Abstract:
The present invention relates to an electrical device for rail mounting. Such electrical devices are mounted in use in a side-by-side relationship on a carrier rail. An electrical device for rail mounting comprises a housing having a rail receiving recess therein, at least one edge of said recess being defined by an end region of a latch member that is movable relative to the housing in a direction transverse to the recess, and resilient means biasing the latch member to move in a direction to reduce the width of said recess. Said resilient means comprise a torsion spring having a first leg and a second leg deflectable around a hinge region, said first leg resting against a protrusion of the housing, and said second leg resting against the latch member.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an electrical device for rail mounting. Such electrical devices are mounted in use in a side-by-side relationship on a carrier rail, the so-called DIN rail (also known as top hat rail according to the European standard EN 50022, in the USA called TS35 rail). The present invention can be used with devices such as circuit breakers, fuse units, safety switching and control devices, and rail mounted power supplies, to name only a few. 
         [0002]    Conventional rail mounted electrical devices comprise an electrically insulating housing that is shaped at one side to engage a carrier rail. At this side, the housing has a dovetail shaped transversely extending recess within which the rail is gripped in use. One side of this recess is defined by a latch member movable relatively to the housing to permit a snap-on engagement and disengagement of the housing and the rail, and the latch member is resiliently biased relative to the housing in a direction to reduce the width of the recess and thus to grip the rail in use. 
         [0003]    Where one of the known devices is utilized alone then it is necessary to hold the latch member in a retracted position against the action of its resilient bias, while manipulating the device to disengage it from the rail. Such a mode of operation is inconvenient, but it will be realized that where a device comprises two or more units physically secured together, for handling as a unitary structure, as would be the case with two or more ganged isolators, then it is extremely difficult to hold two or more latches in their retracted position while manipulating the unitary structure relative to the rail. 
         [0004]    In an attempt to overcome this problem it has previously been proposed to incorporate into each device a catch element movable manually, transversely relative to the latch member and cooperating with the housing to permit locking of the latch member in a retracted position. The provision of such a catch member simplifies operation by permitting latch members to be locked in their retracted positions but of course increases the complexity, and therefore cost of each unit. 
         [0005]    Consequently, GB 2216177 A1 proposes an electrical device for rail mounting comprising a housing having a rail receiving recess therein, one edge of said recess being defined by an end region of a latch member movable relative to the housing in a direction transverse to the recess, resilient means biasing the latch member to move relative to the housing in a direction to reduce the width of said recess, and, interengagable surfaces on the housing and the latch member respectively which cooperate, when the latch member is retracted to a predetermined position against the action of said resilient means, to retain the latch member in said retracted position, said surfaces being such that the latch member can be released for movement under the action of said resilient means by a movement of the latch member relative to the housing in a direction transverse to the direction of movement of the latch member under the action of said resilient means. 
         [0006]    This known solution, however, has the disadvantage that a compression spring is needed for biasing the latch member, which is space consuming and complicates the assembly process. 
         [0007]    Consequently, there is still a need for an electrical device that can be mounted on a DIN rail efficiently, securely, and precisely, while the electrical device can be fabricated in a particularly cost-efficient way. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides an electrical device for rail mounting which comprises a housing having a rail receiving recess therein, at least one edge of said recess being defined by an end region of a latch member that is movable relative to the housing in a direction transverse to the recess. Resilient means are provided for biasing the latch member to move in a direction to reduce the width of said recess. 
         [0009]    The present invention is based on the idea that the resilient means comprise a torsion spring having a first leg and a second leg deflectable around a hinge region, said first leg resting against a protrusion of the housing, and said second leg resting against the latch member. This particular design has the advantage that the spring is particularly space saving and yet provides a sufficiently high biasing force. In addition, the assembly of the housing of the electrical device is significantly facilitated. 
         [0010]    Torsion springs are springs that are designed with ends that can be rotated in angular deflection around a hinge region. This offers resistance to externally applied torque. Torsion springs are for instance close wound coils with the legs arranged at each end. Alternatively, the torsion spring may also be formed from a bent metal sheet. There are a variety of options for end configurations of torsion springs to allow the spring to be anchored in several different configurations. Moreover, the legs may be located at varying positions, angles, or distances from the hinge region of the spring. 
         [0011]    According to an advantageous embodiment, the second leg of the torsion spring is at least partly received in a groove provided at the latch member. Thereby safe support of this part of the torsion spring can be achieved and, moreover, the spring can be mounted at the latch member before assembling same within the housing without any danger that the spring gets lost. 
         [0012]    As already mentioned, various angles can be provided between the two legs. According to an advantageous embodiment, an angle between the legs is in a range between 20° and 40° in an unloaded state of the torsion spring. For instance, the angle may be 30°. This value allows a sufficiently long linear movement of the latch member within the housing. 
