fig1 a through 1d illustrate a first embodiment of a trigger mechanism according to the invention . the trigger mechanism is used in an inhaler 1 to actuate delivery of a dose of a drug stored in the inhaler 1 , for example a dry powder , aqueous droplet or metered dose inhaler . successive stages of an actuation process for drug delivery are shown to explain the operation of the trigger mechanism . the trigger mechanism comprises an actuation flap 11 , a first lever 12 , a second lever 13 , a first spring 14 and a second spring 15 . the actuation flap 11 is located in a breathing channel 10 through which a user inhales . the actuation flap 11 and the levers 12 , 13 are pivoted around pivots 111 , 121 , 131 at one of their ends respectively . the actuation flap 11 is equipped with a first ring segment 112 located at its pivot . the first lever 12 is equipped with a second ring segment 122 located at its pivot . the ring segments 112 , 122 extend about one third of a circle around the centre of the respective pivot 111 , 121 and extend from the surface of the respective pivot 111 , 121 . the pivots 111 and 121 of the actuation flap 11 and of the first lever 12 are separated by a distance l 1 corresponding to a length of the first lever 12 . the pivots 121 and 131 of the levers 12 , 13 are separated by a distance l 2 corresponding to a length of the second lever 13 . the pivots 111 , 121 , 131 are located in a common plane . hence , when the actuation flap 11 and the levers 12 , 13 are rotated to this plane and likewise oriented from their respective pivots 111 , 121 , 131 as shown in fig1 a , the first lever 12 extends to the pivot 111 of the actuation flap 11 , and the second lever 13 extends to the pivot 121 of the first lever 12 . furthermore , in this position the first ring segment 112 restrains the first lever 12 from rotating upwards while the second ring segment 122 restrains the second lever 13 from rotating downwards . the first lever 12 is coupled to the first spring 14 near to the pivot 111 of the actuation flap 11 at a distance x 1 to the pivot 121 of the first lever 12 . the second lever 13 is coupled to the second spring 15 near to the pivot 121 of the first lever 12 at a distance x 2 to the pivot 131 of the second lever 13 . thereby the first spring 14 is located below the first lever 12 while the second spring 15 is located above the second lever 13 . the stiffness of the second spring 15 exceeds the stiffness of the first spring 14 . fig1 a shows an initial state of the trigger mechanism with the actuation flap 11 and the levers 12 , 13 located in the same plane as described above . in this state both springs 14 , 15 are compressed , the second spring 15 storing more energy than the first spring 14 . when no force is acting on the actuation flap 11 , a rotation of the actuation flap 11 and the levers 12 , 13 are restrained by the ring segments 112 , 122 respectively . the levers 12 , 13 are thus pre - stressed by the springs 14 , 15 respectively , the pre - stressing of the second lever 13 exceeding the pre - stressing of the first lever 12 . fig1 b shows the trigger mechanism when a user just has started to inhale . the inhaling causes an airflow b and a pressure drop p which suffices to rotate the actuation flap 11 downwards . a detailed quantitative discussion of this mechanism is given below . fig1 c shows the trigger mechanism when the actuation flap 11 has been rotated sufficiently so that the first ring segment 112 releases the first lever 12 . as a consequence , the first spring 14 expands and rotates the first lever 12 upwards . this mechanism is also discussed in detail below . fig1 d shows the trigger mechanism when the first lever 12 has been rotated sufficiently so that the second ring segment 122 releases the second lever 13 . as a consequence , the second spring 15 expands and rotates the second lever 13 downwards . during the actuation process illustrated by the fig1 a through 1d an actuation force fa exerted by the pressure drop p on the actuation flap 11 releases energy stored in the first spring 14 which in turn is used to release energy stored in the second spring 15 . thereby the actuation force fa can be considerably amplified to