Abstract:
One aspect according to the invention can include a headrest operating device including a backside collision detecting device ( 20 ) that can produce an operational force upon detection of occurrence of backside collision. The operational force of the backside collision detecting device ( 20 ) can be transmitted to an operation member ( 25, 40, 50 ) via an interlocking member ( 23 ). A switching device ( 24 ) can switch a path of movement of the interlocking member ( 23 ) between an inoperative path and an operative path. When no backside collision occurs, the interlocking member ( 23 ) can move along the inoperative path, so that the operational force of the backside collision detecting device ( 20 ) cannot be transmitted to the operation member ( 25, 40, 50 ). When the backside collision occurs, the interlocking member ( 23 ) can move along the operative path, so that the operational force of the backside collision detecting device ( 20 ) can be transmitted to the operation member ( 25, 40, 50 ).

Description:
[0001]    This application claims priority to Japanese patent application serial number 2008-096036, the contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to headrest operating devices that are operable to move a headrest toward a head portion of a person seated on a vehicle seat of a vehicle when a backside collision occurs. In this specification, the term “back side collision” is used to mean the situation where an object, such as another vehicle, collides with the backside of the vehicle. 
         [0004]    2. Description of the Related Art 
         [0005]    A vehicle seat is known that has a mechanism for supporting a head portion of a seated person by moving a headrest toward the head portion of the seated person when a backside collision occurs. For example, Japanese Laid-Open Patent Publication No. 2005-95237 teaches a headrest operating device for moving a headrest based on a load that may be applied to a seat back by a seated person when a backside collision occurs. 
         [0006]    According to the headrest operating device of the above publication, a pressure receiving mechanism disposed within the seat back for receiving the load from the seated person is configured not to operate unless the backside collision occurs. Therefore, the headrest may not be accidentally moved even if the seated person leans on the seat back to apply a large load to the seat back. More specifically, the pressure receiving mechanism is coupled to a weight that can move by an inertia force when the backside collision occurs. The weight is coupled to a lock mechanism for locking and unlocking the headrest, so that the lock mechanism maintains the lock condition of the headrest unless the inertia force moves the weight. 
         [0007]    However, the headrest operating device of the above publication requires a relatively large installation space for the pressure receiving mechanism, the lock mechanism and a device for detecting the backside collision. 
         [0008]    Thus, there is a need in the art for a headrest operating device that has an accidental operation preventing device and has a compact construction. 
       SUMMARY OF THE INVENTION 
       [0009]    One aspect according to the invention can include a headrest operating device including a backside collision detecting device that can produce an operational force upon detection of occurrence of backside collision. The operational force of the backside collision detecting device can be transmitted to an operation member via an interlocking member. A switching device can switch a path of movement of the interlocking member between an inoperative path and an operative path. When no backside collision occurs, the interlocking member can move along the inoperative path, so that the operational force of the backside collision detecting device cannot be transmitted to the operation member. When the backside collision occurs, the interlocking member can move along the operative path, so that the operational force of the backside collision detecting device can be transmitted to the operation member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a general perspective view of a headrest operating device according to an embodiment of the present invention; 
           [0011]      FIG. 2  is a view showing the operation for releasing an initial position holding state of a headrest of the headrest operating device when a backside collision occurs; 
           [0012]      FIG. 3  is a front view showing an initial position of a pressure receiving mechanism for detecting the backside collision of the headrest operating device; 
           [0013]      FIG. 4  is a schematic side view showing the initial position of the pressure receiving mechanism; 
           [0014]      FIG. 5  is a side view showing the operation of the pressure receiving mechanism when a leaning load is applied to a backrest under a normal condition; 
           [0015]      FIG. 6  is a side view showing the operation of the pressure receiving mechanism when a leaning load is applied to the backrest at the occurrence of the backside collision; 
           [0016]      FIG. 7  is a side view showing the operation of pulling an operation cable of the backrest operating device; 
           [0017]      FIG. 8  is a front view showing the operation of pulling the operation cable; 
           [0018]      FIG. 9  is an exploded perspective view showing the operation for inserting the operation cable and a stay; 
           [0019]      FIG. 10  is a perspective view showing the state where the operation cable is inserted into a support; 
           [0020]      FIG. 11  is a perspective view showing the state where the stay is inserted into the support; 
           [0021]      FIG. 12  is a sectional view taken along line XII-XII in  FIG. 11 ; 
           [0022]      FIG. 13  is a side view showing the state where the headrest is held at the initial position; 
           [0023]      FIG. 14  is a view showing the internal structure of the headrest and is a sectional view taken along line XIV-XIV in  FIG. 19 ; 
           [0024]      FIG. 15  is a side view showing the state where the headrest is positioned at a midpoint of during the forward movement; 
           [0025]      FIG. 16  is a side view showing the state where the headrest is moved to a collision accommodating position; 
           [0026]      FIG. 17  is a side view showing the state where the headrest is positioned at a midpoint during the movement for returning to the initial position; 
           [0027]      FIG. 18  is a side view showing the state where the headrest moves toward the initial position under the guide of a lever; and 
           [0028]      FIG. 19  is a view of the headrest as viewed from a direction indicated by arrow XIV in  FIG. 16 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    Each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide improved headrest operating devices. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in conjunction with one another, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Moreover, various features of the representative examples and the dependent claims may be combined in ways that are not specifically enumerated in order to provide additional useful embodiments of the present teachings. 
         [0030]    An embodiment of the present invention will now be described with reference to  FIG. 1  through  FIG. 19 .  FIG. 1  shows a vehicle seat  1  incorporating a headrest operating device of the embodiment. The vehicle seat  1  is constituted by a seat back  2  serving as a backrest of a seated person, a seat cushion  3  serving as a seating portion, and a headrest  4  serving as a head support. Further, according to the vehicle seat  1 , when another vehicle collides with the backside of the vehicle incorporating the headrest operating device, the headrest  4  can move toward the head portion of the seated person instantaneously by the operation of the headrest operating device as will be explained later. In this specification, the situation where another vehicle collides with the backside of the vehicle will be called “backside collision.” 
         [0031]    In  FIG. 1  and some of the other figures, in order to enable the inner structures of the headrest  4  and the seat back  2  to be easily understood, illustration of the skin structures of the headrest  4  and the seat back  2  is omitted. Here, the headrest  4  is mounted to and supported on an upper face portion of the seat back  2  by inserting two pipe-like stays  4 B extending downwardly from the headrest  4  into tubular supports  2 S provided at the upper face portion of the seat back  2 . 
