Method and system for reducing hoist distance

Abstract

A method of controlling a wheeled 1320 load-handling device 1300 comprising a gripping assembly (1210 Fig 12), within a grided and tracked 1330 framework structure (Fig 1) of a storage and retrieval s

Description

Technical Field The present disclosure relates generally to the field of automated storage and retrieval systems, and in particular reducing hoisting distance for a load handling device used in the automated storage and retrieval system. Background Some commercial and industrial activities require systems that enable the storage and retrieval of a large number of different products. For example, WO2015 / 185628A2 (Ocado) describes an automated storage and fulfilment system (ASRS) in which stacks of storage containers are arranged within a grid storage structure. The containers are accessed from above by load-handling devices operative on rails or tracks located on the top of the grid storage structure. The load-handling devices may be those described in WO2015 / 019055A1 (Ocado). Within the storage and fulfilment system, it is important that the load-handling devices can reduce hoisting distance when accessing the grid storage structure. It is against this background that the present invention has been devised. Summary In a first aspect, there is a method of controlling a load-handling device for lifting and moving storage containers stacked in a grid framework structure comprising: a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising: a container-receiving space located above the rails for receiving and transporting a container to / from a storage location; a container-gripping assembly configured to releasably grip a container to attach and / or release a container; a container-lifting assembly configured to raise and lower the container gripping assembly to and from the container-receiving space, the method comprising: controlling, when a container is not attached to the container-gripping assembly, the container-lifting assembly to hold the container gripping assembly in a position above and as close as possible to a top surface of the grid without the container gripping assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks. By holding the container-gripper assembly as close to the grid as possible, this means the hoisting distance required to access a container in a storage location is reduced. The collective reduction in hoisting distance over a period of time can be significant. The container-receiving space may comprise a cavity to receive the container-gripping assembly and a container attached to the container-gripping assembly, and the container-lifting assembly may be located above the container-receiving space. The container-receiving space may comprises a cantilever assembly, wherein the cantilever assembly defines a space below to receive the container-gripping assembly and a container attached to the container-gripping assembly, and the cantilever assembly may comprise the container-lifting assembly. This means the collective reduction in hoisting distance over a period of time can be achieved regardless of the type of load-handling device used. The method may comprise controlling, when a container is attached to the container gripping-assembly, the load-handling device to move adjacent to a target grid space, wherein the container-lifting assembly is controlled to position the container attached to the container gripping-assembly above and as close as possible to the top surface without the container attached to the container gripping-assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks, controlling the container-lifting assembly to lower the container attached to the container gripping assembly into a storage location beneath the target grid space, controlling the container-gripping assembly to release the container attached to the container gripping assembly, and controlling the container-lifting assembly to raise the container-gripping assembly to the position above and as close as possible to the top surface of the grid. This means the hoisting distance can be reduced when both lowering an attached container and returning the container-gripping assembly above the grid. The method may comprise controlling, when a container is not attached to the container gripping-assembly, the load-handling device to move adjacent to a target grid space, wherein the container-gripping assembly is held at the position above and as close as possible to the top surface of the grid, controlling the container-lifting assembly to lower the container gripping-assembly from the position above and as close as possible to the top surface of the grid into a storage location beneath the target grid space, controlling the container-gripping assembly to attach a container, and controlling the container-lifting assembly to raise the container-gripping assembly to position the container attached to the container-gripping assembly above and as close as possible to the top surface without the container attached to the container-gripping assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks. This means the hoisting distance can be reduced when both lowering the container-gripping assembly and returning an attached container above the grid. The load-handling device and / or the container-lifting assembly may comprise a sensor, such as an encoder, to determine a position of the container-gripping assembly. The container-lifting assembly may comprises at least one tether connected to the container-gripping assembly, and at least one winding means, such as a motor and spool, to wind and unwind the at least one tether. The container-lifting assembly may comprises four tethers, wherein each tether is connected to a respective corner of the container-gripping assembly. The top surface of the grid may defines a plane and the position above and as close as possible to the top surface of the grid is vertically above the plane by a predefined offset. The predefined offset may be about 10 mm or 5-35 mm. The predefined offset is set so that no part of the container-gripping assembly contacts or crosses the plane. This ensures a maximum reduction in hoisting distance is achieved without the container-gripping assembly contacting the grid. The container-gripping assembly may comprises a frame, and one or more gripping elements on the frame, wherein the one or more gripping elements are each configured to attach and release a container thereto, and optionally one or more guiding elements on the frame to guide the container-gripping assembly onto a container, and / or optionally one or more sensors to detect whether a container is attached to the gripping assembly. The load-handling device may comprise a body or skeleton mounted on a first set of wheels being arranged to engage with the first set of parallel tracks and a second set of wheels being arranged to engage with the second set of parallel tracks. The loadhandling device may comprises a drive assembly configured to drive the first or second sets of wheels to move the load-handling device along the first or second set of parallel rails in the respective parallel directions, and / or a direction-change assembly configured to raise or lower the first set of wheels and / or lower or raise the second set of wheels with respect to the body or skeleton to engage and disengage the wheels with the parallel tracks. In a second aspect, there is a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of the first aspect. In a third aspect, there is a load-handling device configured to operate on a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising: a container-receiving space located above the rails for receiving and transporting a container to / from a storage location, the container-receiving space comprising a container-lifting assembly configured to raise and lower a container gripping assembly, the container-gripping assembly configured to releasably grip a container to attach and / or release a container; and a controller configured to carry out the method of the first aspect. In a fourth aspect, there is a system comprising: a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces; and a load-handling device according to the third aspect. Brief Description of Drawings The invention is described with reference to the accompanying drawings, wherein: Figure 1 shows a known automated storage and retrieval system that uses load-handling devices; Figure 2 shows a known single load-handling device with a container-lifting assembly in a lowered configuration; Figure 3 shows a method of reducing hoisting distance when a load-handling device accesses the grid storage structure; Figure 4 shows a method of reducing hoisting distance when a load-handling device accesses the grid storage structure; Figure 5 shows a method of reducing hoisting distance when a load-handling device accesses