Patent Publication Number: US-2023137430-A1

Title: Headrest with an adjustment device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase of PCT Application No. PCT/EP2020/078586 filed on Oct. 12, 2020, which claims priority to German Patent Application No. DE 10 2020 203 159.2, filed on Mar. 11, 2020, the disclosures of which are hereby incorporated in their entirety by reference herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a headrest with an adjustment device, a method of assembling a headrest, and a method of controlling a headrest. 
     BACKGROUND 
     A headrest of a vehicle seat may be adjustable by means of an adjustment device in a depth direction along the axis of the direction of travel so that the distance of an impact element to the back of the head of a vehicle occupant can be varied. At least two components are moved relative to each other by means of such an adjustment device. 
     One problem with many types of headrest adjustment devices is that the respective adjustment device mechanism has play. Such play can lead to low accuracy of adjustability and generally to reduced comfort. For example, an adjustment device with play in a vehicle may have a tendency to produce irritating rattling noises when driving. Furthermore, adjustment mechanisms can sometimes take up a large amount of space, which is too large for some applications. 
     An approach would be the use of a spindle drive with a spindle oriented in the displacement direction, the direction of travel of the vehicle. However, spindles that are oriented in the direction of travel would have to be enclosed in a highly secure housing to avoid a potential danger in the case of a crash for the head of a seat occupant. 
     SUMMARY 
     The object of the present disclosure is to provide an improved headrest. 
     Accordingly, a headrest for a vehicle seat is provided. The headrest includes an adjustment device, the adjustment device may include at least two adjusting parts and one drive element movable along an axis. It is provided that the drive element is coupled to each of the adjusting parts via meshing drive contours (the drive contours engage one another). Therein, the drive contours describe an adjustment path for each of the adjusting parts, the adjustment path of at least one of the adjusting parts having a twist with respect to the axis and differing from the adjustment path of at least one other of the adjusting parts in such a way that a movement (such as an axial movement) of the drive element along the axis causes a relative movement (e.g., rotation) between the adjusting parts, for example a rotational movement (about the axis). 
     This makes it possible to exert a force on two or more (e.g., three) adjusting parts with a single and easily adjustable drive element in order to adjust the adjusting parts relative to each other, e.g. one of the adjusting parts relative to the other(s) (e.g., two) or each of the adjusting parts relative to each of the others. No opposing (with respect to ta sense of a twist) adjustment paths are necessary in this case, and for example no such paths are provided. Possible tolerances can be kept free of play in a simple manner by means of an external preload. This headrest can provide adjustability in the x-direction of the vehicle coordinate system by means of the adjustment device and can be held free of play in a relatively simple manner. The headrest can be set to a substantially flat configuration because the adjustment device can be designed to be flat. In the case of adjustable headrests, adjustment is typically made in an unloaded or lightly loaded condition. For example, in the proposed configuration, a force acting in the axial direction on the drive element when a torque is applied can be reduced for the same overall transmission ratio compared with a configuration with symmetrical counter-rotating twist. This makes it possible to reduce a load on a drive mechanism and a motor housing, thus making it possible to reduce the weight. 
     For example, the adjustment path is the path along which a point on the corresponding adjusting part is moved relative to the drive element when the drive element is displaced relative to the adjusting part along the axis. The movement of the drive element along the axis optionally causes a movement of one of the adjusting parts to the other of the adjusting parts, whereby the other adjusting parts are moved together, i.e. not relative to each other. Alternatively, each of the adjusting parts is movable relative to each other of the adjusting parts. The adjustment device serves for example to adjust at least two components relative to each other. 
     The adjustment device of the headrest may comprise at least three adjusting parts. This allows a symmetric application of forces. 
     Optionally, the drive element has an internal thread for engagement by a spindle extending along the axis, the axis being an axis of rotation. The drive element may serve as a spindle nut. The orientation of the internal thread thus defines the position of the axis of rotation. This provides a reliable and efficient adjustment. 
     The adjustment paths of all adjusting parts of the adjustment device either have a twist with the same sense of rotation as the adjustment path of the one of the adjusting parts, or alternatively run parallel to the axis of rotation. In other words, none of the drive contours describes a twist in the opposite direction to the other drive contours. In addition to the measures already described above, this also enables simplified production. 
     In an example design, the adjustment paths of two outer of the adjusting parts have a twist and/or the adjustment path of an inner adjusting part arranged between the outer adjusting parts runs parallel to the axis of rotation (and thus does not have a twist). Alternatively, it can be provided that the adjustment paths of the two outer ones of the adjusting parts run parallel to the axis of rotation and the adjustment path of the inner adjusting part arranged between the outer adjusting parts has a twist. 
     According to a further development, the twist of the adjustment paths of the two outer adjusting parts each have the same pitch. For example, each of these adjustment paths describes a spiral around the axis of rotation. One turn around the axis of rotation can have the same length along the axis of rotation for both spirals. Alternatively (e.g. for the two outer adjusting parts) different pitches are provided. This makes it possible to adjust two adjusting parts at different speeds relative to a third adjusting part. 
     In one or more embodiments, it can be provided that the adjustment path for the inner adjusting part runs parallel to the axis of rotation. 
     Optionally, the at least two adjusting parts each form at least one scissor lever. The scissor levers cross each other at a crossing point. At the crossing point the two adjusting parts with their scissor levers can be rotated relative to each other about the axis of rotation. This makes it possible to exert a force on two or more adjusting parts with a single drive element, a simple design and relatively little play in order to adjust the adjusting parts relative to each other. No opposing adjustment paths are necessary, especially not provided for. Since the drive element can be moved along the axis of rotation which passes through the intersection point, a relatively compact design is possible. The scissor levers form a scissor kinematics. Here it is possible to drive the scissor kinematics at the optimal point and to lock it (e.g. by holding the spindle). This also allows an optimum rigidity to be achieved. 
     For example, the headrest may include a component in the form of a base having mounting areas, and mounting areas of the scissor levers are mounted or mountable on the mounting areas of the base. This allows a modularized construction. A module with the adjustment device may be mounted on the base, and, alternatively, a non-adjustable front pad may be mounted on the base. Hence, the same design of the base may be used for a large number of different use cases. 
     Thus, optionally, a preassembled x-adjustment module may include the adjusting parts may be mounted or mountable on the base by means of the mounting areas. The x-adjustment module allows an adjustment along an x axis. The x axis may be, in use, aligned horizontally and in the direction of the back of a seat user&#39;s head. 
     According to an embodiment, the mounting areas of the scissor levers are arranged at longitudinal ends of the scissor levers. This enables a simple design occupying only little space. 