         [0032]    The supports  2 S are integrally fixed to an upper ann portion (upper frame Fu) of a back frame  2 F of a gate type constituting a skeleton of the seat back  2 . Further, the upper frame Fu is integrally coupled to both side frames Fs in such a manner that upper end portions of the both side frames Fs of the back frame  2 F are connected to each other. The above-described headrest  4  is normally held at an upper portion of the seat back  2  for receiving the head portion of the seated person on a rear side thereof when the backside collision does not occur. 
         [0033]    When the backside collision occurs, only a support portion  4 A on a front side of the headrest  4 , which receives the head portion, can instantaneously move forwardly. Therefore, when the backside collision occurs, only the support portion  4 A for receiving the head portion can first move to a position proximate to the seated person, whose body may take a position floating forwardly from the headrest  4  and the seat back  2 , and a movement of the head portion inclining rearward by a momentum produced due to the collision form the backside can be prevented at an early stage. Therefore, a load that may be applied to the neck portion of the seated person can be alleviated, and therefore, a whiplash injury can be prevented. 
         [0034]    The forward movement of the support portion  4 A performed when the backside collision occurs is enabled by a headrest moving mechanism  10  assembled within the headrest  4 . As shown in  FIG. 13 , the headrest moving mechanism  10  normally locks and holds the support portion  4 A at an initial position by restricting the forward movement of the support portion  4 A when the backside collision does not occurs. Here, the support portion  4 A is urged in a direction toward the head portion normally by an biasing force of a tension spring  16  connected between the support portion  4 A and a headrest base portion  4 C constituting a main body of the headrest  4 . The support portion  4 A is normally held at the initial position against the urging force or the biasing force the tension spring  16  when the backside collision does not occur. 
         [0035]    Further, the headrest moving mechanism  10  can move the support portion  4 A forwardly by the biasing force of the tension spring  16  by releasing the state of restricting the movement of the support portion  4 A when the backside collision occurs. More specifically, the headrest moving mechanism  10  moves the support portion  4 A to a position proximate to the rear side of the head (hereinafter called “collision accommodating position”) as shown in  FIG. 15  by moving the support portion  4 A forwardly and upwardly along shapes of elongated holes  11 H formed in the headrest base portion  4 C that will be explained later. 
         [0036]    Here, according to the headrest moving mechanism  10 , because the support portion  4 A can move to the collision accommodating position, the support portion  4 A may not be pushed back rearwardly even in the case that a rearward inclining load of the head portion caused by the backside collision is applied to the support portion  4 A. Therefore, when the back collision occurs, the head portion can be supported in stable by the support portion  4 A positioned at the collision accommodating position. 
         [0037]    Meanwhile, referring back to  FIG. 1 , the release of the state of restricting the movement of the support portion  4 A in the case of the backside collision is carried out by the operation of pushing up a push rod  50  that is inserted into the right side tubular stay  4 B of the headrest  4 . As shown by  FIG. 2 , an upper end portion of the push rod  50  is connected to a releasing member  15  provided for a lock releasing operation of the headrest moving mechanism  10 . Further, a lower end portion of the push rod  50  is connected to an upper end portion of an operation cable  40  wired within the seat back  2 . 
         [0038]    Here, as shown in  FIG. 1 , the operation cable  40  has an inner cable  41  and an outer cable  42 . A lower end portion of the inner cable  41  is connected to a pressure receiving mechanism  20  serving as a backside collision detecting mechanism and arranged within the seat back  2 . Further, a lower end portion of the outer cable  42  is engaged with and fixed to a base plate  2 B that is fixedly mounted to a lower reinforcement plate  2 D extending between the both side frames Fs of the back frame  2 F. 
         [0039]    Here, the pressure receiving mechanism  20  includes a formed wire  21  extended in a vertically prolonged manner at a center portion of the seat back  2 , and a pressure receiving plate  22  arranged at a lower portion to be opposed to the waist portion of the seated person. The pressure receiving mechanism  20  is constructed such that, when the backside collision occurs, the occurrence of the back side collision is detected as an operation movement amount of the pressure receiving plate  22  that is pushed toward the rear side by the back portion of the seated person, which may be inclined rearward by the momentum produced by the backside collision. 
         [0040]    Specifically, the above-described formed wire  21  is formed by folding and bending a single wire member into a frame-like shape for contacting with the back portion or the waist portion of the seated person. Further, upper ends of the formed wire  21  arranged on the left and right sides are attached to an upper reinforcement plate  2 U extending between the both side frames Fs by clips Cr such that the upper ends can move only in a height direction or a vertical direction. 
         [0041]    Further, the formed wire  21  is held in a position for contacting with a cushioning pad (not illustrated) arranged on a front side thereof because a lower frame-like portion of the formed wire  21  is normally pushed forwardly by biasing forces of bent springs  2 T interleaved between the formed wire  21  and a lower reinforcement plate  2 D. Here, the rear end portions of the bent springs  2 T are respectively engaged with cut-to-rise portions Dc formed by cutting and rising portions of the lower reinforcement plate  2 D, and the front end portions of the bent springs  2 T are integrally coupled and fixed to the frame shape portion of the formed wire  21 . 
         [0042]    Therefore, the formed wire  21  can resiliently support the back portion of the seated person while the formed wire  21  deforms as it is pushed rearwardly about upper fulcrum points (support points by the clips Cr) by a load applied by the seated person who leans on the seat back  2 . 
         [0043]    Next, as shown in  FIG. 3  and  FIG. 4 , the pressure receiving plate  22  is arranged such that its face is oriented toward the front side of the seat back  2 , and a lower end portion of the pressure receiving plate  22  is rotatably connected to the above-described base plate  2 B by a connecting shaft  22 A. As shown in  FIG. 4 , the pressure receiving plate  22  is normally forced in a rotational direction for tilting forwardly in the counterclockwise direction by a biasing force of a torsional spring  22 S wound around the connecting shaft  22 A. 
