Abstract:
The invention relates to a releasable holding mechanism which may be used to hold and release a roll bar in a roll over protection system. The roll bar is often made part of the seat structure of vehicle, or a part of the chassis of the vehicle. The releasable holding mechanism will be fastened rigidly or slightly loose to the roll bar frame, seat frame or the chassis of the vehicle. Between the roll bar and the releasable holding mechanism there will be a connection element that is rigidly or slightly loosely fastened to the roll bar. The arrangement of fastening the connection element rigidly or slightly loosely depends on the tolerances for positioning the actuator towards the roll bar. The connection element is locked or released by the releasable holding mechanism. The releasable holding mechanism is locked/-engaged when the roll bar is pushed downwards whereby the connection element is forced into the releasable holding mechanism. The releasable holding mechanism is to release the connecting element when a signal is sent to the release element of the holding mechanism. The releasable holding mechanism can be activated by one short duration pulse, for instance a pulse from an electronic control unit for activation of airbags in a vehicle. The system can be activated with low energy demands.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to a releasable holding mechanism with means for retaining the mechanism in a locked condition and to a method for actuating the mechanism, and is concerned particularly, although not exclusively, with a releasable holding mechanism for use in automotive roll-over protection systems (ROPS), releasable headrest, backrest, raisable hood for pedestrian protection and other automotive applications. 
     The invention is designed especially for ROPS, but it may be applied otherwise where there is a need for releasing a secured load, e.g. safety systems at sea, life rafts (as an alternative to rope-cutter devices), securing and releasing cargo, and release mechanisms for aero/space, etc. 
     II. Description of the Related Art 
     The Roll Over Protection System (ROPS) is installed in a vehicle to protect the driver and passengers if the vehicle should roll over. During normal driving the roll over bar or the protection member is stored in a lower, retracted position. The roll bar is only released and raised to an upper position when a roll over situation occurs and a signal is received from one or several sensors in the vehicle. 
     Known roll bars are normally sprung towards the upper position, but are held back in the lower position by a releasable holding mechanism, holding the roll bar via one or more connecting elements. Thus the holding mechanism in a locked position will keep the roll bar in the lower position and the holding mechanism in a released position allows the roll bar to be raised to the upper position. 
     The ROPS may comprise one or more separate roll bars or protection members using one holding mechanism for each. Several protection members may also be connected together using one holding mechanism. The holding mechanism to be used in a roll over protection system faces several problems:
         Rough environmental conditions such as vibrations, tilting, dust and moisture that can cause the roll bars to either fasten in a lower position or release unintentionally.   Tolerances and deflections in the vehicle chassis create positioning problems and might cause the roll bar to release unintentionally or fasten.   The power and energy consumption of the release mechanism should be very low to avoid the need for extra power supplies like electrical capacitors. Ideally the mechanism could be powered directly by the crash control unit.   The holding mechanism should be regenerative. That means that the roll bar connection element should snap into locked position again after being released when it is pushed\pulled down manually or by another force.       

     According to relevant prior art, releasable holding mechanisms to be used in a roll over protecting system are either activated by electromechanical or pyrotechnical devices. The roll bar is kept in its lower position by a mechanical catch holding element until a signal is given and the roll bar is released. Such mechanisms rely on an active release arrangement whereby some form of actuator is energized in order to move a piston or rod element to unlock the catch holding element which in turn releases the roll bar for deployment. 
     The pyrotechnical actuators are based on the principle that a small amount of propellant is ignited, and the pressure energy is used to disconnect the locking mechanism. The main problem is that they are not regenerative and after being used they must be replaced. 
     The electromechanical actuators such as the one disclosed in DE 100 02 723 are known to have a connection rod between the roll bar and the actuator. This connection rod is more or less directly connected to a release solenoid using e.g. a ramp (cone) and ball(s) to lock the roll bar in the lower position. This solution is disadvantageous in that a minor tilting of the connection rod leads to movement directly in the release mechanisms locking balls. As a result the roll bar may be released unintentionally. The mechanism is also partly open for pollution. The direct connection between the connection rod and the release mechanism requires high power and energy consumption when the actuator releases, because the actuator must work against most of the spring load. 
     Another example of a release mechanism comprising a solenoid actuator is a release mechanism as described in the patent specification U.S. Pat. No. 5,626,361, in the name Bayerische Motoren Werke AG. 
     In U.S. Pat. No. 5,228,730 there is disclosed an apparatus for converting mechanical locks to operate electrically using momentary power. A lock knob can be arrested by means of a locking pin. The movement of the locking pin is generated by means of a combined electromagnet and a permanent magnet together with a spring. By alternating the polarity of the power, the position of the locking pin can be alternated between a locked position and an unlocked position. This solution does not relate to hold and release of a pre-tensioned connecting element. 
