Background to the Invention

Some commercial and industrial activities require systems that enable the storage and retrieval of a large number of different products. One known system for the storage and retrieval of items in multiple product lines involves arranging storage bins or containers on rows of shelves arranged in aisles. Each bin or container holds a plurality of products of one product type. The aisles provide access between the rows of shelves, so that the required products can be retrieved by operatives or robots that circulate in the aisles. It will be appreciated, however, that the need to provide aisle space to access the products means that the storage density of such systems is relatively low. In other words, the amount of space actually used for the storage of products is relatively small compared to the amount of space required for the storage system as a whole.

In an alternative approach, which offers a significant improvement in storage density, containers are stacked on top of one another and the stacks are arranged in rows. The containers are accessed from above, removing the need for aisles between the rows and allowing more containers to be stored in a given space.

Methods of handling containers stacked in rows have been well known for decades. In some such systems, for example as described in US 2,701 ,065, free-standing stacks of containers are arranged in rows in order to reduce the storage volume associated with storing such containers while still providing access to a specific container if required. Access to a given container is made possible by providing relatively complicated hoisting mechanisms which can be used to stack containers and to remove given containers from stacks. The cost of such systems are, however, impractical in many situations and they have mainly been commercialised for the storage and handling of large shipping containers. The concept of using freestanding stacks of containers and providing a mechanism to retrieve and store specific containers has been developed further, for example as described in EP 0 767 1 13 B (Cimcorp). Cimcorp discloses a mechanism for removing a plurality of stacked containers using a robotic load handler in the form of a rectangular tube which is lowered around the stack of containers, and which is configured to be able to grip a container at any level in the stack. In this way, several containers can be lifted at once from a stack. The movable tube can be used to move several containers from the top of one stack to the top of another stack, or to move containers from a stack to an external location and vice versa. Such systems can be particularly useful where all of the containers in a single stack contain the same product (known as a single- product stack). The load handler can be used to move containers between single-product stacks, for example to add a plurality of containers containing a single type of product to the store, and to pick up one or more containers from two or more single-product stacks to create a multi-product output stack. An example of this is the picking of vegetable crates in a central warehouse to create a multi-product order for delivery to retail stores. In the system described in Cimcorp, the height of the tube has to be as least as high as the height of the largest stack of containers, so that that the highest stack of containers can be extracted in a single operation. Accordingly, when used in an enclosed space such as a warehouse, the maximum height of the stacks is restricted by the need to accommodate the tube of the load handler. Furthermore, the system is not well adapted for the selection of a single container from a multi-product stack.

Online retail businesses selling multiple product lines, such as online grocers and supermarkets, require systems that are able to store tens or even hundreds of thousands of different product lines. The use of single-product stacks in such cases can be impractical, since a very large floor area would be required to accommodate all of the stacks required. Furthermore, it can be desirable only to store small quantities of some items, such as perishables or infrequently-ordered goods, making single-product stacks an inefficient solution.

Accordingly, for some applications, the use of multi-product stacks, in which the containers making up each stack may hold different products, is favoured in order to maximise the storage density of the system. The stored items must remain accessible reasonably quickly and easily, so that a plurality of different items required to fulfil a customer order can be picked from the storage system in an efficient way, even if some of the items required are stored in a lower level of a stack, underneath several other containers.

International patent application WO 98/049075A (Autostore), the contents of which are incorporated herein by reference, describes a system in which multi-product stacks of containers are arranged within a frame structure. A system of this type is illustrated schematically in Figures 1 to 4 of the accompanying drawings.

As shown in Figures 1 and 2, stackable containers, known as bins 10, are stacked on top of one another to form stacks 12. The stacks 12 are arranged in a grid frame structure 14 in a warehousing or manufacturing environment. Figure 1 is a schematic perspective view of the frame structure 14, and Figure 2 is a top-down view showing a stack 12 of bins 10 arranged within the frame structure 14. Each bin 10 typically holds a plurality of product items (not shown), and the product items within a bin 10 may be identical, or may be of different product types depending on the application.