         [0044]    Therefore, the pressure receiving plate  22  is maintained in a state where it contacts with the cushion pad (not illustrated) arranged on the front side thereof as it is pushed forwardly. Further, the torsional spring  22 S is assembled in a previously twisted state, one end of the torsional spring  22 S is engaged by a front face portion of the base plate  2 B, and the other end thereof is engaged by a back face (rear face) of the pressure receiving plate  22 . Therefore, as shown in  FIG. 5  through  FIG. 7 , the pressure receiving plate  22  is pushed toward the rear side about a fulcrum (connecting shaft  22 A) on a lower side thereof by a load of the seated person who leans on the seat back  2 . 
         [0045]    Further, referring back to  FIG. 4 , a connecting arm  23  is connected between the pressure receiving plate  22  and the base plate  2 B and can slide along the base plate  2 B to follow the movement of the pressure receiving plate  22  that is pushed to pivot toward the rear side. Here, the connecting arm  23  serves as an interlocking member. The connecting arm  23  is connected to the pressure receiving plate  22  by a connecting shaft  23 A and can rotate about an axis. In addition, the connecting arm  23  is connected to the base plate  2 B by a slide pin  23 P such that it can slide and rotate within an elongated through hole Bh having a configuration elongated in a vertical direction. Here, the slide pin  23 P serves as an engaging member. 
         [0046]    Further, in the state where the pressure receiving plate  22  is held at the initial position, the connecting arm  23  is held to take a position inclined upwardly to the right in which a point of connection with the base plate  2 B (slide pin  23 P) is positioned higher than a point of connection with the pressure receiving plate  22  (connecting shaft  23 A). Further, at this occasion, the point of connecting between the connecting arm  23  and the base plate  2 B (slide pin  23 P) is positioned within a lower end portion of the elongated hole Bh. 
         [0047]    Here, the connecting arm  23  is normally urged in a rotational direction for tilting forwardly in the counterclockwise direction relative to the pressure receiving plate  22  by a biasing force of a torsional spring  23 S wound around the connecting shaft  23 A. Therefore, the slide pin  23 P is held normally in a state where it is pressed against a front face Fr of an inner peripheral face of the elongated hole Bh. Here, the position of the pressure receiving plate  22  shown in  FIG. 4  is taken when the pressure receiving plate  22  is pushed and moved by the seated person who leans on the seat back  2  in an ordinarily manner. 
         [0048]    As shown in  FIG. 4 , the elongated hole Bh formed in the base plate  2 B has a configuration in which a middle portion of the elongated hole Bh has a width widened in a left and right direction in  FIG. 4  (front and rear direction). Further, a lower end portion of the elongated hole Bh has a width narrowed in the left and right direction in  FIG. 4  (front and rear direction) toward the side of the front face Fr in comparison with the width of the middle portion. Further, a rear face Re of a width changing portion positioned between the middle portion and the lower end portion of the elongated hole Bh is formed as an inclined face, and therefore, the width changes smoothly at the width changing portion. 
         [0049]    As shown in  FIG. 4 , when the pressure receiving plate  22  is pushed to move slightly rearward by the back leaning load of the seated person, the slide pin  23 P of the connecting arm  23  is brought to take a position where the slide pin  23 P is slightly slid upward from the lower end portion of the elongated hole Bh. Further, as shown in  FIG. 5 , when the pressure receiving plate  22  is pushed to move rearward by a stronger force, for example, in the case that a person having a large physical constitution is seated, the slide pin  23 P is slid further upward to follow the rotation of the connecting arm  23 . 
         [0050]    At this occasion, the slide pin  23 P is slid upward while maintaining the contacting state with the front face Fr within the elongated hole Bh by the biasing force of the torsional spring  23 S interleaved between the above-described connecting arm  23  and the pressure receiving plate  22 . However, as shown in  FIG. 6  and  FIG. 7 , when the pressure receiving plate  22  is pushed to move rearward by the occurrence of the backside collision, the slide pin  23 P is slid upward while maintaining the pressing state against the rear face Re of the elongated hole Bh as it is moved reward against the biasing force of the torsional spring  23 S by receiving the inertia movement of a weight  24  attached to the slide pin  23 P. 
         [0051]    Here, referring back to  FIG. 3 , a pivoting plate  25  in a shape of an elongated plate is rotatably connected to the above-described base plate  2 B by a connecting shaft  25 A. The pivoting plate  25  has a right arm portion and a left arm portion. The right arm portion is formed with a receiving portion  25 B folded in a plate thickness direction. The left arm portion is formed with an elongated hole  25 C, within which a lower end portion of the inner cable  41  of the operation cable  40  engages. Here, the operation cable  40  has a double structure in which the inner cable  41  in a shape of a linear solid cable that is more flexible than the outer cable  42  is inserted into the outer cable  42  in a shape of a tubular flexible cable. 
         [0052]    Further, a lower end portion of the outer cable  42  is engaged with and fixed to the base plate  2 B, and the lower end portion of the inner cable  41  is engaged with and fixed to an elongated hole  25 C formed in the above-described pivoting plate  25 . Therefore, the operation cable  40  is operated to be pulled in such a manner that the inner cable  41  is pulled out from the lower end portion of the outer cable  42  by the movement of the pivoting plate  25  that is operated to rotate in the counterclockwise direction about the connecting shaft  25 A as shown in  FIG. 3 . 
         [0053]    Further, in  FIG. 4 , the operation for rotating the pivoting plate  25  in the counterclockwise direction is carried out in accordance with the sliding movement of the above-described slide pin  23 P within the elongated hole Bh caused due to the backside collision. Specifically, the receiving portion  25 B formed at the above-described pivoting plate  25  is normally held in a position extending to be exposed within the elongated hole Bh. Here, referring back to  FIG. 3 , the pivoting plate  25  is normally maintained in the initial rotational position shown in  FIG. 3  due to connection with the inner cable  41  because an amount of pulling out the inner cable  41  from the outer cable  42  is limited at the upper end portion of the operation cable  40 . 
         [0054]    As shown in  FIG. 5 , the receiving portion  25 B of the pivoting plate  25  does not interfere with the slide pin  23 P and does not carry out any operation since the slide pin  23 P is slid on an inoperative path along the front face Fr of the elongated hole Bh even in the event that the pressure receiving plate  22  is pushed to move to the rear side under the normal condition without occurrence of the backside collision. However, as shown in  FIG. 6  and  FIG. 7 , when the pressure receiving plate  22  is pushed to move to the rear side due to occurrence of backside collision, the receiving portion  25 B is pushed to move by the slide pin  23 P since the slide pin  23 P is slid on an operative path along the rear face Re of the elongated hole Bh in accordance with the inertia movement of the weight  24 . 
