TECHNICAL FIELD AND PRIOR ART

The invention relates to a support element for a modular transport plane and to a modular transport plane for a laboratory distribution system. The invention further relates to a laboratory distribution system and to a laboratory automation system comprising a laboratory distribution system.

A laboratory automation system comprises a plurality of pre-analytical, analytical and/or post- analytical stations, in which specimens, for example blood, saliva, swab and other specimens taken from human or animal bodies, are processed. For specimen processing and movement, it is generally known to provide various containers, such as test tubes or vials, containing the specimen. The test tubes are also referred to as specimen tubes or sample tubes. In the context of the application, containers such as test tubes or vials for containing specimen are referred to as specimen containers.

For a movement or distribution of specimen containers, laboratory distribution system comprising a transport plane and a plurality of carriers are known. The carriers are configured to retain one or more specimen container in an upright or vertical position, and each comprise at least one magnetically active element such as, for example, at least one permanent magnet. The transport plane is configured to support the carriers on a driving surface and comprises a plurality of electromagnetic actuators stationary arranged below the driving surface, wherein the electromagnetic actuators can be controlled to move a carrier placed on the driving surface by applying a magnetic force to the carrier.

EP 3 211 430 A1 shows a modular or tiled transport plane with a plurality of transport modules, each transport module comprising a base plate assembly, an actuator assembly, and a driving surface assembly. The transport module units have a regular polygonal basic shape, in particular a square basic shape with four corners. For assembling the transport plane from a plurality of transport modules, at first a plurality of base plate assemblies is mounted to a support structure, wherein adjacent base plate assemblies are aligned and connected to each other at corner regions by means of support elements in the form of corner support elements. The actuator assemblies are mounted to the base plate assemblies. Finally, the driving surface assemblies are mounted to the base plate assemblies via the corner support elements. US 10,288,634 shows a modular or tiled transport plane with a plurality of transport modules, each transport module having a regular polygonal basic shape with three, four or six corners and comprising an actuator assembly, and a driving surface assembly. The tiled transport plane is mounted by means of a plurality of support brackets to a support frame in a grid pattern, wherein corners of neighboring transport modules define nodes of the grid pattern, wherein each support bracket is provided with a support structure arranged at one of the nodes of the grid pattern. The support structures each have a corner support element with a cross element configured to support all corner regions of neighboring actuator assemblies of the node and an upper support surface with apertures configured to receive connection pins of neighboring driving surface assemblies of the node.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a support element for a modular transport plane allowing a reliable alignment of neighboring driving surface assemblies, even on misalignment of a support structure or base plate assemblies and/or thermal expansion. It is a further object of the invention to provide a modular transport plane comprising such support elements.

According to a first aspect, a support element for a modular transport plane with a plurality of transport module units, each transport module unit comprising a driving surface assembly, is provided, wherein the support element comprises a lower part having a mounting structure and an upper part having an upper support surface, wherein the upper support surface is configured to support the driving surface assemblies of at least two neighboring transport module units, wherein the upper support surface is provided with driving surface assembly interfaces configured to engage with complementary interfaces of the supported driving surfaces, wherein the upper part is connected lengthwise to the lower part via a connection structure, and wherein the connection structure is configured to restrain a relative movement between the lower part and the upper part in the longitudinal direction of the support element and to allow a limited relative movement between the lower part and the upper part in a plane perpendicular to the longitudinal direction of the support element.

A driving surface assembly for example comprises a driving surface, a driving surface support and a sensor board. The sensor board at least forms part of a device for sensing a presence or position of a carrier moved across the upper side of the driving surface. In one embodiment, the driving surface is transparent to IR light, wherein the sensor board can be equipped with multiple IR based reflection light barriers arranged in a grid, and the carriers can be adapted to reflect IR radiation emitted by the light barriers. In other embodiments, the sensor board is equipped with inductive sensors adapted for detecting a position of a carrier moved across the upper side of the driving surface. The transport module unit may further comprise an actuator assembly with back iron, a plurality of coils mounted to the back iron and controller circuit board. The transport module units in embodiments are mounted above a base plate assembly with cabling for a power supply, cabling for a communication, and a cooling infrastructure.

The support element, more particular the lower part of the support element, comprises a mounting structure. In one embodiment, the mounting structure is configured to mount the lower part to a support frame. In other embodiments, base plate assemblies are provided, and the mounting structure is configured to mount the lower part to the base plate assemblies.