The frame structure 14 comprises a plurality of upright members 16 that support horizontal members 18, 20. A first set of parallel horizontal members 18 is arranged perpendicularly to a second set of parallel horizontal members 20 to form a plurality of horizontal grid structures supported by the upright members 16. The members 16, 18, 20 are typically manufactured from metal. The bins 10 are stacked between the members 16, 18, 20 of the frame structure 14, so that the frame structure 14 guards against horizontal movement of the stacks 12 of bins 10, and guides vertical movement of the bins 10.

The top level of the frame structure 14 includes rails 22 arranged in a grid pattern across the top of the stacks 12. Referring additionally to Figures 3 and 4, the rails 22 support a plurality of robotic load handling devices 30. A first set 22a of parallel rails 22 guide movement of the load handling devices 30 in a first direction (X) across the top of the frame structure 14, and a second set 22b of parallel rails 22, arranged perpendicular to the first set 22a, guide movement of the load handling devices 30 in a second direction (Y), perpendicular to the first direction. In this way, the rails 22 allow movement of the load handling devices 30 laterally in two dimensions in the horizontal X-Y plane, so that a load handling device 30 can be moved into position above any of the stacks 12.

The load handling devices 30 are further described in Norwegian patent number 317366, the contents of which are incorporated herein by reference. Figures 3(a) and 3(b) are schematic perspective views of a load handling device 30 from the rear and front, respectively, and Figure 3(c) is a schematic front perspective view of a load handling device 30 lifting a bin 10.

Each load handling device 30 comprises a vehicle 32 which is arranged to travel in the X and Y directions on the rails 22 of the frame structure 14, above the stacks 12. A first set of wheels 34, consisting of a pair of wheels 34 on the front of the vehicle 32 and a pair of wheels 34 on the back of the vehicle 32, is arranged to engage with two adjacent rails of the first set 22a of rails 22. Similarly, a second set of wheels 36, consisting of a pair of wheels 36 on each side of the vehicle 32, is arranged to engage with two adjacent rails of the second set 22b of rails 22. Each set of wheels 34, 36 can be lifted and lowered, so that either the first set of wheels 34 or the second set of wheels 36 is engaged with the respective set of rails 22a, 22b at any one time. When the first set of wheels 34 is engaged with the first set of rails 22a and the second set of wheels 36 is lifted clear from the rails 22, the wheels 34 can be driven, by way of a drive mechanism (not shown) housed in the vehicle 32, to move the load handling device 30 in the X direction. To move the load handling device 30 in the Y direction, the first set of wheels 34 is lifted clear of the rails 22, and the second set of wheels 36 is lowered into engagement with the second set of rails 22b. The drive mechanism can then be used to drive the second set of wheels 36 to achieve movement in the Y direction.

The load handling device 30 is equipped with a crane device 40. The crane device 40 comprises a cantilever arm 42 that extends laterally from the top of the vehicle 32. A gripper plate 44 is suspended from the cantilever arm 42 by four cables 46. The cables 46 are connected to a winding mechanism (not shown) housed within the vehicle 32. The cables 46 can be spooled in or out from the cantilever arm 42, so that the position of the gripper plate 44 with respect to the vehicle 32 can be adjusted in the Z direction.

The gripper plate 44 is adapted to engage with the top of a bin 10. For example, the gripper plate 44 may include pins (not shown) that mate with corresponding holes (not shown) in the rim that forms the top surface of the bin 10, and sliding clips (not shown) that are engageable with the rim to grip the bin 10. The clips are driven to engage with the bin 10 by a suitable drive mechanism housed within the gripper plate 44, which is powered and controlled by signals carried through the cables 46 themselves or through a separate control cable (not shown).

