Method and system for reducing hoist time of a gripping assembly

Abstract

A grided storage structure (14, Fig 3) in which containers / bins (10) are stacked (12) and on top of which load handling devices (30) move on a grided rail / track arrangement (22a, 22b). Devices (30)

Description

Technical Field The present invention relates to a method and system for reducing hoist time of a container-gripping assembly, such as one used in a load-handling device. Background Some commercial and industrial activities require systems that enable the storage and retrieval of a large number of different products. WO2015 / 185628A describes a storage and fulfilment system 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 are further described in WO2015 / 019055A1. A load-handling device comprises a container-lifting assembly that uses a motor to raise and lower a container-gripping assembly. It is important to reduce the time (i.e. hoisting time) taken for raising and lowering the container-gripping assembly. It is against this background that the present invention has been devised. Summary In a first aspect, there is 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 pattern 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, the load-handling device comprising: 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; a container-gripping assembly configured to releasably grip a container; a container-lifting assembly comprising a motor to raise and lower the container-gripping assembly; and a controller configured to: determine a load carried by the container-gripping assembly; and apply field weakening to the motor to increase a speed of the motor beyond a base speed if the load is below a threshold. This means the raising and / or lowering time of the container-lifting assembly can be reduced. The controller may be configured to determine the load is below a threshold if the containergripping assembly is not carrying a container, or the container-gripping assembly is carrying a container, wherein the container is below a predetermined weight. This means overheating and / or stalling of the motor can be avoided. The container-gripping assembly may be configured to lower the container-gripping assembly in the vertical direction, and / or raise the container-gripping assembly in the vertical direction. This means the containers can be raised from and / or lowered to the grid in minimum time. The motor may comprise a permanent magnet synchronous motor, PMSM. A PMSM lends itself to precise speed control. The controller may be further configured to drive the motor at or below the base speed if the load is at or above the threshold. This means overheating and / or stalling of the motor can be avoided. The controller may be further configured to use vector control or field orientated control to control the motor. This allows the motor’s speed to be precisely controlled. The load-handling device may further comprising a load sensor to determine the load carried by the container-gripping assembly, wherein the load sensor comprises, a weight sensor configured to determine the weight of a container, or a proximity sensor configured to determine whether the container gripping assembly is carrying a container, or the controller, wherein the controller is configured to correlate a current draw of the motor to the load. The controller may be configured to receive data from an external controller, wherein the data indicates the load carried by the container-lifting assembly. This means the field weakening can be selectively applied based on a precisely determined load. In a second aspect, there is a method for raising and / or lowering the container-gripping assembly of the load-handling device of any preceding aspect, wherein the method comprises: using the motor to raise and / or lower the container-gripping assembly; and using the controller to: determine a load carried by the container-gripping assembly; and apply field weakening to the motor to increase a speed of the motor beyond a base speed if the load is below a threshold. In a third 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 second aspect. Brief Description of Drawings The present invention is described with reference to one or more exemplary embodiments as depicted in the accompanying drawings, wherein: Figure 1 shows a storage structure and containers; Figure 2 shows track on top of the storage structure illustrated in Figure 1; Figure 3 shows load-handling devices on top of the storage structure illustrated in Figure 1; Figure 4 shows a single load-handling device with container-lifting means in a lowered configuration; Figures 5A and 5B show cutaway views of a single load-handling device with container-lifting means in a raised and a lowered configuration; Figure 6 shows a load-handling device; Figure 7 shows a motion profile used by the load-handling device of figure 6 when hoisting a container-gripping assembly; Figure 8 shows a system according to the invention; Figure 9 shows a method according to the invention; and Figure 10 shows a motion profile used by the invention. Detailed Description Online retail businesses selling multiple product lines, such as online grocers and supermarkets, require systems that can store tens or hundreds of thousands of different product lines. The use of single-product stacks in such cases can be impractical since a vast floor area would be required to accommodate all of the stacks required. Furthermore, it can be desirable to store small quantities of some items, such as perishables or infrequently ordered goods, making single-product stacks an inefficient solution. 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. PCT Publication No. WO2015 / 185628A (Ocado) describes a further known storage and fulfilment system in which stacks of containers are arranged within a grid framework structure (or a grid storage 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 Figures 1 to 3 of the accompanying drawings. As shown in Figures 1 and 2, stackable containers 10, also known as “bins”, are stacked on top of one another to form stacks 12. The stacks 12 are arranged in a grid framework structure 14, e.g. in a warehousing or manufacturing environment. 