the grid storage structure; Figure 6 shows a method of reducing hoisting distance when a load-handling device accesses the grid storage structure; Figures 7A and 7B show how a load-handing device can reduce hoisting distance when accessing the grid storage structure; Figures 8A and 8B show how a load-handing device can reduce hoisting distance when accessing the grid storage structure; Figures 9A and 9B show how a load-handing device can reduce hoisting distance when accessing the grid storage structure; Figures 10A and 10B show how a load-handing device can reduce hoisting distance when accessing the grid storage structure; Figure 11 shows a load-handling device; Figure 12 shows a container-gripping assembly and container used by the load-handling device of Figure 11; and Figures 13A, 13B, 14A, and 14B show how the load-handling device of Figure 11 can reduce hoisting distance when accessing the grid storage structure. Detailed Description WO2015 / 185628A (Ocado), hereby incorporated by reference, describes a known ASRS in which stacks of containers are arranged within a grid framework structure. The containers are accessed by one or more load-handling devices, otherwise known as “bots”, operative on tracks located on the top of the grid framework structure. A system of this type is illustrated schematically in Figure 1. As shown in Figures 1, stackable containers 10, also known as “bins” or “totes”, are stacked on top of one another to form stacks 12. The stacks 12 are arranged in a grid framework structure 14. The grid framework structure 14 is made up of a plurality of storage columns or grid columns. Each grid in the grid framework structure has at least one grid column to store a stack of containers. Each bin 10 typically holds a plurality of product items (not shown). The grid framework structure 14 comprises a plurality of upright members 16 that support horizontal members 18, 20. A first set of parallel horizontal grid members 18 is arranged perpendicularly to a second set of parallel horizontal members 20 in a grid pattern comprising respective grid spaces to form a horizontal grid structure 15 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 grid framework structure 14, so that the grid framework structure 14 guards against horizontal movement of the stacks 12 of bins 10 and guides the vertical movement of the bins 10. The top level of the grid framework structure 14 comprises a grid or grid structure 15, including rails 22 arranged in a grid pattern comprising respective grid spaces across the top of the stacks 12. The rails or tracks 22 guide a plurality of load-handling devices 30. A first set 22a of parallel rails 22 guide movement of the robotic load-handling devices 30 in a first direction (e.g. an X-direction along track 22a) across the top of the grid framework structure 14. 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 (e.g. a Y-direction along track 22b), perpendicular to the first direction. In this way, the rails 22 allow the robotic load-handling devices 30 to move laterally in two dimensions in the horizontal X-Y plane. A load-handling device 30 can be moved into position above any of the stacks 12. A known form of load-handling device 30 shown in Figure 2 is described in WO2015 / 019055 (Ocado), hereby incorporated by reference. The load-handling device 30 comprises a vehicle 32, which is arranged to travel on the rails 22 of the frame structure 14. 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, by way of a direction-change assembly, 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. For example, 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 first set of wheels 34 can be driven, by way of a drive assembly housed in the vehicle 32, to move the load-handling device 30 in the X-direction. To achieve movement 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 22b of rails 22. The drive assembly can then be used to drive the second set of wheels 36 to move the load-handling device 30 in the Y-direction. The load-handling device 30 is equipped with a container-lifting device or assembly, e.g. a crane mechanism, to lift a storage container from above. The container-lifting device comprises a winch tether or cable 38 wound on a spool or reel and a container-gripping device or assembly 39. The container-lifting device shown in Figure 2 comprises a set of four lifting tethers 38 extending in a vertical direction. The tethers 38 are connected at or near the respective four corners of the container-gripping device 39, e.g. a lifting frame, for releasable connection to a storage container 10. The container-gripping device 39 is configured to releasably grip the top of a storage container 10 to lift it from a stack of containers in a storage system of the type shown in Figure 1. That is, the containergripping device is configured to releasably grip a container to attach and / or release a container, which may be detected by a sensor. To remove a bin 10 from the top of a stack 12, the load-handling device 30 is first moved in the X- and Y-directions to position the gripper device 39 in a respective grid space above the stack 12. The container-gripping device 39 is then lowered vertically in the Z-direction to engage with the container 10 on the top of the stack 12. The container-gripping device 39 grips the container 10, and is then pulled upwards by the cables 38, with the container 10 attached. At the top of its vertical travel, the container 10 is held above the rails 22 accommodated within the vehicle body (or skeleton) 32. In this way, the load-handling device 30 can be moved to a different position in the X-Y plane, carrying the container 10 along with it, to transport the container 10 to another location. On reaching the target location (e.g. another stack 12, an access point in the storage system, or a conveyor belt) the container 10 can be lowered from the container receiving portion and released from the gripper device 39. It will be appreciated that the ASRS and the load-handling devices are under the control of a master controller, where each loadhandling device has a controller its operation. Typically, the container-gripping device is raised to the top of its vertical travel both when lifting a container and returning from lowering a container. That is, when the load handling device is moving along the first and / or second set of tracks, the container-gripping assembly is held as high as possible in the Z-direction. This means that the distance the container-gripping assembly has to be lowered to attach to a container, is maximised. Put another way, the hoisting time of the container-gripping assembly is maximised. According to an embodiment, the container-gripping assembly may instead be held above and closer to the grid when a container is not attached to the container-gripping assembly. That is, instead of container-gripping assembly being held as high as possible in the Z-direction, it is held lower but above the grid. Therefore, when the load-handling device is moved adjacent to a target grid space, the distance to lower the container-gripping assembly to access a container below the target grid space is reduced. That is, the load-handling device travels along the tracks with the containergripping assembly as close to a top surface of the grid as possible. Therefore, the container-gripping assembly, when not holding a container, is held as close as possible to a top surface of the grid without contacting the grid when the loadhandling device moves along the first and / or second set of tracks. The container-gripping assembly has a range of movement above the grid defined by maximum and minimum values in the Z-direction. In general, provided the container-gripping assembly is held in a region away from the maximum value, a reduction in hoisting distance can be achieved. Therefore any region below the maximum value that allows an advantageous reduction in hoisting distance to be achieved can be said to be as close as possible, provided the region selected does not result in container-gripping assembly contacting the grid when the load-handling device moves along the first and / or seconds sets of tracks. It will be appreciated that the maximum reduction in hoisting distance occurs when the region is closer to the minimum value above the grid in the Z-direction. In theory, a negligible minimum value, e.g. ~0, above the grid in the Z-direction may be chosen. However, the minimum value above the grid in the Z-direction may have to account for a pendulum effect of the container-gripping assembly as the load-handling device moves along the first and / or second set of tracks. That is, no part of the container-gripping assembly is below a plane defined by the top surface of the grid when the load-handling device is stationary. However, a part of the container-gripping assembly above the grid in the Z-direction may breach or contact