         [0055]    Therefore, as shown in  FIG. 8 , the operation cable  40  is operated in such a manner that the inner cable  41  is pulled out from the lower end portion of the outer cable  42  by the rotation of the pivoting plate  25  in the clockwise direction. Further, the operation movement amount of the pulled inner cable  41  is transmitted to the push rod  50  as an operation movement amount for pushing the push rod  50  in  FIG. 2 , so that the headrest  4  is operated to move. 
         [0056]    Next, a structure of transmitting the operating force from the operation cable  40  to the push rod  50  will be explained. The operation cable  40  is wired within the seat back  2 , and a wiring position is fixed by a bundling band B that can bundle the operation cable  40  together with the formed wire  21 . Further, as shown in  FIG. 2 , an upper end portion of the operation cable  40  is inserted into the tubular support  2 S from its lower side, so that the operation cable  40  is capable of pushing the push rod  50  within the stay  4 B inserted into the tubular support  2 S from its upper side. 
         [0057]    Here, the operation cable  40  has engaging projections  41 P projecting in a T-like shape and formed at the upper end portion of the inner cable  41 . The engaging projections  41 P extend radially outward through elongated holes  42 S formed to penetrate a peripheral wall of an upper end of the outer cable  42 , respectively. Therefore, the inner cable  41  can move in an axial direction relative to the outer cable  42  within a range in which the engaging projections  41 P projecting in the T-like shape can move in an axial direction along the elongated holes  42 S. The engaging projections  41 P as well as the elongated holes  42 S are formed at two positions along the circumferential direction of the inner cable  41  and the outer cable  42  and are positioned to be symmetrical. 
         [0058]    Further, the upper end portion of the outer cable  42  is formed with a head portion  42 H in a shape of closing the open end portion of the inner space of the outer cable  42 . As shown in  FIG. 9 , the operation cable  40  can be temporally held in a downwardly suspended state at the upper end portion by the tubular support  2 S when the upper end portion of the operation cable  40  is inserted into the tubular support  2 S (refer to  FIG. 10 ). By inserting the stay  4 B into the support  2 S from the upper side in the suspended state of the operation cable  40 , the operation cable  40  is shifted from the state suspended by the support  2 S to a state suspended by the stay  4 B. 
         [0059]    Further, because the operation cable  40  is brought to the state suspended by the stay  4 B in this way, the operation cable  40  can reversely transmit an operation force (operation movement amount) of pulling from the lower end side to the push rod  50  inserted into the stay  4 B as a push operation force (push operation amount). That is, the operation cable  40  and the push rod  50  are brought into a state coupled to each other to enable transmission of the operation force to each other. 
         [0060]    An explanation will be given to a connecting structure of connecting the above-described operation cable  40  and the push rod  50  to enable transmission of the operation force to each other. Here, as shown in  FIG. 9 , a peripheral wall of the support  2 S is formed with insertion holes Sd each extending in a slit-like shape in an axial direction from the lower end to the upper end of the peripheral wall. The insertion holes Sd are formed at two positions along the circumferential direction of the support  2 S and positioned to be symmetrical with respect to the axis of the support  2 S. 
         [0061]    The insertion holes Sd can receive the respective engaging projections  41 P formed on the inner cable  41  described above as the operation cable  40  is inserted into the support  2 S. Here, the upper end portions defining terminal end portions of the insertion holes Sd are bent in left and right directions, respectively, that may correspond to the circumferential direction of the support  2 S. 
         [0062]    Specifically, the terminal end portion of the insertion hole Sd indicated by a solid line in  FIG. 9  is bent leftward. The terminal end portion of the insertion hole Sd indicated by a chain line is bent rightward and is symmetrical with the insertion hole Sd indicated by the solid line. Here, the shapes of the terminal end portions of the insertion holes Sd are formed to extend obliquely downward from a horizontal plane. Therefore, when the engaging projections  41 P are inserted into the insertion holes Sd and positioned at the terminal end portions of the insertion holes Sd, the engaging projections  41 P (operation cable  40 ) can be held in a state suspended from the support  2 S and incapable of dropping by the weight of the operation cable  40 . 
         [0063]    Here, the upper end portion of the outer cable  42  is formed by a synthetic resin, and a middle portion thereof is formed with a guide portion  42 D that is radially outwardly bulged. The guide portion  42 D has a serration-like shape with alternate concave and convex potions arranged in the circumferential direction and an outer diameter of the guide portion  42 D is set to be matched with an inner diameter of the support  2 S. Therefore, when the operation cable  40  is inserted into the tubular support  2 S, the guide portion  42 D gently follows the configuration of the inner circumference of the support  2 S, and the operation cable  40  can be smoothly inserted without being rattled. 
         [0064]    On the other hand, as shown in  FIG. 10 , a peripheral wall of the stay  4 B inserted into the support  2 S from the upper side has receiving holes Bd each extending upwardly in a slit-like shape in an axial direction from a lower end portion of the peripheral wall. The receiving holes Bd are formed at two portions along the circumferential direction of the stay  4  and are symmetrical with each other with respect to the axis of the stay  4 B. When the stay  4 B is inserted into the tubular support  2 S from the upper side, the receiving holes Bd can receive the respective engaging projections  41 P of the inner cable  41  suspended downwardly within the support  2 S. 
         [0065]    Further, when the stay  4 B is inserted further into the support  2 S, the engaging projections  41 P move upward in accordance with the shapes of the respective receiving holes Bd. Here, the upper end portions defining the terminal end portions of the receiving holes Bd are bent in directions opposite to bending directions of the respective insertion holes Sd formed in the support  2 S. 
         [0066]    Further, the receiving holes Bd are bent in the circumferential direction such that their orientations gradually and smoothly changed from the axial direction to the horizontal direction. Therefore, when a push operation force is applied in the axial direction for inserting the stay  4 B into the support  2 S, the receiving holes Bd enables the respective engaging projections  41 P disposed within the support  2 S to smoothly move to positions within the terminal end portions of the receiving holes Bd that are oriented substantially horizontally. 