         [0013]    In order to safely store the spring in a space saving manner, said torsion spring is arranged in a recess of the latch member that follows the outer contour of the torsion spring in the unloaded state. 
         [0014]    According to an advantageous embodiment of the present invention, said torsion spring is a flat spring. Like sheet metal stampings, flat springs are parts that are stamped, laser cut, or photo-chemically etched from a sheet of material. In most cases, the blanks are then formed, heat treated as necessary and finished to have their final shape. In particular, the flat torsion spring has a V-shaped form. 
         [0015]    In order to allow an efficient interaction between the housing, the latch member, and the torsion spring, said first leg has a shorter length than said second leg extending from said hinge region. 
         [0016]    According to the present invention, the latch member further comprises a detent lug that engages with a first detent recess at the housing for securing the latch member in a closed position. Thereby the latch member can be prevented from being released inadvertently. In order to remove the electric device from the DIN rail, the latch member has to be releasable. This can be achieved by providing an opening through which the detent lug is accessible from the outside of the housing for disengaging the detent lug from the housing. In particular, the housing may comprise an opening for inserting a tool for disengaging the detent lug from the housing. Without using the special tool the latch member cannot be unlocked, so that a particularly secure mounting of the electrical device on the DIN rail can be achieved. 
         [0017]    According to an advantageous embodiment of the present invention, said housing further comprises a second detent recess for securing the latch member in an open position. Such a second locking mechanism allows a facilitated mounting of the electrical device on the DIN rail. In particular, it is not necessary to manually hold open the latch member against the restoring force of the torsion spring. 
         [0018]    Furthermore, said housing comprises a bottom shell and a cover, wherein said latch member is arranged at said bottom shell. By using such a modular design, an improved flexibility regarding the adaptation to various constructional needs can be achieved. 
         [0019]    According to one embodiment of the present invention, the housing comprises an integrally formed fixed rail gripping hook which is arranged opposite to said latch member for gripping the rail. This design allows for a particular simple and cost effective assembly because only one latch member is needed. Alternatively, in order to allow for an enhanced flexibility regarding the mounting position, the housing may of course also comprise two opposing latch members for gripping the rail. Advantageously, the latch members are formed identically. 
         [0020]    A particularly effective actuating of the latch member can be achieved when the housing comprises a retention pin that compresses the torsion spring in an open position of the latch member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The accompanying drawings are incorporated into the specification and form a part of the specification to illustrate several embodiments of the present invention. These drawings, together with the description serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used, and are not to be construed as limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the embodiments may form—individually or in different combinations—solutions according to the present invention. The following described embodiments thus can be considered either alone or in an arbitrary combination thereof. Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings, in which like references refer to like elements, and wherein: 
           [0022]      FIG. 1  is a schematic exploded perspective view of an electrical device for rail mounting; 
           [0023]      FIG. 2  is a schematic perspective sectional view of a bottom shell according to a first embodiment; 
           [0024]      FIG. 3  is a detail of  FIG. 2 ; 
           [0025]      FIG. 4  is a top view of the bottom shell according to the first embodiment; 
           [0026]      FIG. 5  is a sectional view along the line A-A of  FIG. 4 ; 
           [0027]      FIG. 6  is a sectional view along the line B-B of  FIG. 4 ; 
           [0028]      FIG. 7  is a detail of  FIG. 5 ; 
           [0029]      FIG. 8  is a detail of  FIG. 6 ; 
           [0030]      FIG. 9  is a top view of a latch member as shown in  FIG. 1 ; 
           [0031]      FIG. 10  is a sectional view along the line C-C of  FIG. 9 ; 
           [0032]      FIG. 11  is a sectional view along the line D-D of  FIG. 9 ; 
           [0033]      FIG. 12  is an exploded perspective view of the latch member with the torsion spring as shown in  FIG. 1 ; 
           [0034]      FIG. 13  is a perspective view of the latch member with the assembled torsion spring as shown in  FIG. 1 ; 
           [0035]      FIG. 14  is a top view of a flat torsion spring as shown in  FIG. 1 ; 
           [0036]      FIG. 15  is a side view of the flat torsion spring of  FIG. 14 ; 
           [0037]      FIG. 16  is a bottom view of an electrical device for rail mounting having two opposing latch members in a closed position; 
           [0038]      FIG. 17  is a sectional view along the line E-E of  FIG. 16 ; 
           [0039]      FIG. 18  is a detail of  FIG. 17 ; 
           [0040]      FIG. 19  is a bottom view of an electrical device for rail mounting having two opposing latch members in an open position; 
           [0041]      FIG. 20  is a sectional view along the line F-F of  FIG. 19 ; 
           [0042]      FIG. 21  is a detail of  FIG. 20 ; 
           [0043]      FIG. 22  is a side view of the bottom shell according to a second embodiment; 
           [0044]      FIG. 23  is a top view of the bottom shell of  FIG. 22 ; 
           [0045]      FIG. 24  is another side view of the bottom shell of  FIG. 22 ; 
           [0046]      FIG. 25  is a side view of a cover as shown in  FIG. 1 ; 
           [0047]      FIG. 26  is a top view of the cover shown in  FIG. 25 ; 
           [0048]      FIG. 27  is another side view of the cover shown in  FIG. 25 ; 
           [0049]      FIG. 28  is a perspective view of a latch member according to a second embodiment; 
           [0050]      FIG. 29  is a bottom view of the latch member of  FIG. 28 ; 
           [0051]      FIG. 30  is a side view of the latch member of  FIG. 28 ; 
           [0052]      FIG. 31  is another side view of the latch member of  FIG. 28 . 
       
