Patent Publication Number: US-11377007-B2

Title: Adjustment mechanism for motorized multi-way seat adjustment

Description:
FIELD OF THE INVENTION 
     The present invention relates to an adjustment mechanism for a seat and to a seat equipped with such adjustment mechanism. The invention relates in particular to an adjustment mechanism for motorized multi-way adjustment of a seat. 
     BACKGROUND OF THE INVENTION 
     In view of optimizing comfort of a seat, it is known to provide the seat with various kinds of adjustability. By way of example, it is known to adjust a vehicle seat with respect to inclination of a backrest portion of the seat, with respect to height of a headrest of the seat, or with respect to arching and/or vertical position of a lumbar support of the seat. 
     To facilitate adjustment of the seat, it is also known to implement adjustment of the seat in a motorized manner. For example, WO 2017/032390 A1 describes a motorized adjustment mechanism which can be used for adjusting various seat components. EP 2 698 278 A1 describes a motorized adjustment mechanism for a headrest which may be used for shifting the headrest in a forward/backward direction or for adjusting the position of side bolsters of the headrest. 
     However, in such known adjustment mechanisms, a separate adjustment mechanism and motor is may need to be provided for each adjustment degree of freedom. For example, EP 2 698 278 A1 describes usage of two separate actuators in the headrest of the seat, one being provided for adjustment of the headrest in a forward/backward direction, the other being provided for height adjustment of the headrest. 
     Accordingly, there is a need for adjustment mechanisms which allow for efficiently adjusting a seat according to multiple degrees of freedom. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an adjustment mechanism according to claim  1  and a seat according to claim  14 . The dependent claims define further embodiments. 
     Accordingly, an adjustment mechanism according to an embodiment has the purpose of being used in a seat, for adjusting the seat according to at least two degrees of freedom. Examples of such degrees of freedom are tilting of one or more flap elements, such as side bolsters or a leg support, horizontal movement of a headrest of the seat in a forward/backward direction, vertical movement of the headrest of the seat, length adjustment of a seat cushion portion of the seat, height adjustment of a lumbar support of the seat, or adjustment of arching of a lumbar support of the seat. 
     According to an embodiment, the adjustment mechanism comprises at least one screw shaft and a motor for driving the at least one screw shaft. Further, the adjustment mechanism comprises at least one nut element engaged with the at least one screw shaft in such a way that a rotation of the at least one screw shaft caused by the motor translates into a linear motion of the at least one nut element. In a first range of the linear motion of the at least one nut element the linear motion translates into adjustment of the seat according to a first degree of freedom, and in a second range of the linear motion of the at least one nut element the linear motion translates into adjustment of the seat according to a second degree of freedom. Accordingly, the same motor and the same screw shaft may be used for implementing adjustment according to different degrees of freedom. The adjustment mechanism may thus be implemented in a compact and efficient manner. 
     According to an embodiment, the first degree of freedom corresponds to displacement of a headrest of the seat in a horizontal direction or to tilting of the headrest. The headrest may then comprise at least one flap element, e.g., side bolsters, which is pivotable with respect to a main portion of the headrest, and the second degree of freedom may correspond to pivoting of the at least one flap element with respect to the main portion of the headrest. Accordingly, horizontal displacement or tilting of the headrest may be efficiently combined with tilting of the flap element(s). 
     According to an embodiment, the headrest comprises a first flap element which is pivotable with respect to the main portion of the headrest and second flap element which is pivotable with respect to the main portion of the headrest. The first and second flap elements may for example correspond to right and left side bolsters of the headrest. The second degree of freedom may then correspond to pivoting of the first flap element and the second flap element with respect to the main portion of the headrest. In this case the adjustment mechanism may comprise a first nut element engaged with the at least one screw shaft and a second nut element engaged with the at least one screw shaft. The first nut element and the second nut element may be engaged with a first threaded portion and a second threaded portion of the same screw shaft, with one of the first threaded portion and the second threaded portion being right handed and the other being left handed. Rotation of the screw shaft in one direction would thus cause the linear movements of the first and second nut elements to be in opposite directions. Alternatively, the first nut element and the second nut element may be engaged with different screw shafts. In this case, the direction of the linear movements of the first and second nut elements may also be determined by the respective rotation direction of the screw shafts and/or which each may either have a right handed thread or a left handed thread. 
     In the above case of using the first nut element and the second nut element, a rotation of the at least one screw shaft caused by the motor translates into a linear motion of the first nut element and a linear motion of the second nut element. In a first range of the linear motion of the first nut element and a first range of the linear motion of the second nut element, the linear motion of the first nut element and of the second nut element may translate into adjustment of the seat according to the first degree of freedom. Further, in a second range of the linear motion of the first nut element the linear motion of the first nut element may translate into pivoting of the first flap element with respect to the main portion of the headrest, while in a second range of the linear motion of the second nut element the linear motion of the second nut element translates into pivoting of the second flap element with respect to the main portion of the headrest. Accordingly, the two flap elements may be tilted in an efficient manner by using the linear motion of the two nut elements in the second range. 
     The tilting of the at least one flap element may be about a vertical tilt axis. In this case, the adjustment mechanism may be efficiently implemented by arranging the at least one screw shaft in a horizontal direction. 
     According to an embodiment, the adjustment mechanism comprises at least one further screw shaft and a clutch mechanism for selectively engaging the motor with one or more of the at least one screw shaft and the at least one further screw shaft. In this case, a rotation of the at least one further screw shaft caused by the motor may translate into adjustment of the seat according to a third degree of freedom. For example, the adjustment mechanism may comprise at least one further nut element engaged with the at least one further screw shaft such that rotation of the at least one further screw shaft translates into a linear motion of the at least one further nut element. The linear motion of the at least one further nut element may then translate into adjustment of the seat according to the third degree of freedom. The third degree of freedom may for example correspond to displacement of a headrest of the seat in a vertical direction, i.e., to a height adjustment of the headrest. In this case, it may be beneficial if the at least one screw shaft is arranged in a horizontal direction and the at least one further screw shaft is arranged in a vertical direction. 
     Accordingly, additional degrees of freedom may be efficiently supported by providing the clutch mechanism. 
     According to an embodiment, the clutch mechanism comprises at least one solenoid actuator for switching the clutch mechanism between a first state in which the motor is engaged with one or more of the screw shafts and second state in which the motor is not engaged with said one or more of the screw shafts. The solenoid actuator may be controlled electronically. 
     According to an embodiment, the clutch mechanism comprises at least one shape memory alloy (SMA) actuator for switching the clutch mechanism between a first state in which the motor is engaged with one or more of the screw shafts and second state in which the motor is not engaged with said one or more of the screw shafts. The SMA actuator may be controlled electronically. 
     According to an embodiment, the adjustment mechanism is configured to be accommodated within the headrest. Accordingly, the adjustable seat may be implemented in a compact manner, by efficiently using space which is available within the headrest. 