The upper part is mounted so as to be moveable with respect to the lower part in a plane perpendicular to the longitudinal direction. In other words, the upper part is connected in a floating manner to the lower part. This allows for a limited relative movement between the lower part and the upper part, and, thus, between the lower part and driving surface assemblies supported by the upper support surface and in engagement with the upper part.

Upon installation of a modular transport plane and/or in use, misalignments between base plate assemblies and/or other support structures can be caused for example by manufacturing tolerances, assembly tolerances, installation tolerance, and/or thermal expansion or contraction. By providing a multi-part support element, an alignment of driving surface assemblies can be achieved using the upper parts of a set of support elements despite a misalignment between base plate assemblies and/or other support structures engaged with the lower parts of said set of support elements.

Throughout this description and the claims, the indefinite article "a" or "an" means "one or more". Reference to "a first element" does not mandate presence of “a second element”. Further, the expressions “first” and “second” are only used to distinguish one element from another element and not to indicate any order of the elements.

The connection structure in one embodiment comprises a pair of interlocking hook and eye elements, in particular three or four pairs of interlocking hook and eye elements evenly distributed about a circumference of the support element, wherein the hook element and the eye element of the pair of interlocking hook and eye elements have contacting cross braces which are configured to allow a sliding movement between the hook element and the eye element in at least one direction in a plane perpendicular to the longitudinal direction of the support element. ln the context of the application, a hook element is defined as a curved or bent element having a free end and which is configured to interlock with an eye element configured to catch the hook element for restricting a movement in a longitudinal direction. The eye element in one embodiment is also provided with a free end and similar in design to the hook element. In other embodiments, the eye element is a closed, loop-shaped element. The hook element and the eye element contact each other in cross braces, which cross braces according to an embodiment are configured to allow a sliding movement between the hook element and the eye element in at least one direction in a plane perpendicular to the longitudinal direction of the support element.

In one embodiment, the hook elements are formed at the upper part and the eye element are formed at the lower part. In other embodiments, the hook elements are formed at the lower part and the eye element are formed at the upper part.

In one embodiment, in order to allow a relative sliding movement, the hook element and the eye element each have a cross brace with a flat contact surface, wherein in particular the cross braces are arranged perpendicular to one another. In one embodiment, the cross braces are in the form of bars, wherein an extension of the cross brace of the hook element in its longitudinal direction is larger than an extension of the cross brace of the eye element perpendicular to the longitudinal direction. Likewise, an extension of the cross brace of the eye element in its longitudinal direction is larger than an extension of the cross brace of the hook element perpendicular to the longitudinal direction. This allows a relative movement between the hook element and the eye element along the longitudinal directions of the two cross braces, and, thus, a relative movement between the lower part and the upper part in a plane perpendicular to the longitudinal direction of the support element.

In order to limit a relative movement, and to secure a connection between the hook element and the eye element, in one embodiment the cross brace of the hook element is provided with a lock pawl at its free end.

In one embodiment, the connection structure comprises one pair of hook and eye elements, which is arranged at a center line of the support element. In order to provide a more stable connection, in other embodiments the connection structure comprises four pairs of interlocking hook and eye elements. In one embodiment, the cross braces of the hook elements are arranged at four sides of a virtual square with a common orientation in a circumferential direction of the virtual square. This arrangement allows a connection of the upper part with the lower part by twisting the upper part about a center line. As mentioned above, in one embodiment, the transport module unit further comprises an actuator assembly with a back iron. In one embodiment, the actuator assembly is supported by a base plate assembly arranged below the actuator assembly. In alternative or in addition, in one embodiment, the upper part of the support element is provided with a back iron interface configured to interlock with back irons of actuator assemblies of two neighboring transport module units.

The back iron interface in one embodiment comprises two opposing force application surfaces, wherein each force application surface is configured to interlock with the back iron of one of the actuator assemblies of the two neighboring transport module units, such that such that a relative movement of neighboring transport module units away from each other is restrained and in response to a restrained relative movement forces are transmitted via the back irons and the force application surfaces . The force transmission allows for a distribution of the relative movement over the modular transport plane.

In order to avoid that a liquid accidently spilled on the upper surface of the driving surface assembly enters the transport module unit, in one embodiment the upper part is provided with a central cavity for liquid collection.

In one embodiment, the support element is arranged at side-to-side junctions between exactly two neighboring transport module units. In other embodiments, the support element is a corner support element, wherein the upper support surface is provided with up to three, four or six driving surface assembly interfaces configured to engage with complementary interfaces of the supported driving surface assemblies having a hexagonal, square or triangular basic shape, respectively. Corner support elements allow a stable support of transport module units and a precise alignment of neighboring transport module units with a minimum number of support elements.