    
    
     DETAILED DESCRIPTION 
       [0053]    The present invention will now be explained in more detail with reference to the Figures and firstly referring to  FIG. 1 .  FIG. 1  shows a partly exploded perspective view of an electrical device  100  according to the present invention. According to the present invention, the electrical device  100  may be any kind of rail mounted electrical device, such as a circuit breaker, fuse unit, safety switching and/or control device, or a rail mounted power supply. Because the electric functionality is not important for the mechanical principle of the present invention, no further details on the electronic components are shown in the drawings. For instance, one or more connector receptacles  110  may be provided for connecting external electrical components to the electrical device  100 . 
         [0054]    According to an advantageous embodiment, an electrically insulating housing of the electrical device  10  comprises a bottom shell  102  and a cover  104 . The cover  104  is fixed on the bottom shell  102  by means of four snap hooks  106  which engage with corresponding recesses  108  in order to form a closed protective housing. However, it is clear for a person skilled in the art that the housing may also be formed from a single part or from more than two parts. The bottom shell  102  and the cover  104  advantageously are fabricated from an electrically insulating plastic material. 
         [0055]    The bottom shell  102  is formed to grip a rail (not shown in the Figures) within a rail recess  112 . According to the shown embodiment of the present invention, the bottom shell  102  is adapted to be mounted on a so-called DIN rail (also known as top hat rail according to the European standard EN 50022, in the USA called TS35 rail). However, it is clear for a person skilled in the art that the mounting technique according to the present invention can of course also be employed with any other cross-sectional form of a mounting rail. 
         [0056]    For gripping the rail, the rail recess  112  comprises two opposing rail gripping projections  116 , at least one of which is movable in a direction across to a longitudinal axis  114  of the rail for mounting and dismounting the electrical device to/from the rail. As shown in  FIG. 1 , at least one of the rail gripping projections  116  is an integral part of a latch member  118 . The latch member  118  is held slidably within the bottom shell  102 . For pushing the latch member  118  in a direction indicated by the arrow  120  the latch member  118  has an actuating region  122  which passes through an actuating opening  124  provided at the bottom shell  102 . In the fully locked state, the actuating region  122  is received within the actuating opening  124  and the outer surface  126  of the actuating region is flush with the outer surface  128  of the bottom shell  102 . In the unlocked state, the actuating region  122  extends outside the actuating opening  124  and is accessible for an operator. In this position, a flat torsion spring  130  is compressed in order to resiliently bias the latch member  118  to automatically move in the direction  120  when released. 
         [0057]    According to the present invention and as will become more apparent from  FIGS. 16 to 21 , the latch member  118  is secured in the locked as well as the unlocked position by means of a snap fit between the latch member  118  and the bottom shell  102 . For this purpose, the latch member comprises a resilient catch arm  132 . 
         [0058]      FIG. 2  shows a schematic sectional view of the bottom shell  102  with two latch members  118  each in a locked position, where the rail gripping projections  116  grip the DIN rail. In this position, the surface is  126  and  128  are flush, so that the latch member  118  is safely stored inside the housing. As can be seen more clearly from the enlarged detail shown in  FIG. 3 , the torsion spring  130  has a first leg  134  and a second leg  136  which are connected to each other via a hinge region  138 . The first leg  134  rests  142  a retention pin  140  which is arranged at that part of the housing which is cut away in the view of  FIGS. 2 and 3  and extends towards the latch member  118 . The second leg  136  in held within a groove  142  provided at the latch member  118 . 
         [0059]    Consequently, when the latch member  118  is moved from the locked position shown in  FIGS. 2 and 3  along direction  121  into the unlocked position, the retention pin  140  holds back the first leg  134  so that the torsion spring  130  is compressed to include a smaller angle between the first leg  134  and the second leg  136 . Thereby energy is stored in the torsion spring  130  that will cause the latch member  118  to move back into the locked position when it is not actively restrained in the unlocked position. 