     According to a further embodiment, a seat is provided, e.g., a vehicle seat. The seat comprises an adjustment mechanism as defined above. Using this adjustment mechanism, the seat can be adjusted according to two or more degrees of freedom. As mentioned above, examples of such degrees of freedom are tilting of one or more flap elements, such as side bolsters or a leg support, horizontal movement of a headrest of the seat in a forward/backward direction, vertical movement of the headrest of the seat, length adjustment of a seat cushion portion of the seat, height adjustment of a lumbar support of the seat, or adjustment of arching of a lumbar support of the seat. 
     Accordingly, in some embodiments the seat comprises a headrest. In this case the adjustment mechanism may be used for adjustments according to degrees of freedom related to the headrest and may be accommodated within the headrest. In this way, a compact design of the seat may be achieved. Further, distances over which force and/or torque needs to be transmitted may be limited, which may help to improve efficiency, durability, and reliability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the invention will be described with reference to the accompanying drawings. 
         FIGS. 1A and 1B  illustrate an adjustable seat according to an embodiment of the invention. 
         FIGS. 2A, 2B, and 2C  illustrate an adjustment mechanism for adjustment of the headrest of the seat according to multiple degrees of freedom. 
         FIGS. 3A, 3B, and 3C  illustrate adjustment of the headrest by horizontal displacement of the headrest. 
         FIGS. 4A, 4B, and 4C  illustrate adjustment of the headrest by tilting flap elements of the headrest. 
         FIGS. 5A and 5B  illustrate adjustment of the headrest by vertical displacement of the headrest. 
         FIG. 6  illustrates a clutch mechanism of the adjustment mechanism. 
         FIGS. 7A and 7B  illustrate different states of the clutch mechanism. 
         FIG. 8  illustrates a further clutch mechanism which may be used in an adjustment mechanism according to an embodiment of the invention. 
         FIG. 9  illustrates a still further clutch mechanism which may be used in an adjustment mechanism according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments of the invention will be described with reference to the drawings. While some embodiments will be described in the context of specific fields of application, such as in the context of a vehicle seat, the embodiments are not limited to this field of application. The features of the various embodiments may be combined with each other unless specifically stated otherwise. 
       FIGS. 1A and 1B  schematically illustrate an adjustable seat  10 . Specifically,  FIGS. 1A and 1B  illustrated various degrees of freedom of adjusting the seat  10 . In the illustrated example, it is assumed that the seat  10  is a vehicle seat, in particular a driver&#39;s seat or a passenger seat for a car. However, it is noted that similar configurations could also be used for other types of seats, e.g., seats for other types of vehicles, such as trucks, aircrafts or trains, or seating furniture. 
     As illustrated, the seat  10  comprises a seat cushion portion  20 , a backrest portion  30 , and a headrest  50 . The backrest portion  30  may be provided with a lumbar support  40 , e.g., configured as a wire basket or as a flexible plastic element. The seat  10  is assumed to be adjustable according to various degrees of freedom, illustrated by double-headed arrows. As illustrated, these degrees of freedom may include: displacement of the headrest  50  in a horizontal direction, denoted by HH, displacement of the headrest  50  in a vertical direction, denoted by HV, displacement of the lumbar support  40  in a horizontal direction, denoted by LH, adjustment of arching of the lumbar support  40 , denoted by LA, adjustment of a horizontal length of the seat cushion portion  20 , denoted by SL, and tilting of a leg support at a front edge of the seat cushion portion  20 , denoted by LF. Here, a “horizontal” direction refers to a normal installation position of the seat and substantially corresponds to a direction within a plane of the seat cushion portion  20 . In particular, a horizontal displacement as described herein may correspond to a displacement along a forward/backward direction, the forward direction corresponding to a direction from a back edge BE of the seat cushion portion  20 , where the backrest portion  30  is attached, to an opposing front edge FE of the seat cushion portion  20 , and the backward direction corresponding to a direction from the front edge FE to the back edge BE. A “vertical” direction refers to a direction along the backrest portion  30 . The vertical direction is typically substantially perpendicular to the horizontal direction. The vertical direction may also be referred to as upward/downward direction, with the upward direction corresponding to a direction from the seat cushion portion  20  towards the headrest  50  and the downward direction corresponding to a direction from the headrest  50  towards the seat cushion portion  20 . It is noted that in some situations, e.g., when the backrest portion  30  is tilted backwards into a sleeping position, the vertical or upward/downward direction may also deviate from a direction which is perpendicular to the horizontal direction. 
     As illustrated in  FIG. 1A , the adjustability of the length of the seat cushion portion  20  may be implemented by providing the seat cushion portion  20  with a seat cushion element  25  which is slidable in the forward/backward direction with respect to a main part of the seat cushion portion  20 . The above-mentioned tiltable leg support may be provided by a flap element  26  on a front edge side of the slidable seat cushion element  25 . 
       FIG. 1B  shows a schematic top view of the headrest  50 . As further illustrated in  FIG. 1B , a further degree of freedom for adjustment of the seat  10  may correspond to adjustment of side bolsters of the headrest  50 , denoted by BA. In particular, the headrest  50  may be provided with a main portion  51 , a first flap element  52  on a right side of the main portion  51 , and a second flap element  53  on a left side of the main portion  51 . As illustrated, the flap elements  52 ,  53  can be tilted about a vertical axis with respect to the main portion  51  of the headrest  51 . The flap element  52  thus defines an adjustable right side bolster of the headrest, and the flap element  53  defines an adjustable left side bolster of the headrest  50 . 
     It is noted that the above degrees of freedom for adjustment of the seat  10  are to be understood as a non-exhaustive list of examples of adjustability. Accordingly, adjustment according to all of these degrees of freedom does not need to be supported by the seat  10 , or the seat  10  could also support adjustment according to one or more other degrees of freedom. 
     In the examples as further explained in the following, it is assumed that adjustment of the seat  10  according to at least two degrees of freedom is implemented in a motorized manner, using a single motor. An example of a corresponding adjustment mechanism will now be further explained with reference to  FIGS. 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B, 6, 7A, and 7B . In this example, it is assumed that the adjustment mechanism provides motorized adjustment of the seat  10  by horizontal displacement of the headrest  50 , i.e., according to the degree of freedom denoted by HH, by vertical displacement of the headrest  50 , i.e., according to the degree of freedom denoted by HV, and tilting of the side bolsters of the headrest  50 , i.e., according to the degree of freedom denoted by BA. However, it is to be noted that a similar configuration of the adjustment mechanism could also be used for adjustment according to other combinations of two or more degrees of freedom. 