     Optionally, a latching element, e.g., with a lead-in chamfer, is provided at least at one of the mounting areas of the base. The lead-in chamfer may be designed so as to elastically deflect the corresponding scissor lever aside. The scissor lever may then snap into a receptacle defined by the latching element. Thereby, the assembly of the headrest can be further simplified. 
     For example, the base is mounted or mountable on a headrest rod, for example on a pair of headrest rods, so as to be displaceable relative thereto, for example along a z axis, and the z axis may be different from the x axis, for example perpendicular or substantially perpendicular thereto. By this, a two-way adjustable headrest can be provided, for example as a part of a construction set allowing to assemble, as an alternative, the base to a front pad to provide a headrest that is adjustable along the z axis, but not along the x axis. 
     For example, the adjusting parts together form a tubular receptacle for the drive element. For example, the drive element can be moved along the axis of rotation within the receptacle. Together with the drive element, the adjusting parts can thus form a tubular drive. 
     At least one additional component can be coupled with the adjusting parts, such as movable. As an example, two components can be provided which are coupled to the adjusting parts, and the two components can be moved relative to one another by a movement of the adjusting parts relative to one another. One of the components may be the base. The components are connected, for example, to each of the adjusting parts via a joint, e.g. via a swivel joint or a sliding swivel joint. In this way, it is possible, by means of the movement of the drive element along the axis of rotation, to effect a rotation of the adjusting parts relative to each other in a first stage, which is then converted in a second stage into a movement which comprises a translation or consists of a translation. One of the components is a support element which optionally comprises or carries a cushion, e.g. an impact element of the headrest. 
     By moving the two components relative to each other, especially the distance between the two components can be changed. In one configuration, one of the two components can be extended, e.g. in relation to the other of the two components it can be moved between a retracted position and an extended position, for example by means of the adjusting parts. For example, the extendable component is the impact element of the headrest. 
     The adjustment device optionally comprises a drive unit with a motor. Activation of the motor may effect a movement of one of the two components with respect to the other of the two components. This allows an automatic adjustment and an improved comfort for a user. 
     The retracted position and the extended position define a kinematical range. According to an embodiment, a control unit (e.g., of the headrest, for example of the adjustment device or of the drive unit) is provided and adapted to control operation of the motor such that a stationary (fixed) adjustment of the one of the two components with respect to the other of the two components is restricted to the retracted position and an adjustable range, the adjustable range being smaller than the kinematical range. On the other hand, a portion of the kinematical range between the retracted position and the adjustable range may be disallowed by the control unit. For example, the control unit does not stop operation of the motor in the disallowed range. For example, when the user attempts to adjust the headrest in the disallowed range, the control unit continues displacement of the one of the two components until the component is in the retracted position or within the adjustable range. This allows to avoid portions that define a mechanical end stop (defining an end of the kinematical range) to contact one another in use. Thereby, in turn, rattling noises can be effectively avoided. In other words, the control unit is adapted to prevent a (stationary) setting of the headrest in a disallowed range, the disallowed range being between the retracted position and the adjustable range. The control unit may be adapted to terminate an adjustment process only when the one of the two components with respect to the other of the two components is arranged in the retracted position or in the adjustable range. 
     According to another embodiment, a headrest is provided. The headrest may include two components, one of the two components being movable with respect to the other of the two components between a retracted position and an extended position. The headrest further may include a drive unit provided with a motor, and activation of the motor effects a movement of the one of the two components with respect to the other of the two components. Therein, the retracted position and the extended position define a kinematical range, and a control unit is provided and adapted to control operation of the motor such that a (stationary, steady, persistent) adjustment (that is assumed and maintained) of the one of the two components with respect to the other of the two components is restricted to the retracted position and an adjustable range, the adjustable range being smaller than the kinematical range. Regarding the further details of the control unit of this headrest reference is made to the description above and below. 
     An end stop may define the retracted position or the extended position. For example, one end stop defines the retracted position and another end stop defines the extended position. 
     Further, the end stop(s) may be defined by stop portions of the scissor levers. A contact of the stop portion of one of the scissor levers with the stop portion of another one of the scissor levers prevents a continued motion of the scissor levers with respect to one another. By providing such stop portions, a relatively high stiffness and a retracted position being free of any play can be achieved. 
     According to an embodiment, the stop portions are arranged at longitudinal ends of the scissor levers. This allows a relatively high stiffness in the retracted position. 
     Optionally, a return spring is provided which preloads the two components in one direction, preferably in the direction in which a load acts during use, for example in the direction towards one another. In general, a return spring may be provided to provide a preload between at least two of the adjusting parts. The return spring may act on the adjusting parts or on the components, or at one side on one adjusting part and on the other side on one of the components. By means of the return spring the adjustment device can easily be kept free of play. 
     One or more of the drive contours can be in the form of a toothing. In this way, e.g. tilting can be prevented, as a large contact surface is made possible. Those drive contours which describe an adjustment path with a twist are, for example, designed as twist toothing and/or have at least one twist groove. The toothing can describe a helical line. For example, at least one of the toothing is designed to extend spirally around the axis of rotation. The toothing(s) describing a twist can each have a thread pitch which is in each case e.g. larger than a thread pitch of the internal thread of the drive element. Alternatively or additionally, one or more toothing are axially aligned, i.e. they have an infinite thread pitch. An axially aligned toothing can usually be produced relatively easily, e.g. with an open-close tool. 
     For example, for each of the adjusting parts, a drive contour is formed on the respective adjusting part, and a drive contour respectively engaging with it is formed on the drive element. For example, the drive contours on at least one or all of the adjusting parts are designed in the form of internal toothing and the associated drive contour(s) on the drive element are designed in the form of external toothing. 
     For example, the drive contours of the drive element for the respective adjusting parts are arranged next to each other along the axis of rotation. This makes it possible to easily provide different adjustment paths for the adjusting parts. 
     Optionally, the drive element is designed in one part. Alternatively, the drive element is designed in several parts, for example two parts. This allows an optimization of the material in certain areas of the drive element and a simplified production of the adjustment device. The two parts can be connectable or connected to each other in a rotationally-fixed manner. For example, the drive element comprises at least two parts, one part being made of a different material or a different material combination than the other part. For example, the drive element comprises an input part and an output part. The input part has the internal thread. The output part has at least one of the drive contours. For example, both parts each consist of a plastic, for example of different plastics. A one-piece drive element can also consist of a plastic. 
     The drive part can be firmly connected to the output part by means of scraper ribs, for example without play. This enables a relatively secure connection of the parts to each other. 