     SUMMARY OF THE INVENTION 
     The present invention sets out to overcome the problems of the known mechanisms. 
     In accordance with the present invention, there is provided a mechanism that can be operated with a minimum of energy. Further, in automotive applications the release action can be controlled by management systems already available in the vehicle. 
     In accordance with one embodiment of the invention there is provided a releasable holding mechanism that does not necessarily require a mechanical catch to keep the mechanism in a locked position but uses a magnetic force to retain the holding means of the mechanism in a secured condition. 
     Preferably, the releasable holding mechanism comprises a permanent magnet that provides the first magnetic force for retaining the holding means in a locked position. 
     Alternatively, the releasable holding mechanism preferably comprises an electro-magnet and a power source that provides the first magnetic force for keeping the holding means in a locked position. 
     The releasable holding mechanism preferably comprises release means comprising a second magnetic force that is useable to opposes the first magnetic force, the arrangement being such that, in use, the resultant magnetic force of the first magnetic force and second magnetic force is insufficient to prevent the holding means moving from the locked position to the unlocked position wherein the connecting element is released. 
     The releasable holding mechanism preferably comprises an electromagnet and a power source for providing the second magnetic force and control means for activating the electromagnet. 
     The present invention sets out to overcome the problems of the known mechanisms by providing a releasable holding mechanism that does not necessarily require a solenoid or other types of movable actuator to initiate the release function of the mechanism but uses a second magnetic force to reduce the first magnetic force to such an extent that resultant magnetic force is not sufficient to prevent movement of the holding means and the connecting element is released. 
     The holding means preferably comprises at least one pivotable element, the arrangement being such that in the locked position a first portion of the pivotable element is in operative engagement with the connecting element and a second portion of the pivotable element is in operative association with, and retainable by, the first magnetic force. 
     In an embodiment of the present invention it is preferable that the releasable holding mechanism comprises auxiliary means for initiating the turning of the lever from the locked position to the unlocked position in conjunction with the second magnetic force. It is also preferable that the auxiliary means for initiating the turning of the lever comprises a spring element that urges the lever from the locked position to the unlocked position. 
     The spring element preferably comprises a helical spring, the arrangement being such that in the locked position of the holding mechanism the helical spring applies a turning force to the second distal end of the lever, which urges the lever to the unlocked position. Preferably, the force exerted on the lever by the connecting element is in a general direction extending away from the second distal end of the lever. 
     In order to help achieve an good contact between the lever and the first magnetic field it is preferable that the lever comprises a second pivot region disposed between the first pivot and the second distal end, the second distal end being pivotable about the second pivot region and arrangement being such that in locked position the second pivot region allows the second distal end to self align with the magnetic force for preventing movement of the holding means. 
     The holding mechanism may be situated in a unclean environment and it may be preferable that the releasable holding mechanism comprises a sealed housing that contains the second distal end of the lever, the first magnetic force for preventing movement of the lever in the locked position and the release means comprising the second magnetic force that is useable to oppose the first magnetic force. The housing will protect the contact surfaces of the magnet and the second distal end of the lever and therefore improve the magnetic attraction between the two elements. 
     Preferably, the second magnetic force is provided by an electromagnet and a power source. The electromagnet preferably comprises a flexible attachment to the sealed housing and the arrangement being such that in the locked position the flexible attachment allows the electromagnet to self align with the second distal end. 
     Another feature that may improve the magnetic attraction between the second distal end and the magnet is that the lever is preferably formed with a hole through which the pivot extends, the hole being of a diameter that is sufficiently greater than the diameter of the pivot such that the second distal end may self align with the first magnetic force. 
     The permanent magnet is preferably attached stationary to the housing, and preferably it is a part of a electromagnet unit, the arrangement being such that, in use, the second distal end of the lever is separated from the permanent magnet and moves in a direction away from the permanent magnet when the lever moves from the locked position to the unlocked position. Alternatively, the permanent magnet is attached to, or forms part of, the second distal end of the lever, the arrangement being such that, in use, the second distal end of the lever and the permanent magnet move in a direction away from the origin of second magnetic force when the lever moves from the locked position to the unlocked position. 
     The skilled person shall appreciate that the general principle of the present invention may be used for a wide variety of releasable holding mechanisms. However, it is preferable that the holding mechanism comprises a mechanical reduction arrangement for reducing the force that the connecting element applies to the releasable holding mechanism such that the turning force that urges the holding means towards the unlocked position is reduced hence the magnetic force required to prevent movement of the holding means does not have to be so large. 
     Preferably, the arrangement for reducing the effective magnetic force required to prevent movement of the holding means comprises a mechanical reduction arrangement having a ratio of 5:1 or more. 