       FIGS. 2A, 2B, and 2C  show top views of the headrest  50  in different adjustment states, with the adjustment mechanism being exposed for purposes of illustration. Specifically,  FIG. 2A  illustrates the headrest  50  in a neutral position,  FIG. 2B  illustrates the headrest  50  with the main portion of the headrest  50  being displaced out of the neutral position in the forward direction, and  FIG. 2C  illustrates the headrest  50  with the flap elements  52 ,  53  being tilted out of the neutral position. The adjustment mechanism couples the main portion  51  and the flap elements  52 ,  53  to a bracket  55 , by means of which the headrest  50  is attached to the backrest portion  30  of the seat  10 . The adjustment mechanism is based on a spindle drive and uses screw shafts  111 ,  112  (or spindles) for driving the motorized adjustment. As illustrated, the screw shafts  111 ,  112  extend horizontally in opposite directions from a central part of the adjustment mechanism. A first nut element  121  is engaged with the first screw shaft  111 , and a second nut element  122  is engaged with the second screw shaft  112 . The first nut element  121  and the second nut element  122  are movable along the respective screw shaft  111 ,  112 , but locked with respect to a rotation about the screw shaft  111 ,  112 . Accordingly, a rotation of the screw shaft  111 ,  112  translates into a linear movement of the respective nut element  121 ,  122  along the screw shaft  111 ,  112 . In the illustrated example, it is assumed that the first screw shaft  111  and the second screw shaft  112  are formed on opposite ends of a single drive shaft. Accordingly, rotation of this drive shaft produces a corresponding rotation of the first screw shaft  111  and the second screw shaft  112 , which in turn translates into the linear movement of the nut elements  121 ,  122 . One of the first screw shaft  111  and the second screw shaft  112  is assumed to have a left-handed thread, while the other of the first screw shaft  111  and the second screw shaft  112  has a right handed thread. Accordingly, if the drive shaft is rotated in a certain direction, the linear movements of the not elements  121 ,  122  will be in opposite directions. 
     As further illustrated, the main portion  51  of the headrest  50  is coupled by levers  131 ,  132  to the bracket  55 . The levers  131 ,  132  are each supported to be rotatable about a vertical tilt axis. In a first range of the linear movement of the nut elements  121 ,  122 , the nut element  121  is engaged with the lever  131 , and the nut element  122  is engaged with the lever  132 . This has the effect that the linear movements of the nut elements  121 ,  122  translate into rotation of the levers  131 ,  132 , which in turn displaces the main portion  51  of the headrest  50  in the forward/backward direction, as illustrated in  FIG. 2B . 
     As further illustrated, the flap element  52  of the headrest  50  is coupled by a lever  141  to the bracket  55 , and the flap element  53  of the headrest is coupled by a lever  142  to the bracket  55 . The levers  141 ,  142  are each supported to be rotatable about a vertical tilt axis. In a second range of the linear movement of the nut elements  121 ,  122 , the nut element  121  is engaged with the lever  141 , and the nut element  142  is engaged with the lever  142 . This has the effect that the linear movements of the nut elements  121 ,  122  translate into rotation of the levers  141 ,  142 , which in turn tilts the flap elements  52 ,  53  of the headrest  50 , as illustrated in  FIG. 2C . 
     It is noted that while in  FIGS. 2A, 2B, and 2C  the displacement of the main portion  51  of the headrest  50  in the/backward direction is illustrated as occurring without moving the flap elements  52 ,  53 , it is also possible to couple movement of the flap elements  52 ,  53  in the forward/backward direction to the movement of the main portion  51  in the forward/backward direction. This may for example be achieved by coupling the flap elements  52 ,  53  to the main portion  51 , e.g., by upholstery elements and/or a common cover, and/or by providing a support on the main portion  51 , such as illustrated by short dotted lines in  FIG. 2A . Accordingly, the adjustment mechanism may also be used for horizontal displacement of the headrest  50  as whole. 
       FIGS. 3A, 3B, and 3C  further illustrate the adjustment of the headrest  50  by horizontal displacement of the main portion  51  of the headrest  50 . Specifically,  FIGS. 3A, 3B, and 3C  further illustrate actuation of the lever  131  by the nut element  121 . Here, it is to be understood that the lever  132  will be actuated in a corresponding manner by the nut element  122 . 
       FIG. 3A  illustrates a state in which the nut element  121  and the lever  131  are in a neutral position and not yet engaged with each other. By rotation of the screw shaft  111  in a first direction, the nut element  121  may be moved towards the lever  131 , until it eventually engages with the lever  131 , as illustrated in  FIG. 3B . By further rotation of the screw shaft  111  in the first direction, the nut element  121  is urged against the lever  131 , causing rotation of the lever  131  and forward displacement of the main portion  51  of the headrest  50 , until reaching a maximum displacement position as illustrated in  FIG. 3C . When rotating the screw shaft  111  in the opposite direction, the lever  131  is gradually allowed to return to its neutral position. This return movement may for example be driven by spring force. For example, the lever  131  could be coupled to a spring element which is deformed when rotating the lever  131  out of its neutral position, or the lever  131  could itself exhibit a spring characteristic. 
       FIGS. 4A, 4B, and 4C  illustrate adjustment of the headrest by tilting the flap elements  52 ,  53  of the headrest  50 . Specifically,  FIGS. 4A, 4B, and 4C  further illustrate actuation of the lever  141  by the nut element  121 . Here, it is to be understood that the lever  142  will be actuated in a corresponding manner by the nut element  122 . 
       FIG. 4A  illustrates a state in which the nut element  121  and the lever  141  are in a neutral position and not yet engaged with each other. By rotation of the screw shaft  111  in a second direction which is opposite to the above-mentioned first direction, the nut element  121  may be moved towards the lever  141 , until it eventually engages with the lever  141 , as illustrated in  FIG. 4B . By further rotation of the screw shaft  111  in the second direction, the nut element  121  is urged against the lever  141 , causing rotation of the lever  141  and tilting of the flap element  52  of the headrest  50 , until reaching a maximum tilt position as illustrated in  FIG. 4C . When rotating the screw shaft  111  in the opposite direction, the lever  141  is gradually allowed to return to its neutral position. This return movement may for example be driven by spring force. For example, the lever  141  could be coupled to a spring element which is deformed when rotating the lever  141  out of its neutral position, or the lever  141  could itself exhibit a spring characteristic. 
     Accordingly,  FIGS. 3B and 3C  define limits of the above-mentioned first range of the linear movement of the nut element  121 , and  FIGS. 4B and 4C  define limits of the above-mentioned second range of the linear movement of the nut element  121 . Here, it is noted that in the illustrated example the first range and the second range are separated, i.e., do not overlap. However, it would also be possible to have some overlap of the first range and the second range. In this case, a part of the displacement of the main portion  51  would occur simultaneously with the tilting of the flap elements  52 ,  53 . 