In one embodiment, the mounting structure of the lower part is provided with up to three, four or six base plate assembly interfaces configured to engage with complementary interfaces at corner regions of base plate assemblies having a hexagonal, square or triangular basic shape, respectively. In one embodiment, the transport module unit further comprises a base plate assembly. In other embodiments, base plate assemblies are provided below each transport module unit comprising an actuator module and a driving surface module. In both cases, the number of base plate assemblies supported by one support element is the same as the number of driving surface assemblies supported by said support element. However, embodiments are conceivable in which base plate assemblies and driving surface assemblies are not provided on a one by one basis.

In one embodiment, each base plate assembly interface comprises a guiding pin or a guiding recess configured to engage with a guiding recess or guiding pin of a base plate assembly, respectively, wherein the mounting structure further comprises a plurality of legs arranged between the guiding pins or guiding recesses, each leg having an upward facing engagement surface for a snap-fit connection with complementary snap-on elements of the base plate assemblies of two neighboring transport module units. At the base plate assembly, the guiding pins or guiding recesses are in one embodiment arranged along angle bisectors at the corner regions, wherein the snap-on elements are arranged symmetrically at either side of the guiding pin or guiding recess.

In one embodiment, the lower part is provided with a central body, wherein a lower end of the central body is configured to compress seals of base plate assemblies the interfaces of which are engaged with base plate assembly interfaces of the lower part. The seal in one embodiment is configured to seal an interior of the base plate assembly airtight against the environment for an air recirculation within the interior. By compressing the seal using the support element, in particular a support element in the form of a corner support element, a seal extending between base plate assemblies acts as a plug, and a sealing effect can be improved.

According to a second aspect, a modular transport plane with a plurality of transport module units, each transport module unit comprising a driving surface assembly, and with a number of multipart support elements is provided.

According to a third aspect, a laboratory distribution system with a modular transport plane and a plurality of carriers is provided, wherein the carriers each comprise at least one magnetically active device, preferably at least one permanent magnet, and are configured to carry a specimen container.

According to a fourth aspect, a laboratory automation system with a plurality of pre-analytical, analytical and/or post-analytical stations, and with a laboratory distribution system is provided. BRIEF DESCRIPTION OF THE DRAWINGS

In the following, an embodiment of the invention will be described in detail with reference to the schematic drawings. Throughout the drawings, the same elements will be denoted by the same reference numerals.

Fig. 1 is a top view of transport plane for a laboratory distribution system build from several transport module units.

Fig. 2 is an exploded view of a transport module unit.

Fig. 3 is a perspective view of a corner support element for connecting neighboring transport module unit.

Fig. 4 is a perspective view of the corner support element of Fig. 3 upon installation.

Fig. 5 is a detail of the corner support element of Fig. 3.

Fig. 6 is a sectional view of the corner support element of Fig. 3 together with corner regions of two base plate assemblies.

Fig. 7 shows a top part of the corner support element of Fig. 3 together with a corner region of one driving surface assembly.

Fig. 8 shows an upper part of the corner support element of Fig. 3 in isolation.

Fig. 9 is a perspective view of four base plate assemblies interconnected by corner support elements and back irons of associated actuator assemblies.

Fig. 10 is a detail X of Fig. 9 showing a force distribution via back irons when moving the transport module units apart from each other.

Fig. 11 is a detail X of Fig. 9 showing a force distribution via back irons when moving the transport module units towards each other. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig. 1 schematically shows a top view of an embodiment of a modular transport plane 10 build from several, in the embodiment shown twenty transport module units 1. The transport module units 1 are connected to an infrastructure system 11 comprising support struts 12. Each of the transport module units 1 shown has a square basic shape allowing building of transport planes 10 of various designs by adding additional transport module units 1 at either side of already existing units 1 and/or removing transport module units 1 from the transport plane 10 shown in Fig. 1. In other embodiments, the transport module units have a different basic shape, for example a triangular basic shape or a hexagonal basic shape. Preferably, all transport module units 1 have the same basic shape, wherein the shape is a tessellating shape. However, in specific embodiments, a transport device is composed of transport module units 1 having different basic shapes.