         [0067]    Therefore, by inserting the stay  4 B into the support  2 S, the engaging projections  41 P moves to return m the circumferential direction away from the terminal end positions of the insertion holes Sd of the support  2 S while being guided by the bent shapes of the respective receiving holes Bd formed in the stay  4 B. Further, by further continuing the insertion movement of the stay  4 B, the engaging projections  41 P,  41 P are brought to reach the bent terminal end positions of the respective receiving holes Bd of the stay  4 B and to return to positions within axially extending hole portions of the respective insertion holes Sd of the support  2 S. 
         [0068]    Therefore, the engaging projections  41 P,  41 P are shifted from the state suspended by the support  2 S to the state suspended by the stay  4 B. Hence, the engaging projections  41 P are brought into a state where the engaging projections  41 P are prevented from moving in the axial direction relative to the stay  4 B but are permitted to move in the axial direction relative to the support  2 S. At this occasion, the engaging projections  41 P are positioned within the axially extending hole portions of the respective insertion holes Sd, and therefore, the engaging projections  41 P are prevented from moving in the circumferential direction relative to the support  2 S. 
         [0069]    Therefore, the engaging projections  41 P are held within the terminal end positions of the respective receiving holes Bd. Further, the stay  4 B can move in the axial direction relative to the support  2 S as the stay  4 B is connected with the inner cable  41  to be able to move together therewith in the axial direction. 
         [0070]    Here, as shown in  FIG. 12 , a locking claw St in a plate-like shape is disposed within an insertion hole Sa of the support  2 S and is engageable with an engaging groove Bk formed in an outer peripheral wall of the stay  4 B. The locking claw St is normally urged to be held in a state projecting into the insertion hole Sa. The locking claw St can move out of the insertion hole Sa by a pushing operation of a knob Sb in a transverse direction. 
         [0071]    Therefore, as the stay  4 B is inserted into the insertion hole Sa, the engaging groove Bk is brought to be opposed to the locking claw St, so that the locking claw St can engage the locking groove Bk by the urging force. Therefore, the movement of the stay  4 B in the inserting direction can be restricted. This restricting state can be released by removing the looking claw St from the engaging groove Bk by the pushing operation of the knob Sb. 
         [0072]    A plurality of the engaging grooves Bk may be formed in the stay  4 B and arranged in the axial direction. With this arrangement, the installation height of the headrest  4  can be freely adjusted by operating the knob Sb. Further, when the lower end portion of the stay  4 B and the upper end portion of the inner cable  41  are connected to each other in the axial direction by the operation of inserting the stay  4 B, a state where the head portion  42 H of the outer cable  42  is inserted into the stay  4 B from a lower side is resulted. 
         [0073]    Therefore, as shown in  FIG. 12 , a lower end portion of the push rod  50  inserted into the stay  4 B and the head portion  42 H of the outer cable  42  are brought to be proximate to each other in the axial direction. Further, a small gap may be formed between the lower end portion of the push rod  50  and the head portion  42 H. With this small gap, the push rod  50  can be prevented from being accidentally pushed by the head portion  42 H of the outer cable  42  during the operation for inserting the stay  4 B. 
         [0074]    Further, when the head portion  42 H of the outer cable  42  and the lower end portion of the push rod  50  are positioned to be proximate to each other in the axial direction, as described above with reference to  FIG. 2 , the operation force produced by pulling the operation cable  40  on the lower end side is transmitted reversely from the head portion  42 H of the outer cable  42  to the push rod  50  as the push operation force. This is because the upper end portion of the inner cable  41  of the operation cable  40  is connected to the stay  4 B in the axial direction by the above-described operation of inserting of the stay  4 B. 
         [0075]    Further, the upper end portion (head portion  42 H) of the outer cable  42  can move in the axial direction relative to the inner cable  41 . Therefore, by pulling the inner cable  41  on the lower side, in accordance with the operation movement amount of pulling the inner cable  41 , the upper end portion of the outer cable  42  is pushed upward relative to the upper end portion of the inner cable  41  that is connected with the stay  4 B. 
         [0076]    Therefore, as shown in  FIG. 2 , when the backside collision occurs, the operation force of pulling the operation cable  40  can be transmitted as the push operation force to the push rod  50  positioned within the stay  4 B, and the releasing member  15  can be operated for releasing the lock condition of the headrest moving mechanism  10 . 
         [0077]    Meanwhile, referring back to  FIG. 1 , the headrest  4  can be removed from the seat back  2  by pulling the stays  4 B out from the supports  2 S. As the stays  4 B are pulled out, the connecting state between the inner cable  41  and the stay  4 B mentioned above can be released. Specifically, as is best shown in  FIG. 10 , when the stay  4 B is pulled upward out from the support  2 S, the engaging projections  41 P move in the circumferential direction under the guide of the bent shapes of the respective insertion holes Sd of the support  2 S. 
         [0078]    Therefore, the engaging projections  41 P,  41 P return from the state held at the terminal end positions of the respective receiving holes Bd in a direction away from the terminal end positions, reach the terminal end positions of the respective insertion holes Sd under the guide of their bent shapes, and return to the axially extending hole portions of the respective receiving holes Bd. Therefore, as shown in  FIG. 10 , the engaging projections  41 P return from the state suspended by the stay  4 B to the state suspended by the support  2 S. Further, the axially connected state between the head portion  42 H of the outer cable  42  and the push rod  50  is released. 
         [0079]    Next, the headrest moving mechanism  10  will be explained with reference to  FIG. 13  through  FIG. 19 . Further, the construction of the headrest moving mechanism  10  is best shown in  FIG. 16 , and therefore, components of the headrest moving mechanism  10  will be explained primarily with reference to  FIG. 16 . The headrest moving mechanism  10  is configured to be able to move the support portion  4 A forward relative to the headrest base portion  4 C when the backside collision occurs. 
         [0080]    Specifically, the headrest moving mechanism  10  includes a pair of left and right link members  12  connecting the support portion  4 A and the headrest base portion  4 C, a pair of left and right arm members  13  extending rearward from the support portion  4 A, hooks  14  for releasably locking the support portion  4 A at an initial position, the releasing member  15  for releasing the lock state achieved by the hooks  14 , the tension spring  16  for urging the support portion  4 A forward, and lever members  17  for guiding the support portion  4 A when the support portion  4 A returns to the initial position. 