       FIGS. 5A and 5B  illustrate further adjustment of the headrest  50  by vertical displacement. Specifically,  FIGS. 5A and 5B  show rear views of the headrest  50  in different vertical positions, with the adjustment mechanism being exposed for purposes of illustration.  FIG. 5A  illustrates the headrest  50  its lowermost position and  FIG. 5B  illustrates the headrest  50  being displaced upwardly from the lowermost position. 
     As illustrated in  FIGS. 5A and 5B , the adjustment mechanism includes a third screw shaft  113 , which extends in vertical direction. A third nut element  123  is engaged with the third screw shaft  113  and connected to the bracket  55 . The third screw shaft  113  and the third nut element  123  provide a vertically adjustable coupling of the headrest  50  to the bracket  55 . In particular, rotation of the third screw shaft  113  will cause a linear movement of the third nut element  123  of the third screw shaft  113 , thereby vertically displacing the headrest  50 , as illustrated by  FIGS. 5A and 5B . 
       FIGS. 5A and 5B  further illustrate the motor  150 , which in the illustrated example is used for driving the first screw shaft  111 , the second screw shaft  112 , and the third screw shaft  113 . The motor  150  may be an electric motor and may be electronically controlled to adjust the seat  10 . The motor  150  may for example be a brushless motor, e.g., using an electronic commutation scheme. Implementing the motor  150  as a brushless motor may help to reduce noise and/or improve durability and reliability. Since motor  150  is used for driving both screw shafts  111 ,  112  and usage of multiple motors is therefore not necessary, excessive costs for using a brushless motor can be avoided. 
     For coupling the motor  150  to the screw shafts  111 ,  112 ,  113 , the adjustment mechanism is provided with gears  155 ,  180  and a clutch mechanism  160  as further explained below. The gear  155  is a two-stage worm gear which facilitates a compact implementation of the adjustment mechanism and at the same time allows for efficient adaptation of a revolution speed of the motor  150  to a desired rotation speed of the screw shafts  111 ,  112 ,  113 . The gear  180  is a worm gear used for translation of a rotation along the horizontal axis of the first and second screw shafts  111 ,  112  and the vertical axis of the third screw shaft  113 . The clutch mechanism  160  allows for selectively engaging the motor  150  either with the first and second screw shafts  111 ,  112  or with the third screw shaft  113 . The clutch mechanism  160  is provided with an actuator  170  which allows for electronically controlling a state of the clutch mechanism  160 . In particular, the actuator  170  may be used for electronically switching the clutch mechanism  160  between a first state, in which the motor  150  is engaged with the first and second screw shafts  111 ,  112  and a second state, in which the motor  150  is engaged with the third screw shaft  113 . In the illustrated example, the actuator  170  is assumed to be a solenoid actuator. However, other types of actuator could be used as well. As for example further explained below, also an SMA based actuator could be used for switching between the states of the clutch mechanism  160 . 
     By using the clutch mechanism  160  to engage the motor  150  with the third screw shaft  113  while disengaging the motor  150  from the first and second screw shafts  111 ,  112 , the motor  150  can be used for exclusively driving the third screw shaft  113 , thereby allowing to adjust the vertical position of the headrest  50 , without vertically displacing the headrest  50  or tilting the flap elements  52 ,  53  of the headrest  50 . Similarly, by using the clutch mechanism  162  engage the motor  150  with the first and second screw shafts  111 ,  112  while disengaging the motor  150  from the third screw shaft  113 , the motor  150  can be used for exclusively driving the first and second screw shafts  111 ,  112 , thereby allowing to vertically displace the main portion  51  of the headrest  50  and/or to tilt the flap elements  52 ,  53  of the headrest  50 , while maintaining the vertical position of the headrest  50 . 
       FIG. 6  further illustrates the clutch mechanism  160  of the adjustment mechanism. As illustrated, the clutch mechanism  160  includes a first gear wheel  161  which is driven by the two-stage worm gear  155 . As mentioned above, the first and second screw shaft  111 ,  112  are formed as different portions of the same drive shaft. The first gear wheel  161  is arranged concentrically with this drive shaft, but is rotatable about the drive shaft. Accordingly, a rotation of the first gear wheel  161  does not necessarily translate into a rotation of the drive shaft. Next to the first gear wheel  161 , also concentrically with the axis of the drive shaft, a first clutch wheel  163  is arranged on the drive shaft. The first clutch wheel  163  rotates together with the drive shaft. The first gear wheel  161  and the first clutch wheel  163  have opposing axial faces which are configured to be used for selectively engaging the first gear wheel  161  and the first clutch wheel  163  by bringing the opposing axial faces together. As further explained below, the first gear wheel  161  and the first clutch wheel  163  may be provided with axially extending teeth on their respective axial face, and the first gear wheel  161  and the first clutch wheel  163  may be engaged by interlocking of these axially extending teeth. For facilitating engagement and/or disengagement of the axial teeth, the axial teeth may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. However, it is noted that other ways of engaging the first gear wheel  161  and the first clutch wheel  163  could be used as well, e.g., engagement by surface friction. If the first clutch wheel  163  is engaged with the first gear wheel  161 , the drive shaft and thus also the first and second screw shafts  111 ,  112  rotate together with the first gear wheel  161 , i.e., are driven by the motor  150 . If the first clutch wheel  163  is disengaged from the first gear wheel  161 , the driveshaft and thus also the first and second screw shafts  111 ,  112  do not rotate together with the first gear wheel  161 , i.e., are not driven by the motor  150 . 
     The clutch mechanism further includes a second gear wheel  164  arranged on a further drive shaft  181  which is arranged in parallel to the drive shaft of the first and second screw shafts  111 ,  112 . The second gear wheel  164  is engaged with the first gear wheel  161 . Accordingly, the motor  150  drives the second gear wheel  164  through the first gear wheel  161 . The second gear wheel  164  is arranged concentrically with the further drive shaft  181  while being rotatable about the further drive shaft  181 . Accordingly, a rotation of the second gear wheel  164  does not necessarily translate into a rotation of the further drive shaft  181 . Next to the second gear wheel  164 , concentrically with the axis of the further drive shaft  181 , a second clutch wheel  166  is arranged on the further drive shaft  181 . The second clutch wheel  166  rotates together with the further drive shaft  181 . The second gear wheel  164  and the second clutch wheel  166  have opposing axial faces which are configured to be used for selectively engaging the second gear wheel  164  and the second clutch wheel  166  by bringing the opposing axial faces together. As further explained below, also the second gear wheel  164  and the second clutch wheel  163  may be provided with axially extending teeth on their respective axial face, and the second gear wheel  164  and the second clutch wheel  166  may be engaged by interlocking of these axially extending teeth. For facilitating engagement and/or disengagement of the axial teeth, the axial teeth may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. However, it is noted that other ways of engaging the second gear wheel  164  and the second clutch wheel  166  could be used as well, e.g., engagement by surface friction. If the second clutch wheel  166  is engaged with the second gear wheel  164 , the further drive shaft  181  rotates together with the second gear wheel  164 , i.e., is driven by the motor  150 . Through the worm gear  180 , the further drive shaft  181  then drives the third screw shaft  113 . If the second clutch wheel  166  is disengaged from the second gear wheel  164 , the further drive shaft  181  and thus also the third screw shaft  113  does not rotate together with the second gear wheel  166 , i.e., is not driven by the motor  150 . 