Fig. 2 shows a transport module unit 1 , a base plate assembly 2 and support elements for building a transport plane 10 of Fig. 1 in an exploded view. The transport module unit 1 shown in Fig. 2 comprises two assemblies, namely an actuator assembly 3 and a driving surface assembly 4. The actuator assembly 3 comprises a plurality of electro-magnetic actuators 30 mounted to a back iron 37, wherein the back iron 37 is supported by a handle protection 31 . The driving surface assembly 4 comprises a driving surface 40, a driving surface support with an interface 42 and a sensor board (not visible). In the embodiment shown, the transport module units 1 are provided on a one-by-one with base plate assemblies 2, which are located underneath the transport module units 1 and configured for connecting the transport module unit 1 to the support struts 12 (see Fig. 1).

Adjacent transport module units 1 are connected by means of support elements. In the embodiment shown, the support elements are in the form corner support elements 5 connecting corner regions of up to four neighboring transport module units 1. In an alternative embodiment, in which the transport module units have a triangular basic shape or a hexagonal basic shape, the corner support elements are designed to support the corner regions of up to six or up to three neighboring transport module units, respectively. In alternative or in addition, in still another embodiment, support elements connecting neighboring transport module units 1 at the side surfaces are provided.

The base plate assembly 2 shown comprises a base plate 20 having an essentially square basic shape with four sides and four corners. The corner support elements 5 are provided with a mounting structure for mounting the corner support elements 5 to the corners of the base plates 20 of neighboring base plate assemblies 2, wherein the base plates 20 are aligned by means of the corner support elements 5. The corner support elements 5 are further provided with an upper support surface 55 having a driving surface assembly interface 56 configured to engage with complementary interfaces 42 of the supported driving surface assemblies 4. In other words, in the embodiment shown the corner support element 5 functions as a connection node for up to four base plate assemblies 2 and for up to four driving surface assemblies 4. Further, the corner supports 5 are provided with back iron interfaces 57. The back iron interfaces 57, the mounting structure, and the upper support surface 55 will be described in more detail below.

A corner support element 5 is shown in isolation in Fig. 3. Fig. 4 is a perspective view of the corner support element 5 upon assembly. Fig. 5 is a detail of the corner support element of Fig. 3 on a larger scale.

The corner support element 5 shown is a two-part element and comprises a lower part 50 and an upper part 51 , which upper part 51 is connected lengthwise to the lower part 50 via a connection structure 52. The connection structure 52 is such that the upper part 51 is connected in a floating manner to the lower part 50. This allows a relative movement between base plate assemblies 2 and driving surface assemblies 4 connected to the corner support element 5 in a horizontal plane.

The connection structure 52 shown in Fig. 3 to 5 comprises four pairs of interlocking hook and eye elements. In the embodiment shown, four hook elements 521 (only two visible in Fig. 3) are provided on a top surface 500 of the lower part 50, and four eye elements 522 are provided at a bottom surface of the upper part 51.

The connection structure 52 comprising the hook element 521 and the eye element 522 allows to restrain a relative movement between the lower part 50 and the upper part 51 in the longitudinal direction of the corner support element 5 and to permit a limited relative movement between the lower part 50 and the upper part 51 a plane perpendicular to the longitudinal direction of the corner support element 5.

As best seen in Fig. 5, the hook element 521 and the eye element 522 are provided with contacting cross brackets 5210, 5220. The hook element 521 protrudes from the top surface 500 of the lower part 50, wherein the cross bracket 5210 is arranged in parallel to the top surface 500 and the cross bracket 5220 of the eye element 522 enters between the cross bracket 5210 of the hook element 521 and the top surface 500, thereby restricting a movement between the lower part 50 and the upper part 51 in the longitudinal direction of the corner support element 5. At the free ends of the cross brackets 5210 pawls 5212 are provided. An extension of the cross bracket - IQ -

5210 of the hook element 521 in its longitudinal direction is larger than an extension of the cross bracket 5220 of the eye element 522 perpendicular to the longitudinal direction. Likewise, an extension of the cross bracket 5220 of the eye element 522 in its longitudinal direction is larger than an extension of the cross bracket 5210 of the hook element 521 perpendicular to the longitudinal direction. This allows a relative movement between the lower part 50 and the upper part 51 along the longitudinal directions of the two cross brackets 5210, 5220. The relative movement is limited by dimensioning of the cross brackets 5210, 5220. In addition, U-shaped rims 501 protruding from the top surface 500 are provided, wherein each rim 501 partly surrounds one hook element 521 at its two side faces and at its end opposite to its free end. The rim 501 also limits a movement of the eye element 522 with respect to the hook element 521 , thereby avoiding high bending forces on the hook element 521 upon a relative movement in the direction of the cross bracket 5220 of the eye element 522.