         [0081]    Here, the headrest base portion  4 C is formed of a synthetic resin and has a rear face portion  11 B, a bottom face portion  11 D, both side face portions  11 S and an upper face portion  11 U each having a plate-like configuration. More specifically, the bottom face portion  11 D extends forwardly from a lower edge of the rear face portion  11 B. Further, the side face portions  11 S extend vertically upward from opposite side portions with respect to a width direction of the headrest base portion  4 C. Further, the upper face portion  11 U connects between upper edge portions of the side face portions  11 S. 
         [0082]    Here,  FIG. 19  shows a view as viewed in a direction of arrow XIX in  FIG. 16 , that is, a view of the headrest  4  as viewed from an obliquely downward front side. As shown in  FIG. 19 , between the both side face portions  11 S of the headrest base portion  4 C, a plurality of vertical ribs  11 R are form in parallel with the side face portions  11 S for reinforcement of the headrest base portion  4 C. Further, upper end portions of the stays  4 B are inserted into the bottom face portion  11 D of the headrest base portion  4 C so as to be fixedly joined thereto. 
         [0083]    The stays  4 B have tubular shapes and have open upper ends. The stays  4 B are fixed to the bottom face portion  11 D, while the open upper ends of the stays  4 B are exposed on the upper side of the bottom face portion  11 D. Further, referring back to  FIG. 16 , the both side face portions  11 S of the headrest base portion  4 C have elongated holes  11 H each formed throughout the thickness of the side face portion  11 S and having a wavy shape. Each of the elongated holes  11 H is formed with a first stopper groove H 1  and a second stopper groove H 2  positioned between a lower end portion H 0  and an upper end portion H 3 . The first and second stopper grooves H 1  and H 2  are recessed toward the rear side (right side in  FIG. 16 ) in a wavy shape. 
         [0084]    The pair of link members  12 ,  12  are formed of a synthetic resin and serve as connecting members for connecting between the headrest base portion  4 C and the support portion  4 A. The link members  12  are arranged to arranged in a width direction and have respective end portions that are pivotally connected to portions proximate to an upper end side of the headrest base portion  4 C and portions on a rear face side of the support portion  4 A. 
         [0085]    More specifically, rear end portions of the link members  12  are pivotally supported by a connecting shaft  12 A extending through and between the side face portions  11 S of the headrest base portion  4 C. Here, as shown in  FIG. 19 , each of the rear end portions of the link members  12  connected to the headrest base portion  4 C are positioned between the side face portion  11 S and the rib  11 R. 
         [0086]    Further, referring back to  FIG. 16 , front end portions of the link members  12  for connecting with the support portion  4 A are pivotally supported by connecting shafts  12 B that are connected to portions on a rear face side of the support portion  4 A and extend in a width direction. The connecting shafts  12 B extended in a width direction in parallel with each other. As will be understood from  FIG. 14 , the link members  12  may contact with the upper face portion  11 U of the headrest base portion  4 C as they pivot in the clockwise direction about the connecting shaft  12 A supporting the rear end portions thereof, and therefore, the clockwise pivoting movement of the link members  12  is restricted. 
         [0087]    Next, referring back to  FIG. 16 , the pair of arm members  13  are configured as extended members extending from the rear face portion of the support portion  4 A and formed integrally therewith. The arm members  13  are arranged in the width direction of the support portion  4 A. Here, the support portion  4 A is formed in a shape of a plate curved along its entire length and may be integrally molded by a synthetic resin Further, the rear end portions of the arm members  13  formed with the rear face portion of the support portion  4 A are connected to each other by a connecting shaft  13 A extending in the width direction. 
         [0088]    More specifically, as shown in  FIG. 19 , the rear end portions of the arm members  13  is positioned between the outer side ribs  11 R and the inner side ribs  11 R. Further, the connecting shaft  13 A connecting between the rear end portions of the arm members  13  is arranged in parallel with the connecting shaft  12 A and the connecting shafts  12 B. 
         [0089]    The both end portions of the connecting shaft  13 A are inserted into the respective elongated holes  11 H formed in the side face portions  11 S of the headrest base portion  4 C. Therefore, the connecting shaft  13 A can slide in forward and rearward directions and upward and downward directions only within ranges of shapes of the elongated holes  11 H. Further, the ribs  11 R (see  FIG. 19 ) formed between the side face portions  11 S are configured not to interfere with the connecting shaft  13 A that may move within the elongated holes  11 H. 
         [0090]    Next, with reference to  FIG. 14 , the hooks  14  are formed in a cam shape and arranged in the width direction at positions proximate to a lower end of the headrest base portion  4 C. The hooks  14  serve as holding members for holding the connecting shaft  13 A at the lower end portions H 0  that define the initial position of the connecting shaft  13 A. Here, as shown in  FIG. 19 , the hooks  14  are arranged between the side face portions  11 S and the ribs  11 R disposed on the inner sides thereof and are pivotally supported by connecting shafts  14 A inserted therebetween. 
         [0091]    Further, referring back to  FIG. 14 , at positions along the circumferential direction of outer peripheral edge portion of each hook  14 , an upper jaw portion  14 B and a lower jaw portion  14 C are formed to project radially outward. Therefore, a radially inwardly concaved recess is formed between the upper jaw portion  14 B and the lower jaw portion  14 C and serves to receive the connecting shaft  13 A. 
         [0092]    Further, a torsional spring  14 S is interleaved between each hook  14  and the headrest base portion  4 C. The torsional spring  14 S is assembled in a twisted state and urges the hook  14  in the counterclockwise direction from a position shown in  FIG. 14 . Further,  14 , a locking groove  14 D configured as a stepped recess is formed in the outer peripheral edge portion of each hook  14 . 
         [0093]    A pair of engaging arm portions  15 C that are components of the releasing member  15  are engaged with and abut to the respective locking grooves  14 D of the hooks  14 . Therefore, the hooks  14  are normally held not to pivot in the counterclockwise direction irrespective of the biasing force of the torsional springs  14 S. Further, in the state where the hooks  14  are prevented from pivoting in the counterclockwise direction, the connecting shaft  13 A is received within the recesses formed between the upper jaw portions  14 B and the lower jaw portions  14 C of the hooks  14 , so that the connecting shaft  13 A is held to be positioned within the lower end portions HO of the elongated holes  11 H. 