         [0060]    Moreover, the bottom shell  102  further comprises stop angles  144  that limit the movement of the latch member  118  in the direction  121  to a distance X. Consequently, the retention pin  140  also deflects the first leg  134  of the torsion spring  130  by a maximum distance of X when the latch member  118  is slid from the locked into the unlocked position. When the spring  130  is deformed, the second leg  136  is fixed in the groove  142  and additionally rests against a support wall  146 . Thereby it can be ensured that the torsion spring biases the latch member  118  with a uniformly distributed force (directed in the direction  120 ) across the latch member&#39;s complete breadth for allowing a smooth sliding movement back into the locked position. 
         [0061]      FIG. 4  shows a top view of the bottom shell shown in  FIG. 2 .  FIGS. 5 and 6  are sectional views along the lines A-A and B-B of  FIG. 4 , respectively. According to this particular embodiment, two latch members  118  can be inserted into latch member receptacles  148 . In order to be able to use only one identical design of the latch member  118  for both sides, the latch member receptacles  148  are not formed symmetrical to each other, but are formed identical and turned by 180°. 
         [0062]      FIGS. 7 and 8  show as enlarged details a first snap recess  150  and a second snap recess  152  formed at the housing for retaining the latch member  118  in the locked position and the unlocked position, respectively. The first and second snap recesses  150 ,  152  engage with a detent lug formed at the catch arm  132  of the latch member  118 . 
         [0063]    In order to be able to access the latch member  118  from the outside while it is in the final locked position, the bottom shell  102  comprises an opening  154 . The opening  154  preferably only gives access for a suitable tool, for instance a screw driver or the like, so that unintentional releasing of the latch member  118  can be avoided. 
         [0064]      FIGS. 9 to 11  show different views of the latch member  118  which is fabricated as a part separate from the housing. According to the present invention, one end of the latch member  118  is formed as a rail gripping projection  116  which in the locked state of the latch member  118  grips behind the edge of a top hat rail as this is known to a person skilled in the art. The opposing end of the latch member  118  is formed as an actuating region  122  for pushing the latch member  118  from the outside into the housing (as will be explained in more detail below). 
         [0065]    For securing the latch member  118  in the locked as well as in the unlocked state, the latch member  118  comprises a resilient catch arm  132 . The catch arm  132  has a detent lug  156  that engages with either the first snap recess  150  or with the second snap recess  152  of the bottom shell  102  for fixing the latch member  118  in a fully inserted locking position or a partly extracted unlocked position, respectively. For actuating the catch arm through the opening  154 , the catch arm has an essentially planar actuating section  158 . 
         [0066]    Stop edges  160  that are provided close to the rail gripping projection  116  interact with the stop angles  144  in order to prevent the latch member  118  to be pushed outside too far when being moved into the unlocked position. 
         [0067]    According to the present invention, the latch member  118  is embedded in a depression  162  that essentially follows the outer contour of the torsion spring  130  in a relaxed or only slightly compressed state. Thereby a support wall  146  is formed that experiences an evenly distributed pressure force exerted by the second leg  136  of the torsion spring  130  when in a compressed state. Moreover, at the peripheral end of the support wall  146 , a groove  142  is formed which firmly holds the end of the second leg  136 . Thus, the torsion spring  130  can be assembled at the latch member  118  without getting lost. 
         [0068]      FIGS. 12 and 13  show the mounting are the slack torsion spring  130  inside the depression  162 . As already mentioned, the flat torsion spring  130  may be slightly compressed in the state shown in  FIG. 13 , so that both legs are resting against the walls of the depression  162 . The outer end of the second lead  136  is firmly held in the groove  142 . In contrast thereto, the first leg  134  is movable towards the second leg  136 . In particular, the peripheral end of the first leg  134  is accessible for engaging with the retention pin  140  that is provided at the bottom shell  102  of the housing. 
         [0069]    The torsion spring  130  is depicted in  FIGS. 14 and 15 . According to the embodiment shown in the Figures, the torsion spring  130  is a flat spring that is formed from a cut and bent metal sheet. It is clear for a person skilled in the art, that of course also a wire spring, advantageously with some coil windings in the hinge region  138 , can be provided. 