As shown in Fig. 3, the cross brackets 5210 of the hook elements 521 are arranged along four sides of a virtual square with a common orientation in a circumferential direction of the corner support element 5. The cross brackets 5220 of the eye elements 522 are arranged perpendicular to the four sides of the virtual square. As shown in Fig. 4, for connecting the upper part 51 to the lower part 50, the hook elements 521 are threaded into the eye elements 522 by twisting the upper part 51 with respect to the lower part 50 as schematically indicated by an arrow.

Fig. 6 is a sectional view of the corner support element 5 together with corner regions of two base plate assemblies 2. As shown in Fig. 6, the lower part 50 of the corner support 5 has a central body 502 and the upper part 51 of the corner support 5 has a central body 512, wherein a lower end of the central body 512 of the upper part 51 is rotatable and with play received in recess of the central body 502 of the lower part 50.

It will be apparent to the person skilled in the art that other embodiments are conceivable, wherein for example hook elements are provided on the upper part, or wherein interlocking eye elements having free ends instead of closed loops are provided.

As shown in Fig. 3 and 6, the lower part 50 is provided with the mounting structure, which in the embodiment shown comprises four base plate assembly interfaces 53 configured to engage with complementary interfaces 24 (see Fig. 2) at corner regions of base plate assemblies 2 having a rectangular, in particular square basic shape.

In the embodiment shown, at corner regions of each base plate assembly 2, upwardly protruding pins (not visible in the figures) are provided on virtual diagonal lines of the base plates 20. Further, at each corner region two snap-on elements 25, arranged symmetrically with regard to the virtual diagonal lines are provided.

Each base plate assembly interface 53 comprises a guiding recess 530 configured to receive one guiding pin provided at a corner of a base plate assembly 2 from below. In the embodiment shown, the central body 502 of the lower part 50 has an essentially square cross section with four corners and four sides, and the guiding recesses 530 are arranged at the four corners of the central body 502. Further, the mounting structure comprises a plurality of legs 531 protruding from the sides of the central body 502, in the embodiment four legs 531 , which are arranged between the guiding recesses 530. Each leg 531 has an upward facing engagement surface 532 provided at a wedgeshape end.

As shown in Fig. 6, the legs 531 having the upward facing engagement surfaces 532 can be placed on the base plate 20 of the base plate assembly 2 and are configured for a snap-fit connection with complementary snap-on elements 25 of the base plate assemblies 2 of two neighboring transport module units 1 , wherein in the sectional view shown in Fig. 6, in each case only one snap-on element 25 supported by the respective leg 531 provided in the drawing plane on the left and the right of the corner support element 5 is shown.

As further shown in Fig. 3 and 6, a lower end of the central body 502 is configured to compress seals 26 of base plate assemblies 2. When assembled, the corner support element 5 compresses the seals 26 in the corners between base plate assemblies 2 acting as a plug to create a seal in the corners between neighboring base plate assemblies 2. As shown in Fig. 3, the upper support surface 55 having four driving surface assembly interfaces 56 is provided at the upper part 51 . Each driving surface assembly interface 56 comprises an alignment pin 560.

Fig. 7 shows the upper support surface 55 of the corner support element 5 having four alignment pins 560 together with a corner region of one driving surface assembly 4. As shown in Fig. 7, each alignment pin 560 is configured to receive from above an alignment hole 44 provided at a corner of a driving surface assembly 4. Further, below the upper support surface 55 a plurality of protrusions 551 are provided, which are configured for a snap-fit connection with complementary snap-on elements 420 of the driving surface assemblies 4.

As shown in Fig. 3 and 6, the upper part 51 is provided with a central cavity 54 for liquid collection. The central cavity 54 is formed in the central body 512 of the upper part 51. The multi-part design of the corner support element 5 allows a removal of the upper part 51 of the corner support element 5 for discarding the trapped liquid, without the necessity to unmount the base plate assemblies 2 connected by the lower part 50 of the corner support element 5.

Fig. 8 shows the upper part 51 of the corner support element 5 in isolation. In the embodiment shown, the corner support element 5 is further configured to support the back irons 37 (see Fig. 2) of up to four actuator assemblies 3. For this purpose, in the embodiment shown the upper part 51 of the corner support element 5 is provided with four back iron interfaces 57. Each back iron interface 57 comprises a U-shaped support surface 570 configured to support the back irons 37 from below, and two parallel vertical force application surfaces 571 , 572 facing each other.