forces exerted by the springs 14 , 15 . this will be shown in the following quantitative analysis of the trigger mechanism described qualitatively above . with a denoting the area of the actuation flap 11 , the actuation force fa exerted by the pressure drop p on the actuation flap 11 is fa = p · a . the actuation force fa exerts an actuation torque ta = p · a · z on the actuation flap 11 where z is the distance between the pivot 111 of the actuation flap 11 and the effective application point of the actuation force fa . denoting the spring force exerted by the first spring 14 on the first lever 12 by f 1 , a reaction force y 1 at the pivot 111 is y 1 =( x 1 / l 1 )· f 1 . in the initial state of the trigger mechanism shown in fig1 a , the rotation of the actuation flap 11 is restrained by a static friction between the first ring segment 112 and the corresponding end of the first lever 12 . this static friction is mue y 1 = μ ·( x 1 / l 1 )· f 1 with μ a friction coefficient . denoting the radius of the first ring segment 112 from the centre of the pivot 111 by r 1 , the rotation of the actuation flap 11 is thus restrained by a first restraining torque t 1 = r 1 · μ ·( x 1 / l 1 )· f 1 . in order for the trigger mechanism to operate according to fig1 b , i . e . in order to rotate the actuation flap 11 , this first restraining torque t 1 must be exceeded by the actuation torque ta , i . e . t 1 & lt ; ta and thus r 1 · μ ·( x 1 / l 1 )· f 1 & lt ; z · p · a . therefore , the force f 1 of the first spring 12 that can be restrained by the trigger mechanism , and still released by actuation flap 11 is restricted by f 1 & lt ; z · p · a · ( l 1 / x 1 )/( r 1 · μ ). [ 1 ] and the maximal amplification of the actuation force fa provided by the first spring 14 is restricted by f 1 / fa & lt ; z · ( l 1 / x 1 )/( r 1 · μ ). [ 2 ] inserting typical values a = 100 mm 2 , z = 5 mm , p = 1 kpa , l 1 = 40 mm , x 1 = 20 mm , r 1 = 1 mm and μ = 0 . 5 , this results in an analogous consideration applies to the coupling of the first lever 12 to the second lever 13 through the second ring segment 122 . denoting the spring force on the second lever 13 due to the second spring 15 by f 2 , a reaction force y 2 at the pivot 121 is y 2 =( x 2 / l 2 )· f 2 . the rotation of the first lever 12 is restrained by a static friction between the second ring segment 122 and the corresponding end of the second lever 13 . this static friction is μ · y 2 = μ ·( x 2 / l 2 )· f 2 . denoting the radius of the second ring segment 122 from the centre of the pivot 121 by r 2 , the rotation of the first lever 12 is restrained by a second restraining torque t 2 = r 2 · μ ·( x 2 / l 2 )· f 2 . in order for the trigger mechanism to operate according to fig1 c , i . e . in order to rotate the first lever 12 , the second restraining torque t 2 must be exceeded by the torque x 1 · f 1 provided by the first spring 14 on the first lever 12 , i . e . r 2 · μ ·( x 2 / l 2 )· f 2 & lt ; x 1 · f 1 . therefore , the additional force amplification f 2 / f 1 is restricted by f 2 / f 1 & lt ; x 1 ·( l 2 / x 2 )/( r 2 · μ ). [ 5 ] inserting the same typical values as above with x 1 = x 2 = 20 mm , l 1 = l 2 = 40 mm , r 1 = r 2 = 1 mm , μ = 0 . 5 , this yields f 2 could thus be up to about 160 n . this is a significant force and the energy released from the springs 14 , 15 can indeed be used dose delivery through the inhaler 1 . additional cascaded trigger elements and springs could be added to enhance the force amplification even further . a further use for the trigger mechanism could be that each lever 12 , 13 could be connected to a separate part of the inhaler mechanism . for example , the first lever 12 could trigger opening of a dose container , the second lever 13 could trigger dose delivery . by adding damping to either the first lever 12 or the second lever 13 it would also be possible to introduce a time delay between the initial breath actuation of the actuation flap 11 and the release of the second lever 13 . this could be used to introduce a “ staged ” response to the breath actuation . after the levers 12 , 13 have been released the user would have to reset both levers 12 , 13 before the trigger mechanism could be used