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. Figure 1 is a schematic perspective view of the grid framework structure 14, and Figure 2 is a schematic top-down view showing a stack 12 of bins 10 arranged within the framework structure 14. Each bin 10 typically holds a plurality of product items (not shown). The product items within a bin 10 may be identical or different product types depending on the application. 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 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 across the top of the stacks 12. Referring to Figure 3, 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) 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), 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 Figures 4, 5A and 5B - is described in PCT Patent Publication No. WO2015 / 019055 (Ocado), hereby incorporated by reference, where each load-handling device 30 covers a single grid space 17 of the grid framework structure 14. This arrangement allows a higher density of load handlers and thus a higher throughput for a given sized system. 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 assembly comprises a raising and lowering assembly (an example of which is shown in Figure 9) with a winch tether or cable 38 wound on a spool or reel, and a container-gripping assembly 39. The raising and lowering assembly also comprises a motor to rotate the spools and thus wind and / or unwind the tethers. The raising and lowering assembly shown in Figure 4 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 assembly 39, e.g. a lifting frame, for releasable connection to a storage container 10. For example, a respective tether 38 is arranged at or near each of the four corners of the container-gripping assembly 39. The container-gripping assembly 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 Figures 1 and 2. For example, the container-gripping assembly 39 may include pins (not shown) that mate with corresponding holes (not shown) in the rim that forms the top surface of 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 container-gripping assembly 39, powered and controlled by signals carried through the cables 38 themselves or a separate control cable (not shown). 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 container-gripping assembly 39 above the stack 12. The container-gripping assembly is then lowered vertically in the Z-direction, by the raising and lowering assembly to engage with the bin 10 on the top of the stack 12, as shown in Figures 4 and 5B. The container-gripping assembly 39 grips the bin 10, and is then pulled upwards by the cables 38, with the bin 10 attached. At the top of its vertical travel, the bin 10 is held above the rails 22 accommodated within the vehicle body 32. In this way, the loadhandling 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. On reaching the target location (e.g. another stack 12, an access point in the storage system, or a conveyor belt) the bin or container 10 can be lowered from the container receiving portion and released from the container-gripping assembly 39. The cables 38 are long enough to allow the load-handling device 30 to retrieve and place bins from any level of a stack 12, e.g. including the floor level. As shown in Figure 3, a plurality of identical load-handling devices 30 is provided so that each load-handling device 30 can operate simultaneously to increase the system’s throughput. The system illustrated in Figure 3 may include specific locations, known as ports, at which bins 10 can be transferred into or out of the system. An additional conveyor system (not shown) is associated with each port so that bins 10 transported to a port by a load-handling device 30 can be transferred to another location by the conveyor system, such as a picking station (not shown). Similarly, bins 10 can be moved by the conveyor system to a port 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. The load-handling device 30 has a container-receiving cavity or recess 40, in its lower part. The recess 40 is sized to accommodate the container 10 when lifted by the lifting mechanism, as shown in Figures 5A and 5B. When in the recess, the container 10 is lifted clear of the rails 22 beneath, so that the vehicle 32 can move laterally to a different grid location. If it is necessary to retrieve a bin 10b (“target bin”) that is not located on the top of a stack 12, then the overlying bins 10a (“nontarget bins”) must first be moved to allow access to the target bin 10b. This is achieved by an operation referred to hereafter as “digging”. Referring to Figure 3, during a digging operation, one of the load-handling devices 30 lifts each non-target bin 10a sequentially from the stack 12 containing the target bin 10b and places it in a vacant position within another stack 12. The target bin 10b can then be accessed by the load-handling device 30 and moved to a port for further transportation. Each of the provided load-handling devices 30 is remotely operable under the control of a central computer. Each individual bin 10 in the system is also tracked so that the appropriate bins 10 can be retrieved, transported and replaced as necessary. For example, during a digging operation, each non-target bin location is logged so that the non-target bin 10a can be tracked. Wireless communications and networks may be used to provide the communication infrastructure from the central computer, e.g. via one or more base stations, to one or more load-handling devices operative on the grid structure. In response to receiving instructions from the central computer, a controller in the load-handling device is configured to control various driving mechanisms to control the movement of the load-handling device. For