spring  321  is shared by both inertia levers, although of course each lever could have its own spring. Normal operation is shown in  FIGS. 3   b  and  3   c , with both inertia levers coupling to their respective coupling catches. Abnormal operation is shown in  FIG. 3   d , in which excessive acceleration causes both inertia levers to move closer to the central longitudinal axis, and to bypass the catches.  
      A third embodiment of the invention is shown in  FIGS. 4   a  and  4   b . In addition to the decoupling of the drive by virtue of the rotation of the inertia lever  43 , which corresponds to lever  31  of  FIGS. 2   a  to  2   c , abnormal acceleration of cable  25  causes motion of the door handle (or other actuator) to be locked against the housing  23  and therefore the vehicle. Along one side of the housing there are notches  40 ,  41  and  42  forming a ratchet longitudinally of the housing. An engagement surface  44  on the inertia lever, opposite to the operative surface which engages the coupling catch, is shaped so as to lock against one or other of the notches of the ratchet. The engaging surfaces are shaped so as to retain the inertia lever  43  in its locked position against the ratchet, provided tension is maintained on cable  25 . Depending on the degree of excessive acceleration above the predetermined threshold, the apparatus will lock the door handle (or other actuator) in one or other of the ratchet notches  40 ,  41  and  42 . Engagement against the first notch  40  is shown in  FIG. 4   b.    
      It will be appreciated that a single notch, or any number of notches could replace the ratchet shown in  FIGS. 4   a  and  4   b.    
      The arrangement of this third embodiment shown in  FIGS. 4   a  and  4   b  provides an extra fail-safe mechanism, against faulty operation of the door latch. Depending on the orientation at which the coupling apparatus is secured to the vehicle, it is conceivable that, in exceptional circumstances, an accelerating force acting transversely to the housing could cause the inertia lever to engage against the coupling catch despite accelerative tension on cable  25 . The arrangement with the notch or ratchet should ensure that the inertia lever is unable to return to a position at which it locks against the coupling catch and re-engages the drive between cables  21  and  25 .  
      A fourth embodiment of the invention is shown in  FIGS. 5   a  and  5   b . This is similar in operation to that of  FIGS. 4   a  and  4   b , except that in this example there are two inertia levers and two coupling catches, operating in tandem, as described with reference to  FIGS. 3   a  to  3   d . Also, there are correspondingly two ratchets, one on each side of the housing. In the event of a transverse acceleration  50  on the housing, causing the lower inertia lever  43  to swing downwardly as shown, despite tension on cable  25  in the direction of the arrow  51 , inadvertent coupling is prevented by the engagement of inertia lever  43  against the lower ratchet, by virtue of the engagement of its operative surface  44  with notch  40 .  
      In any of the embodiments, the coil spring  32 ,  321  could be replaced with some alternative means for ensuring the inertia lever is aligned correctly to couple with the coupling lever. With the single lever example of  FIG. 2 , it may be sufficient to rely on the weight of the lever itself, provided the coupling apparatus is mounted at the correct orientation to the vehicle i.e. the reverse of that shown in  FIG. 2 .  
      A fifth embodiment of the coupling apparatus according to the invention is shown in  FIGS. 8   a  to  8   c . This functions in a similar way to the third embodiment, shown in  FIGS. 4   a  and  4   b , in that the operation of the door handle is blocked in the event of an excessive side impact on the frame  23  in one direction.  
      As shown in  FIG. 8   a , a coupling catch  827  has its pivot point slidable axially along a groove  231 . An elongate arm of the coupling catch  827  has upwardly projecting bosses  807  and  804  which are guided to slide axially along an elongate groove  836  formed in the lid  60 . A further boss  802 , adjacent boss  804 , is arranged to slide along a parallel and adjacent guiding slot  837  formed in the lid  60 . The face of the coupling catch  827  which faces the lid  60  is formed with a recess between the bosses  807  and  804 , for accommodating the inertia lever  831 .  
      Transversely extending abutment surfaces  801  and  802  are formed in the lid  60 , in order to block the movement of the door handle cable  25  in the event of excessive lateral impact or acceleration on the frame  23 , as described below. A transversely extending, but angularly inclined, abutment surface  808  on the coupling catch  827  is formed as a shoulder, defining the forward wall of the recess mentioned above, and this serves as an abutment surface for locking the inertia lever  831  against the coupling catch  827  under normal operation for door release.  
      A dual return spring  806  is mounted over the pivot bracket for the inertia lever  831 , in place of the coil spring  32  of  FIG. 2 . This resiliently biases the inertia lever  831  to the middle position as shown in  FIG. 8   a . It causes the lever  831  to return to that middle position if it has swung to either of the rotational positions shown in  FIGS. 8   a  and  8   c.    
      Normal operation of the coupling apparatus of  FIG. 8   a  will now be described. Provided the acceleration applied by the door handle to its cable  25  is less than the predetermined threshold, for example 2 G, the inertia lever  831  will not have swung counter clockwise sufficiently for it to bypass the abutment surface  808  on the coupling catch  827 . Thus the tendency for the massive cylinder  33  to move towards the longitudinal axis through the pivot point of the inertia lever is sufficiently countered by the clockwise spring force of the spring  806 . Once the gap between the respective engagement surfaces of the inertia lever and the coupling catch has closed, upon translation of the cable  25 , the two elements lock together and allow the latch to operate to open the door, as described with reference to other embodiments of the invention. At the same time, bosses  834   a  and  834   b  projecting from the surface of the inertia lever which faces the lid  60  slide axially along the slot  837 .  