     The present invention may be used to hold and raise a roll bar in a roll over protection system. The roll bar is often made part of the seat structure of vehicle, or a part of the chassis of the vehicle. The releasable holding mechanism will be fastened rigidly or slightly loose to the roll bar frame, seat frame or the chassis of the vehicle. Between the roll bar and the releasable holding mechanism there will be a connection element that is rigidly or slightly loosely fastened to the roll bar. The arrangement of fastening the connection element rigidly or slightly loosely depends on the tolerances for positioning the actuator towards the roll bar. The connection element is locked or released by the releasable holding mechanism. The releasable holding mechanism is locked/engaged when the roll bar is pushed downwards whereby the connection element is forced into the releasable holding mechanism. The releasable holding mechanism is to release the connecting element when a signal is sent to the release element of the holding mechanism. 
     In accordance with the present invention there is achieved one releasable holding mechanism that can be released fast with low energy demands. The releasing action is activated by the movement of a lever, for instance a pivoting lever securing the locking of a connecting member at one distal end. The movement of the lever is balanced between a static holding force provided for instance by a permanent magnet that generates a holding torque, and one opposing releasing force provided for instance by a pre-charged spring, generating a unlocking torque that is lower than said holding torque. In the releasing action, one electromagnet can be energized to act upon the lever with an assisting unlocking torque that together with the pre-charged spring overcomes said holding torque. If the connecting element is pre-tensioned in its release direction or if it affects the torque balance of the lever in other ways, this can be accounted for by introducing an off-center arrangement of the pivot axis of the lever. 
     These and further improvements and advantages can be achieved with the present invention as defined in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention shall be further described by examples and figures where: 
         FIG. 1  is a side view of a partial cross-section of a firsts first embodiment of a release holding mechanism and a connecting element in a locked position; 
         FIG. 2  is a side view of a partial cross-section of the releaseable holding mechanism and the connecting element shown in  FIG. 1 , in an unlocked position; 
         FIG. 3  is an isometric, perspective view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 1  in a locked position; 
         FIG. 4  discloses details regarding the hold/release function, represented by an enlarged cut-out of  FIG. 1 , 
         FIG. 5  is an side view of a partial cross-section of a second embodiment of a release holding mechanism and a connecting element in a locked position; 
         FIG. 6  is a side view of a partial cross-section of the releaseable holding mechanism and the connecting element shown in  FIG. 5 , in an unlocked position; 
         FIG. 7  is an isometric, perspective view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 5  in an unlocked position 
         FIG. 8  is a schematic view of a partial cross-section of a third embodiment of a releasable holding mechanism and a connecting element in a locked position; 
         FIG. 9  is a schematic view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 8  in an unlocked position; 
         FIG. 10  is a schematic view of a partial cross-section of a fourth embodiment of a releasable holding mechanism and a connecting element in a locked position; 
         FIG. 11  is a schematic view of a partial cross-section of the releasable holding is mechanism and a connecting element of  FIG. 10  in an unlocked position; 
         FIG. 12  is a schematic view of a partial cross-section of a fifth embodiment of a releasable holding mechanism and a connecting element in a locked position; 
         FIG. 13  is a schematic view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 12  in a partially unlocked position; 
         FIG. 14  is a schematic view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 12  in an unlocked position; 
         FIG. 15  is a schematic view of a partial cross-section of a sixth embodiment of a releasable holding mechanism and a connecting element in a locked position; 
         FIG. 16  is a schematic view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 15  in an unlocked position; 
         FIG. 17  is a schematic view of a partial cross-section of a seventh embodiment of a releasable holding mechanism and a connecting element in a locked position; 
         FIG. 18  is a schematic plan view of a cross-section through X-X of the lower section of the releasable holding mechanism and a connecting element shown in  FIG. 17 ; 
         FIG. 19  is a schematic view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 17  in an unlocked position; 
         FIG. 20  is a schematic plan view of a cross-section through Y-Y of the lower section of the releasable holding mechanism and a connecting element shown in  FIG. 19 ; 
         FIG. 21  is a schematic view of a partial cross-section of an eight embodiment of a releasable holding mechanism and a connecting element in a locked position; 
         FIG. 22  is a schematic view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 21  in a partially unlocked position; and 
         FIG. 23  is a schematic view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 21  in an unlocked position; 
         FIG. 24  is a schematic view of an arrangement for activation of the releasable holding mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIGS. 1-3 , there is shown a first embodiment of a releasable holding mechanism  302  for locking and releasing a connecting element  304 . The releasable holding mechanism  302  comprises holding means or unit for retaining the connecting element  304  in a locked position in the form of a protrusion P and a pivotable lever  306  with at least one notch  310  and further means for preventing movement of the lever  306  from the locked position (as shown in  FIGS. 1 and 3 ) to an unlocked position (as shown in  FIG. 2 ) (e.g. a release unit) is provided by the attractive magnetic force for instance of a permanent magnet. The lever  306  is capable of being influenced by a magnetic force of said permanent magnet; this magnetic force retains the lever  306  in the locked position. The axis  326  of said pivot is preferably arranged in the same base element or housing as the protrusion P. The releasable holding mechanism  302  comprises a combined magnet unit  308  comprising a permanent magnet and an electromagnet. The electromagnet can be connected to a power source (not shown) via one or more wires and is activated by a control unit (not shown). As in previous embodiments, the holding mechanism can be provided with a spring  342  acting upon the lever  306 . The spring urges the lever to rotate about its pivot  326  in a direction away from the magnet and thus serving as an assisting force when the magnet is activated to perform a releasing action. The force represented by the spring and acting upon the lever  306  must be lower than the holding action performed by the magnet, but sufficient to (in co-operation with the force generated by the electromagnet) move the lever  306  to act with its notch  309  upon the connecting element  304  to move the connecting element  304  out of the position beneath the protrusion P, thus allowing the connecting element to be released. The connecting element should be allowed to move slightly in its transversal direction. 