     The actuator  170  is used for controlling engagement of the first clutch wheel  163  with the first gear wheel  161  and engagement of the second clutch wheel  166  with the second gear wheel  164 . This is accomplished by axial shifting of the first clutch wheel  163  on the drive shaft of the first and second screw shafts  111 ,  112 , and by axial shifting of the second clutch wheel  166  on the further drive shaft  181 . This is further illustrated in  FIGS. 7A and 7B . 
       FIG. 7A  illustrates the clutch mechanism  160  in the first state, in which the motor  150  is engaged with the first and second screw shafts  111 ,  112  and disengaged from the third screw shaft  113 . As can be seen, in the first state an effector end  171  of the actuator  170  is extended by a travel distance D. The effector end  171  is coupled to both the first clutch wheel  163  and the second clutch wheel  166 . As a result, the first clutch wheel  163  is shifted towards the first gear wheel  161 , and the second clutch wheel  166  is shifted away from the second gear wheel  164 . Accordingly, the first clutch wheel  163  engages with the first gear wheel  161 , while the second clutch wheel  166  is disengaged from the second gear wheel  164 . As illustrated, the engagement of the first clutch wheel  163  with the first gear wheel  161  is achieved by interlocking of axial teeth  162  of the first gear wheel  161  with complementary axial teeth of the first clutch wheel  163 . For facilitating engagement and/or disengagement of the axial teeth  162  of the first gear wheel  161  with the complementary axial teeth of the first clutch wheel  163 , the axial teeth  162  and complementary axial teeth may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth  162  have an outer shape which tapers toward the first clutch wheel  163  and fits into a narrowing gap between two neighboring complementary teeth of the first clutch wheel  163 . However, other ways of engaging the first clutch wheel  163  with the first gear wheel  161  could be used as well. 
       FIG. 7B  illustrates the clutch mechanism  160  in the second state, in which the motor  150  is engaged with the third screw shaft  113  and disengaged from the first and second screw shafts  111 ,  112 . As can be seen, in the second state the effector end  171  of the actuator  170  is retracted. As a result, the first clutch wheel  163  is shifted away from the first gear wheel  161 , and the second clutch wheel  166  is shifted towards the second gear wheel  164 . Accordingly, the first clutch wheel  163  is disengaged from the first gear wheel  161 , while the second clutch wheel  166  engages with the second gear wheel  164 . As illustrated, the engagement of the second clutch wheel  166  with the second gear wheel  164  is achieved by interlocking of axial teeth  165  of the second gear wheel with complementary axial teeth of the second clutch wheel  166 . For facilitating engagement and/or disengagement of the axial teeth  165  of the second gear wheel  165  with the complementary axial teeth of the second clutch wheel  166 , the axial teeth  165  and complementary axial teeth may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth  165  have an outer shape which tapers toward the second clutch wheel  166  and fits into a narrowing gap between two neighboring complementary teeth of the second clutch wheel  166 . However, other ways of engaging the second clutch wheel  166  with the second gear wheel  164  could be used as well. 
     As can be seen, the adjustment mechanism of the illustrated example may be used for efficiently controlling adjustment of the seat  10  according to the different degrees of freedom by a single motor, namely for controlling horizontal displacement of the headrest  50 , in particular for controlling displacement of the main portion  51  of the headrest  50  in the forward/backward direction, for adjusting the side bolsters of the headrest  50  by tilting the flap elements  52 ,  53 , and for controlling displacement of the headrest  50  in the vertical direction, i.e., for adjusting the height position of the headrest  50 . The adjustment mechanism may be implemented in a compact manner which allows for accommodating the adjustment mechanism within the headrest  50 . 
     It is noted that while the above example referred to adjustment of the seat  10  according to the different degrees of freedom, a similar adjustment mechanism could also be used for controlling the seat  10  according to only two degrees of freedom, e.g., only for controlling only the horizontal displacement of the headrest  50  and the tilting of the flap elements  52 ,  53 . Still further, as an alternative or in addition to controlling the horizontal or vertical displacement of the headrest  50 , the adjustment mechanism could also be used for controlling tilting of the headrest  50 . Furthermore, in addition or as an alternative to the above examples of controlling degrees of freedom related to adjustment of the headrest  50 , the adjustment mechanism could also be used for controlling degrees of freedom related to adjustment of the backrest portion  30 , such as adjustment of the lumbar support  40  by controlling vertical displacement of the lumbar support  40  and/or arching of the lumbar support  40 , and/or degrees of freedom related to the seat cushion portion  20 , such as controlling the length of the seat cushion portion  25  by controlling horizontal displacement of the seat cushion element  25  and/or adjustment of leg support by controlling tilting of the flap element  26 . Depending on the controlled degrees of freedom, the adjustment mechanism could also be accommodated within the backrest portion  30  or the seat cushion portion  20  of the seat  10 . 
     The location for accommodating the adjustment mechanism may also depend on the degrees of freedom to be controlled. For example, in the case of a degree of freedom related to the backrest portion  30 , such as controlling lumbar support or adjustment of side bolsters of the backrest portion  30 , the adjustment mechanism could be arranged in the backrest portion  30  of the seat  10 . Similarly, in the case of controlling a degree of freedom related to the seat cushion portion  20 , such as adjustment of seat cushion length or adjustment of leg support, the adjustment mechanism could be arranged in the seat cushion portion  20  of the seat  10 . However, in some scenarios the adjustment mechanism could also be located differently. For example, for controlling a degree related to the headrest  50 , a part of the adjustment mechanism could also be arranged in the backrest portion  30 . For controlling degrees of freedom relating to the headrest  50  and the backrest portion  30 , the adjustment mechanism could also have components in accommodated in the headrest  50  and components accommodated in the backrest portion  30 . 
     In some implementations, the adjustment mechanism may use more than three screw shafts for controlling adjustment of the seat  10 . In such cases, each of the screw shaft may be used for controlling adjustment of the seat  10  according to at least one degree of freedom. As explained above, different ranges of linear motion of a nut element engaged with one of the screw shafts may be used for controlling multiple degrees of freedom with the same screw shaft. A clutch mechanism may be used for selectively controlling one or more of multiple screw shafts, and/or other types of drive shafts, by the same motor. 