this plane due a swinging motion when the load-handling device moves along the first and / or second set of tracks. Similarly, the height of the container-gripping assembly may be controlled by a sensor, such as an encoder, that has a sensor tolerance. The tolerance may mean there is a limit on how accurately a current height of the container-gripping assembly can be determined so choosing a negligible minimum value, e.g. ~0, above the grid in the Z-direction risks contact with the grid. Additionally, the grid may not be perfectly horizontal so choosing negligible minimum value, e.g. ~0, above the grid in the Z-direction risks contact with the grid. Further, the load-handling device may no longer operate within precise engineering tolerance so once again choosing a negligible minimum value, e.g. ~0, above the grid in the Z-direction risks contact with the grid. Therefore, in a preferred embodiment to account for all of these considerations the container-gripping assembly is controlled to position at a height about 10 mm above a top surface of the grid (compared to an example load-handling device where the maximum height of the container-gripping assembly is about 330 mm above a top surface of the grid). That is, no part of the container-gripping assembly is held closer than about 10 mm to the top surface of the grid. This value provides an optimal trade-off between maximising the reduction in hoisting distance whilst reducing the risk of the container-gripping assembly contacting the grid as the load-handling device moves along the first and / or second sets of tracks. However, other trade-offs may be chosen. In one example, any value between 5-35 mm is acceptable. The upper value is not as constrained as the lower value and can be viewed as a linear relationship where increasing the upper value increases the hoisting time. Therefore 5-45 mm, 5-55 mm, 5-65 mm, 5-75 mm, 5-85 mm, 5-95 mm, 5-105 mm, 5-115 mm, 5-125 mm, 5-135 mm, 5-145 mm, 5-155 mm, 5-165 mm, 5-175 mm, 5-185 mm, 5-195 mm, 5-205 mm, 5-215 mm, 5-225 mm, 5-235 mm, 5-245 mm, 5-255 mm, 5-265 mm, 5-275 mm, 5-285 mm, 5-295 mm, 5-305 mm, 5-315 mm, and 5-325 mm are all viable provided the increase in hoisting time is acceptable. Figure 3 shows a method 300 of using the above embodiment in normal operation of the ASRS. In step 310, when a container is attached to the container gripping-assembly, the load-handling device is controlled to move adjacent to a target grid space. In this scenario, the container-gripping assembly is held at or as close as possible to the maximum height in the Z-direction above the top surface of the gird, or at the second position described below. Therefore, the container-lifting assembly is controlled to position the container attached to the container gripping-assembly above and as close as possible to the top surface without the container attached to the container grippingassembly contacting the grid when the load-handling device moves along the first and / or second set of tracks. In step 320, the container-lifting assembly is controlled to lower the container attached to the container gripping-assembly into a storage location beneath the target grid space. In step 330, the container-gripping assembly is controlled to release the container attached thereto. In step 340, the container-lifting assembly is then controlled to raise the containergripping assembly to the position above and as close as possible to the top surface of the grid as described above. In this way, there is both a reduction in hoisting distance when raising the container-gripping device, and a reduction in hoisting distance when the container-gripping device is next lowered. The collective reduction in hoisting distance over a period of time can be significant. Figure 4 shows a method 400 that uses the above embodiment and / or follows on from the method of Figure 3. In step 410, when a container is not attached to the container gripping-assembly, the load-handling device is controlled to move adjacent to a target grid space. The container-gripping assembly is held at the position above and as close as possible to the top surface of the grid as described above. This way, the hoisting distance required to lower the container-gripping assembly is reduced. The collective reduction in hoisting distance over a period of time can be significant. In step 420, the container-lifting assembly is controlled to lower the container-gripping assembly from the position above and as close as possible to the top surface of the grid into a storage location beneath the target grid space. In step 430, the container-gripping assembly is controlled to attach a container. In step 440, the container-lifting assembly is controlled to raise the container-gripping assembly to position the container attached to the container-gripping assembly above and as close as possible to the top surface without the container attached to the container-gripping assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks. In this scenario, the container-gripping assembly is held at or as close as possible to the maximum height in the Z-direction above the top surface of the gird, or at the second position described below. The method of Figure 3 may then occur after step 440. The above embodiments are described in respect of a position of the container-gripping assembly that is selected to reduce hoisting distance. This position in respect of when a container is not attached can be denoted nominally as a first position. That is, Figures 3 and 4 are used in respect of the first position. It will be appreciated that the above embodiments can equally be applied to the scenario when a container is attached and that the position of the container-gripping assembly can be denoted nominally as a second position. The second position is thus selected using the same considerations as the selection of the first position, with the difference being that no part of the container should contact the grid when the load-handling device moves along the first and / or second sets of tracks. The second position is constrained by a size of the container, but is set so that the distance between a bottom of the container and a top of the grid can be minimised whilst accounting for the same considerations when selecting the first position. The reduction in hoisting distance will be smaller compared to when no container is attached to the container-gripping assembly and will be a function of the height of the container in the Z-direction. Nonetheless, the collective reduction in hoisting distance over a period of time can be significant. Therefore, in a preferred embodiment to account for all of these considerations the container-gripping assembly is controlled to be held at position (i.e. at a second position) such that the container is at a height about 10 mm above a top surface of the grid (compared to an example load-handling device where the container is held at a maximum height of about 40mm above a top surface of the grid). That is, no part of the container is held closer than about 10 mm to the top surface of the grid. This value provides an optimal trade-off between maximising the reduction in hoisting distance whilst reducing the risk of the container contacting the grid as the load-handling device moves along the first and / or second sets of tracks. However, other trade-offs may be chosen. In one example, any value between 5-35 mm is acceptable. The upper value is not as constrained as the lower value and can be viewed as a linear relationship where increasing the upper value increases the hoisting time. However, the extent to which the upper value can be increased is a function of the vertical height of the container. Therefore other 5-45 mm, 5-55 mm, 5-65 mm, 5-75 mm, 5-85 mm, 5-95 mm, 5-105 mm are all viable provided the increase in hoisting time is acceptable, and the vertical height of the container does not provide a constraint. Figure 5 shows a method 500 that uses the second position described above. In step 510, when a container is attached to the container-gripping assembly, the load-handling device is controlled to move adjacent to a target grid space. In this scenario the container-gripping assembly is held at the second position above and as close as possible to the top surface of the grid. In step 520, the container-lifting assembly is controlled to lower the container attached to the container-gripping assembly from the second positon into a storage location beneath the target gird space. Due to the container-gripping assembly being held at the second position, the hoisting distance is reduced. In step 530, the container-gripping assembly is controlled to release the container attached thereto. In step 540, the container-lifting assembly is controlled to return the container-gripping assembly to above and as close as possible to the top surface without the containergripping assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks (i.e. the first position). This way, the container-gripping assembly will have a reduced hoisting distance when next used to retrieve a container from a storage location within the grid. The collective reduction in hoisting distance over a period of time can be significant. Figure 6 shows a method 600 that uses that uses the second position described above and / or follows on from the method of Figure 5. In step 610, when a container is not attached to the container-lifting assembly, the load-handling device is controlled to move adjacent to a target grid space. In this scenario, the container-lifting assembly is controlled to position the container gripping-assembly above and as close as possible to the top surface without the container gripping-assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks (i.e. the containerlifting assembly is held at the first position. In step 620, the container-lifting assembly is controlled to lower the container-gripping assembly from the first positon into a storage location beneath the target gird space. Due to the container-gripping assembly being held at the first position, the hoisting distance is reduced. The collective reduction in hoisting over a period of time can be significant. In step 630, the container-gripping assembly is controlled to attach a container. In step 640, the container-lifting assembly is controlled to raise container-gripping assembly to the second position. This way, the container-gripping assembly will have a reduced hoisting distance when next used to lower the attached container into a storage location within the grid. The collective reduction in hoisting distance over a period of time can be significant. The method of Figure 5 may then occur after step 640. It will be appreciated that the methods of Figures 3 and 4 with can be combined with the methods of Figures 4 and 6 to reduce the overall hoisting distance. Figures 7A and 7B show a load-handling device 700 that carries out the above methods. The load-handling device is shown with respect to a horizontal direction along the grid 750. In this example, the load-handling device is similar to that shown in Figures 1 and 2 and described above. The load-handling device has a container-lifting assembly 710 that may comprise tethers, spools, and a motor to lower and raise container-gripping assembly 720. The load-handling device also has a container-receiving space 730, which in this example is a cavity within a body of the load-handling device. The container-receiving space can be used to temporally receive and store a container so that it can be transported. The load handling device has wheels 735 for movement along either the first or second set of tracks 750. It will be appreciated that four wheels may be needed, only two of which are shown, to move in one of the horizontal directions. In this example, a container is not attached to the container-gripping assembly. When a controller of load-handling device 700 is configured to carry out the above methods, the container-gripping assembly 720 is lowered and held in the first position as shown in Figure 7B. In this example, tethers 740 are used to raise and lower the containergripping assembly. The container-gripping assembly is held at distance 760 from a top surface of the grid that achieves an optimal trade-off between maximising the reduction in hoisting distance whilst reducing the risk of the container-gripping assembly contacting the grid as the load-handling device moves along the first and / or second sets of tracks. The top of the grid 750 may be thought of as a plane and distance 760 is offset from that plane so that no part of the container-gripping assembly contacts or crosses the plane. It will be appreciated that the reduction in hoisting is a function of the load-handling device’s design, and in particular the height of the container-receiving space, which in turn is a function of the container height. Therefore in principle, whilst the absolute value of a reduction in hoisting may reduce as the height of the container reduces, it is nonetheless a relative reduction which adds up with repeated hoisting. Figures 8A and 8B show a load-handling device 800 that carries out the above methods. The load-handling device is shown with respect to a horizontal direction along the grid 850. In this example, the load-handling device is similar to that shown in Figures 1 and 2 and described above. The load-handling device has a container-lifting assembly 810 that may comprise tethers, spools, and a motor to lower and raise container-gripping assembly 820. The container-gripping assembly is attached to a container 825 which is received within container-receiving space 830, which in this example is a cavity within a body of the load-handling device. The container-receiving space can be used to temporally receive and store a container so that it can be transported. The load handling device has wheels 835 for movement along either the first or second set of tracks 850. It will be appreciated that four wheels may be needed, only two of which are shown, to move in one of the horizontal directions. In this example, a container is attached to the container-gripping assembly. When a controller of load-handling device 800 is configured to carry out the above methods, the container-gripping assembly 820 is lowered and held in the second position 870 as shown in Figure 8B. In this example, tethers 840 are used to raise and lower the container-gripping assembly. The second position ensures that the container is held at a distance 860 above the grid that achieves an optimal trade-off between maximising the reduction in hoisting distance whilst reducing the risk of the container contacting the grid as the load-handling device moves along the first and / or second sets of tracks. The top of the grid 850 may be thought of as a plane and distance 870 is offset from that plane so that no part of the container contacts or crosses the plane. It will be appreciated that the reduction in hoisting is function of the load-handling device’s design, and in particular the height of the container-receiving space, which in turn is a function of the container height. Therefore in principle, whilst the absolute value of a reduction in hoisting may reduce as the height of the container reduces, it is nonetheless a relative reduction which adds up with repeated hoisting. Figures 9A and 9B show a load-handling device 900 that carries out the above methods. The load-handling device is shown with respect to a horizontal direction along the grid 950. In this example, the load-handling device is similar to that described in NO317366B1, which is hereby incorporated by reference. The load-handling device has a cantilever arm 910 which in this example comprises the container-lifting assembly, which may comprise tethers, spools, and a motor to lower and raise container-gripping assembly 920. The load-handling device also has a container-receiving space 930, which in this example is a space below the cantilever arm 910. The container-receiving space can be used to temporally receive and store a container so that it can be transported. The load handling device has wheels 935 for movement along either the first or second set of tracks 950. It will be appreciated that four wheels may be needed, only two of which are shown, to move in one of the horizontal directions. In this example, a container is not attached to the container-gripping assembly. When a controller of load-handling device 900 is configured to carry out the above methods, the container-gripping assembly 920 is lowered and held in the first position as shown in Figure 9B. In this example, tethers 940 are used to raise and lower the containergripping assembly. The container-gripping assembly is held at distance 960 from a top surface of the grid that achieves an optimal trade-off between maximising the reduction in hoisting distance whilst reducing the risk of the container-gripping assembly contacting the grid as the load-handling device moves along the first and / or second sets of tracks. The top of the grid 950 may be thought of as a plane and distance 960 is offset from that plane so that no part of the container-gripping assembly contacts or crosses the plane. It will be appreciated that the reduction in hoisting is a function of the load-handling device’s design, and in particular the height of the container-receiving space, which in turn is a function of the container height. Therefore in principle, whilst the absolute value of a reduction in hoisting may reduce as the height of the container reduces, it is nonetheless a relative reduction which adds up with repeated hoisting. Figures 10A and 10B show a load-handling device 1000 that carries out the above methods. The load-handling device is shown with respect to a horizontal direction along the grid 