again . the reset action could occur simultaneously when the user performs some other action with the inhaler 1 , for example opening it to remove an empty dose container or load a new dose container , or in a priming action of the inhaler 1 prior to use . the limit of how far a force could be amplified by the trigger mechanism is likely to be how much energy the user can put back into the system when resetting the trigger mechanism . the embodiment shown in fig1 a through 1d has the disadvantage that the springs 14 , 15 have to be reset in opposite directions . this disadvantage is overcome by an alternative embodiment of the trigger mechanism shown in fig2 a and 2b . again , the trigger mechanism is used in an inhaler 1 . a difference of this embodiment as compared to the first embodiment is that the levers 12 , 13 are arranged such that they are stacked one above the other in an initial state of the trigger mechanism shown in fig2 a . furthermore , the first lever 12 is fixed to the actuation flap 11 , both having the same pivot 111 so that they can only rotate simultaneously . the springs 14 , 15 are located on the same side of the levers 12 , 13 , and the second lever 13 is equipped with a third ring segment 133 of the same type as the ring segments 112 , 122 of the first embodiment . the first lever 12 now extends from its pivot 111 to the pivot 131 of the second lever 13 . again , the pivot 111 is equipped with a first ring segment 112 ( not visible in the fig2 a and 2b ) to which the second lever 13 extends in its initial position . in the initial state of the trigger mechanism shown in fig2 a , the third ring segment 133 restrains the actuation flap 11 and the first lever 12 from rotating through the friction between the third ring segment 133 and the corresponding end of the first lever 12 and the first ring segment 112 restrains the second lever 13 from rotating through the friction between the first ring segment 112 and the corresponding end of the second lever 13 . when a user exerts a sufficient actuation force fa on the actuation flap 11 through inhaling , the levers 12 13 are released and both rotate upwards as shown in fig2 b . in order to reset the trigger mechanism both levers 12 , 13 are pushed downwards to reengage the ring segments 112 , 133 . fig3 a through 3d illustrate a third embodiment of a trigger mechanism according to the invention . the trigger mechanism is used in an autoinjector 2 to actuate delivery of a dose of a drug 242 stored in a cartridge 24 through a dispensing element 243 of the autoinjector 2 located at the bottom of the cartridge 24 . the cartridge 24 is sealed by plug 241 . the trigger mechanism comprises a manually operated actuation lever 21 , an intermediate lever 22 , a piston 23 , a first spring 26 and a second spring 25 . the actuation lever 21 is pivoted around a pivot 211 at one of its ends and is equipped with a trigger button at its opposite end . the distance between the centres of the pivot 211 and of the trigger button is denoted by x 5 . the actuation lever 21 is equipped with a first ring segment 212 which is located at the pivot 211 and is of the same type as the ring segments 112 , 122 , 133 of the first and second embodiment . the intermediate lever 22 is hook - shaped with a bend located at the pivot 211 of the actuation lever 21 . a first end of the intermediate lever 22 is directed towards the piston 23 , the second end contains a pivot around which the intermediate lever 22 is pivoted . the intermediate lever 22 is connected to the first spring 26 at a distance x 3 from its pivot . the distance between the bend and the pivot of the intermediate lever 22 is denoted by x 4 . one end of piston 23 is directed towards the plug 241 of the cartridge 24 , the other end is connected to the second spring 25 . the surface of the piston 23 is equipped with a notch in which the first end of the intermediate lever 22 can engage . the operation of the trigger mechanism is now described first qualitatively with reference to fig3 a through 3d and afterwards analysed quantitatively . fig3 a shows an initial state of the trigger mechanism . both springs 25 , 26 are compressed . the first end of the intermediate lever 22 engages