       FIG. 8  illustrates a further example of a clutch mechanism  260  for an adjustment mechanism of the seat  10 . The clutch mechanism to  160  may be used as an alternative or in addition to the above-mentioned clutch mechanism  160 . As illustrated, the clutch mechanism  260  may be used for selectively driving multiple screw shafts  211 ,  212 ,  213 ,  214 ,  215  with a single motor  250 , e.g., an electronically controlled electric motor. The motor  250  may for example be a brushless motor, e.g., using an electronic commutation scheme. Implementing the motor  250  as a brushless motor may help to reduce noise and/or improve durability and reliability. Since the motor  250  is used for driving all screw shafts  211 ,  212 ,  213 ,  214 ,  215  and usage of multiple motors is therefore not necessary, excessive costs for using a brushless motor can be avoided. For at least one of the screw shafts  211 ,  212 ,  213 ,  214 ,  215 , different ranges of linear motion of a nut element engaged with one of the screw shafts may be used for controlling multiple degrees of freedom, such as explained in connection with  FIGS. 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B, and 5C . 
     In the clutch mechanism  260  the motor  250  uses a two-stage worm gear  255  to drive a first gear wheel  261  and a first clutch wheel  262  on a drive shaft of the screw shaft  211 . A second gear wheel  263  is arranged next to the first clutch wheel  262  while being rotatable about the drive shaft of the screw shaft  211 . Accordingly, a rotation of the second gear wheel  263  does not necessarily translate into a rotation of the drive shaft of the screw shaft  211 . The first clutch wheel  262  and the second gear wheel  263  have opposing axial faces which are configured to be used for selectively engaging the first clutch wheel  262  and the second gear wheel  263  by bringing the opposing axial faces together. As illustrated, this engagement may be achieved by interlocking of axial teeth of the first clutch wheel  262  and complementary axial teeth of the second gear wheel  263 . For facilitating engagement and/or disengagement of the axial teeth with the complementary axial teeth, the axial teeth of the first clutch wheel  262  and the complementary axial teeth of the second gear wheel  263  may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth have an outer shape which tapers toward the second gear wheel  263  and fits into a narrowing gap between two neighboring complementary teeth of the second gear wheel  263 . However, other ways of engaging the opposing axial surfaces could be used as well. 
     Next to the second gear wheel  263 , a second clutch wheel  264  is arranged on the drive shaft of the screw shaft  211 . The second clutch wheel  264  rotates together with the screw shaft  211 . The second gear wheel  263  and the second clutch wheel  264  have opposing axial faces which are configured to be used for selectively engaging the second gear wheel  263  and the second clutch wheel  264  by bringing the opposing axial faces together. As illustrated, this engagement may be achieved by interlocking of axial teeth of the second clutch wheel  264  and complementary axial teeth of the second gear wheel  263 . For facilitating engagement and/or disengagement of the axial teeth with the complementary axial teeth, the axial teeth of the second clutch wheel  264  and the complementary axial teeth of the second gear wheel  263  may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth have an outer shape which tapers toward the second gear wheel  263  and fits into a narrowing gap between two neighboring complementary teeth of the second gear wheel  263 . However, other ways of engaging the opposing axial surfaces could be used as well. 
     A third gear wheel  271  and a third clutch wheel  272  are arranged on a drive shaft of the screw shaft  212 . The third gear wheel  271  is engaged with and driven by the first gear wheel  261 . The third clutch wheel  272  rotates together with the third gear wheel  271  and is thus also driven by the first gear wheel  261 . A fourth gear wheel  273  is arranged next to the third clutch wheel  272  while being rotatable about the drive shaft of the screw shaft  212 . Accordingly, a rotation of the fourth gear wheel  273  does not necessarily translate into a rotation of the drive shaft of the screw shaft  212 . The third clutch wheel  272  and the fourth gear wheel  273  have opposing axial faces which are configured to be used for selectively engaging the third clutch wheel  272  and the fourth gear wheel  273  by bringing the opposing axial faces together. As illustrated, this engagement may be achieved by interlocking of axial teeth of the third clutch wheel  272  and complementary axial teeth of the fourth gear wheel  273 . For facilitating engagement and/or disengagement of the axial teeth with the complementary axial teeth, the axial teeth of the third clutch wheel  272  and the complementary axial teeth of the fourth gear wheel  273  may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth have an outer shape which tapers toward the fourth gear wheel  273  and fits into a narrowing gap between two neighboring complementary teeth of the fourth gear wheel  273 . However, other ways of engaging the opposing axial surfaces could be used as well. 
     Next to the fourth gear wheel  273 , a fourth clutch wheel  274  is arranged on the drive shaft of the screw shaft  212 . The fourth clutch wheel  274  rotates together with the screw shaft  212 . The fourth gear wheel  273  and the fourth clutch wheel  274  have opposing axial faces which are configured to be used for selectively engaging the fourth gear wheel  273  and the fourth clutch wheel  274  by bringing the opposing axial faces together. As illustrated, this engagement may be achieved by interlocking of axial teeth of the fourth clutch wheel  274  and complementary axial teeth of the fourth gear wheel  273 . For facilitating engagement and/or disengagement of the axial teeth with the complementary axial teeth, the axial teeth of the fourth clutch wheel  274  and the complementary axial teeth of the fourth gear wheel  273  may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth have an outer shape which tapers toward the fourth gear wheel  273  and fits into a narrowing gap between two neighboring complementary teeth of the fourth gear wheel  273 . However, other ways of engaging the opposing axial surfaces could be used as well. 
     A fifth gear wheel  275  is arranged on a drive shaft of the screw shaft  213 . The fifth gear wheel  275  is engaged with and driven by the fourth gear wheel  273 . The fifth gear wheel  275  is rotatable about the drive shaft of the screw shaft  213 . Accordingly, a rotation of the fifth gear wheel  275  does not necessarily translate into a rotation of the screw shaft  213 . Next to the fifth gear wheel  275 , a fifth clutch wheel  276  is arranged on the drive shaft of the screw shaft  213 . The fifth clutch wheel  276  rotates together with the screw shaft  213 . The fifth gear wheel  275  and the fifth clutch wheel  276  have opposing axial faces which are configured to be used for selectively engaging the fifth gear wheel  275  and the fifth clutch wheel  276  by bringing the opposing axial faces together. As illustrated, this engagement may be achieved by interlocking of axial teeth of the fifth clutch wheel  276  and complementary axial teeth of the fifth gear wheel  275 . For facilitating engagement and/or disengagement of the axial teeth with the complementary axial teeth, the axial teeth of the fifth clutch wheel  276  and the complementary axial teeth of the fifth gear wheel  275  may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth have an outer shape which tapers toward the fifth gear wheel  275  and fits into a narrowing gap between two neighboring complementary teeth of the fifth gear wheel  275 . However, other ways of engaging the opposing axial surfaces could be used as well. 