1050. In this example, the load-handling device is similar to that described in NO317366B1, which is hereby incorporated by reference. The load-handling device has a cantilever arm 1010 which in this example comprises the container-lifting assembly, which may comprise tethers, spools, and a motor to lower and raise container-gripping assembly 1020. The container-gripping assembly is attached to a container which is received within container-receiving space 1030, which in this example is a space below the cantilever arm 1010. The container-receiving space can be used to temporally receive and store a container so that it can be transported. The load handling device has wheels 1035 for movement along either the first or second set of tracks 1050. It will be appreciated that four wheels may be needed, only two of which are shown, to move in one of the horizontal directions. In this example, a container is attached to the container-gripping assembly. When a controller of load-handling device 1000 is configured to carry out the above methods, the container-gripping assembly 1020 is lowered and held in the second position 1070 as shown in Figure 10B. In this example, tethers 1040 are used to raise and lower the container-gripping assembly. The second position ensures that the container is held at a distance 1060 above the grid that achieves an optimal trade-off between maximising the reduction in hoisting distance whilst reducing the risk of the container contacting the grid as the load-handling device moves along the first and / or second sets of tracks. The top of the grid 1050 may be thought of as a plane and distance 1070 is offset from that plane so that no part of the container contacts or crosses the plane. It will be appreciated that the reduction in hoisting is function of the load-handling device’s design, and in particular the height of the container-receiving space, which in turn is a function of the container height. Therefore in principle, whilst the absolute value of a reduction in hoisting may reduce as the height of the container reduces, it is nonetheless a relative reduction which adds up with repeated hoisting. Figure 11 shows a specific embodiment of a load-handling device 1100 that can use the above methods. Load-handling device 1100 has wheels 1110, and is similar to that further described in PCT / EP2022 / 051652, herby incorporated by reference. Figure 12 shows a combination 1200 of a specific container-gripping assembly 1210 and container 1250 that may be used by the load-handling device of Figure 11. The container-gripping assembly comprises a frame which has gripper elements such as element 1230. The elements 1230 interact with slots 1260 to attach and / or release container 1250 to the container-gripping assembly. The container-gripping assembly may also comprise guide elements 1240 to engage with contours 1270 in the container and thus align the gripper elements 1230 with slots 1260. The bottom of the gripper elements, or the guide elements if used, can define a lowermost point which should not contact or cross the plane defined by the top surface of the grid when the containergripping assembly is held in the first position. Similarly, the bottom of container can define a lowermost point which should not contact or cross the plane defined by the top surface of the grid when the container-gripping assembly is held in the second position. Figure 13 shows a side view of a load-handling device 1300 similar to that in Figure 11 that uses a container-gripping assembly similar to that shown in Figure 12. The loadhandling device has wheels 1320 that engage with tracks 1330. In Figure 13A, the container-gripping assembly, with no container attached, is held at its maximum height in the Z-direction. In Figure 13B, the load-handling device, carrying out the above methods, holds the container-gripping device in the first position at an offset 1340 from a plane defined by a top surface of the grid. Figure 14 shows a side view of a load-handling device 1400 similar to that in Figure 11 that uses a container-gripping assembly similar to that shown in Figure 12. The loadhandling device has wheels 1420 that engage with tracks 1430. In Figure 14A, the container-gripping assembly, with container 1425 attached, is held at its maximum height in the Z-direction. In Figure 14B, the load-handling device, carrying out the above methods, holds the container-gripping device in the second position 1440 from a plane defined by a top surface of the grid to such that no point of the container 1425 contacts or crosses the plane. That is the bottom of the container is at an offset 1450 from the top of the grid. In this document, “controller” is intended to include any hardware which is suitable for controlling (e.g. providing instructions to) one or more other components. For example, a processor equipped with one or more memories and appropriate software to process data relating to a component or components and send appropriate instructions to the component(s) to enable the component(s) to perform its / their intended function(s). Furthermore, the invention can take the form of a computer program embodied as a computer-readable medium having computer executable code for use by or in connection with a computer. Reference to an element in the singular is not intended to mean only one unless specifically so stated, but rather “one or more.” The subsequent use of a definite article (e.g., “the” or “said”) with an element (e.g., “the processor”) is not intended to invoke a singular meaning (e.g., “only one”) on the element unless otherwise specifically stated. For example, reference to an element (e.g., “a processor,” “a controller,” “a memory,” “a transceiver,” “an antenna,” “the processor,” “the controller,” “the memory,” “the transceiver,” “the antenna,” etc.), unless otherwise specifically stated, should be understood to refer to one or more elements (e.g., “one or more processors,” “one or more controllers,” “one or more memories,” “one more transceivers,” etc.). The terms “set” and “group” are intended to include one or more elements, and may be used interchangeably with “one or more.” Where reference is made to one or more elements performing functions (e.g., steps of a method), one element may perform all functions, or more than one element may collectively perform the functions. When more than one element collectively performs the functions, each function need not be performed by each of those elements (e.g., different functions may be performed by different elements) and / or each function need not be performed in whole by only one element (e.g., different elements may perform different sub-functions of a function). Similarly, where reference is made to one or more elements configured to cause another element (e.g., an apparatus) to perform functions, one element may be configured to cause the other element to perform all functions, or more than one element may collectively be configured to cause the other element to perform the functions. It will be understood that the above description is given by way of example only and that various modifications may be made by those skilled in the art. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this invention. The following is a non-exhaustive list of embodiments which may be or are claimed. Embodiments - Set A 1. A method of controlling a load-handling device for lifting and moving storage containers stacked in a grid framework structure comprising: a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising: a container-receiving space located above the rails for receiving and transporting a container to / from a storage location; a container-gripping assembly configured to releasably grip a container to attach and / or release a container; a container-lifting assembly configured to raise and lower the container gripping assembly to and from the container-receiving space, the method comprising: controlling, when a container is attached to the container-gripping assembly, the container-lifting assembly to hold the container gripping assembly in a position above and as close as possible to a top surface of the grid without the container contacting the grid when the load-handling device moves along the first and / or second set of tracks. 2. The method of claim 1, wherein the container-receiving space comprises a cavity to receive the container-gripping assembly and a container attached to the containergripping assembly, and optionally wherein the container-lifting assembly is located above the container-receiving space. 3. The method of claim 1, wherein the container-receiving space comprises a cantilever assembly, wherein the cantilever assembly defines a space below to receive the container-gripping assembly and a container attached to the container-gripping assembly, and optionally wherein the cantilever assembly comprises the container-lifting assembly. 