     The input part can be positively connectable or connected to the output part by means of a snap-in connection (or in any other way). For example, the two parts can be or are plugged into each other. This enables an easy assembly. Alternatively or additionally a material-locking connection is provided, e.g. a welded connection. As an example, it may be provided that the connection is first made by the scraper ribs and that this connection is secured by an additional positive fit and/or a material connection. 
     In a further development, the input part is optimized for sliding. For example, the input part has a lower coefficient of sliding friction compared to the output part (especially on steel and/or on the material of the spindle). This allows the internal thread to engage with the spindle relatively smoothly. 
     As an alternative or in addition, the output part has a higher strength than the input part (and/or consists e.g. of a high-strength plastic). This allows a safe and long-lasting engagement of the corresponding drive contours. Furthermore, the part of the drive element which is made of a stronger material can have a drive contour which (perpendicular to the axis of rotation) has a smaller maximum outside diameter than the other part. This in turn makes it possible to insert the assembled drive element from one side into the tubular receptacle of the adjusting parts. 
     Optionally, the output part (perpendicular to the axis of rotation) has the same maximum outside diameter as the input part. Due to the two-part design, the still separate parts of the drive element can then be inserted on both sides into the receptacle of the adjusting parts and fastened to each other, for example locked together, therein. An intermediate drive contour (e.g. an axially aligned toothing), for example, has a smaller outer diameter than the outer drive contours. 
     According to an aspect, a headrest for a vehicle seat is provided, and the headrest may be designed in accordance with any aspect or embodiment described herein. The headrest comprises two components and an adjustment device with at least two adjusting parts and a drive unit including a motor. It is provided that an activation of the motor effects a movement of one of the two components with respect to the other of the two components. Therein, by means of the adjusting parts, the one of the two components is movable with respect to the other of the two components between a retracted position and an extended position defining a kinematical range, and a control unit is adapted to control operation of the motor such that a stationary adjustment (of a position that the component assumes and maintains) of the one of the two components with respect to the other of the two components is restricted to the retracted position and an adjustable range. The adjustable range is smaller than the kinematical range. The adjustable range is displaced from the retracted position. 
     According to an aspect, a construction set for assembling a headrest is provided. The construction set comprises a height-adjustment module with a component having mounting areas, a depth-adjustment module having mounting areas that are mountable on the mounting areas of the height-adjustment module so as to assemble a height-and-depth-adjustable headrest, for example according to any aspect or embodiment described herein, and a front pad having mounting areas that are, as an alternative, also mountable on the mounting areas of the height-adjustment module so as to assemble a height-adjustable headrest (not being depth-adjustable). This modularized construction allows to use the same height-adjustment module for an increased number of use cases. 
     According to an aspect, a method for assembling a headrest is provided. The method comprises providing a height-adjustment module with a component having mounting areas, providing a depth-adjustment module having mounting areas that are mountable on the mounting areas of the height-adjustment module so as to assemble a height-and-depth-adjustable headrest, for example according to any aspect or embodiment described herein. The method further comprises providing a front pad having mounting areas that are (also, as an alternative) mountable on the mounting areas of the height-adjustment module so as to assemble a height-adjustable (height-only-adjustable) headrest. The method further comprises mounting a selection of either the depth-adjustment module or the front pad on the height-adjustment module (by means of the mounting areas) to assemble the height-and-depth-adjustable headrest or the height-adjustable (height-only-adjustable) headrest. 
     According to an aspect, a method for controlling adjustment of a headrest for a vehicle seat, for example of a headrest according to any aspect or embodiment described herein, is provided. The headrest comprises two components and an adjustment device with at least two adjusting parts and a drive unit including a motor. Therein, activation of the motor effects a movement of one of the two components with respect to the other of the two components, and, by means of the adjusting parts, one of the two components is movable with respect to the other of the two components between a retracted position and an extended position defining a kinematical range. The method comprises controlling operation of the motor such that a stationary adjustment of the one of the two components with respect to the other of the two components is restricted to the retracted position and an adjustable range, the adjustable range being smaller than the kinematical range. 
     According to an aspect, a vehicle seat is provided which includes a headrest with an adjustment device according to any configuration described herein. For the advantages of this vehicle seat, reference is made to the advantages indicated for the headrest. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The idea underlying the invention will be explained in more detail below using the embodiments shown in the figures. In schematic representations: 
         FIG.  1    shows a perspective view of a headrest with an adjustment device in a retracted position; 
         FIG.  2    shows a perspective and partially cut open view of the headrest as shown in  FIG.  1   ; 
         FIG.  3 A  shows a side view of the headrest as shown in  FIGS.  1  and  2   ; 
         FIG.  3 B  shows a sectional view of the headrest according to the sectional plane A-A shown in  FIG.  3 A ; 
         FIG.  4    shows a perspective view of the headrest as shown in  FIGS.  1  to  3 B , the adjustment device being shown in an extended state; 
         FIG.  5 A  shows a side view of the headrest as shown in  FIG.  4   ; 
         FIG.  5 B  shows a sectional view of the headrest according to the sectional plane B-B shown in  FIG.  5 A ; 
         FIGS.  6 A- 6 C  show views of a drive element of the adjustment device of the headrest according to  FIGS.  1  to  5 B ; 
         FIG.  7    shows a sectional view of a drive element for the adjustment device of the headrest according to  FIGS.  1  to  5 B ; 
         FIGS.  8 A and  8 B  show views of a drive element for the adjustment device of the headrest according to  FIGS.  1  to  5 B ; 
         FIG.  9    shows a perspective view of a headrest with an adjustment device in a retracted position; 
         FIG.  10 A  shows a side view of the headrest according to  FIG.  9   ; 
         FIG.  10 B  shows a sectional view of the headrest according to the sectional plane F-F shown in  FIG.  10 A ; 
         FIG.  11    shows a vehicle seat with a headrest; 
         FIGS.  12 A- 12 C  show different steps of a method to assemble a headrest with a height-adjustment module and a depth-adjustment module; 
         FIG.  13    shows an assembled headrest being height-adjustable and depth-adjustable; 
         FIGS.  14 A- 14 C  show details of the attachment of the depth-adjustment module to the height-adjustment module according to  FIGS.  12 A- 12 C ; 
         FIGS.  15 A- 15 C  show different adjustment positions of two components of a headrest with respect to one another; and 
         FIGS.  16 A,  16 B  show a kinematic end position of the headrest of  FIGS.  15 A- 15 C . 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     EP 1 661 753 A2 describes a headrest of a vehicle seat including a head support portion for supporting a head of a passenger, and a drive unit for moving the head support portion. Therein, a pair of cross bar links is provided, wherein each of the cross bar links is composed of outer and inner cross bars that are rotatably interconnected via a pivot pin. 