     As shown in  FIG. 1 , there is a gap between the inward leg (notch  309 ) of the “U” shaped extension of the lever  306  and the connecting element  304 . The gap is preferably designed to allow the lever  306  to obtain a certain velocity before abutting the connecting element  304 . Thus the lever  306  will be charged with momentum and kinetic energy by means of the spring and the magnetic resultant force before it abuts onto and moves the connecting element in a transversal direction. 
     Preferably, the protrusion P can be provided with one or more insert p ( FIG. 3 ) having a shape complementary to that of the connecting element  304 , and being of a low friction material, to secure a proper holding and a smooth release action. 
     Further, a plate  360  ( FIG. 2 ) is attached to one lower part of the lever  306 . The plate should be attached to the lever in a tilting manner, to secure that a planar contact with the magnet unit  308  can be achieved. Alternatively, the magnet unit  308  can be fixed in the housing in a flexible manner to secure said planar contact. 
     In a further alternative embodiment (not shown) of the present invention the second distal end  312  of the lever comprises the permanent magnet. In this arrangement the permanent magnet will rotate with the lever when the electromagnet is activated. 
     As can be seen in  FIG. 3 , the connecting element  304  can be formed as a bar with a straight part having arms or studs  352  at its ends (only one shown). These studs can be rotably fixed to a movable part, such as a releasable roll bar of a roll over protection system in automotive applications. Preferably the center of the stud axis and the straight part  354  can have a minor offset to generate a forcing action upon the connecting element in the direction opposite to that of the releasing direction relative the protrusion P. 
     In this embodiment the connecting element is shown in a “standing” arrangement, here the straight part represents one upper part and the studs  352  (only one shown) lower parts of the element. However, one alternative to this arrangement is to arrange the connecting element in a “hanging” manner where the connecting element is rotated 180 degrees relative the axis of the straight part, where the studs are extending upwards. 
       FIG. 4  discloses details regarding the hold/release function in an enlarged cut-out of  FIG. 1 . Only a part of the lever  306  is shown, where the two upper notches  309 ,  310  are shown. In between the notches the connection element  304  is shown, held by the protrusion P with its insert p. Preferably there is one off set distance “A” between the center of the connecting element and the edge of the protrusion/insert PIp to ensure a proper holding function. A play is clearly shown between the notch  309  and the connecting element ensuring that the lever  306  will be sufficiently energized in its releasing action. 
     The system can be regenerated after one releasing action by pushing the connecting element  304  into the slot “S” in the base element (see  FIG. 1 ). The lever  306  will then be rotated counterclockwise until its plate  360  ( FIG. 2 ) abuts the magnetic unit  308  and is held by said unit. 
     The releasable holding mechanism  302  may also have a manual release function. Such a manual function may be required if there is an electrical power failure or malfunction. Preferably one distal end of the lever  306  is accessible by a tool, which will rotate the lever  306  from the locked position to the unlocked position and release the connecting element  404 . The releasable holding mechanism  302  may also provide access to the inside of the housing  328  such that distal end  312  is accessible by a tool, which will rotate the lever  306  from the locked position to the unlocked position and release the connecting element  404 . 
       FIG. 5  is a side view of a partial cross-section of a further alternative embodiment of a release holding mechanism and a connecting element in a locked position.  FIG. 6  is a side view of a partial cross-section of the releaseable holding mechanism and the connecting element shown in  FIG. 5 , in an unlocked position.  FIG. 7  is an isometric, perspective view of a partial cross-section of the releasable holding mechanism and a connecting element of  FIG. 5  in an unlocked position. 