         [0070]      FIG. 14  shows the flat torsion spring  130  in its tension free state. In this state, the angle α between the first and second legs  134 ,  136  is approximately 30°. Of course, also other angles α can be chosen according to the actual required displacement distances X. 
         [0071]    The operation of the latch member  118  according to the present invention will now be explained in more detail with reference to  FIGS. 16 to 21 . 
         [0072]    More specifically,  FIGS. 16 to 18  refer to the latch members  118  being in a locked position (“Position 1”), where the electric device  100  is firmly attached to the rail. On the other hand,  FIGS. 19 to 21  show the completely unlocked position (“Position 2”), where both latch members  118  are retracted from the rail so that the electrical device  100  and can be removed from the rail. Of course, it may also be sufficient to move only one of the latch members  118  into the unlocked position in order to be able to detach the electrical device  100  from the rail (this position is not shown in the Figures). 
         [0073]    In the locked position, both latch members  118  are pushed towards the rail recess  112  so that a DIN rail can be firmly gripped by the rail gripping projections  116 . The actuating regions  122  are completely within the bottom shell  102 . 
         [0074]    By inserting a suitable tool, such as a screwdriver, into the opening  154 , the underlying catch arm  132  can be actuated and the latch member  118  can accordingly be moved into Position 2 (shown in  FIGS. 19 to 21 ). In this position, the actuating region  122  is accessible from the outside. The detent lug  156  is held (with only a low force) in the second snap recess  152 . In order to ensure that for mounting the electrical device  100  on the rail only a minor manual force is needed to lock the electrical device  100  safely on the rail, the latch member  118  is biased against the housing by means of the compressed torsion spring  130  when the latch member  118  is in the unlocked state. 
         [0075]    Consequently, an operator can release the latch member  118  by pushing same inside. The spring force of the torsion spring  130  (which is compressed in Position 2) ensures that the latch member  118  slides back into the fully locked position. In the locked position the detent lug  156  snaps deeply into the first snap recess  150 , so that the latch member  118  can only be released by actuating the resilient catch arm  132  upwardly through the opening  154 . 
         [0076]    With reference to the previous Figures, an embodiment has been explained in detail where the bottom shell  102  comprises two opposing latch members  118 , so that two movable rail gripping projections  116  are extending into the rail recess  112 . 
         [0077]    In contrast to such a solution having two slidable rail gripping projections  116 ,  FIGS. 22 to 24  show a further embodiment of the present invention, where the bottom shell  102  comprises only one latch member receptacle  148  according to the principles of the present invention. For safely attaching the electrical device  100  to the DIN rail, the bottom shell  102  according to the present invention comprises a fixed rail gripping hook  164 . 
         [0078]    Independent from the particular embodiment of the bottom shell  102  of the housing, a cover  104  as shown in  FIGS. 25 to 26  can be clicked on to the bottom shell  102  to form a closed electrically insulating housing. As already mentioned above, the electrically insulating housing may of course also be formed from one integral part or from more than two parts. According to the embodiment shown in the Figures, the cover is fixed by resilient flaps  166  having recesses  108  for the snap hooks  106 . However, it is clear for a person skilled in the art that apart from a snap fixed connection also laser welding or gluing or any other suitable connection technique can be employed. 
         [0079]      FIGS. 28 to 31  show an alternative embodiment of the latch member  118 . According to this embodiment, the latch member  118  does not have a detent lug  156  which interacts with a first or second snap recess  150 ,  152  arranged on the lower surface of the bottom shell and which can be actuated through the opening  154  by means of a suitable tool. Instead, the resilient catch arm  132  has a catch hook arranged on the side oriented towards the cover  104 . This catch hook interacts with an inner wall  170  (as indicated in  FIG. 6 ) for ensuring that the latch member does not get lost prior to the assembly of the electrical device  100  on the DIN rail. According to this embodiment, the latch member  118  is not snap fixed in the open position, but needs to be held back by gripping a recess  172  for instance by means of a suitable tool. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 REFERENCE NUMERALS 
               