To remove a bin 10 from the top of a stack 12, the load handling device 30 is moved as necessary in the X and Y directions so that the gripper plate 44 is positioned above the stack 12. The gripper plate 44 is then lowered vertically in the Z direction to engage with the bin 10 on the top of the stack 12, as shown in Figure 3(c). The gripper plate 44 grips the bin 10, and is then pulled upwards on the cables 46, with the bin 10 attached. At the top of its vertical travel, the bin 10 is accommodated beneath the cantilever arm 42 and is held above the level of the rails 22. In this way, the load handling device 30 can be moved to a different position in the X-Y plane, carrying the bin 10 along with it, to transport the bin 10 to another location. The cables 46 are long enough to allow the load handling device 30 to retrieve and place bins from any level of a stack 12, including the floor level. The vehicle 32 is sufficiently heavy to counterbalance the weight of the bin 10 and to remain stable during the lifting process. The weight of the vehicle 32 may be comprised in part of batteries that are used to power the drive mechanism for the wheels 34, 36.

As shown in Figure 4, a plurality of identical load handling devices 30 are provided, so that each load handling device 30 can operate simultaneously to increase the throughput of the system. The system illustrated in Figure 4 includes two specific locations, known as ports 24, at which bins 10 can be transferred into or out of the system. An additional conveyor system (not shown) is associated with each port 24, so that bins 10 transported to a port 24 by a load handling device 30 can be transferred to another location by the conveyor system, for example to a picking station (not shown). Similarly, bins 10 can be moved by the conveyor system to a port 24 from an external location, for example to a bin-filling station (not shown), and transported to a stack 12 by the load handling devices 30 to replenish the stock in the system. Each load handling device 30 can lift and move one bin 10 at a time. If it is necessary to retrieve a bin 10 ("target bin") that is not located on the top of a stack 12, then the overlying bins 10 ("non- target bins") must first be moved to allow access to the target bin 10. Each of the load handling devices 30 is under the control of a central computer. Each individual bin 10 in the system is tracked, so that the appropriate bins 10 can be retrieved, transported and replaced as necessary.

The system described with reference to Figures 1 to 4 has many advantages and is suitable for a wide range of storage and retrieval operations. In particular, it allows very dense storage of product, and it provides a very economical way of storing a huge range of different items in the bins 10, while allowing reasonably economical access to all of the bins 10 when required for picking. For high-volume systems in which speed of operation is critical, it is important to maximise the performance of each of the load handing devices, in terms of speed of operation, battery life, reliability, lifting capacity, stability and so on. It may therefore be desirable to provide load- handling devices that offer improved performance in one or more of these areas. It may also be desirable to increase the number of load handling devices in use at any one time, to allow an increase in the speed with which items can be retrieved from the storage system. For example, the Applicant's co-pending International Patent Application No. PCT/GB2013/051215, the content of which is incorporated herein by reference, describes a storage system in which a plurality of each of two different types of load handling device are provided. One type of load handling device is adapted to lift a plurality of bins from a stack in one operation, to allow a target bin in the stack to be accessed by a single-bin load handling device of the second type. In such cases, it may be desirable to reduce the size of the load handling devices in order to minimise instances in which the optimum movement path for one device is hindered by the presence of other devices.

It is against this background that the present invention has been devised.

Summary of the invention From one aspect, the present invention concerns load handling devices for use in storage systems comprising a grid frame containing a plurality of stacks of containers. The load handling devices are arranged above the stacks of containers and are capable of lifting a container from a stack and moving the container laterally to another location. Advantageously, each load handling device occupies substantially only a single grid space in the storage system. Accordingly, the present invention provides a load handling device for lifting and moving containers stacked in a storage system comprising a plurality of rails or tracks arranged in a grid pattern above the stacks of containers, the grid pattern comprising a plurality of grid spaces and each stack being located within a footprint of substantially only a single grid space, the load handling device being configured to move laterally on the rails or tracks above the stacks, and the load-handling device comprising: a container-receiving space located above the rails or tracks in use and a lifting device arranged to lift a container from a stack into the container-receiving space; wherein the load handling device has a footprint that, in use, occupies substantially only a single grid space in the storage system.

A load handling device according to an embodiment of the invention includes a container- receiving space into which a container can be lifted. The container-receiving space is arranged beneath a vehicle module, in which components such as power components, control components, drive components and lifting components are housed.