       FIGS.  1  to  5 B  show different views of a headrest  2  for a vehicle seat. The headrest  2  comprises an adjustment device  1  which can be displaced from a retracted position to an extended position.  FIGS.  1  to  3 B  show the retracted position and  FIGS.  4  to  5 B  the extended position. The headrest  2  can also be adjusted to several intermediate positions between the retracted position and the extended position. The adjustment device  1  comprises a tubular drive, which is explained in more detail further below. 
     The headrest  2  comprises a base  20 , which is mounted on a headrest rod  22 . The headrest rod  22  is used to attach the headrest  2  to a vehicle seat. The headrest rod  22  has two parallel sections. In the present example, the headrest rod  22  is U-shaped. Furthermore, the headrest  2  comprises an impact element  21 , which can be designed in the form of a plate, for example. The impact element  21  can be seen in the side view of  FIG.  3 A . The impact element  21  defines a bumper surface provided to support the head of a seat user, such as in the event of a vehicle crash. In order to absorb an impact of the head against the impact element  21  as well as possible, the impact element  21  can be adjusted relative to the base  20  by means of the adjustment device  1 , in this case along a longitudinal direction X. This adjustability also enables a relatively ergonomic and comfortable adjustment of the headrest  2 . Additional elements such as a separate front pad (e.g. as a foam support) can be attached to the impact element  21 . 
     The adjustment device  1  comprises a double scissor mechanism. This is formed by several, in this case three, adjusting parts  10 A,  10 B,  11 A. Two outer adjusting parts  10 A,  10 B are provided, each forming a scissor lever  100 . Furthermore, an inner adjusting part  11 A is provided, which is arranged between the two outer adjusting parts  10 A,  10 B. The inner adjusting part  11 A forms two scissor levers  110 . The inner adjusting part  11 A is H-shaped and has a cross member  111  which connects the two scissor levers  110 . The inner adjusting part  11 A is designed in one piece in the example shown. The first adjusting parts  10 A,  10 B are also each designed in one piece. 
     The scissor lever  100  of each of the outer adjusting parts  10 A,  10 B is located adjacent to one of the scissor levers  110  of the inner adjusting part  11 A. The adjacently arranged scissor levers  100 ,  110  cross each other at a crossing point K. The scissor levers  100 ,  110  are each elongated with two longitudinal ends and the crossing point K is located between the longitudinal ends of each of the scissor levers  100 ,  110 . The scissor levers  100 ,  110  can be rotated relative to each other about an axis of rotation D. The axis of rotation D passes through the crossing points K of the two pairs of scissor levers  100 ,  110 . 
     Each of the scissor levers  100 ,  110  is connected to the base  20  via one of the longitudinal ends. In this case the scissor levers  110  of the inner adjusting part  11 A are pivotally mounted to the base  20  via swivel joints DG. The scissor levers  100  of the outer adjusting parts  10 A,  10 B are each mounted on a sliding swivel joint SG on the base  20 , which in the example shown is formed in each case by a projection which is rotatably mounted in a slotted guide. The swivel joints DG are each formed by a dome  16  rotatably mounted in a receptacle. In the example shown, the domes  16  are each formed on the corresponding scissor lever  100 ,  110 . Optional stiffening struts  17  connect the ends of the jointly movable scissor levers  100 ,  110 . 
     In the present case, the swivel joints DG are arranged at the top and the sliding swivel joints SG at the bottom in relation to the intended use of the headrest  2 , although other arrangements are also conceivable. It would also be possible to mount the inner adjusting part  11 A via sliding swivel joints and the outer adjusting parts  10 A,  10 B via swivel joints at the base  20 . 
     At the respective other longitudinal ends, the scissor levers  100 ,  110  of the outer adjusting parts  10 A,  10 B and of the inner adjusting part  11 A are connected to the impact element  21 , in this case the scissor levers  100  of the outer adjusting parts  10 A,  10 B (at the top) via swivel joints DG and the scissor levers  110  of the inner adjusting part  11 A (at the bottom) via sliding swivel joints SG. By turning the outer adjusting parts  10 A,  10 B relative to the inner adjusting part  11 A the adjustment device  1  can thus be displaced between the retracted position and the extended position. 
     To drive a corresponding adjusting movement, the adjustment device  1  comprises a drive unit  14 . The drive unit  14  comprises an electric motor. The drive unit  14  is attached to one of the adjusting parts  10 A,  10 B,  11 A, here to an outer adjusting part  10 A (alternatively e.g. to the inner adjusting part  11 A). If the adjusting parts  10 A,  10 B,  11 A are thus adjusted relative to each other, then the drive unit  14  is also moved relative to the base  20  (and relative to the impact element  21 ). 
     The drive unit  14  drives (via a gearbox) a spindle  12  (see especially  FIGS.  3 B,  4  and  5 B ). The spindle  12  extends along the axis of rotation D. Activating the drive unit  14  causes the spindle  12  to rotate about the axis of rotation D. It is worth noting that the spindle  12  is not arranged in parallel to the adjustment direction, but vertical thereto. Correspondingly, when mounted in a vehicle with the adjustment direction parallel to the (normal, straight) direction of travel, the spindle  12  is arranged vertical thereto. 
     The spindle  12  has an external thread  120 . The external thread  120  is engaged with an internal thread  130  of a drive element in the form of a spindle nut  13 A. The spindle nut  13 A has several drive contours  15 A- 15 C (see especially  FIGS.  2  and  4   ). The drive contours  15 A- 15 C are formed on an outer shell surface of the spindle nut  13 A (side by side as seen along the axis of rotation D). One drive contour  15 A,  15 B,  15 C is assigned to each of the adjusting parts  10 A,  10 B,  11 A. The adjusting parts  10 A,  10 B,  11 A each have a drive contour  15 D- 15 F. The drive contour  15 D- 15 F of each of the adjusting parts  10 A,  10 B,  11 A is in engagement (with respect to a relative rotation about the axis of rotation D in a positive connection) with the drive contour  15 A- 15 C of the spindle nut  13 A assigned to the respective adjusting part  10 A,  10 B,  11 A. 
     It is provided that the drive element  13 A is coupled to the adjusting parts  10 A,  10 B,  11 A via the meshing drive contours  15 A- 15 F, which describe at least one adjustment path B 1 -B 3  for each of the adjusting parts  10 A,  10 B,  11 A, and the adjustment path B 1 -B 3  of at least one of the adjusting parts  10 A,  10 B,  11 A has a twist with respect to the axis of rotation D and differs from the adjustment path B 1 -B 3  of at least one other of the adjusting parts  10 A,  10 B,  11 A, so that a movement of the drive element  13 A- 13 C along the axis of rotation D causes a relative movement between the adjusting parts  10 A,  10 B,  11 A. 