     A sixth gear wheel  281  is arranged on a drive shaft of the screw shaft  214 . The sixth gear wheel  281  is engaged with and driven by the second gear wheel  263 . The sixth gear wheel  281  is rotatable about the drive shaft of the screw shaft  214 . Accordingly, a rotation of the sixth gear wheel  281  does not necessarily translate into a rotation of the screw shaft  214 . A seventh gear wheel  282  is arranged next to the sixth gear wheel  281  while being rotatable about the drive shaft of the screw shaft  214 . Accordingly, a rotation of the seventh gear wheel  282  does not necessarily translate into a rotation of the screw shaft  214 . The sixth gear wheel  281  and the seventh gear wheel  282  have opposing axial faces which are configured to be used for selectively engaging the sixth gear wheel  281  and the seventh gear wheel  282  by bringing the opposing axial faces together. As illustrated, this engagement may be achieved by interlocking of axial teeth of the sixth gear wheel  281  and complementary axial teeth of the seventh gear wheel  282 . For facilitating engagement and/or disengagement of the axial teeth with the complementary axial teeth, the axial teeth of the sixth gear wheel  281  and the complementary axial teeth of the seventh gear wheel  282  may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth have an outer shape which tapers toward the seventh gear wheel  282  and fits into a narrowing gap between two neighboring complementary teeth of the seventh gear wheel  282 . However, other ways of engaging the opposing axial surfaces could be used as well. 
     Next to the seventh gear wheel  282 , a sixth clutch wheel  283  is arranged on the drive shaft of the screw shaft  214 . The sixth clutch wheel  283  rotates together with the screw shaft  214 . The seventh gear wheel  282  and the sixth clutch wheel  283  have opposing axial faces which are configured to be used for selectively engaging the seventh gear wheel  282  and the sixth clutch wheel  283  by bringing the opposing axial faces together. As illustrated, this engagement may be achieved by interlocking of axial teeth of the sixth clutch wheel  283  and complementary axial teeth of the seventh gear wheel  282 . For facilitating engagement and/or disengagement of the axial teeth with the complementary axial teeth, the axial teeth of the sixth clutch wheel  283  and the complementary axial teeth of the seventh gear wheel  282  may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth have an outer shape which tapers toward the seventh gear wheel  282  and fits into a narrowing gap between two neighboring complementary teeth of the seventh gear wheel  282 . However, other ways of engaging the opposing axial surfaces could be used as well. 
     An eighth gear wheel  284  is arranged on a drive shaft of the screw shaft  215 . The eighth gear wheel  284  is engaged with and driven by the seventh gear wheel  282 . The eighth gear wheel  284  is rotatable about the drive shaft of the screw shaft  215 . Accordingly, a rotation of the eighth gear wheel  284  does not necessarily translate into a rotation of the screw shaft  215 . Next to the eighth gear wheel  284 , a seventh clutch wheel  285  is arranged on the drive shaft of the screw shaft  215 . The seventh clutch wheel  285  rotates together with the screw shaft  215 . The eighth gear wheel  284  and the seventh clutch wheel  285  have opposing axial faces which are configured to be used for selectively engaging the eighth gear wheel  284  and the seventh clutch wheel  285  by bringing the opposing axial faces together. As illustrated, this engagement may be achieved by interlocking of axial teeth of the seventh clutch wheel  285  and complementary axial teeth of the eighth gear wheel  284 . For facilitating engagement and/or disengagement of the axial teeth with the complementary axial teeth, the axial teeth of the seventh clutch wheel  285  and the complementary axial teeth of the eighth gear wheel  284  may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth have an outer shape which tapers toward the eighth gear wheel  284  and fits into a narrowing gap between two neighboring complementary teeth of the eighth gear wheel  284 . However, other ways of engaging the opposing axial surfaces could be used as well. 
     The clutch mechanism  260  can be brought into various different states, depending on whether the above-mentioned clutch wheels or gear wheels are engaged with each other. The screw shaft  211  is driven by the motor  250  if the first clutch wheel  262  is engaged with the second gear wheel  263  and the second clutch wheel  264  is engaged with the second gear wheel  263 . The screw shaft  212  is driven by the motor  250  if the third clutch wheel  272  is engaged with the fourth gear wheel  273  and the fourth clutch wheel  274  is engaged with the fourth gear wheel  273 . The screw shaft  213  is driven by the motor  250  if the fifth clutch wheel  276  is engaged with the fifth gear wheel  275  and the fourth gear wheel  273  is engaged with the third clutch wheel  272 . The screw shaft  214  is driven by the motor  250  if the sixth clutch wheel  283  is engaged with the seventh gear wheel  282 , the seventh gear wheel  282  is engaged with the sixth gear wheel  281 , and the second gear wheel  263  is engaged with the first clutch wheel  262 . The screw shaft  215  is driven by the motor  250  if the seventh clutch wheel  285  is engaged with the eighth gear wheel  284 , the seventh gear wheel  282  is engaged with the sixth gear wheel  281 , and the second gear wheel  263  is engaged with the first clutch wheel  262 . 
     Further, each of the screw shafts  211 ,  212 ,  213 ,  214 ,  215  can be disengaged from the motor  250 : By disengaging the second clutch wheel  264  from the second gear wheel  263 , the screw shaft  211  can be disengaged from the motor  250 , while at the same time allowing any other of the screw shafts  212 ,  213 ,  214 ,  215  to be engaged with the motor  250 . By disengaging the fourth clutch wheel  274  from the fourth gear wheel  273 , the screw shaft  212  can be disengaged from the motor  250 , while at the same time allowing any other of the screw shafts  211 ,  213 ,  214 ,  215  to be engaged with the motor  250 . By disengaging the fifth clutch wheel  276  from the fifth gear wheel  275 , the screw shaft  213  can be disengaged from the motor  250 , while at the same time allowing any other of the screw shafts  211 ,  212 ,  214 ,  215  to be engaged with the motor  250 . By disengaging the sixth clutch wheel  283  from the seventh gear wheel  282 , the screw shaft  214  can be disengaged from the motor  250 , while at the same time allowing any other of the screw shafts  211 ,  212 ,  213 ,  215  to be engaged with the motor  250 . By disengaging the seventh clutch wheel  285  from the eighth gear wheel  284 , the screw shaft  215  can be disengaged from the motor  250 , while at the same time allowing any other of the screw shafts  211 ,  212 ,  213 ,  214  to be engaged with the motor  250 . 
     Accordingly, by providing each of the screw shafts  211 ,  212 ,  213 ,  214 ,  215  with at least one gear wheel which is rotatable about a drive shaft of the respective screw shaft  211 ,  212 ,  213 ,  214 ,  215  and with a clutch wheel which rotates together with the screw shaft and can be selectively engaged with the respective gear wheel, the clutch mechanism  260  can flexibly support various states. As further can be seen from the clutch mechanism  260 , gear wheels on one drive shaft may be used for driving other drive shafts and at the same time be used for selective engagement with a clutch wheel or other gear wheel. This allows for a compact and efficient implementation of the clutch mechanism  260 . 