In preferred embodiments of the invention, the load handling device has an external housing that substantially encloses the container-receiving space. The external housing preferably has the shape of a cuboid. By arranging the bulky components of the load handling device above the container-receiving space, the footprint of the load handling device is reduced compared to the cantilever designs shown in Figures 3(a) to 3(c) and described in N0317366, in which the bulky components are housed in a vehicle module disposed to one side of the container-receiving space. Advantageously, the load handling device of the invention occupies the space above only one stack of containers in the frame, in contrast to the cantilever design shown in Figures 3(a) to 3(c) which occupies the space above two stacks. This means that, by virtue of the invention, the efficiency of operation of the storage system can be improved, because the reduced footprint allows more load handling devices to be accommodated and reduces the likelihood of one device obstructing the optimum path of another.

The load handling device preferably includes a set of wheels for supporting the load handling device above the stacks. For example, lateral movement of the load handling device may be guided by rails disposed above the frame. The rails may be arranged in a grid pattern, allowing two-dimensional movement of the load handling device in the horizontal plane. The wheels may engage with the rails. Two sets of wheels may be provided, with one set being arranged to engage with a first set of rails to guide movement of the load handling device in a first direction, and another set being arranged to engage with a second set of rails to guide movement of the load handling device in a second direction. In an embodiment of the invention, the wheels are arranged at the periphery of the container- receiving space. The wheels may be driven by one or more motors housed in the vehicle module. Drive may be transferred from the motors in the vehicle module to the wheels by drive transfer means disposed around the container-receiving space. For example, the drive transfer means may comprise a suitable arrangement of pulleys and drive belts.

Alternatively, the wheels may include integrated motors, for example motors integrated within the wheel hubs. In this way, each wheel is a self-container drive unit, and drive belts are not required. This arrangement is advantageous as it reduces the size of the load-handling device and facilitates servicing.

One or both sets of wheels may be configured to be raised and lowered with respect to the other set of wheels. One or more wheel lift motors or other wheel lift devices may be housed in the vehicle module for this purpose.

The vehicle module may house a winch or crane device for lifting the container into the container- receiving space. The crane device may include one or more motors for lifting the container, and the or each motor of the crane device may be housed in the vehicle module. The crane device may include a gripper device configured to grip a container from above. The gripper device may be suspended from cables that can be extended and retracted from the vehicle to move the gripper device vertically.

In another embodiment, the load handling device is equipped with a lifting device arranged to lift a single container from the stack into the container-receiving space. The lifting device may comprise a pair of lifting arms arranged on either side of the container-receiving space, in which case the lifting device may comprise a gripper device mounted between the ends of the arms and arranged to grip a container from above. The load-handling device preferably has a centre of mass that is located substantially directly above the gripper device when the gripper device is lowered below the container-receiving space.

In another embodiment, the lifting device comprises rods or cables arranged to engage with vertical channels formed in the side walls of the containers. The channels may be accessed by apertures in a top face of each container. In such an arrangement, vertically-extending spaces in the storage system are not necessary.

The rods or cables may carry an anchor mechanism arranged to engage releasably with a container. For example, the anchor mechanism may comprise one or more laterally-extendable arms for engaging a surface of the container. The anchor mechanism may be operated remotely, for example by a wire that extends through a tubular bore of the rod or cable.

A load handling device according to another embodiment of the invention comprises an upper part, a lower part including a container-receiving space, and winch means for lifting a container into the container-receiving space. The winch means preferably comprises a winch motor which is housed in the upper part, above the container-receiving space. The lower part preferably includes a wheel assembly to facilitate lateral movement of the load handling device with respect to the frame, and the upper part also includes at least one motor for driving one or more wheels of the wheel assembly.

The lower part may comprise a frame structure for supporting the wheels of the wheel assembly. The frame structure may be arranged around the container-receiving space. For example, the container-receiving space may be bounded on four sides by the frame structure. One or more elements of the frame structure may be moveable to raise and lower a first set of the wheels with respect to a second set of the wheels, thereby to facilitate engagement of either the first set of wheels or the second set of wheels with a first or a second set of rails or tracks, respectively. The moveable elements of the frame structure may be driven by a motor housed in the upper part of the load handling device.