             
          
           
               
                   
                 Reference 
                   
               
               
                   
                 Numeral 
                 Description 
               
               
                   
                   
               
               
                   
                 100 
                 Electrical device 
               
               
                   
                 102 
                 Bottom shell 
               
               
                   
                 104 
                 Cover 
               
               
                   
                 106 
                 Snap hook 
               
               
                   
                 108 
                 Recess for snap hook 
               
               
                   
                 110 
                 Connector receptacle 
               
               
                   
                 112 
                 Rail recess 
               
               
                   
                 114 
                 Longitudinal axis of the rail 
               
               
                   
                 116 
                 Rail gripping projection 
               
               
                   
                 118 
                 Latch member 
               
               
                   
                 120 
                 Direction into locked position (towards middle  
               
               
                   
                   
                 axis of the rail) 
               
               
                   
                 121 
                 Direction into unlocked position (away from  
               
               
                   
                   
                 middle axis of the rail) 
               
               
                   
                 122 
                 Actuating region at the latch member 
               
               
                   
                 124 
                 Actuating opening at the housing 
               
               
                   
                 126 
                 Outer surface of actuating region 
               
               
                   
                 128 
                 Outer surface of bottom shell 
               
               
                   
                 130 
                 Flat torsion spring 
               
               
                   
                 132 
                 Catch arm 
               
               
                   
                 134 
                 First leg 
               
               
                   
                 136 
                 Second leg 
               
               
                   
                 138 
                 Hinge region 
               
               
                   
                 140 
                 Retention pin 
               
               
                   
                 142 
                 Groove at the latch member 
               
               
                   
                 144 
                 Stop angle 
               
               
                   
                 146 
                 Support wall 
               
               
                   
                 148 
                 Latch member receptacle 
               
               
                   
                 150 
                 First snap recess at housing 
               
               
                   
                 152 
                 Second snap recess at housing 
               
               
                   
                 154 
                 Opening for actuating the catch arm 
               
               
                   
                 156 
                 Detent lug 
               
               
                   
                 158 
                 Actuating section 
               
               
                   
                 160 
                 Stop edge 
               
               
                   
                 162 
                 Depression 
               
               
                   
                 164 
                 Fixed rail gripping hook 
               
               
                   
                 166 
                 Resilient flap 
               
               
                   
                 168 
                 Catch hook 
               
               
                   
                 170 
                 Inner wall of bottom shell 
               
               
                   
                 172 
                 Recess for holding open latch member 
               
               
                   
                 X 
                 Distance of movement between locked and  
               
               
                   
                   
                 unlocked position 
               
               
                   
                 α 
                 Angle between first and second leg