     The drive contours  15 A- 15 F are each designed in the form of (respectively interlocking) toothing. The drive contours  15 A- 15 C on the spindle nut  13 A are designed in the form of external toothing, the drive contours  15 D- 15 E on the adjusting parts  10 A,  10 B,  11 A each in the form of internal toothing. Due to the engagement, the drive contours  15 A,  15 C,  15 D,  15 F for the external adjusting parts  10 A,  10 B each describe a plurality of adjustment paths B 1 -B 3 . Along the adjustment paths B 1 -B 3 , the adjusting parts  10 A,  10 B,  11 A and the spindle nut  13 A slide along each other when the spindle nut  13 A is displaced relative to the adjusting parts  10 A,  10 B,  11 A along the axis of rotation D. Since the drive contours  15 A- 15 F are each formed by interlocking toothing, such an adjustment path B 1 -B 3  is each described by a tooth engaging in a groove. This enables a relatively precisely guided motion with little play and a long life, as well as good planar power transmission and thus high strength. In general, however, an adjustment path described by a projection engaging in a single groove would be sufficient in principle. 
     In this example, the drive contours  15 A,  15 C,  15 D,  15 F for the outer adjusting parts  10 A,  10 B each run spirally around the axis of rotation D. As a result, the adjustment paths exhibit a twist in relation to the axis of rotation D. The twist of the adjustment paths of the two outer adjusting parts  10 A,  10 B is in the same direction (the helixes have the same sign). The adjustment paths B 1 , B 3  each have the same pitch, so that the two outer adjusting parts  10 A,  10 B are adjusted synchronously. As an alternative it would be possible that these pitches are different (but, for example, still with the same direction), in order to achieve faster turning of one of the outer adjusting parts  10 A,  10 B, which e.g. would allow a shorter design of the corresponding swivel arm  100 . 
     The drive contours  15 B,  15 E for the inner adjusting part  11 A, i.e., drive contour  15 B on the spindle nut  13 A and drive contour  15 E on the inner adjusting part  11 A, are each aligned parallel to the axis of rotation D, they have no twist, in other words an infinitely large pitch, and thus form an axial guidance. The drive contours  15 B,  15 E for the inner adjusting part  11 A thus serve as axial guide. 
     The corresponding adjustment paths B 2  run parallel to the axis of rotation D. The spindle nut  13 A can thus be moved along the axis of rotation D relative to the inner adjusting part  11 A without being turned relative to it. In contrast, a displacement of the spindle nut  13 A along the axis of rotation D relative to the two outer adjusting parts  10 A,  10 B causes a rotation of the outer adjusting parts  10 A,  10 B relative to the spindle nut  13 A. This means that when the spindle nut  13 A is displaced along the axis of rotation D as a result of a rotation of spindle  12 , the outer adjusting parts  10 A,  10 B are rotated relative to the inner adjusting part  11 A. 
     The adjusting parts  10 A,  10 B,  11 A together form a tubular receptacle A, in which the spindle nut  13 A can be displaced along the axis of rotation D. The outer adjusting parts  10 A,  10 B are each rotatably mounted on the inner adjusting part  11 A. For this purpose, the outer adjusting parts  10 A,  10 B each have a cylinder section  101 ,  102 , which is inserted into a corresponding bushing  112 ,  113  of the inner adjusting part  11 A. The drive contours  15 D,  15 F formed on the outer adjusting parts  10 A,  10 B are arranged on these cylinder sections  101 ,  102 . The drive contour  15 E formed on the inner adjusting part  11 A is arranged on the cross member  111 . 
     In the example shown in  FIG.  1 - 5 B , the drive contours  15 A- 15 C, viewed in one direction, have outer diameters that decrease stepwise in comparison to each other. An outer first drive contour  15 A has the largest outer diameter, a middle second drive contour  15 B has a smaller outer diameter and an outer third drive contour  15 C has an even smaller outer diameter. This allows the spindle nut  13 A to be inserted from one side into the receptacle A formed by the adjusting parts  10 A,  10 B,  11 A. In addition, a stop surface for an end stop  138  of the spindle nut  13 A for the extended position can be easily realized. This stop surface is formed by a corresponding constriction at the end of the drive contour  15 D of the (motor-side) outer adjusting part  10 A (see e.g.  FIG.  3 B ). The end stop  138  is formed by a step of the spindle nut  13 A (see e.g.  FIGS.  6 B and  6 C ). 
     In the retracted position (see e.g.  FIG.  2   ), the spindle nut  13 A is completely accommodated in the receptacle A. In the extended position (see e.g.  FIG.  4   ) the spindle nut  13 A protrudes from the receptacle A. An end stop  137 A adjacent to the drive unit  14  defines the retracted position. 
     In the example shown, the drive contours  15 A- 15 F can also be referred to as toothing sections. It should be noted, however, that other types of drive contours can also be used. For example, instead of axially aligned toothings for the drive contours  15 B,  15 E for the inner adjusting part  11 A, other axially extended positive locking contours can also be used, e.g. polygonal contours, such as a square contour or a hexagonal contour, or a star contour. 
     A pretensioning element pretensions the adjustment device  1  in the retracted position. Here the pretensioning element is designed in the form of a return spring  18 , for example in the form of a spiral spring. The return spring  18  (see for example  FIG.  5 A ) is fixed with one side to the base  20  and with the other side to the impact element  21 , specifically, hooked in. Since none of the drive contours  15 A- 15 F or adjustment paths B 1 -B 3  runs in the opposite direction to another of the drive contours  15 A- 15 F or adjustment paths B 1 -B 3 , this one return spring  18  is sufficient to keep all gear stages and also the bearing points DG, SG free of play. 
       FIGS.  6 A to  6 C  show further details of the spindle nut  13 A. The spindle nut  13 A is multi-part, in this example it is made of two parts. It comprises an input part  131 A and an output part  132 A. The input part  131 A comprises the internal thread  130 . The output part  132 A is firmly (especially torsionally rigid) connected to the input part  131 A. For this purpose, the input part  131 A comprises several passages for latching hooks  133  of the output part  132 A (see for example  FIG.  6 C ). For assembly, the output part  132 A is inserted into the input part  131 A (for example pressed in) until the latching hooks  133  reach through the passages and engage positively with latching edges  134  of the input part  131 A. Thereby a stop  136  on the output part  132 A strikes against a stop  135  on the input part  131 A. 