example, the load-handling device may be instructed to retrieve a container from a target storage column at a particular location on the grid structure. The instruction can include various movements in the X-Y plane of the grid structure 15. As previously described, once at the target storage column, the container-lifting assembly can be operated to grip and lift the storage container 10 using the raising and lowering assembly and the container-gripping assembly 39. Once the container 10 is accommodated in the container-receiving space 40 of the load-handling device 30, it is subsequently transported to another location on the grid structure 15, e.g. a “drop-off port”. At the drop-off port, the container 10 is lowered to a suitable pick station to allow retrieval of any item in the storage container. Movement of the loadhandling devices 30 on the grid structure 15 can also involve the load-handling devices 30 being instructed to move to a charging station, usually located at the periphery of the grid structure 15. To manoeuvre the load-handling devices 30 on the grid structure 15, each of the load-handling devices 30 is equipped with motors for driving the wheels 34, 36. The wheels 34, 36 may be driven via one or more belts connected to the wheels or driven individually by a motor integrated into the wheels. For a single-cell load-handling device (where the footprint of the load-handling device 30 occupies a single grid cell 17), and the motors for driving the wheels can be integrated into the wheels due to the limited availability of space within the vehicle body. For example, the wheels of a single-cell load-handling device are driven by respective hub motors. Each hub motor comprises an outer rotor with a plurality of permanent magnets arranged to rotate about a wheel hub comprising coils forming an inner stator. The system described with reference to Figures 1 to 5 has many advantages and is suitable for a wide range of storage and retrieval operations. In particular, it allows very dense storage of products and provides a very economical way of storing a wide range of different items in the bins 10 while also allowing reasonably economical access to all of the bins 10 when required for picking. An example container-lifting assembly (further described in PCT application no. PCT / EP2022 / 081364 (Ocado)) is shown in Figure 6. In Figure 6, a container-lifting assembly 600 has a raising and lowering assembly 602 including four spools 610 to wind and unwind respective tethers 38. A drive belt 620 is driven by a motor (not shown) to rotate the spools on drive shaft 605 in an opposite direction to drive shaft 606. By rotating drive shafts 605 and 606 in opposite directions, respective tethers 38 can be located at or near the corners of the raising and lowering assembly. In particular, as shown in Figure 6, the point at which each tether winds or unwinds to or from a spool is at or near a respective corner of the raising and lowering assembly. This allows the tethers to connect to the container-gripping assembly 39 at a respective corner of the container-gripping assembly, which increases stability when raising and lowering the container-gripping assembly 39. During storage and retrieval operations, the container-lifting assembly (examples of which are shown in Figures 4, 5A, 5B, and 6) uses the raising and lowering assembly to raise and / or lower the container-gripping assembly in the Z-direction. The extent to which a container-gripping assembly is raised or lowered varies across the grid storage structure 14. Each stack of containers of the grid storage structure 14 will have a current dimension / height in the Z-direction defined by the number of containers currently in that stack. The current dimension / height in the Z-direction can be determined by tracking, via the central computer for example, containers that have been raised from and / or lowered to each stack of containers. For example, if a stack currently has 10 containers, where the containers are of a fixed dimension / height in the Z-direction, it can be determined that the current dimension / height of the stack in the Z-direction is 10 times the fixed dimension / height of a container in the Z-direction. It will be appreciated that the current dimension / height of the stack in the Z-direction can be expressed in an absolute sense for example n metres from ground, or a relative position, for example n metres from the bottom of the grid storage structure 14, or n metres from the top of the grid storage structure. The current dimension / height in the Z-direction of a stack of containers over which a load-handling device is located can be communicated to that load-handling device. The raising and lowering assembly of the load-handling device can use the current dimension / height in the Z-direction of the stack of containers to control the raising and lowering of the container-gripping assembly throughout an operation to obtain or replace a container from or to the grid storage structure. This way, the container-gripping assembly can be controlled appropriately using a motion profile, such as accelerating after being lowered from the load-handling device, and decelerating when approaching the top of the top container in the stack of containers. Similarly, the container-gripping assembly can, according to a motion profile, accelerate after being raised from the top of the top container in the stack of containers, and decelerate when approaching the load-handling device. One such example motion profile is shown in Figure 7. Motion profile 700 is used to move the container-gripping assembly 8 metres. As shown in Figure 7, motion profile has an acceleration phase 710 between 0 and 2 seconds, followed by a constant velocity phase 720 between 2 and 4 seconds, followed by a deceleration phase 730 between 4 and 6 seconds. The distance the container-lifting assembly moves due to motion profile 700 is shown by the dashed line 740 (this is shown as