         [0094]    Here, as shown in  FIG. 13 , the connecting shaft  13 A is urged normally in a direction toward the connecting shaft  12 A by a biasing force of the tension spring  16  interleaved between the connecting shaft  13 A and the connecting shaft  12 A, and therefore, the connecting shaft  13 A is urged toward the upper end portions H 3  along shapes of the elongated holes  11 H. However, the connecting shaft  13 A is normally held at the lower end portions H 0  of the elongated holes  11 H (initial position) by the hooks  14  against the biasing force of the tension spring  16 . 
         [0095]    Further, referring back to  FIG. 14 , the hooks  14  can pivot in the counterclockwise direction by the biasing forces of the torsional springs  14 S when the engaging arm portions  15 C pivot in the counterclockwise direction to release the engagement with the hooks  14 . Therefore, as indicated by imaginary lines in  FIG. 14 , the upper jaw portions  14 B of the hooks  14  move to a position outside of the elongated holes  11 H (i.e., a position not to oppose to the elongated holes  11 H), and the lower jaw portions  14 C move to a position exposed to the elongated holes  11 H (i.e., a position opposed to the elongated holes  11 H) as if they are pushed up from the lower side. 
         [0096]    Therefore, the state of locking the connecting shaft  13 A by the hooks  14  is released, and therefore, as shown in  FIG. 15  and  FIG. 16 , the connecting shaft  13 A moves forwardly upwardly along shapes of the elongated holes  11 H by the biasing force of the tension spring  16 . Hence, the support portion  4 A moves forwardly upwardly relative to the headrest base portion  4 C to follow the pivoting movement of the link members  12 . 
         [0097]    Here, referring back to  FIG. 14 , the engaging ann portions  15 C for preventing the hooks  14  from being pivoted in the counterclockwise direction are arranged in the width direction for engagement with the respective hooks  14 . As shown in  FIG. 19 , similar to the hooks  14 , the engaging ann portions  15 C are arranged between the side face portions  11 S and the ribs  11 R disposed on the inner sides. 
         [0098]    Further, a connecting shaft  15 B extending through and between the side face portions  11 S supports the engaging arm portions  15 C. The connecting shaft  15 B is fixedly connected to the engaging arm portions  15 C and is pivotally supported by the side face portions  11 S. Further, the connecting shaft  15 B is arranged in parallel with the connecting shafts  12 A,  12 B and the connecting shaft  13 A. 
         [0099]    Further, a torsional spring  15 S is interleaved between one of the engaging arm portions  15 C and the headrest base portion  4 C. The torsional spring  15 S is assembled in a twisted state and urges the engaging ann portions  15 C to pivot in the clockwise direction as shown in  FIG. 14 . Therefore, the engaging arm portions  15 C are normally held to be pressed against outer peripheral face portions of the hooks  14 , so that the front end portions of the engaging arm portions  15 C are held to engage with the locking grooves  14 D recessed in a stepped manner of the hooks  14 . 
         [0100]    In the state where the engaging ann portions  15 C are in engagement with the locking grooves  14 D, respectively, the engaging ann portions  15 C and the hooks  14  are in abutment to each other, and therefore, they mutually prevent the pivotal movement relative to each other against the biasing forces applied to them. Further, as shown in  FIG. 14  and  FIG. 15 , an end portion of the connecting shaft  15 B connected to the engaging arm portions  15 C is integrally connected with an operating arm portion  15 A that can be pivot by the push rod  50 . 
         [0101]    When backside collision occurs, the operating arm portion  15 A pivots in the counterclockwise direction because the push rod  50  is pushed upward. Therefore, as shown in  FIG. 14 , the operating arm portion  15 A operates to pivot the engaging arm portions  15 C in the same direction to release the engagement of the engaging arm portions  15 C with the hooks  14 . Hence, the holding state of the support portion  4 A at the initial position is released, and the support portion  4 A moves forwardly upwardly by the biasing force of the tension spring  16 . 
         [0102]    The forward and upward movement of the support portion  4 A is stopped when the connecting shaft  13 A reaches the upper end portions H 3  of the elongated holes  11 H as shown in  FIG. 16 . When the movement of the support portion  4 A is stopped, the support portion  4 A is prevented from being pushed back toward the rear side even in the case that a load is applied by the head portion of the seated person. That is, when the connecting shaft  13 A reaches the upper end portions H 3  of the elongated holes  11 H, the link members  12  connected to the support portion  4 A are brought into contact with the upper face portion  11 U of the headrest base portion  4 C, so that the link members  12  are prevented from pivoting in the clockwise direction. 
         [0103]    Further, in this pivotal movement prevented state, if a force is applied in a direction of pushing the support portion  4 A rearward, the link members  12  receive an operation force in such a direction that the link members  12  are pressed against the upper face portion  11 U of the headrest base portion  4 C. Therefore, the support portion  4 A is not pushed back in the counterclockwise direction even if the force described above is applied. Therefore, the support portion  4 A can receive the head portion of the seated person while the support portion  4 A is positioned at the collision accommodating position. Further, as shown in  FIG. 15 , during the forward movement of the support portion  4 A, the support portion  4 A is prevented from being pushed back rearward even in the event that the head portion of the seated person presses the support portion  4 A. 
         [0104]    Specifically, if a force is applied in a direction of pushing back the connecting shaft  13 A rearward during the forward movement the support portion  4 A, the connecting shaft  13 A moves into the first stopper grooves H 1  or the second stopper grooves H 2 , which are recessed rearward (rightward in the drawings), of the elongated holes  11 H so as to be locked therewithin. Therefore, the rearwardly pushing back movement of the connecting shaft  13 A can be prevented, and the support portion  4 A can be held in a position that is taken during the forward movement. 
         [0105]    Therefore, even in the case that the support portion  4 A is at a midpoint before reaching the collision accommodating position, the head portion of the seated person can stably be received by the support portion  4 A.  FIG. 15  shows a state where the connecting shaft  13 A enters into the second stopper grooves H 2 . Here, referring back to  FIG. 13 , the left and right lever members  17  are disposed at the headrest base portion  4 C and each has an arm-like shape. 
         [0106]    The lever members  17  are arranged in the width direction, and rear end portions thereof are pivotally supported by the headrest base portion  4 C. More specifically, as shown in  FIG. 19 , the rear end portion of each of the lever members  17  is positioned between the rib  11 R positioned on its outer side and the rib  11 R positioned on its inner side, and is pivotally supported by a connecting shaft  17 A extending through and between the outer and inner ribs  11 R. 