     Due to the fact that the spindle nut  13 A has a multi-piece design, individual adaptation to different applications is easily possible. Furthermore, the spindle nut can be mounted directly in receptacle A. This means that the outer diameter that grips last when pushed in can be made relatively large. 
     In the circumferential direction, the passages alternate with webs, via which an inner section with the internal thread  130  is connected to an outer section of the input part  131 A. These webs prevent rotation of the parts  131 A,  132 A relative to each other. As an alternative or in addition to a form-fitting anti-rotation means, an interference fit between the output part and the input part can serve as anti-rotation means. As an alternative or in addition, interlocking gear teeth are provided on the contact surfaces of the two parts. 
     The input part  131 A and the output part  132 A are inserted into each other over a length that ensures coaxial alignment. 
     The input part  131 A and the output part  132 A are made of different materials. In this case both parts are made of plastic. As an example, the input part  131 A can be made of a (slide-optimized) POM (polyoxymethylene). This allows the input part to be easily guided along the fast rotating spindle. The output part  132 A, for example, has a high-strength plastic and/or a plastic which has a higher strength than the material of the input part  131 A. In this way, e.g. a smaller outer diameter of the drive contour  15 C of the output part  132 A compared to the drive contour  15 A of the input part  131 A can be compensated. In an optional design, the output part  132 A is made of steel, e.g. as a cold extruded part. 
     It can also be seen that an end face of the spindle nut  13 A forms the end stop  137 A. 
       FIG.  7    shows a spindle nut  13 B for adjustment device  1 , where the output part  132 B is designed with a larger outside diameter compared to the spindle nut according to FIG.  6 A- 6 C. In this case, the corresponding drive contours for the outer adjusting parts  10 A,  10 B have the same outer diameter. During assembly, the input part  131 A and the output part  132 B are inserted into the two opposite openings of the receptacle A of the adjustment device  1  (with correspondingly adapted inner diameters of the adjusting parts) and locked together inside the receptacle A. 
       FIGS.  8 A and  8 B  show a spindle nut  13 C for the adjustment device  1 , which is designed in two parts, but where the input part  131 B is completely accommodated in the output part  132 C and does not protrude from the output part in the direction of the axis of rotation D, as in the examples shown in  FIGS.  6 A to  7   . In the example of  FIGS.  8 A and  8 B , all drive contours  15 A- 15 C are formed on the output part  132 C, specifically, on its lateral surface. 
     The input part  131 B and the output part  132 C are pressed axially into each other. A relatively precise coaxial alignment of the parts is possible via the guide length, which corresponds to the entire length of the input part  131 B. The input part  131 B has a cross-section that describes a polygon, here, an octagon. The output part  132 C has a matching inner shape. Thus the parts are secured against rotation. In this example (and also generally possible with multi-part spindle nuts) an oversize joint with scraper ribs is also provided. This ensures that the parts are held securely together in the axial direction. A snap-in connection of the parts is also provided. Alternatively or additionally, in addition to pressing the parts together, it is possible to glue and/or weld the parts together. 
     The input part  131 B forms the end stop  137 B (for the retracted position). 
       FIGS.  9  to  10 B  show a headrest  2 ′ similar to headrest  2  as shown in  FIGS.  1  to  5 B  and including an adjustment device  1 ′. The adjustment device  1 ′ according to  FIGS.  9  to  10 B  comprises, in contrast, only for one of the outer adjusting parts  10 C,  10 D drive contours  15 G- 15 J. 
     Accordingly, it is provided that the drive element  13 D is coupled with (only) two adjusting parts  10 C,  11 B via meshing drive contours  15 G- 15 J, which describe at least one adjustment path B 4 , B 5  for each of the two adjusting parts  10 C,  11 B, and the adjustment path B 4  of one (outer) of the adjusting parts  10 A has a twist with respect to the axis of rotation D and is different from the adjustment path B 5  of the other (inner) of the adjusting parts  11 B, so that movement of the drive element  13 D along the axis of rotation D causes a relative movement between the adjusting parts  10 A,  11 B. 
     In general, the adjustment paths B 1 -B 5  of at least two adjusting parts  10 A- 10 D,  11 A,  11 B (especially in the form of teeth) can have unidirectional but different pitches. The drive contours  15 A- 15 J of at least two adjusting parts  10 A- 10 D,  11 A,  11 B (especially in the form of teeth) can have different diameters. Thus the spindle nut  13 A- 13 D can be used directly as an adjustment stop in both directions. 
     The drive contours  15 H,  15 J for the inner adjusting part and the corresponding adjustment paths B 5  are straight and parallel to the axis of rotation. The outer adjusting parts  10 C,  10 D are coupled via the base  10 , the impact element  21  and the stiffening strut  17 . 
     In the case of the adjustment device  1 ′, a springing of the inner adjusting part  11 B is provided, in the form of a return spring  18 , which is attached on one side to the inner adjusting lever  11 B and on the other side to the base  20 . 
     A vehicle seat  3  of a vehicle shown schematically in  FIG.  11    has a seat part  30 , a backrest  31  and several adjustment devices. 
     The vehicle seat  3  comprises an optional height adjustment device  32 , with which the seat part  30  (here, together with the backrest  31 ) is adjustable (at least) in a height axis relative to a vehicle floor of the vehicle in a state installed in the vehicle. 
     The backrest  31  can be adjusted to the seat part  30  by means of an optional arrangement of swivel fittings  33 . By means of the arrangement of swivel fittings  33 , the backrest  31  can be swivelled relative to the seat part  30  about a pivot axis in order to adjust the tilt position of the backrest  31  in relation to the seat part  30  or to bring the backrest  31  into a pre-swivelled, for example flat position, for example to increase storage space in the vehicle. 
     The vehicle seat  3  further comprises a longitudinal adjustment device  34  for adjusting the vehicle seat  3  in the direction of a longitudinal axis. By means of the longitudinal adjustment device  34 , the seat part  30  together with the backrest  31  is adjustable along the longitudinal axis relative to the vehicle floor of the vehicle in a state installed in the vehicle. The longitudinal axis is perpendicular to the height axis. By means of the longitudinal adjustment device  34  the (remaining) vehicle seat  3  can be connected to the vehicle floor and is connected in the example shown. 
     Such a vehicle seat  3  may be designed as a front seat in a vehicle. However, such a vehicle seat  3  may also be used as a rear seat in a second or third row of seats in a vehicle. 