The load-handling device of the invention is preferably a self-propelled robot vehicle.

From another aspect, the invention resides in a storage system comprising a frame containing a plurality of stacks of containers, and one or more load handling devices as described above. Each load handling device occupies substantially a single grid space, corresponding to the area occupied by only one stack of containers.

Accordingly, the present invention provides a storage system comprising: a first set of parallel rails or tracks and a second set of parallel rails or tracks extending transverse to the first set in a substantially horizontal plane to form a grid pattern comprising a plurality of grid spaces; a plurality of stacks of containers located beneath the rails, and arranged such that each stack occupies a footprint of substantially a single grid space; a load handling device as described above and arranged to move laterally above the stacks on the rails, the load handling device comprising a container-receiving space located above the rails and a lifting device arranged to lift a single container from a stack into the container-receiving space; wherein the load handling device has a footprint that occupies substantially only a single grid space in the storage systems.

In another aspect, the invention comprises a storage system comprising a frame containing a plurality of stacks of containers, a first handling device capable of lifting a plurality of containers from a stack in a single operation, and a second handling device capable of lifting a single container and moving the container laterally. The first and second handling devices are disposed above the frame and are independently moveable to access different stacks. The second handling device is of the type described above, and occupies a space corresponding substantially to only one stack of containers.

In this aspect, the provision of a first handling device capable of lifting a plurality of containers from a stack in a single operation along with a second handling device capable of lifting a single container and moving the container laterally provides an optimum solution when seeking to retrieve a container which is located in the middle or bottom of a stack. In such a case, only two lifting operations need be carried out to retrieve the target container, which greatly increases the speed and efficiency of the retrieval process compared to prior art arrangements in which only one container can be lifted at a time.

The storage system may further comprise one or more port locations at which containers can be removed from and/or added to the storage system. The load handling device of the invention may be capable of transporting a target container from a stack to a port location. The containers may comprise open-topped bins. The containers may be arranged to interlock or engage with one another in the vertical direction when formed in a stack. In a typical application, multiple handling devices may be employed so that multiple containers can be lifted and moved simultaneously. The handling devices may be of different types, and may be selected to balance the cost and energy consumption of the system with the speed and flexibility of operation. One benefit of the present invention is that, because the load handling devices occupy the space above only one stack, the efficiency of a multiple-device system can be improved compared to prior art load handling device designs which occupy two or more stack spaces. The gain in efficiency may arise from being able to accommodate more load handling devices in a given system, from optimising the routing of the device using the space gained by the reduced device footprints, or from a combination of these factors. Preferred and/or optional features of each aspect of the invention may be used, alone or in appropriate combination in the other aspects of the invention also.

Brief description of the drawings Figure 1 is a schematic perspective view of a frame structure for housing a plurality of stacks of bins in a known storage system;

Figure 2 is a schematic plan view of part of the frame structure of Figure 1 ; Figures 3(a) and 3(b) are schematic perspective views, from the rear and front respectively, of a known load handling device for use with the frame structure of Figures 1 and 2, and Figure 3(c) is a schematic perspective view of the known load handling device in use lifting a bin; and Figure 4 is a schematic perspective view of a known storage system comprising a plurality of load handling devices of the type shown in Figures 3(a), 3(b) and 3(c), installed on the frame structure of Figures 1 and 2.

Embodiments of the present invention will now be described, by way of example only, with reference to the remainder of the accompanying drawings, in which like reference numerals are used for like features, and in which:

Figure 5 is a schematic perspective view of a load handling device according to an embodiment of the invention;

Figures 6(a) and 6(b) are schematic perspective views of the load handling device of Figure 5 with part of the load handling device being cut-away in Figures 6(a) and 6(b) to show the inside of the device, and Figure 6(c) shows one possible system architecture of the device; Figure 7 is a schematic perspective view of a storage system comprising a plurality of known load handler devices of the type shown in Figures 3(a), 3(b) and 3(c) and a plurality of load handler devices of the type shown in Figure 5, installed on the frame structure of Figures 1 and 2;