4. The method of embodiments 1-3, further comprising: controlling, when a container is attached to the container gripping-assembly, the load-handling device to move adjacent to a target grid space, wherein the containergripping assembly is held at the position above and as close as possible to the top surface of the grid; controlling the container-lifting assembly to lower the container attached to the container gripping assembly from the position above and as close as possible to the top surface of the grid into a storage location beneath the target grid space; controlling the container-gripping assembly to release the container attached to the container gripping assembly; and controlling the container-lifting assembly to raise the container-gripping assembly to position the container-gripping assembly above and as close as possible to the top surface without the container-gripping assembly contacting the grid when the loadhandling device moves along the first and / or second set of tracks. 5. The method of embodiments 1-4, further comprising: controlling, when a container is not attached to the container-lifting assembly, the load-handling device to move adjacent to a target grid space, wherein the containerlifting assembly is controlled to position the container gripping-assembly above and as close as possible to the top surface without the container gripping-assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks; controlling the container-lifting assembly to lower the container-gripping assembly into a storage location beneath the target grid space; controlling the container-gripping assembly to attach a container; controlling the container-lifting assembly to raise the container-gripping assembly to the position above and as close as possible to the top surface of the grid. 6. The method of embodiments 1-5, wherein the load-handling device and / or the container-lifting assembly comprise a sensor, such as an encoder, to control a position of the container-gripping assembly. 7. The method of embodiments 1-6, wherein the container-lifting assembly comprises: at least one tether connected to the container-gripping assembly; and at least one winding means, such as a motor and spool, to wind and unwind the at least one tether. 8. The method of embodiments 1-7, wherein the container-lifting assembly comprises four tethers, wherein each tether is connected to a respective corner of the container-gripping assembly. 9. The method of embodiments 1-8, wherein the top surface of the grid defines a plane and the position above and as close as possible to the top surface of the grid is such that the container is vertically above the plane by a predefined offset. 10. The method of embodiment 9, wherein the predefined offset is about 10 mm or 5-35 mm. 11. The method of embodiments 9 or 10, wherein the predefined offset is set so that no part of the container contacts or crosses the plane. 12. The method of embodiment 11, wherein the container-gripping assembly comprises: a frame; and one or more gripping elements on the frame, wherein the one or more gripping elements are each configured to attach and release a container thereto; and optionally one or more guiding elements on the frame to guide the containergripping assembly onto a container; and / or optionally one or more sensors to detect whether a container is attached to the gripping assembly. 13. The method of embodiments 1-12, wherein the load-handling device comprises a body or skeleton mounted on a first set of wheels being arranged to engage with the first set of parallel tracks and a second set of wheels being arranged to engage with the second set of parallel tracks. 14. The method of embodiment 13, wherein the load-handling device comprises: a drive assembly configured to drive the first or second sets of wheels to move the loadhandling device along the first or second set of parallel rails in the respective parallel directions; and / or a direction-change assembly configured to raise or lower the first set of wheels and / or lower or raise the second set of wheels with respect to the body or skeleton to engage and disengage the wheels with the parallel tracks. 15. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any preceding embodiment. 16. A load-handling device configured to operate on a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising: a container-receiving space located above the rails for receiving and transporting a container to / from a storage location, the container-receiving space comprising a container-lifting assembly configured to raise and lower a container gripping assembly, the container-gripping assembly configured to releasably grip a container to attach and / or release a container; and a controller configured to carry out the method of embodiments 1-14. 17. A system comprising: a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces; and a load-handling device according to embodiment 16. Embodiments: Set B 1. A method of controlling a load-handling device for lifting and moving storage containers stacked in a grid framework structure comprising: a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising: a container-receiving space located above the rails for receiving and transporting a container to / from a storage location; a container-gripping assembly configured to releasably grip a container to attach and / or release a container; a container-lifting assembly configured to raise and lower the container gripping assembly to and from the container-receiving space, the method comprising: controlling, when a container is not attached to the container-gripping assembly, the container-lifting assembly to hold the container-gripping assembly in a first position above and as close as possible to a top surface of the grid without the container gripping assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks; and / or controlling, when a container is attached to the container-gripping assembly, the container-lifting assembly to hold the container-gripping assembly in a second position above and as close as possible to a top surface of the grid without the container contacting the grid when the load-handling device moves along the first and / or second set of tracks. 2. The method of claim 1, wherein the container-receiving space comprises a cavity to receive the container-gripping assembly and a container attached to the containergripping assembly, and optionally wherein the container-lifting assembly is located above the container-receiving space. 3. The method of claim 1, wherein the container-receiving space comprises a cantilever assembly, wherein the cantilever assembly defines a space below to receive the container-gripping assembly and a container attached to the container-gripping assembly, and optionally wherein the cantilever assembly comprises the container-lifting assembly. 4. The method of embodiments 1-3, further comprising: controlling, when a container is attached to the container gripping-assembly, the load-handling device to move adjacent to a target grid space, wherein the containergripping assembly is held at the second position above and as close as possible to the top surface of the grid; controlling the container-lifting assembly to lower a container attached to the container gripping assembly from the second position above and as close as possible to the top surface of the grid into a storage location beneath the target grid space; controlling the container-gripping assembly to release the attached container; controlling the container-lifting assembly to raise the container-gripping assembly to the first position above and as close as possible to the top surface of the grid. 5. The method of embodiments 1-4, further comprising: controlling, when a container is not attached to the container gripping-assembly, the load-handling device to move adjacent to a target grid space, wherein the containergripping assembly is held at the first position above and as close as possible to the top surface of the grid; controlling the container-lifting assembly to lower the container gripping-assembly from the first position above and as close as possible to the top surface of the grid into a storage location beneath the target grid space; controlling the container-gripping assembly to attach a container; controlling the container-lifting assembly to raise the container-gripping assembly to the second position above and as close as possible to the top surface of the grid. 6. The method of embodiments 1-5, wherein the load-handling device and / or the container-lifting assembly comprise a sensor, such as an encoder, to control a position of the container-gripping assembly. 7. The method of embodiments 1-6, wherein the container-lifting assembly comprises: at least one tether connected to the container-gripping assembly; and at least one winding means, such as a motor and at least one spool about which a respective the, to wind and unwind the at least one tether. 