     Furthermore, the vehicle seat includes a headrest  2  as shown in  FIG.  1 - 5 B , alternatively a headrest  2 ′ as shown in  FIG.  9 - 10 B . The impact element  21  is thus adjustable in the longitudinal direction relative to the base  20  by means of the adjustment device  1  (alternatively by means of the adjustment device  1 ′). 
       FIG.  12 A to  12 C  show different steps in a method to assemble a headrest  2 ″. The headrest  2 ″ corresponds to the headrest  2  as shown in  FIGS.  1  to  5 B , with the difference that the base  20  of the headrest  2 ″ of  FIGS.  12 A- 12 C  is designed with mounting areas  201  in the form of open-ended slots allowing to slide in domes  16  of the scissor levers  100  of the outer adjusting parts  10 A,  10 B, and mounting areas  200  that allow to snap in the domes  16  of the scissor levers  110  of the inner adjusting part  11 A. It is worth noting that the headrest  2  of  FIGS.  1  to  5 B  could, as an option also be provided with such mounting areas  200 ,  201 , as well as the headrest  2 ′ of  FIGS.  9  to  10 B . 
       FIG.  12 A  shows a pre-assembled depth-adjustment module MX with a (double) scissor kinematics, and a pre-assembled height-adjustment module MY. Further,  FIG.  12 A  shows a front pad  4 , and the front pad  4  does not provide any depth-adjustment. 
     Together, the pre-assembled depth-adjustment module MX, the pre-assembled height-adjustment module MY and the front pad  4  form a construction set. The construction set allows to either mount the depth-adjustment module MX or the front pad  4  to the height-adjustment module MY. Thus, the same design of the height-adjustment module MY may be used both for use cases where depth-adjustment is required, and for use cases where depth-adjustment is not required. 
     The depth-adjustment module MX comprises the inner and outer adjusting parts  10 A,  10 B,  11 A, the drive element  13 A, the impact element  21  (which may have substantially the same shape as the front pad  4 ) as described above with reference to  FIGS.  1  to  5 B . The depth-adjustment module MX (optionally) also comprises the drive unit  14  or, alternatively, a manual drive. Further parts, such as a cushion and trim parts may also be pre-assembled. At longitudinal ends of the scissor levers  100 ,  110  facing away from the impact element  21 , domes  16  (as one example of suitable mounting areas) are arranged. 
     To mount the depth-adjustment module MX on the height-adjustment module MZ, first the domes  16  of the outer adjusting parts  10 A,  10 B are sled into the mounting areas  201  of the base  20  being designed as slots. For this purpose, the slots have one closed end, and one open end. The closed end faces the other (snap-in) mounting areas  200  of the base  20 , and the closed ends face away therefrom. Thus, said domes  16  are introduced into the slots from below. 
     A state where the domes  16  of the outer adjusting parts  10 A,  10 B are sled into the slot-shaped mounting areas  201  of the base  20  is shown in  FIG.  12 B . Next, the depth-adjustment module MX is rotated about the domes  16  in the slot-shaped mounting areas  201  so that the domes  16  of the inner adjusting part  11 A approach the (upper) mounting areas  200 . The upper mounting areas  200  each define a receptacle. The upper mounting areas  200  allow the domes  16  to snap in so as to be retained in the receptacles with a positive fit in a rotatable manner. 
       FIG.  12 C  shows the assembled state of the headrest  2 ″. Therein, the domes  16  of the inner adjusting part  11 A are snapped into the mounting areas  200  defining the receptacles. The headrest  2 ″ is adjustable in depth along an x axis indicated in  FIG.  13   . When adjusting the depth, the outer adjusting parts  10 A,  10 B rotate relative to the inner adjusting part  11 A, and the domes  16  of the outer adjusting parts  10 A,  10 B slide along the slot-shaped mounting areas  201  of the base  20 . 
       FIG.  12 C  also shows the return spring  18  being mounted on the base  20  with one end and on the impact element  21  with the other end. 
     When a headrest shall be assembled that is not depth-adjustable, instead of the depth-adjustment module MX the front pad  4  may be mounted on the height-adjustment module MZ in the same manner as described with respect to the depth-adjustment module MX. 
       FIG.  13    shows a different height (along a z axis perpendicular to the x axis) of the base  20  with respect to the headrest rod  22 . For the height-adjustment, the headrest  2 ″ is provided with a height-adjustment mechanism  19 . It is worth noting that all other embodiments described herein may likewise be provided with such a height-adjustment mechanism  19 . 
     The height-adjustment mechanism  19  comprises slide bearings  192  by means of which the base  20  is slideably mounted on the parallel sections of the headrest rod  22 . 
       FIGS.  14 A and  14 B  further show a spindle  190  and a drive unit  191  of the height-adjustment mechanism  19 . The spindle  190  extends parallel to the parallel sections of the headrest rod  22  and parallel to the z axis. The drive unit  191  is mounted on the base  20 , but could alternatively also be mounted on the headrest rod  22 . In the present example, the drive unit  191  rotates a spindle nut rotatably mounted on the base  20  which travels along the spindle  190 , the spindle being fixed with respect to the headrest rod  22 . Alternatively, the drive unit  191  could rotate the spindle  190 , and a spindle nut would be fixed with respect to the headrest rod  22 . 
       FIGS.  14 B and  14 C  show further details of the upper mounting areas  200  of the base  20 , and the domes  16  of the inner adjusting part  11 A. Therein,  FIG.  14 C  is an enlarged view of the box H indicated in  FIG.  14 B . 
     It can be seen that each mounting area  200  is provided with a latching element  202  positively locking the dome  16  in the receptacle of the mounting area  200 . The latching element  202  is formed as a protrusion. The scissor lever  110  is resilient and may be elastically bent aside when the dome  16  is pushed into the receptacle. To simplify this connection, the latching element  202  has a lead-in chamfer  203  at its free end on which the dome  16  glides aside before snapping into the receptacle. 
     Further, the mounting area  200  comprises a securing element  204 . The securing element  204  is formed on the base  20 . The securing element  204  is resilient. When the dome  16  is pushed into the receptacle, the corresponding scissor lever  110  of the inner adjusting part  11 A pushes against the securing element  204  and elastically bends it in the pushing direction. The mounting area  200  is designed such that when the dome  16  snaps (sideways, vertical to the pushing direction) into the receptacle at the latching element  202 , the securing element  204  snaps back (opposite the pushing direction) to the side of the scissor lever  110  to secure the dome  16  in the receptacle. In the attached state, the latching element  202  and the securing element  204  retain the dome  16  inside the receptacle from two orthogonal directions. 