Figures 8, 9 and 10 are schematic side, perspective and top views of a load handling device according to another embodiment of the invention and with the outer casing omitted;

Figure 1 1 is a schematic perspective view of the load handling device of Figures 8 to 10 with the outer casing omitted; Figure 12 is a side view of the load handling device of Figures 8 to 1 1 ;

Figure 13 is a schematic perspective view of a wheel suitable for use in the load handling device of Figures 8 to 12; Figure 14 is a schematic perspective view of part of a load handling device according to another embodiment of the invention;

Figures 15 and 16 are schematic perspective and side views of internal components of the load handling device of Figure 14; and Figure 17 is a perspective view of a load handling device according to a further embodiment of the invention.

Detailed description of embodiments of the invention

Figure 5 shows a load handling device 100 according to an embodiment of the invention. The load handling device 100 comprises a vehicle 102 equipped with a winch or crane mechanism 104 to lift a storage container or bin 106, also known as a tote, from above. The crane mechanism 104 includes winch cables 108 and a grabber plate 1 10. The grabber plate 1 10 is configured to grip the top of the container 106 to lift it from a stack of containers 106 in a storage system of the type shown in Figures 1 and 2.

Referring also to Figures 6(a) and 6(b), the vehicle 102 comprises an upper part 1 12 and a lower part 1 14.

The lower part 1 14 is fitted with two sets of wheels 1 16, 1 18, which run on rails provided at the top of the frame of the storage system. At least one wheel of each set 1 16, 1 18 is driven to enable movement of the vehicle 102 in X- and Y-directions respectively along the rails. As will be explained below, one or both sets of wheels 1 16, 1 18 can be moved vertically to lift each set of wheels clear of the respective rails, thereby allowing the vehicle 102 to move in the desired direction.

The wheels 1 16, 1 18 are arranged around the periphery of a cavity or recess 120, known as a container-receiving recess, in the lower part 1 14. The recess 120 is sized to accommodate the bin 106 when it is lifted by the crane mechanism 104, as shown in Figure 6(a). When in the recess 120, the bin 106 is lifted clear of the rails beneath, so that the vehicle 102 can move laterally to a different location. On reaching the target location, for example another stack, an access point in the storage system or a conveyor belt, the bin 106 can be lowered from the recess 120 (as shown in Figure 6(b)) and released from the grabber plate 1 10.

The upper part 1 12 of the vehicle 102 houses all of the significant bulky components of the load handling device, as shown in Figure 6(c). The upper part 1 12 houses the battery and associated electronics, controllers and communications devices, motors for driving the wheels 1 16, 1 18, motors for driving the crane mechanism 104, and other sensors and systems.

In this way, the footprint of the vehicle 102 is larger than the size of a bin 106 only enough to accommodate the wheels 1 16, 1 18 either side of the recess 120. In other words, the vehicle 102 occupies a single grid space in the storage system. In this way, the vehicle 102 therefore takes up the minimum possible amount of space in the X-Y plane, and has a footprint approximately half that of the prior art cantilever design shown in Figure 3. For comparison, Figure 7 shows load handling devices 100 according to the invention in use in a storage system of the type shown in Figures 1 and 2, alongside prior art cantilever-type load handling devices 30 of the type shown in Figure 3. It can be seen that the prior art devices 30, although less tall, occupy two stack spaces compared to the taller but smaller-footprint devices 100 of the invention.

The load handling devices 100 of the invention can also offer improved stability, increased load handling capacity and reduced weight compared to the cantilever-type prior art load handling devices 30, because in the invention the load of the containers is suspended between the pairs of wheels on each side of the vehicle. In contrast, the prior-art devices 30 must have a relatively heavy vehicle module to counterbalance the load in the cantilever configuration.