     With reference to the  FIGS. 5-7 , there is shown an alternative releasable holding mechanism  402  for locking and releasing a connecting element  404 . The releasable holding mechanism  402  comprises holding means for retaining the connecting element  404  in a locked position in the form of a protrusion or notch  410  at the distal end of a pivotable lever  406 . The lever  406  has means for preventing movement of said lever  406  from the locked position (as shown in  FIG. 5  to an unlocked position (as shown in  FIG. 6 ) and is provided by the attractive magnetic force, for instance of a permanent magnet. 
     The lever  406  is capable of being influenced by a magnetic force of said permanent magnet; this magnetic force retains the lever  406  in the locked position. The axis  426  of said pivot is preferably arranged in the same base element or housing as the magnet. The releasable holding mechanism  402  comprises a combined magnet unit  408  with a permanent magnet and an electromagnet. The electromagnet can be connected to a power source (not shown) via one or more wires and is activated by a control unit (not shown). 
     As in the previous embodiment, the holding mechanism can be provided with a spring  442  acting upon the lever  406 . The spring urges the lever to rotate about its pivot  426  in a direction away from the magnet and thus serving as an assisting force when the magnet is activated to perform a releasing action. The torque represented by the spring and acting upon the lever  406  must be lower than the holding torque or action performed by the magnet, but sufficient to (in co-operation with the torque generated by the electromagnet) move the lever  406  to retract its notch  410  that holds the connecting element  404 , thus allowing the connecting element  404  to move out of the locked position beneath the notch  410 , following an upwardly oriented slot “8” in the base element. A secondary notch  409  positioned beneath the connecting element will assist the regeneration of the holding action when the connecting element  404  is pushed downwardly in the slot “8”, thus causing the lever  406  to rotate counterclockwise similarly to that of previous embodiment. 
     The release action of the connecting element  404  may be further improved by forming a slope or chamfer on the surface of the notch  410  that is in contact with the connecting element. In this embodiment of the present invention the vertical cross sectional thickness of the notch  410  reduces in the direction towards the end of the notch  410 . The use of a chamfer or sloped surface will help to reduce the contact frictional force between the connecting element and the notch. The skilled person shall appreciate that the optimal angle of the chamfer may be calculated using the specific load applied to the notch by the connecting element  404  and the surface characteristics of the contacting elements. 
     In order to release the connecting element  404 , the attractive magnetic force of the permanent magnet is reduced by creating an opposing magnetic force using the electromagnet. The electromagnet is turned on and a magnetic field is generated which counteracts and lessens the magnetic field of the permanent magnet to such an extent that the sum of the attractive force of the permanent magnet and the lateral friction force between the lever  406  and the connecting element  404  are overcome by the spring force and a turning force generated by the connecting element  404 . Then the lever will rotate in a clock-wise direction to the unlocked position and release the connecting element  404  (as shown in  FIGS. 6 and 7 ). The spring  442  will hold the second distal end  412  of lever  406  in the unlocked position away from the permanent magnet. 
     In case the connecting element is not pre-tensioned by the object to be locked, the secondary notch may assist the connecting element to be released as the lever  406  is put into rotation. As shown in  FIG. 5  there is a gap between the secondary notch  409  in the extension of the lever  406  and the connecting element  404 . The gap is preferably designed to allow the lever  406  to obtain a certain velocity before abutting the connecting element  404 . Thus the lever  406  will be charged with momentum and kinetic energy by means of the spring and the magnetic resultant force before it abuts onto and moves the connecting element due to its cantilevered shape. 
     It should be understood that in the releasable holding mechanism, in its assembled locked position, the contact point of the connecting element on the lever can be offset from the central axis of the pivot of the lever such that the connecting element applies a turning force to the lever about the central axis of the pivot and the turning force urges the lever from the locked position to the unlocked position, or alternatively the turning force can be designed to assist the locking function. 
     With reference to  FIGS. 8 and 9 , there is shown a releasable holding mechanism  60  for locking and releasing a connecting element  4  that incorporates an adapter bar  61 . The releasable holding mechanism  60  comprises holding means for retaining the connecting element  4  in a locked position in the form of a tiltable roll bar adapter  62  and means for preventing movement of the roll bar adapter  62  from the locked position to an unlocked position is provided by the attractive magnetic force of a permanent magnet, the arrangement being such that in the locked position the roll bar adapter  62  is urged towards the unlocked position by the adapter bar  61  and the connecting element  4 . The roll bar adapter  62  is capable of being influenced by a magnetic force of the permanent magnet; this magnetic force retains the roll bar adapter  62  in the locked position. The releasable holding mechanism  60  comprises a combined magnet unit  8  comprising a permanent magnet and an electromagnet. The electromagnet is connected to a power source (not shown) via a wire  11  and is activated by a control unit (not shown). 
     The releasable holding mechanism  60  comprises a housing  64  formed with a ramp extension  66  on the inner wall of the housing. At the lower distal end of the adapter bar  61  there is formed a foot section  68 . The foot section  68  is adapted to be in a slideable contact with the ramp extension  66 . 