      If the acceleration applied to the cable  25  exceeds the threshold, then, as shown in  FIG. 8   b , the inertia lever  831  swings counter clockwise so that it bypasses the abutment surface  808  on the coupling catch by the time the gap between its engagement surfaces has closed. The inertia lever is then free to slide axially until the forward boss  834   b  abuts against the abutment surface  802  in the lid  60 . This engagement of the boss  834   b  is shown in  FIG. 8   b . The arcuate shape of the abutment surface  802  locks the boss  834   b  against counter clockwise movement until such time as tension on the cable  25  is released. At that point, the dual return spring  806  moves the inertia lever back to its middle position. The effect of this is to block movement of the door handle cable  25 .  
      Under very exceptional circumstances, the acceleration applied to the cable  25  may be below the predetermined threshold, even though the vehicle is impacted, for example in a direction  50  transverse to the frame  23 . In this situation, unsafe operation of the door release mechanism is prevented by blocking the door handle cable  25 , as shown in  FIGS. 8   b  and  8   c  for different directions of the acceleration or impact transversely of the frame  23 . It will be understood from the description above of  FIG. 8   b  that acceleration of the frame  23  on the axis  50  would have the effect of swinging the inertia lever  831  either clockwise or anticlockwise. Counter clockwise swinging would cause it to block as shown in  FIG. 8   b . Clockwise swinging motion would cause it to move to the position shown in  FIG. 8   c , at which the rear boss  834   a  on the inertia lever slides into abutment against the abutment surface  831  on the lid  60 . Again, once the acceleration or impact has stopped, and tension on the cable  25  has been released, the dual return spring  806  will return the inertia lever to its middle position.  
      A sixth embodiment of the invention is shown in  FIGS. 9   a  and  9   b , and this is similar in operation to that of the fifth embodiment shown in  FIGS. 8   a  to  8   c , except that there are dual inertia levers  931   a  and  931   b  operating in tandem. There is also a pair of coupling catches  927   a  and  927   b , pivoted on a common mounting point which slides along an axial slot  931  formed in the base of the housing. A pair of coil springs  906   a ,  906   b  operate independently on the same mounting point, to resiliently bias the respective inertia levers into locking engagement against their respective coupling catches, in a similar way to the second embodiment shown in  FIG. 3 .  
      Each coupling catch has an elongate projection (not shown) which guides it to slide along the elongate slot  936   a ,  936   b  formed in the lid  60 . A boss  934   a ,  934   b  formed on each inertia lever guides the lever to slide axially along a groove  937   a ,  937   b  respectively in the lid  60 . As shown in  FIG. 9   a , rearwards facing abutment or engagement surfaces  981   a  and  981   b  are formed respectively on the coupling catches  927   a ,  927   b  for locking engagement with corresponding engagement surfaces on the inertia levers.  
      Normal operation of the coupling apparatus is shown in  FIG. 9   a , in which acceleration on the cable  25  below the threshold allows the axial gap between the inertia levers and the coupling catches to close whilst the springs ensure that the inertia levers lock against their respective coupling catches.  
      Excessive acceleration applied to the door handle cable  25  causes the inertia levers to swing towards the centre of the frame, to cause the respective bosses  934   a ,  934   b  to lock against respective abutment surfaces  901  formed in the lid  60 . This blocks further movement of the door handle cable  25 .  
      In the event of excessive lateral impact or acceleration  50  as shown in  FIG. 9   b , applied to the frame  23 , a corresponding one of the inertia levers will swing to an extreme position at which it engages against one of the abutment surfaces  901 . Depending on the direction of the acceleration along the axis  50 , this will be one or other of the inertia levers. Thus safe operation in the event of such a side impact is ensured, regardless of its direction. Once again, the springs reset the coupling apparatus once accelerations have stopped.  
      The arrangements shown in FIGS.  1  to  9  can be used in a number of different systems for controlling latches for doors or tailgates or other closure mechanisms, as shown in FIGS.  10  to  12 .  
      In the system shown in  FIG. 10 , a conventional door handle  10  controls a conventional latch  80  through Bowden cables  21 ,  25  in which the apparatus embodying the invention is disposed in line, i.e. in series.  
      In the arrangement shown in  FIG. 11 , an electrical actuator  90  is also disposed in line, between the handle  10  and the cable  25 . This provides for electrical control of the door latch  80 , in addition to manual control through the handle. The electrical control  90  is controlled by control electronics unit  92  and by a switch  91  mounted on or adjacent to the exterior door handle  10  or the interior.  
      In the arrangement shown in  FIG. 12 , there is no manual door handle for the exterior, and instead entry is controlled by an electrical switch  91 , for example using keyless entry systems or a microswitch. A door release electrical actuator and its control electronics are shown as box  1000 , containing a mechanical gearing and indexing system  1001 , a motor  1002 , a microprocessor  1003  and a control electronics unit  1004 .  
      With electric actuators, there is a possibility of a fault condition developing, or of interference for example by criminal activity, which might cause incorrect actuation, i.e. at an acceleration over the predetermined threshold such as 2 G. This could happen if an electric motor power supply is not correctly modulated by control circuitry, so that the motor within the actuator is driven at maximum power to apply excessive force.  
      The component parts of the coupling apparatus are preferably made of plastics wherever possible—i.e. probably excluding the spring and the massive cylinder. Conveniently they may be plastics mouldings.  
      The invention has been illustrated in its application to the control of a door latch, but it is also applicable to a wide range of other mechanically actuable mechanisms where safety in the event of an impact is important.  
      The preferred embodiments are linear actuators, with the inertia lever and catch both following a linear path in the housing. However, this could be modified to a rotary arrangement in which both inertia lever and catch follow arcuate paths. In this case when the inertia lever is at a position at which it locks against the catch to couple drive from the actuator to the cable, its centre of mass is shifted transversely from a line through its pivotal mounting on the bracket parallel at that point to the path of the inertia lever.