Fig. 9 is a perspective view of four back irons 37 of associated actuator assemblies 3 (see Fig. 2), which are supported by corner support elements 5 .and interconnected by the corner support elements 5 for a force transmission. Fig. 10 and 11 are a detail X of Fig. 9.

As shown in Fig. 9, the back irons 37 each have a grid structure with intersecting straight elements 371 , 372. The straight elements 371 , 372 of the back irons 37 of neighboring actuator assemblies 3 are aligned.

As best seen in Fig. 10 and 11 , free ends of straight elements 371 , 372 arranged at sidelines of the grid structure are provided with pawls 373, extending perpendicular to the longitudinal direction of the respective straight element 371 , 372. The pawls 373 are configured to interlock with the force application surfaces 571 , 572, wherein two pawls 373 of neighboring back irons 37 are received between two opposing force application surfaces 571 , 572 of one back iron interface 57. The pawls 373 are arranged between the force application surfaces 571 , 572 leaving a small gap between the two pawls 373 under standard conditions.

When forces are acting on transport module units 1 , which cause one transport module unit 1 to move apart from a neighboring transport module unit 1 as shown by an arrow in Fig. 10, for example due to thermal contraction or thermal extension within the modular transport plane 10, the interlocking connection between force application surfaces 571 , 572 and the pawls 373 received between the force application surfaces 571 , 572 causes a force transmission to the back iron 37 of the neighboring transport module unit 1 as schematically shown by a dotted arrow in Fig. 10. The force transmission causes a chain effect in the modular transport plane 10, wherein by pulling on the back iron 37 of the neighboring transport module unit 1 , a relative movement is distributed in the transport plane 10. When forces are acting on transport module units 1 , which cause one transport module unit 1 to move closer towards a neighboring transport module unit 1 as shown by an arrow in Fig. 11 , for example due to thermal contraction or thermal extension within the modular transport plane 10, the back irons 37 will contact each other, thereby causing a force transmission to the back iron 37 of the neighboring transport module unit 1 as schematically shown by a dotted arrow in Fig. 11. The force transmission causes a chain effect in the modular transport plane 10, wherein by pushing on the back iron 37 of the neighboring transport module unit 1 , a relative movement is distributed in the transport plane 10.

In embodiments of the invention, all straight elements 371 , 372 of neighboring back irons 37 are arranged with a gap there between, wherein the gaps are identical in size across the whole length of the back iron 37, such that upon a relative movement of the back irons 37 towards one another all straight elements 371 , 372 of neighboring back irons 37 at least essentially simultaneously come into contact.

For an installation of a modular transport plane 10, first the base plate assemblies 2 can be mounted to a support struts 12. Then the lower parts 50 of the corner support elements 5 can be mounted to the corner regions of the base plate assemblies 2 by inserting the guiding pins 241 of up to four neighboring base plate assemblies 2 of a corner junction into the guiding recesses 530 of the base plate assembly interfaces 53 of the lower part 50. At the same time, the wedge-shaped end of the legs 531 move the snap-on elements 25 of the neighboring base plate assemblies 2 apart and after reaching a final position, the snap-on elements 25 contact the engagement surfaces 532 of the legs 531 for a snap-fit connection. Next, the upper part 51 can be connected to the lower part 50 by inserting the lower end of a central body 512 of the upper part 51 into a recess of the central body 502 of the lower part 50 and twisting the upper part 51 for threading the hook elements 521 into the eye elements 522. Then, the actuator assemblies 3 are mounted, wherein pawls 373 provided at corner regions of the back irons 37 of the actuator assemblies 3 are received between the force application surfaces 571 , 572 of the upper parts 51 of the corner support elements 5. Finally, the driving surface assemblies 4 are mounted by inserting alignment pins 560 of associated corner supports 5 into the alignment holes 44 provided at the corner regions of the driving surface assemblies 4, wherein in a final position, the snap-on elements 420 of the driving surface assemblies 4 are in a snap-fit connection with the protrusions 551 protruding below the upper surface 55.

Upon mounting the driving surface assemblies 4, the upper parts 51 of the corner support elements 5 can be moved relative to the lower parts 50 within limits to ensure a good alignment of the driving surface assemblies 4. It will be apparent to the person skilled in the art, that in an alternative installation procedure the corner support elements 5 can also be pre-assembled before mounting the corner support elements 5 to the base plate assemblies 2.

The embodiments shown are only exemplary and various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.