     In the locked position the foot section  68  of the adapter bar  61  is locked between the ramp extension  66  and a wheel  69  which forms part of the roll bar adapter  62 . The roll bar adapter  62  is retained in the locked position by the permanent magnet. 
     To release the connecting element  4  the electromagnet is energised to generate a magnetic field that opposes the field of the permanent magnet and the resultant magnetic field is not sufficiently strong to hold the roll bar adapter  62 . The roll bar adapter  62  will then rotate away from the adapter bar  61 . The adapter bar  61  slides along the ramp extension  66  until the adapter bar  61  has cleared the ramp extension  66  and the connection element  4  and adapter bar  61  are released. A spring may be used to hold the roll bar adapter  62  in the open unlocked position. In the open unlocked position the bar adapter  61  may be inserted back into the housing  64  and the bar adapter  61  will contact the roll bar adapter  62  and rotate the roll bar adapter  62  back into the locked position. 
     With reference to  FIGS. 10 and 11 , there is shown a releasable holding mechanism  110  for locking and releasing a connecting element  4 . The releasable holding mechanism  110  comprises holding means for retaining the connecting element  4  in a locked position in the form of two pivotable levers  112 ,  114  and means for preventing movement of two pivotable levers  112 ,  114  from the locked position (as shown in  FIG. 10 ) to an unlocked position (as shown in  FIG. 11 ) is provided by the attractive magnetic force of a permanent magnet, the arrangement being such that in the locked position the two pivotable levers  112 ,  114  are urged towards the unlocked position by the connecting element  4 . The pivotable lever  112  is capable of being influenced by a magnetic force of the permanent magnet; this magnetic force retains the two pivotable levers  112 ,  114  in the locked position. The releasable holding mechanism  110  comprises a combined magnet unit  8  comprising a permanent magnet and an electromagnet. The electromagnet is connected to a power source (not shown) via a wire  11  and is activated by a control unit (not shown). 
     The releasable holding mechanism  110  comprises a housing  116  formed with an access slot  117 . The pivotable lever  112  is pivotable about a pivot shaft  118  that is not fixed to the housing  116  and one end of the pivotable lever  114  is pivotable about the pivot shaft  118  and the other end of the lever  114  is pivotable about a pivot shaft  113  that is fixed to the housing  116 . The mechanism  110  also comprises two guides  119 ,  121  each disposed either side of, and are in slideable contact with, the lever  112 . 
     To release the connecting element  4  the electromagnet is energised to generate a magnetic field that opposes the field of the permanent magnet and the resultant magnetic field is not sufficiently strong to hold the two pivotable levers  112 ,  114 . The pivotable levers  112 ,  114  will then rotate away from the adapter bar magnet unit  8  and the connecting element  4  will be released and move in direction  52 . A spring may be used to hold the pivotable levers  112 ,  114  in the open unlocked position. In the open unlocked position the connecting element  4  may be inserted back into the access slot  117  of the housing  116  and the connecting element  4  will contact the lever  112  and the two pivotable levers  112 ,  114  can rotate back into the locked position. 
     With reference to  FIGS. 12 to 14 , there is shown a releasable holding mechanism  120  for locking and releasing a connecting element  4 . The releasable holding mechanism  120  comprises holding means for retaining the connecting element  4  in a locked position in the form of two pivotable levers  122 ,  124  and a stop element  126  and means for preventing movement of two pivotable levers  122 ,  124  from the locked position (as shown in  FIG. 12 ) to an unlocked position (as shown in  FIG. 14 ) is provided by the attractive magnetic force of a permanent magnet, the arrangement being such that in the locked position the two pivotable levers  122 ,  124  are urged towards the unlocked position by the connecting element  4 . The pivotable lever  124  is capable of being influenced by a magnetic force of the permanent magnet; this magnetic force retains the two pivotable levers  122 ,  124  in the locked position. The releasable holding mechanism  120  comprises a combined magnet unit  8  comprising a permanent magnet and an electromagnet. The electromagnet is connected to a power source (not shown) via a wire  11  and is activated by a control unit (not shown). 
     The releasable holding mechanism  120  comprises a housing  128  formed with an access slot  127 . The pivotable levers  122 ,  124  are pivotable about a pivot shaft  130  that is not fixed to the housing  128 . One end of the pivotable lever  124  is pivotable about the pivot shaft  130  and the other end of the lever  124  is pivotable about a pivot shaft  132  that is fixed to the housing  128 . The mechanism  120  also comprises the stop element  126  that is disposed at one end of the lever  122  and is in slideable contact with, the lever  122 . 