         [0107]    Further, torsional springs  17 S are interleaved between the lever members  17  and the headrest base portion  4 C. As shown in  FIG. 13 , the torsional springs  17 S are wound around the connecting shafts  17 A, respectively. One end of each of the torsional springs  17 S is engaged with the corresponding lever member  17 , and the other end thereof is engaged with the headrest base portion  4 C. Therefore, the lever members  17  are normally held at positions exposed to (or opposed to) the elongated holes  11 H where the biasing forces of the torsional springs  17 S are balanced. 
         [0108]    Further, the front end portions (left side end portions as viewed in the drawings) of the lever members  17  exposed to (or opposed to) the elongated holes  11 H of the respective are respectively formed with receiving portions  17 B each having a folk-like shape. As shown in  FIG. 15 , when the connecting shaft  13 A is moved upward from the lower end portions H 0  of the elongated holes  11 H, the receiving portions  17 B move out of the elongated holes  11 H in such a manner that they are pushed away by the connecting shaft  13 A. 
         [0109]    However, as shown in  FIG. 16 , when the connecting shaft  13 A reaches the upper end portions  113  of the elongated holes  11 H, the receiving portions  17 B,  17 B are returned to positions where they are exposed to the elongated holes  11 H again by the biasing forces of the torsional springs  17 S. Further, as shown in  FIG. 17 , when the connecting shaft  13 A is transferred downwardly from the upper end portions H 3  toward the elongated holes  11 H, the receiving portions  17 B can receive and catch the connecting shaft  13 A by the fork shaped end portions. 
         [0110]    As the connecting shaft  13 A moves further downward, the lever members  17  are pushed by the connecting shaft  13 A caught by the receiving portions  17 B and pivots in the counterclockwise direction. Therefore, as shown in  FIG. 18 , the connecting shaft  13 A is transferred to positions proximal to the lower end portions HO while being moved and guided by the lever members  17 , and the connecting shaft  13 A is then removed from the receiving portions  17 B. 
         [0111]    In this way, when the connecting shaft  13 A is transferred downwardly from the upper end portions H 3  along the elongated holes  11 H, the connecting shaft  13 A is smoothly transferred to the lower end portions H 0  while being moved and guided by the lever members  17  without entering the first stopper grooves H 1  and the second stopper grooves H 2 . Further, because the connecting shaft  13 A is moved to reach the lower end portions H 0  in such a manner that the connecting shaft  13 A is pushed into the lower end portions H 0 , the lower jaw portions  14 C of the hooks  14  exposed to (opposed to) the lower end portions H 0  are pushed downward by the connecting shaft  13 A. 
         [0112]    Therefore, as shown in  FIG. 14 , the hooks  14  are pushed to pivot in the clockwise direction and are brought into a position where the upper jaw portions  14 B are positioned on the upper side of the connecting shaft  13 A. In this position, the hooks  14  are engaged with the respective engaging arm portions  15 C and are locked to hold the connecting shaft  13 A again at the initial position (lower end portions H 0 ). Hence, the support portion  4 A returns to the initial position and held at this position. 
         [0113]    Next, the operation of the above embodiment will be explained. Referring to  FIG. 1 , the support portion  4 A of the headrest  4  is normally held in the initial position when no backside collision occurs. When the backside collision occurs, the pressure receiving mechanism  20  is pushed to move by the back leaning load of the seated person applied to the seat back  2 , and this movement of the pressure receiving mechanism  20  is transmitted to the releasing member  15  by way of the operation cable  40  and the push rod  50 , so that the releasing member  15  is pivoted. 
         [0114]    Therefore, the holding state at the initial position of the support portion  4 A is released, and as shown in  FIG. 16 , the support portion  4 A is transferred to the collision accommodating position by the biasing force of the tension spring  16 . The support portion  4 A moved to the collision accommodating position can receive and support, at an early stage of the back collision, the head portion of the seated person that may be inclined rearward by the momentum produced due to the back collision. 
         [0115]    In this way, according to the headrest operating device of the embodiment, when no backside collision occurs, the connecting arm  23  (interlocking member) moves the slide pin  23 P on the inoperative path along the front face Fr of the elongated hole Bh even in the event that the connecting arm  23  is moved by the operation of the pressure receiving plate  22 . Therefore, when no the backside collision occurs, the operation cable  40  is not operated to be manipulated, and therefore, the headrest  4  may not be accidentally operated. 
         [0116]    When the backside collision occurs, the connecting arm  23  moves on the operative path along the rear face Re of the elongated hole Bh in response to the operation of the pressure receiving plate  22 . In other words, the connecting arm  23  is brought to interlock with the pressure receiving plate  22 . Therefore, the operation cable  40  is operated by the connecting arm  23 , so that the headrest  4  is operated. In this way, the connecting arm  23  for operating the operation cable  40  has a function of preventing an accidental operation of the headrest  4 , and therefore, necessary space for arrangement can be saved and the headrest operating device may have a compact construction. 
         [0117]    The present invention may not be limited to the above embodiment and the above embodiment may be modified in various ways. For example, as disclosed in JP-A-2005-104259, the contents of which is incorporated herein by reference, the headrest moving mechanism can also be constituted such that the support portion  4 A of the headrest  4  is operated to move in an advancing direction by an amount in correspondence with a moving amount of the operation cable  40 . 
         [0118]    Further, although the inertia force of the weight  24  provided at the connecting arm  23  has been used as switching means for switching the operation path of the connecting arm  23  moved in response to the operation of the pressure receiving mechanism  20  (cooperatively moving member) between the path for the non-occurrence of the backside collision and the path for the occurrence of the backside collision, the operation path can be switched by electrically detecting the occurrence of backside collision. For example, the pressure receiving plate  22  and the connecting arm  23  may be connected by a solenoid that normally maintains the positional relationship with regard to a rotational angle between the pressure receiving plate  22  and the connecting arm  23 . When the backside collision is detected, the solenoid operates to change the positional relationship between the pressure receiving plate  22  and the connecting arm  23 , so that the operation path of the connecting arm  23  can be changed. 
         [0119]    Further, the switching means may be configured to switch the operation path of the connecting arm  23  by using a guide portion having different guide paths that are selected between the occurrence and non-occurrence of the backside collision.