     As can be seen in  FIGS.  12 B and  12 C , the securing element  204  may have the shape of a spring plate, e.g., having a free end and formed by two parallel slits. In  FIG.  13    securing element  204  is omitted solely for simplicity. 
       FIGS.  15 A- 15 C  show different positions of the impact element  21  as one component with respect to the base  20  as another component of the headrest  2 ″. Here, the headrest  2 ″ of  FIGS.  12 A- 14 C  is shown, but the following description may correspondingly apply to any one of the headrests  2 ;  2 ′;  2 ″ described above. 
     The adjustment device  1 ′ allows an adjustment of the impact element  21  relative to the base  20  along the x axis within a kinematical range RK which is delimited by the retracted position PR shown in  FIG.  15 A , and the extended position PE shown in  FIG.  15 C . 
     The retracted position PR is defined by an end stop. The end stop is defined by a stop portion  104  at an end of a scissor lever  100  of one of the outer adjusting parts  10 A,  10 B, and a stop portion  114  at an end of one of the scissor levers  110  of the inner adjusting part  11 A by making contact with one another. A corresponding end stop is (optionally) provided at the other pair of scissor levers  100 ,  110 . Further, another end stop is (optionally) provided at the other end of the scissor lever(s)  100 ,  110 , see  FIGS.  16 A and  16 B . 
     Since the end stops are provided at the ends of the scissor levers  100 ,  110 , a relatively stiff stop is achieved. However, when driving a vehicle with the headrest  2 , the stop portions  104 ,  114  may rattle. To avoid such rattling, one might consider interposing a damper such as a rubber or foam. Such a damper, however, would increase the thickness of the headrest  2 ″. Herein, a different solution is provided that does not need any additional damper component which would add weight and may have a limited lifetime, and at the same time the solution described herein allows a very slim package in the rearmost adjustable position. 
     The drive unit  14  comprises a motor  140  and a control unit  141  (see, e.g.,  FIG.  14 A ), but it is worth noting that the control unit  141  may also be located elsewhere. The control unit  141  controls operation of the motor  140 . The control unit  141  is adapted to control the motor  140  such that the scissor levers  100 ,  101  and the impact element  21  are not adjusted in a disallowed range RD. The disallowed range RD extends between the retracted position PR and a rear adjustment position PA, see  FIG.  15 B . Before entering the disallowed range RD, the control unit  141  stops operation of the motor  140  at the rear adjustment position PA. Alternatively or in addition, the control unit  141  is adapted to continue operation of the motor  140  (e.g., irrespective of an input signal indicating to stop) as long as the scissor levers  100 ,  101  and the impact element  21  are within the disallowed range RD. 
     Between the rear adjustment position PA and the extended position PE, the scissor levers  100 ,  101  and impact element  21  are adjustable by means of the drive unit  14 . In the present example the adjustment is stepless. The rear adjustment position PA and the extended position PE define an allowed, adjustable range RA. That is, the control unit  141  allows a stationary adjustment within the adjustable range RA, prevents a stationary adjustment in the disallowed range RD, and allows a stationary adjustment in the retracted position. As an example, starting from the retracted position PR, and receiving a signal to displace the scissor levers  100 ,  101  and the impact element  21  in the direction of the extended position PE, the control unit  141  may be adapted to at least displace the scissor levers  100 ,  101  and the impact element  21  until entering the adjustable range. Alternatively or in addition, starting from the adjustable range RA in the direction of the retracted position PR, when entering the disallowed range RD, the control unit  141  may be adapted to at least displace the scissor levers  100 ,  101  and the impact element  21  until assuming the retracted position PR. Alternatively or in addition, the control unit  141  may be adapted to displace the scissor levers  100 ,  101  and the impact element  21  to the closer one of the retracted position PR and the rear adjustment position PA when receiving a signal to stop. 
     The adjustable range RA is smaller than the kinematical range RK. The kinematical range RK equals the sum of the adjustable range RA and the disallowed range RD. Hence, the control unit  141  defines a virtual end stop, and the stop portions  104 ,  114  define a mechanical end stop (by mechanically contacting one another). By means of the control unit  141  it is made sure that a gap of, e.g., several mm, e.g., 3 mm (or at least 3 mm), between the stop portions  104 ,  114  defining the end stop is maintained to effectively avoid any rattling. The disallowed range RD may be preconfigured or configurable, and it may be defined by a number of motor revolutions. Optionally, a calibration may be performed by the control unit  141  and the motor  140  is controlled to retract until reaching the mechanical end stop and then extend by a (preconfigured) number of revolutions to assume the rear adjustment position PA. 
     The following is a list of reference numbers shown in the Figures. However, it should be understood that the use of these terms is for illustrative purposes only with respect to one embodiment. And, use of reference numbers correlating a certain term that is both illustrated in the Figures and present in the claims is not intended to limit the claims to only cover the illustrated embodiment. 
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               1 ;  1 ′ adjustment device 
               10 A- 10 D outer adjusting part 
               100  scissor lever 
               101 ,  102  cylinder section 
               103  axis 
               104  stop portion 
               11 A,  11 B inner adjusting part 
               110  scissor lever 
               111  cross member 
               112  bushing 
               113  bushing 
               14  stop portion 
               2  spindle 
               120  external thread 
               13 A- 13 D spindle nut (drive element) 
               130  internal thread 
               131 A,  131 B input part 
               132 A- 132 CA output part 
               133  latching hook 
               134  latching edge 
               135  counterstop 
               136  stop 
               37 A;  137 B,  138  end stop 
               14  drive unit 
               140  motor 
               141  control unit  15 A- 15 J drive contour 
               16  mounting area (dome) 
               17  stiffening strut 
               18  reset spring 
               19  height-adjustment mechanism 
               90  spindle 
               91  drive unit 
               192  slide bearing 
               2 ;  2 ′;  2 ″ headrest 
               20  base 
               200  mounting area (receptacle) 
               201  mounting area (slot) 
               202  latching element 
               203  lead-in chamfer 
               204  securing element 
               21  impact element 
               22  headrest rod 
               3  vehicle seat 
               30  seat part 
               31  backrest 
               32  height adjustment device 
               33  swivel fitting 
               34  length adjustment device 
               4  front pad 
               40  mounting area 
             B 1 -B 5  adjustment path 
             A receptacle 
             D axis of rotation 
             DG swivel joint 
             DS sliding swivel joint 
             K crossing point 
             MX depth-adjustment module 
             MZ height-adjustment module 
             PA rear adjustment position 
             PE extended position 
             PR retracted position 
             RA adjustable range 
             RD disallowed range 
             RK kinematical range 
             X longitudinal direction 
           
         
       
    
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.