Figures 8 to 12 show one embodiment of the invention. The upper part 1 12 of the vehicle 102 houses three main motors: a Z-drive motor 150 used to raise and lower the winch cables 108, which are wound onto spools 109 mounted on drive shafts situated at opposite sides of the vehicle 102; an X-drive motor 152 which drives the first set of wheels 1 16, and a Y-drive motor 154 which drives the second set of wheels 1 18. The upper part 1 12 of the vehicle also houses a battery 156 to power the motors, and controllers, sensors and other components as described above with reference to Figure 6(c). Drive is transferred from the X- and Y-drive motors 152, 154 to the respective sets of wheels 1 16, 1 18 by means of belt drive mechanisms. The X-drive motor 152 drives a pulley 160 connected to a short drive shaft 162 that extends across the vehicle body. Drive is transferred from the short drive shaft 162 to each wheel in the first set of wheels 1 16 by an X drive belt 164. The Y-drive motor 154 drives a pulley 170 connected to a long drive shaft 172 that extends across the vehicle body in a direction perpendicular to the short drive shaft 162. Drive is transferred from the long drive shaft 172 to each wheel in the second set of wheels 1 18 by a Y drive belt 174.

The belt-driven wheels 1 16, 1 18 are mounted at the bottom of the lower part 1 14 of the vehicle 102. The use of drive belts 164, 174 to transfer drive from the motors to the wheels enables the motors 152, 154 to be mounted in the upper part 1 12 of the vehicle.

In this embodiment, the first set of wheels 1 16 can be raised clear of the rails or lowered onto the rails by means of a wheel positioning mechanism, as shown most clearly in Figures 9, 1 1 and 12. Each wheel 1 16 is mounted on an arm 180 that is pivotally mounted at its outer end. An inner end of each arm 180 is connected to the lower end of a respective linkage 182. The upper ends of both linkages 182 are connected to the lower end of a common linkage 184. In turn, the upper end of the common linkage 184 is connected to a lever arm 186 that is moved by a motor 188. By operating the motor 188 to draw the common linkage 184 upwards, the first set of wheels 1 16 can be raised so that the second set of wheels 1 18 alone is engaged with the rails, allowing movement of the vehicle 102 in the Y-direction. By operating the motor 188 to push the common linkage 184 downwards, the first set of wheels 1 16 move downwards to engage with the rails and to lift the vehicle so that the second set of wheels 1 18 is lifted clear of the rails, as shown in Figures 9, 1 1 and 12. The vehicle 102 can then move in the X-direction. The wheels 1 18 of the second set are mounted to fixed T-pieces 190 disposed at either end of the lower part 1 14 of the vehicle 102.

Figures 8, 9 and 12 show the load handling device 100 with a bin 106 lifted into the recess 120. Figure 1 1 shows the load handling device 100 with the bin 106 beneath the device 100 and the grabber plate 1 10 about to engage with the bin 106. The wheels 1 16, 1 18 and the associated support pieces, linkages and drive belts 164, 174 are arranged around the edges of the recess 120, so that the upper part 1 12 of the vehicle 102 is solidly supported.

Figure 13 shows a wheel 200 suitable for use as one of the wheels 1 16, 1 18 of the load handling device 100. The wheel 200 has a toothed central channel 202 that forms a pulley for cooperating with a drive belt 164, 174. The channel 202 is bounded by two rubber tyres 204, which bear upon the rails in use. The wheel 200 can be mounted to an arm 180 by way of an axle (not shown) that extends through an axial hole 206 in the wheel 200. This wheel design is compact and balanced, to minimise wear, and the tyres 204 serve to keep the drive belt 164, 174 in alignment in use.

Figure 14 shows two wheels 200 mounted in a frame structure 210 of a load handling device according to another embodiment of the invention. As in the previous embodiments, in this embodiment the load handling device comprises a vehicle with an upper part 1 12 that houses the major components of the device and a lower part having a recess 120 for accommodating a bin, with the wheels 200 being arranged on four sides of the recess (the wheels on only one side are shown in Figure 14).

In this case, the frame structure 210 comprises two parallel panels that accommodate the wheels 200 therebetween. A drive belt 212 is provided to transfer drive to the wheels 200 from a motor housed in the upper part 1 12 of the vehicle.