8. The method of embodiments 1-7, wherein the container-lifting assembly comprises four tethers, wherein each tether is connected to a respective corner of the container-gripping assembly. 9. The method of embodiments 1-8, wherein the top surface of the grid defines a plane and the first position above and as close as possible to the top surface of the grid is vertically above the plane by a first predefined offset, and / or the second position above and as close as possible to the top surface of the grid is such that the container is vertically above the plane by a predefined offset. 10. The method of embodiment 9, wherein the first predefined offset is about 10 mm or 5-35 mm and / or the second predefined offset is about 10 mm or 5-35 mm. 11. The method of embodiments 9 or 10, wherein the first predefined offset is set so that no part of the container-gripping assembly contacts or crosses the plane, and / or the second predefined offset is set so that no part of the container contacts or crosses the plane. 12. The method of embodiments 1-11, wherein the container-gripping assembly comprises: a frame; and one or more gripping elements on the frame, wherein the one or more gripping elements are each configured to attach and release a container thereto; and optionally one or more guiding elements on the frame to guide the containergripping assembly onto a container; and / or optionally one or more sensors to detect whether a container is attached to the gripping assembly. 13. The method of embodiments 1-12, wherein the load-handling device further comprises a body or skeleton mounted on a first set of wheels being arranged to engage with the first set of parallel tracks and a second set of wheels being arranged to engage with the second set of parallel tracks. 14. The method of embodiment 13, wherein the load-handling device comprises: a drive assembly configured to drive the first or second sets of wheels to move the loadhandling device along the first or second set of parallel rails in the respective parallel directions; and / or a direction-change assembly configured to raise or lower the first set of wheels and / or lower or raise the second set of wheels with respect to the body or skeleton to engage and disengage the wheels with the parallel tracks. 15. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any preceding embodiment. 16. A load-handling device configured to operate on a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising: a container-receiving space located above the rails for receiving and transporting a container to / from a storage location, the container-receiving space comprising a container-lifting assembly configured to raise and lower a container gripping assembly, the container-gripping assembly configured to releasably grip a container to attach and / or release a container; and a controller configured to carry out the method of embodiments 1-14. 17. A system comprising: a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces; and a load-handling device according to embodiment 16. Embodiments: Set C 1. A method of controlling a load-handling device for lifting and moving storage containers stacked in a grid framework structure comprising: a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising: a container-receiving space located above the rails for receiving and transporting a container to / from a storage location; a container-gripping assembly configured to releasably grip a container to attach and / or release a container; a container-lifting assembly configured to raise and lower the container gripping assembly to and from the container-receiving space, the method comprising: controlling, when a container is not attached to the container-gripping assembly, the container-lifting assembly to hold the container-gripping assembly in a first position such that a lowermost part of the container-gripping assembly is about 10 mm or 5-35 mm above a top surface of the grid; and / or controlling, when a container is attached to the container-gripping assembly, the container-lifting assembly to hold the container gripping assembly in a second position such that a lowermost part of the container is about 10 mm or 5-35 mm above a top surface of the grid. 2. The method of claim 1, wherein the container-receiving space comprises a cavity to receive the container-gripping assembly and a container attached to the containergripping assembly, and optionally wherein the container-lifting assembly is located above the container-receiving space. 3. The method of claim 1, wherein the container-receiving space comprises a cantilever assembly, wherein the cantilever assembly defines a space below to receive the container-gripping assembly and a container attached to the container-gripping assembly, and optionally wherein the cantilever assembly comprises the container-lifting assembly. 4. The method of embodiments 1-3, further comprising: controlling, when a container is attached to the container gripping-assembly, the load-handling device to move adjacent to a target grid space, wherein the containergripping assembly is held at the second position; controlling the container-lifting assembly to lower a container attached to the container gripping assembly from the second position into a storage location beneath the target grid space; controlling the container-gripping assembly to release the attached container; controlling the container-lifting assembly to raise the container-gripping assembly to the first position. 5. The method of embodiments 1-4, further comprising: controlling, when a container is not attached to the container gripping-assembly, the load-handling device to move adjacent to a target grid space, wherein the containergripping assembly is held at the first position; controlling the container-lifting assembly to lower the container gripping-assembly from the first position into a storage location beneath the target grid space; controlling the container-gripping assembly to attach a container; controlling the container-lifting assembly to raise the container-gripping assembly to the second position. 6. The method of embodiments 1-5, wherein the load-handling device and / or the container-lifting assembly comprise a sensor, such as an encoder, to control a position of the container-gripping assembly. 7. The method of embodiments 1-6, wherein the container-lifting assembly comprises: at least one tether connected to the container-gripping assembly; and at least one winding means, such as a motor and at least one spool about which a respective the, to wind and unwind the at least one tether. 8. The method of embodiments 1-7, wherein the container-lifting assembly comprises four tethers, wherein each tether is connected to a respective corner of the container-gripping assembly. 9. The method of embodiments 1-8, wherein the container-gripping assembly comprises: a frame; and one or more gripping elements on the frame, wherein the one or more gripping elements are each configured to attach and release a container thereto; and optionally one or more guiding elements on the frame to guide the containergripping assembly onto a container; and / or optionally one or more sensors to detect whether a container is attached to the gripping assembly. 10. The method of embodiments 1-9, wherein the load-handling device further comprises a body or skeleton mounted on a first set of wheels being arranged to engage with the first set of parallel tracks and a second set of wheels being arranged to engage with the second set of parallel tracks. 11. The method of embodiment 10, wherein the load-handling device comprises: a drive assembly configured to drive the first or second sets of wheels to move the load-handling device along the first or second set of parallel rails in the respective parallel directions; and / or a direction-change assembly configured to raise or lower the first set of wheels and / or lower or raise the second set of wheels with respect to the body or skeleton to engage and disengage the wheels with the parallel tracks. 12. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any preceding embodiment. 13. A load-handling device configured to operate on a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising: a container-receiving space located above the rails for receiving and transporting a container to / from a storage location, the container-receiving space comprising a container-lifting assembly configured to raise and lower a container gripping assembly, the container-gripping assembly configured to releasably grip a container to attach and / or release a container; and a controller configured to carry out the method of embodiments 1-11. 14. A system comprising: a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces; and a load-handling device according to embodiment 13.