in the notch of the piston 23 and prevents the piston 23 from moving towards the plug 241 . the bend of the intermediate lever 22 is coupled to the first ring segment 212 which restrains the intermediate lever 22 from rotating . fig3 b shows the trigger mechanism when a user presses the trigger button of the actuation lever 21 sufficiently so that the actuation lever 21 is rotates around its pivot 211 . as the actuation lever 21 rotates , the first ring segment 212 eventually disengages and releases the intermediate lever 22 . fig3 c shows the trigger mechanism after the intermediate lever 22 has been released . the first spring 26 expands and rotates the intermediate lever 22 . the first end of the intermediate lever 22 disengages from the notch in the surface of the piston 23 which releases the piston 23 . the piston 23 is now free to move towards the plug 241 under the action of the fourth spring 25 . fig3 d shows the trigger mechanism after the piston 23 has been released . the piston 23 has moved to the plug 241 and pressed it towards the bottom of the cartridge 24 . thereby it exerts a pressure on the drug inside the cartridge 24 which forces delivery of the drug through the dispensing element 243 . to discuss the trigger mechanism quantitatively the spring forces of the third spring 26 and of the fourth spring 25 exerted on the intermediate lever 22 and the piston 23 in the initial state of the trigger mechanism are denoted by f 3 and f 4 respectively . assuming that the thickness of the intermediate lever 22 thickness is negligible compared to its length , the approximate reaction force provided by the first spring 26 between the intermediate lever 22 and the second ring segment 212 in the initial state of the trigger mechanism is f 3 ·( x 3 / x 4 ). a third restraining torque t 3 caused by friction between the intermediate lever 22 and the fourth ring segment 212 is therefore approximately t 3 = r 3 · μ · f 3 ·( x 3 / x 4 ) where r 3 is the radius of the first ring segment 212 from the centre of the pivot 211 . a user must provide a sufficient actuation force u to the trigger button to overcome this resistance . the actuation torque resulting from u is u · x 5 . the actuation lever 21 starts to rotate when this torque exceeds the third retraining torque t 3 , i . e . when u · x 5 & gt ; r 3 · μ · f 3 ·( x 3 / x 4 ). hence , for the actuation lever 21 to rotate , the force f 3 of the third spring 26 is restricted by f 3 & lt ;( x 4 · x 5 / x 3 )· u /( r 3 · μ ). [ 8 ] in order to release the piston 23 the force f 3 ·( x 3 / x 4 ) provided by the first spring 26 at the bend of the intermediate lever 22 must overcome the friction between the piston 23 and the intermediate lever 22 which is μ f 4 . therefore , the piston 23 is released if f 3 ·( x 3 / x 4 )& gt ; μ · f 4 . hence , for the trigger mechanism to operate , the force f 4 of the second spring 25 is restricted by inserting typical values x 5 = 25 mm , x 3 = 15 mm , x 4 = 30 mm , μ = 0 . 5 , r 3 = 2 . 5 mm and u = 1 n , one obtains as compared to the actuation force u = 1 n this gives a force amplification up to a factor of 40 . the amplification can be further enhanced by different arrangements of the intermediate lever 22 and / or the use of further intermediate levers and springs and / or a “ rolling ” coupling of the intermediate lever 22 to the piston 23 in place of the coupling through the notch in the surface of the piston 23 .
the invention relates to a trigger mechanism for a drug delivery device comprising at least one energy storing element , an actuation element and a series of cascaded trigger elements . the trigger elements are pre - stressed with increasing pre - stressing and coupled to the at least one energy storing element such that the trigger elements , upon exerting a sufficient actuation force on the actuation element , cause a cascaded release of increasing portions of energy stored in the at least one energy storing device . at least one of the trigger elements is equipped with a latch element directly coupling at least two trigger elements such that the latch element restrains at least one of these trigger elements to its pre - stressed state before exerting the actuation force .