being linear for simplicity). As shown in Figure 7, the motion profile ensures the container-lifting assembly moves the required 8 metres. The motion profile is constrained by the maximum acceleration, maximum velocity, and maximum deceleration that can be provided by the motor used to raise and lower the container-lifting assembly. The maximum velocity is constrained by the base speed of the motor. The base speed is generally defined as the maximum speed at which the motor can operate at under at a rated torque of the motor. In Figure 7, the base speed of the motor is achieved in phase 720. Assuming the motor operates at its base speed, maximum acceleration, and maximum deceleration, it is not possible to reduce the time taken to move the container-lifting assembly the required distance, without compromising the torque provided by the motor. Furthermore, if a container carried by the container-lifting assembly carries a significant load, a minimum level of torque is necessary to lift the container. Operating below a minimum level of torque can result in overheating and / or stalling of the motor. It is therefore advantageous to reduce the time taken to raise and lower the container-gripping assembly a required distance. Figure 8 shows a schematic 800 of load-handling device 30 in accordance with the invention. The dashed lines show the vehicle body 32 of a load-handling device that travels on grid 22 via wheels 34 / 36. A raising and lowering assembly 810 (such as that shown in Figures 4, 5A, 5B, and 6) driven by a motor 830 raises and lowers container-gripping assembly 39 by winding and unwinding tethers 38. One or more load sensors 840 is configured to detect a load carried by the container-gripping assembly. Load-handling device 800 can use processor or controller 850 to receive and transmit data from and to each of the raising and lowering assembly 810, motor 830, and one or more load sensors 840. This data can be stored in storage 860. The data in storage 860 can be periodically transmitted for further processing via one or more networks, such as base stations. Figure 9 shows the steps of a method 900 for use in a load-handling device (such as that described above) 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 pattern 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, the load-handling device comprising 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, and a container-gripping assembly configured to releasably grip a container. It would be appreciated that the method of Figure 9 could be carried out using a controller (such as controller 850 of a load-handling device of Figure 8 for example). In step 910, a container-lifting assembly comprising a motor is used to raise and lower a container-gripping assembly 39, such as that shown in Figures 4, 5A, 5B, and 6. In step 920, a load currently carried by the container-gripping assembly is determined. In one implementation, the load can be determined by positioning a load sensor 820 between the container-gripping assembly 39 and an element (such as the pin or clips mentioned above) that grips the container. A load sensor used in this implementation converts the weight of the container to an electrical signal, and includes weight transducers, or strain gauges, or load cells. In an additional or alternative implementation, the load sensor 820 may detect that a container is currently being carried by the container-gripping assembly. A load sensor used in this implementation is any suitable proximity sensor. In an additional or alternative implementation, a current draw on the motor during an initial movement of the container-lifting device can be correlated by the controller to the load currently being carried by the containergripping assembly. A larger current draw indicates a larger load. In an additional or alternative implementation, the controller receives the data about the load of the current container from the central computer that has knowledge of the contents of the current container and thus the load. In step 930, the load currently carried by the container-gripping assembly is compared to a threshold. The threshold is chosen to strike a balance between increasing the speed of the motor and reducing the risk of overheating and / or stalling of the motor. In one implementation, the threshold corresponds to the container-gripping assembly not carrying a container, as detected by any of the load sensor configurations set out above, or via the central computer. Therefore, the threshold in this implementation ensures that the loading on the containergripping assembly is effectively zero and the speed of the motor can be increased without risk of overheating and / or stalling of the motor. In an alternative implementation, the threshold corresponds to a predetermined weight of the container, as detected by any of the load sensor configurations set out above, or via the central computer. The predetermined weight can be set depending on the motor characteristics to ensure that speed of the motor can be increased without risk of overheating and / or stalling of the motor. For example, the higher a power rating of the motor, the higher the predetermined weight. If the load is below the threshold, field weakening is applied to the motor to increase the speed of the motor beyond a base speed of the motor. In general, field weakening can be applied using vector control. In one implementation, the controller uses Field Orientated Control, FOC, on a motor, such as a permanent magnet synchronous motor, PMSM. Under normal operation, i.e. the motor having a base speed as its maximum speed, the FOC precisely aligns the motor’s stator’s magnetic field to be perpendicular with the torque producing vector. Field weakening, can be applied via the FOC, to adjust the stator’s magnetic field to be slightly angled or offset to the torque vector. This reduces the back EMF that the motor produces and thus the speed of the motor can be increased beyond the base