Claims

1. A method of controlling a load-handling device for lifting and moving storage containers stacked in a grid framework structure comprising:a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising:a container-receiving space located above the rails for receiving and transporting a container to / from a storage location;a container-gripping assembly configured to releasably grip a container to attach and / or release a container;a container-lifting assembly configured to raise and lower the container gripping assembly to and from the container-receiving space, the method comprising:controlling, when a container is not attached to the container-gripping assembly, the container-lifting assembly to hold the container gripping assembly in a position above and as close as possible to a top surface of the grid without the container gripping assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks.

2. The method of claim 1, wherein the container-receiving space comprises a cavity to receive the container-gripping assembly and a container attached to the containergripping assembly, and optionally wherein the container-lifting assembly is located above the container-receiving space.

3. The method of claim 1, wherein the container-receiving space comprises a cantilever assembly, wherein the cantilever assembly defines a space below to receive the container-gripping assembly and a container attached to the container-gripping assembly, and optionally wherein the cantilever assembly comprises the container-lifting assembly.

4. The method of claims 1-3, further comprising:controlling, when a container is attached to the container gripping-assembly, the load-handling device to move adjacent to a target grid space, wherein the containerlifting assembly is controlled to position the container attached to the container grippingassembly above and as close as possible to the top surface without the container attached to the container gripping-assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks;controlling the container-lifting assembly to lower the container attached to the container gripping assembly into a storage location beneath the target grid space;controlling the container-gripping assembly to release the container attached to the container gripping assembly; andcontrolling the container-lifting assembly to raise the container-gripping assembly to the position above and as close as possible to the top surface of the grid.

5. The method of claims 1-4, further comprising:controlling, when a container is not attached to the container gripping-assembly, the load-handling device to move adjacent to a target grid space, wherein the containergripping assembly is held at the position above and as close as possible to the top surface of the grid;controlling the container-lifting assembly to lower the container gripping-assembly from the position above and as close as possible to the top surface of the grid into a storage location beneath the target grid space;controlling the container-gripping assembly to attach a container; andcontrolling the container-lifting assembly to raise the container-gripping assembly to position the container attached to the container-gripping assembly above and as close as possible to the top surface without the container attached to the container-gripping assembly contacting the grid when the load-handling device moves along the first and / or second set of tracks.

6. The method of claims 1-5, wherein the load-handling device and / or the containerlifting assembly comprise a sensor, such as an encoder, to determine a position of the container-gripping assembly.

7. The method of claims 1-6, wherein the container-lifting assembly comprises:at least one tether connected to the container-gripping assembly; andat least one winding means, such as a motor and spool, to wind and unwind the at least one tether.

8. The method of claims 1-7, wherein the container-lifting assembly comprises four tethers, wherein each tether is connected to a respective corner of the container-gripping assembly.

9. The method of claims 1-8, wherein the top surface of the grid defines a plane and the position above and as close as possible to the top surface of the grid is vertically above the plane by a predefined offset.

10. The method of claim 9, wherein the predefined offset is about 10 mm or 5-35 mm.

11. The method of claims 9 or 10, wherein the predefined offset is set so that no part of the container-gripping assembly contacts or crosses the plane.

12. The method of claims 1-11, wherein the container-gripping assembly comprises: a frame; andone or more gripping elements on the frame, wherein the one or more gripping elements are each configured to attach and release a container thereto; andoptionally one or more guiding elements on the frame to guide the containergripping assembly onto a container; and / oroptionally one or more sensors to detect whether a container is attached to the gripping assembly.

13. The method of claims 1-12, wherein the load-handling device comprises a body or skeleton mounted on a first set of wheels being arranged to engage with the first set of parallel tracks and a second set of wheels being arranged to engage with the second set of parallel tracks.

14. The method of claim 13, wherein the load-handling device comprises:a drive assembly configured to drive the first or second sets of wheels to move the load-handling device along the first or second set of parallel rails in the respective parallel directions; and / ora direction-change assembly configured to raise or lower the first set of wheels and / or lower or raise the second set of wheels with respect to the body or skeleton to engage and disengage the wheels with the parallel tracks.

15. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any preceding claim.

16. A load-handling device configured to operate on a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces, wherein the load-handling device is configured to move along the first and / or second set of tracks, the load-handling device comprising:a container-receiving space located above the rails for receiving and transporting a container to / from a storage location, the container-receiving space comprising a container-lifting assembly configured to raise and lower a container gripping assembly, the container-gripping assembly configured to releasably grip a container to attach and / or release a container; anda controller configured to carry out the method of claims 1-14.

17. A system comprising:a grid framework structure comprising a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of rails or tracks in a substantially horizontal plane to form a grid comprising a plurality of grid spaces, wherein the grid is supported by a set of uprights to form a plurality of vertical storage locations beneath the grid for containers to be stacked between and be guided by the uprights in a vertical direction through the plurality of grid spaces; anda load-handling device according to claim 16.s

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