Referring additionally to Figures 15 and 16, the wheels 200 in this embodiment can be raised and lowered by moving the frame structure 210 relative to the upper part 1 12 of the vehicle. The frame structure 210 is mounted to a body 230 of the upper part 1 12 of the vehicle by way of rails 232. The rails 232 are fixed to the body 230 in a vertical orientation, and the frame structure 210 is slidably mounted to the rails 232.

The frame structure 210 is retained by a pair of linkages 240 that extend between the panels. The bottom ends of the linkages 240 are attached to respective shafts 242 that bridge the gap between the panels. The top ends of the linkages 240 are rotatably attached to threaded bosses 246 that are mounted on a threaded horizontal driveshaft 244. The bosses 246 are slidably attached to horizontal rails 248.

The driveshaft 244 is driven by a motor 250 by way of a drive belt (not shown). When the driveshaft 244 is rotated in a first direction, the top ends of the linkages 240 move apart to push the frame structure 210 downwards, thereby to lower the wheels 200 onto a rail. When the driveshaft 244 is rotated in a second, opposite direction, the top ends of the linkages 240 move together to pull the frame structure 210 upwards, lifting the wheels 200. Although only one frame structure 210 with two wheels 200 is shown in Figures 14 to 16, it will be appreciated that an identical frame structure 210 would be provided on the opposite side of the vehicle. Both frame structures 210 are raised and lowered by a common motor, so the four wheels 200 can be lifted and lowered in unison to control engagement of this first set of wheels 200 with rails extending in a first direction across the frame. Although not shown in Figures 14 to 16, the vehicle includes another set of wheels arranged to engage with rails extending in a second, perpendicular direction across the frame when the first set of wheels is lifted.

It will be appreciated that many different variations and modifications are possible. For example, both sets of wheels may be powered by a single motor, with a suitable transfer arrangement to direct power to the appropriate set of wheels. In other embodiments, one or more of the wheels may include an integrated motor or a motor located adjacent the wheel. An example of this is shown in Figure 17.

Referring to Figure 17, this shows a load-handling device 252 according to a further embodiment of the invention. The device 252 has a cuboid-shaped external housing 254 to which a plurality of wheels 256 are mounted near a lower edge 258 of the housing 254. The wheels 256 are motorised hub wheels, with each wheel 256 having a motor integrated within a hub 260 of the wheel 256. The motors are used to drive the respective wheels 256 directly, and hence this embodiment does not require drive belts connected between the wheels and drive motors.

In this example the motors are powered by batteries located within side walls 262 of a lower part 264 of the housing 254, adjacent to a container-receiving space 266 of the device 252. Locating the batteries low down in this way has the advantageous effect of lowering the centre of gravity of the device 252, thereby increasing its stability and allowing higher acceleration and deceleration. The device 252 is otherwise similar to the previous embodiments and contains similar mechanisms for raising and lowering the wheels 256, and a similar lifting device for lifting a container into the container-receiving space 266. The batteries located in the side walls 262 are also used to power these components. In any of the previously-described embodiments, the mechanism used to lift containers into the container-receiving space could take any suitable form. For maximum stability and load capacity, it is desirable to provide four lifting cables, with one cable disposed near each of the corners of the device, but a different arrangement, for example with fewer cables, could be used if desired. Conveniently, all of the cables are spooled and unspooled using a single motor, but more than one motor could be used if desired.

Instead of a motor, the mechanism used to lift the wheels may use linear actuators, such as linear motors or hydraulic rams. Instead of using battery power, other means of powering the load- handling devices will be apparent to persons skilled in the art, for example using overhead power or by supplying power via the rails on which the devices run.

It will be appreciated that features described in relation to one particular embodiment are interchangeable with features described in relation to the other embodiments. For example, the motorised hub wheels described in relation to Figure 17 could be used on any of the other embodiments and/or the batteries could be located low down adjacent the container-receiving space in any of the embodiments to improve stability and increase acceleration/deceleration. Other variations and modifications not explicitly described above will also be apparent to the skilled reader.