speed. Field weakening can be applied upon the motor operating at its base speed. In general, any field weakening algorithm can be used provided it reduces the back EMF of the motor to allow a motor speed beyond the base speed. As a result of the method of Figure 9, the time taken to raise and lower the container-gripping assembly a required distance can be reduced. With reference to Figure 10, a comparison 1000 between the application of the field weakening algorithm to the motor to allow a motor speed beyond the base speed, and base speed operation of the motor is provided. Line 1010 shows the increased speed achieved by the field weakening application compared to base speed operation shown by line 1020. In this case, the maximum speed of the motor with field weakening applied is 50% greater than the base speed. Also shown in Figure 10, line 1030 shows that the time taken to move a required 8m with field weakening applied is 14 / 3 (4.66666 ) seconds. In comparison, line 1040 shows that the time taken to move a required 8m with base speed operation is 6 seconds. Lines 1030 and 1040 are shown as being linear for simplicity. Therefore, the application of field weakening reduces the time taken by 4 / 3 (1.33333 ) seconds (or 22.22222 %). When several load-handling devices are operated using the method of Figure 9, the overall time saving when raising and lowering container-lifting assemblies is significant and system throughput is increased. In other words, the total hoist time of the system is significantly reduced. It will also be appreciated that when the load is at or above the threshold, field weakening is not applied. That is, the motor has a base speed as its maximum speed. An asymmetric hoisting profile can thus be implemented. For example, when the load is at or above the threshold, the container-gripping assembly can lower a container with the motor operating at its base speed. As soon as the container has been deposited in the grid-structure, the load is below the threshold, and the container-gripping assembly can be raised with the motor operating above its base speed. Conversely, when the load is below the threshold in absence of a container, the container-gripping assembly is lowered with the motor operating above its base speed. As soon as a container has been gripped by the container-gripping assembly, and the load is at or above the threshold, the container-gripping assembly can be raised with the motor operating at its base speed. In effect, one part of a rising and lowering operation is implemented based on line 1010, and the other part is implemented based on line 1020. The system throughput can thus be maximised by using asymmetric hosting profiles. In this document, the language “movement in the n-direction” (and related wording), where n is one of x, y and z, is intended to mean movement substantially along or parallel to the n-axis, in either direction (i.e. towards the positive end of the n-axis or towards the negative end of the n-axis). In this document, the word “connect” and its derivatives are intended to include the possibilities of direct and indirection connection. For example, “x is connected to y” is intended to include the possibility that x is directly connected to y, with no intervening components, and the possibility that x is indirectly connected to y, with one or more intervening components. Where a direct connection is intended, the words “directly connected”, “direct connection” or similar will be used. Similarly, the word “support” and its derivatives are intended to include the possibilities of direct and indirect contact. For example, “x supports y” is intended to include the possibility that x directly supports and directly contacts y, with no intervening components, and the possibility that x indirectly supports y, with one or more intervening components contacting x and / or y. The word “mount” and its derivatives are intended to include the possibility of direct and indirect mounting. For example, “x is mounted on y” is intended to include the possibility that x is directly mounted on y, with no intervening components, and the possibility that x is indirectly mounted on y, with one or more intervening components. In this document, the word “comprise” and its derivatives are intended to have an inclusive rather than an exclusive meaning. For example, “x comprises y” is intended to include the possibilities that x includes one and only one y, multiple y’s, or one or more y’s and one or more other elements. Where an exclusive meaning is intended, the language “x is composed of y” will be used, meaning that x includes only y and nothing else. 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). As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and / or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The invention can take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment containing both hardware and software elements. In a preferred embodiment, the invention is implemented in software. 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. For the purposes of this description, a computer readable medium can be any tangible apparatus that can contain, store, communicate, propagate, or transport the program for use by or in connection with the computer. Moreover, a computer-readable medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk- read only memory (CD-ROM), compact disk-read / write (CD-R / W) and DVD. The flow diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of methods according to various embodiments of the present invention. In this regard, each block in the flow diagram may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be performed substantially concurrently, or the blocks may sometimes be performed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the flow diagrams, and combinations of blocks in the flow diagrams, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 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.