     To release the connecting element  4  the electromagnet is energised to generate a magnetic field that opposes the field of the permanent magnet and the resultant magnetic field is not sufficiently strong to hold the two pivotable levers  122 ,  124 . The pivotable levers  122 ,  124  will then rotate away from the adapter bar magnet unit  8  and the connecting element  4  will be released and move in direction  52 . A spring may be used to hold the pivotable levers  122 ,  124  in the open unlocked position. In the open unlocked position the connecting element  4  may be inserted back into the access slot  127  of the housing  128  and the connecting element  4  will contact the lever  122  and the two pivotable levers  122 ,  124  can rotate back into the locked position. 
     With reference to  FIGS. 15 and 16 , there is shown a releasable holding mechanism  140  for locking and releasing a connecting element  4 . The releasable holding mechanism  140  comprises holding means for retaining the connecting element  4  in a locked position in the form of three pivotable levers  142 ,  144 ,  146  and means for preventing movement of the pivotable levers  142 ,  144 ,  146  from the locked position (as shown in  FIG. 15 ) to an unlocked position (as shown in  FIG. 16 ) is provided by the attractive magnetic force of a permanent magnet, the arrangement being such that in the locked position the pivotable levers  142 ,  144 ,  146  are urged towards the unlocked position by the connecting element  4 . The pivotable lever  146  is capable of being influenced by a magnetic force of the permanent magnet; this magnetic force retains the two pivotable levers  142 ,  144 ,  146  in the locked position. The releasable holding mechanism  140  comprises a combined magnet unit  8  comprising a permanent magnet and an electromagnet. The electromagnet is connected to a power source (not shown) via a wire  11  and is activated by a control unit (not shown). 
     The releasable holding mechanism  140  comprises a housing  148  formed with an access slot  147 . The pivotable lever  142  is pivotable about a pivot shaft  150  that is fixed to the housing  148 . One end of the pivotable lever  144  is pivotable about a pivot shaft  152  that is fixed to the housing  148  and the other end of the lever  144  is pivotable about a pivot shaft  154  that is not fixed to the housing  148  but is connected to a midsection of the lever  146 . One end of the lever  146  is connected to the lever  142  by a pivot  156 . The pivot  156  is not connected to the housing  148 . 
     To release the connecting element  4  the electromagnet is energised to generate a magnetic field that opposes the field of the permanent magnet and the resultant magnetic field is not sufficiently strong to hold the three pivotable levers  142 ,  144 ,  146 . The pivotable levers  142 ,  144 ,  146  will then rotate about their respective pivots and the connecting element  4  will be released and move in direction  52 . A spring may be used to hold the pivotable levers  142 ,  144 ,  146  in the open unlocked position. In the open unlocked position the connecting element  4  may be inserted back into the access slot  147  of the housing  148  and the connecting element  4  will contact the lever  142  and the three pivotable levers  142 ,  144 ,  146  can rotate back into the locked position. 
     With reference to  FIGS. 17 to 20 , there is shown a releasable holding mechanism  170  for locking and releasing a connecting element  4 . The releasable holding mechanism  170  comprises holding means for retaining the connecting element  4  in a locked position in the form of a pivotable lever  172  and a slideable element  174  and means for preventing movement of the pivotable lever  172  and the slideable element  174  from the locked position (as shown in  FIG. 17 ) to an unlocked position (as shown in  FIG. 19 ) is provided by the attractive magnetic force of a permanent magnet, the arrangement being such that in the locked position the pivotable lever  172  and the slideable element  174  are urged towards the unlocked position by the connecting element  4  and a spring element  176 . The slideable element  174  is capable of being influenced by a magnetic force of the permanent magnet; this magnetic force retains the pivotable lever  172  and the slideable element  174  in the locked position. The releasable holding mechanism  170  comprises a combined magnet unit  8  comprising a permanent magnet and an electromagnet. The electromagnet is connected to a power source (not shown) via a wire  11  and is activated by a control unit (not shown). 
     The releasable holding mechanism  170  comprises a housing  178  formed with a substantially vertical access slot  177  in the upper part and a lower channel  182  extending substantially perpendicular to the slot  177 . The lower channel  182  is formed with step  184  about half away along the length of the channel  182  such that the channel  182  has a narrower section extending from the step  184  in a direction towards the end of the housing  178 . The slideable element  174  is contained within the wider part of the lower channel  182 . The slideable element  174  is formed with a sloped distal end  190 . The pivotable lever  172  is pivotable about a pivot shaft  180 , which is attached to the housing  178 . The spring  176  is disposed between the magnet unit  8  and the sloped distal end  190  and in the locked position the compressed spring  176  urges the slideable element  174  in a direction away from the magnet unit  8 . In the locked position, a lower end of the lever  172  is in contact with the step  184 . 