     In the clutch mechanism  260 , multiple actuators may be used for selectively engaging the clutch wheels and gear wheels as described above. These actuators may be implemented as solenoid actuators, similar as described for the above-mentioned actuator  170 . However, other types of actuators could be used as well, such as SMA based actuators. The clutch mechanism  260  could also use a combination of two or more different types of actuators, such as a combination of solenoid based actuators and SMA based actuators. Further, it is also possible to use one actuator for engaging more than one pair of wheels, such as described for the above-mentioned actuator  170 . 
       FIG. 9  illustrates a still further clutch mechanism  360  which may be used in an adjustment mechanism as explained above, e.g., in addition or as an alternative to the above-mentioned clutch mechanism  160  or  260 . As illustrated, the clutch mechanism  260  may be used for selectively driving multiple screw shafts  311 ,  312  with a single motor  350 , e.g., an electronically controlled electric motor. The motor  350  may for example be a brushless motor, e.g., using an electronic commutation scheme. Implementing the motor  350  as a brushless motor may help to reduce noise and/or improve durability and reliability. Since the motor  350  is used for driving both screw shafts  311 ,  312  and usage of multiple motors is therefore not necessary, excessive costs for using a brushless motor can be avoided. For at least one of the screw shafts  311 ,  312 , different ranges of linear motion of a nut element engaged with one of the screw shafts may be used for controlling multiple degrees of freedom, such as explained in connection with  FIGS. 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B, and 5C . 
     In the clutch mechanism  360 , the motor  350  uses a two-stage worm gear  355  to drive the screw shaft  311 . A first gear wheel  361  rotating together with the screw shaft  311  is used to drive a second gear wheel  362 . The second gear wheel  362  is arranged on a drive shaft of the screw shaft  312 , while being rotatable about the drive shaft of the screw shaft  312 . Accordingly, a rotation of the second gear wheel  362  does not necessarily translate into a rotation of the screw shaft  312 . Next to the second gear wheel  362 , a clutch wheel  363  is arranged on the driveshaft of the screw shaft  312 . The clutch wheel  363  rotates together with the screw shaft  312 . The second gear wheel  362  and the clutch wheel  363  have opposing axial faces which are configured to be used for selectively engaging the clutch wheel  363  with the second gear wheel  362  by bringing the opposing axial faces together. As illustrated, this engagement may be achieved by interlocking of axial teeth of clutch wheel  363  and complementary axial teeth of the second gear wheel  362 . For facilitating engagement and/or disengagement of the axial teeth with the complementary axial teeth, the axial teeth of the clutch wheel  363  and the complementary axial teeth of the second gear wheel  362  may have sloped mating faces, e.g., inclined by an angle of 5° or less with respect to the axial direction. Due to the sloped mating faces, the axial teeth have an outer shape which tapers toward the second gear wheel  362  and fits into a narrowing gap between two neighboring complementary teeth of the second gear wheel  362 . However, other ways of engaging the opposing axial surfaces could be used as well. 
     In the clutch mechanism  360 , the engagement of the clutch wheel  363  with the second gear wheel  362  is controlled by an SMA based actuator  370 . As illustrated, the SMA based actuators  370  includes an SMA wire  371  and an electronically controlled heater  372 . In response to heating of the SMA wire  371  by the heater  372 , the SMA wire  371  changes its length. For example, the SMA wire  371  may shorten in response to heating by the heater  372 . Shortening of the SMA wire  371  actuates a lever  373  which pushes the second gear wheel  362  in an axial direction towards the clutch wheel  363 , thereby engaging the clutch wheel  363  with the second gear wheel  362 . In response to cooling of the SMA wire  371 , the length of the SMA wire  371  increases, thereby moving the second gear wheel  362  away from the clutch we have  363  and thus disengaging the clutch wheel three and  63  from the second gear wheel  362 . In the illustrated example, the lever  373  is formed of two parts which are flexibly connected to each other. This allows for avoiding excessive stress, e.g., if the lever  373  pushes the second gear wheel  362  towards the clutch wheel  363  while the axial teeth of the second gear wheel  362  and the complementary axial teeth of the clutch wheel  363  are misaligned and thus impair movement of the second gear wheel  362  towards the clutch wheel  363 . 
     In the clutch mechanism  360 , both screw shafts  311 ,  312  are driven by the motor  350  if the clutch wheel  363  is engaged with the second gear wheel  362 . If the clutch wheel  363  is disengaged from the second gear wheel  362 , only the screw shaft  311  is driven by the motor  350 . This may for example be useful for both controlling vertical position of the lumbar support  40  of the seat  10  and arching of the lumbar support  40  of the seat: Driving both screw shafts  311  and  312  may be used for adjustment of the vertical position of the lumbar support  40 , while driving only one of the screw shafts  311  and  312  may be used for adjustment of the arching. For this purpose, nut elements on the screw shafts  311  and  312  may be coupled to different engagement points on the lumbar support  40 , so that relative movement of these engagement points with respect to each other causes parking of the lumbar support  40 . Different ranges of movement of at least one of the nut elements may be used for controlling one or more additional degrees of freedom for adjustment of the seat  10 , 
     While exemplary embodiments have been described in the context of a vehicle seat, the adjustment mechanisms and seats according to embodiments of the invention are not limited to this particular application. Rather, adjustment mechanisms as explained above may be employed in a wide variety of seats. Further, it is noted that the illustrated adjustment mechanisms may be modified in various ways. By way of example, the adjustment mechanisms could include various numbers of screw shafts and be accommodated within various parts of the seat. Further, one or more of the screw shafts could be replaced by other kinds of shafts, e.g., torsion shafts, or other additional types of shafts could be used to supplement the above-mentioned screw shafts and be driven by the same motor. Further, one or more of the screw shafts could or other shafts could also be flexible or include flexible portions. Accordingly, the above-described clutch mechanisms may be used for driving screw shafts, but also for driving various other types of drive shafts, in addition or as an alternative to screw shafts as described above. In some cases such screw shafts or drive shafts may also extend from one part of the seat to another part of the seat, e.g., from the backrest portion to the headrest or vice versa. Further, it is also possible to use an adjustment mechanism which is distributed over multiple parts of the seat  10 , e.g., by accommodating some components of the adjustment mechanism in the backrest portion  30  of the seat  10  and accommodating other components of the adjustment mechanism in the headrest  50  and/or the seat cushion portion  20  of the seat  10 . Further, adjustment mechanisms as explained above may be used for adjustment of a seat with respect to various combinations of degrees of freedom. For example, using similar principles as explained above for the tilting of the side bolsters, tilting of the headrest, e.g., as denoted by HT in  FIG. 1A , could be implemented by using different ranges of linear movement of a nut element on the same screw shaft. Forward tilting of the headrest  50  could then for example start when the headrest  50  has reached its maximum upward position.