Claims

1. 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 pattern 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, the load-handling device comprising: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;a container-gripping assembly configured to releasably grip a container;a container-lifting assembly comprising a motor to raise and lower the container-gripping assembly; anda controller configured to:determine that a load carried by the container-gripping assembly is below a threshold; andapply field weakening to the motor to increase a speed of the motor beyond a base speed to lower the container-gripping assembly in the vertical direction;grip a container using the container-gripping assembly;determine that the load carried by the container-gripping assembly is above or at the threshold; andraise the container-gripping assembly in the vertical direction by driving the motor at or below the base speed; and / ordetermine that a load carried by the container-gripping assembly is at or above a threshold; andlower the container-gripping assembly in the vertical direction by driving the motor at or below a base speed of the motor;release a container using the container-gripping assembly;determine that the load carried by the container-gripping assembly is below the threshold; andapply field weakening to the motor to increase a speed of the motor beyond the base speed to raise the container-gripping assembly in the vertical direction.

2. The load-handling device of claim 1, wherein the controller is configured to determine the load is below a threshold if the container-gripping assembly is not carrying a containerthe container-gripping assembly is carrying a container, wherein the container is below a predetermined weight.

3. The load-handling device of any preceding claim, wherein the motor comprises a permanent magnet synchronous motor, PMSM.

4. The load-handling device of any preceding claim, wherein the controller is further configured to use vector control or field orientated control to control the motor.

5. The load-handling device of any preceding claim, further comprising a load sensor to determine the load carried by the container-gripping assembly, wherein the load sensor comprises:a weight sensor configured to determine the weight of a container; ora proximity sensor configured to determine whether the container gripping assembly is carrying a container; orthe controller, wherein the controller is configured to correlate a current draw of the motor to the load.

6. The load-handling device of any preceding claim, wherein the controller is configured to receive data from an external controller, wherein the data indicates the load carried by the container-lifting assembly.

7. A method for raising and / or lowering the container-gripping assembly of the load-handling device of any preceding claim, wherein the method comprises:using the motor to raise and / or lower the container-gripping assembly; andusing the controller to:determine a load carried by the container-gripping assembly; andapply field weakening to the motor to increase a speed of the motor beyond a base speed if the load is below a threshold; ordrive the motor at or below the base speed.

8. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of claim 7.

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