     To release the connecting element  4  the electromagnet is energised to generate a magnetic field that opposes the field of the permanent magnet and the resultant magnetic field is not sufficiently strong to hold the slideable element  174  against the force of the spring  176 . The slideable element  174  moves in a direction away from the magnet unit  8  and raises the pivotable lever  172  over the step  184  and the lever  172  will then rotate away from the adapter bar magnet unit  8  in direction  186  and the connecting element  4  will be released and move in direction  52 . 
     In the open unlocked position, the connecting element  4  may be inserted back into the access slot  177  of the housing  178  and the connecting element  4  will contact the lever  172  and rotate the pivotable lever  172  back into the locked position abutting the step  184 . The lever  172  is formed with a substantially ‘C’ shaped end  173  and the element  4  comes into contact with the lower part of the ‘C’ shaped end as the element  4  is inserted back into the slot  177 . It shall be appreciated that the majority of the embodiments described include a similar ‘C’ shaped end that is utilized to move the mechanism from the unlocked position to the locked position. 
     With reference to  FIGS. 21 to 23 , there is shown a releasable holding mechanism  190  for locking and releasing a connecting element  4 . The releasable holding mechanism  190  comprises holding means for retaining the connecting element  4  in a locked position in the form of two pivotable levers  192 ,  194  and means for preventing movement of two pivotable levers  192 ,  194  from the locked position (as shown in  FIG. 21 ) to an unlocked position (as shown in  FIG. 23 ) is provided by the attractive magnetic force of a permanent magnet, the arrangement being such that in the locked position the two pivotable levers  192 ,  194  are urged towards the unlocked position by the connecting element  4 . The pivotable lever  192  is capable of being influenced by a magnetic force of the permanent magnet; this magnetic force retains the two pivotable levers  192 ,  194  in the locked position. The releasable holding mechanism  190  comprises a combined magnet unit  8  comprising a permanent magnet and an electromagnet. The electromagnet is connected to a power source (not shown) via a wire  11  and is activated by a control unit (not shown). 
     The releasable holding mechanism  190  comprises a housing  196  formed with an access slot  197 . The two pivotable levers  192 ,  194  are pivotable about respective pivot shafts  198 ,  200  that are each connected to the housing  196 . 
     To release the connecting element  4  the electromagnet is energised to generate a magnetic field that opposes the field of the permanent magnet and the resultant magnetic field is not sufficiently strong to hold the two pivotable levers  192 ,  194 . The pivotable lever  192  will then rotate away from the adapter bar magnet unit  8  in an anticlockwise direction  202  and the pivotable lever  194  will be released by the lever  192 . The lever  194  will then rotate in a clockwise direction  204  and the connecting element  4  will be released and move in direction  52 . Respective spring may be used to hold each of the pivotable lever  192 ,  194  in the open unlocked position. 
     In the open unlocked position, the connecting element  4  may be inserted back into the access slot  197  of the housing  196  and the connecting element  4  will contact the lever  194  and the two pivotable levers  192 ,  194  can rotate back into the locked position. 
       FIG. 24  discloses main components in the releasable holding mechanism such as a lever  306  and a connecting element  304 . Here, the lever  306  is kept in a locked position by means of a permanent magnet. Further, there is shown an arrangement for activation of the releasable holding mechanism, by energizing the coil of the electromagnet by means of an electrical pulse. The pulse can be provided by an airbag electronic controller unit (ECU). The main components in the embodiment shown are one electronic controller unit  212 , being powered by a source of voltage  210 . One diode  211  of the freewheeling type is arranged in parallel with and connected to two leads interconnecting the ECU and the coil in the magnet unit  308  in the low energy actuator (LEA)  213 . 
     The coil and associated ferromagnetic material have an inherent electrical resistance and inductance. Some of the applied energy in a short transient pulse from the airbag ECU is lost in eddy induced currents in the ferromagnetic material in the coil, some is lost into heat, and the remainder contributes to an electromagnetic field around the coil. When the airbag ECU triggers due to a sensor input (charge region), the ECU seeks to provide a defined electrical current (or voltage) across the coil of the LEA. 
     The current increases toward the target current and finally stabilizes at that current, thus enabling the electromagnet to establish a magnetic field that in whole or partially neutralizes the permanent magnet field in the LEA. 
     When the trigger output from the airbag ECU ends (discharge region), the stored electrical inductance energy in the LEA electromagnet is allowed to maintain a current due to the diode. This effectively extends the duration of release signal for LEA since the electromagnetic field is extended in time, although decreasing with decreasing current. 
     It should be understood that the LEA could also be energized by other controlling units than that of the above mentioned embodiment, where appropriate. 
     Further, it should be understood that the holding force in the abovementioned embodiments is preferably sustained by a permanent magnet. However, such holding force can also be generated by means of snap fasteners or snap couplings, “Velcro” locks, non-hardening glue or any other appropriate regenerative fastening means.